Advances in Photochemistry

ADVANCES IN PHOTOCHEMISTRY Volume 29

ADVANCES IN PHOTOCHEMISTRY Volume 29 Editors

DOUGLAS C. NECKERS Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio

WILLIAM S. JENKS Department of Chemistry, Iowa State University, Ames, Iowa

THOMAS WOLFF Technische Universita¨t Dresden, Institut fu¨r Physikalische Chimie und Elektrochimie, Dresden, Germany

WILEY-INTERSCIENCE A JOHN WILEY & SONS, INC., PUBLICATION

Copyright # 2007 by John Wiley & Sons, Inc. All rights reserved Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http:// www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats For more information about Wiley products, visit our web site at www.wiley.com. Library of Congress Cataloging-in-Publication Data: Library of Congress Catalog Card Number: 63-13592 ISBN 13: 978-0-471-68240-0 ISBN 10: 0-471-68240-3

Printed in the United States of America 10 9

8 7 6

5 4 3 2

1

CONTRIBUTORS

F. C. De Schryver KULeuven Department of Chemistry Celestijnenelaan 200F Heverlee B-3001 Belgium Mamoru Fujitsuka The Institute of Scientific and Industrial Research Osaka University Mihogaoka 8-1 Ibaraki, Osaka 567-0047 Japan

Tetsuro Majima The Institute of Scientific and Industrial Research Osaka University Mihogaoka 8-1 Ibaraki, Osaka 567-0047 Japan G. Schweitzer KULeuven Department of Chemistry Celestijnenelaan 200F Heverlee B-3001 Belgium

J. Hofkens KULeuven Department of Chemistry Celestijnenelaan 200F Heverlee B-3001 Belgium

Bernd Strehmel Kodak Polychrome Graphics Research and Development Division An der Bahn 80 D-37520 Osterode Germany

M. Lor KULeuven Department of Chemistry Celestijnenelaan 200F Heverlee B-3001 Belgium

Veronika Strehmel University of Potsdam Applied Polymer Chemistry Karl-Liebknecht Str. 24/25 D-14476 Golm Germany

v

vi

CONTRIBUTORS

M. van der Auweraer KULeuven Department of Chemistry Celestijnenelaan 200F Heverlee B-3001 Belgium

PREFACE

Volume 1 of Advances in Photochemistry appeared in 1963. The stated purpose of the series was to explore the frontiers of photochemistry through the eyes of experts and pioneers. As editors we have solicited articles from scientists who have strong identifications with the work presented, and strong points of view. Photochemistry has expanded enormously since those first days. A serious percentage of the papers in any single volume of the Journal of the American Chemical Society, for instance, can rightly fall in its purview. The emergence of the laser and the evolution of theoretical methods strongly influenced research. With new computational methodology almost no intermediate lives too short a time to be detected and its dynamics characterized. The fundamental objective of our field, elucidation of the history of a molecule that absorbs radiation, is now within reach in even the most complicated cases. We hope that the series continues to reflect the frontiers of photochemistry as it evolves into the future. We report, sadly, that one of the founding Editors, George S. Hammond, passed away in Portland, Oregon on October 5, 2005. George will be sorely missed. With the publication of this volume, Douglas Neckers will be leaving the post of Senior Editor. He has served, first as Associate Editor and more recently as Editor, for nearly half of the volumes in the Advances series. He wishes to express his appreciation for all of the cooperation he has received from everyone involved in the series. He will remain as a consultant, and Pavel Anzenbacher will take over as Editor beginning with Volume 30. Bowling Green, Ohio, USA Dresden, Germany Ames, Iowa, USA

Douglas C. Neckers Thomas Wolff William S. Jenks

vii

CONTENTS

Ensemble Photophysics of Rigid Polyphenylene Based Dendritic Structures M. LOR, G. SCHWEITZER, M. VAN DER AUWERAER, J. HOFKENS, AND F. C. DE SCHRYVER Photochemistry of Short-Lived Species Using Multibeam Irradiation MAMORU FUJITSUKA AND TETSURO MAJIMA

1

53

Two-Photon Physical, Organic, and Polymer Chemistry: Theory, Techniques, Chromophore Design, and Applications BERND STREHMEL AND VERONIKA STREHMEL

111

Index

355

Cumulative Index Volumes 1–29

379

ix

ENSEMBLE PHOTOPHYSICS OF RIGID POLYPHENYLENE BASED DENDRITIC STRUCTURES M. Lor, G. Schweitzer, M. van der Auweraer, J. Hofkens, and F. C. De Schryver KULeuven Department of Chemistry, Celestijnenelaan 200F, Heverlee B-3001, Belgium

CONTENTS I. II. III. IV.

V. VI.

Introduction Electronic Excitation Transfer Stationary Measurements Single-Photon Timing Measurements A. Time-Resolved Fluorescence Measurements Performed Under Magic Angle Polarization Condition 1. Para-substituted Carbon Core Dendrimers 2. Meta-substituted First Generation Carbon Core Dendrimers B. Time-Resolved Fluorescence Polarization Measurements 1. Meta-substituted First Generation Carbon Core Dendrimers 2. Para-substituted First Generation Carbon Core Dendrimers Femtosecond Fluorescence Upconversion Measurements Femtosecond Transient Absorption Measurements A. p-C1P1 and m-C1P1 B. p-C1P3 and m-C1P3

Advances in Photochemistry, Volume 29 Edited by Douglas C. Neckers, William S. Jenks, and Thomas Wolff # 2007 John Wiley & Sons, Inc.

1

2

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

C. m-C1P3 D. p-C2P1 and p-C2P4 VII. Conclusions Acknowledgments References

I. INTRODUCTION Excited state processes in multichromophoric systems have attracted attention for a long time [1], since these processes are of great importance in biological and material science. Indeed, in the light harvesting system as well as in polyconjugated polymers, multiple chromophores are present and the efficiency of the system in the energy cascade to the reaction center or in the efficiency of the charge generation is influenced by excitation and electron transport along the multichromophoric system. Because of the controllable incorporation of various functional groups in different parts of their structure, dendrimers have attracted much attention recently as model systems for the study of photoinduced intramolecular energy and electron transfer. Dendrimers can act as scaffolds that tether the donor and acceptor chromophores [2], providing versatility such that additional features can easily be introduced by simply changing the various components of the dendrimer. Alternatively, the dendrimer backbone itself can concurrently be used as the energy donor or acceptor. Several types of chromophoric dendrimer backbones such as poly(phenylacetylene) [3], poly(phenylene) [4], and poly(benzylether) [5] have been used as light absorbers, and the energy was efficiently transferred to the core acceptor. While most of these systems have high energy transfer efficiencies, they still suffer from a weak fluorescence or a low fluorescence quantum yield. However, polyphenylene dendrimers composed of tens or hundreds of out-of-plane twisted phenyl units can be used as chromophoric backbones [6] carrying highly luminescent dyes at the periphery. The earliest work on intramolecular energy transfer in dendritic macromolecules originates from Moore and co-workers [7], who synthesized dendritic structures based on phenylacetylene units with perylene in the center. The excitation of the phenylacetylene units at the rim at a wavelength of 310 nm leads to fluorescence emitted by the center perylene unit, indicating intramolecular excitation energy transfer. A significant increase in the rate of excitation energy transfer was achieved by modifying the dendrimer skeleton. This was done in such a way that additional phenylacetylene units with lower excited state energy and larger conjugation length toward the core were introduced near the perylene unit. Recently, Bardeen and co-workers

3

INTRODUCTION

1-H:

R¢ R

1-TMS:

R = H, R¢ = H R = Si(CH3)3, R¢ = H

1-Ph:

R = 3,5-di-t-butylphenyl, R¢ = t-butyl

R¢ R R

R

R

R 2-H:

R=H

2-TMS: R = Si(CH3)3 2-Ph:

R = 3,5-di-t-butylphenyl

3-H:

R=H

3-TMS:

R = Si(CH3)3

3-Ph:

R = 3,5-di-t-butylphenyl

Figure 1.1. Building blocks of phenylacetylene dendrimers studied by Bardeen and co-workers [8].

reported the role of Fo¨ rster, Dexter, and charge transfer interactions in phenylacetylene dendrimers [8]. They demonstrated by steady state spectroscopy, picosecond time-resolved emission and anisotropy measurements, and ab initio calculations that while the subunits of polyacetylene dendrimers (Fig. 1.1) are weakly coupled in their equilibrium ground state geometry, they can become strongly coupled in the excited state. This geometry-dependent electronic coupling will affect the modeling of energy transfer in these molecules. They found that the variation of the electronic coupling V with molecular geometry is due to the throughbond or charge transfer type of interaction rather than due to variation of the more familiar dipole–dipole and Dexter terms. These dendritic structures are rigid systems in which the branches are also the absorbers and the Bardeen study underlines the complexity of these systems in terms of excitation transfer. Most of the dendritic molecules investigated for excitation transfer between chromophores attached at the periphery belong to a class in which the arms are rather flexible. This of course leads to data related to excitation transfer, which are averaged over all the possible branch conformations leading to a distribution in distances between donor and acceptor. Balzani et al. [9] reported metal-containing dendrimers, where the core and branching unit are built up from ruthenium complexes of a polypyridine

4

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

O

O

O

O

O O

O

O

O

O

O

O

O

O O

N

N

O

O

O Ru

N N

O

2+ N

O O

N

O O O

O

O

O

O

O

O

O

O O

O O

O

Figure 1.2. Molecular structure of a metal-containing dendrimer investigated by Balzani and co-workers [10].

ligand serving as core and branching units. By varying the ligands and metals used, different directional excitation energy transfer processes were observed, either from the center to the rim or from the rim to the core [10]. The molecular structure of such a dendrimer with a ruthenium complex in the center is depicted in Figure 1.2. Recently, Balzani and co-workers published results on dendrimers consisting of a benzophenone core and branches containing four and eight naphthalene units (Fig. 1.3) [11]. In both dendrimers, excitation of the peripheral naphthalene units is followed by fast singlet–singlet energy transfer to the benzophenone core; but on a longer time scale a back energy transfer takes place from the triplet state of the benzophenone core to the triplet state of the

5

INTRODUCTION

O

O

O

(a)

O O O

O

O

O

O

(b) O

O

O O

O

O

O O O

O

O O

Figure 1.3. Molecular structures of dendrimers with 4 (a) and 8 (b) peripheral naphthalene units and a benzophenone core investigated by Balzani and co-workers [11].

peripheral naphthalene units. Selective excitation of the benzophenone unit is followed by intersystem crossing and triplet–triplet energy transfer to the peripheral naphthalene units, which could be observed by nanosecond transient absorption. Using a similar type of branch, developed by the Fre´ chet group, they have published extensively on chromophore labeled dendrimers [12]. The dendrimers possessing coumarin-2 dyes at the periphery, and either coumarin-343 (Fig. 1.4) or a heptathiophene dye at the core, were studied by time-resolved fluorescence and transient absorption spectroscopy. It was revealed that upon excitation of the rim chromophores almost no direct fluorescence occurred from these initially excited chromophores. Instead, only the center

6

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES O

O O

O O

O O

N

O

N N N O O O

O

O

O

N O

N

O O

O

N O O O

O

N O

O

O O

N

Figure 1.4. Molecular structure of a third generation dendrimer with coumarin-343 at the center investigated by Fre´ chet and co-workers [12].

chromophore showed emission; thus proving efficient excitation energy transfer within this dendrimer. The efficiency of the excitation energy transfer decreased by increasing the generation number from 3 to 4. This comes from the fact that increasing the generation number increases the average distance between the chromophores and thus the overall efficiency of excitation energy transfer decreases. Recently, Fre´ chet and co-workers reported intramolecular energy transfer in dendritic systems containing one or more two-photon absorbing chromophores at the periphery, which act as energy donors, and a Nile Red chromophore at the core that acts as energy acceptor as well as fluorescence emitter [13]. The two-photon energy absorbed by the chromophores at the periphery was transfered to the core, where the core’s emission was strongly enhanced. The emission from the core chromophore in these dendritic systems was significantly greater than the emission from the core itself when the core was not connected to the donor chromophores. This increased emission arises from the much larger two-photon absorbing cross section of the donor chromophores compared to the core acceptor at the excitation wavelength.

INTRODUCTION

7

N

Figure 1.5. Molecular structure of a second generation triaryl dendrimer investigated by Goodson and co-workers [15].

Meijer and co-workers investigated the dynamics of excitation energy transfer for a series of spherical porphyrin arrays based on different generations of poly(propylene-imine) dendrimers using time-resolved fluorescence anisotropy measurements in a glass environment [14]. They demonstrated that the multiporphyrin functionalized dendrimers were able to absorb light and efficiently distribute the excitation energy by hopping over the chromophore arrays with minimal loss during the energy migration process. Goodson and co-workers investigated excitation energy transfer processes in nitrogen cored distyrylbenzene and triarylamine dendrimer systems (Fig. 1.5) by photon echo and polarized fluorescence upconversion spectroscopy. Observed components of less than 1 ps were attributed to a coherent energy transport mechanism. The contributions from his group were recently summarized [15]. De Cola and co-workers recently published [16] a study of the photophysical properties of a molecular system consisting of a bay-functionalized perylene

8

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

O

O O

O

O O

O O

N

O

O

N

N

O

N

O O O

O O

O O

Figure 1.6. Molecular structure of a bichromophoric pyrene–perylene bisimide system investigated by De Cola and co-workers [16].

bisimide, containing four appended pyrene and two coordinating pyridine units (Fig. 1.6) using steady state, time-resolved emission and femtosecond transient absorption spectroscopy. Analysis of the data showed the presence of a fast intramolecular photoinduced energy transfer process from pyrene*–perylene to pyrene–perylene* (ken
6:2  109 s1 ) with a high yield (>90%), followed by efficient intramolecular electron transfer from pyrene–perylene* to pyrene.þ–perylene. (70%, ket
6:6  109 s1 ). Both processes occur from the pyrene unit to the perylene moiety. The Fo¨ rster distance was calculated to be 3.4 nm and the corresponding donor–acceptor distance was calculated from the energy transfer rate as 0.9 nm. No indications for energy hopping between different pyrene moieties were observed. Similarly, a number of terrylenediimide core dendrimers with semiflexible arms were investigated by our research group at the ensemble [17] and at the single molecule level [18]. Different generations of a polyphenyl dendrimer containing a terrylenediimide core with peryleneimide chromophores at

9

INTRODUCTION

O O

N

N

O

O O

N

O

O

O

O

O

O

N

O

O N

O N

O O

Figure 1.7. First generation polyphenylene dendrimer with terrylene as a luminescent core.

the periphery (first generation depicted in Fig. 1.7) have been studied with respect to intramolecular energy transfer processes. Excitation of the peryleneimide at 480 nm resulted in fluorescence of the terrylenediimide chromophore at 700 nm with an almost complete disappearance of the fluorescence of the peryleneimide chromophore at 550 nm, indicating a very efficient energy transfer process between the peryleneimide and terrylenediimide chromophore. Single molecule data measured at room temperature indicated that a distri bution of excitation transfer rate constants could be observed [18], while Basche´ and co-workers [19] showed, studying linewidths at low temperature, that the observed rates are larger than expected from classical Fo¨ rster excitation transfer theory and suggested that in these systems through-bond interaction might play a role. Similarly, phenylacetylene based dendrimers [7, 8] and those investigated by Goodson and co-workers [15] show substantial coupling between the branches while all others discussed above, due to flexibility of the connecting arms, have an undefined three-dimensional structure and hence variable donor– acceptor distances.

10

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES R1 R1

O N O

R2

O

C

C

N

R2

O

R3

R3

O N O

p -C1Px

m -C1Px

PI 1

p-C1P1 p-C1P3 p-C1P4

R H PI PI

2

R H PI PI

1

3

R H H PI

m-C1P1 m-C1P2 m-C1P3 m-C1P4

R H PI PI PI

2

R H H PI PI

3

R H H H PI

Figure 1.8. Molecular structures of p-C1Px ðx ¼ 1; 2; 4Þ, para-substituted first generation dendrimers, and m-C1Px ðx ¼ 1; 2; 3; 4Þ, meta-substituted first generation dendrimers; PI, peryleneimide chromophore.

In the present contribution we want to focus on rigid dendritic structures in which the coupling between the chromophores is weak and in which the distance between the chromophores involved is fixed in space. To achieve this goal together with the Mu¨ llen group (MPI Mainz), a series of molecules was developed based on the general structure in Figure 1.8. Besides these first generation dendrimers, second generation dendritic structures p-C2Pn were also investigated (p-C2P1, p-C2P2, p-C2P3, p-C2P4) (see Fig. 1.9).

II.

ELECTRONIC EXCITATION TRANSFER

One of the basic mechanisms in multichromophoric systems, electronic excitation transfer has been in the past and still is in many studies largely described using Fo¨ rster theory. As stated by Fo¨ rster [20], this model is developed for the weak coupling limit as it is based on an equilibrium Fermi Golden Rule

11

ELECTRONIC EXCITATION TRANSFER

O

N

O

O O

N

C

O

N O

O

N

O

Figure 1.9. Molecular structure of p-C2P4.

approach and the derived Fo¨ rster equation is valid provided a number of conditions are fullfilled as recently discussed by Scholes [21]: ‘‘(a) A dipole–dipole (or convergent multipole–multipole) approximation for the electronic coupling can be employed appropriately for the donor–acceptor interaction. (b) Neither the donor fluorescence lifetime, emission line shape, acceptor absorption line shape, nor oscillator strength is perturbed because of interactions among donors or acceptors, respectively. (c) Static disorder (inhomogeneous line broadening) is absent in the donor and acceptor line shapes. (d) the energy transfer dynamics are incoherent.’’ Different complicating factors led to the development of a more generalized approach [22, 23] in which the Coulomb interaction is now considered in terms of local interactions between donor and acceptor transition densities. This is

12

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

(a)

(c) (b)

Figure 1.10. (a) Chemical structure of p-C1P1. (b) Two-dimensional (2D) representation of where chromophores can be attached to the dendrimer and (c) three-dimensional (3D) representation of isomer 2A2B of p-C1P2. The arrows indicate the possible substitution patterns.

particularly important when the donor–acceptor ‘‘chromophores’’ are large compared to their center-to-center separation. To verify if the above-mentioned boundary conditions are valid for the molecular structures reported in Figure 1.8, electronic coupling constants were calculated. Doing this, one needs to take into account that, as a result of the asymmetric building blocks used in the Diels–Alder cycloaddition in the course of the reaction, the attachment of the chromophores leads to structural isomers. Therefore, if multiple chromophores are present, small differences can occur in the efficiencies of photophysical properties among different isomers. An example of possible structural isomers (2D picture) and one example of a 3D isomer of p-C1P2 (2A2B) are given in Figure 1.10. As depicted in Figure 1.10b, there are four attachment places for the chromophores and this normally results in four possible isomers for p-C1P2. However, there is an asymmetry in the four polyphenyl branches resulting in two possible ways in which the two chromophores can be attached. The arrows indicate the possible substitution patterns of the chromophores in the different structural isomers. The positions where a chromophore can be attached are A2, A3, B2, B3, C2, C3, D2, and D3, where A, B, C, and D represent the different branches and 2 and 3 the second or third phenyl group within each branch where a chromophore can be attached. For p-C1P1, however, the two different structural isomers that can be formed will show similar photophysical behavior. Also, for p-C1P4 there are a number of possible structural isomers as can be seen in Figure 1.11a, b. These two

ELECTRONIC EXCITATION TRANSFER

13

Figure 1.11. (a, b) Two-dimensional representation of the two structural isomers p-C1P4A and p-C1P4B of p-C1P4. ða0 ; b0 Þ Three-dimensional representation of the two structural isomers of p-C1P4.

minimized structures were obtained using a molecular mechanics optimization method (Merck molecular force field) present in SPARTAN1. Geometry optimization of p-C1P4A shows that the center-to-center distance between the chromophores is on average 3.17 nm. For two structural isomers of this compound (p-C1P4A and isomer p-C1P4B, respectively), the difference in interaction between the chromophores in each isomer was calculated. Calculations of the electronic transitions of the two depicted structural isomers of p-C1P4 were done by using the CEO-INDO/S procedure [24]. Besides revealing the energy of the electronic transitions, this method allows for the calculation of the electronic coupling constants between the transition dipole moments of the chromophores. All reported values apply to a molecule in vacuum at 0 K. CEO calculations were performed on two isomers (p-C1P4A and p-C1P4B) of p-C1P4,

14

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

obtained by energy minimization (see Fig. 1.11a and 1.11b). The results of the CEO calculations on both isomers show an average value for this coupling of the chromophores of p-C1P4A to be 22.6 cm1. In p-C1P4B, the average distance between the chromophores is 3.3 nm except for pair 1–4, where the distance is only 1.7 nm. The average value for the coupling constants is 21.22 cm1, except for pair 1–4 for which a value of 62.6 cm1 is obtained. However, one needs to take into account that all the calculations are done assuming a temperature of 0 K, and hence at room temperature these couplings will be minimal. Furthermore, in collaboration with Beljonne and co-workers, transition densities were calculated [25] for excitation transfer between two peryleneimide chromophores coupled by a fluorene trimer (separation 3.4 nm) and found to be in line with the Fo¨ rster approximations.

III.

STATIONARY MEASUREMENTS

The steady state absorption and fluorescence spectra of all first generation dendrimers in toluene are depicted in Figure 1.12. Within experimental error, the former ones are identical for all compounds. In the emission spectra, however,

Figure 1.12. Steady state absorption and emission spectra of the first generation dendrimers in toluene: p-C1P1, p-C1P3, (solid lines,—), m-C1P1 (short dashes, - - -), and m-C1P3 (long dashes, – – – ).

SINGLE-PHOTON TIMING MEASUREMENTS

15

a small shift and broadening of the meta-substituted compounds spectra relative to the ones of the para-substituted compounds can be seen. Moreover, a change in the intensity ratio between the two vibronic maxima is also visible. For the meta compounds, the vibronic maximum at 595 nm is relatively more pronounced as compared to the one for the para compounds. The para coupling allows for a better conjugation of the p-electrons of the peryleneimide over the aromatic phenyl ring of the branch. As this effect is more important in the excited state than in the ground state, it will alter the perpendicular orientation of the neighboring phenyls in the excited state compared to the ground state. The width of the fluorescence band at half maximum (FWHM) increases slightly with the number of chromophores from 2680 cm1 for m-C1P1 to 2750 cm1 for m-C1P4. The fluorescence spectra of the first generation para-substituted dendrimers p-C1Px (x ¼ 1--4) are independent on the number of PI chromophores. Similarly, the absorption and emission spectra of the second generation rigid dendrimers (p-C2P1, p-C2P2, p-C2P3, p-C2P4) were found to be independent of the number of chromophores present in the dendrimers. The fluorescence quantum yield (
f ) is calculated to be 0.98 0.05 and is identical within experimental error for all compounds. The similarity of the fluorescence properties of all the para-substituted dendrimers in terms of spectral shape, fluorescence maxima, and fluorescence quantum yield suggests that the emission occurs from the same state in all the dendrimers. Triplet formation is very inefficient in these chromophores: the rate constant of intersystem crossing could be measured using single molecule spectroscopy and was found to be equal to 7  103 s1 [25, 29].

IV. SINGLE-PHOTON TIMING MEASUREMENTS A. Time-Resolved Fluorescence Measurements Performed Under Magic Angle Polarization Condition In order to examine the properties of the fluorescent states for the dendrimers more closely, fluorescence decay times for all first generation dendrimers were determined in toluene by single-photon timing detecting the emission under magic angle condition. 1. Para-substituted Carbon Core Dendrimers Table 1.1 shows that the lifetimes of p-C1P1, p-C1P3, and p-C1P4 are identical with the fluorescence decay measured for an adequate model containing a peryleneimide chromophore. A representative plot of the fluorescence decay of the first generation para-substituted dendrimers is given in Figure 1.13 for p-C1P4.

16

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

TABLE 1.1 Fit Parameters of the Fluorescence Magic Angle and Anisotropy Decays Measured for p-C1Px (x ¼ 1; 3; 4) in Toluene with kexc ¼ 488 nm and kflu ¼ 600 nm and Average Peryleneimide–Peryleneimide Distances (dDA) Compound p-C1P1 p-C1P3 p-C1P4

t (ns)

r0

y1 (ns)

y2 (ps)

b1

b2

4.2 4.2 4.2

0.34 0.31 0.34

1.4 1.6 2.0

— 70 50

0.34 0.09 0.07

— 0.33 0.37

b2/r0 (%) dDA (nm) — 71 79

— 2.7 2.7

The corresponding decay parameters are collected in Table 1.1. Similarly, the decays of the second generation dendrimers were measured and all decays could be fitted globally by a single exponential with a time constant of 4.2 ns (Table 1.2).

Figure 1.13. Time-resolved fluorescence decays of p-C1P4 with fits at 600 nm and 700 nm detection wavelengths. The upper panel shows the weighted distribution of residuals (Ri) and the lower panel represents the autocorrelation (ac) function for the decays. Inset reports on a shorter time scale.

17

SINGLE-PHOTON TIMING MEASUREMENTS

TABLE 1.2 Fit Parameters of the Fluorescence Magic Angle and Anisotropy Decays Measured for p-C2Px (x ¼ 1; 2; 3; 4) in Toluene with kexc ¼ 488 nm and kflu ¼ 600 nm and Average Peryleneimide–Peryleneimide Distances (dDA) Compound p-C2P1 p-C2P2 p-C2P3 p-C2P4

t (ns)

r0

y1 (ns)

4.2 4.2 4.2 4.2

0.32 0.36 0.36 0.35

2.7 3.1 2.7 3.0

y2 (ps) — 410 310 280

b1

b2

0.32 0.18 0.14 0.08

— 0.18 0.22 0.27

b2/r0 (%) dDA (nm) — 50 61 77

— 3.6 3.7 3.8

2. Meta-substituted First Generation Carbon Core Dendrimers The corresponding decay parameters are collected in Table 1.3. The fluorescence intensity of the dendrimer having only one chromophore (m-C1P1) decays single exponentially with a decay time of 4.25 0.05 ns. However, as the number of chromophores is increased in the dendrimer, a small contribution of an additional long decay component of 7.4 0.6 ns is found essential to fit the experimental data. It has to be noted, however, that the amplitude of this long decay component is very small in m-C1P2 and m-C1P3. Thus, in order to minimize the error in the fit procedure, an additional component with a fixed decay time of 7.4 ns, as obtained for m-C1P4, was introduced in the analysis of the fluorescence decays of m-C1P2 and m-C1P3 to allow a better comparison of the corresponding amplitudes. It was furthermore observed that the relative amplitude of the longer decay time is larger at the red edge of the fluorescence spectrum for all multichromophoric dendrimers as shown in Table 1.3 by the comparison of results obtained at 600 nm and 725 nm emission. From the small difference in the spectral width (vide supra), the assumption of an excited state excimer-like (or dimer) chromophore–chromophore interaction is possible but not conclusive. Better insight into the extent of excimer-like emission is obtained from the fluorescence decays, where only for the multichromophoric dendrimers is a long decay component of 7.4 ns observed along

TABLE 1.3 Fluorescence Decay Times (si ) and Associated Relative Amplitudes (ai ) for m-C1Px (x ¼ 1–4) Measured in Toluene at Room Temperature Using kexc ¼ 488 nm Compound m-C1P1 m-C1P2 m-C1P3 m-C1P4

t1 (ns)

t2 (ns)

4.25 4.25 4.25 4.25

0.32 0.36 0.36 0.35

a1-600 (%) 100.0 99.2 98.1 96.0

a2-600 (%) 0.0 0.8 1.9 4.0

a1-725 (%) — 98.7 96.2 93.7

a2-725 (%) — 1.3 3.8 6.3

18

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

with the typical peryleneimide fluorescence decay time of 4.25 ns as obtained for the monochromophoric model compound m-C1P1. The attribution of this long time constant can be made to an ‘‘excimer-like’’ species as the decay time is similar to that reported for the higher generation dendrimers having a flexible biphenyl core [26]. Further evidence for this assignment can be derived from the dependence of the amplitude a2 connected with the 7.4 ns component on the number of chromophores and the dependence on the emission wavelength (lflu), respectively. As reported in Table 1.3, this amplitude is 0.8% for m-C1P2 and increases to 4% for m-C1P4 at lflu ¼ 600 nm. This is reasonable as the probability of formation of the ‘‘excimer-like’’ entity increases as the number of chromophores in the dendrimer increases. By detecting at lflu ¼ 725 nm, a2 increases to 1.3% for m-C1P2 and to 6.3% for m-C1P4. The larger contribution of that component at longer emission wavelengths is also consistent with a red-shifted fluorescence from ‘‘excimerlike’’ entities. This suggests that a fraction of the molecules have a substitution pattern in which two of the PI chromophores are relatively close in space. No such long decay component of 7.4 ns is observed for para-substituted dendritic structures p-CnPn. The absence of the long decay component is therefore due to the different position of substitution leading to a better spatial separation of the individual chromophores. This is also supported by a comparison of the molecular structures of the para- and meta-substituted dendrimers obtained from molecular modeling, since the average center-to-center distance among the chromophores is 2.9 nm for the para series but only 2.6 nm for the meta series in the first generation series.

B. Time-Resolved Fluorescence Polarization Measurements From time-resolved fluorescence depolarization measurements, the anisotropy decay times () and the associated anisotropy (b) have been determined for all first generation dendrimers using Eq.(1): X X bi ð1Þ rðtÞ ¼ bi expðt=i Þ with r0 ¼ i

The sum of all bi is called the limiting anisotropy r0. 1. Meta-substituted First Generation Carbon Core Dendrimers For the monochromophoric meta-substituted dendrimer (m-C1P1), a monoexponential fit of the anisotropy decay function is sufficient, which gives a relaxation time of 1 ¼ 950 30 ps with b1 ¼ r0 ¼ 0:38 (Table 1.4). However, the anisotropy decay functions for the meta-substituted dendrimers having more than one chromophore (m-C1P2 to m-C1P4) can only be fitted with two exponential decay

19

SINGLE-PHOTON TIMING MEASUREMENTS

TABLE 1.4 Fit Parameters of the Fluorescence Anisotropy Decays Measured for m-C1Px (x ¼ 1–4) in Toluene with kexc ¼ 488 and kflu ¼ 600 nm at Which There Is Only Monomer Emission and Average Chromophore–Chromophore Distances (dDA) Compound m-C1P1 m-C1P2 m-C1P3 m-C1P4

r0 0.38 0.31 0.28 0.24

y1 (ns) 0.9 1.1 1.2 1.3

y2 (ps) — 200 130 110

b1

b2

0.38 0.16 0.10 0.08

— 0.15 0.18 0.16

b2/r0 (%) — 48 63 66

dDA (nm) — 2.6 2.6 2.7

functions (Table 1.4). The long depolarization time constant is similar to that obtained for m-C1P1. The value of this long time component increases with the number of chromophores from 1:1 0:04 ns for m-C1P2 to 1.3 0.07 ns for m-C1P4, while the value of the fast component (2) changes from 200 30 ps for m-C1P2 to 110 20 ps for m-C1P4 (Table 1.4). The sum of the bi for m-C1P3 and m-C1P4 is substantially smaller than the limiting anisotropy. This strongly suggests that, at a time shorter than the resolution of single-photon timing, there is already a process leading to loss of fluorescence polarization in the meta-substituted dendritic systems. 2. Para-substituted First Generation Carbon Core Dendrimers For the para-substituted dendrimer with one chromophore (p-C1P1), a monoexponential function is found to be sufficient to fit the anisotropy decay trace, which can be related to the relaxation time of 1 ¼ 1:4 ns 30 ps with b1 ¼ r0 ¼ 0:34 0:04 (Table 1.1). However, the anisotropy decay traces for the dendrimers having more than one chromophore (p-C1P3 and p-C1P4) can only be fitted with two exponential decay functions (Table 1.1). The amplitude of the component with a long depolarization time increases with the number of chromophores increasing from 1 to 4, while the value of the fast component (2) changes from 70 ps for p-C1P3 to 50 ps for p-C1P4 (Table 1.1). From fluorescence depolarization measurements, anisotropy relaxation times and the associated anisotropy values have been determined for p-C2P1, p-C2P2, p-C2P3, and p-C2P4. For the dendrimers with more than one chromophore, a two-exponential function was found to be necessary to fit the experimental anisotropy decay traces (Table 1.2). The multichromophoric dendrimers present two-exponential decays in the anisotropy traces. The fast component (410 ps to 280 ps) of the anisotropy decay (Table 1.2) is found to decrease from p-C2P2 to p-C2P4. Contrary to the meta-substituted dendrimers m-C1Pn, the sum of the bi is now always close to the limiting value of the anisotropy even if n is larger than one. Energy transfer processes can be revealed by time-resolved anisotropy data. The large value for the limiting anisotropy (r0) of p-C1P1, p-C2P1, and m-C1P1

20

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

(Tables 1.1, 1.2, and 1.4, respectively) confirms the parallel orientation of the absorption and emission transition dipole moment for a single chromophore. In contrast to the fluorescence anisotropy decay of m-C1P1, which contains only one peryleneimide chromophore, an additional shorter picosecond anisotropy decay component is observed in the multichromophoric dendrimers p-C1Pn , p-C2Pn , and m-C1Pn ðn > 1Þ. Therefore, this fast depolarization process can unambiguously be related to excitation energy hopping among the identical chromophores. The time scale of a hundred picoseconds for these processes suggests that the observed energy hopping occurs in terms of fluorescence resonance energy transfer. Within the framework of the Fo¨ rster formulation [20, 21, 27], a rate constant for excitation transfer between donor D and acceptor A can be expressed as kET ¼

 6 R0 kD R

ð2Þ

where R is the interchromophoric distance, kD is the inverse of the decay time of the donor, and R0 (the Fo¨ rster radius) is the distance at which the efficiency equals 50%, that is, the distance at which an equal probability exists for the excited chromophore to relax to the ground state or to undergo energy transfer. R0 depends on the relative orientation of the transition dipoles toward each other (k2), the spectral overlap (J(l)) of the absorption spectrum of the acceptor, and the normalized emission spectrum of the donor,
D, that represents the quantum yield of fluorescence of the donor, and n that represents the refractive index of the solvent as can be seen in Eq. (3): R0 ¼ 0:211ð^e2 n4
D Jð€eÞÞ1=6

ð3Þ

The calculated value of J ¼ 2:5  1014 M1 cm3 and R0 ¼ 3:8 nm using the spectral data (e ¼ 38; 000 M1 cm1 ,
F ¼ 0:98) of the monochromophoric m-C1P1 model compound have the typical order of magnitude. The efficiency as a function of relative distance, E, is E¼

R60 R60 þ r 6

ð4Þ

Information about the rate constant of hopping (khopp) through excitation energy transfer can be derived from the fast anisotropy decay time (2). In order to take into account the possibility of multiple energy transfer channels in the case of a multichromophoric system containing identical chromophores, among which efficient dipole–dipole interactions occur, the measured anisotropy decay time 2 can be related to khopp by Eq. (5), where the value of i represents the number

SINGLE-PHOTON TIMING MEASUREMENTS

21

of chromophores fully interacting in both forward and backward directions [28]. khopp ¼

1 1  i2 i1

ð5Þ

If we take m-C1P2 as the model system for energy hopping between two peryleneimide chromophores and further assume that the energy transfer occurs in both directions, then the rate constant (khopp) calculated from Eq. (5) with i ¼ 2 results in khopp ¼ 2:0 ns1 . Using this value for khopp, we can calculate the expected anisotropy decay time (2) for the case of equally distributed and interacting chromophores in m-C1P3 and m-C1P4, which gives 133 ps and 67 ps, respectively. These results are in good agreement with the experimentally observed 2 ¼ 130 ps of m-C1P3 and 2 ¼ 110 ps of m-C1P4. This indicates the suitability of the proposed model of energy hopping among all chromophores. Within the Fo¨ rster formulation, the donor–acceptor distances (dDA) can be calculated by Eq. (6) and are listed in Table 1.4: 6 dDA ¼

R60 kET tD

ð6Þ

where tD is the fluorescence decay time of the donor chromophore. All calculated values of dDA for the peryleneimide dendrimers are on the order of 2.6 nm, which is in good agreement with the average distance between two chromophores in different conformations obtained from molecular modeling results [28, 29]. This agreement further substantiates the suitability of the above proposed model for an energy hopping mechanism in the present dendrimers. However, with an increasing number of chromophores r0 decreases from 0.38 in m-C1P1 to 0.31 in m-C1P2 and to 0.24 in m-C1P4. This means that an additional fast depolarization process on a time scale below the time resolution of 30 ps for the time-correlated single-photon counting anisotropy experiments takes place. This loss in initial anisotropy can be explained by the occurrence of ultrafast energy hopping between neighboring chromophores, which can approach one another to distances on the order of 1 nm or by dimer formation within the temporal resolution, also observed in transient absorption anisotropy (vide infra). Hence, the Fo¨ rster approach for m-C1Pn is a first approximation, which is not fully adequate for that fraction of molecules in the ensemble of the constitutional isomers for which the closer distance leads to electronic coupling values no longer negligible and cannot be described with a weak coupling model. This restriction, however, is not applicable for the p-C1Pn series. In contrast to the monoexponential anisotropy trace of monochromophoric p-C1P1, the corresponding traces of the multichromophoric dendrimers p-C1P3 and p-C1P4 reveal a second and fast anisotropy decay component on the order of 50–80 ps (Table 1.1). Within the framework of the Fo¨ rster

22

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

formulation, an effective interaction radius (R0) can be calculated from the steady state spectra and the fluorescence quantum yield of the donor chromophore (fD), yielding a value of R0 ¼ 3:8 nm. On the basis of Eq. (6), a value of around 4:6 ns1 is obtained for khopp of p-C1P3 and p-C1P4. It might seem surprising that this value is more than twice as large as that (khopp ¼ 2 ns1 ) obtained for the meta-substituted dendrimers, even though the interchromophoric distances are 0.2 nm larger in the para series. In fact, by employing the excited state lifetime tD ¼ 4 ns, the above derived values of R0 ¼ 3:8 nm, and khopp, the calculation of the distance between two chromophores by Eq. (6) yields too small values of dDA. The obtained interchromophoric distances dDA for the peryleneimide chromophores are on the order of 2.3 nm in spite of the expected 2.8 nm from molecular modeling structures. The only reason for this large discrepancy can be the wrongly estimated value of R0 due to the too simplified assumption of the dipole–dipole orientation factor k2 value of 2/3, which is strictly valid only for a random orientation of the chromophores. Here, this assumption is not true anymore because of the attachment of chromophores into the dendrimer backbone. The real values of k2 are described by Eq. (7) k ¼ sinðdD Þ sinðdA Þ cosðjDA Þ  2 cosðdD Þ cosðdA Þ

ð7Þ

where jDA is the azimuthal angle between the involved transition dipole moment directions of the energy donor D and acceptor A, and dD and dA are the angles between the corresponding dipole directions of D and A with the internuclear D–A axis, respectively. For m-C1Pn, the values of k2 have been calculated using geometrical data derived from a 3D molecular mechanics calculation and leading to average values of around 0.8 for the chromophore orientations in the meta-substituted dendrimers, confirming that the approximation of k2 ¼ 2=3 (vide infra) was reasonable. However, for the para-substituted dendrimers p-C1Pn, the average k2 is determined as 2.1 and thus is much larger. The ratio of the calculated k2 values for the para versus those of the meta series is about 2.6. This value is in good agreement with the respective ratio of the experimentally determined hopping rate constants being about khopp(para)/khopp(meta) ¼ 2.3 or, if the slightly different interchromophoric distances (d ffi 2.6 nm for meta and d ffi 2.8 nm for para) are taken into account, with the ratios khoppd 6(para)/ khoppd 6(meta) ¼ 3.5. Consequently, the faster energy hopping kinetics in the para series can directly be traced back to a better orientation of the peryleneimide chromophores toward each other, yielding a much larger Fo¨ rster interaction radius R0 of 4.4 nm than in the meta series. Using this value of R0 in Eq. (6) indeed leads to values of dfret ¼ 2:7--2:8 nm, which are in good agreement with the average interchromophoric distances found in molecular mechanics modeling.

23

SINGLE-PHOTON TIMING MEASUREMENTS

50

S1

S0

S0

S1 Fluorescence Intensity (a.u.)

ε (103/ M cm)

40

energy transfer

30

20

10

0 350

400

450

500

550

600

650

700

750

Wavelength (nm)

Figure 1.14. Stationary absorption and emission spectra of p-C1P4 in toluene. The spectral overlap is depicted in gray. Inset: The scheme representing singlet–singlet excitation hopping.

Energy hopping is a Fo¨ rster-type process that is present in the multichromophoric dendrimer such as p-C1P4 and can be related to the spectral overlap as depicted in Figure 1.14. Using the values of dDA and R0 mentioned above for p-C1P4, efficiencies of 97.5% are obtained for energy hopping. The efficiency of energy hopping and singlet–singlet annihilation in p-C1P4 as a function of distance is shown in Figure 1.15. The figure clearly indicates that 50% efficiency is reached for a distance of 4.5 nm. It also allows us to see where in this three-dimensional picture the p-C2Pn series is situated. As the attachment of the chromophores to the dendrimer backbone in p-C2Pn cannot be taken as random, the value of k has been calculated from the threedimensional molecular structure using Eq. (7). The average value of about 2.7 has been found for the dendrimers where the chromophores are at large distances from each other (Fig 1.16a, a0 ). However, for the isomer of p-C2P4 with a short distance pair of chromophores (Fig. 1.16b, b0 ), the average k2 for all couplings between pairs of two chromophores is obtained as 1.5. The presence of two sites in each branch where the formation of different constitutional isomers is possible will lead to a much broader relative distribution of the distances and angles between the chromophores compared to p-C1P4. Hence, the hopping rate constant

24

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

1.0 Energy hopping 0.8

Efficiency

0.6

0.4

0.2

0.0 0

2

4

6

8 10 Distance (nm)

12

14

16

Figure 1.15. Schematic representation of the efficiency of the energy hopping process present in p-C1P4 as a function of distances expressed in Eq. (4).

khopp obtained from experimental results should be considered as an average hopping for the different possible constitutional isomers in the dendrimer. Taking into account the possibility of random hopping in the multichromophoric systems containing identical chromophores, an average hopping rate constant (khopp) according to the energy hopping model is given by Eq. (5), where 1 and 2 are the experimental extracted decay times and the value of i represents the number of chromophores. Using Eq. (5), a value of 0.85 ns1 for khopp is obtained for these dendrimers. This value is more than five times smaller than that of the corresponding first generation dendrimers. Based on the excited state lifetime (tD), the derived values of R0 (4.5 nm) with a value of 2.7 for k2 and khopp the distance between the two chromophores have been calculated from Eq. (6). This yields a value of d ¼ 3:7 nm, which is in good agreement with the average interchromophoric distance obtained from molecular modeling. From the sixth power dependence of khopp on the average interchromophoric distance (dDA) and using the ratio of the values of khopp for first and second generation series, an average for dDA for the second generation series is found to be 3.7 nm. As shown in Figure 1.15, the multichromophoric second generation dendrimer is still inside the active sphere in which energy hopping can take place with high efficiencies. The decrease in hopping rate constant in these molecules thus scales with the sixth power of the distance difference as expected within the Fo¨ rster model.

FEMTOSECOND FLUORESCENCE UPCONVERSION MEASUREMENTS

(a)

O

N

25

(a¢)

O

O N

O C

O

N O

Isomer p-C2P4

O

N

O

(b)

(b¢)

O N O O N O

O N C O

Isomer p-C2P4

O

N

O

Figure 1.16. Molecular structures of p-C2P4 isomers: (a) isomer with a long distance pair of chromophores; ða0 Þ 3D structure of isomer with a long distance pair of chromophores; (b) isomer with a short distance pair of chromophores; ðb0 Þ 3D structure of isomer with a short distance pair of chromophores.

V. FEMTOSECOND FLUORESCENCE UPCONVERSION MEASUREMENTS To reveal possible ultrafast processes occurring on a time scale less than 30 ps, femtosecond fluorescence upconversion experiments were performed [30] in toluene under magic angle polarization. To extract complete information of the decay times and their amplitudes in function of detection wavelength, the measurements were performed in three time windows of 5 ps, 50 ps, and 420 ps. In order to reveal properties that are independent of potential chromophore– chromophore interactions, p-C1P1 was investigated in a first series of measurements as a model compound, since it contains only one chromophore. Figure 1.17a shows a typical result for p-C1P1 at two different detection

26

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

Normalized Fluorescence Intensity

(a)

620 nm

540 nm

0 0

10

(b)

20 30 Delay (ps)

40

50

Normalized Intensity

1.0 590 nm

0.8

p -C1P4

0.6

p -C2P4

0.4 0.2

(a)

0.0 0

100

200

300

400

Normalized Intensity

1.0 0.8

p -C1P1 p -C2P1

590 nm

0.6 0.4 0.2

(b)

0.0 0

100

200 Delay (ps)

300

400

Figure 1.17. (a) Time-resolved fluorescence intensity of p-C1P1 detected at 540 nm and 620 nm as indicated. (b) Comparison of the time-resolved fluorescence intensity recorded at 590 nm. (a) Multichromophoric first generation p-C1P4 and multichromophoric second generation p-C2P4. (b) Monochromophoric first generation p-C1P1 and monochromophoric second generation p-C2P1.

FEMTOSECOND FLUORESCENCE UPCONVERSION MEASUREMENTS

27

TABLE 1.5 Decay Times Resulting from Global Analysis for All Dendrimers Investigated in Toluene Compound

t1 (ps)

t2 (ps)

t3 (ps)

t4 (ns)

p-C1P1 p-C1P3 p-C1P4 m-C1P1 m-C1P3 m-C1P4 p-C2P1 p-C2P4

0.5–2.0 0.5–2.0 0.5–2.0 0.5–2.0 0.5–2.0 0.5–2.0 0.5–2.0 0.5–2.0

6.3 4.6 4.0 10.0 8.0 7.5 6.0 5.8

110 45 45 188 137 83 50 40

4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2

wavelengths excited at 495 nm, showing a clear wavelength dependence of the fluorescence decay and a complex multiexponential decay consisting of several components. Especially in the first few picoseconds pronounced wavelength dependence is evident. These data are then compared to those of dendrimers containing 3 and 4 peryleneimide chromophores at the rim. By global analysis, four decay components were revealed in both first and second generation compounds. Decay traces from second and first generation dendrimers p-C1P4, p-C2P4, p-C1P1, and p-C2P1 are compared in Figure 1.17b. The resulting time constants obtained by the global analysis procedure for the various compounds are summarized in Table 1.5. The values for t1 are not constant at different analysis wavelengths throughout the spectrum, so these decay times could not be linked globally. The second component (t2, a2) exhibits a fast time constant on the order of a few picoseconds for all compounds and represents 15–40% of the total amplitude, depending on the wavelength and the compound. The third component contributes at most 10%, and in most cases even less to the total amplitude, but is found necessary to obtain good fits. The largest part of the amplitude, however, is found in the nanosecond component 4 (t4, a4) for all compounds. Figure 1.18a shows the partial amplitudes for p-C1Px for the ultrafast decay component 1 as a function of the detection wavelength. The related decay component t1 is the only decay time out of the four resolved in our analysis that is wavelength dependent as shown in Figure 1.18c for p-C1P3. It clearly demonstrates the increase of the decay time with increasing fluorescence detection wavelength. This shortest time constant is measured at the shortest detection wavelength and has a value of 500 fs evolving as shown in Figure 1.18c to 2 ps from shorter to longer wavelengths. While this decay time remains more

28

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

(a) 0.2

a1

0.0

–0.2

520

540

560

580

600

620

640

660

680

540

560

580

600

620

640

660

680

600

620

640

660

680

(b)

a1

0.0

–0.2

520 (c)

p-C1P3

τ1/ps

2.0

1.5

1.0

0.5 520

540

560

580

Wavelength / nm

Figure 1.18. Dependence of the intramolecular vibrational reorganization process amplitude a1 on the detection wavelength, (a) for the p-C1Px dendrimers (p-C1P1 [&], pC1P3[*], p-C1P4 [~]) and (a) comparison of p-C1P1 [&] and m-C1P1 [&]. (c) Time constant t1 as a function of the detection wavelength for p-C1P3.

FEMTOSECOND FLUORESCENCE UPCONVERSION MEASUREMENTS

29

or less constant between 500 and 700 ps between 540 and 640 nm, it increases rapidly at longer wavelengths. Thus, the time constant could not be kept constant in the global analysis, although the values obtained for the partial amplitudes are still the result of the global analysis procedure in which the three other decay times were linked. A second observation that can be made for this decay component is that it has negative partial amplitudes at all detection wavelengths above 540 nm. This means a growing-in of the decay curves at the early times after excitation due to the population of a fluorescing state from the initially populated vibronic level. For all compounds p-C1P1, p-C1P3, and p-C1P4, a similar behavior with respect to partial amplitudes and decay times could be observed at the measured fluorescence wavelengths (Fig. 1.17a). This behavior of the negative partial amplitudes, the order of magnitude and change in decay time depending on detection wavelength is typical for an intramolecular vibrational reorganization process in the electronically excited state of the chromophore [31]. This component is found in first and second generation dendrimers discussed here as well in the mono- and multichromophoric ones and is a combination of various processes resulting from the static and dynamic response of the environment of the chromophore [32]. Also, a fast relaxation of vibrationally excited levels (max. 2000 cm1 ) of the first singlet excited state in the peryleneimide cannot totally be excluded as a part of this component [32]. The second decay component that could be found in the para-substituted dendrimers has a value of 6.3 ps to 4 ps, depending on the compound (Table 1.5). Figure 1.19a shows the partial amplitudes of p-C1Px for this component as a function of the detection wavelength. First, considering only the monochromophoric compound p-C1P1 (Fig. 1.19a [&]) with a t2 of 6.3 ps, a change of sign of the partial amplitude can be observed. Taking into account the shape and the positive/negative behavior of this kinetic component, it is attributed to a vibrational relaxation in the electronically excited state of the peryleneimide chromophore. This process is coupled to a relaxation of the solvation shell around the chromophore, as the solvent molecules have to accommodate for the newly populated S1 state of the peryleneimide [33]. At fluorescence detection wavelengths close to the excitation, this will be seen as a fast decay component, whereas at longer wavelengths the fluorescence is detected from a state that first has to be populated with the time constant resolved. In the kinetic analysis, this is found as a rise term with the corresponding time constant. Thus, it can be concluded that this kinetic component is related to the single chromophore itself and its interaction with the surrounding solvent toluene molecules. The finding of a 6.3 ps component and its attribution is in line with literature, where an ultrafast stimulated transient absorption spectroscopy setup [34] was used to determine a vibrational population relaxation time in the same order of magnitude for

30

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

(a) 0.6

a2

0.4 0.2 0.0 –0.2 520

540

560

580

600

620

640

660

680

540

560

580

600

620

640

660

680

(b) 0.6

a2

0.4 0.2 0.0 –0.2 520

Wavelength (nm)

Figure 1.19. Wavelength dependence of the amplitude a2 of the second component in toluene (a) for the p-C1Px dendrimers (p-C1P1 [&], p-C1P3 [*], p-C1P4 [~]) and (b) comparison of p-C1P1 [&] versus m-C1P1 [&] and p-C1P4 [~] versus m-C1P4 [~].

molecules such as perylene in toluene solution. In many other investigations [35], time constants of a few picoseconds were found and attributed to a vibrational relaxation process for various chromophores in toluene and other solvents. To study the influence of the number of chromophores attached to the dendrimer on this second component, the multichromophoric compounds p-C1P3 and p-C1P4 were also studied. As can be seen in Figure 1.19a, the typical shape and wavelength dependence of the partial amplitude is persistent for all three dendrimers, but an additional positive shift can clearly be observed which increases with the number of chromophores. This clearly indicates the contribution of more than one process to this second kinetic component, meaning a more complex attribution compared to the one in the monochromophoric compound. Thus, for the interpretation of these results, two different contributions 2a and 2b to this component are assumed, which are related to different kinetic processes,

FEMTOSECOND FLUORESCENCE UPCONVERSION MEASUREMENTS

31

both of which exhibit time constants that are very close to each other and because of this cannot be separated by the global analysis. First, contribution 2a is present in all compounds and can be attributed to the vibrational and solvent relaxation, responsible for the typical shape and wavelength dependence and appearing purely only in the analysis of the monochromophoric compound. Contribution b of component 2 (2b) can only be observed if more than one chromophore is present and hence if there is an intramolecular interaction possible between two or more chromophores. It is superimposed on the wavelengthdependent contribution a of time constant 2 (2a) and is almost wavelength independent and increasing in amplitude with the number of chromophores. If more than one chromophore per molecule can get excited, a singlet–singlet annihilation process could occur, eventually resulting in a first excited singlet state and a ground state chromophore [30]. When the photon flux available in the laser focus at the sample position is calculated, a value of several tens of photons per chromophore and per laser pulse can be found; hence, intensity dependence of the photophysics can be expected. The estimated distance obtained by means of molecular modeling between two chromophores is about 2.9 nm [28]. The combination of this estimated distance and the fact that more chromophores can get excited simultaneously in one molecule yields the possibility for an intramolecular singlet–singlet annihilation of two excited chromophores resulting eventually in a first excited state and a ground state [36]. This annihilation process has been reported [36, 37] and has been the experimental topic in different investigations in various systems such as pigment–protein complexes [38] and J aggregates [39]. Thus, it is assumed that contribution 2a can be attributed to vibrational and solvent relaxation whereas 2b, only present in multichromophoric dendrimers, is attributed to singlet–singlet annihilation. In order to distinguish and separate these two kinetic decay channels, an excitation energy-dependent study was performed on the mono- and multichromophoric para compounds. The excitation energy imposed onto the sample was systematically varied between 20 and 400 nJ, corresponding to several tens and several hundreds of photons per laser pulse and per chromophore, respectively, and a clear dependence of the amplitude of the total second component (2a þ 2b) could be observed. Figure 1.20 shows the decay curves for p-C1P4 at two different well chosen wavelengths, namely, 590 nm (a) and 630 nm (b) at two different excitation energies. These detection wavelengths were selected because of the values of the amplitude of the vibrational relaxation process observed in the data obtained for p-C1P1 (Fig. 1.19a). At 590 nm, it is close to zero, while at 630 nm it has a clear negative value. In contrast, for the multichromophoric compounds, the amplitudes are positive and substantially larger at these selected wavelengths, which is due to the admixture of annihilation process 2b. Its partial amplitude should decrease as the excitation energy diminishes, and hence the partial amplitudes as a function of wavelength of component 2 of the

32

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

Figure 1.20. Comparison of the time-resolved fluorescence intensity I recorded at low and high excitation energy (as indicated). (a) Multichromophoric compound p-C1P4 detected at 590 nm. (b) Multichromophoric compound p-C1P4 detected at 630 nm. (c) Monochromophoric compound p-C1P1 detected at 590 nm.

multichromophoric compounds should converge to those observed for p-C1P1 (component 2a) at low excitation energies. This means that at 630 nm detection the total amplitude of this second decay component (2a þ 2b) should turn from a positive into a negative value with decreasing excitation energy. This is exactly what is observed (Figure 1.20b). At 590 nm, there is a clear decrease of the contribution of the annihilation process 2b upon lowering the excitation energy. Since the partial amplitude of the vibrational relaxation at this wavelength is also zero (vide infra), the total amplitude (2a þ 2b) of component 2 will vanish, yielding a decay consisting

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of a nanosecond component that only appears as a constant in this short time window (Figure 1.20b). In order to cross-check these findings, a similar energy series has also been performed for the monochromophoric compound p-C1P1. The results shown in Figure 1.20c show no detectable excitation energy dependence within the measured range as expected and in contrast with the multichromophoric compound measured at this detection wavelength. Thus, at all excitation intensities, the partial amplitudes a2 are constant, which is a clear indication that in this monochromophoric compound the contribution b of time constant 2 is nonexistent. Figure 1.21 shows the partial amplitude a2 for p-C1P1 (&) and p-C1P4 () at the two selected detection wavelengths (Fig. 1.21a at 590 nm and Fig. 1.21b at 630 nm) as a function of the excitation energy. The data of the multichromophoric dendrimers ( p-C1P4 shown) contain the typical dependence of an annihilation process, while those of the monochromophoric p-C1P1 do not exhibit excitation energy dependence variations (Fig. 1.21a, b).

Figure 1.21. Dependence of the partial amplitude a2 of the second component from the laser excitation energy for the para-substituted p-C1Px (p-C1P1 [&], p-C1P4 [~]) and meta-substituted peryleneimide dendrimers m-C1Px (m-C1P1 [&], m-C1P4 [~]). (a) Detection done at 580 nm (meta compounds) and 590 nm (para compounds). (b) Detection done at 620 nm (meta compounds) and 630 nm (para compounds).

34

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

This is a clear indication that in this monochromophoric compound the second contribution b of kinetic component 2 is not present and that this amplitude spectrum is showing only the contribution a of kinetic component 2 (related time constant 6.3 ps), which is attributed to the vibrational/solvent relaxation of the molecule. Looking at decay times for this component 2 (Table 1.5), one can observe a decrease in decay time upon increasing number of chromophores. This can be explained by the fact that the decay time determined by the analysis is a weighted combination of these two separate decay times t2a and t2b of the vibrational/solvent relaxation and the annihilation process as shown in the previous paragraph. Since the relaxation process is the slower of these two processes, the more chromophores present, the more important the annihilation becomes and the shorter the overall decay times. Similar observations were made for the meta-substituted dendrimers m-C1Pn, when n is larger than 1. In the second generation dendrimers, the second decay component that could be recovered has a decay time of 6 ps (Table 1.5). Figure 1.22 shows the partial amplitudes for the monochromophoric second generation compound p-C2P1 as a function of the detection wavelength. The positive offset of the partial amplitude curves of p-C2P4 (Fig. 1.19c) compared to p-C2P1 indicates that more than one process is contributing to the apparent component 2 of the multichromophoric compound as also observed in the first generation dendrimers. The first contribution (2a) to this process in both compounds has been attributed to a relaxation process (vide supra). The second contribution (2b) to this process is again an intramolecular singlet–singlet annihilation process that is independent of detection wavelength and exists only in compounds with multiple chromophores. It is, however, clearly less important than in the first generation p-C1P4. To further underpin the hypothesis formulated for p-C2P1 and p-C2P4 and to be able to separate these two processes discussed above, an excitation energy0.6

a2

0.4 0.2 0.0 –0.2 520

540

560

580

600

620

640

660

680

Figure 1.22. Wavelength dependence of the amplitude a2 of the second component for the compounds p-C2P4 [&], p-C1P4 [&], p-C2P1 [~], and p-C1P1 [].

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dependent study was also performed on both compounds. By varying the excitation energy impinging on the sample between 20 and 420 nJ, a clear dependence of the amplitude of the 6 ps component could be observed. This energydependent study was performed at the strategically chosen detection wavelength of 590 nm (vide supra). For the monochromophoric compound (p-C2P1), the partial amplitude for the component 2 is close to zero. However, for the multichromophoric compound (p-C2P4), the amplitude of the apparent component 2 becomes positive. Thus, all intensity dependence observed at 590 nm detection wavelength can be attributed to the intramolecular singlet–singlet annihilation process. The partial amplitudes for the multichromophoric compound (p-C2P4) are shifted to a higher value over the entire detection wavelength range. In Figure 1.23, the decays recorded at the 590 nm detection wavelength and at different excitation energies are depicted for the multichromophoric p-C2P4. Because the relative importance of the annihilation process should increase as the excitation energy increases, the partial amplitudes as a function of the detection wavelength of the 6 ps component of the multichromophoric compound should at low excitation energy resemble the one for the monochromophoric compound. This is exactly what is observed.

Figure 1.23. Dependence of the partial amplitude a2 of the second component from the laser excitation energy for the compounds p-C2P4 [&], p-C2P1 [~], p-C1P4 [&], and p-C1P1 [~] at detection wavelengths (a) 630 nm and (b) 590 nm.

36

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

The dependence of the partial amplitude a2 of this 6 ps component on the incident laser energy is shown in Figure 1.23a, b for p-C2P4 and p-C2P1 at 630 nm and 590 nm detection wavelength, respectively. This is a clear indication that in p-C2P1 the annihilation process is absent and that the amplitude spectrum is only showing the vibrational/solvent relaxation of the chromophore itself. The positive amplitude offset of the multichromophoric compounds with respect to the monochromophoric compounds is more pronounced for the first generation p-C1P4 than for the second generation p-C2P4, as seen in Figure 1.23 where the amplitude a2 is displayed as a function of the excitation energy at the detection wavelengths 630 nm (Fig. 1.23a) and 590 nm (Fig. 1.23b). At both detection wavelengths, the curve for the first generation compound p-C1P4 has larger partial positive amplitude compared to the second generation compound p-C2P4. The third kinetic component that could be recovered for all para-substituted peryleneimide dendrimers p-C1Px at all detection wavelengths has a time constant on the order of 100 ps and a relatively low partial amplitude. By checking a possible concentration effect between 105 M and 106 M on the different kinetic components by diluting the samples, this was the only component that was found to be dependent on the concentration. As a time constant on the order of 100 ps was also retrieved in SPT measurements performed on concentrated solutions of p-C1Px, this component can be attributed to an intermolecular process. For the second generation p-C2P4, however, this partial amplitude is intensity dependent as can be seen in Figure 1.24. Figure 1.24 depicts the partial amplitude a3 for p-C2P1, p-C2P4, and p-C1P1 at 590 nm as a function of the excitation energy. The monochromophoric p-C2P1 and p-C1P1 show no dependence on the excitation energy at the selected

Partial Amplitude α3

0.3 0.2 0.1 0.0

–0.1 0

50

100

150 200 250 Excitation Energy (nJ)

300

350

Figure 1.24. Dependence of the amplitude a3 of component 3 from the laser excitation energy for the compounds p-C2P4 [&], p-C2P1 [~], and p-C1P1 [&] at detection wavelength 590 nm.

FEMTOSECOND TRANSIENT ABSORPTION MEASUREMENTS

37

detection wavelength while p-C2P4 clearly shows a dependence at these wavelengths (Fig. 1.24). In view of the typical power dependence, the 40 ps component of p-C2P4 can also be attributed to an annihilation process. Because of the dependence of a2 and a3 on the excitation energy in p-C2P4, both the 5 and 40 ps components can be attributed to singlet–singlet annihilation processes. Since this is a Fo¨ rster allowed excitation energy transfer process, it has to be distance dependent. The appearance of two annihilation processes in p-C2P4 probably relates to the presence of constitutional isomers, which gives a broader distribution of distances between neighboring chromophores compared to that of p-C1P4. As a result, besides a fast (5 ps) annihilation process occurring between chromophores at short distances, similar to p-C1P4 but less important in p-C2P4, an additional annihilation process (50 ps) is resolved, which can be attributed to interactions between chromophores at longer distance. Two possible structures for isomers with a short- and a long-distance pair of chromophores are depicted in Figure 1.16a, a0 and 1.16b, b0 , respectively. The relative contribution of the short annihilation process indicates approximately 10–15% of isomers where the two chromophores are at shorter distances. The fourth and the longest component (t4, a4) is in the range of a few nanoseconds and thus cannot be determined precisely in the time windows used here. Instead, the actual values were taken from measurements performed using a single-photon timing detection setup and is attributed to the intrinsic fluorescence lifetime of the peryleneimide chromophore equal to 4.2 ns.

VI.

FEMTOSECOND TRANSIENT ABSORPTION MEASUREMENTS

So far, the photophysical properties of the dendrimers were investigated using the fluorescence signal either by single-photon timing (SPT) or fluorescence upconversion. Transient absorption is used to validate the presence of the annihilation process, to allow quantifying the spectral overlap between emission and absorption of the S1 state, the basis of singlet–singlet annihilation, and to evaluate the influence of the substitution pattern and the number of PI chromophores on the transient absorption properties of these dendrimers [40].

A. p-C1P1 and m-C1P1 The wavelength-dependent absorption changes are presented in Figure 1.25 for a number of different delay times after excitation. At positive times, two different parts in the transient spectrum can be seen: a negative signal extending from

38

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

Figure 1.25. Transient absorption spectra of p-C1P1 at different delay times: 10 ps (&), 1 ps (*), 2 ps (~), 5 ps (q), 10 ps (^), and 30 ps (3). Inset: Detailed display of the 520–580 nm region.

450 to 600 nm and a positive signal beyond 600 nm with a maximum approximately at 660 nm. In first approximation, both features can be seen instantaneously after excitation and decay on a nanosecond time scale. Since the signal in the transient absorption spectrum above 600 nm is positive, it can predominantly be attributed to an excited state absorption (ESA) process. From previous studies, it is known that p-C1P1 has a fluorescence quantum yield of almost unity and a fluorescence lifetime of 4.2 ns; thus, the ESA found here can be attributed to S1–Sn absorption within the peryleneimide chromophore. As the steady state absorption spectrum shows no intensity above 560 nm while the fluorescence spectrum extends from 510 to 750 nm, the negative signal in the transient spectrum cannot solely be attributed to ground state bleaching. It seems reasonable to assume that ground state bleaching dominates the signal between 450 and 510 nm. Above 510 nm, both ground state bleaching and stimulated emission are responsible for the negative signal, whereas the signal in the range between 560 and 600 nm is dominated by stimulated emission. There is no reason to assume that stimulated emission would only occur in the very blue part of the fluorescence spectrum, so it must be considered that

FEMTOSECOND TRANSIENT ABSORPTION MEASUREMENTS

39

there also is a contribution of stimulated emission above 600 nm. However, as the absolute value of the cross section for excited state absorption at these wavelengths exceeds the stimulated emission, the net result of transient absorption and stimulated emission is a large positive signal in this wavelength range. As stated earlier, the transient signal mainly decays on a nanosecond time scale. However, a detailed decay analysis of the transient absorption intensities as a function of delay time for different wavelengths reveals an additional picosecond relaxation process, which can most clearly be seen in the inset of Figure 1.25. Within the first 20 ps, the transient absorption intensity drops at a wavelength of 530 nm, at about 555 nm the intensity remains the same, while at 570 nm it rises. This relaxation process has been described before in detail [41] and is interpreted as a combination of vibrational and solvent relaxation. This feature, where the intensity decays at a given wavelength and rises at another wavelength with an identical time constant (6.3 ps), had been found (vide supra) in fluorescence upconversion experiments [30]. In the transient absorption data discussed, the same feature can be observed; however, the signs of the amplitudes are of course reversed. The results of the measurements for m-C1P1 are very similar to those of the para compound and the data sets can be interpreted identically. The obtained time constant of this vibrational/solvent relaxation process is 10 ps as found previously in fluorescence upconversion experiments [30]. For the compound m-C1P1, the maximum of the positive transient absorption band attributed to the S1–Sn absorption is shifted about 5 nm to the blue and also the zero crossing point is shifted from 610 nm in the case of p-C1P1 to 602 nm for m-C1P1.

B. p-C1P3 and m-C1P3 Another series of experiments was performed on p-C1P3, which contains three peryleneimides at the rim. Comparing the transient absorption spectra of this compound (see Fig. 1.26 top) to those of p-C1P1 (Fig. 1.26, bottom), one can see that the general shape is identical. Since the same chromophore is involved, the attribution of the signals in p-C1P3 can be the same as for p-C1P1. However, the transient absorption signal of p-C1P3 is two times lower in intensity than that of p-C1P1. This suggests the occurrence of an additional decay channel, which in view of the results discussed earlier can be attributed to an ultrafast singlet–singlet annihilation process. The temporal evolution of the transient spectra of p-C1P3 and p-C1P1 is grossly different. It seems that the signal in the multichromophoric dendrimer at 530 and 650 nm decays faster when compared to p-C1P1. This feature is demonstrated in Figure 1.27, where the transient absorption intensity as a function of time is plotted for p-C1P3 and p-C1P1 at detection wavelengths of 530 nm (top) and 650 nm (bottom). In

40

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

Figure 1.26. Transient absorption spectra of p-C1P3 (top) and p-C1P1 (bottom) at different delay times: 10 ps (&), 1 ps (*), 2 ps (~), 5 ps (q), 10 ps (^), and 30 ps (3).

accordance with previous findings [25, 30], this feature is attributed to singlet– singlet annihilation between two excited states within one dendrimer, leading to a first excited state and a ground state. In order to confirm this attribution, an additional series of experiments was performed in which, in analogy to the upconversion experiments, only the excitation intensity impinging on the sample was decreased by a factor of 5. These measurements were performed at 530 nm (maximum of the negative part of the transient signal) and 650 nm (maximum of the positive part of the transient spectrum). The results of these measurements are also collected in Figure 1.27. The decays of p-C1P1 are independent of the excitation intensity in contrast to the ones of p-C1P3. This is further strong support for the earlier made assumption of singlet–singlet annihilation. Thus, at 530 nm the annihilation process reduces the number of excited peryleneimide chromophores, leading to a decrease in both stimulated emission and ground state bleaching. At 650 nm it can be understood as a decrease in the amount of peryleneimides in the excited state. This singlet–singlet annihilation process is the additional decay channel in the transient absorption measurements of p-C1P3 compared to p-C1P1. The relative decrease of the signal due to the singlet–singlet

FEMTOSECOND TRANSIENT ABSORPTION MEASUREMENTS

41

Figure 1.27. Excitation intensity-dependent plot of the normalized transient absorption signals as a function of time at high (*, &, q, ~) and low (*, & ,s, ~) excitation power recorded at 530 nm (top) and at 650 nm (bottom) for the dendrimers p-C1P3 (*, *) and p-C1P1 (&, &).

42

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

annihilation at longer times is, at high excitation intensity, only 70% of the signal observed for p-C1P1.

C. m-C1P3 A similar series of experiments were performed on m-C1P3, which contains three peryleneimides (PI) connected in meta position to the outer phenyl ring, leading to exactly the same picture as derived from the comparison of p-C1P3. This includes the same general attribution of the transient bleaching and absorption signals as in m-C1P1 and the occurrence of singlet–singlet annihilation in m-C1P3, which is also evidenced by an additional excitation intensity-dependent study at a detection wavelength of 650 nm. The differences in the photophysical properties due to the different substitution patterns can be determined by comparing the compounds (m-C1P1, 3) with the compounds (p-C1P1,3). Already in the emission spectra, a bathochromic shift of the latter can be observed while the ground state absorption spectra are identical. They also show a less pronounced vibrational structure of the emission spectra. Differences can also be seen in the transient behavior of these compounds. The maximum of the positive transient absorption band for the compound m-C1P1 is shifted about 5 nm to the blue and also the zero crossing point is shifted from 610 nm in the case of p-C1P1 to 602 nm in the case of m-C1P1. The influence of the different substitution pattern upon the fluorescence dynamics of these dendrimers was discussed in detail using SPT and fluorescence upconversion detection (vide supra). The transient absorption measurements reported here show very similar features, thus confirming the above interpretation. The para coupling leads to a better conjugation of the p-electrons of the peryleneimide over the aromatic phenyl ring of the branch. This better conjugation lowers the excited state energy, leading to a bathochromic shift of the emission spectrum. This is illustrated by the 5 nm shift of the zero crossing point of the transient absorption spectrum from m-C1P1 compared to p-C1P1. In the meta-substituted compounds m-C1Pn, steric hindrance between the hydrogens of PI and the 2,6-phenyl rings on the second phenyl of the dendritic arm will disrupt the conjugation between PI and its 9-phenyl ring (Fig. 1.8).

D. p-C2P1 and p-C2P4 In order to reveal the influence of the generation number on the photophysical properties, the compound p-C2P1 was studied. It is a monochromophoric second generation dendrimer consisting of an interior building block and one

FEMTOSECOND TRANSIENT ABSORPTION MEASUREMENTS

43

Figure 1.28. Transient absorption spectra of p-C2P4 (top) and p-C2P1 (bottom) at different delay times: 10 ps (&), 1 ps (*), 5 ps (~), 20 ps (q), 50 ps (^), and 400 ps (3).

peryleneimide chromophore attached in a para position to the outer phenyl ring at the rim. The time-dependent transient absorption spectra are shown in Figure 1.28 (bottom). At positive times, two different parts in the transient spectrum can be seen: a negative signal that reaches from 450 to 600 nm with different maxima at about 500 nm, and a positive signal beyond 600 nm with a maximum at approximately 660 nm. Both features appear instantaneously after excitation and decay on a nanosecond time scale. Another series of experiments were performed on p-C2P4, which contains four peryleneimides at the rim. The time-dependent transient absorption spectra are displayed in Figure 1.28 (top ). Comparing the transient absorption spectra of p-C2P4 to those of p-C2P1 (Figure 1.28, bottom), one can see that the general shape is identical; thus, the attribution of the signals in p-C2P4 can be the same as in p-C2P1. The initial drop in transient absorption signal for p-C2P4 compared to p-C2P1 is smaller than in their first generation counterpart. This can be explained by the smaller relative contribution of the singlet–singlet annihilation process in p-C2P4 compared to p-C1P4. These general features are exactly the same as those observed for the first generation monochromophoric dendrimer p-C1P1. Since the chromophore involved, the steady state spectra, and quantum

44

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

yields are identical, the above information leads to the identical attributions of the negative signal to ground state bleaching/stimulated emission and of the positive signal to S1–Sn absorption as in the case of p-C1P1. In the short wavelength part of the signal related to ground state bleaching, however, we find a more intense signal as compared to the transient absorption spectrum of p-C1P1. This negative spectrum also resembles the ground state absorption spectrum in a more precise way, which seems reasonable assuming there is no excited state absorption in this spectral region. The temporal evolution of the spectra, however, is different. The change is completely analogous to the one reported above for the comparison of first generation mono- to multichromophoric dendrimers. In fluorescence upconversion (vide supra), two intensity-dependent annihilation processes—one with a time constant of 40 ps and by far the main component and a second, minor amplitude process with a time constant of about 5 ps—were observed. In the transient absorption measurements reported here, only two different excitation intensities were investigated from which we can neither exclude nor claim the presence of a second minor annihilation component. The occurrence of more than one annihilation process might relate to the presence of different isomers resulting in a broad distribution of rate constants, which under certain conditions would be analyzed as two annihilation processes. The influence of the generation number can be deduced by comparing the results of the second generation dendrimers (p-C2P1, 4) to those of the first generation dendrimers (p-C1P1, 3). While in the monochromophoric compounds no difference can be observed between the first and second generation dendrimer, in the multichromophoric dendrimers a clear dependence of the annihilation process on the generation number can be observed. Although this process is seen in both generations, the corresponding time scales are grossly different: while in p-C1P3 the annihilation process relates to a decay time of 4.2 ps, it corresponds to a decay time of 53 ps for p-C2P4. These two decay times result from global analysis of the transient absorption decays obtained at different probe wavelengths. For p-C1P3 and p-C2P4, the decay times of the annihilation processes determined by fluorescence upconversion were 4.6 ps and 40 ps, respectively [30, 42], which is in good agreement with the transition absorption data. Why is singlet–singlet annihilation faster than energy hopping? Since the distribution of distance is identical for both processes, one can visualize the difference based on the overlap between the emission and absorption. The transient absorption data allow extracting the absorption spectrum of the S1 to Sn transition and if we compare the spectral overlap for this transition with the emission (Fig. 1.29) with that for the S0 to S1 transition (Fig. 1.14), one immediately sees that the overlap integral is substantially larger for the annihilation process, hence leading to larger rate constants. As due to residual induced emission at longer wavelength, the extinction coefficient of the S1–Sn absorption can only

45

CONCLUSIONS

60 energy

S1

transfer

internal

S0

S2

conversion

S0

S1

Fluorescence Intensity (a.u.)

S1

50

30

ε (103/Mcm)

40

20

10

350

400

450

500

550

600

650

700

0 750

Wavelength (nm)

Figure 1.29. Overlap between the fluorescence spectrum of p-C1P1 and its transient absorption spectrum of the S1–Sn transition. Inset: The singlet–singlet annihilation process.

be underestimated; the rate for singlet–singlet annihilation can even exceed the values estimated here.

VII. CONCLUSIONS Ensemble photophysics of two series of rigid dendrimers with an identical rigid central sp3 core and substituted with peryleneimide chromophores at the meta (m-C1Px) and para (p-CnPx) position of the outer phenyl ring have been investigated by steady state and nanosecond to femtosecond time-resolved spectroscopic techniques. This series of molecules were synthesized to investigate chromophore–chromophore interactions and to validate models describing such processes. A complicating factor in the synthesis due to two different modes of Diels–Alder addition led to a mixture of constitutional isomers, which could not be separated. This means that even if all prerequisites for the application of the Fo¨ rster model are fulfilled, the resulting rate constants will be average values. Similar limitations will exist for all nonrigid dendritic structures, where in

46

ENSEMBLE PHOTOPHYSICS OF POLYPHENYLENE BASED STRUCTURES

solution a distribution of conformations present in a bulk experiment will lead to a distribution of distances. This was observed in directional excitation transfer in the semirigid dendrimers represented in Figure 1.7 [17]. This distribution could be resolved, however, in single molecule experiments [18]. In the time-resolved single-photon counting measurements, the meta-substituted dendrimers showed a contribution of an emission from an ‘‘excimer-like’’ species resulting from chromophore–chromophore interaction with a decay time of 7.4 ns, beside the emission of the individual phenyl-substituted peryleneimide chromophores with 4.2 ns, whereas for the para-substituted ones no such state has been observed. This suggests that a fraction of the molecules containing more than one chromophore show a stronger interaction and consequently can not be described using the weak coupling condition. Study of the same molecules at the single molecule level [43] underpins this conclusion. The excitation of the peryleneimide chromophore results in excitation energy hopping among similar chromophores for both dendrimer series. In the first generation para-substituted dendrimers, this energy hopping takes place among all peryleneimide chromophores with a hopping rate constant experimentally determined to be khopp ¼ 4:6 ns1 . This value is in accordance with rate constants theoretically derived on the basis of molecular modeling structures. By comparing to polyphenylene dendrimers, where the peryleneimide chromophores are attached in meta instead of para position, the importance of the dipole orientation factor k2 could experimentally be demonstrated in excellent agreement with the theoretical Fo¨ rster equation. While the value of k2 ¼ 0:8 in the meta series yields a hopping rate constant of khopp ¼ 2 ns1 , the improved orientation of peryleneimide chromophores in the para series yields a larger k2 value of about 2.1, leading to a more than two times faster hopping dynamics in spite of a larger average distance between the chromophores. To determine the influence of the distance between the chromophores in these dendrimers on intramolecular energy hopping, a series of second generation para-substituted peryleneimide dendrimers with rigid tetrahedral core (p-C2Px) were investigated. The energy transfer process could be explained in terms of Fo¨ rster-type energy transfer and the average values obtained for khopp scale properly with the sixth power of the distance ratio between first and second generation. The short time scale dynamics have been studied by means of femtosecond fluorescence upconversion. For all dendrimers these measurements revealed size-independent kinetic processes related to an internal vibrational redistribution, a vibrational/solvent relaxation. Singlet–singlet annihilation, only present in the multichromophoric compounds, was established by an excitation energydependent study. It has been shown that this type of process contributes to a larger extent in the para-substituted dendrimers compared to the meta-substituted ones. These differences between the meta- and para-substituted dendrimers

REFERENCES

47

demonstrate the important role of the spatial distribution of the chromophores at the periphery in the dynamics of the photophysical processes involved. Moreover, in the multichromophoric second generation p-C2P4, a dual annihilation process was observed. The fast annihilation process occurs between a short distance pair of chromophores comparable in distance to the one in p-C1P4, while the longer time annihilation process occurs among the more prevalent pair of chromophores at longer distance. The origin of this can be traced back to the distribution of constitutional isomers as a result of the synthesis as mentioned earlier. The presence of a generation-dependent annihilation process and the influence of the substitution pattern have been validated by femtosecond timeresolved transient absorption measurements. Two other Fo¨ rster allowed processes can occur in multichromophoric systems under condition of multiple excitations, namely, singlet–triplet quenching and singlet ion/radical quenching if either the triplet or ion/radical absorption spectra do overlap with the fluorescence spectrum of the donor. These processes were not observed for these systems at the ensemble level because of the low probability of formation of these species resulting in a small relative abundance at the ensemble level. However, at the single molecule level they could be visualized [25, 44].

ACKNOWLEDGMENTS All compounds discussed were synthesized in the research group of Prof. K. Mu¨ llen to whom we are greatly indebted. This fruitful and exciting collaboration was made possible through a Max Planck Research Award to FDS and a IAPV-03 grant by the Fedral Science Policy Agency. We are also indebted to many co-workers whose names are mentioned in the references to the original papers and to D. Beljonne and S. Mukamel for computational support.

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION Mamoru Fujitsuka and Tetsuro Majima The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan

CONTENTS I. Introduction II. Pulse Radiolysis–Laser Flash Photolysis A. Excited Radical Cations B. Fluorescence from Excited Radical Cations C. Excited Radical Anions III. Two-Color Two-Laser Flash Photolysis A. Higher Triplet Excited States ðTn Þ 1. Energy Transfer from the Tn State 2. Energy Gap Law 3. Substituent Effect on the Tn State Lifetimes 4. Bond Dissociation from the Tn State 5. Electron Transfer from the Tn State 6. Direct Observation of the Tn State B. Ketyl Radicals in the Excited State C. Excited Radical Cations D. Other Reactive Intermediates

Advances in Photochemistry, Volume 29 Edited by Douglas C. Neckers, William S. Jenks, and Thomas Wolff # 2007 John Wiley & Sons, Inc.

53

54

PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

1. 2.

Two-Color Two-Laser DNA Damaging Two-Color Laser Photolysis for Determination of the Rate Constant from the Product Analysis 3. Two-Color Laser Control of Photocatalytic Reaction on TiO2 Surface IV. Three-Color Three-Laser Flash Photolysis A. Three-Laser Control of Intermediate Population B. Stepwise Bond Cleavage of Two C—O Bonds Via the Tn State V. Conclusions Acknowledgments References

I. INTRODUCTION One important technology development is the laser. Today, we see various lasers in use everywhere. In science, pulse lasers have been used for the generation of various chemical intermediates by photoexcitation [1]. From laser flash photolysis experiments, various interesting reactions have been revealed. Recent improvements in the stability and pulse duration of lasers have made experiments quite easy to perform. For example, pulse duration decreased almost six orders of magnitude, going from the nanosecond to femtosecond regime. Furthermore, the pump and probe method, using a stable femtosecond laser, gives reliable transient absorption spectra even when spectral change is quite subtle, on the order of 103 absorbance. Because operation of pulse lasers is quite easy, scientists in various fields can use laser systems with the expectation of high performance. An electron beam from a linear accelerator is another powerful tool to generate reactive intermediates, such as radical cations and radical anions [2]. By selecting appropriate reaction conditions, including the solvent and gas atmosphere, selective generation of an intermediate can be achieved, since the reaction pathways generating these intermediates are well established. Furthermore, concentration of generated intermediates can be increased in relatively large volume even when the intermediate is difficult to generate with other methods. These points are quite useful; however, use of pulse radiolysis is not common among scientists. By combination of these pulse techniques (multibeam irradiation), multistep excitation of intermediates can be achieved. For example, laser excitation of an intermediate generated during pulse radiolysis can be realized. For years, chemistry based on multibeam irradiation methods has been investigated [3]. There are several advantages for the multibeam irradiation method.

55

INTRODUCTION

LUMO HOMO

M(S0)

M(S1)

M(Tn )

M(T1)

M.+(D1)

M.+(D0)

Figure 2.1. Electronic structures of M(S0), M(S1), M(T1), MðTn Þ, M þ(D0), and M þ*(D1). 



First, by employing multibeam irradiation, higher excited states, which cannot be accessed by the single-pulse excitation method, can be generated. Excited doublet and higher triplet excited states are examples of such intermediates (Fig. 2.1). For these higher excited states, various reactions, which do not proceed from the lowest excited states, are expected (Fig. 2.2). Isomerization, bond dissociation, rearrangement, and ionization are expected as intramolecular reactions via the higher excited states [4–14]. As for intermolecular reactions via the higher excited states, energy transfer, electron transfer, and hole transfer processes have been investigated [15–17]. Second, it should be pointed out that yield and selectivity of these photoinduced processes caused by the multibeam irradiation depend largely on the delay time of the second beam irradiation with respect to the first beam. Because an intermediate generated by the first beam excitation has a finite lifetime, concentration of the targeted intermediate depends on the delay time of the second laser irradiation after the first beam excitation. That is, chemical reaction induced by the multibeam irradiation is a ‘‘time-selective’’ process (Fig. 2.3). This feature is

3

M**

h v2 1

M*

Products x x

3

h v1

M*

M

Figure 2.2. Schematic illustration of a reaction caused by multibeam excitation. Reaction from the higher triplet excited state generated during two-color two-laser flash photolysis was representative.

56

PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

hv2

hv1 deactivation

hv2 B

C

B

low

A* concentration

A

C

selectivity

high

0 Second beam delay from first beam A*

First beam irradiation

time

Figure 2.3. Schematic illustration describing the ‘‘time-selective’’ process of multibeam chemistry. When the second beam produces different products B and C from A and A*, respectively, as indicated in the scheme on the left, selectivity depends on the second beam delay from the first beam irradiation.

one of the unique points of the chemical reactions induced by multibeam irradiation and can be regarded as a distinct advantage. Third, since a chemical reaction induced by multibeam irradiation only proceeds at the position where both beams overlap, a chemical process induced by multibeam irradiation is a ‘‘site-selective’’ process. This third advantage is important in photodynamic therapy, in which damage to healthy cells must be avoided. One of the most advanced techniques is to perform selective damage of cancer tissues deep beneath the skin surface (Fig. 2.4). Molecular memory is also possible when taking this feature into account. Because various reactive

Figure 2.4. Schematic illustration describing the ‘‘site-selective’’ process of multibeam chemistry. The damaged area caused by the multibeam laser irradiation can be limited to the overlap of two beams.

PULSE RADIOLYSIS–LASER FLASH PHOTOLYSIS

57

intermediates such as radicals have strong absorption bands in the visible region, bleaching and reappearance of the visible absorption band by the multibeam irradiation are equivalent to chromism control, which provides for information storage and read-out. From these characteristics, multibeam irradiation using various excitation sources can be applied to various fields, not only to the basic chemistry of the higher excited states. For years, various groups have investigated the reactions induced by multibeam irradiation [3] and various compounds, including basic molecules, supramolecules for molecular devices, semiconductor nanoparticles, and biomolecules like DNA. Because of their availability, nanosecond lasers have been combined to achieve multibeam irradiation, although excited intermediates have lifetimes usually shorter than nanoseconds. Thus, the properties determined by the nanosecond technique are indirect ones, including various ambiguities. Furthermore, the nanosecond technique does not allow investigation of photoswitching faster than the nanosecond regime. For detailed investigations, use of ultrashort pulse lasers is important. A series of studies on molecular switching by Wasielewski and co-workers [18–22] and a study of the higher excited states of thin films of conjugated polymers by Masuhara and co-workers [23] are examples of utilizing multibeam irradiation with ultrashort pulse lasers. These examples indicate the importance of multibeam irradiation in the picosecond regime, since the estimated kinetic parameters directly facilitate design of functional molecules. Studies using multibeam irradiation with ultrashort pulse lasers will become more important in the near future. In this chapter, we summarize recent progress in the photochemistry of shortlived species by use of multibeam excitation, including our recent achievements in this field. Our research group has employed various multibeam irradiation methods to reveal reaction processes of various excited intermediates, including basic molecules and biomolecules. We also achieved direct observation of shortlived species utilizing ultrashort pulse lasers. These results are interesting recent examples of reactions induced by multibeam irradiation. Based on the excitation method, this chapter is divided into the following sections: Pulse Radiolysis– Laser Flash Photolysis (Section II), Two-Color Two-Laser Flash Photolysis (Section III), and Three-Color Three-Laser Flash Photolysis (Section IV). Each section is further divided into subsections based on the topics.

II. PULSE RADIOLYSIS–LASER FLASH PHOTOLYSIS Pulse radiolysis is a powerful tool for generating various kinds of intermediates, such as radical cations and radical anions. Since an electron and a hole are

58

PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

generated from the initial radiolytic reaction during the pulse radiolysis of solutions, the electron or hole can be selectively trapped by solvents. Therefore, a solute radical cation is generated during the pulse radiolysis in alkylhalides, such as 1,2-dichloroethane or butyl chloride, and a solute radical anion is generated in basic solvents such as N,N-dimethylformamide and 2-methyltetrahydrofuran [24]. Selective generation of radical ions is quite useful for investigating the excited states of radical ions on the time scale of nanoseconds to 100 ms, because of the easy subsequent excitation. In the present section, we introduce some examples of studies on the excited states of radical ions using the pulse radiolysis–laser flash photolysis combined method [25–29].

A. Excited Radical Cations Upon photoexcitation, several organic compounds undergo isomerization. It is well known that cis (c)–trans (t) isomerization of stilbene (St) occurs via twist C double bond in the singlet or triplet excited states ing about the central C  upon irradiation with UV light [30]. Lewis and co-workers [31] have reported that a St radical cation (St þ) undergoes thermal c–t one-way isomerization via the St dimer radical cation (St2 þ) as an intermediate. On the other hand, photochemical c–t one-way isomerization of c-St þ to t-St þ occurs in rigid matrices at 77 K [32] and in solution at room temperature based on the laser flash photolysis of c-St þ formed during pulse radiolysis in 1,2-dichloroethane or secondary electron transfer (ELT) in acetonitrile [14, 33]. The photochemical c–t isomerization has been reported to take place in the second doublet excited (D2) state but not in the lowest doublet excited (D1) state of c-St þ[14]. Characterizations of St þ in the D2 state (St þ*) are necessary to elucidate the isomerization mechanism. c-St þ generated during pulse radiolysis of c-St in 1,2-dichloroethane showed D0 and D1 D0 transitions at 515 and the absorption bands due to the D2 780 nm, respectively. In the case of t-St þ, the corresponding peaks appeared at 480 and 760 nm [14, 34, 35]. The excitation of St þ(D0) at 532 nm produces St þ* with excitation energies of 50 and 53 kcal mol1 for c-St þ* and t-St þ*, respectively, that were calculated from the red edges of the absorption bands at 420–580 nm. The irradiation of t-St þ with a laser flash at 532 nm exhibited no change in the transient absorption spectra and time profiles of
O.D.480, where t-St þ shows an absorption peak (Fig. 2.5a). Therefore, t-St þ* does not isomerize to c-St þ in the ground (D0) state with the irradiation but decays to t-St þ in the D0 state with the rate constant of internal conversion (IC) of tSt þ* (Scheme 2.1). The irradiation of t-St þ in the presence of anisole (ANS) caused a decrease in the
O.D.480 immediately after the flash (Fig. 2.5b). The
O.D.480 increased 









































59

PULSE RADIOLYSIS–LASER FLASH PHOTOLYSIS

Figure 2.5. Kinetic traces of
O.D.480 during the pulse radiolysis–laser flash photolysis experiment of t-St (5  103 M) in the absence (a) and presence (b) of ANS (1.0 M) in Ar-saturated 1,2-dichloroethane.

with increasing concentration of ANS ([ANS]). It is obvious that t-St þ* is quenched by ANS via hole transfer quenching to give t-St and ANS þ with the bimolecular rate constant of kA (Scheme 2.1). The rise of
O.D.480 after the laser flash corresponds to the hole transfer from ANS þ to t-St to produce ANS and t-St þ [35], which occurs at the rate constant of 7.8  109 M1 s1, equivalent to the diffusion-controlled rate (kdiff) in 1,2-dichloroethane. The chemical yield of [t-St þ]disapp ðYt Þ in the presence of ANS is represented by Eq. (1): 









Yt ¼ ð½t-Stþ disapp =½t-Stþ 0 Þ ¼ I0 kA ½ANS =ðt1 t þ kA ½ANS Þ

ð1Þ

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

t-St * (τt = 240 ps) kA[ANS] hν

kdt

t-St + ANS

t-St

Scheme 2.1. Photochemistry of t-St þ involving the internal conversion at the rate constant of kdt and hole transfer quenching of t-St þ* by ANS at the bimolecular rate constant of kA. 



where [t-St þ]0 and [t-St þ]disapp are the concentrations of t-St þ before irradiation of the laser flash and t-St þ disappeared immediately after the laser flash, is the reciprocal of the I0 is the efficiency of the formation of t-St þ*, and t1 t 1 versus lifetime of t-St þ*. According to Eq. (1), the Stem–Volmer plots of Yt 1 1 [A] produced a linear line with an intercept of I0 and a slope of ðI0 kA tt Þ1 . Consequently, tt ¼ 240  50 ps was obtained. Contrary to t-St þ*, c–t one-way isomerization of c-St þ* to t-St þ* was observed within a laser flash. The chemical yield of t-St þ from c-St þ* is approximately (75  15)% per flash, and isomerization of c-St þ to t-St þ proceeds as the main process. Because only the decay of c-St þ and formation of t-St þ were observed in Figure 2.6, the remaining (25  15)% of c-St þ that disappeared is considered to convert to c-St, t-St, or a radical cation as a product 































Figure 2.6. (a) Transient absorption spectra recorded before the laser flash (open circles) and immediately after (filled circles) and 200 ns (open triangles) and 1 ms (filled triangles) after the laser flash during pulse radiolysis–laser flash photolysis of c-St (5  103 M) in Ar-saturated 1,2-dichloroethane. (b, c) Kinetic traces of
O.D.480 and
O.D.515, respectively, as a function of time after the electron pulse.

PULSE RADIOLYSIS–LASER FLASH PHOTOLYSIS

c-St * (τc = 120 ps)

kp

product

61

kA[ANS] ki

c

hν

kd

c-St + ANS

c-St

t-St

Scheme 2.2. Photochemistry of c-St þ involving the internal conversion at the rate constant of kdc , c–t isomerization to t-St þ at the rate constant of ki, product formation at the rate constant of kp, and hole transfer quenching of c-St þ* by ANS at the bimolecular rate constant of kA. 





showing little or weak absorption obscured by the strong absorption bands of t-St þ and c-St þ over the range of 400 –700 nm. An electrocyclic product such as dihydrophenanthrene is assumed as an intermediate of the phenanthrene that is formed as an oxidation product in the photolysis of c-St [36]. In order to determine the lifetime of c-St þ* ðtc Þ, c-St þ was irradiated in the presence of ANS. The transient phenomena of c-St þ* involving the isomerization and hole transfer quenching of c-St þ* with ANS are shown in Scheme 2.2. According to the Stem–Volmer plots, tc was calculated to be approximately 120  30 ps and found to be one-half of tt . The measurement of the lifetime of the D2 state using the hole transfer quenching was also examined for 1,2-diphenylcyclobutene radical cation in the D2 state (CB þ*). Because CB has a rigid planar structure with the c-St chromophore structurally constrained by the cyclobutene ring and is stable for geometrical isomerizations, CB þ is expected to have the same rigid planar structure as CB. The irradiation of CB þ with a laser flash at 532 nm exhibited no change in the transient absorption spectra and the time profile of
O.D.480. Therefore, CB þ* is deactivated to CB þ in the D0 state within the laser flash, which is similar to t-St þ* (Scheme 2.1). The lifetime of CB þ* was estimated to be 380  30 ps. The shorter lifetime of c-St þ* is attributed to isomerization and conversion to another product via twisting about the central C C double bond. The analogous process in CB þ* is severely hindered by structural constraints. 





























B. Fluorescence from Excited Radical Cations Fluorescence is a quite sensitive probe widely used in various fields. Although most radical cations and anions are nonemissive, there are several exceptions. In this section, we introduce the fluorescence detection of radical cations generated during pulse radiolysis. Fluorescence of the radical cations gave us unique information on their reactivities [26, 27].

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

Figure 2.7. Transient absorption spectra observed at 0 ns, 100 ns, and 1 ms after the 8-ns electron pulse, and transient fluorescence spectrum of TMB þ* observed at 300 ns after the electron pulse during pulse radiolysis–laser flash photolysis of TMB (1  102 M) in Ar-saturated 1,2-dichloroethane. Excitation wavelength, 532 nm. Laser pulse energy, 140 mJ pulse1. 

1,3,5-Trimethoxybenzene radical cation (TMB þ), which is generated during pulse radiolysis of TMB in 1,2-dichloroethane, gave a transient fluorescence spectrum around 600–750 nm upon photoexcitation at 532 nm with the second harmonic pulse of a Nd:YAG laser as shown in Figure 2.7. The transient fluorescence spectrum of TMB þ in the doublet excited state (TMB þ*) showed a mirror image symmetry to the absorption spectrum of free TMB þ [37]. Since the duration of the fluorescence was almost the same as that of the laser pulse, the fluorescence lifetime of TMB þ* is shorter than 1 ns. The fluorescence intensity monitored at 620 nm decreased with the increase of the delay time of the laser pulse to the 8-ns electron pulse, similar to the temporal profile of the transient absorption monitored at 590 nm (Fig. 2.8). However, the decay observed around 532 nm was much slower than that at 590 nm because of the ion pair formation ([TMB þ Cl]). When TMB þ was excited with the 532-nm laser pulse, a permanent depletion of the absorption was observed. Although the fluorescence spectra did not change with changing the delay time, the depleted absorption spectrum changed with the delay time. Immediately after the laser photolysis at an early stage shorter than 100 ns after pulse radiolysis, the depletion of the transient absorption of free TMB þ around 590 nm was small (less than 10%). However, greater depletion of the transient absorption of the ion pair at 520 nm (15–25%) was observed at a later stage longer than 100 ns after 















63

PULSE RADIOLYSIS–LASER FLASH PHOTOLYSIS

Figure 2.8. Kinetic trace of
O.D. at 590 nm of TMB þ during the pulse radiolysis followed by the consecutive irradiation of a 532-nm laser pulse. Fluorescence intensity (open circle) as a function of the delay time of the 532-nm laser pulse relative to the electron pulse is superimposed on the decay curve. 

pulse radiolysis. These results suggest that TMB þ* is quenched by Cl within the ion pair. From the quenching experiment with tetrabutylammonium chloride, the transient absorption spectrum after 30 ns was no longer assigned to free TMB þ. The estimated fluorescence quantum yield (f) was 3.1  105 and was constant in the time range longer than 50 ns. Since the lifetime of TMB þ* seems to be too short for the quenching of TMB þ* by quencher molecules at a diffusion-controlled rate, the estimated f is considered to be TMB þ* in the ion pair. On the other hand, f for free TMB was estimated from the experiment involving intermolecular hole transfer. Pulse radiolysis of a 1,2-dichloroethane solution containing biphenyl and TMB gave the radical cation of biphenyl þ immediately after the electron pulse. Since the oxidation potential of biphenyl is higher than TMB, biphenyl þ is quenched by the hole transfer to TMB at a nearly diffusion-controlled rate. Although the transient absorption of TMB þ showed a rise corresponding to the hole transfer, f decreased monotonously with the increase of the delay time. The estimated f value was 1.1  103 at 10 ns. At this stage, TMB þ can be considered a free ion because of the low yield of TMB þ and Cl (approximately 105 M), although the rate constant of collision between TMB þ and Cl is quite large (2.6  1011 M1 s1). Therefore, this value was considered to be the f value for free TMB þ*. 























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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

The radical cation of 3,5-dimethoxyphenol showed fluorescence in a similar manner [27]. The f value and lifetime were estimated to be (2  0.3)  103 and 350 ps, respectively.

C. Excited Radical Anions Radical anions of organic compounds can be generated selectively during pulse radiolysis in solvents such as N,N-dimethylformamide. Generated radical anions can be excited selectively by employing a laser at an appropriate wavelength. The c–t isomerization is also reported for the radical anion of St (St ). It is unclear whether or not the photoisomerization of St  occurs via a mechanism similar to that for St þ. In order to characterize St  in the D2 state (St *), we have investigated the selective ELT quenching of isomeric St * using biphenyl as an electron acceptor and estimated the lifetimes of St * using the pulse radiolysis–laser flash photolysis combined method [28]. From the selective ELT from St * to biphenyl, kbiphenyl t ¼ 10:7 and 17.8 M1 were obtained for c-St  and t-St , respectively, where kbiphenyl and t denote the rate constants of the ELT and the lifetimes of St *, respectively. The t values are estimated to be approximately 1.5  0.4 ns and 2.5  0.7 ns for c-St * and t-St *, respectively, from kbiphenylt assuming the diffusion-controlled rate constant for kbiphenyl. The shorter t of c-St * is attributed to the c–t isomerization * and via twisting about the central C C double bond. The t values of c-St * þ* þ* t-St are one order of magnitude larger than those of c-St and t-St [25]. The selective ELT quenching of the radical anions of dicyanoanthracene, phenazine, and anthraquinones in the higher doublet excited ðDn Þ state by electron quenchers such as fumaronitrile or dicyanobenzene is also investigated in N,N-dimethylformamide at room temperature using the pulse radiolysis–laser flash photolysis combined method [29]. The radical anions generated during the pulse radiolysis do not change upon irradiation with a laser flash at 532 nm. The radical anions in the Dn state decay into the D0 state within the laser flash (5 ns). Lifetimes of approximately 4 ns are estimated for three radical anions in the Dn state assuming a diffusion-controlled rate constant for the ELT quenching. The shorter lifetimes of 1.0–1.4 ns for methyl and chloro substituents on anthraquinone can be explained in terms of IC from the Dn to the D0 state of the radical anions accelerated by rotation of the substituents. The energy gap between the Dn and D0 states of the radical anions is a significant factor for the rate of IC. The quencher radical anion–neutral molecule pair is suggested as an intermediate in the ELT quenching of the radical anions in the Dn state by the electron quencher. In the present section, we introduced some examples of excited radical ions generated during pulse radiolysis. As pointed out previously, pulse radiolysis is a 



































65

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS hν

N

M

M

M

∆

hν

+ e–

– e–

N

Q

N

hν

∆

hν

∆

∆

hν

D

Q

Scheme 2.3. Reaction pathways applicable to the multibit molecular memory utilizing the multibeam chemistry of a molecule (M), in which isomerization of M to N and charge transfer from M to quencher (Q) play important roles to realize the multibit memory.

powerful tool for generating radical ions selectively and in a high yield. Furthermore, an oxidant or a reductant generated during pulse radiolysis as the initial process reacts with various kinds of compounds even though their oxidation or reduction is difficult with other methods. At the present stage, a nanosecond laser has been employed to excite radical ions generated during pulse radiolysis. Therefore, estimation of the properties of the excited radical ions was carried out in a rather indirect manner. For direct observation, detection and excitation systems with picosecond resolution should be developed. We summarized the present section from the viewpoint of a basic study to reveal the properties of excited radical ions. On the other hand, the present reaction system employing pulse radiolysis and laser photolysis gives a basis of a multibit molecular memory composed of various charged states and isomeric structures. Reaction pathways applicable to the multibit molecular memory utilizing multibeam chemistry are indicated in Scheme 2.3, in which isomerization and charge transfer to a quencher play important roles in achieving the multibit memory. It should be pointed out that a rather fast response is expected for multibit molecular memories utilizing an adequate chromophore-linked system or solid-state support for these chromophores. Furthermore, various intermediates, such as a highly charged state, are also applicable to this approach.

III. TWO-COLOR TWO-LASER FLASH PHOTOLYSIS A pulse laser is another excitation source for generating various reactive intermediates efficiently. In the case of pulse lasers, selective excitation of the ground and excited states is easy because laser pulses with various wavelengths can be obtained by harmonic generation with nonlinear crystals. Recent development of a laser utilizing optical parametric oscillation, which emits a variable wavelength, enlarged the scope of study. Thus, two-color two-laser flash photolysis has been adapted to a wide variety of fields [3]. Furthermore, utilization of

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

ultrashort pulse lasers is beneficial for direct observation of higher excited states with quite short lifetimes. We recently succeeded in the synchronization of nanosecond and picosecond lasers. In this section, we summarize recent results of two-color two-laser flash photolysis studies on the higher triplet excited states, excited radicals, excited radical ions, and other reactive intermediates.

A. Higher Triplet Excited States (Tn) 1. Energy Transfer from the Tn State Upon the photoexcitation of a chromophore with a doubly occupied highest molecular orbital, almost all chromophores generate the lowest triplet excited (T1) state through the intersystem crossing (ISC) process (Fig. 2.1). Since the lifetime of the T1 state is usually on the order of a microsecond or millisecond, the excitation to the higher triplet excited ðTn Þ state is feasible by combining nanosecond lasers. By employing adequate timing circuits, synchronization of two nanosecond lasers can be achieved rather easily. Thus, explorations of the Tn state have been carried out for years. In the late 1960s, Liu and co-workers demonstrated the energy transfer (ENT) process from the Tn state of compounds such as anthracene based on the product analysis, which can be obtained only from the Tn states [38–42]. They determined the lifetime of anthracene(T2) using high-concentration benzene, which acts as the triplet energy quencher and triplet energy carrier to endo-dicyclopentadiene, giving norbornene from the T1 state. Saltiel et al. [43] reported a similar approach in which the sensitized photoisomerization of St or 2,4-hexadiene is used as a probe of the triplet energy quenching of anthracene(T2). Kokubun and co-workers measured fluorescence from the S1 state after ISC from the Tn state [44–46]. The research groups of Scaiano and McGimpsey revealed various reactions for the Tn states [7, 9–11, 15, 16, 47, 48]. For the systematic understanding of the Tn state, we have studied intermolecular ENT processes of several aromatic hydrocarbons in the Tn states. In this section, studies on the Tn state properties of naphthalene (Np), a fundamental molecule, are described [49]. Since the quantum yield of fluorescence of Np (f ¼ 0.19) is not negligible [50], Np in the lowest triplet excited state (Np(T1)) was obtained by the triplet sensitization [51, 52]. As a triplet sensitizer, we employed benzophenone (BP), which yields the T1 state quantitatively with higher triplet energy than that of Np(T1). The generation of Np(T1) was confirmed by the growth of the transient absorption band at 415 nm (Fig. 2.9). Np(Tn) was obtained by the excitation of Np(T1) with irradiation of laser light at 425 nm. (Scheme 2.4 shows an energy diagram involving Np(S0), Np(T1), Np(Tn), Q(S0), and Q(T1).) No change of the transient absorption of Np(T1) was observed during the second laser irradiation. It is

67

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

0.4

0.2 [CCl4] = 0M 0.60 M 0.0 0

100

200

300

Time (ns)

Figure 2.9. Kinetic traces of
O.D.415 during two-color two-laser flash photolysis of Np in the absence and presence of CCl4— 0.25, 0.35, 0.50, and 0.60 M—in Ar-saturated acetonitrile solution at room temperature.

suggested that the fast IC of the Tn !T1 transition occurs within the laser flash duration of 5 ns. However, entirely different consequences were observed when the solution included CCl4. In the presence of CCl4, the second laser irradiation caused bleaching of the transient absorption of Np(T1). Furthermore, the bleaching increased with increasing concentration of CCl4. No change of transient absorption of Np(T1) was observed under the irradiation by one laser at 355 or 425 nm Sn

Tn ENT

IC

S1

T1

IC

hν2

ISC

reaction

T1 T1

ENT

hν1

ENT

ISC ISC

S0

S0

S0 BP

Np

Q

Scheme 2.4. Energy diagram of BP(S0), BP(S1), BP(T1), Np(S0), Np(T1), Np(Tn), Q(S0), and Q(T1) involving the triplet ENT from NpðTn Þ to Q(S0) giving Np(S0) and Q((T1), and from Q(T1) to Np(S0) giving Q(S0) and Np(T1).

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

in the presence of CCl4. The bleaching was observed only in the presence of CCl4 and with irradiation of two lasers as shown in Figure 2.9. Similar experimental results were observed in the presence of CH2Cl2. On the other hand, the formation of Cl–benzene complex was confirmed by detection of the transient absorption with a peak at 490 nm in the presence of benzene in CCl4 solution [53, 54], indicating that NpðTn Þ donated its energy to CCl4, causing the cleavage of the C—Cl bond of CCl4, that is, CCl4* ! CCl3 þ Cl. In order to elucidate the mechanism involving these phenomena, several quenchers such as dichlorobenzene (DCB) and dicyanobenzene (DCNB) were used. The bleaching and recovery of the transient absorption of Np(T1) were observed in the presence of these quenchers. The bleaching increased with increasing concentration of the quenchers (0.3 < [Q] < 1.0 M) as shown in Figure 2.10. The recovery was accomplished in 100% yield without formation of a new peak. If DCNB could act as an electron acceptor for the quenching of NpðTn Þ, Np radical cation and DCNB radical anion would be observed. Therefore, no ELT quenching occurred, but ENT quenching did. The absence of the radical cation and anion also indicated that ISC from the Tn to S1 (or Sn ) can also be neglected in the present processes, since it is known that Np(S1) causes the ELT with DCNB. It is well established that the triplet ENT occurs at the diffusion-controlled rate (kdiff) when ET of the triplet energy donor is 13 kJ mol1 higher than that of the triplet energy acceptor [41, 52]. In the present case, the ET values of DCB and DCNB in the T1 state (335 and 305 kJ mol1, respectively) are much higher

0.4

0.40

[DCB]= 0M 0.50 M 0.70 M 0.90 M

.D.415

0.2

0.35 100.0

150.0

200.0

Time (ns)

0.0 0

100

200

300

Time (ns)

Figure 2.10. Kinetic traces of
O.D.415 during two-color two-laser flash photolysis of Np in the absence and presence of DCB—0.50, 0.70, and 0.90 M—in Ar-saturated acetonitrile at room temperature.

69

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

than ET of BP(T1) and Np(T1) (289 and 253 kJ mol1, respectively) [50], and the recovery rate of transient absorption of Np(T1) is almost independent of the acceptor concentration ([DCB] or [DCNB]). Therefore, it can be concluded that the triplet ENT from quencher(T1) to Np occurred at kdiff to give Np(T1) and quencher(S0). Since the formation and decay of NpðTn Þ occurred within the duration of the 425-nm laser flash, the transient phenomena of NpðTn Þ cannot be monitored directly using nanosecond lasers. The bleaching of
O.D.415 upon laser irradiation (

O.D.415 ¼
O.D.before 
O.D.after) in the presence of CCl4 resulted from the ENT from NpðTn Þ to CCl4 and increases with an increase of the concentration of CCl4. The inverse of

O.D.415 is represented by concentration of CCl4 as in Eq. (2) [25, 28]: ð

O:D:415 Þ1 ¼ b þ bðkENT t½CCl4 Þ1

ð2Þ

where b is a constant depending on the experimental condition, kENT is the rate constant of the triplet ENT from NpðTn Þ to CCl4, and t is the lifetime of NpðTn Þ. According to Eq. (2), the Stern–Volmer plots of (

O.D.415)1 versus [CCl4]1 gave a linear line with an intercept of b and slope of b(kENTt)1 as shown in Figure 2.11. From ET of Np(T1), 253 kJ mol1 [50], ET of NpðTn Þ is estimated to be 534 kJ mol1 under the 425-nm laser excitation in our experiments. ET of NpðTn Þ is much higher than those of Qs(T1). Therefore, it is reasonable to suggest that the ENT from NpðTn Þ to Qs(S0) occurs at kdiff. However, the ENT kinetics from NpðTn Þ to Q(S0) is different from that from Q(T1) to Np(S0). The concentration of Q (0.3 < [Q] < 1.0 M) is high and t is short, while the

25

20

15

10 1

2

3 [CCl4]–1 (M –1 )

4

Figure 2.11. Plots of (

O.D.415)1 versus [CCl4]1 in two-color two-laser photolysis of Np in the presence of CCl4.

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

concentration of Np (6 mM) is low and the lifetime of Q(T1) is long (several hundred microseconds [50]). Consequently, lifetime-dependent quenching should be considered in the case of quenching of short-lived species such as NpðTn Þ [16, 55]. In such cases, kENT can be expressed by the lifetime-independent and lifetime-dependent terms shown in Eq. (3): kENT ¼ kdiff þ kdiff s0 =ðpDtÞ0:5

ð3Þ

where kdiff ¼ 4pNs0 D, N is Avogadro’s number, s0 is the reaction distance, and D is the sum of the diffusion coefficients for the excited molecule and quencher molecule. Therefore, t of NpðTn Þ was found to be 4.5 ps at s0 ¼ 0.6 nm and D ¼ 2.0  107 dm2s1 [16]. Similar results were obtained when CCl4 was replaced by DCB or DCNB. This is the first report on t ¼ 4.5 ps of NpðTn Þ. 2. Energy Gap Law For other aromatic hydrocarbons (AH) such as chrysene (CHR) [56] and dibenzanthracene (DBA) [57], lifetimes of the Tn states were estimated. For anthracene, the Tn state lifetime was reported by Scaiano and co-workers [16]. Employing these estimated values, the factors governing the lifetime were discussed [58, 59]. Since neither unimolecular reaction nor luminescence of AHðTn Þ was observed, AHðTn Þ is suggested to be deactivated through IC to AH(T1). Therefore, the rate constant (kIC) of IC of AHðTn Þ is defined to be the inverse of t. Generally, kIC depends on the energy gap between two states, following the energy gap law for IC as shown in Eq. (4) [52]: t1 ¼ kIC  1013 expða
EÞ

ð4Þ

where a (eV1) is a constant and
E (eV) is the energy gap between the zero point vibrational levels of the states undergoing IC. The a value is usually smaller than 5 eV1 and does not change so much for rigid AHs [16, 52, 60]. For example, a ¼ 3.3 eV1 was calculated from the data of anthracene(T2), which decays through radiationless processes with the lifetime of the T2 state (tT2) of 11 ps and
E value between the T1 and Tn states (
ET2-T1) of 1.39 eV [16]. With a ¼ 3.3 eV1 and t of AH ðTn Þ calculated from the experimental results, the
E values between the T1 and Tn states ð
ETnT1 Þ of Np, DBA, and CHR were calculated to be 1.15, 1.54, and 1.94 eV, respectively. The
ET2-T1 values between the T1 and T2 states for Np and CHR are theoretically calculated to be 1.17 and 1.75 eV, while
ET3-T1 values of Np and CHR are 1.29 and 2.03 eV, respectively [61–63]. Therefore, NpðTn Þ and CHRðTn Þ are assigned to Np(T2) and CHR(T2), respectively. Since no theoretical study has been reported on DBAðTn Þ, the triplet manifolds of DBAðTn Þ cannot be determined. However, from the results of Np, anthracene, and CHR, DBAðTn Þ is tentatively assigned to be DBA(T2). It is suggested that the AHðTn Þ initially generated decays through fast IC to a lower triplet excited state, the T2 state, which has

71

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

the longest lifetime among the Tn states. NpðTn Þ having a large transition dipole moment was found to be Np(T10) according to theoretical calculation T1 (2 < n < 10) tranusing the QCFF/PI þ CISD method [61]. All other Tn sitions are forbidden or have very small oscillator strengths. Therefore, the T–T T1 transition. In other words, absorption spectrum is assigned to the T10 NAP(T10) is generated by the 425-nm laser excitation of Np(T1) and decays through fast IC within picoseconds to Np(T2) with t of 4.5 ps. The t values of Np(T2), DBA(T2), and CHR(T2) calculated from the ENT quenching experiments increased in the order of Np(T2) (4.5 ps) < DBA(T2) (16 ps) < CHR(T2) (60 ps). This order is consistent with the energy gap law for the transition from AH(T2) to AH(T1). Therefore, AH(T2) with the longest lifetime among AHðTn Þ is responsible for the ENT quenching. 3. Substituent Effect on the Tn State Lifetimes By employing a method similar to the one described in Section III.A.2, the lifetime of the T2 state of BP was estimated to be 450 ps [64]. For a series of BP derivatives, the T2 state lifetimes were estimated as listed in Table 2.1 [65]. The introduction of the substituent tends to decrease the T2 state lifetime. In the case of Np derivatives (NpD), the electron-donating substituent increases the t value, while the electron-withdrawing substituent decreases t [66]. The t values of NpD(T2) in cyclohexane increase in the order of OCH3 > CH2CH3 > CH3 > H ¼ CH2CN > CN. This is the same order as the Hammet constant sp . As shown in Figure 2.12, the ET1 values of NpD(T1) are almost constant. In contrast, the ET2 values of NpD(T2) depend on sp significantly. The T2 state (B2u) consists mainly of two electronic configurations, namely, HOMO 1! LUMO and HOMO ! LUMO þ 1 [61]. The electron-withdrawing group is expected to reduce the antibonding character of the LUMO þ 1 reducing ET2. Therefore, substitution of the electron-withdrawing group seems to decrease
E, leading to shorter t.

TABLE 2.1 Lifetimes of Benzophenone (BP) and Substituted Benzophenone (BPD) in the T1 and Tn States (sT1 and sTn , respectively), Triplet Excited State Energies of BP(T1) and BPD(T1) (ET1), and the Hammett Constants of the para Substituents (rp) for BP and BPD (4-X-C6H4COC6H4-Y-40 ) X Y

H H

CH3O H

CH3 H

F H

Cl H

CF3 H

CN H

CH3O CH3 CH3O CH3

ET1(kJ mol1) sp tT1 (ms) tTn (ps)

289

290

290

292

288

285

280

292

0.00 0.71 450

0.27 0.23 240

0.17 0.06 0.27 0.21 250 260

0.23 0.54 0.66 0.50 0.41 0.44 280 140 110

290

F F

CN CN

294 276

0.27 0.17 0.06 0.66 0.19 0.26 0.72 0.67 350 300 300 250

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

4.5

E (eV)

4.0

3.5

3.0

2.5 –0.2

0.0

0.2

0.4

0.6

p

Figure 2.12. Plots of ET1 (closed triangle) and ET2 (closed circle) of 1-substituted naphthalene and ET1 (open triangle) and ET2 (open circle) of 2-substituted naphthalene versus the Hammett constant sp in cyclohexane.

4. Bond Dissociation from the Tn State For the Tn state, reaction that cannot be observed for the T1 states can be expected. A bond dissociation process is one of the reactions, which can be expected for the Tn states. Here, we introduce the formation of naphthylmethyl radical from the Tn states [67]. Naphthylmethyl radical (NpCH2) is a typical organic radical and has been studied extensively [68–74]. It is well established that NpCH2 is produced from the photolysis of naphthylmethylhalides through the cleavage of the naphthylmethyl–halogen bond [69, 71]. The cleavage also occurs in other naphthylmethyl compounds under the laser irradiation. Steenken and co-workers also found that the C—O bond cleavage occurred in the S1 state but not in the T1 state of 1-[(4-benzoylphenoxy)methyl]naphthalene (1-NpCH2-OBP) [74]. They assumed that the C—O bond cleavage could occur if the Np moiety is excited to the Tn state through further photon excitation of the T1 state. When 1-NpCH2-OBP was irradiated at 355 nm using a Nd:YAG laser in cyclohexane, the transient absorption spectrum observed immediately after the laser flash (Fig. 2.13a) was coincident to that of Np(T1) with a peak at 420 nm [72, 74]. Because the Np chromophore has no absorption at 355 nm, the first 355-nm photon is absorbed by the BP chromophore to give BP(S1), from which ISC occurs to give BP(T1) in quantum yield of 1.0 [50]. The intramolecular triplet ENT from BP(T1) to the Np chromophore occurs to give Np(T1) within

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

73

Figure 2.13. Transient absorption spectra obtained at 1.1 ms after the 355-nm first laser irradiation (a) (broken line) and at 1 ms after the second 430-nm laser irradiation (b) (solid line) during two-color two-laser flash photolysis of 1-NpCH2-OBP in Ar-saturated cyclohexane at room temperature. The delay time of the second laser after the first laser was 100 ns. Inset: Spectrum obtained by (b)(a) in the region of 350–380 nm.

a laser flash of 5 ns. No appearance of the peak at 365 nm assigned to 1-NpCH2 indicates that the C—O bond cleavage did not occur from 1-Np(T1)CH2-OBP [74]. This observation is adequate since the triplet energy of Np(T1) (254 kJ mol1) is lower than the dissociation energy of the C—O bond (285 kJ mol1) [74]. The absorption peak at 365 nm assigned to 1-NpCH2 was observed with the bleaching of the 420-nm peak within a laser flash, when the second 430-nm OPO laser flash was irradiated to 1-Np(T1)CH2-OBP with a delay time of 100 ns after the first 355-nm laser flash (Fig. 2.13b). Because only 1-Np(T1)CH2-OBP has an absorption at 430 nm, 1-Np(T1)CH2-OBP can be excited to 1-NpCH2-OBPðTn Þ by the second laser. The 430-nm photon (278 kJ mol1) supplies sufficiently high energy into 1-Np(T1)CH2-OBP, giving 1-NpCH2-OBPðTn Þ, from which a rapid C—O bond cleavage occurred within the laser flash (Scheme 2.5). The quantum yield () of 1-Np(T1)CH2-OBP disappeared and 1-NpCH2 formed was calculated to be 0.042  0.004 from the slopes of the linear lines of the plots of j

O.D.420 j and j

O.D.365 j versus 430-nm laser power. The small  value indicates that IC from 1-NpCH2-OBPðTn Þ to 1-Np(T1)CH2-OBP is the predominant process (95.8% yield). Similar experimental results to those of 1-NpCH2-OBPðTn Þ were obtained for 2NpCH2-OBPðTn Þ. The bleaching of the transient absorption of 2-Np(T1) CH2-OBP

74

PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

O

CH2

O

hν355

CH2

< 100 ps ISC

O

O

S1 O

CH2

O

< 5 ns

CH2

hν430

ENT

IC

O

O T1

T1 O

CH2

O

< 5 ns

H2C +

O

O Tn

Scheme 2.5. Two-color two-laser photochemistry of 1- and 2-NpCH2-OBP involving intramolecular triplet ENT, selective excitation of Np(T1)CH2-OBP to NpCH2-OBPðTn Þ, and cleavage of the C—O bond from NpCH2-OBPðTn Þ. Dotted square shows the excitation energy delocalization.

at 420 nm and the formation at 380 nm assigned to 2-NpCH2 were observed during the two-color two-laser flash photolysis of 2-NpCH2-OBP [75, 76]. However, the bleaching of the transient absorption at 420 nm and the formation at 380 nm observed in 2-Np(T1)CH2-OBP were less than those in 1-Np(T1)CH2-OBP. The  value of 2-Np(T1)CH2-OBP disappeared and 2-NpCH2 formed was calculated to be 0.020  0.002, which was almost half of that of 1-Np(T1)CH2-OBP. This result indicates that the C—O bond cleavage of 1-NpCH2-OBPðTn Þ occurs more efficiently than that of 2-NpCH2-OBPðTn Þ. Although the T1 state is localized on the Np chromophore of 1- and 2-Np(T1)CH2-OBP, the Tn state could be delocalized in 1- and 2-NpCH2-OBPðTn Þ including the C—O bond. It is expected that the extent of the delocalization is more prominent in 1-NpCH2-OBPðTn Þ than in 2-NpCH2OBPðTn Þ. Similar bond cleavage was observed for the Tn state of the other compounds. For example, p-phenoxymethylbenzophenone (BPCH2OPh) and p-methoxymethylbenzophenone (BPCH2OCH3) undergo bond cleavage to generate benzoylbenzyl radical and phenoxy or methoxy radical through the Tn state of BP during the two-color two-laser flash photolysis [77]. The cleavage yield of BPCH2OPh was higher than that of BPCH2OCH3. We also found that the C—Si bond is another target of the bond cleavage from the Tn state generated by the two-color two-laser photolysis [78]. The

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

75

two-color two-laser flash photolysis of p-trimethylsilylmethylacetophenone generated p-acetylbenzyl radical, indicating the bond cleavage from the Tn state of acetophenone. On the other hand, p-trimethylsilylmethylbenzophenone did not generate the bond cleavage products, in spite of the higher Tn state energy than the C—Si bond dissociation energy. These results indicate that the existence of a bond cleavage crossing between the potential surfaces of the Tn state and a dissociative state of the C—Si bond is also an important factor for the bond dissociation in addition to the energetic consideration. 5. Electron Transfer from the Tn State Photoinduced ELT, one of the fundamental processes in physical, chemical, and biological aspects, is an attractive subject and has received much attention. Usually the lowest singlet and triplet excited states participate in the photoinduced ELT. Although the ELT from the higher excited states is also energetically possible, studies are limited. Intermolecular ELT from the higher singlet excited (S2) state of zinctetraphenylporphyrin to dichloromethane was reported by Okada and co-workers [79]. LeGourrie´ rec et al. [80] reported the intramolecular ELT from the S2 state of porphyrin in a covalently linked zinc porphyrin–ruthenium(II) tris-bipyridine dyad. Systematic study of the intramolecular ELT from the S2 state of porphyrins was carried out by Mataga et al. [81–83]. The dependence of the intramolecular ELT rate from the S2 state on the free energy change and solvent was confirmed. Intermolecular ELT quenching of the S2 states of azulene, benz[a]azulene, and xanthone by several electron donors was investigated by Muller and Vauthey [84]. Furthermore, intermolecular ELT from St þ in the D2 state was reported by our group [25, 28]. These studies clearly demonstrate that the ELT from the higher excited states is possible even when the lifetime of the higher excited state is as short as a few picoseconds. Therefore, intermolecular ELT from the higher triplet excited states ðTn , n  2) seems to be also possible. Because large excitation energy of the Tn state affords larger driving force of the ELT even when the ELT from the T1 state is energetically unfavorable, the ELT is expected to occur from the Tn state. Limited numbers of studies have been reported on the properties of molecules in the Tn states. Two-color two-laser flash photolysis can be applied to study photoinduced reactions from the Tn states. It has been reported that the main reaction path from the Tn states is the triplet ENT to the triplet quenchers. To the best of our knowledge, there has been only one report on the ELT from the Tn state. Wang et al. [48] reported the ELT from anthracene(T2) to ethyl bromoacetate. However, no detailed mechanism of the ELT from the Tn state has been reported. In this section, we summarize our recent systematic study of the intermolecular ELT from a series of substituted naphthalenes (NpD) in the Tn state to electron acceptors [85, 86]. 

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

NpD(T1) was generated from the triplet sensitized reaction during the first laser (355 nm, 3 mJ pulse1) irradiation to a mixture of BP (7.0  103 M) and NpD (7.0  103 M) in Ar-saturated acetonitrile at room temperature. NpDðTn Þ was generated by the excitation of NpD(T1) with the second laser (425 nm, 9 mJ pulse1) at 100 or 150 ns after the first laser. In the presence of CCl4, bleaching of the transient absorption of 1-methoxynaphthalene(T1) and growth of new transient absorption peaks at 385 and 702 nm were observed immediately after the second laser irradiation as shown in Figure 2.14. The new absorption bands were assigned to 1-methoxynaphthalene radical cation [35, 87]. The inset of Figure 2.14 shows the kinetic traces of
O.D. at 440 and 702 nm. Trace a shows the second laser-induced bleaching of the absorption of 1-methoxynaphthalene(T1) at 440 nm within the second 425-nm laser flash duration of 5 ns. This bleaching indicates that 1-methoxynaphthaleneðTn Þ generated with the second laser irradiation did not reproduce 1-methoxynaphthalene(T1). Trace b shows the growth of the radical cation absorption at 702 nm within the laser pulse duration. Thus, it is clearly indicated that the ELT from 1-methoxynaphthaleneðTn Þ to CCl4 occurred to give the 1-methoxynaphthalene 0.15 0.12 second laser fire 0.08

a 440 nm

0.10 0.04

b 702 nm

0.00 0

0.05

100

200 300 Time (ns)

400

500

0.00 350

400

450

500 550 600 Wavelength (nm)

650

700

750

Figure 2.14. Transient absorption spectra obtained during two-laser (first 355-nm and second 425-nm) excitation (filled circles) and one-laser (355-nm) excitation (open circles) of BP (7.0  103 M) with 1-methoxynaphthalene (7.0  103 M) in Ar-saturated acetonitrile in the presence of CCl4 (1.0 M) at room temperature. The second laser was irradiated at 200 ns after the first laser pulse. Inset: Kinetic traces of
O.D. at 440 (a) and 702 nm (b) with and without the second 425-nm laser irradiation. The initial growth of
O.D in the 50-ns time scale corresponds to the formation of NpD(T1) by the ENT from BP(T1).

77

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

radical cation within the laser flash duration. The ELT from the Tn state was also observed for other NpD such as 1-methylnaphthalene and 1-ethylnaphthalene. It should be noted that the radical cation was not observed without the second laser irradiation even in the presence of CCl4, indicating the contribution of the Tn state to the occurrence of the ELT. Since no fluorescence from NpD(S1) was observed after the second laser irradiation in the absence of CCl4, back ISC (Tn ! S1) can be neglected. Therefore, NpD(S1) is not involved during the second laser irradiation although the ELT from NpD(S1) to CCl4 was reported [88]. The efficiency of the ELT from NpD(T2) to CCl4 obviously depends on the properties of NpD(T2). Both ENT and IC are involved in the NpD(T2) decay process [49]. Therefore, the efficiency of the ELT (fELT) from NpD(T2) is represented by fELT ¼

O.D.702/ (eNPDþ [NpD(T2)]0), where

O.D.702 is the difference of
O.D. at 702 nm with and without the second laser irradiation in the presence of CCl4 (1.0 M), eNPDþ is the molar absorption coefficient of NpD radical cation, and [NPD(T2)]0 is the initial concentration of NpD(T2). The [NpD(T2)]0 value can be estimated from the rate constant of IC and the bleaching of transient absorption of NpD(T1) in CCl4 upon the second laser irradiation. The estimated fELT values for NpD(T2) are listed in Table 2.2. The rate of the ELT depends on the driving force (
GELT). It is necessary to estimate
GELT to explain the difference of fELT. CCl4 undergoes the C—Cl bond cleavage following the one-electron reduction [89–93]. According to the ELT mechanism involving the formation of NpD radical cation and the

TABLE 2.2 Lifetimes of Naphthalene and Substituted Naphthalenes in the T2 State (NpD(T2)) (sT2), Energies of the T1 and T2 States (ET1 and ET2, respectively), the Driving Force of the ELT (
GELT), Half-Wave Oxidation Potentials (Eox) in Acetonitrile, Efficiencies of the ELT from NpD(T2) to CCl4 ( fELT), and Calculated Efficiencies ( fELT(calcd)) NpD Naphthalene 1-Methyl1-Ethyl1-Isopropyl1-Methoxy2-Methyl2-Ethyl2-Methoxy-

tT2(ps)

ET1 (eV)

ET2 (eV)

9.4  2.0 9.5  0.8 19  4.3 45  1.2 61  8.6 34  6.5 36  9.2 63  13

2.65 2.67 2.59 2.58 2.66 2.62 2.62 2.62

3.8 3.9 4.0 4.2 4.3 4.2 4.2 4.3

Eox (V) 
GELT vs SCE (eV) 1.78 1.63 1.63 1.63 1.38 1.65 1.64 1.41

1.3 1.5 1.6 1.8 2.2 1.8 1.8 2.2

fELT

fELT (calcd)

0 0.11  0.1 0.16  0.1 0.31  0.1 0.50  0.1 0.26  0.1 0.29  0.1 0.59  0.1

0.06 0.14 0.21 0.31 0.34 0.28 0.29 0.34

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

dissociative electron attachment to CCl4 leading to the C—Cl bond cleavage [89–93],
GELT is represented by Eq. (5):
GELT ¼ Eox ERCl=R þCl wp  ET2

ð5Þ

where Eox is the oxidation potential of NpD, ERCl=R þCl is the reduction potential of the RCl/R þ Cl couple (for CCl4, ERCl=R þCl ¼ 0:825 V vs. SCE) [93], wp is a Coulombic energy (0.06 eV), and ET2 is the energy level of the T2 state given by the sum of the energy of the T1 state (ET1) and
E. The estimated 
GELT values are summarized in Table 2.2. As shown in Figure 2.15, the fELT value increases with an increase of the 
GELT value. From the theoretical calculation (vide infra), the faster ELT is expected for larger 
GELT in this system. Since the ELT from NpD(T2) occurs competitively with other fast processes such as IC (T2 ! T1) and ENT, the large 
GELT value is necessary for the occurrence of the ELT. Since the ELT to CCl4 leads to the concerted C—Cl bond cleavage, contribution of the bond breaking should be considered. The dissociative ELT model, in which the Morse potential curve is employed, has been developed to describe 

0.8 0.7 0.6

f ELT

0.5 0.4 0.3 0.2 0.1 0.0 1.2

1.4

1.6 1.8 –∆GELT (eV)

2.0

2.2

Figure 2.15. Plots of fELT versus 
GELT for the ELT from NpDðTn Þ to CCl4 during the 355- and 425-nm two-color two-laser flash photolysis of BP with NpD in Ar-saturated acetonitrile in the presence of CCl4.

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

79

such ELT [89–93]. According to the ‘‘sticky’’ dissociative ELT model, the activation energy (
G*) can be represented by Eq. (6) [91–93]: " #2 pffiffiffiffiffiffi pffiffiffiffiffiffi ð DR  Dp Þ2 þ l0
GELT  Dp 1 þ pffiffiffiffiffiffi pffiffiffiffiffiffi 2
G ¼ 4 ð DR  Dp Þ þ l0 

ð6Þ

where DR is a bond dissociation energy of reactant RX, Dp is an interaction energy of the radical ion pair, and l0 is the solvent reorganization energy independent of bond breaking. In the present work, the following parameters were employed: DR ¼ 2.99 eV, Dp ¼ 0.161 eV [93], and l0 ¼ 1.48 eV [92]. In simplified form, the activation-energy-controlled ELT rate constant can be expressed by Eq. (7):   
G ð7Þ kELT ¼ n exp RT where v is the frequency factor. Here, v is assumed to be 5.0  1013 M1 s1 [90]. For the bimolecular reaction, the ELT rate constant (k0 ELT) can be given by Eq. (8) [94]: 1 1 1 ¼ þ 0 kELT kdiff kELT

ð8Þ

taking the formation of an encounter complex into account. Because of the high concentration of CCl4 and short lifetime of the Tn state, the ELT rate constant 00 ) from NpDðTn Þ to CCl4 can be expressed by Eq. (3). According to the lit(kELT erature, D0 ¼ 2  105 cm2 s1 [50, 55]. The efficiency of the ELT from NpDðTn Þ (fELT(calcd)) can be calculated by Eq. (9): fELT ðcalcdÞ ¼

00 ðkELT

00 kELT ½CCl4

þ kENT Þ½CCl4 þ kIC

ð9Þ

where kIC is the IC rate constant given by the reciprocal of t, and [CCl4] is the concentration of CCl4 as a quencher ([CCl4] ¼ 1.0 M). The estimated fELT(calcd) values are summarized in Table 2.2. As shown in Figure 2.16, the plots of fELT versus fELT(calcd) showed a good correlation. It is suggested that the efficient ELT from NpD(T2) to CCl4 and the inefficient ELT from NpD(T1) to CCl4 are explained qualitatively by the ‘‘sticky’’ dissociative ELT model [91–93]. The rate constant of the ELT from 1-ethylnaphthalene(T2) to CCl4 is calculated to be 5.9  1010 s1, while the rate constant of the ELT from 1-ethylnaphthalene(T1) to CCl4 is calculated to be 660 s1. Thus, the dissociative ELT from the triplet excited NpD to CCl4 occurs only from the T2 state.

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

Figure 2.16. Plots of fELT versus fELT(calcd) for the ELT from NpDðTn Þ to CCl4 during the 355- and 425-nm two-color two-laser flash photolysis of BP with NpD in Ar-saturated acetonitrile in the presence of CCl4.

When some polychlorobenzene, such as chlorobenzene, 1,4-dichlorobenzene, and 1,2,4-trichlorobenzene, was employed as the electron acceptor, it was found that the ELT proceeds from the NpDðTn Þ, in which the dissociative ELT is not operative. Since the C—Cl bond dissociation rate of chlorobenzene radical anion was reported to be 1.8  109 to >2  1010 s1, the radical ion pair should be formed after the ELT from NpD(T2) [95, 96]. For the stepwise mechanism (Scheme 2.6), the dissociative ELT theory for the concerted dissociation cannot be applied [55]. The Marcus theory is adequate for the present system. According to the Marcus ELT theory, the free energy change for the ELT can be given by Eq. (10):
GELT ¼ Eox  Ered  wp  ET2

Stepwise mechanism D(Tn) + RX D·+ + RX·– Ion pair Concerted mechanism D·+ + R· + X– D(Tn) + RX

ð10Þ

D·+ + R· + X–

Scheme 2.6. Stepwise and concerted mechanisms for the dissociative ELT from DðTn Þ to RX to give D þ, R , and X. 



TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

The activation energy (
G*) can be represented by Eq. (11) [94, 97]:   l
G 2
G ¼ 1þ 4 l

81

ð11Þ

where l is an intrinsic barrier corresponding to the bond length change and solvent reorganization. According to the literature, we employed l ¼ 1.9 eV [98]. By employing Eqs. (3) and (7)–(9), the fELT(calcd) values are estimated. The plots of fELT versus fELT(calcd) showed good correlation, indicating that the efficiency of the ELT from NpD(T2) to chlorobenzenes is explained qualitatively by the Marcus ELT theory. The ELT from the Tn state was inefficient in several NpD–polychlorobenzene pairs, where no NpD radical cation was detected, although bleaching and recovery of NpD(T1) were observed upon the irradiation of the second laser. Figure 2.17a shows the transient spectra obtained during the two-color two-laser experiment of the Np–1,2,4-trichlorobenzene system. The absorption band around 350 nm is reasonably assigned to the corresponding Cl—CHD and chlorine atom [99, 100]. Figure 2.17b shows simultaneous growth of Cl—CHD at 350 nm. Thus, it is suggested that Cl—CHD was generated from the homolytic C—Cl bond cleavage after the ENT from NpðTn Þ. It should be noted that the homolytic C—Cl bond dissociation energies of 1,2,4-trichlorobenzene, 1,2-dichlorobenzene, and chlorobenzene were reported to be 3.7, 3.8, and 3.8 eV, respectively, which are higher than the energy levels of chlorobenzenes(T1) (3.4–3.5 eV) [101]. Thus, it is concluded that the C—Cl bond dissociation takes place from 1,2,4-trichlorobenzeneðTn Þ (n > 1) after the ENT from NpðTn Þ (Scheme 2.7). This is the first example of the intermolecular ENT from the molecules in the Tn state to quenchers giving the quenchers in the Tn state. 6. Direct Observation of the Tn State The existence of the Tn states has been indirectly indicated by the bimolecular ENT from the Tn states to acceptors with the T1 energies higher than the T1 energy of the donor. Direct observation of the Tn states is indispensable for the study of the photoinduced reactions specific to the Tn state which has a much larger excitation energy than the T1 state. Since the lifetimes of the Tn states are reported to be on the order of picoseconds, utilization of short-pulse lasers is necessary to detect them. In this section, we present examples of direct observations of the Tn states using two-color two-laser flash photolysis employing a picosecond laser. For the detection of the Tn state, we employed the pump and probe method to attain better time resolution [102–104]. Oligothiophenes have been selected as the target molecule, since their photoexcitation processes are well established and they are known as electron donors with large extinction coefficients of the T1 state and radical cation in the D0 state [105–108]. The S0 and T1 states of the trimer, tetramer, and

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

(a)

0.20

0.15

0.10

0.05

0.00 340

360

380

400

420

440

Wavelength (nm) (b)

0.08

0.06

second laser fire

0.04

0.02

0.00 0

200

400 600 Time (ns)

800

1000

Figure 2.17. (a) Transient absorption spectra obtained during the one-laser (355-nm) excitation (solid line) and the two-laser (first 355-nm and second 425-nm) excitation (broken line) of BP (7.0  103 M) with naphthalene (7.0  103 M) in Ar-saturated acetonitrile in the presence of 1,2,4-trichlorobenzene (1.0 M). The second laser irradiation was at 150 ns after the first laser pulse. These spectra were obtained at 200 ns after the first laser. (b) The kinetic traces of
O.D. at 350 nm with (dotted line) and without (solid line) the second laser irradiation.

pentamer of thiophene (3T, 4T, and 5T, respectively) can be excited selectively with the 355-nm nanosecond laser and the 532-nm picosecond laser, respectively. Figure 2.18a shows the transient absorption spectra of 4T in toluene at 40 ps before and at 20 and 150 ps after the second laser excitation;

83

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

NpD(Tn)+A IC

.

NpD+A(Tn)

.

NpD + Cl-CHD +Cl

IC

NpD(T2)+A

ENT1

NpD+A(T1)

ELT

(NpD·+ A· –)3

IC ENT2

NpD(T1)+A

NpD(S0)+A

A: triplet energy acceptor

Scheme 2.7. Schematic energy diagram for ELT from NpDðTn Þ to polychlorobenzene (A). As a competitive process to ELT, ENT generating AðTn Þ was confirmed.

Figure 2.18. (a) Transient absorption spectra observed at 40 ps before and 20 and 150 ps after the laser flash during two-color two-laser flash photolysis of 4T in toluene employing a nanosecond YAG laser (355 nm, FWHM 5 ns, 7 mJ pulse1) and a picosecond YAG laser (532 nm, FWHM 30 ps, 21 mJ pulse1). (b) Difference spectra of transient absorption spectra at 20 and 150 ps. (c) Kinetic traces of
O.D. at 650 and 600 nm. Thick lines are fitted curves.

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

in the present study, the probe time was indicated with respect to the second laser excitation. At 40 ps, transient absorption peaks at 614 and 580 nm indicated the generation of 4T(T1) by the first laser excitation [105–107]. At 20 ps, the absorption bands of 4T(T1) became low and structureless and a new absorption band appeared around 650 nm, as shown in Figure 2.18b. The new absorption band at 650 nm with an absorption tail extending to around 800 nm can be attributed to the Tn state of 4T. In the present case, generation of 4T þ can be excluded since 4T þ showed a rather sharp absorption band at 640 nm [105–108]. Thermal effects can also be excluded from the quenching of Tn by the ENT. It should be noted that at 150 ps, the transient absorption spectrum had almost recovered to that before the second laser irradiation, indicating the quantitative relaxation to 4T(T1) from the Tn state by IC. The kinetic traces of
O.D. at 650 and 600 nm (Fig. 2.18c) indicate that 4TðTn Þ (650 nm) decayed within 100 ps after the second laser irradiation with the concomitant recovery of 4T(T1) (600 nm). From the deconvolution fitting of a single exponential function to these kinetic traces, the rate constant of IC to 4T(T1) (kIC) was estimated to be (2.6  0.8)  1010 s1, which corresponds to a 38-ps lifetime of 4TðTn Þ. In the cases of 5T and 3T in toluene, the lifetimes of the Tn states were estimated to be 31 and 38 ps, respectively. Rentsh et al. [109] reported that the T2–T1 gaps of 3T and 4T were 1.48 and 1.37 eV, respectively. Although a comparison of oligothiophenes with ArH is difficult due to their different molecular structures, the estimated Tn state lifetimes of the oligothiophenes are close to those of ArH(T2) described earlier when taking the T2–T1 gaps into account. Thus, IC between T2–T1 is the ratedetermining step for the oligothiophenes. The solvent polarity effect was examined by employing acetonitrile as the solvent. The difference spectrum of 4T in acetonitrile at 20 ps showed a broad absorption band at 650 nm, which is essentially the same as that observed in toluene. At 150 ps, on the other hand, a sharp absorption band was confirmed at 640 nm, which can be attributed to 4T þ, indicating the ionization from 4TðTn Þ [105–108]. In the kinetic trace of
O.D. at 600 nm, bleaching of 4T(T1) showed incomplete recovery, indicating that 38% of 4TðTn Þ changed to 4T þ. The radical cation formation in acetonitrile seems to be an adequate process since polar solvents stabilize the radical ion. The generation of a radical cation from the Tn state in acetonitrile was also confirmed for 5T in acetonitrile at 700 nm [105–108]. The Tn states of oligo(p-phenylenevinylene)s (OPVn, n denotes the number of phenyl rings, n ¼ 3, 4) were also investigated by means of the nanosecond– picosecond two-color two-laser flash photolysis. The lifetimes of the Tn states were estimated to be 35 and 30 ps for OPV3 and OPV4, respectively. Based on the ENT from OPVnðTn Þ to a series of triplet energy quenchers, the T2–T1 energy gaps of OPV3 and OPV4 were estimated to be 1.3 and 1.1 eV, respectively [103]. 







TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

85

The T2 state lifetime was also estimated for CHR(T2). By combining the estimated lifetime (45 ps) and the ENT rate from the T2 state to a triplet energy quencher, the reaction distance of the bimolecular ENT was estimated to be ˚ in cyclohexane and acetonitrile, respectively, indicating that the 3.8 and 3.7 A collision between CHR(T2) and quenchers occurs in accordance with the exchange mechanism [104]. Direct measurement of the Tn state lifetime by nanosecond–picosecond twocolor two-laser flash photolysis was applied to BP derivatives with various substituents [110]. The Tn state lifetime of BP was estimated to be 37 ps, which is shorter than the value estimated by the energy transfer process discussed earlier, indicating the oversimplification of the assumption and the importance of the direct measurements. On the other hand, the tendency that the introduction of the substituents decreases the Tn state lifetime was also confirmed in the direct measurements. Furthermore, the ENT process from the Tn state to the quencher was investigated by analyzing the kinetic traces during the second picosecondlaser flash, from which lifetime-independent and lifetime-dependent processes were distinguished. It was revealed that the contribution of the lifetimedependent term on the ENT rate became larger as the size of the energy quencher increased.

B. Ketyl Radicals in the Excited State The nanosecond–picosecond two-color two-laser flash photolysis method is also useful to study the excited state of radicals, that is, the D1 state. We applied nanosecond–picosecond two-color two-laser flash photolysis to detect the absorption and fluorescence spectra of the ketyl radical of benzophenone and its derivatives (BPH and BPDH ) in the excited state in the UV–visible region [111], since BPH and BPDH are well investigated radicals in various fields. Since BPH and BPDH are generated from irreversible ways, such as photoionization, we employed a streak camera to realize the single-shot detection of these intermediates. BPH and BPDH were generated from the photoreduction of BP and BPD in cyclohexane. BP and BPD in the lowest triplet excited states (BP(T1) and BPD(T1)) decayed through the hydrogen abstraction from cyclohexane to produce BPH and BPDH after the first 266-nm nanosecond-laser irradiation. The generated BPH was excited at the visible absorption band using the second laser (532 nm, 30 ps FWHM) with the delay time of 1 ms after the first laser. Upon the excitation, BPH showed fluorescence with a peak at 564 nm. Similarly, fluorescence of BPDH was observed with the second laser irradiation. The measured tf of BPH is 2.0 ns, which is close to the reported value [112–114]. 4Chloro-, 4,40 -dichloro-, 4-bromo-, 4,40 -dibromo-, and 4-trifluoromethyl-substituted

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

(a)

0.15 c

a 0.10 0.05 b (b)

0.00 0.08 0.04 0.00 –0.04

350

400

450

500

550

600

Wavelength (nm)

Figure 2.19. (a) Transient absorption spectra observed at 0.5 (broken line a) and 20 (dotted line b) ns after the second laser irradiation during two-color two-laser photolysis (266 and 532 nm) and the spectrum observed during one-laser photolysis (266 nm, solid line c) of BP (1.0  104 M) in Ar-saturated cyclohexane. The second laser irradiation was at 1 ms after the first laser pulse. The transient absorption spectrum of BPH (D1) (b) was given by subtracting spectrum c from spectrum a. The blank around 532 nm in the spectra is due to the residual SHG of the Nd3þ:YAG laser.

BPDH showed larger tf values than others. It is generally admitted that tf increased with the decrease of the Stokes shift [115]. Immediately after the second laser irradiation, the bleaching of the absorption of BPH and growth of new transient absorption peaks at 350 and 480 nm were observed as shown in Figure 2.19a. The spectral shape of the new transient species was given by subtracting the spectrum observed before the second laser irradiation from that observed at 0.5 ns after the irradiation (Fig. 2.19b). The t estimated from the absorption decay was essentially the same as tf estimated from the fluorescence decay of BPH . Therefore, these transient absorption D1 transition (Scheme 2.8). Similar spectral bands can be attributed to the Dn changes were observed for other BPDH . The intermolecular reaction of BPH (D1) with the solvent molecules and the unimolecular cleavage of the O—H ketyl bond of BPH (D1) yielding BP and a hydrogen atom have been observed in the microsecond time scale [112–114]. Thus, the decay of BPH(D1) can be attributed to the combination of a chemical reaction and nonradiative and radiative transition processes

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

87

3.5 eV Dm 2.4 eV

Dn D2

D1

2.2 eV

kC hν532

kIC

kf

Product D0

OH

Scheme 2.8. Deactivation processes of BPH (D1) including radiative and nonradiative deactivation processes and chemical reactions.

(Scheme 2.8). Because the chemical reaction does not regenerate BPDH (D0), reaction rate constants can be determined based on the lifetime and yield of the recovery of BPDH(D0). Generally, the lifetime increased with the decrease of the Stokes shift (Table 2.3). The
E(D1  D0) values of BPDH are similar to each other, indicating that the lifetime was not affected by the
E(D1  D0) values. Therefore, it is suggested that BPDH (D1) such as 4-fluoro-, 4,40 -difluoro-, 4-methoxy-, and 4,40 -dimethoxy-substituted BPDH (D1) have shorter lifetimes and larger Stokes shift, showing higher reactivity with large kC. The distorted conformation in the D1 state may enhance the reactivity of such BPDH (D1). A similar relationship between the Stokes shift and the lifetime of the D1 state was confirmed for other BP analogues such as 4-benzoylbiphenyl and bis(biphenyl-4-yl)methanone [116]. In the case of ketyl radical of 4,40 -dimethoxybenzophenone, generation of the bis(4-methoxyphenyl)methanol cation and 4,40 -dimethoxybenzophenone radical anion was confirmed upon excitation of ketyl radical [117]. The generation of cation and radical anions indicates the ELT between the excited ketyl radical and parent molecule in the ground state. For the ELT mechanism, the following two pathways were indicated: (1) two-photon ionization of the ketyl radical followed by electron capture by the parent molecule and (2) an intermolecular collisional ELT process between the excited ketyl radical and

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TABLE 2.3 Stokes Shifts for BPDH (m SS), Lifetimes of Transient Absorption (s) and Fluorescence (sf), Rate Constants of the Chemical Reaction (kC), Rate Constants of the Radiative (kf) and Nonradiative (kIC) Relaxation Processes, and the Energy Gaps of BPDH Between the D1 and D0 States (
E(D1  D0)) Ketyl Radical

t nSS (103cm1) (ns)

Benzophenone 4-Fluoro 4,40 -Difluoro4-Chloro4,40 -Dichloro4-Bromo4,40 -Dibromo4-Trifluoro4-Methyl4-Methoxy4,40 -Dimethyl4,40 -Dimethoxy

0.75 0.96 1.5 0.75 0.72 0.78 0.69 0.73 0.79 1.2 0.78 1.7

a

2.0  0.1 1.4 0.47 3.3  0.1 3.4  0.4 2.1 3.5  0.1 4.3  0.3 1.9  0.1 0.86  0.04 1.7  0.1 0.34  0.02

tf (ns) 2.0  0.1 1.3  0.1 0.47 3.2  0.1 3.4  0.1 2.1 3.6  0.1 4.3  0.3 1.8 0.89 1.6 0.35

kC (108 s1)

kf þ kIC (108 s1)

1.7  0.1

3.4  0.1

a

a

a

a

a

a

1.4  0.1 2.2  0.1 1.8  0.1

1.5  0.1 2.8  0.1 1.1  0.1

a

a

a

a

a

a

a

a

b

b

Not determined because BPDH (D0) was not observed due to the overlap of the Dn The recovery was not observed.


E (D1D0)(eV) 2.20 2.18 2.12 2.16 2.11 2.16 2.10 2.15 2.18 2.11 2.14 2.06 D1 absorption.

b

parent molecule (Scheme 2.9). The two-photon ionization of the ketyl radical in process (1) was confirmed by the laser power dependence of the absorption band of the cation. Furthermore, a fluorescence decay rate that depends on the concentration of the parent molecule supports the ELT process (2) at the diffusion-limiting rate. These dual ELT pathways were also confirmed for BP [116]. The D1 state properties were also examined with xanthone ketyl radical (XnH ). The absorption and fluorescence of XnH (D1) were observed for the first time by using the nanosecond–picosecond two-color two-laser photolysis [118]. Several factors governing the deactivation processes of XnH(D1) such as interaction and reaction with solvent molecules were pointed out. The remarkable change of reactivity of XnH (D1) compared with that in the ground state (XnH (D0)) was indicated from the experimental results. The rapid halogen abstraction of XnH (D1) from some halogen donors, such as carbon tetrachloride (CCl4), was found to occur. The halogen abstraction occurred more efficiently in the polar solvents than in the nonpolar solvents. It is suggested that the polar solvents promote the spin distribution of XnH (D1) of the phenyl ring favorable to the halogen abstraction. Time-resolved absorption and fluorescence spectra of azaxanthone (AX) ketyl radical (AXH ) in the excited state (AXH ðDn Þ (n ¼ 1 or 2)) were also

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

89

Scheme 2.9. Dual ELT pathways of ketyl radical of 4,40 -dimethoxybenzophenone (1 in this scheme).

observed during nanosecond–picosecond two-color two-laser flash photolysis [119]. AXH showed dual fluorescence peaks at 460 and 645 nm, which were assigned to the D2 ! D0 and D1 ! D0 transitions, respectively (Fig. 2.20). The lifetime of the D2 ! D0 fluorescence (1.0 ns) was longer than that of the D1 ! D0 fluorescence (0.4 ns). Fluorescence quantum yields (f) of the D1 ! D0 and D2 ! D0 fluorescence were estimated to be 0.0008  0.0002 and 0.05  0.02, respectively. These anomalous emitting properties can be attributed to the pyridine ring in AX. As discussed earlier, the excited radical has an enhanced reactivity. Especially, excited ketyl radicals can be used as excellent reducing agents, which can be generated by the two-laser excitation. Thus, we employed excited ketyl radicals as reducing agents to generate metal nanoparticles [120]. In order to generate gold nanoparticles (AuNps) in a polymer matrix, a two-color laser was irradiated to the poly(vinyl alcohol) (PVA) film including BP and  AuCl 4 . The first laser irradiation generated the BPH , which was confirmed by the transient absorption spectrum. Upon excitation of BPH with the second laser, BPH showed fluorescence, although the fluorescence intensity became  weak in the presence of AuCl 4 , indicating the reduction of AuCl4 by the  excited ketyl radical. Actually, the PVA film including BP and AuCl4 changed color upon the two-color laser irradiation, indicating the surface plasmon band due to the formation of AuNps, while one laser irradiation did not change the color of the film. The formation of AuNps with a 2.5–4 nm diameter was

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

0.10 OH

0.08

N

460 nm

O

AXH Fluor. Int.

0.06 645 nm 0.04

0

2 4 Time (ns)

6

0.02

0.00 400

500 600 Wavelength (nm)

700

800

Figure 2.20. Absorption (black line) and fluorescence spectra of the AXH in Ar-saturated cyclohexane at room temperature. Fluorescence spectra were obtained during the 266- and 355-nm (dark gray line) or 266- and 532-nm (light gray line) two-color two-laser flash photolysis. The absorption spectrum was obtained during onelaser photolysis (266-nm, black line) of AX (4.0  104 M). The second laser irradiation was at 1 ms after the first laser pulse. All the fluorescence spectra of AXH were normalized with the corresponding absorption peaks. Inset: Kinetic traces of the fluorescence intensity of AXH at 460 and 645 nm.

also confirmed by TEM (Fig. 2.21). This fact indicates that the enhanced reducing power of the excited BPH efficiently reduced AuCl 4 generating AuNps. Since the reaction of the excited radical can be limited to a rather small region by controlling the overlapping volume of the two-laser light,

Figure 2.21. (a) TEM image of AuNps in HAuCl4-doped PVA film containing BP. (b) Distribution of diameter of AuNps formed in PVA films.

91

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

three-dimensional control of the AuNps fabrication can be achieved using the present method.

C. Excited Radical Cations Fluorescence of radical cation gives important information during various reaction processes. For example, distance between the ion pair can be estimated based on the Fo¨ rster-type ENT theory under the condition where a pair of radical ions generated from the photoinduced ELT are fluorescent and there is a good spectral overlap of fluorescence and absorption for the pair. Here, we introduce an example in which two-color two-laser flash photolysis was employed to estimate the distance between TMB þ and the radical anion of 1,4-dicyanonaphthalene (DCN ) [121], since TMB þ* shows fluorescence as indicated earlier [26]. Fluorescence intensity of TMB þ generated during the laser flash photolysis of a N2-purged acetonitrile solution containing TMB and DCN with a XeCl laser pulse (308 nm, 30 ns FWHM) was measured with various delay times of the second laser pulse (532 nm, 5 ns FWHM). The fluorescence intensity of TMB þ immediately after the first pulse was unexpectedly weak, although its concentration was at the maximum as observed in the trace of the transient absorption of TMB þ. The fluorescence intensity increased with the increase of the delay time and reached maximum at approximately 220 ns, and decreased in a second-order kinetics. On the other hand, the fluorescence in an aerated solution did not show the initial increase and decreased monotonously in a second-order kinetics, in accordance with the decay of the transient absorption of TMB þ. Since DCN  was readily quenched by oxygen within 50-ns transient absorption measurements, the rise of the fluorescence intensity corresponds to the decrease of the ENT quenching of TMB þ* by DCN  with the increase of the distance between TMB þ* and DCN . The transient absorption due to TMB þ showed no depletion by the 532-nm excitation, indicating not ELT quenching but Fo¨ rster-type ENT quenching of TMB þ* by DCN . From the time-dependent fluorescence intensity, it was revealed that the distance between ions increased from 0.6 nm at 50 ns to 1.7 nm at 200 ns. From these values, the apparent diffusion constant was estimated to be 8.5  108 cm2 s1, which is much smaller than the typical value, 2  105 cm2 s1, for acetonitrile. It could be attributed to the contribution of the Coulombic interaction between the radical ions. 



























D.



Other Reactive Intermediates

1. Two-Color Two-Laser DNA Damaging Photodynamic therapy (PDT) is a promising treatment for cancer based on the photosensitized oxidative reaction at the diseased tissues producing cell death, and DNA is considered as a potential target [122]. Compared with surgery and chemotherapy, the

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Scheme 2.10. Mechanism of DNA damage by two-color two-laser irradiation. The first laser irradiation causes charge separation between sensitizer (S) and DNA. The second laser causes photo ejection from S , making charge recombination impossible. Then, chemical reaction of DNA þ occurs leading to DNA damage. 



combination of a photosensitizer (S) uptake in malignant tissues and selective light delivery offers the advantage of a selective method of destroying diseased tissues without damaging surrounding healthy tissues. Excitation of DNAbound sensitizers produces the S / DNA þ charge-separated state through photoinduced ELT. However, the efficiency of producing photosensitized DNA damage is low because the charge recombination rate is usually much faster than the process leading to DNA damage, such as the reaction of G þ with water [123–126]. Here, we introduce the first study of nanosecond-laser DNA damaging, using a combination of two-color two-laser pulses as a promising new strategy to reach a high DNA damaging efficiency. The first laser pulse was applied for the production of S  and DNA þ, and the second laser pulse for the electron ejection from S , making the reaction irreversible (Scheme 2.10) [127]. In this study, naphthaldiimide (NDI) was selected as an S that can be excited with the first laser at a wavelength of 355 nm [128–132]. First, to assess the feasibility of electron ejection from S  bound to DNA, the pulse radiolysis–laser flash photolysis of NDI-conjugated oligodeoxynucleotide (NDI-ODN) was performed (Fig. 2.22) [133]. NDI  with a maximum absorption peak at 495 nm [129] was generated from the electron attachment during pulse radiolysis of NDI-ODN. Since S  often absorbs light at a longer wavelength compared with its nonreduced form, laser pulses with a longer wavelength can be used for the excitation of S , and a 532-nm laser was applied as the second laser. Irradiation of NDI  in NDI-ODN with a 532-nm laser pulse caused a decrease in
O.D. of NDI  and the formation of absorption at 630 nm assigned to a solvated electron (eaq) immediately after the flash (Fig. 2.22, inset), demonstrating the successful electron ejection from NDI  to the solvent water. Figure 2.23 shows the time profile of NDI  in the one-color laser photolysis of NDI-ODN. Upon the first laser excitation, hole transfer via consecutive fast 



























93

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

Figure 2.22. Electron ejection from NDI  promoted by a 532-nm laser pulse. NDI  was generated from the electron attachment during pulse radiolysis of NDI-ODN (NDIAAAAAAGTGCGC/TTTTTTCACGCG) (gray), and photoirradiated with a 532-nm laser pulse at 2 ms after the electron pulse (black). Inset: Formation and decay of the solvated electron monitored at 630 nm. 



adenine hopping leads to a charge-separated state within the laser duration (5 ns), and the charge recombination proceeds by the single-step superexchange from the ˚ away from NDI  with a lifetime of guanine (G) radical cation (G þ) about 14 A 240 ns [123, 134, 135]. Figure 2.23 also shows the consumption of G as a function 



Figure 2.23. Formation and decay of NDI  and the effect of the delay time between two laser pulses on the consumption of G during the laser flash photolysis of NDI-ODN (NDITTTCGCGCTT/AAAGCGCGAA).The transient absorption of NDI  was monitored at 495 nm following the 355-nm excitation (left axis). The consumption of G is plotted as a function of the delay of the 532-nm pulse with respect to the 355-nm pulse ( , right axis). The dashed line shows the consumption of G in the absence of the 532-nm pulse. 



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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

of the delay time of the second laser pulse in the time-delayed two-color photolysis. The delay time dependence of the consumption of G agreed well with the decay of the transient absorption of NDI  obtained in one-color laser photolysis. Thus, the acceleration caused by the second laser is clearly based on the excitation of NDI . The experiments were performed under single-hit (i.e., low-conversion) conditions where, on average, each duplex reacts once or not at all, and the consumption of G was linearly correlated with the irradiation time and the power of the second laser in the present experimental arrangement. 



2. Two-Color Laser Photolysis for Determination of the Rate Constant from the Product Analysis o-Quinodimethane is one of the extensively studied intermediates. Cycloaddition of o-quinodimethane with alkenes and alkynes is one of the well-investigated fields [136–138], but the reported kinetic studies are mainly of some substituted o-quinodimethanes [139], and, to the best of our knowledge, the rate constant of the cycloaddition of parent o-quinodimethane in room-temperature solutions has not been reported so far. The lack of such important and basic kinetic data seems to be due to the difficulty in conducting the experiments by spectroscopic means. In the cycloaddition reactions, it is possible to observe the decay of o-quinodimethane that is generated from conventional precursors by the flash photolysis technique because in most cases the absorption of o-quinodimethane appears at a longer wavelength than the precursors. However, the decay does not simply reflect the formation of the cycloadduct because o-quinodimethane also gives other products, such as its dimers, oligomers, and polymers [140, 141]. There are also difficulties in tracing the formation of the cycloadduct spectroscopically because the absorption of the cycloadduct appears at the same wavelength region as the precursors (a large amount of the precursor remains intact even after the laser pulse irradiation due to the low photochemical efficiency) and the above-mentioned dimers, oligomers, and polymers. The strategy used in our experiment for determination of the rate constant of the cycloaddition of o-quinodimethane and maleic anhydride in room-temperature solutions was the fast and efficient generation of o-quinodimethane during the first laser pulse irradiation and the quenching of the reaction by the decomposition of remaining o-quinodimethane during the second laser pulse irradiation (Scheme 2.11) [142]. When o-quinodimethane is generated during the first KrF laser pulse irradiation in the presence of maleic anhydride, the cycloaddition of o-quinodimethane with maleic anhydride gives cis-1,2,3,4-tetrahydro-2,3-naphthalenedicarboxylic anhydride (path 1 with the rate constant of k1) together with thermal products of o-quinodimethane [140, 141], which are formed by second-order kinetics [138] (path 2 with the bulk rate constant of k2). The reaction is quenched by the second XeCl laser pulse after a particular time in the course of the reaction to decompose the remaining o-quinodimethane, partly forming benzocyclobutene [bicyclo(4.2.0)-octa-1,3,5-

95

TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

O O O cis-1,2,3,4-tetrahydro-2,3-naphthalenedicarboxylic anhydride O O

∆

path 1; k1

O maleic anhydride

SePh

KrF excimer laser 2hν -2PhSe

SePh

∆

path 2; k2

o-quinodimethane + hν

other photochemical products

XeCl excimer laser

other thermal products

+ benzocyclobutene

Scheme 2.11. Determination of the rate constants of the cycloaddition reaction of o-quinodimethane based on the product analysis depending on the delay time between the first 248-nm laser irradiation and second 308-nm laser irradiation. o-Quinodimethane was generated by the first 248-nm laser pulse irradiation. The cycloaddition reaction with maleic anhydride (path 1 with the rate constant of k1) and the dimerization (path 2 with the bulk rate constant of k2) were quenched by the photochemical conversion of o-quinodimethane into benzocyclobutene with the second 308-nm laser pulse irradiation.

triene] as a photochemical product. By analyzing the dependence of delay time between the first and second lasers (0–0.1 s) on the yield of tetrahydronaphthalenedicarboxylic anhydride, the rate constant of the cycloaddition of o-quinodimethane and maleic anhydride (path 1: o-quinodimethane þ maleic anhydride ! tetrahydronaphthalenedicarboxylic anhydride) was determined to be 2.1  105 M1 s1. To the best of our knowledge, this is the first report on the determination

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

of the rate constant of the cycloaddition of parent o-quinodimethane with an alkene. Therefore, this is an interesting example of the application of two-color laser photolysis to determine the rate constant even under the condition where the spectroscopic analysis is difficult [143]. 3. Two-Color Laser Control of Photocatalytic Reaction on TiO2 Surface Photocatalytic reactions on the semiconductor surface attract quite a lot of attentions today. Especially, the photocatalytic reaction on the TiO2 surface has been widely used in areas such as environmental purification, hydrogen production, gas sensors, and dye-sensitized solar cells. Typically, the reaction processes are initiated by the band-gap excitation of TiO2 particles with UV irradiation to generate reactive intermediates such as hole, electron, and various oxygen-related species. The reactivity and lifetime of these oxidizing species play an important role in controlling the overall kinetics of the oxidative processes. Recently, Keggin-type polyoxometalates (POMs) have been applied to TiO2 photocatalytic systems as electron scavengers to retard fast charge recombination between the hole and electron to enhance the reactivity of the hole. In addition, POM, which is generated by a one-electron reduction of POM by the conduction band electron of TiO2 nanoparticles, absorbs visible light to form the excited POM i.e., POM*, which synergistically catalyzes the reduction process of solute in the solution [144]. Thus, by employing two-color two-laser flash photolysis, the reduction process via POM* can be examined. By employing methylviologen (MV2þ) as an indicator of the reduction via POM*, we studied two-color laser control of the photocatalytic reaction of the TiO2/POM/MV2þ ternary system [145]. The electron transfer from the conduction band of TiO2 to POM was confirmed by the increase of the visible absorption band during the laser flash photolysis of TiO2/POM colloidal solution. The efficiency of the electron transfer from TiO2 to POM was on the order of H2W12O406 < SiW12O406 < PW12O403, depending on the reduction potential of the POMs. Electron injection from PW12O404* to the conduction band of TiO2 was clearly observed as the bleaching of the absorption band due to PW12O404 upon excitation of PW12O404 with the second laser (Fig. 2.24). In the presence of MV2þ, the extent of the bleaching decreased with an increase in the concentration of MV2þ. Direct electron transfer from TiO2 to MV2þ is negligible under the present condition, while complex formation is possible between POM and MV2þ. Thus, the present observation can be attributed to the electron transfer from PW12O404* to MV2þ generating POM and MV þ. Because the reduction potential of MV2þ is more negative than that of PW12O403, cascade electron transfer from MV þ to PW12O403 is possible, which is in accordance with the absence of the absorption band of MV þ in Figure 2.24. Scheme 2.12 summarizes the energy diagram for the TiO2/POM photocatalytic redox process. By employing two-color two-laser flash photolysis, electron 





TWO-COLOR TWO-LASER FLASH PHOTOLYSIS

97

Figure 2.24. (a) Transient absorption spectra obtained at 0.1 ms (solid squares) before and 0.3 ms (open circles), 4 ms (solid triangles), and 30 ms (open inverted triangles) after a second 532-nm laser flash following the first 355-nm laser flash with a delay time of 1 ms during two-color two-laser flash photolysis of an argon-saturated TiO2/PW12O403 colloidal solution in the presence of MV2þ. (b) Time traces observed at 605 nm during two-color two-laser flash photolysis of TiO2/PW12O403 colloidal solution in the absence and the presence of MV2þ.

transfer from POM* became possible. This process is interesting in terms of mimicking the ‘‘Z-scheme’’ of the natural photosynthesis system. In this section, we introduced several examples of studies employing twocolor two-laser flash photolysis. In many cases, the second laser has been employed to excite intermediates, such as triplet excited states, radicals, and radical ions. Thus, two-color two-laser flash photolysis can be regarded as a powerful tool to produce the higher excited intermediates, which cannot be accessed by the single laser flash. As indicated earlier, the higher excited

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

Scheme 2.12. Energy diagram for the TiO2/POM photocatalytic redox processes. Dotted and broken arrows represent the deactivation and charge recombination processes, respectively.

intermediates are generally short-lived species. Thus, investigation in the picosecond regime is important to elucidate the properties of the higher excited state. To detect such short-lived species, we introduced two methods—streak camera detection and the pump and probe method. The detection method using a streak camera is useful for detecting an intermediate generated from an irreversible process. Fluorescence detection is also possible by using a streak camera. Thus, we employed streak camera detection for studies on the excited ketyl radicals. On the other hand, the pump and probe method gives kinetic parameters of the reversible processes with better time resolution than the streak camera detection. We employed it for studying the Tn state. By employing a femtosecond laser, dynamic properties of higher excited states in the subpicosecond regime will become clear in the near future. It should be stressed that two-color two-laser flash photolysis is not limited to the study of higher excited states. For example, the study on DNA cleavage described earlier is one such example. Two-color two-laser flash photolysis is an advantageous DNA cleavage method, which realizes site selectivity to avoid damage to healthy cells and to perform selective damage of cancer tissue deep beneath the skin surface. Application of two-laser photolysis to photodynamic therapy is also possible by other approaches, since two-laser photolysis generates site selectivity for highly reactive intermediates, such as radicals, which are expected to induce damage on various diseased tissues. Generation of highly reactive intermediates by the second laser excitation is also beneficial in the decomposition of harmful pollutants in the air. Therefore, application of two-laser photolysis to semiconductor catalytic reactions is another interesting approach. Two-color two-laser photolysis is also an interesting approach to realize ultrafast control over the function of molecular devices.

99

THREE-COLOR THREE-LASER FLASH PHOTOLYSIS

IV. THREE-COLOR THREE-LASER FLASH PHOTOLYSIS Combination of pulse lasers can be achieved rather easily by using delay circuits as indicated earlier. We have already reported the studies using three-color threelaser photolysis. In this section we briefly introduce the results to show the availability of the multibeam irradiation method.

A. Three-Laser Control of Intermediate Population Three-color three-laser photochemistry of di(p-methoxyphenyl)methyl chloride ((p-CH3OC6H4)2CHCl ¼ An2CHCl) was studied by three-step excitation using 308-, 355-, and 495-nm lasers with delay times of 100 ns to 3 ms (Scheme 2.13) [146]. Di(p-methoxyphenyl)methyl radical (An2CH ) was produced together with An2CH in the excited state (An2CH*) and di(p-methoxyphenyl)methyl cation (An2CHþ) in the quantum yields of 0.09, 0.12, and 0.12, respectively, after a laser flash during the 308-nm laser (first laser) photolysis of An2CHCl in acetonitrile. Excitation of An2CH with the 355-nm laser (second laser) resulted in the formation of transient absorption of An2CH* and An2CHþ and fluorescence of An2CH* with a peak at 550 nm. The formation of An2CHþ from An2CH requires two-photon energy at 355 nm and proceeds by the resonant two-photon ionization (RTPI) of An2CH through sequential excitation of An2CH *. Excitation of An2CHþ with the 495-nm laser (third laser) produced fluorescence with D0

S1 tio

za

hν495

ni

io

hν355

I. P.

n

Dn S1

hνf560

S0

IC

hν308

e

ag av

cle

hν355

D1 hνf550

D0 S0 An2CHCl

An2CH .

An2CH+

Scheme 2.13. First laser generates di(p-methoxyphenyl)methyl radical (An2CH ) together with An2CH in the excited state (An2CH *) and di(p-methoxyphenyl)methyl cation (An2CHþ). Second laser excitation of An2CH resulted in the formation An2CH *, which shows fluorescence at 550 nm ðhnf 550 Þ, and An2CHþ. Excitation of An2CHþ with the 495-nm laser (third laser) produced fluorescence with a peak at 560 nm ðhnf 560 Þ. 



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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

a peak at 560 nm. Although the fluorescence of An2CHþ was also observed without the second laser excitation because of the initial formation of An2CHþ during the first 308-nm laser photolysis, the fluorescence intensity of An2CHþ increased approximately 1.2 times with the second 355-nm laser excitation of An2CH . Therefore, the second laser excitation can perform the conversion of An2CH to An2CHþ through RTPI within the laser flash duration, and the fluorescence intensity of An2CHþ can be controlled by the second laser irradiation.

B. Stepwise Bond Cleavage of Two C—O Bonds Via the Tn State As described previously, (4-benzoylphenoxy)methylnaphthalene exhibited C—O bond dissociation via the Tn state. The multiple laser excitation technique allows us to study stepwise cleavage of two equivalent bonds in a molecule. To provide clear evidence for the stepwise photocleavage of two equivalent bonds in a molecule, we introduce here two C—O bond cleavages of 1,8-bis[ (4-benzoylphenoxy)methyl]naphthalene (1,8-(BPO-CH2)2Np) to give acenaphthene with the three-step excitation using three-color three-laser flash photolysis (Scheme 2.14) [147].

O

O

O

O

ENT

O

O

O

O hν430

T1

hν308

+ H2C D0

< 5 ns, 8.5%

< 20 ns

O

hν355

O

O

O H2C

CH2

H2C < 5 ns

< 5 ns

O

CH2 + O

1,8-(BPO-CH2)2Np

acenaphthene

Scheme 2.14. Three-color three-laser photochemistry of 1,8-(BPO-CH2)2Np involving intramolecular triplet ENT, selective excitation of 1,8-(BPO-CH2)2Np(T1) to 1,8-(BPOCH2)2NpðTn Þ, the C—O bond cleavage from 1,8-(BPO-CH2)2NpðTn Þ to give 1-(BPOCH2)NpCH2 , and selective excitation and C—O bond cleavage of 1-(BPO-CH2)NpCH2 to give acenaphthene as the stable product through 1,8-( CH2)Np. Dotted square shows the excitation energy delocalization. 





THREE-COLOR THREE-LASER FLASH PHOTOLYSIS

101

Figure 2.25. Transient absorption spectra obtained at 300 ns after the first (308 nm) laser (a, broken line) and at 100 ns after the second (430 nm) laser (b, solid line) during laser flash photolysis of 1,8-(BPO-CH2)2Np in Ar-saturated acetonitrile. The delay time of the two lasers was 200 ns. Top Inset: Time profiles of transient absorptions obtained at 415 and 345 nm during one-laser irradiation at 308 nm (c and e) and two-laser irradiation at 308 and 430 nm (d and f). The growth of the transient absorption in the time scale of a few tens of nanoseconds was due to the formation of Np(T1) through intramolecular triplet ENT from BP(T1) to Np. Bottom Inset: Plots of

O.D.345 of 1,8-(BPOCH2)2Np versus 430-nm laser intensity (I430).

A transient absorption spectrum was observed during the 308-nm laser irradiation of 1,8-(BPO-CH2)2Np (6.8  105 M) in acetonitrile (Fig. 2.25a). Absorption peaks at 400 and 425 nm were assigned to 1,8-(BPO-CH2)2Np(T1), whose energy was localized at the Np moiety because of an intramolecular triplet ENT from BP(T1) to Np(S0) producing Np(T1). Immediately after the second 430-nm laser (7 mJ pulse1) flash, the formation of a transient absorption band around 345 nm and bleaching of the peaks of 1,8-(BPO-CH2)2Np(T1) at 400 and 425 nm were observed (Fig. 2.25b). Because 1,8-(BPO-CH2)2Np(S0) has no absorption at 430 nm, only 1,8-(BPO-CH2)2Np(T1) was excited to give 1,8-(BPO-CH2)2NpðTn Þ during the second 430-nm laser irradiation. The absorption around 345 nm was assigned to 1-(BPO-CH2)NpCH2 [67, 74]. These results reasonably show that the Tn state energy is higher than the C—O bond dissociation energy and it delocalizes in the molecule including the C—O bonds. The quantum yield () of the formation of 1-(BPO-CH2)NpCH2 from the photoreaction of 1,8-(BPO-CH2)2Np(T1) was calculated to be 0.085  0.004. Because 1-(BPO-CH2)NpCH2 has an absorption at 355 nm, 1-(BPOCH2)NpCH2 was irradiated by the third 355-nm laser (10 mJ pulse1) at 200 ns after the second 430-nm laser irradiation. The results of the three-color three-laser photolysis of 1,8-(BPO-CH2)2Np are shown in Figure 2.26. Bleaching

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PHOTOCHEMISTRY OF SHORT-LIVED SPECIES USING MULTIBEAM IRRADIATION

Figure 2.26. Three-color three-laser photolysis of 1,8-(BPO-CH2)2Np (a, spectra; b, time profiles detected at 345 nm) in Ar-saturated acetonitrile. The transient spectra observed during the irradiation of the 308-nm laser (at 500 ns after the laser pulse) (a), successive irradiation with the 308- and 430-nm lasers (at 300 ns after the second laser pulse; delay time between the two lasers: 200 ns) (b), successive irradiation of the 308- and 355-nm lasers (at 100 ns after the second laser pulse; delay time between the two lasers: 400 ns) (c), successive irradiation of the 308-, 430-, and 355-nm lasers (at 100 ns after the third laser pulse; delay time between the lasers: 200 and 200 ns) (d), and irradiation of the 355nm laser (at 100 ns after the laser pulse) (e). The inset in (a) shows the spectra b  a (f) and d  c (g). In panel (b), 1, 2, and 3 refer to the irradiation sequence order of the 308-, 430-, and 355-nm lasers, respectively.

of the transient absorption at 345 nm during the third 355-nm laser irradiation was clearly observed, indicating the C—O bond cleavage from 1-(BPO-CH2)NpCH2 ðDn Þ. It is reported that the molecular orbital of the Dn state is delocalized not only on the Np chromophore but also on the C—O s orbital. Formation of acenaphthene from 1-(BPO-CH2)NpCH2 ðDn Þ may be explained by the radical backside attack mechanism or the second C—O bond cleavage to give the 1,8( CH2)2Np biradical, which rapidly cyclizes to form acenaphthene. However, since similar results were observed for 1,4-(BPO-CH2)2Np [148], the formation of acenaphthene is most likely explained by the 1,8-(CH2)2Np biradical mechanism (Scheme 2.14). The absorption of 1,8-(CH2)2Np, which was reported to appear at 500 nm [149], was not observed probably due to the small absorption coefficient, the formation of 1,8-(CH2)2Np at a low concentration, and fast cyclization of 1,8-( CH2)2Np that occurred within the laser duration (5 ns). Similar stepwise bond cleavage was confirmed for 1,8-bis(phenoxymethyl)naphthalene (1,8-(PhOCH2)2Np) and 1,4-(PhOCH2)2Np, while not with 1,8bis(hydroxymethyl)naphthalene (1,8-(HOCH2)2Np) and 1,4-(HOCH2)2Np. Furthermore, the cleavage yields of 1,8-(PhOCH2)2Np and 1,4-(PhOCH2)2Np were larger than those of 1,8-(BPOCH2)2Np and 1,4-(BPOCH2)2Np. In addition, cleavage yields with 1,8-substituted compounds were larger that those of 1,4subsituted compounds, when the compounds bear the same substituents. These

ACKNOWLEDGMENTS

103

facts indicate that the cleavage yield depends on the types and position of the substituents [148]. We introduced two examples of three-color three-laser photolysis. The former example employed the third laser as an excitation source to evaluate the amount of intermediates generated by the first and second lasers irradiation. On the other hand, the latter example used the second and third lasers to promote bond dissociation from the respective Tn and Dn states. The role of each laser is quite different. These examples indicate that one can control reactions by selecting laser wavelength and delay time based on the properties of each intermediate. Combination of pulse radiolysis and two-laser flash photolysis will be another interesting subject of three-beam excitation chemistry to be investigated in the near future. In this case, radical ions generated during pulse radiolysis will be excited by the successive two lasers. That is, one can introduce additional pathways to the reaction scheme indicated in Scheme 2.3. Utilization of a dyad or triad molecular system will realize fast intramolecular reaction systems applicable to the multibit molecular memory.

V.

CONCLUSIONS

In this chapter, several examples of photochemistry of short-lived species by multibeam irradiation are introduced. In many cases, the properties of excited intermediates have been investigated by using nanosecond lasers. Since the lifetime of excited intermediates is usually quite short, investigations employing ultrashort laser pulses are intrinsically important. Various properties estimated by the direct manner will appear in the near future. Recent progress of ultrashort pulse lasers and detection systems will make this possible. On the other hand, utilization of multiple excitations is not limited to the basic study of excited intermediates. These are applicable to biological and environmental fields. Further fruitful results are expected for these explorations.

ACKNOWLEDGMENTS We thank our collaborators, particularly Dr. Xichen Cai, Dr. Masanori Sakamoto, Dr. Kiyohiko Kawai, Dr. Takashi Tachikawa, Dr. Akihiko Ouchi, Dr. Minoru Yamaji, Dr. Nobuyuki Ichinose, Dr. Michihiro Hara, and Mrs. Sachiko Tojo and Mr. Yosuke Oseki, for their contribution to this work, as well as the members of the Radiation Laboratory of SANKEN, Osaka University, for running the linear accelerator. This work has been partly supported by a Grant-in-Aid for Scientific Research (Project 17105005, Priority Area (417), 21st Century COE

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Research, and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.

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139. Y. Takahashi, K. Miyamoto, K. Sakai, H. Ikeda, T. Miyashi, T. Ito, and K. Tabohashi, Tetrahedron Lett. 1996, 37, 5547. 140. M. P. Cava and A. A. Deana, J. Am. Chem. Soc. 1959, 81, 4266. 141. L. A. Errede, J. Am. Chem. Soc. 1961, 83, 949. 142. A. Ouchi and Y. Koga, J. Org. Chem. 1998, 63, 6780. 143. A. Ouchi, Z. Li, M. Sakuragi, and T. Majima, J. Am. Chem. Soc. 2003, 125, 1104. 144. M. Yoon, J. A. Chang, Y. Kim, J. R. Choi, K. Kim, and S. J. Lee, J. Phys. Chem. B 2001, 105, 2539. 145. T. Tachikawa, S. Tojo, M. Fujitsuka, and T. Majima, Chem. Eur. J. 2006, 12, 3124. 146. M. Hara, S. Tojo, and T. Majima, J. Phys. Chem. A 2003, 107, 4778. 147. X. Cai, M. Sakamoto, M. Hara, S. Tojo, A. Ouchi, K. Kawai, M. Endo, M. Fujitsuka, and T. Majima, J. Am. Chem. Soc. 2004, 126, 7432. 148. X. Cai, M. Sakamoto, M. Hara, S. Tojo, A. Ouchi, A. Sugimoto, K. Kawai, M. Endo, M. Fujitsuka, and T. Majima, J. Phys. Chem. A. 2005, 109, 3797. 149. M. C. Biewer, M. S. Platz, M. Roth, and J. Wirtz, J. Am. Chem. Soc. 1991, 113, 8069.

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY: THEORY, TECHNIQUES, CHROMOPHORE DESIGN, AND APPLICATIONS Bernd Strehmel Kodak Polychrome Graphics GmbH, Research and Development, An der Bahn 80, D-37520 Osterode, Germany Veronika Strehmel University of Potsdam, Applied Polymer Chemistry, Karl-Liebknecht Str. 24/25, D-14476 Potsdam-Golm, Germany

CONTENTS I. Introduction II. Two-Photon Absorption: Theory, Mechanism, and Quantification A. Theoretical Background 1. Relation Between Two-Photon Absorption and Nonlinear Optical Parameters 2. Theoretical Methods for Description of Two-Photon Absorption 3. Symmetry Considerations 4. Bond Length Alternation B. Mechanistic Consideration 1. Two-Level Model Advances in Photochemistry, Volume 29 Edited by Douglas C. Neckers, William S. Jenks, and Thomas Wolff # 2007 John Wiley & Sons, Inc.

111

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2. Three-Level Model 3. Vibrational Contributions 4. Surface Plasmons C. Evaluation of Two-Photon Absorbing Materials 1. Experimental Techniques in Nonlinear Absorption 2. Absolute Evaluation of Two-Photon Absorbing Materials 3. Relative Evaluation of Two-Photon Absorbing Materials by Two-Photon Excited Fluorescence D. Pulse Propagation III. Chromophore Design and Optimization of Two-Photon Absorption A. Chromophores with Large p-Systems 1. General p-Structures 2. Conjugated Polymers and Oligomers B. Dipolar Chromophores 1. Neutral Donor-p-Acceptor Compounds 2. Ionic Donor-p-Acceptor Compounds C. Symmetric Chromophores with Large Two-Photon Absorptivities 1. Donor-p-Donor Compounds 2. Acceptor-p-Acceptor Compounds 3. Donor-Acceptor-Donor Compounds D. Octupolar/‘‘Propeller Shaped’’ Chromophores with Large Two-Photon Absorptivity E. Polymers with Nonlinear Absorbing Chromophors IV. Two-Photon Excited Fluorescence V. Organic Reactions Upon Two-Photon Excitation A. Isomerization B. Cycloaddition C. Singlet Oxygen VI. Simultaneous Two-Photon Initiated Polymerization and Crosslinking A. Radical Two-Photon Initiated Polymerization B. Cationic Two-Photon Initiated Polymerization VII. Applications A. Three-Dimensional Micro- and Nanofabrication B. Three-Dimensional Data Storage C. Optical Band-Gap Materials D. Waveguide Materials E. Fluorescence Imaging F. Two-Photon Medical Applications G. Upconverted Lasing H. Optical Power Limiting VIII. Outlook for Two-Photon Photosciences References

INTRODUCTION

I.

113

INTRODUCTION

The first theoretical prediction of a simultaneous two-photon absorption (TPA) event was reported by Maria Go¨ ppert-Mayer in 1931 [1]. The development of high-energy pulsed lasers experimentally allowed confirmation of TPA a few decades later [2] because simultaneous multiphoton excitation requires high peak intensities. Such large photon densities can be achieved by (1) use of a collimating lens and (2) a laser providing large photon flux. At least 1028 photons/ (cm2
s) are needed to access the excited state by simultaneous multiphoton excitation. Since the experimental discovery of TPA, multiphoton excitation has become a popular tool in the photochemical sciences to determine the excitation energy of states with parity forbidden transition [3–54]. Transitions that are parity forbidden by one-photon (OP) excitation can thus become allowed by two-photon (TP) excitation. TP excitation spectroscopy localizes the energetic position of TP excited states, which cannot be observed by OP excitation. These pioneering works confirmed many quantum chemical studies predicting the existence of TP excited states and therefore experimentally completed the pattern of electronic transitions in organic compounds. In general, TP excitation had been mainly limited to academic interest until the end of the 1980s [2–24, 26–45, 47–52, 55–69]. Modern photochemistry based on multiphoton excitation uses femtosecond NIR-lasers, resulting in population of an excited state absorbing either in the UV or visible spectral range as described by an upconversion mechanism [16, 23, 31, 41, 70–73]. However, practical applications require the use of inexpensive laser sources, which are in most cases diode lasers emitting continuous wave (cw) light around 800 nm. The use of such laser sources requires the development of TP absorbing materials with extraordinarily large nonlinear absorptivity. In this chapter we introduce possible ways to accomplish these requirements. Perhaps future research will advance even in TP photosciences if inexpensive and miniaturized pulsed femtosecond (fs) lasers become available for industrial applications. Thus, large TP absorptivity of the chromophore and improved efficient excitation sources may spur interest even in industrial applications that need a high spatial resolution. TP excitation results in a significantly better spatial resolution, compared to OP excitation, because excitation occurs with high accuracy at the focal point, and the efficiency of TPA decreases with increase of the distance from the focal point by the power of 4. This clearly shows an advantage of TP excitation. It is attractive for applications requiring high spatial resolution, which is not accomplished by OP excitation. Thus, a spatial resolution significantly below the diffraction limit of the excitation light can be achieved by TP excitation. This spatial resolution is >120 nm and results in higher storage

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densities needed for some industrial applications [70, 72–76]. Furthermore, no significant attenuation of the excitation light occurs by its traveling through even thick samples if the linear absorption coefficient vanishes at the excitation wavelength. This allows selective excitation of a small volume in thick samples to store/read information more densely with higher accuracy than possible using OP excitation. Since the early 1990s, development of fs lasers [77] has opened new directions for technologies, which can be based on TPA. These light sources offer significantly higher photon densities in a short pulse, in comparison with traditional high-energy nanosecond (ns) or cw lasers. This allows the excitation power of the fs laser to be maintained at a level where material destruction does not occur, while the high photon density in a short laser pulse does not significantly damage the chromophore. This can be seen as an additional important benefit of using fs excitation. However, high cost and the complicated setup, compared to inexpensive cw laser sources (diode lasers), have limited fs lasers as excitation sources mostly to the academic landscape. Perhaps future laser development will result in less expensive and more compact fs lasers even for industrial uses in TP applications. TP excited materials have been successfully applied for imaging applications [78–112], holography [70, 113–117], data storage [118, 119], optical power limiters [53, 120–130], three-dimensional (3D) submicro- and nanofabrication [74–76, 88, 113, 114, 116, 131–146], optical band gap materials [145, 147–155], and upconverted lasing materials [126, 156–178]. Optical power limiters are important for development of eye protecting materials [127]. This is caused by the rapid development of new laser systems. There is currently a need to protect eyes against any laser frequency that may result in an accident. Thus, materials are needed that are transparent under ambient light conditions but whose transparency rapidly changes in <1 ps to absorb/filter intense light over a broad frequency including NIR frequencies. Presumably, TP absorbing materials may close the gaps in this field. More details are summarized in Ref. [127]. TP excitation has also been used to manufacture singlet oxygen [179–191]. This can become important in photodynamic therapy (PDT) in biomedicine [90, 91, 95, 97, 100, 101, 107, 109, 170, 171, 181, 186, 187, 192–204]. PDT applies the capability of some sensitizers to generate singlet oxygen in cancer cells, resulting in destruction of the tumor cells. The use of OP excitation diminishes the penetration depth of excitation light and limits PDT mainly to a few types of biological tissues. Thus, PDT can become more generally applicable using TP excitation. This technique results in deeper penetration of the exciting NIR photons into the biological tissue because biological material is more transparent in this spectral region. Furthermore, major advances in cancer treatment have introduced the use of TP chromophores in a ‘‘magnetic nanoclinic.’’ This ‘‘nanoclinic’’ was designed with a multifunctional Fe2O3 nanoparticle core (<50 nm)

INTRODUCTION

115

for magnetocytolysis using a DC magnetic field and a TP fluorescent probe to function in optical tracking [70, 192, 194, 195, 200]. These applications clearly demonstrate the potential of TP photochemistry. Selective spatial excitation can be achieved with resolution on the nanoscale in even thick samples. These efforts have sparked interest in the use of TP excitation in photochemical sciences, since TP chromophores have been successfully applied as imaging materials in the biophysical sciences [46, 80, 84–86, 103, 109–111, 187, 188, 195–197, 200, 205–212]. However, photochemistry based on TP excitation has not been a common tool in many photochemical fields because of some practical limitations. There is a substantial need to give greater attention to chromophore design, experimental techniques to quantify efficiency of TPA, and theories describing TPA. Consequently, this chapter first reviews models used to describe TPA. It is also intended as a practical guide for the manufacture of efficient TP chromophores. TP absorbing materials are furthermore grouped into several classes to show the relationships between chromophore size, symmetry, and substitution pattern affecting the efficiency of TPA. TP excitation can start several photochemical reactions. These are isomerization [28, 32, 34, 39, 40, 44, 48, 88, 116, 213–224], photocycloaddition [217], photogeneration of singlet oxygen [179–191], intermolecular charge transfer [133, 225], and intramolecular charge transfer [131, 219, 224, 226–237]. These photochemical processes are well known with OP excitation [57, 238–241]. However, the crucial point is to have a chromophore that can be efficiently excited by TPA in order to populate either directly or indirectly the first excited singlet (S1) state from which photochemistry can then occur. Photochemistry from the S1 state is the prerequisite for three-dimensional data storage [114, 118, 119, 134, 138, 143–145, 148, 170, 171, 242–248], holography [113, 114, 116, 117, 249–252], photodynamic therapy [170, 184–189, 195–198, 202, 206, 207, 210, 253–256], and manufacture of 3D objects [114, 133, 134, 137, 138, 142, 148, 171, 189, 203, 204, 246, 247, 257–265]. The latter can be achieved by TP photoinitiated polymerization and photocrosslinking [74–76, 88, 114, 134–136, 139–142, 147, 149, 150, 266–271]. It possesses the largest impact for the manufacture of photonic band-gap materials, which are often called photonic crystals [145, 147–150]. Although TP photochemistry has been successfully applied for some applications, it is still in its infancy. The results summarized in this chapter are systematically grouped to assist in further development and use of TP absorbing materials and related applications in material science and engineering. The interdisciplinary relations described are useful for future work in physical, organic, and polymer chemistry as well as experimental physics. Some selected reviews are complementary to this chapter [16, 23, 31, 35, 41, 45, 50, 53, 70, 71, 86, 127, 129, 272–277].

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II.

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION A. Theoretical Background

1. Relation Between Two-Photon Absorption and Nonlinear Optical Parameters TPA occurs by simultaneous absorption of two photons in the same quantum act promoting the chromophore from the ground state (S0) into the excited state (Si , i > 0). When a higher excited state (Si , i  2) is populated, the higher excited state travels to the first excited state S1 by internal conversion (IC). The S1 state can deactivate to the ground state either by fluorescence or nonradiative deactivation. Furthermore, the S1 state can change the spin by intersystem crossing (ISC), resulting in formation of the triplet state T1 that gives access to photochemistry occurring from state T1. In addition, TP excitation can cause photochemical reactions (isomerization, cyclization, bond cleavage, and interand intramolecular charge transfer). These reactions occur mostly from the S1 state as described for OP excitation experiments [57, 238–241]. In other words, the photochemistry occurs from the S1 state as long as this state is populated either directly or indirectly from a higher excited singlet state. TP excitation of higher excited states requires electronic coupling with the photoactive S1 state to get access for photochemical processes. If no electronic coupling occurs, the higher TP excited state deactivates by the release of two photons. Figure 3.1a describes TPA in a simple form. It is not general because the detailed description of TPA requires the inclusion of many states. The scheme in Figure 3.1a uses one virtual state and one excitation frequency (o1 ¼ o2 ), often called the one-color experiment that is needed for excitation with the first photon while the second photon simultaneously excites the chromophore to its final excited state. Simultaneous absorption of two photons is scaled by the lifetime of the virtual state and is on the order of electronic excitation (1014–1015 s). More theoretical detail regarding the function of virtual states in multiphoton spectroscopy can be found elsewhere [278]. The amplitudes of each transition are scaled by the absorption cross section from the ground state to the virtual state (sgi ) and the absorption cross section from the virtual state to the excited state (sif ). These quantities have the unit cm2. Both sgi and sif follow, qualitatively, the rules known for OP excitation; that is, parity must change in order to have an allowed transition. The Gaussian curve represents the lifetime distribution of the virtual state. TP excitation with two distinct excitation frequencies is depicted in Figure 3.1b. This is called the two-color experiment with the condition o1 6¼ o2 . There is no distinction whether the chromophore is excited with a photon of higher (upper graph in Fig. 3.1b) or lower excitation frequency (lower diagram in Fig. 3.1b).

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

(a) ω

excited state 2nd photon (σif)

(b) ω

τ ≈ 1-10 ns

1st photon (σgi) ω ground state

(c) τ ≈ 1-10 ns

τ << 1 ns

ω excited state

ω2

ω2

excited state

virtual state

virtual state

117

τ ≈ 1 -10 fs

ω1

2nd photon (σ12) ω2 τ ≈ 1-10 ns 1st photon (σ01) ω1

1

τ=∞

ω

ω2 virtual state

ω1

Figure 3.1. Simplified mechanistic pattern of simultaneous TPA ((a) and (b)) in comparison with stepwise excitation (c) with two photons (o1 and o2 ¼ excitation frequencies, t ¼ lifetime of the state considered, dashed line ¼ energy distribution of the virtual state according to the Heisenberg relationship, solid line ¼ energy levels of the real states, i.e., the ground state and the excited states, sgi ¼ absorption cross section from ground state to the virtual state, sif ¼ absorption cross section from the virtual state to the final excited state). (a) Simultaneous excitation with one color, o1 ¼ o2. (b) Simultaneous excitation with two colors, o1 6¼ o2. (c) Stepwise excitation with two photons in which the first excited state operates as an intermediate state.

Moreover, excited state absorption (ESA) as shown in Figure 3.1c also requires two photons. However, this mechanism is based on the consecutive absorption of two photons. In this case, the intermediate state is a physical state with no significant lifetime broadening compared to the virtual state depicted in Figure 3.1a,b. The optical transition between the ground state and the intermediate excited state, which is usually S1, is scaled by the one-photon absorption (OPA) cross section s01 known from the Lambert–Beer law. This results first in population of the S1 state with the first photon if the excitation energy matches the optical transition. It is followed by consecutive excitation of the S1 state into a higher excited state scaled by the OPA cross section s12 . The lifetime of the final excited state is smaller in comparison with the S1 state because internal conversion occurs on the picosecond (ps) time scale. The spatial resolution of such consecutive excitation is diminished in comparison to simultaneous TP excitation because light is attenuated according to the Lambert–Beer law through the quantities s01 and s12 . However, we do not focus on the stepwise multiphoton excitation shown in Figure 3.1c, although photochemistry based on this resulted in multiphoton gated photochromic reactions [279]. Instead,

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we focus on simultaneous excitation with two photons because it results in improved spatial resolution using excitation frequencies located either in the red or NIR spectral region where no OPA of the chromophore occurs. TP excitation requires at least one virtual state. The latter is not a physically observable state. The lifetime ti is set to several femtoseconds. Lifetime broadening of the virtual state can be approximated by the Heisenberg relationship, Eq. (1):
E ¼

h 2ti

ð1Þ

where  h ¼ Planck’s constant divided by 2p,
E ¼ energy difference according to line broadening, and ti ¼ lifetime of the virtual state. Furthermore, both absorption cross sections sgi and sif must become significantly larger than zero. Thus, the virtual state functions as an operator that changes parity upon TP excitation, and the lifetime of whom scales the simultaneous arrival of two photons. The product of sgi  sif  ti results in the unit of the TPA cross section d—cm4
s
molecule1
photon1. The size of 1050 cm4
s
molecule1
photon1 is equal to 1 GM. Such quantities were observed in pioneering studies in the 1960s and 1970s for small unsaturated compounds investigated mostly as neat liquids [16, 23, 41]. The unit GM stands for Go¨ ppertMayer. It was introduced in TP photosciences to honor Maria Go¨ ppert-Mayer, who first predicted the event of simultaneous TPA [1]. TPA cross sections >50 GM are considered large [131]. Semiclassical perturbation theory [56, 59] is applicable to describe TPA as a second-order phenomenon. Light can be seen as an electromagnetic wave perturbing the stationary wavefunctions of a molecule. Quantities related to these wavefunctions are the absorption cross sections sgi and sif (Fig. 3.1). Furthermore, high excitation power/photon density requires additional higher order terms according to perturbation theory. The spatial (Cartesian coordinates, r) and time (t) dependent wavefunction cð2Þ g ðr; tÞ is shown for second-order perturbation in Eq. (2) [23]: cð2Þ g ðr; tÞ

¼

cð1Þ g ðr; tÞ

  1  exp½iðofg  2oÞt q2 A20 o2 X 0 þ


þ Sgf cf ðrÞ ofg  2o h f ð2Þ

This relation contains the wavefunction for first-order perturbation cð1Þ g ðr; tÞ, describing the interaction of light with matter at low intensities, the excitation frequency (o), the transition frequency from the ground state to the final excited state (ofg), the amplitude of the electromagnetic wave (A0), the electronic charge

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

119

(q), the coordinate wavefunction1 ðc0f ðrÞÞ, and the transition probability for the TP transition (Sgf). The latter is defined by Eq. (3):  Xhgj~ e rjiihij~ e rjf i ð3Þ Sgf ¼ ogi  o þ  i where hgj ¼ ground state, hij ¼ intermediate/virtual state, h f j ¼ final state, o ¼ excitation frequency, ogi ¼ excitation frequency from the ground state to the intermediate/virtual state, and  ¼ damping factor often set as 0.1 eV as a reasonable approximation. This relation is applicable for excitation with photons of identical frequency (Fig. 3.1a). Thus, the excitation frequency of the virtual state is about one-half e is the complex polarization vector. that of the TP excited state Sn . The term ~ This term is needed to describe the orientation and polarization affecting TP excitation [23, 238]. Equation (3) contains the possibility of a new type of resonance that is not available in the wavefunction cð1Þ g ðr; tÞ related to OP excitation. If an excited state exists with the near resonant condition ogf < 2o, excitation of energy 2 ho appears in the same quantum act. The terms hgj~ e
rjii and hij~ e
rj f i are the transition density from the ground state to the virtual state and of the virtual state to the final excited state. Sgf is a second rank tensor. The square of this quantity is proportional to d depending on the imaginary part of the second-order nonlinear susceptibility Im(w3 ). The rate constant for the two-photon transition ð2Þ kgf (in s1) is calculated by Eq. (4), in which A20 can easily be substituted by the ð2Þ photon flux2 (F)[16, 23]. This yields an expression for kgf showing the square ð2Þ dependence of kgf on the incident photon flux. The fine structure constant af is dimensionless with af ¼ 1=137. In other words, Eq. (4) demonstrates dependence of excited state evolution on the square of photon flux in the case of TP excitation. In general, this relationship shows the possibility of excited state population by simultaneous absorption of two photons if Sgf is greater than zero. The remaining parameters are either a physical constant (af) or given by the excitation equipment (o, F). The strength of simultaneous TP excitation (Sgf ) depends on molecular parameters. These are the transition density between the ground state and virtual state, the transition density between the virtual and final excited state, and the excited state energy. These parameters can be optimized by change of the TP chromophore structure [23].   qA0 o 4 ð2Þ jSgf ðoÞj2 gM ð2oÞ ¼ 128p3 a2f o2 f 2 jSgf ðoÞj2 gM ð2oÞ ð4Þ kgf ¼ 8p h  The coordinate wavefunction c0f ðrÞ is part of the wavefunction cg ðr; tÞ ¼ c0f ðrÞeEt=h (where E ¼ energy difference between the ground state and the excited state). 2 This treatment is justified because A20 / F=o2 (F in cm2
s1). 1

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

A measurable quantity for TPA is the TPA cross section d, Eq. (5), representing the absorptivity of two identical photons: ð2Þ

d¼

kgf

F2

¼ 128p3 a2f o2 jSgf ðoÞj2 gM ð2oÞ

ð5Þ

It is therefore a molecular parameter for the absorption strength when two photons are absorbed in the same quantum act by one molecule (Fig. 3.1a). d has the unit cm4
s
molecule1
photon1. The term gM(2o) stands for spectral line profile of the material. Furthermore, Eq. (6) describes the number of molecules promoted into the excited state by simultaneous absorption of two photons, ð2Þ showing the quadratic dependence on the incident photon flux; Nph is the number of photons involved in the TPA process, and Nch stands for the number of chromophores. More details are in the original reference [23]. ð2Þ

dNph dt

ð2Þ

¼ Nch kgf ¼ Nch dF 2

ð6Þ

The TPA cross section (d) is proportional to the second-order nonlinear polarizability g. This is derived by consideration of interactions of light with matter causing a change in the dipole moment that is the induced dipole moment ~ mind . mind and the field strength ~ E of the Induced polarization ~ Pind is proportional to ~ incident light, Eq. (7) [50, 73]: ~ E ¼ jN ch~ rj Pind ¼ wð1Þ ~ mind j ¼ jN ch e~

ð7Þ

The vector ~ r describes the overall changes of the chromophore’s Cartesian coordinates upon interaction with light. The macroscopic constant wð1Þ is the linear susceptibility, related to the dielectric constant e according to Eq. (8): e ¼ 1 þ 4pwð1Þ

ð8Þ

The optical response of the material is defined by the complex refractive index nc(o), Eq. (9): n2c ðoÞ ¼ eðoÞ ¼ 1 þ 4pwð1Þ ðoÞ

ð9Þ

The complex refractive index is a sum of the real and imaginary part (nc ¼ n þ ik), where n is the real part corresponding to dispersion, and k is the imaginary part corresponding to an absorption. A high field intensity can be achieved by high laser intensity. Hence, ~ Pind is no longer proportional to the field strength of the incident light as shown in

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

121

Eq. (7). Higher order terms must be added in the form of a Taylor series to E, Eq. (10): disclose the dependence of ~ Pind as a function of the field strength ~ ~ E þ wð2Þ ~ E~ E þ wð3Þ~ E~ E~ E þ


Pind ¼ wð1Þ~

ð10Þ

Under conditions in which the material is isotropic only, the odd nonlinear susceptibilities contribute to TPA. The even terms vanish because orientation is required. Therefore, the lowest nonlinear absorption is described by the imaginary part of w(3), corresponding to Stokes–Raman, anti-Stokes–Raman scattering, and TPA [16, 22, 23, 35, 45, 50, 53, 70, 73, 280, 281]. Equation (11) expresses the energy exchange rate for monochromatic waves: dW 1 ¼ o Imð~ E~ PÞ dt 2

ð11Þ

For TP excitation conditions where wð1Þ  0 (almost no linear absorption), E~ E~ E if the medium is isotropic. Insertion of Eq. (10) simplifies to ~ Pind  wð3Þ ~ 3 this relationship into Eq. (11) results in Eq. (12) for simultaneous absorption of two photons with the same frequency (o1 ¼ o2 ) [50, 73]: dW 8p2 o ð2Þ ¼ kgf ¼ 2 2 I 2 Imðw3 Þ dt nc

ð12Þ

where n ¼ refractive index, c ¼ velocity of light in vacuum, ~ E ¼ electric field vector, dW=dt ¼ energy exchange per time unit, and o ¼ excitation frequency. The intensity I is defined in Eq. (13): I¼~ E~ E

nc 8p

ð13Þ

The relation between d and the number of photons absorbed per unit time is introduced in Eq. (6) [50, 73]. With the condition that F ¼ I=ho and dW=dt ¼ hoðNph ¼ number of photons), one obtains by substitution of these ðdNph =dtÞ expressions into Eq. (6) and consecutive rearrangement the relation depicted in Eq. (14),4 describing the dependence of d on the material parameter w(3) [50, 53, 55, 73, 129, 272–274], if both photons have the same frequency: d¼

8p2  ho2 Imðwð3Þ Þ n2 c2 Nch

ðin cm4
s
photon1 Þ

ð14Þ

3 Degenerate TPA (interaction of two photons with identical frequency) follows at E~ E~ E, which is replaced in Eq. (11) resulting in Eq. (12) including Eq. (13). t¼0:~ P ¼ 14 wð3Þ ~ 4 Thus, Eq. (12) can be written dNph =dt ¼ ð8p2 I 2 = hn2 c2 ÞImðw3 Þ and Eq. (6) can be rearranged, with the condition F ¼ I= ho, to dNph =dt ¼ ðdI 2 = h2 o2 ÞNch . Combination of both expressions results in Eq. (14).

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Thus, d in Eq. (14) relates to the photons absorbed. The number of excited molecules per time unit must be corrected by a factor because it follows that 1 dNph ¼ dF 2 Nch 2 dt for one absorbing chromophore. Thus, the TPA cross section related to the excited molecule is described by Eq. (15) and has the unit cm4
s
photon1
molecule1: d¼

4p2 ho2 Imðwð3Þ Þ n2 c2 Nch

ðin cm4
s
photon1
molecule1 Þ

ð15Þ

Although both Eqs. (14) and (15) contain the same quantity, they differ by the factor 0.5 due to different convention. The third-order nonlinear susceptibility w(3) is the product of the second hyperpolarizability g, the number of chromophores per cm3 (Nch), and the Lorentz field correction term L to the power of 4 (L ¼ ðn2 þ 2Þ=3Þ, Eq. (16) [132, 274]: Imðwð3Þ Þ ¼ L4 Nch ImðhgiÞ

ð16Þ

Thus, one obtains Eq. (17) for d by substitution of Eq. (16) into Eq. (15): d¼

4p2  ho2 4 L ImðhgiÞ n2 c 2

ð17Þ

Because hgi represents a microscopic quantity, Eq. (17) is the interface between experiment and theory. There are several approaches to calculate g [129, 282, 283]. Representative quantities affecting this are excitation energies from the ground state to the S1 and higher excited states, transition dipole moments M, and the change for the state dipole moment between the first excited singlet state and the ground state
m01 . These data are available from quantum chemical calculations. One can therefore predict TPA by theoretical methods, which enables the organic chemist to get an idea of the molecular engineering for the design of chromophores with large TPA. 2. Theoretical Methods for Description of Two-Photon Absorption The sum over states (SOS) method [56] is one of the most applied theoretical frameworks for calculation of Im½gðo; o; o; oÞ [129]. Equation (18) discloses the dependence of Im½gðo; o; o; oÞ as a function of excitation energies (E),

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123

transition dipole moment (M), and dipole moment changes (
m) using one virtual state. Im½gðo; o; o; oÞ is calculated with Eq. (18)5 [233, 283–285]. 2

½M01
m01 2

3

6 7 6 ½E01   ho  i01 2 ½E01  2ho  i01  7 6 7 6 X 7 ½M01 M1i 2 6 7 Im½gðo; o; o; oÞ ¼ P6 þ 7 6 i¼2 ½E01  ho  i01 2 ½E01  2ho  i0i  7 6 7 6 7 2 4 5 M01  ½E01   ho  i01 2 ½E01  2ho  i01  ð18Þ Substitution of hgi into Eq. (17) results in an expression that can be applied to calculate d based on molecular parameters. P corresponds to a permutation operator over the optical frequencies, M01 corresponds to the transition dipole moment from the S0 state to the first excited singlet state S1, M1i (i  2) is the transition dipole moment between higher excited state S1i and S1,
m01 stands for changes of the state dipole moment between states S0 and S1, and E01 is equal to the excitation energy from state S0 to S1. The parameter ho is equal to the excitation energy. The terms 01 and 0i are damping factors. In calculations, they were often set to 0.1 eV as a reasonable approximation [283, 286]. The energy of the ho. The first part in Eq. 18 tunes TP excited state E0i is equal to 2 Im½gðo; o; o; oÞ of dipolar compounds (dipolar term). Thus, chromophores with large M01 and
m01 become attractive candidates as chromophores with large TPA. Thus, intensive fluorescence caused by the large M01 and solvatochromism as a result of large
m01 may become an experimental hint for a large TPA cross section. The second term describes electronic coupling between higher states and state S1. Because even higher excited states can be involved in the interaction of light with matter, it must be considered as a sum of all excited states contributing to Im½gðo; o; o; oÞ. The third part is a negative term containing OP resonances. The general scheme in Fig. 3.2 exhibits two general excitation mechanisms depending on the substitution pattern. The TP excited state can be either a higher excited singlet state (Fig. 3.2a) or the lowest excited singlet state (S1, Fig. 3.2b). The distinct energetic position of the TP excited state can have a significant impact on the emission behavior of the chromophore. Because the S1 state is 5

Comparison of calculated d with those measured in solution requires evaluation of orientational averaged (isotropic) values of g. P This quantity P is defined as the orientational average 1 of the polarizability tensor, hgi ¼ 15 ½3 i ðgiiii Þ þ i6¼j ðgiijj þ gijij þ gijji Þ, in which indices i and j refer to the spatial directions x, y, and z [236].

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Energy

(a)

(b)

symmetric

nonsymmetric

quadrupolar

dipolar

S2 (two-photon excited state)

M12

S1 (one-and two-photon excited state)

electronic coupling

S1 (one-photon excited state) virtual state

M01

S0

virtual state

M01

S0

Figure 3.2. Mechanistic description of TP absorption occurring from the ground state into a higher excited state (a) and to the lowest excited state (b). Mechanism (a) is representative for centrosymmetric compounds, while mechanism (b) is often discussed for dipolar chromophores with donor (D)–acceptor (A) pattern.

indirectly6 populated by TPA in Figure 3.2a, deactivation of this state by fluorescence (OP event) can occur with high emission efficiency if M01 is large. Strong electronic coupling between the higher TP excited state and the OP excited state (S1) finally results in efficient population of the S1 state.7 In centrosymmetric compounds, the coupling can be understood as an excitation density flow from the terminal groups toward the center of the p-chain or vice versa. Under these conditions, the second term mainly tunes TPA of compounds with centrosymmetry because
m01 vanishes. Consequently, it follows that d / ½M01 M12 =ðE01  E02 =2Þ2 if the two-photon excited state is S2 for the case that E02  2hoðE02 corresponds to the excitation energy of the TP excited state). However, the situation changes if the chromophore bears a dipolar substitution pattern and the TP excited state is 6 Indirect population means that first the energetically higher TP allowed excited state is populated and electronic coupling results in final population of the OP excited state (S1). 7 If photochemistry is desired, fluorescence of the chromophore is not a prerequisite. If the S1 state is populated, it shows the well-known deactivation processes discussed in photochemistry textbooks.

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almost S1 (mechanism depicted in Fig. 3.2b). Therefore, M12 becomes negligible and mainly the first term of Eq. (18) is important to tune d. TPA is a resonance phenomenon. The selectively probed excited state dominates the response and d is proportional to ½M01
m01 =E01 2 [131, 132, 233, 283–285]. The necessary parameters for the calculation of Im½gðo; o; o; oÞ can be obtained from quantum chemical calculations. Ground state geometry was often calculated using either the semiempirical AM1 [131, 132, 233, 283–285, 287–290] approach or the density functional method [224, 291, 292]. The most applied MO method for excited state calculation has been the semiempirical package ZINDO/S [293] using an appropriate multireference double excitation configuration interaction (MRDCI) scheme [131, 132, 224, 233, 283–285, 287, 288, 290, 294]. This is necessary for a correct energetic order of the odd (Bu) and even (Ag) states of centrosymmetric compounds [131, 132, 233, 283–285]. ZINDO/ S-MRDCI calculations provide the necessary molecular parameters
m01 ; M01 ; M02 ; E01 , and E02 to calculate Im½gðo; o; o; oÞ. Reasonable correlation between measured and calculated TPA was observed for a series of chromophores with medium size [123, 131, 132, 224, 227, 233, 284, 285, 289, 295–300]. However, problems can arise for larger molecules requiring larger CI space. Under these circumstances, the calculated quantities
m01 ; M01 ; M02 ; E01 , and E02 may not reliably describe TPA according to Eq. (18). Despite these problems, semiempirical calculations of Im½gðo; o; o; oÞ have been the most applied method for a theoretical design of chromophores with large TP absorptivity [131, 132, 233, 283–285, 287–289, 301]. Hence, ZINDO/S-MRDCI has become an attractive theoretical tool for the calculation of TPA due to the modest calculation time. An additional theoretical framework is the response theory [179, 180, 232, 235, 286, 302–311]. This is a sophisticated method based at the ab initio level for a theoretical description of d, despite the fact that this method needs extensive computation time. Truncations needed in the SOS expressions for molecular properties can be avoided using this theoretical framework. This is important in many calculations of nonlinear optical properties if convergence is insufficient. Response theory screens molecular charge distribution induced by polarization in dielectric media. This gives rise to an extra polarization potential, which is included in the quantum mechanical equations. The basis set contains diffuse functions to describe reliable NLO properties. The TP matrix elements describing an instantaneous, resonant absorption of two photons with identical energy have been computed through a residue of the quadratic response function [303]. The corresponding sum over states expression for the TP matrix elements is given by Eq. (19):

Sab ¼

X h0j~ ma jiihij~ mb jf i i

oi  of =2 þ 

þ

h0j~ mb jiihij~ ma jf i oi  of =2 þ 

 ð19Þ

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

The index (a, b) relates to the distinct energy of each photon, respectively; that is, a ¼ b when both photons have the same excitation frequency;  ¼ damping factor. The parameter of denotes the excitation energies of the final excited states j f i, the summation includes the ground state j0i, and jii corresponds to the virtual state. Definition of the TP matrix element yields the theoretical TPA cross section dtp if a linearly polarized radiation source is applied [16, 23], Eq. (20): dtp ¼

X

ð2Saa Sbb þ 4Sab Sab Þ

ð20Þ

ab

Summation is performed over the molecular axes a; b ¼ fx; y; zg. Furthermore, Eq. (21) enables direct comparison with experiment: d¼

4p3 a50 af o2 gðoÞ dtp 15c f

ð21Þ

g(o) provides the spectral line profile and f is the lifetime broadening of the final state, which can be assumed to be about 0.1 eV. The same value of f is also employed in the determination of the TP cross section in earlier studies [131, 283], which gives a connection between theory and experiment. The unit of d is 1050 cm4
s
molecule1
photon1 provided that cgs (cgs: centimeter–gram–second unit system) units are used for a0 and c, and atomic units are taken for dtp, o, and f. More details can be found in the original literature [232, 235, 286, 304–311]. Time-dependent density functional theory (TDDFT) [291, 292] is an additional theoretical framework applied to calculate OPA and TPA spectra in a series of particularly large donor-acceptor-donor substituted conjugated molecules [236]. Calculated excitation energies corresponding to OPA and TPA maxima were found to be in excellent agreement with experiment focusing on evaluation of frequency-dependent polarizabilties. TDDFT was additionally applied using response theory, emphasizing properties rather than states and energies [312]. In addition, density analysis of ImðgÞ considers spatial contributions of p-electrons to Im(hgi), which according to Eq. (17) is related to TPA [313]. B3LYP geometries, obtained as a result of density functional theory (DFT) calculations, were found to underestimate calculated transition energies compared to experiment in both OPA and TPA spectra (see Fig. 3.3) [236]. In particular, the description of bond length alternation parameters in B3LYPoptimized geometries is believed to be the main factor resulting in overestimated electronic delocalization. Subsequently, bathochromic shifts were obtained for the calculated excited state energies. In contrast, calculations based on

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127

Figure 3.3. Comparison of calculated and experimental excitation energies corresponding to OPA (top) and TPA (bottom) of molecules experimentally investigated [131, 132, 287]. Calculations were done at the TDDFT/B3LYP level using three different optimized geometry sets (B3LYP, HF/planar, and HF/nonplanar). From Ref. [236] with permission of the American Chemical Society.

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Hartree–Fock (HF) geometries agree with crystallographic data. As a result, excited state energies were consistent with experiment. However, reasonable correlation between calculated and experimental data was found for a series of styryl compounds, in which the ground state geometry was optimized by DFT, while excited states were calculated by ZINDO/S-MRDCI using the geometry obtained by DFT calculations [224]. This shows that as of now no universal method is available for accurate description of TPA. There is a great need for theoretical tools to predict reliable TPA according to a given structure. The multivalence-bond state approach was used to perform molecular engineering of dipolar as well as quadrupolar TP chromophores [314]. This is a semiclassical VB-CT model based on a general donor–acceptor pattern [315–318]. This framework gives more insight into the structural evolution of VB-CT energies and therefore on relationships between hyperpolarizabilities and molecular structure. The VB-CT model is based on measurable parameters. Analytical expressions of the resonant TP absorption cross sections are derived for distinct molecular families of chromophores. Thus, the multivalence-bond state model may provide useful guidelines for the design of dipolar and quadrupolar chromophores with enhanced TPA cross sections. 3. Symmetry Considerations Evaluation of transition densities gives a clearer pattern for the amplitudes of OP and TP optical transitions. The transition dipole moment Mnm between the ground state cn (r) and the excited state cm ðrÞ is given by the matrix element of the dipole moment operator ~ m (light wave), Eq. (22): ð mjmi ¼ cn ðrÞ~ mcm ðrÞ d3 r ð22Þ Mnm ¼ hnj~ This equation is well known from OP photochemistry and scales the amplitude of optical transitions [57, 238, 239, 241]. Consequently, Mnm is different from zero if the integrand displays a fully symmetric pattern with respect to all existm cn ðrÞ must be a symmetric function. ing symmetry elements; that is, cm ðrÞ ~ The dipole moment operator ~ m functions as antisymmetry operator. Therefore, the product cm ðrÞ  cn ðrÞ must contain at least one antisymmetric element if the transition is allowed. Therefore, OP transitions are allowed if parity changes from g!u according to Eq. (22).8 However, allowed TP transitions require no change of parity; that the g!g transition becomes allowed. Hence, ˚ strong TP bands are observable with two transition moments greater than 1.5 A under resonance lineshape conditions. Thus, TP spectroscopy is a complemen8

Parity u stands for odd (‘‘ungerade’’) and g for even (‘‘gerade’’). Furthermore, g means symmetric with respect to the inversion center and u is understood as antisymmetric with respect to the inversion center. While a g!u transition is antisymmetric, symmetry of the product cm ðrÞ  cn ðrÞ does not change for the g!g transition.

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TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

One-Photon Excitation ψ∗(r)

ψ(r)

+ - + + + - -

+ + ψ(r)

ψ∗(r)

+ + -

+ +

+ +

+ +

-

µ

Two-Photon Excitation ψ∗(r)

ψ(r)

+ + - + + - -

+ + + + ψ(r) ψ∗(r)

+ + µ

+ + -

+ +

+ +

µ

Figure 3.4. Schematic sketch for necessary changes of parity in order to accomplish strong OP and TP optical transitions.

tary tool to localize electronic transitions, which are not observable by OP excitation (Mnm ¼ 0). These relations are easier to understand by considering the wavefunction of the molecular orbitals needed to describe the optical transition. The simple sketch in Figure 3.4 for ethylene shows the necessary symmetry patterns needed for the integrand to achieve a fully symmetric figure for OP and TP excitation. The wavefunction cn ðrÞ represents the highest occupied molecular orbital (HOMO), while cm ðrÞ represents an unoccupied molecular orbital. cm ðrÞ cn ðrÞ results in a distinct pattern if the symmetry of cm ðrÞ  cn ðrÞ is either different (OP excitation) or equal (TP excitation). Consequently, cm ðrÞ  cn ðrÞ displays a symmetry with one nodal plane when both involved wavefunctions possess different symmetry (upper part of Fig. 3.4). However, a fully symmetric product for cm ðrÞ  cn ðrÞ is obtained if both cm ðrÞ and cn ðrÞ have the same m results in a fully symmetric funcsymmetry. Treatment of cm ðrÞ  cn ðrÞ with ~ tion for OP excitation; that is, such an optical transition becomes allowed. Full symmetry of cm ðrÞ  cn ðrÞ results in an asymmetric function after treating with the first photon (~ m) for the example depicted in the bottom part of Figure 3.4, but it remains fully symmetric after interacting with a second photon. This is a representative example of an allowed TP transition. Although Figure 3.4 is only a simple sketch, it shows how symmetry affects the amplitude of an optical transition as indicated by the parameter Mmn in Eq. (22). This can be extended to higher conjugated systems, resulting in the same results as shown for OP and TP excitation in the bottom part of Figure 3.4 [238]. However, a substituent and additional vibrational modes can

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

destroy the derived symmetry relations [16, 23, 43, 45, 58, 238, 284]. Under these conditions, the amplitude of TPA is no longer controlled by symmetry. 4. Bond Length Alternation Bond length alternation (BLA) has been discussed as one of the main factors describing the polarization of the p-system in polyenes [318–326]. Bond length alternation is the difference between the average length of all double bonds and the average length of all single bonds according to Eq. (23): P  P  d dCC  C  n C  ð23Þ  n BLA ¼  nS   nD where dC¼C ¼ bond length of the double bond, dCC ¼ bond length of the single bond, nD ¼ number of double bonds, and nS ¼ number of single bonds. This is the maximum number that can be reached if an external field (light) is applied. For BLA, the nature of the chemical bond switches from a single bond to a double bond and vice versa. For BLA  0, the chromophore possesses bond lengths comparable to cyanines because all bond lengths are approximately equal; compare Figure 3.5. A change in the nature of the bond is schematically drawn for a polyene chain in Figure 3.5, showing the shift of p-electron density resulting in a change of bond order if an external field is applied. Starting from a polyene, the p-system goes through a structure in which bond lengths are similar to those for cyanines [327] and partial charges are positioned at the ends. The final zwitterionic structure is drawn in Figure 3.5c. These bond order changes are accompanied by dipole moment changes in the presence of an external field. Bond length alternation affects the second hyperpolarizability g, because molecular parameters such as
m01 , excitation energies, and transition dipole E

(a)

δ

+

δ

−

(b)

(c)

Figure 3.5. Schematic sketch of bond length alternation starting from a polyene structure (a), passing a pattern with similar bond lengths and partial charges at the ends (b), to the final charge separated form (c) with a zwitterionic pattern.

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

131

moments are part of this microscopic moiety according to Eq. (18). Electric field-dependent calculations indicate significant change of g accompanied by changes in the geometry of the molecules. In particular, g can possess a maximum at a specific geometry if BLA 6¼ 0. This is shown in Figure 3.6, which

Figure 3.6. Plot of g versus bond length alternation (a) and contribution of each  CH)4—CHO. Some individual term of Eq. (18) to g (b) for the structure (CH3)2N—(CH  authors prefer to use the term bond order alternation (BOA) that is somehow related with bond length alternation; that is, change of bond length results in change of bond order. The triangles represent dipolar contributions (first term in Eq. (18)); diamonds are for the coupling term (second part of Eq. (18)); and the squares are related to the negative term (third part in Eq. (18)). (From Ref. [318] with permission of the American Chemical Society.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.7. Definition of regions from A to E of bond length alternation with the percentage contribution of neutral (structure I) and charge separated (structure II) resonance structures to the ground state. This pattern demonstrates the evolution of the charge separated structure from the neutral from passing through polar structure with almost equal bond lengths (region C). (From Ref. [319] with permission of the American Association for the Advancement of Science.)

displays the changes of g as a function of bond length alternation. When all bond lengths are equal (BLA ¼ 0), g possesses a minimum. Figure 3.6b shows the individual contribution of the terms contained in Eq. (18). The first term (triangles) is a dipolar contribution exhibiting a minimum at BLA  0, while two maxima are observable if BLA 6¼ 0. The large dipolar contribution is caused (CH CHO used by the dipolar structure of the chromophore (CH3)2N CH)4 for these theoretical studies. The coupling term (diamonds), which is the second part in Eq. (18), peaks at BLA  0. It contributes positively to g, compensating a little for the negative contribution to g (squares) of the third term in Eq. (18). Bond length alternation occurs through different molecular regions. Mixing of the zero-order neutral form (left structure in Figure 3.7) and charge separated form (right pattern for the zwitterion) results in structures controlling electronic properties of the chromophore. BLA furthermore depends on the difference between the energy of the zero-order states for the neutral form and charge separated form (
U 0 ). This parameter depends on the strength of donor and acceptor substituents, the difference of p-electronic energy of the two forms, and the magnitude of the dipolar stabilization energy. Though BLA explains the relationship between donor and acceptor strength of substituents and the nature of the p-system on the nonlinear hyperpolarizability g, it must be considered as a model within its limits.

B. Mechanistic Consideration 1. Two-Level Model A two-level model can appropriately describe TPA under resonant conditions if the molecule bears a dipolar substitution pattern. A repre-

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

133

sentative mechanism is shown in Figure 3.2b. Direct population of state S1 occurs by simultaneous absorption of two photons. No coupling with higher excited states is necessary. Therefore, TPA of dipolar chromophores is favored by the first term of Eq. (18). Substitution of this relation into Eq. (17) results in Eq. (24)9 if electronic coupling (second term of Eq. (18)) is of minor importance. 2 2 4p2 E01 4 1 ½M01
m01  L d/ h n2 c 2  5 ½E01 =22 01

! ð24Þ

Thus, d becomes proportional to ½M01
m01 2 for a dipolar compound. Both, M01 and
m01 can be evaluated by OP experiments. Change of state dipole moment
m01 can be quantified from solvatochromic measurements using the Lippert–Mataga treatment, Eqs. (25a) and (25b) [59, 328–335].    2me me  mg e  1 1 n2  1 nflu ¼  
þ const: 2e þ 1 2 2n2 þ 1 4pe0 r3   2ðme  mg Þ2 e  1 n2  1 þ const:  ðnabs  nflu Þ ¼ 4pe0 r3 2e þ 1 2n2  1

ð25aÞ ð25bÞ

where nabs ¼ maximum for absorption, nflu ¼ maximum for fluorescence, mg ¼ dipole moment of the ground state, me ¼ dipole moment of the excited state, r ¼ Onsager radius, e0 ¼ dielectric constant in vacuum, e ¼ dielectric constant, and n ¼ refractive index. Estimation of
m01 from the solvatochromic slope is roughly possible applying the Onsager model [59, 334–336]. Large changes of
m01 can be accomplished by introduction of appropriate donor and acceptor groups at the p-skeleton of the chromophore. Extension of the p-system results additionally in an increase of
m01 . A summary of solvatochromic data is compiled for dipolar compounds elsewhere [334, 335, 337]. The second parameter M01 also scales d of dipolar compounds and can be evaluated from the absorption spectrum using the Strickler–Berg treatment [338]. Reorganization of this relation results in Eq. (26) [338–340], showing the inverse proportionality between ðM01 Þ2 and excitation energy E01, but a direct relation exists between ðM01 Þ2 and the integral over the absorption band. 2 M01

9

ð 9:19  103 eðnÞ dn ¼ n E01

This relationship holds for the case E01  2 ho.

ð26Þ

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

TABLE 3.1 Excitationa

Compilation of Photophysical Data for 1–3 Obtained by OP and TP

f 1 0.022 2 0.025 3 0.026

iso

tf lOPE max (ps) (nm)

0.50 60 0.31 80 0.11 100

370 396 412

 e(M

1

1

cm )

32500 45100 52100

2me ðme mg Þ 4pe0 hca3 1

(cm )

11800 12600 15900


m01 (D) me(D) 7.4 8.4 10.0

13.6 15.4 17.7

lTPE max (nm) 750 825 850

d(GM) 120 300 500

f ¼ fluorescence quantum yield, iso ¼ quantum yield for trans–cis isomerization, lOPE max ¼ maximum for OPA in toluene, e ¼ molar extinction coefficient in toluene, tf ¼ fluorescence decay time in toluene, 2me ðme  mg Þ=4pe0 hca3 ¼ solvatochromic slope of Eq. (25a),
m01 ¼ difference between the dipole of the excited state and the ground state, me ¼ dipole moment of the excited state, lTPE max ¼ maximum for TP excitation, d ¼ TPA cross section. a

where eðnÞ ¼ molar extinction coefficient in M1cm1 at the wavenumber n (in cm1), E01 ¼ excitation energy in cm1, and n ¼ refractive index. Thus, chromophores with large extinction coefficient are favored materials for TPA if the dipolar substitution pattern results in a large
m01 as well. Because the amplitude of both transitions is tuned by the same parameters, that is, M01 and
m01 , dipolar chromophores can have the same excitation energy for both OP and TP excitation. Influence in extension of chromophore conjugation on TPA was studied for the fluorinated polyenes 1–3. Pentafluorosubstitution reduces p-electron density at the left aromatic benzene ring. This results in an electron-deficient substituted benzene with a p-electron density comparable with that of cyanobenzene [341]. This was concluded from the reduction potentials of 1,2,4,5-tetrafluorobenzene 1=2 1=2 (Ered ¼ 2:4 V vs. SCE [342]) and cyanobenzene (Ered ¼ 2:3 V vs. SCE [341]). The larger p-conjugation in 3 causes an increase of e compared to 1 but the change of
m01 was rather modest compared to the increased size of the chromophore [224]. TPA data and representative photophysical parameters are summarized in Table 3.1. Table 3.1 shows the dependence between d and OPA data. The larger e and
m01 , the larger the TPA cross section, as shown by Eq. (24). Increase of both TPE lOPE max and lmax is caused by the gradual increase of the p-system by stepwise extension of the polyene pattern. Chromophores 1–3 are not strong emitters. They exhibit, on the other side, reasonable quantum yields for photoisomerization. Thus, these chromophores are good TP chromophores undergoing reactions through p-bond isomerization. Furthermore, normalization of the excitation energy yields the spectra of 1 shown in Figure 3.8. This picture shows that both OP and TP excitations result in spectra exhibiting maxima at approximately the same position. Similar relations were found for 2 and 3 as well. Energy differences between OP and TP

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TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

F

F

F

F

F

F

N

N F

F F

F 1

2

N F

F

F

F

F

3

excitations are 0.04, 0.1, and 0.08 eV for 1–3, respectively, showing that the S1 is accessible by both OP and TP excitations. Thus, the same excited state is directly populated by either OP or TP excitation and excludes mechanisms proposing the existence of two distinct excited states with similar excitation energy. The fluorescence decay time and solvatochromic behaviors were almost the same with either OP or TP excitation. Figure 3.2b appropriately discloses the TPA of 1–3.

30 × 103

ε (M–1 cm–1)

25

100 80

20 60

15 10

40

5

20

0

0 360

380

400

420 440 λ (nm)

460

480

δ (10–50 cm4 s photon–1)

One-Photon Excitation (ε) Two-Photon Excitation (δ)

500

Figure 3.8. Comparison of OP and TP excitation spectra for the dipolar compound 1; solvent is toluene. (Adapted from Ref. [224].)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

2. Three-Level Model A three-level model appropriately describes twophoton absorption of centrosymmetric chromophores in which a higher excited state is directly populated by TP excitation (Figure 3.2a). Thus, the TP excited state is a higher excited state, which is populated under resonant conditions as long as Eq. (27)10 fulfills the necessary conditions.  2 ! 2 M01 M12 4p2 E02 4 4 L d/ h n2 c2 5 ½E01  E02 =22 01

ð27Þ

This expression is derived from Eq. (18) for the case when the second term dominates control of electronic coupling between two excited states. The first term vanishes for centrosymmetric chromophores in Eq. (18) because
m01  0. Substitution of Eq. (18) into Eq. (17) results in Eq. (27) for centrosymmetric compounds. This relation shows the proportionality of d to the square of M01
M12 and the excitation energy of the TP excited state E02 , respectively. The TP excited state is in most cases S2. A further important term in Eq. (27) is the detuning energy E01  E02 =2 resulting in an increase of d if E01  E02 =2 approaches a minimum. The coupling term M12 , described by Eq. (28), was derived from Eq. (27) [132]. M12 ¼

nc ½E01  E02 =2 pffiffiffiffiffiffiffiffiffiffiffiffi 5 h d 4p E01 L2 M01

ð28Þ

where n ¼ refractive index, c ¼ speed of light, E01 ¼ excitation energy for state S1, E02 ¼ excitation energy for the TP excited state, and  ¼ overall bandwidth for the TP transition (  0:1 eVÞ: M01pisffiffiffi explained through Eq. (26) [132] and M12 must therefore be proportional to d. ZINDO/S-MRDCI calculations of the quadrupolar chromophore 4 demonstrate electronic interaction of states S1 and S2. Plotting the difference for the partial charges between states S1 and S2 results in the multipolar structure shown in Figure 3.9 [224]. Such quadrupolar patterns are believed to tune the strength of the S2 !S1 transition in centrosymmetric chromophores bearing electrondonating and electron-withdrawing groups. The electronic interaction of S2 and S1 is tunable through the substituents. These patterns are related to MO considerations describing the S1 !S2 optical transition of 4. The evaluation of the transition density is more appropriate [284]. Detailed theoretical studies regarding the enhancement of d and the coupling of excited states were performed on the stilbene derivatives 5–8 [284, 290]. 10

This relationship holds for the case E02  2 ho.

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

F

137

F N

N F F

4

F F

N N F

F

Figure 3.9. Change of the partial charges between states S1 and S2 for the centrosymmetric compound 4. (Adapted from Ref. [224].)

A nearly tenfold increase of d in 7 relative to 5 was reported as a primary result of the larger transition dipoles. In particular, M12 is about twice as large in going from 5 to 7. This was confirmed by experimental studies [132]. These effects become even stronger when adding donor and acceptor substituents to the stilbene moiety; compare structure 8. NC

CN 5

6 NC N

N N

N

CN 7

8

The corresponding transition densities for the S0 !S1 and S1 !S2 transitions are plotted in Figure 3.10 [284]. The relatively high transition dipole for the S0 !S1 transition in stilbene can be understood as a complete distribution of the transition density over the entire conjugated path. There are large transition densities for atoms far away from the center of inversion. Most of the transition densities in the right part of the molecule exhibit the same signs while those in the left half display opposite signs. Contributions to M01 are additive according to the pattern of 5 in Figure 3.10. However, contributions from the two inner p-carbon atoms (vinylene C-atoms and 1,10 carbon of the adjacent phenyl rings) are subtractive. Furthermore, the transition density for the S1 !S2 transition possesses the largest contribution only near the vinylene unit; that is, the corresponding coordinate vector is small. For donor and acceptor substitution as

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Figure 3.10. INDO/MRDCI calculated transition densities for 5–8 between the S0 and S1 on the left-hand side and between the S1 and S2 on the right-hand side. The radii of the circles are proportional to the zero differential overlap (ZDO) transition densities associated with each atom. (From Ref. [284] with permission of the American Institute of Physics.)

shown in 8, the subtractive contributions of the inner p-carbon atoms decrease for the S0 !S1 transition and for the vinylene carbons for the S1 !S2 transition. Moreover, one observes a shift of transition density to the nitrogen atoms of the dimethylamino groups in both 7 and 8 because of the larger coordinate vector. The transition densities at these outer atoms are mainly responsible for the large increase of M12 in 7 and 8. Since the increased electronic coupling between excited states is the main origin of the enhanced TPA response when going from 5 to 8, enhanced d values are expected in compounds bearing stronger donor and acceptor groups. This is experimentally supported by data discussed in Sections IIIB and IIIC. Justification for the use of a three-state model was obtained by comparison of OPA and TPA data from a series of centrosymmetric compounds [131, 132, 224, 287]. The TPA of 9 is significantly blue shifted compared to OPA (Figure 3.11) [224]. Strong electronic coupling between states S2 and S1 as shown by ZINDO/ S-MRDCI calculations explains the strong OP fluorescence generated by TP excitation. Although the OP excited state cannot be directly populated by simultaneous absorption of two photons, indirect population according to the mechan-

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

One-Photon Excitation (ε) Two-Photon Excitation (δ)

1000 800

40

600 20

400

δ (10–50 cm4 s photon–1)

ε (M–1 cm–1)

1400 1200

60 × 103

139

200 0 350

400

450 λ (nm)

500

550

Figure 3.11. Comparison of OP and TP excitation spectra for the centrosymmetric compound 9. (Adapted from Ref. [224].)

ism depicted in Figure 3.2a is enhanced by electronic coupling between states S2 and S1. These results and additional contributions support a three-state model to describe TPA in centrosymmetric compounds. F

F

C 4 H9 N

C 4 H9 N C 4 H9

C 4 H9

9 F

F

3. Vibrational Contributions Contribution of vibrational modes has been described for TPA [5–9, 11–17, 19, 22, 23, 31, 37, 61, 235, 309, 343–345] and for other nonlinear optical processes [346]. One classical example is the 1 A1g ! 1B2u TP transition of benzene, the so-called green band. This electronic transition is allowed due to a vibronic coupling mechanism [346]. Semiempirical [60, 61] as well as ab initio response theory calculations using the Herzberg– Teller expansion [344] demonstrate the role of vibronic coupling. Such contributions can either enhance an allowed transition or intensify a symmetry-forbidden transition. Rewriting of the Born–Oppenheimer approximation for the probability of TPE, Eq. (19), results in Eq. (29): Sab ¼

X h0j~ ma jv; iihi; vj~ mb jFi; f i v;i

ovi  o þ 

þ

 h0j~ mb jv; iihi; vj~ ma jFi; f i ovi  o þ 

ð29Þ

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The pairs of indices (n, i) and (Fi, f) refer to the intermediate and final vibronic states, respectively. They are presented by products of the electronic and vibrational wavefunctions. Since the optical frequency o is well separated from the intermediate higher vibrational levels that may significantly contribute in the summation, one can rewrite oni ffi on0  on . Therefore, the closer relation shown in Eq. (30) is used for vibrational states belonging to a certain intermediate electronic state. Sab ¼ h0jSeab jf i

ð30Þ

Seab denotes the electronic part of the TP transition moment shown in Eq. (31): $ % b a X M0n Mnfb M0n Mnfa e þ Sab ¼ ð31Þ on  o þ  on  o þ  n Expansion of the electronic part for the TP transition in a Taylor series with respect to the normal coordinates results in Eq. (32): Sab ¼ Seab ðQeq Þhgj f Fi i þ

X @Seab a

@Qeq

h0jQa j f i

ð32Þ

Seab ðQeq Þcorresponds to the pure electronic contribution evaluated at equilibrium geometry, while @Seab =@Qeq h0jQa j f i discloses contributions of higher vibrational modes resulting in an additional enhancement for the transition. The linear coupling model is an additional approach for describing the contribution of higher vibrational levels to the TPA response. In this model, one takes the linear term in an expansion of the final state potential around the equilibrium geometry of the ground state. Physically, the potential surface of the excited state is thus approximated by a shift in the ground state potential surface without any scrambling of the normal coordinates or a change in the harmonic frequencies. Good performance of the linear coupling model was reported in relation to harmonic models for multidimensional Franck–Condon factors. Calculations show the suitability of the linear coupling approximation for large conjugated systems [347]. Summation over all vibrational excited states results in expressions for the overall contributions of the overall TPA response. Summation over all vibrational modes results in an expression that does not depend on the potential surface of the final electronic excited state. Physically, this corresponds to broadband or white-light excitation. More details can be found in the original contributions [235, 343, 344, 347]. 4. Surface Plasmons A surface plasmon is a collective excitation of electrons at the interface between a conductor and an insulator. Significant enhanced TPA

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141

surface

metal

vacuum

electron

e–

image charge

z

Figure 3.12. Simplified sketch of image formation charge if an electron approaches a metal surface [354].

was observed if chromophores were localized at the surface of noble metals. This can be by orders of magnitude higher compared to the isolated chromophore [62, 63, 261, 348–353]. Thus, a significant intensity increase of TP excited fluorescence was additionally observed if a gold tip was illuminated [89]. In general, surface plasmon effects are understandable with image potential states shown by the simplified sketch in Figure 3.12 [354]. Electrons in metals can move, resulting in positive charges. Therefore, the approach of an electron that is, for example, part of a p-system on a metal surface results in reorganization of the metal’s field lines. An electrical field is formed with a pattern of field lines perpendicular to the metal surface. The field in front of the surface becomes identical with the situation in which a metal atom is replaced with a charge of opposite sign at the same distance. This image charge is attracted by the metal surface. The electron cannot travel into the metal if it is caught at the metal surface, resulting in quantum bound states. Typically, the plasmon resonance frequency of nanometer-sized metal particles is in the visible range. As the particle size increases, the resonance broadens and shifts toward longer wavelengths [355, 356]. Nanoparticles with a diameter of about 160 nm are needed to match the plasmon resonance of the Ti:sapphire laser. This relatively large size creates a problem when attempting specific labeling with nanoparticles for multiphoton applications. It can be resolved either by the aggregation of several smaller nanoparticles or by chemical enlargement of small nanoparticles [357, 358]. Recently, much smaller nanoengineered particles with a plasmon resonance at near-infrared frequencies have been introduced.

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These include nanoshells [359] composed of a dielectric core coated by a thin layer of metal nanodisks and nanorings [360]. These unique nanoparticles exhibit plasmon resonance frequencies in the near-infrared at dimensions significantly smaller than 100 nm. By variation of the aspect ratio of these particles, it is possible to tune the plasmon resonance frequency as well as the cross sections for scattering and absorption, respectively. Nonlinear optical responses of molecules near fractal metal clusters are expected to be enhanced by many orders of magnitude. They are proportional to a high-order function of the local field [361]. Strong enhancement of multiphoton absorption potentially exists because simultaneous absorption of n photons scales the intensity (I) to the power of n ðI n Þ. Thus, net multiphoton absorption is proportional to the average value of I n over a given volume. It can therefore approach a value that is orders of magnitude larger. Organic chromophores can be attached on silver nanoparticles according to the reaction scheme depicted in Figure 3.13 [349]. Thus, the reaction product of 10 and the silver nanoparticle can be considered as a supramolecular assembly covered by a self-assembled layer of chromophores on the metal nanoparticle core allowing packing of 2500 chromophores within a sphere of <10 nm diameter (11). The resulting nanoparticles function as ultrabright TP fluorescent nanobeacons, resulting in an effective d that is orders of magnitude larger compared to the single chromophore (d for 10: 165 GM), a f of 0.33, and a decreased sensitivity to photobleaching. The TPA cross section of the supramolecular assembly 11 is about 2.7  105 GM. It is one of the largest measures ever reported [348, 349]. In addition, the enhancement of TPA by gold nanoparticles was used to increase the efficiency of TP initiated polymerization in extremely

chromophore

O

10

O2N

:

NO2 O

165 GM

HS

n

+

Ag

Ag

11 2.7 105 GM

Figure 3.13. Formation of self-assembled dye-coated nanoparticles resulting in a tremendous increase of d. (Adapted from Ref. [349].)

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143

small volumes [362]. Thus, an array of gold tips was applied as a model system in these investigations. As a result, application of the surface plasmon technique can be considered as one additional factor to increase TPA cross section. It demonstrates the feasibility for manufacture of TP absorbing materials that may be applied in combination with low cost cw lasers due to their large d value, resulting in partial substitution of high end fs lasers. Instrumental application of surface-plasmon-enhanced fluorescence was applied in using a TP scanning tunneling microscope [363]. This was employed to probe the TP excited fluorescence from organic nanoparticles adsorbed on a silver surface. A size dependence of fluorescence enhancement and photodecomposition was reported as a result of competition between surface-plasmonenhanced TP fluorescence and nonradiative energy transfer from the excited dye molecules to the silver surface. The schematic experimental setup is shown in Figure 3.14 [363].

PMT

photon counter

interference filter PC

collimator

scanning and feedback control

fiber probe

shear-force control scanner nanostructured material excitation light silver film

X, Y, Z control

prism

lens L ND filters polarizer

photodetector

mirror M

Figure 3.14. Schematic setup for a TP tunneling microscope for probing surfaceplasmon-induced local field enhancement of TP excited fluorescence for organic nanoparticles coated on a silver surface. (From Ref. [363] with permission of the American Chemical Society.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

C. Evaluation of Two-Photon Absorbing Materials There are several methods available to quantify the TPA cross section. Most of them are based on a relative method; that is, data are related to a standard. Data reported are often measured based on an application. Thus, chromophore data taken by nonlinear absorption are often related to optical power limiting using long and intense ns-laser pulses ( 5–10 ns). Because the decay time for most of the fluorophores is often significantly lower than such long ns pulses, excited state absorption can occur resulting in a significant higher absorption of photons as related to TPA. Therefore, a comparison of such data needs care and can be done only for chromophores using similar setups although the general information would be more qualitative. On the other side, fs lasers can be considered as reliable excitation sources and excited state absorption can be considered as a minor event affecting the quantitative evaluation of d. Relative data evaluation often uses fluorescence to determine the TPA cross section because this method sensitively relates to the number of excited singlet state S1 populated by TP excitation. The observed TPA cross section by fluorescence is the TPA cross section action. It is therefore necessary to divide this quantity by the quantum yield being responsible for this event; that is the fluorescence quantum yield in this example. Fluorescence is not the only prerequisite to quantify relatively the TPA cross section action. In general, almost all photochemical/photophysical events can be applied to probe the TPA cross section action. Again, dividing the TPA cross section action by the quantum yield related to this event results in d, an intrinsic value of the TP absorbing chromophore, which is related to molecular parameters as shown in Eq. (18). 1. Experimental Techniques in Nonlinear Absorption Measurements of TPA yield nonlinear absorption coefficient/TPA cross section (compare Ref. [54] for further details). The nonlinear absorption coefficient b is related to d as b ¼ d NA cch  103 (b in cm/GW, NA ¼ Avogadro’s number with 6.02  1023 mol1, cch ¼ chromophore concentration in mol/L).11 In general, the following techniques have been applied to quantify the TPA cross section[53, 54], which are complementary methods for determination of either b or d.     11

Nonlinear transmission measurements [364, 365] Two-photon fluorescence [85, 86, 366–368] Thermal lensing [67] Laser calorimetry [69]

When the linear absorption coefficient vanishes at the excitation wavelength but b > 0, attenuation of excitation light in a given sample is described as dI=dz ¼ bI 2 .

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145

 Acousto-optic measurements [68]  Degenerate four-wave mixing [364]  Heterodyned Kerr effect measurements [369] Change of transmission in a nonlinear optical material is the most obvious effect related to TPA [364, 365]. Thus, this technique is often used to determine the TPA cross section of materials. This straightforward method gives the nonlinear absorption parameters. The signal observed is often small with respect to the large background signal, so the nonlinear transmission is only a small fraction of the observed signal. This is a disadvantage because reliable parameters can be obtained mainly from optical materials with large TPA. One can either (1) increase the signal intensity or (2) increase the chromophore concentration in order to obtain a better nonlinear transmission signal. Furthermore, high chromophore concentration can lead to artificial results because of possible aggregation. The latter results in a different optical material, which does not reflect the optical properties of a single molecule (d relates to a single molecule). A beam having either a Gaussian- or sech2-shaped pulse is often employed for nonlinear transmission measurements. In general, two different techniques are employed. In the first, the sample is fixed and the intensity is changed during the experiment. This technique can be used for thick samples. A general setup is shown in Figure 3.15 [53]. The variable attenuator changes the intensity of the

Figure 3.15. Nonlinear absorption measurement of a thick sample. (Adapted from Ref. [53].)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.16. A z-scan experimental setup for determination of TPA of a thin sample. (Adapted from Ref. [53].)

beam, which allows data collection at different intensities. A calibrated beam splitter is integrated in the setup in order to get a quantitative measure for the intensity before the beam passes the sample (I0). An energy detector behind the sample measures the intensity that is reduced by the amount of absorbed photons. In the second, the intensity is kept constant and the sample is moved along the beam axis to increase the intensity at the focal point (Fig. 3.16 [53]). The intensity has the largest value at the focus and thus one can measure the largest nonlinear absorption effect if the focus of the beam is inside the sample. This technique requires the use of thin samples. The measured nonlinear transmittance is normalized with respect to linear transmittance measured at low intensity. Because the sample moves along the z-axis, this technique has been often called z-scan. A typical example for a z-scan is shown for compound 96 (compare Section III.C.2) in Figure 3.17. The normalized transmittance is a function of the sample position (z). It is almost negligible in the pure solvent (DMSO: dimethylsulfoxide) compared to the chromophore 96. This compound shows a sharp signal for the loss of transmittance, which is typical for nonlinear absorption. For TPA, the normalized transmittance TN is given by Eq. (33) [54, 364]: 1 ð

1 TN ¼ aqðÞ

ln½1 þ qðÞf ðxÞdx

ð33Þ

1

The function q() is defined in Eq. (34): qðÞ ¼

bð1  Rf ÞI0 Leff 1 þ 2

ð34Þ

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

Normalized transmittance

1,1

147

Compound 1 Pure DMSO

1,0 0,9

0,8 0,7 0,6 –10

–5

0

5 Z [mm]

10

15

20

Figure 3.17. The z-scan experimental data of compound 96 in DMSO (3  102 M) (squares) and of pure DMSO (circles) in a 1 mm cell, using an input intensity laser beam of 0.35 mJ. (From Ref. [370] with permission of the American Chemical Society.)

b stands for the nonlinear absorption coefficient (b ¼ d=Nch Þ; Rf is the Fresnel reflectance of the front sample surface, I0 is the intensity at the focus, and Leff is defined by Eq. (35): Leff ¼

1  expðaLÞ a

ð35Þ

where L ¼ sample thickness, and a ¼ linear absorption coefficient.  represents the position of the focus with respect to the center inside the sample. It is z/zr with zr being in the Rayleigh range. Furthermore, f(x) describes the shape of the beam. f ðxÞ ¼ expðx2 Þ for a Gaussian beam and f ðxÞ ¼ secant squared pulse. For a Gaussian-shaped pulse it sech2(x) for a hyperbolic pffiffiffi follows that a ¼ 2 while a ¼ 2 for a sech2-shaped pulse. For fixed samples (Fig. 3.15),  ¼ zero and I0 is varied, while in the z-scan technique (Fig. 3.16) I0 is fixed and  is varied. In both cases, data are fitted by Eq. (34) to evaluate b, which requires a numerical treatment. However, approximations can be made for conditions in which q0 ¼ bð1  Rf ÞI0 L < 1. Two-photon excited fluorescence (TPEF) is one of the most useful ways to quantify TPA in optical materials competing with nonlinear absorption. While nonlinear transmission yields absolute quantities describing TPA, TPEF requires knowledge of the collection efficiency if absolute data must be acquired. Thus, the use of standards is the method of choice to determine d in case of TPEF.

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Details are given in the next two sections. Furthermore, TPEF requires a certain emission level for reliable determination of TPA data. This prerequisite limits the use of this method to TP chromophores showing fluorescence with reasonable yields.12 Details are given in the next section. In addition, measurements using incident light that is linearly (lin) and circularly (cir) polarized allow determination of the polarization ratio ð ¼ dcir =dlin Þ. Furthermore, gives information regarding the symmetry of excited states with higher vibrational levels [12, 16, 19, 22, 23, 31, 35, 231, 235, 308, 343, 344, 371, 372]. In some cases, the absorbed energy is converted into heat if the chromophore possesses a large quantum yield for nonlinear deactivation [54, 67–69]. This internal change of the optical material can be measured either as a direct change in temperature or by an optical property depending on temperature. Hence, chromophores with large nonradiative deactivation may allow determination of absorption coefficients by measuring temperature changes. The two general monitoring methods described are thermal blooming, related to changes of the refractive index [54, 67, 68], and laser calorimetry [69]. The heat generated by the nonlinear absorption process depends on the square of the instantaneous pump power. Laser calorimetry can be applied to measure the total absorption and hence the TPA coefficient. This method measures large changes in a small quantity (absorption). Care must be taken to acquire an accurate temperature; the sample must be well separated from the surroundings in order to avoid thermal losses. 2. Absolute Evaluation of Two-Photon Absorbing Materials Two-Photon Excited Fluorescence. TPA can be described by Eq. (36) when neither saturation nor photobleaching occurs [85, 86, 366, 368, 373]. Nabs ðtÞ ¼ cch d I02 ðtÞ

ð

S2 ðrÞ dV

ð36Þ

V

This equation contains a time- and space-dependent term. Concentration of the chromophore (cch), the two-photon absorption cross section (d), and the square of the incident laser beam used for excitation (I02(t)) scale the amplitude of the TPA. Nabs ðtÞ stands for the number of photons absorbed in a TPA event per unit time. Furthermore, SðrÞ and I0 ðtÞ describe the spatial and temporal distribution of the laser beam, respectively.

12

The lower the fluorescence quantum yield the larger the error of d.

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

149

By taking account in Eq. (36) of the fluorescence generated by the TPA, one obtains Eq. (37) [85, 86, 366, 368, 373]: ð 1 2 ð37Þ hFðtÞi ¼ gc cch f Zcol d hI0 ðtÞ i S2 ðrÞdV 2 V

hFðtÞi corresponds to the time-averaged photon flux. The latter is proportional to hI0 ðtÞ2 i because most detectors provide a response proportional to hI0 ðtÞ2 i. f is equal to the fluorescence quantum yield and Zcol is the collection efficiency of the optical setup used. The parameter gc corresponds to the second-order temporal coherence; that is, gc ¼ hI0 ðtÞ2 i=hI0 ðtÞi2 . Therefore, Eq. (37) represents all experimental quantities needed for quantitative evaluation of TPA. These are the Ð spatial distribution of the incident light ( V S2 ðrÞ dV), the degree of the secondorder temporal coherence (gc), the fluorescence collection efficiency (Zcol ), and the fluorescence quantum yield (f). Details are compiled in the literature [85, 86, 366, 368, 373]. Thus, exact characterization of the optical elements is necessary to define the spatial dependence of the intensity of the incident beam. Numerical calculations resulted in Eq. (38) for thick samples, which expresses a relation between Ð 2 S ðrÞ dV and the numerical aperture (NA) of the optical setup used for exciV tation (n ¼ refractive index, l ¼ wavelength). ð 8nl3 S2 ðrÞdV  ð38Þ p3 ðNAÞ4 V!1

Equation (39) expresses the relation between the intensity and the incident power of the beam, P(t). pðNAÞ2 I0 ðtÞ ¼ PðtÞ ð39Þ l2 Inserting Eqs. (38) and (39) into Eq. (37) results in Eq. (40) for thick samples [85, 86, 366, 368]. 1 8n hFðtÞi ¼ gc cch f Zcol dhPðtÞi2 ð40Þ 2 pl Quantitative measurements of d require additional screening of temporal fluctuations of the photon flux—the so-called temporal coherence gc. Because the chromophore is excited in the first cycle of the pulse train, evaluation of gc is needed for only one excitation cycle. Defining tH as the excitation pulse width and fltH as the duty cycle, gc is expressed by Eq. (41). gp ð41Þ gc ¼ f1 tH where fl ¼ repetition frequency of the laser.

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

The parameter gp depends on the shape of the laser pulse and the duty cycle. Pulses with a Gaussian temporal profile result in gp ¼ 0.664 while gp ¼ 0.588 is found for hyperbolic-secant square (sech2) pulses. Combination of Eqs. (40) and (41) results in Eq. (42), showing that pulsed lasers are more efficient excitation sources compared to lasers operating in cw mode. gp 1 8n hPðtÞi2 hFðtÞi ¼ cch f Zcol d f tH 2 pl

ð42Þ

Lasers in cw mode have gc ¼ 1 and require some orders of magnitude more average power in comparison with a mode locked Ti:sapphire laser. This may result in photodestruction of the chromophore and is not desired. The general experimental setup used for quantification of TPA is depicted in Figure 3.18. A mode locked Ti:sapphire laser was used for excitation. The pulse was continuously monitored by an autocorrelator. Pulse repetition was controlled by a photodiode. A CCD camera measures dispersion of the pulse spectrum. Pulsed fs excitation results in a nearly Gaussian distribution of the spectral profile. This treatment is essential to ensure that no additional cw light accompanies the pulsed excitation. If cw light exists in the excitation spectrum, the spectral profile can take on an asymmetric shape as a result of the cw and pulse light contribution. Excitation light passes through a l/2 plate, a beam expander, and an objective lens into the sample. TPEF is controlled by a standard single-

Figure 3.18. Optical setup for determination of d using TPEF. (From Ref. [85] with permission of the American Institute of Physics.)

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

151

photon counting setup. The collection efficiency Zcol is another parameter that has to be determined in the setup described in Figure 3.18. It can be evaluated from OP excited fluorescence (OPEF) experiments. Direct comparison with OP fluorescence experiments yields Zcol . This was shown for fluorescein and rhodamine B [85, 86, 366, 374]. The setup in Figure 3.18 uses TPEF for absolute determination of d. Another setup can be used to determine absolute d by in situ second-order autocorrelation for the absolute quantification of TPA [366]. The approach is to access absolute TPA without knowledge of gc. A general scheme is depicted in Figure 3.19.

Figure 3.19. Absolute determination of d by in situ autocorrelation. Experiments were performed with a mode locked femtosecond Ti:sapphire laser. A prism pair (PC) was used to compensate the group delay dispersion (GDD) of the microscope objective. A longpass filter eliminates residual argon pump light and Ti:sapphire fluorescence. After two sequential beam expanders (BE), the beam was approximately 25 mm in diameter, which was sufficient to overfill the back aperture (10-mm diameter) of the objective. A long-pass dichroic mirror (DC) with reflectivity separates fluorescence from excitation light. The incident power at the sample was measured by recollimating the transmitted beam onto a calibrated power meter. Fluorescence was detected by a photomultiplier tube and recorded as a function of the interferometer delay. (From Ref. [366] with permission of the Optical Society of America.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Thus, two beams with equal intensities perfectly overlap in space but they are temporally shifted by the delays ts resulting in a fluorescence signal depending on ts. The original and the delayed beams are represented by P(t) and P(t þ ts), respectively. As a result, the amount of TPEF recorded is about twice the amount generated by a single pulse of one of the half-beams. The TPA cross section is thus expressed by Eq. (43): 0:5t Ð per

d¼

0:5tper

FðtÞdt  tper FðtÞ1

A cch f Zcol



ð43Þ

2 PðtÞdt

1 Ð 1

Equation (44) defines the fluorescence generated over a period tper. The paraÐ meter A is equal to the integral sample S2 ðxÞdV. It is independent of the numerical aperture in the paraxial approximation and for thick samples, but it depends on the beam shape. 2 FðtÞ1 ¼ A d cch f Zcol 4

1 ð

32 PðtÞdt5

ð44Þ

1

Independence of pulse width and chirp was confirmed by measuring autocorrelation traces under different conditions as shown in Figure 3.20. Data were obtained with an interferometric technique to provide a convenient calibration of the time delay and an assay of the spatial overlap between the two beams. The traces in Figure 3.20a,b were taken with the same laser output pulse width but with (Fig. 3.20a) and without (Fig. 3.20b) compensating for the group delay dispersion (GDD) in the microscope objective. The trace in Figure 3.20c was obtained with a longer laser output pulse width. Furthermore, collection efficiency (Zcol ) was calibrated by measurement of OPEF of a thin and dilute sample in the same setup. Absolute TPA cross sections of 46, 43, 50, and 47 GM (average of 46  10 GM) were found in four different experiments with autocorrelation FWHM (full-width half-maximum) values of 148, 189, 198, and 337 fs, respectively. The small variation between the four measurements and the agreement of these values with the value obtained with a single-mode cw Ti:sapphire laser demonstrate that this method correctly accounts for pulse width and chirp. These efforts resulted in compilation of absolute TPA data for chromophores used as standards for relative determination of d. The most used standard for relative TPA measurements is fluorescein (12) [373]. d was determined in a spectral range from 690 to 960 nm, covering the emission of a Ti:sapphire laser. Data compiled in Table 3.2 are a product of d and f and therefore a measure for

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

153

Figure 3.20. Autocorrelation traces of the excitation pulses in fluorescein water at pH ¼ 13. Fluorescence intensity is normalized to the average background. (a, b) Both measured with wavelength spectrum FWHM
l ¼ 14.5 nm but with different amounts of prechirp; (c)
l ¼ 7.8 nm. (From Ref. [366] with permission of the Optical Society of America.)

TABLE 3.2 TPA Cross Section Action for TPEF (df; f ¼ 0.9) and TPA cross section (d) of 12 Taken in Water at pH ¼ 13 [366] l (nm)

691 700 720 740 760 780 800 820

df (GM) 16 19 d(GM) 18 21

19 21

30 33

36 40

37 41

36 40

29 32

840 860 880 900 920 940 960 13 14

8 11 9 12

16 18

26 29

21 15 23 17

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

the efficiency of TPEF—namely, the TP fluorescence cross section action. f is equal to 0.9 at pH ¼ 13. 2O

O

O

COO

2Na

12

Nonlinear Absorption. A more convenient and time-saving approach to record the TP excitation spectrum is the direct measurement using an intense continuum beam requiring no wavelength scanning mechanism (Figure 3.21) [375, 376]. The nonlinear absorption spectrum can be taken in a single shot. Thus, a powerful beam generated by an amplified Ti:sapphire laser system passed a cell filled with heavy water, resulting in generation of a white-light fs continuum. The collimated light passed through a dispersion prism. It was then focused into the center of the sample cell. This alignment allowed spatial separa-

Figure 3.21. Experimental setup for degenerate TPA spectral measurement by using a single intense continuum-generation beam. (From Ref. [375] with permission of the American Chemical Society.)

TWO-PHOTON ABSORPTION: THEORY, MECHANISM, AND QUANTIFICATION

155

tion of the spectral components of the excitation beam in the sample, resulting in separate excitation of the chromophore. A CCD array detector imaged the intensity distribution. Comparison between the continuum passing through the chromophore sample and the part going through a neat solvent sample allowed determination of the TP excitation spectrum as a function of excitation wavelength if linear absorption was either known or negligible. The use of a fs continuum resulted in a spectrum consisting of significantly more data points compared to a wavelength scan with a Ti:sapphire laser. This provided a higher number of data points with respect to the spectral range considered and therefore more accurate spectral information for the measured chromophore. This method is complimentary to Section II.C.1. 3. Relative Evaluation of Two-Photon Absorbing Materials by Two-Photon Excited Fluorescence Relative measurements of d have mostly been used although they are only as good as the data taken for the standard. It took great effort to achieve the absolute determination of the TPA standard fluorescein (12) with the setup shown in Figures 3.18 and 3.19. Equation (45) shows the parameters needed for the relative determination of d at a given wavelength l. This equation gives the TPA cross section of the sample at the excitation wavelength chosen (ds(l)). It is therefore necessary to scan the wavelengths needed for the TP excitation spectrum [373].

ds ðlÞ ¼

f ðrÞ cch ðrÞ Zcol ðrÞ hPr ðtÞi2 hFðlÞis nr





dr ðlÞ f ðsÞ cch ðsÞ Zcol ðsÞ hPs ðtÞi2 hFðlÞir ns

ð45Þ

where f(r) ¼ fluorescence quantum yield for the standard, f(s) ¼ fluorescence quantum yield for the sample, cch(r) ¼ concentration of the standard, cch(s) ¼ concentration of the sample, Zcol (r) ¼ collection efficiency of the standard, Zcol (s) ¼ collection efficiency of the sample, hPr ðtÞi ¼ incident excitation power used for the standard, hPs ðtÞi ¼ incident excitation power used for the sample, hFðlÞir ¼ fluorescence signal generated for the standard, hFðlÞis ¼ fluorescence signal generated for the sample, ds(l) ¼ two-photon absorption cross section of the sample at the excitation wavelength l, dr(l) ¼ two-photon absorption cross section of the standard at the excitation wavelength l, nr ¼ solvent refractive index for standard dissolved, and ns ¼ solvent refractive index for sample dissolved. Fluorescence is not a prerequisite for relative determination of the TPA cross section. When a standard is available with known d for a photochemical process, the corresponding parameters for fluorescence can be replaced by this event in Eq. (45). One example is the formation of singlet oxygen [179, 180, 182].

156

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

D. Pulse Propagation Theoretical simulation of pulse propagation through a nonlinear absorbing medium has led to the conclusion that the TPA cross section is influenced by off-resonant two-step TPA and saturation effects. The approach is based on a suggested dynamic theory for TPA [377]. Excitation energies, changes of state dipole moments, and transition dipole moments were calculated to compute d by density functional theory, and the wave equations were solved for a long and short pulse. According to this theory, light propagates through a TP active medium, taking into account the influence of both pulse duration and saturation on its absorbing capacity. Numerical simulations were performed for long and short pulses, resulting in distinct d values if pulses with different pulse width were used for the chromophore 13. Thus, the ratio for the TPA cross section using an 8-ns pulse versus a 173-fs pulse is 853, which agrees well with experimental data [378]. These results demonstrate the necessity to include pulse propagation in designing TPA equipment. A careful design of optical confinement is also needed to maintain the scale of a fs pulse upon exposure. Thus, a short pulse may become prolonged under some circumstances, resulting in a decrease of the excitation efficiency. Furthermore, these results show the necessity to maintain the experimental conditions that a fs pulse is kept on its time scale. Moreover, the light source of choice is a fs laser for evaluation of TPE data because excited state absorption, resulting in artificially larger d, is of minor importance. N N O

N

13

III.

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

Many TPA data taken with ns lasers and fs lasers are available in the literature. According to the problems arising with ns lasers applying nonlinear absorption, the fs laser should be the light source of choice to quantify TPA data. However,

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

157

we included data taken by either ns or fs lasers to demonstrate how structural design may affect TPA cross section. A direct comparison is often not possible if excited state absorption results in artificially larger nonlinear absorption.

A. Chromophores with Large p-Systems 1. General p-Structures Several types of unsaturated conjugated systems were investigated regarding TPA [31]. These were substituted aromatics, heteroaromatics, polyenes, dyes, and natural products. Table 3.3 summarizes TPA data of some representative compounds for the investigated p-systems. Aromatic compounds such as benzene and their substituted analogs possess only small d values, that is, 1 GM or less. However, chromophores with large TP absorptivity were shown in the literature to have d > 50 GM. Although the data in Table 3.3 clearly exhibit a gradual increase of d by systematic extension of the conjugated system, the overall data do not exhibit large TPA cross sections. This can be seen, for example, by comparing data of benzene, naphthalene, and anthracene (Table 3.3). Details regarding multiphoton absorption of aromatic hydrocarbons have been reported [5, 8–23, 25, 28–32, 35–39, 43, 45, 49–51, 53, 54, 58, 67–69, 379]. In addition, comparison of data obtained in crystals with data obtained in solution shows 55 times more TPA in the solid state [5]. This can be seen in Figure 3.22. The TPA of the 1A1g ! 1B2u transition, which is forbidden for TP excitation, is vibronically induced by a not totally symmetric vibration of the frequency 1200  400 cm1. Section II.B.3 combines the necessary relationships explaining the increase of d as a result of vibronic coupling. In crystals, one can observe an additional TP state with g-symmetry. This has no counterpart in solution. This state exhibits a charge transfer state formed in the crystal due to the packing. In particular, this charge transfer state was believed to be responsible for increasing the TPA cross section. Furthermore, systematic increase of the TPA cross section was found in the series of trans-diphenylpolyenes 14–17 [29]. Then larger TPA may result from the changes in the electronic nature of the p-moiety from an aromatic to a polyene as in 14–17, resulting finally in an increase of d. This may be explained by BLA, which was discussed as a tuning factor for the TPA of polyenes in Section II.A.4 [131, 132, 233, 283–285, 318, 320–322, 325]. Furthermore, experiments focusing on energetic ordering of the OP and TP excited states indicate a change of the order for the odd (OP) and even (TP) states with increasing chain length of the polyene. While the S1 state is almost the OP excited state in 14 and 15, the TP excited state becomes the lowest excited state in 16 and 17, that is, n  3 [52]. This has been one of the minor examples in which state S1 is directly accessible by TPA if the chromophore exhibits a symmetric pattern. No electronic coupling is necessary between S1 and higher excited states according to the

158

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

TABLE 3.3

Compilation of TPA Data of Several Types of Conjugated p-Sytemsa

Compound

Matrix

l (nm)

d (GM)

Method Reference

Substituted Benzenes Benzene Toluene o-Xylene m-Xylene p-Xylene Mesitylene Aniline Fluorobenzene Chlorobenzene Bromobenzene Phenol

Neat Vapor Neat Neat Neat Neat Neat Vapor Vapor Vapor Vapor Vapor

532 0.00025 525 0 532 0.0036 532 0.028 532 0.035 532 0.096 532 0.096 588 50 528 0.5 538 5 540 7 550 7 Multiple Phenyl Rings

TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF

[51] [37] [51] [51] [51] [51] [51] [37] [37] [37] [37] [37]

Biphenyl Carbazol Dibenzofuran Dibenzothiophen Fluorene Difluorenyl (2,2)-Paracyclophane

Crystal Crystal Crystal Crystal Crystal Crystal Crystal

Qualitative spectroscopic studies; no quantitative data reported presumably due to low signal

TPF TPF TPF TPF TPF TPF TPF

[8, 13, 22] [13] [13] [13] [14] [14] [17]

PTL TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF TPF

[42] [15] [15] [15] [15] [15] [15] [5] [5] [5] [7] [7] [7] [21] [13] [14] [6] [54]

Condensed Aromatic Rings Naphthalene

Anthracene

Azulene Phenanthrene 2,20 -Binaphthyl Chrysene Pyrene

CCl4 606 0.4 Crystal 635 2 Crystal 613 10 Crystal 493 40 Cyclohexane 635 0.02 Cyclohexane 611 0.11 Cyclohexane 597 0.45 Crystal 696 0.3 Crystal 597 7.6 Crystal 576 370 Cyclohexane 641 0.05 Cyclohexane 602 0.45 Cyclohexane 578 0.65 Spectroscopic studies Crystal Crystal Crystal 0.01 - 0.001 694 0.22

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

TABLE 3.3

159

(Continued )

Compound

l (nm)

Matrix

d (GM)

Method Reference

Heteroaromatics Pyridine Acridine

Neat

Stilbene (14)

Cyclohexane CHCl3 Benzene

514 693 608

12.1 50 14.4

TWM [29] NLA [10] TWM [29]

Benzene

608

43.3

TWM [18, 25, 29]

Benzene

608

61.0

TWM [18, 29]

Diphenylbutadiene (15) Diphenylhexatriene (16) Diphenyloctatriene (17) all-trans-Retinol 18

694 Polyenes

EPA (77 K)

704 Distyrylbenzenes

Cyclohexane

600

0.1 2

TL

[26] [54]

20

TPF

[24]

62

TPF

[49, 380]

a

l, wavelength related to the TPA cross section d; TPF, two-photon excited fluorescence; PTL, photothermal lensing; TWM, three-wave mixing; TL, thermal lensing; NLA, nonlinear absorption; EPA, solvent mixture made of ether:isopentane:ethanol ¼ 5:5:2.

mechanism depicted in Figure 3.2a because TP excitation results directly in population of the first excited singlet state.

n n = 1: 14: n = 2: 15: n = 3: 16: n = 4: 17

p-Bis(o-methylstyryl)benzene (18) was investigated as a further standard for TPA measurements between 537 and 694 nm [49, 380]. It is a phenylenevinylene chromophore exhibiting a TPA cross section of 62 GM at 600 nm [380]. Though this value is slightly increased compared to stilbene (14), the larger p-system of 18 does not have the desired impact on d.

18

160

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

TWO-PHOTON ENERGY (eV)

ABSORPTION CROSS SECTION (CM4 SEC PHOTON–1)

3.2

3.6

4.0

4.4

4.8

10–48

10–49

10–50

10–51

10–52

10–53

23

27

31

35

39

–1 TWO-PHOTON ENERGY (103 CM )

Figure 3.22. TPA spectra of anthracene in crystal (;&) and solution (); more details can be found in Ref. [5]. (From Ref. [5] with permission of the American Institute of Physics.)

2. Conjugated Polymers and Oligomers Conjugated polymers bearing distinct unsaturated segments have evoked a lot of interest in TP photoscience [27, 33, 381–394]. Table 3.4 summarizes TPA data of some TP materials [27, 33, 296, 381–394]. These materials exhibit interesting p-moieties and the data in Table 3.4 determine if these substructures are responsible for the increase of d. These materials exhibit a significantly larger p-system in comparison with the compounds in Table 3.3 and should therefore have larger nonlinear hyperpolarizability as well [129]. Conjugated polymers have been known to exhibit large nonlinear hyperpolarizability g [71, 129, 388, 395–399]. Therefore, these materials are expected to exhibit a large d as well, because a linear relationship exists between d and g according to Eq. (17). Thus, two-photon spectroscopy has gained importance in determining localization of TP allowed transitions that

161

459 777

394 844

394 790

394 841

3332

34,000 34,000 44,000 44,000

100 fs 100 fs 35 ps

Film Film Film CHCl3

8 ns 80 fs 100 fs 8 ns 80 fs 8 ns 80 fs 100 fs

100 fs 150 fs

cw

2.6 ns

100 fs

1

1

1

12

12

[381] [387] [389]

[381]

[381]

[384] [384] [381] [384] [384] [384] [384] [381]

[381] [386]

[382]

[383]

Pulse Length Reference

Film

TCE TCE Film TCE TCE TCE TCE Film

CHCl3 CHCl3

451 718

CHCl3

TOL

19,000

458 769

387 625

d b lexc l01 l02 M01 M12 Mn (GM) (cm/GW) (nm) (nm) (nm) (D) (D) f (g
mol1) Solventb nc

Summary of Nonlinear Absorption Data of Conjugated Oligomers and Polymersa

19 20,000 625 R1 ¼ R2 ¼ hexyl R1 ¼ R2 ¼ ethyl-hexyl 20 72,000 R1 ¼ C6H13, R2 ¼ C6H13; R3 ¼ CH3; R4 ¼ CH3; R5 ¼ p-C10H21-Phenyl; R6 ¼ H 21 19 5 800 11 R1 ¼ C6H13; R2 ¼ CH3; R3 ¼ p-C10H21-Phenyl 22a 10,798 1.1 810 R1 ¼ ethylhexyl, R2 ¼ CH3O 42 796 675 180 800 22b 16,443 1.7 810 R1 ¼ ethylhexyl, R2 ¼ 9-phenylanthryl 49 796 22c 2945 0.3 810 18 796 R1 ¼ H, R2 ¼ 9-phenylanthryl 22d 300 80 800 R1 ¼ H, R2 ¼ H 22e 94–300 25–80 800 R1 ¼ CH3O, R2 ¼ CH3O 22f 150 40 800 R1 ¼ C8H17, R2 ¼ C8H17 22g 165 44 800 R1 ¼ OC8H17, R2 ¼ OC8H17 14 1064

Material

TABLE 3.4

162

(Continued)

525 625 726 726 726

710 812

0.2

420 10,000 55 15 130 229 215 109

532 800

3.5 26,520 70,000

500

444 760 708 18 438 710 0.58 534 803 0.01 332 530 525 625 401 726 5.7 5.8 0.74 4255 397 726 6.2 5.7 0.97 5680 390 726 4.2 4.6 0.5 3310

293 460

435 758 619 1059

Film CHCl3 THF CHCl3 Benzene CHCl3 DMSO CHCl3 TOL 1 TOL 1.6 TOL 1

CHCl3

CHCl3 Crystal

Crystal

Mn d b lexc l01 l02 M01 M12 (GM) (cm/GW) (nm) (nm) (nm) (D) (D) f (g
mol1) Solventb nc

ns ns 100 fs 100 fs 100 fs

fs 10 ns

100 fs

7 ns

25 ns

[388] [389] [393] [394] [385] [296, 383] [296] [296] [392] [392] [392]

[390]

[389] [27, 33]

[391]

Pulse Length Reference

d, TPA cross section; b, nonlinear absorption coefficient; lexc , excitation wavelength for d; l01, wavelength for maximum of OPA; l02 , wavelength for maximum of TP excitation; M01 , transition dipole moment for S0 ! S1 optical transition; M12 , transition dipole moment for the optical S1 ! STP transition; f, fluorescence quantum yield; Mn, average number of molecular weight; nc, effective conjugation length used for quantification of d; Pulse Length, discloses the pulse length of the laser used for excitation. b Solvent, discloses the solvent/matrix used for TP experiment; CHCl3, chloroform; TOL, toluene; TCE, 1,1,2,2-tetrachloroethane; THF, tetrahydrofurane; DMSO, dimethylsulfoxide.

a

27 (sulfate) 28a (R1 ¼ H; R2 ¼ C6H13) 28b (R1 ¼ H; R2 ¼ C8H17) 32 33 35 36 37 38 39 40

22h R1 ¼ OC8H17 R2 ¼ carbazolyl 23 24 R ¼ -CH2SO3-phenylene-CH3 26

Material

TABLE 3.4

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

163

are forbidden by OP excitation for conjugated polymers. Various conjugated polymers and oligomers have been investigated regarding their two-photon properties: polyfluorenes (19) [383, 400], ladder-type conjugated poly(phenylenes) (20) [382, 386, 400], which are comparable with poly(indenofluorenes) (21) [381], polyphenylenevinylenes (22) [384, 387–389, 391, 401], polyphenyleneacetylene (23) [389], cumulene-containing polymers (24) [402], polydiacetylenes (25) [33, 403], s-p alternating polymers (26) [390], polyaniline (27) [388], fivemembered heteroaromatic oligomers such as polythiophenes (28) [389, 393], oligothiophene (29), oligopyrrole (30), and oligofurane (31) [305], and rigid polymer consisting of an unsaturated carbon moiety and a heterocyclic group (32) [394]. Polymers 19–32 [27, 33, 296, 381–394] represent some examples used in nonlinear absorption studies. Investigations show that large d can be obtained if the unsaturated chain is stiff. This keeps distortion of the p-system as small as possible, and becomes more clear by comparing the data of the stiff laddertype polymer 20 with the more flexible PPV 22. In other words, the more planar the entire p-system, the larger the amplitude of the TPA (Table 3.4). TPA into the TP allowed excited state of substituted poly(p-phenylene acetylene) 23, poly(p-phenylene vinylene) 22, and poly(thiophene) 28 required more excitation energy (about 0.7–0.8 eV) in comparison with the OPA into the S1. Thus, excitation conditions were comparable with the mechanism depicted in Figure 3.2a. TPA of a methyl-substituted ladder-type poly(p-phenylene) 20 needed the same excitation frequency when using either the two-photon excited fluorescence method or the two-photon excitation pump and probe method [386]. The TP excited state exhibited a much broader spectral feature without resolved vibronic structure. Femtosecond transient absorption into the TP excited state occurs in the time frame of the excitation pulse (140 fs). The TP excited state decayed rapidly into the S1 state, which in this case is almost the OP excited state. State S1 exhibited a fluorescence decay of about 600 fs caused by the large transition dipole moment M01 typical for large conjugated systems with negligible distortion of the p-system. Furthermore, the broad absorption feature observed for the TP excited state was similar to that of electroabsorption. Intensity borrowing of state S1 from the energetically higher TP excited state is achieved by coupling between both states. Coupling can occur by intensity borrowing from higher vibrational modes as shown in Eq. (32). The ratio of the excitation energy for the TP excited state (mAg) with respect to the lowest OP excited state (1Bu) is about 1.25 for 20. This agrees with results found for other conjugated polymers [386]. Figure 3.23 shows the fluorescence spectrum of a film made of the methyl-substituted laddertype poly(para-phenylene) 21 obtained by simultaneous two-photon excitation. A similar spectral pattern was observed for OP excitation, showing that the emission belongs to the same excited state, which is S1. These findings are in agreement with the excitation pattern depicted in Figure 3.2a.

164

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

R4

R1 R1

R5

R3

R2 n

R3

R2 R6

19 R1

R6 R2

R2

R5

nR

1

R4

20 R3

R1

R1

R1

R1 R2

R2

R1

R3

n

n

n R1

R1

21

R2 22

23

R1

R C C C C n

24

R

25

R1

N

n

N

R

R

Si

Si N

R

N

m Ru2+

N N

R m yn

N N

2 PF6xn

26

R1

R1

R1

R2

N R

n

27 C10H21

Z

S n 28

C10H21 N

S

S

N n

32

n 29: Z = S; 30: Z = NH; 31: Z = O

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

165

Figure 3.23. Absorption spectrum (solid line) and photoluminescence spectrum obtained after simultaneous TPA (dashed line) of methyl-substituted ladder-type poly(paraphenylene) film at room temperature under vacuum of <105 Torr using pulses of a Ti:sapphire amplifier laser that have a width of 140 fs and a repetition rate of 1 kHz and intensities of up to 6 mW for pump energies in the range 1.55–2.00 eV. (From Ref. [386] with permission of Elsevier.)

Furthermore, the laser power dependence is intermediate between quadratic and cubic over the range of excitation energies from 1.55 to 2.00 eV. A quadratic increase of fluorescence intensity with dependence on excitation intensity would be expected for purely two-photon excitation [386]. Therefore, three-photon absorption can result in an increase of the exponent for the light intensity of greater than 2 if the incident beam is tightly focused onto the sample and the excitation power is very large. Upon simultaneous three-photon excitation, the phenyl ring is the absorbing component because it matches the absorption of liquid benzene in this spectral region. A representative example is given in Figure 3.24. This picture clearly shows the dependence of TPEF on the square of the excitation intensity. The conjugated polymer 20 was chosen for these investigations. Comparison of OPA and TPA spectra of the ladder-type poly(p-phenylene) 20 containing phenyl substituents (Figure 3.25) exhibits a clear splitting of the TP excitation peak. This result is in contrast to the methyl-substituted compound. The energetic splitting of the TP peak (B0 –B bands) is 165 meV and therefore comparable to that of the OP excited state showing a vibronic splitting of the OP peak of about 193 meV (A0 –A band). This difference in energy is similar

166

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.24. (a) Photoluminescence intensity versus pump power for an excitation energy of 1.61 eV. The line represents a linear fit with a slope of 1.97. (b) Spectra of OP photoluminescence (—) recorded at an excitation energy of 3.1 eV and TP excited photoluminescence (---) measured at an excitation energy of 1.61 eV. (From Ref. [174] with permission of Wiley-VCH.)

to the aromatic ring vibration mode observed for ladder-type poly(p-phenylene) by Raman spectroscopy. The mode at 193 meV (1544 cm1) was assigned to an aromatic ring stretching because of alternating Raman peaks, which are located at 1573 and 1605 cm1. The mode at 1320 cm –1 (165 meV) was assigned to an inter-ring stretching. The Raman lines of the latter are located at 1314 and 1329 cm –1 [382]. Because d is not influenced by the concentration of the ladder-type poly(p-phenylene), the TPA process exhibits an intrachain process [382]. The rise of d with decreasing temperature in the range of 300 K to 6 K was explained by the higher orientation order of neighboring phenylene rings of the ladder-type poly(p-phenylene) at low temperatures. Furthermore, the effective conjugation length (nc) affects d [382]. This number was set to 12 in order to calculate d from the experimental data. Determination of the effective conjuga-

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

167

Figure 3.25. Two-photon excitation spectrum of ladder-type poly(p-phenylene) containing phenyl substituents measured in a 104 M solution in toluene in the spectral range between 690 and 900 nm (i.e., 1.47–1.79 eV) using an Arþ ion laser pumped cw Ti:sapphire laser that works in single TEM00 mode. (From Ref. [382] with permission of Wiley-VCH.)

tion length is a critical point, which is discussed later. However, the fact that cw light from a Ti:sapphire laser was applied as the excitation source may justify the large d of 7.2  104 GM [382]. This is one of the largest TP absorptivities found for an organic compound. In particular, the planarity of this system was discussed as the main factor resulting in large d. Additional examples compiled in this section show that conjugated polymers are the preferred materials to achieve extraordinarily large values for d. Polyfluorenes (19) were reported to peak TP excitation at 625 nm with d  2  104 GM [383, 400]. Again, the effective conjugation length (nc) must be taken to quantify the TPA on a reliable effective chromophore basis. Experiments showed that nc  12 for 19 [383, 404]. In comparison to 20 and 21, polymer 19 has also a low distortion of the p-system. Therefore, 19–21 are preferred materials with large nc that can achieve large TPA. The less distorted the p-skeleton, the larger are both nc and d. In addition, a large TPA cross section was also reported for poly [1,6-bis(3,6dihexadecyl-N-carbazolyl)2,4-hexadiyne] (33) [385]. This polymer exhibits a d of about 1  104 GM at 812 nm (Table 3.4). The large TPA of 33 is the result of

168

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

the contribution of vibronic coupling to the TP forbidden 1Ag ! 1Bu transition. Thus, the amplitude for TP excitation of the 1Bu state can increase by intensity borrowing from higher vibronic modes coupling with the 2Ag state. Higher vibronic modes of the 1Bu state and the 2Ag state should have the same symmetry. Furthermore, a second TP Ag state located 0.33 eV higher compared to the first TP excited state also contributes to TPA, explaining the large observed TPA of 33. Section III.B.3 describes the relationships to increase d as a result of vibronic coupling. C16H33

C16H33 N

N

n

C16H33

C16H33

33

The conjugated backbone structure of 33 is, in general, comparable with that of polydiacetylenes 25. The latter were investigated in the early 1980s in the solid state and show strong TPA [33]. Change of bond order between two mesomeric forms may explain the large nonlinear absorption found in 25 (Fig. 3.26). The acetylene mesomer (Fig. 3.26A) and the butatriene mesomer (Fig. 3.26B) R C C

R C C C

C C

R (a)

C n

R

n

(b)

Figure 3.26. Polydiacetylene mesomers (a) and (b) with different energetic conformation. A single crystal of poly(2,4-hexadiyne-1,6-diol-bis-p-toluene-sulfonate) shows strong TP resonances at about 2.7 eV and 2.1 eV, and a weaker TP state that is centered at 1.8 eV when irradiated with a 2–4 ps Ti:sapphire laser [405].

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

169

are believed to contribute to TPA. Large oscillator strengths for both transitions involved in the TPA combined with strong OP resonance effects can explain the large TPA coefficients determined. Spectroscopic studies of urethane-substituted polydiacetylenes show variation of the UV–Vis as a function of the solvent nature. In particular, intramolecular hydrogen bonding was found as a controlling factor for planarity of the chains [390]. Poor solvents or even solvent mixtures with no interfering of hydrogen-bond interactions, such as chloroform–hexane mixtures, result in hydrogen-bond formation of nearly every polymer unit. However, solvents favoring hydrogen bonding result in a conjugation length of about four repeating units [33]. Again, the less the distortion of the p-system, the higher the TPA cross section. Moreover, the TPA cross section reported for a polydiacetylene crystal is about 500 GM (R ¼ CH2 —SO2 —Ph—CH3) peaking at 1006 nm [27]. These experiments clearly indicate that conjugated polymers are attractive materials in TP photoscience. The excitation energy of the TP excited state is higher (about 0.7–0.8 eV) in comparison with the OP excited state as qualitatively shown for 23 (R1 ¼ R2 ¼ OC4H9), 22 (R1 ¼ R2 ¼ OC8H17), and 28 (R1 ¼ H, R2 ¼ C6H13) (Fig. 3.27) [389]. These experiments qualitatively support the mechanism for electronic coupling between the TP and OP excited states shown in Figure 3.2a. Conjugated polymers bearing the PPV pattern 22 in the main chain are important in TP photosciences, as shown for the chromophores 22a–c [384]. TPA is increased by incorporating branched unsaturated moieties. In this series 22b exhibits the largest TPA. This polymer bears a phenylanthracene moiety and an alkoxy group as a substituent of the phenylene group in the PPV chain. Both 22b and 22c display, from a structural point of view, a conjugated polymer consisting of alternating diphenylanthracene and ethylene units. In particular, this combination results in an increase of d in combination with the low molecular weight diphenylanthracene [384]. Although a slight decrease of d was recorded for 22c, which has no alkoxy group, the dialkoxy PPV 22a possesses the lowest TPA amplitude within this series. Thus, incorporation of unsaturated branches favors the increase of d and overcompensates the electron-donating effect of both alkoxy groups in 22a. Furthermore, TPA data were reported per repeating unit; that is, the molecular weight of one repeating unit was taken to quantify d. The effect of the conjugation length was not considered. Because the latter may become larger than unity in these polymers, data for d may also become larger as reported (Table 3.4). Experiments furthermore indicate higher excitation energy of the TP excited state in comparison with the S1 (OP excited) state, which agrees with the excitation mechanism in Figure 3.2a. The sum over states approach was applied to explain the electronic coupling between both states (Eq. (27)). Furthermore, experimental results support the observation of complete energy transfer from the pendent phenylanthracene moiety to the PPV

170

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.27. TPA (dots with error bars) and OPA spectra (solid line) of the conjugated polymers (a) 23 (R1 ¼ R2 ¼ OC4H9), (b) 22 (R1 ¼ R2 ¼ OC8H17), and (c) 28 (R1 ¼ H, R2 ¼ C6H13). (From Ref. [389] with permission of Elsevier.)

backbone. It indicates coupling of the unsaturated branches with the PPV chain within the delocalized p-system. Normalized TP excitation spectra of 22a–c are plotted in Figure 3.28. Data taken between 710 and 890 nm show the highest excitation energy for 22a, while 22b and 22c exhibit two absorption bands at higher wavelengths. All three spec-

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

O

H3CO

O

n

22a

171

n

22b

n

22c

tra in Figure 3.28 exhibit vibrational features presumably caused by the distinct substitution of the phenylene group. This is evidenced by comparison of the spectra for 22b and 22c. However, a different pattern is depicted for 22a, which has no diphenylanthracene but does have two alkoxy pendants, resulting in a different vibrational pattern. In addition, investigations targeted on polymer properties when it was dissolved in dilute solution or as neat film. The two-photon excitation spectrum of 22h taken as a neat film shows a broad peak with an onset of 1.32 eV and a maximum at 1.63 eV [391]. The larger TP excitation energy (1.32 eV; 2 ho ¼ 2:65 eVÞ compared to OP excitation (2.34 eV) implies that the TP transition has a different selection rule than the OP transition. This is similar to behavior obtained in solution [384]. A similar excitation pattern as disclosed in Figure 3.2a was obtained for such polymeric materials; that is, the TP excited state possesses a higher excitation energy in comparison with the lowest OP excited state. Thus, the results obtained for diphenylpolyenes with more than two ethylene moieties, in which the S1 state is assigned to the TP excited state, are considered as an exception for chromophores with no donor substitution pattern [29]. Theory clearly indicates the potential of conjugated polymers to exhibit large TPA [406]. Calculations for poly(diacetylenes) using the PPP model show a decrease of TP excitation energy with both increasing interaction in the polymer chain and bond length alternation. Excitonic states were discussed for oneelectron excitations while biexcitonic states were found for two-electron excitations. TPA occurring at higher energies was found to have at least one magnitude larger amplitude in comparison with TPA occurring at lower excitation energy. In the case of the latter, mixing between excitonic and biexcitonic states occurs and the distinction between both is lost. PPP studies were additionally applied to investigate even-parity states and to describe TPA spectra of polyenes,

172

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.28. Normalized TP fluorescence excitation spectra for 22a, 22b, and 22c. (From Ref. [384] with permission of the American Chemical Society.)

173

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

polydiacetylenes, polyacetylenes, and polysilanes [406]. Ab initio response theory calculations of oligothiophenes (29), oligopyrroles (30), and oligofuranes (31), R1 ¼ R2 ¼ H for 29 and 30, respectively, demonstrate the relation between oligomer length (OL) from head to tail and d [306]. A power law dependence was found between these two quantities for a small number of repeat units, while a saturation region was indicated for species with larger molecular weight and hence conjugation length. The obtained data imply the exponent z of Eq. (46). d / OLz

ð46Þ

29 : z ¼ 7:0; 31 : z ¼ 6:2; 30 : z ¼ 4:0 The oligothiophenes show within this series the strongest TPA, while oligofuranes are comparable with polyenes. Experiments confirmed the large TPA of several thiophene oligomers (29). These compounds have TPA cross sections greater than 1000 GM with an excitation energy of more than 4 eV for the optical TPA transition [407]. Theoretical considerations indicate a remarkable increase in TPA as a result of covalent bonding of two equal p-units [296]. A significant increase of d occurs for larger conjugated systems bearing biphenyl, bis-fluorene, and bisstilbene. Results of semiempirical calculations are summarized in Figure 3.29, showing the higher excitation energy of the TP excited state compared to the lowest OP excited state. Thus, the mechanism depicted in Figure 3.2a appropriately describes TPA of these unsaturated systems as well. This pattern shows that

n

n

n

45700 cm–1 9.5 D –1

43100 cm–1

42600 cm 12 D

6D

35600 cm–1

43900 cm–1 8.5 D 39500 cm–1 6D

10 D

39500 cm–1 14 D 32900 cm–1 10 D

40150 cm–1 11 D 35900 cm–1 9D

37500 cm–1 16 D 31100 cm–1 13 D

n=1

n=2

n=1

n=2

n=1

n=2

60 GM

420 GM

60 GM

570 GM

210 GM

1300 GM

Figure 3.29. Theoretical three-level model for description of TPA in monomeric (n ¼ 1) and dimeric (n ¼ 2) forms. (From Ref. [296] with permission of Elsevier.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

extension of the p-system results in an increase of the transition dipole moments M01 and M12 , and pffiffiffired shift of the excitation energies. Transition dipole moments increase about 2, which is expected for coherent coupling between monomer units of conjugated polymers/oligomers [383], which agrees also with the exciton model in molecular spectroscopy [64, 408, 409]. This has an impact on d and results in an increase of d as shown by comparison of TPA data for the monomer and dimer. Enhancement of d can result from coupling between two monomer units covalently linked with each other as structural parts of the conjugated polymer/oligomer. Experimental data using the TPEF method and a nanosecond optical parametric oscillator as excitation source support these theoretical results (Figure 3.30) [296]. No significant TPA signal was detected for 9,9-dihexylfluorene (34). However, a remarkable increase of TPA was observed for the corresponding dimer 35, exhibiting a d of 55 GM at 534 nm. The dimeric biphenyl compound 36 possesses a smaller TPA cross section (d ¼ 15 GM) in comparison to 35, although the latter has a similar size of the p-system. The more distorted molecular geometry of 36 therefore results in a decrease of d supporting again the thesis that planarity is a key point in increasing d. Stilbene3 (37), a dimer of o-SO3 stilbene, exhibits with 130 GM a significantly larger d compared to stilbene (14) (Table 3.3). Interestingly, in this series 37 shows the largest TP absorptivity when compared to the fluorene 34 and bifluorenyl model dimer 35. Therefore, the alternating pattern of an aromatic and olefinic moiety results in a more efficient increase of d compared to 35 and 36. BLA discussed in

Figure 3.30. Experimental TP excitation spectra of the chromophores 35–37. (From Ref. [296] with permission of Elsevier.)

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

175

Section II.A.4 may explain these findings. Spectra depicted in Figure 3.30 demonstrate how the different molecular patterns of 35, 36, and 37 affect TPA [296]. C6H13

C6H13

C6H13

C6H13 C6H13

34

C6H13

35 C6H13 C4H9

O

O

C4H9 C6H13

36

O3S SO3

37

In addition, these data also explain the significantly larger d values observed for the polyfluorene 19 when compared with the fluorene dimer 35 [296, 383]. Experiments clearly show the larger excitation energy for the TP excited state compared to the S1 state, which is the OP excited state. Thus, the TP excited state relaxes into the S1 state, which is favored by electronic coupling between both excited states according to the mechanism depicted in Figure 3.2a. Twophoton excitation spectra of the dimer 35 and the polymer 19 depicted in Figure 3.31 indicate an enhancement for TPA amplitude by a factor of 360. TPA of the conjugated polymer 19 is bathochromic shifted (625 nm) with respect to dimer 34 (530 nm). The exciton model of linear aggregates was used to explain the evolution of the electronic state parameters with stepwise incorporation of additional monomer units [64, 408, 409]. In this model, the transition dipole moments have only one component directed along the main molecular axis. Thus, the amplitude of the transition dipole moment is expressed by Eq. (47) for nc  12. Mij ðnc Þ ¼

pffiffiffiffiffi nc Mij ðmÞ

ð47Þ

where Mij ðnc Þ ¼ transition dipole moment for the polymer, and Mij ðmÞ ¼ transition dipole moment for the monomer incorporated.

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.31. TPA absorption spectra of the fluorene dimer 35 and the conjugated polymer 19. (From Ref. [383] with permission of Elsevier.)

The term nc stands for the effective conjugation length and is understood as an effective number of chromophores incorporated in the conjugation length contributing to the photonic properties. Substitution of Eq. (47) in Eq. (27) for each transition dipole moment results in Eq. (48), showing the dependence of d in a conjugated polymer as a function of incorporated monomer units. The effective conjugation length cannot exceed 12 as shown for a series of oligofluorenes [404]. Equation (48) holds for nc  12. Thus, nc is not often equal with the average molecular weight. dðnc Þ ¼ ðnc Þ2 dðnc ¼ 1Þ

ð48Þ

where dðnc Þ ¼ transition dipole moment for the polymer, d(nc ¼ 1) ¼ transition dipole moment for the monomer incorporated, and nc ¼ effective conjugation length. For nc > 12, use of Eq. (49) was suggested to explain the large TPA of 19. dðnc Þ ¼

nm dðnc ¼ 12Þ 12

ð49Þ

This relation contains the TPA cross section for nc ¼ 12 (d(nc ¼ 12)) and the number of monomer units in the conjugated polymer nm. The quantity d(nc ¼ 12) is calculated in Eq. (48) and using this result in Eq. (49) results in a d(nc) of 10,000 GM for 19 with nm of 60. This agrees well with the experimental value of 20,000 GM [383]. These considerations clearly demonstrate the importance of a reliable determination of the effective conjugation length, since steric factors may seriously

177

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

affect nc. A detailed description of the determination of effective conjugation length of conjugated polymers was shown for the poly(arylenevinylene)s containing a biphenyl moiety (38–40), which are called oligomers [392]. The Strickler– Berg relationship, Eq. (50), was applied for evaluation of effective values (nc) contributing to OPA [338]. Thus, Eq. (50) was used to calculate the effective extinction coefficient (eeff max ) at the maximum for the optical S0 !S1 transition, resulting in the rate constant for fluorescence, kf. The latter can be determined by taking both fluorescence quantum yield (f) and fluorescence decay time (tf). ð 9 2 2 eff ð50Þ kf ¼ f =tf ¼ 2:88  10 n nmax emax erel ðnÞ dn where kf ¼ rate constant for fluorescence, n ¼ refractive index of the solvent, eeff max ¼ effective molar extinction coefficient at the maximum for the S0 !S1 optical transition, nmax ¼ absorption energy at the maximum for the S0 !S1 optical transition Ðin cm1, f ¼ fluorescence quantum yield, tf ¼ fluorescence decay time, and erel ðnÞdn ¼ integral of the normalized absorption band with erel ¼ 1. OC6H13 OC6H13

OC6H13 C6H13O

OC6H13 OC6H13

OC6H13

n

38

OC6H13

n

39 OC6H13

OC6H13

n

40

The effective photophysical parameter eeff max is used in a modified Lambert– Beer relationship (Eq. (51)), where m ¼ mass of the sample, ODmax ¼ extinction at the maximum for the optical S0 !S1 transition, V ¼ volume, and d ¼ thickness. M eff ¼

eeff max m d ODmax V

ð51Þ

This equation incorporates the effective molecular weight (M eff ) contributing to OP photonic properties. M eff is often significantly smaller than the number

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

average molecular weight of the sample (M n ). In particular, an increased flexibility of connecting single bonds and thus a lowered planarity are the main factors affecting effective conjugation length [410, 411]. M eff was used to calculate the effective emitter concentration needed for quantification of d. M eff was also taken for determination of the TP quantum yield for irradiation abs [392]. The use of average molecular weights determined by chromatographic or mass spectroscopic methods would result in unreliably large photonic data because of the above-mentioned difference between M n and M eff . Determination of M eff from photophysical measurements permits rough evaluation of nc as shown in Eq. (52): nc ¼

Meff Mr

ð52Þ

where Mr corresponds to the molecular weight of one repeating unit. The results obtained for 38–40 from the OP experiments for nc are included in Table 3.4. The transition dipole for S0 ! S1 (M01 ) was calculated by Eq. (53): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 9:19  103 kf M01 ¼ ð53Þ 2:88  109 n3max n3 Interestingly, nc was approximately unity for 38 and 40. This was notable and presumably due to twisting about the biphenyl moiety. The slightly larger nc of 39 (nc ¼ 1.6) may be caused by its specific alkoxy substitution pattern compared to the other systems. Similarly short effective conjugation lengths and the influence of alkoxy substitution were also reported for poly(2,20 -bipyridine-5,50 -diylethynylene[2,5di(2-ethylhexyl)oxy-1,4-phenylene]ethynylene) [410, 411]. Additional evidence for a small effective conjugation length was supported by the UV–Vis spectra of different molecular weight fractions of 39 using semipreparative GPC. Each fraction exhibits essentially the same absorption maximum, indicating no significant dependence of UV–Vis absorption with molecular weight. By comparison, a series of fractionated oligofluorenes (19) showed a significant dependence of photonic properties on molecular weight [404] and data resulted in nc  12. Two-photon excitation spectra obtained for 38–40 are plotted in Figure 3.32 [392]. The maximum is located at approximately the same excitation wavelength (726 nm) for all three compounds. However, there is a significant difference in the amplitudes. Systems 38 and 39 possess similar d of 229 and 215 GM, respectively, while d of 40 approaches a value about half as large. The better torsional mobility of the biphenyl moiety may cause the smaller d of 40 compared to 38 and 39. This agrees with OP results.ÐFurthermore, the fluorescence signal generated by simultaneous TP excitation ( IðlÞdl) is proportional to the square of the light intensity of excitation (I0) for 38–40. The slope of a

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

179

δ / GM

200 150 100 50 0 750

800

750

800

200

λ / nm

850

900

850

900

δ / GM

150 100 50 0 100

λ / nm

δ / GM

80 60 40 20 0 750

800

850

900

λ / nm

Figure 3.32. TP excitation spectra of 38 (&), 39 (), and 40 (!) obtained in toluene. (Adapted From Ref. [392].)

Ð lnð IðlÞdl) !ln(I0) plot is 2.17 for 38, 1.95 for 39, and 2.16 for 40. This demonstrates the occurrence of a nonlinear absorption process requiring two photons. Simultaneous three-photon excitation does not efficiently occur according to the optical setup chosen [392]. The d values for 38–40 are fairly large (Table 3.4), although the effective conjugation length does not significantly exceed more than one repeating unit. The model based on molecular aggregates [64, 408, 409] was applied to discuss the enhancement of optical transitions. Accordingly, each repetitive unit generates locally excited states resulting in an n-fold band of levels with discrete energy levels for each optical transition (Fig. 3.33). The bandwidth for each excited state depends on the number of coupling monomer units. It can become large,

180 E

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

S2

M 12 S1 xn

M 01 S0

monomer unit

conjugated sample

θ

θ

Figure 3.33. Band formation from locally excited states of repeat units in 38–40. (Adapted from Ref. [392].)

that is, >1000 cm1. The bandwidth is furthermore proportional to the intensity of the optical transition, explaining the enhancement of transition probability. Thus, the higher the number of coupling monomer units, the broader the bandwidth and therefore the higher the transition dipole moment for the optical transition. This has a particularly strong impact on the transition dipole moments between the S2 (TP excited) state and S1 (OP excited) state, M12 , as well as for the transition dipole moment for the S0 !S1 transition, M01 . Both transition dipole moments strongly affect d as shown by Eq. (27). The angle dependence between monomer units is important to maximize the transition dipole moment Mij between two excited states. Equation (54) shows the relation between the angle y of the coupling monomer unit (Fig. 3.33), the transition dipole of the respective monomer unit Mijm , and the number of coupling units N [64, 409]. In a stiff conjugated sample, the angle y can be small depending on molecular constitution. Therefore, Mij often decreases according to Eq. (54) if y slightly becomes larger than zero. pffiffiffiffi ð54Þ Mij ¼ N ðcos yÞMijm The values obtained for M12 according to Eq. (27) are large (Table 3.4) [392]. They support the model of efficient electronic coupling between the OP and TP excited states despite the fact that 38–40 possess short net conjugation lengths. According to Eq. (27), d depends quadratically on the transition dipole moments M01 and M12 . Thus, putting Eq. (54) into Eq. (27) for each transition dipole moment shows that d / N2 (N ¼ number of coupling units). This demonstrates

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

181

250

δ / GM

200 150

1.0 0.8 0.6

100

0.4

50

absorption/ a. u.

Excitation: two-photon one-photon

0.2

0

0.0 360

380 400

420 440 λ / nm

460

480

500

Figure 3.34. Absorption spectra obtained for TP (left axis) and OP (right axis) excitation of 38 in toluene at room temperature. (Adapted from Ref. [392].)

that d can become large in conjugated molecules if coupling between monomers occurs despite the fact that the net conjugation length is short. These results  eff , which is demonstrate also the importance of a reliable determination of M needed for determination of d. The latter requires knowledge of concentration and therefore molecular weight. Comparison of OP and TP excitation spectra is shown for 38 in Figure 3.34. For this class of oligomers, the TP excited state displays a higher excitation energy in comparison to the lowest OP excited state. The results for 38–40 were essentially the same. Thus, the energetic relations depicted in Figure 3.33 are justified. This has a strong impact on the photochemistry, particularly for 40. Either OP or TP excitation results in the same photochemically active state, S1, and therefore the same photochemical pathways. Most TPA data for conjugated polymers suffer from a poor consideration of the effective conjugation length needed for a reliable determination of d for a single molecule. Therefore, TPA data for conjugated polymers/oligomers needs to be carefully considered if distinct structural patterns are to be compared in order to understand the relation between d and structure. Nevertheless, the macroscopic nonlinear optical parameters of conjugated polymers related to nonlinear absorption, such as the nonlinear refractive index (n2) and third-order nonlinear susceptibility (w3), are available in the literature and are important for applications in optoelectronic and photonic devices [35, 71, 129, 398]. The nonlinear refractive index is proportional to the real part of the third-order nonlinear optical susceptibility (w(3)), and the TPA coefficient b as well as the TPA cross section d are proportional to the imaginary part of the third-order nonlinear optical susceptibility (w(3)) (Eq. (15)). Large w(3) values were reported for conjugated polymers/oligomers if a large conjugation length exists for the molecule.

182

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Sometimes it was claimed that w(3) changed as a function of the energy gap between the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital). However, the data summarized in Table 3.4 show that the dependencies of the energy gap between the OP and TP excited states and the third-order nonlinear optical susceptibility are rather more complex. Nevertheless, the energy gap between HOMO and LUMO is influenced by different contributions. This can be aromatic resonance, degree of bond length alternation, contributions of the inductive or mesomeric electronic effects of substituents grafted on the conjugated main chain, contributions of interchain interactions discussed in some cases for neat films, and contributions of geometry. The aromatic resonance and the degree of bond length alternation are characteristic of the conjugated backbone of the polymer. The nonlinear optical parameters for the polymer in solution and the polymer in the solid phase can be considerably different because of the difference in local-field factors, density differences between the polymer and the solvent, and different orientation of the polymer molecules in the liquid and the solid state. These aspects need careful consideration in the quantitative evaluation of TPA properties. Other authors give information about the second hyperpolarizability (g) examined, for example, by the z-scan technique [402]. In the case of cumulene-containing polymers, g was intensity independent [402]. The nonlinear refractive indexes show similar values for the conjugated polymers obtained by z-scan or degenerate four-wave mixing. A more detailed discussion is difficult because different conditions were used concerning the wavelength, the repetition rate of the pulses, and the energy of the laser beam. The z-scan measurements on thin polymer films are quite difficult because of the relatively high light intensities necessary to obtain a measurable signal on a short propagation path.

B. Dipolar Chromophores 1. Neutral Donor-p-Acceptor Compounds Both D-A substitution of unsaturated compounds and large M01 result in an increase of TPA according to Eq. (24) because d / (
m01M01)2. Thus, substitution of a polarizable p-system with appropriate donor and acceptor groups results in materials with intramolecular charge transfer (ICT) showing solvatochromism in distinct dielectric surroundings if the ICT formed radiatively deactivates [224]. Solvatochromism can indicate a large TPA in the case of dipolar chromophores as long as M01 is large [219, 224, 234, 337, 412–414]. The strength of ICT formation is tuned by the oxidation potential for the donor (Eox ) and the reduction potential (Ered ) for the acceptor group. The lower Eox and the higher Ered, the more efficient is ICT formation. TPA data of the neutral D-p-A chromophores 41–52 are compiled in Table 3.5 [88, 90, 224, 230, 234, 237, 415–431].

183

n ¼ 1, R1 ¼ NO2, R2-R5 ¼ H, R6-R7 ¼ CH3

n ¼ 2, R1 ¼ NO2, R2-R5 ¼ H, R6-R7 ¼ CH3 n ¼ 1, R1 ¼ SO2(CH2)6OH, R2-R5 ¼ H, R6-R7 ¼ Ph R1 ¼ NO2, R2 =H, R3 ¼ H

R1 ¼ NO2, R2 ¼ H, R3 ¼ CH2CH2OH, R4 ¼ C2H5

R1 ¼ NO2, R2 ¼ Cl, R3 ¼ CH2CH2OH, R4 ¼ C2H5 R1 ¼ NO2, R2 ¼ H, R3 ¼ R4 ¼ CH3 R1 ¼ NO2, R2 ¼ H, R3 ¼ R4 ¼ H

41g

41h 41i

42b

42c

42d 42e

42a

n ¼ 1, R1-R5 ¼ F, R6-R7 ¼ CH3 n ¼ 2, R1-R5 ¼ F, R6-R7 ¼ CH3 n ¼ 3, R1-R5 ¼ F, R6-R7 ¼ CH3 n ¼ 1, R1 ¼ CN, R2-R5 ¼ H, R6-R7 ¼ CH3 n ¼ 2, R1 ¼ CN, R2-R5 ¼ H, R6-R7 ¼ CH3 n ¼ 1, R1 ¼ NO2, R2-R5 ¼ H, R6-R7 ¼ C4H9

41a 41b 41c 41d 41e 41f

Substituents

15.5

17

38,000

33,100 26,800

19 16

19 20 12

41,000 28,000

26,900 44,000

6.1 6.4 7.6

32,500 45,100 52,100

1

750 825 850

180 1010 410 750 178 973 118 944

501 478

535

490

750 850

185 102

485 512

800

428 452

370 396 412 386

50 980 100 750 110 1000

744

895 920 88 1020 191 904

170

120 300 500

e
m01 lTP lOP max 1 (M cm ) (D) d(GM) (nm) (nm)

0.1

0.02 0.03 0.03 0.06 0.14

f

DMSO DMSO DMSO DMSO

DMSO THF

DMSO DMSO DMSO

DMSO THF CHCl3

THF THF THF

TOL TOL TOL

fs fs fs

z-Scan, z-Scan, z-Scan, z-Scan,

NLT NLT NLT NLT

z-Scan, NLT Kerr ellipsometry

z-Scan, NLT z-Scan, NLT z-Scan, NLT

TPEF

z-Scan

fs fs fs fs

fs

fs fs fs

ns

fs

[430] [430] [431] [431]

[430] [424]

[430] [430] [430]

[224] [224] [224] [429] [422] [424] [420] [237] [431] [422] [90]

Excitation Sourcec Reference

Kerr ellipsometry ps z-Scan, NLT fs

TPEF TPEF TPEF

Solventa Methodb

TABLE 3.5 Compilation of Extinction Coefficient (e), Change of State Dipole Moment (
l01 ), Maximum for Two-Photon Excitation (kTP), Two-Photon Absorption Cross Section (d) at kTP, Maximum for One-Photon Absorption (kOP max ), Fluorescence Quantum Yield (f), and the Light Source Used for Determination of TPA Data

184

R1 ¼ C2H5, R2 ¼ OP(OC2H5)2 R1 ¼ C2H5, R2 ¼ NO2 R1 ¼ C2H5 R1 ¼ C6H13 R1 ¼ C10H21

24,500

R1 ¼ C10H21; n ¼ 0

R1 ¼ C10H21; n ¼ 1

48b

13,300

48a

R1 ¼ C10H21

R1 ¼ C2H5 R1 ¼ C2H5

40,000

29

800 800 800 800 615

605 670 800 800 800 796

6710 800 8450 800 104 800 7940 800 131 800 1163 1064 (880)d 830 850 210 780 250 990 370 780 350 990

3920 2270 9770 30 820

650 1300 9700 10,600 11,600 22

0.9

0.9

f

492 0.45

370 386 389 383 345 500 408 0.39 411 0.04 497 0.46

378

403 389 392

383 414 388 389 390

lTP lOP e
m01 max (M1cm1) (D) d(GM) (nm) (nm)

15,000 36,700 50,000 10,200

44 45 46 47

43f 43g

43c R1 ¼ C2H5 43d R1 ¼ C2H5 43e1 R1 ¼ C2H5 R1 ¼ C10H21 43e2

43a1 43a2 43b1 43b2 43b3

(Continued)

Substituents

TABLE 3.5

THF Hexane THF THF THF THF THF THF TOL DMF TOL TOL TOL TOL

CH3CN CH3CN THF THF THF THF Hexane THF THF THF

TPEF TPEF TPEF TPEF

NLT NLT NLT NLT NLT NLT

WLC

NLT NLT NLT

WLC WLC NLT NLT NLT NLT

Solventa Methodb

ns ns ns ns

ns ns ns fs fs fs ns ns fs ns ns ns

fs fs ns ns ns fs

[416] [416] [415] [415] [234, 415] [418] [417] [230] [230] [230] [418] [88, 419] [417] [230] [230] [421] [234, 415] [415] [423] [426] [426] [425] [425] [425] [425]

Excitation Sourcec Reference

185

R1 ¼ CH3, R2 ¼ Mesityl R1 ¼ C2H5, R2 ¼ Mesityl R1 ¼ Ph, R2 ¼ Mesityl N(R1)2 ¼ N-carbazolyl, R2 ¼ Mesityl R1 ¼ CH3, R2 ¼ Mesityl R1 ¼ C2H5, R2 ¼ Mesityl R1 ¼ Ph, R2 ¼ Mesityl N(R1)2 ¼ N-carbazolyl, R2 ¼ Mesityl R2 ¼ Mesityl R ¼ C2H5 31,500

188 194 300 212 74 93 119 123 239 240 499

403 414 402 359 431 444 428 397 417 418

0.55 0.60 0.91 0.79 0.35 0.35 0.82 0.84 0.65 0.03

b

TOL, toluene; THF, tetrahydrofurane; DMSO, dimethylsulfoxide; CH3CN, acetonitrile; CHCl3, chloroform. TPEF, two-photon excited fluorescence; WLC, white light continuum; NLT, nonlinear transmission. c fs, femtosecond; ps, picosencond; ns, nanosecond. d From quantum chemical calculations.

a

49a 49b 49c 49d 50a 50b 50c 50d 51 52

THF THF THF THF THF THF THF THF THF CHCl3

TPEF TPEF TPEF TPEF TPEF TPEF TPEF TPEF TPEF TPEF

fs fs fs fs fs fs fs fs fs fs

[427] [427] [427] [427] [427] [427] [427] [427] [427] [428]

186

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Charge Separation

Excitation D

A

D

A ∆µ01

M 01

Solvatochromism

Figure 3.35. Sketch about the relationship between chromophore size and both transition dipole moment M01 and change of the state dipole moment
m01 .

TPA of dipolar chromophores depends on the same parameters tuning OPA. The optical S0 !S1 transition is therefore tunable by the same parameters (M01 and
m01 ), showing both OPA and TPA of dipolar chromophores. Size of the psystem and planarity of the conjugated system affect M01 . Planarity can be adjusted by a stiff planar molecular pattern resulting in optimal conditions for the conjugated p-system. Furthermore,
m01 depends on the distance of both photon-induced partial charges for donor and acceptor, respectively, and the amplitude of each induced partial charge. The higher the strength of donor and acceptor, the larger the size of the charges generated upon ICT (Fig. 3.35). However, the low fluorescence quantum yield f of many dipolar compounds diminishes the utility of such chromophores for TP applications based on fluorescence imaging [92–112]. f is important to scale the efficiency for TPEF Ztp f . This quantity is called TPA cross section action for fluorescence is a product of the intrinsic TPA cross section d and f (Eq. (55)). [224]. Ztp f Ztp f ¼ f d

ð55Þ

Donor–acceptor substituted polyenes (41) have been well investigated regarding ICT [219] and nonlinear optical properties [429, 432–438]. These materials show solvatochromism upon change of the dielectric surrounding [337]. Data compiled in Table 3.5 demonstrate the relationship between chromophore size, dipole moment change, and TPA. Examples 41a–c, which are equal to 1–3, show the feasibility of increasing d by enhancing the size of the p-system [224]. It appears from the data that the molecular extinction coefficient, which is related to M01 , has a greater impact on d than
m01 . The pentafluorobenzene moiety in 41a–c exhibits an interesting electron-deficient moiety having a similar electron deficiency compared to benzonitrile, as concluded from electrochemical data [341, 342, 439]. Introduction of fluorinated phenyl rings in unsaturated p-systems results in solvatochromism if an electron-rich unit is

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

187

additionally inserted at the chromophore [219, 224, 412, 413, 440]. This was first reported in 1999 [412]. Furthermore, the chemical reactivity of fluorine is small in comparison with many other electron-withdrawing substituents having nucleophilic properties. Therefore, chromophores with fluorinated moieties can be used in the manufacture of fluorescent probes because they possess a better chemical stability in materials having either electrophilic or nucleophilic properties [219, 337, 412, 413]. R3 R6 N R7

R2 R1

n R5

R4

41

Azobenzenes with D-A pattern (42) have been well investigated for NLO purposes [430, 431]. Thus, it is desirable to have TPA data for these chromophores. Examples 42a–c show an increase of d if strength of the donor increases by comparison of 42a and 42b (Table 3.5). Inserting an additional electron-withdrawing substituent at the azobenzene chromophore results in a further increase of d due the increased
m01 of 42c. Furthermore, a higher TP excited state was observed for the general azobenzene 42 as shown for the TP excitation data measured at 750 nm. The peak observed at around 1000 nm corresponds to twice the wavelength needed for OPA. This supports the observations made for 41 that S1 is almost accessible by both OP and TP excitation. Moreover, TPA of 42b is smaller in comparison with the corresponding stilbene 41g (Table 3.5). These studies are complementary to 42d–e (Table 3.5) [431]. R4 N R3

N

R1

N R2

42

Fluorenes with D-A substitution (43) are important in TP photosciences as well. Thus, change of strength for the electron-withdrawing group affects TPA if a diphenylamino group is used as the donor. The differences measured for TPA between 43a1 and 43a2 (Table 3.5) are mainly caused by the different strengths of the acceptor group R2. The nitro group, according to Hammett’s constant (sp ¼ 0:78; sm ¼ 0:71½441), is a stronger acceptor in comparison to the phosphonate (sp ¼ 0:6; sm ¼ 0:55½441). Furthermore, TPA of dipolar compounds can be tuned by the introduction of heterocyclic moieties with electron-withdrawing (A) properties resulting in 43b–f [230, 415, 417–419, 421]. For this purpose, fluorene chromophores bearing a distinct D-p-A substitution pattern were investigated for TPA. Although 43b–f comprise different acceptor units with distinct acceptor strengths, there is no

188

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

R1

R1

N

R

N

N R1

R:

N N

43a

43b

43c

O 43d

N N

N

S O 43e

43f

43g

clear correlation between d and acceptor strength as concluded from Hammett’s substitution constants [442]. Furthermore, nonlinear transmission was applied to explore TPA of 43b–f. This may cause an extremely large value for the absorption because the excited state populated by TPA absorbs additional intensity during the lifetime of the pulse that is not needed for TPA (compare Section II.D). Thus, one observes a higher value for the absorption of excitation intensity than needed for TPA. This becomes clearer by considering the data measured by using a fs laser. These values may become smaller compared to data measured by a ns laser [418]. Therefore, we recommend comparing only those data obtained by a similar physical method. This was confirmed by an additional study in which results of several methods were compared [443]. Fluorescence of 43b3 (Table 3.5) was investigated in solvents of different solvent polarity [234]. A significant shift to lower emission energy was reported upon increase of the solvent polarity (Fig. 3.36). The energy of the fluorescence decreases by 0.7–0.8 eV upon change of the surrounding from hexane to ethanol. Treatment of the solvatochromic data according to Eq. (25b) results in a straight line. However, authors reported this equation did not work when comparing the larger chromophore 44 bearing the same substituents as 43b3 but exhibiting a significantly larger chromophore due to incorporation of two fluorene units. Not being able to use Eq. (25b) to compare solvatochromic data for 44 and 43b3 can be seen in the Onsager model [336], which may fail for long molecules [444]. This theory requires point dipoles. However, the structure of these large

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

189

Figure 3.36. Solvent-dependent fluorescence of 43b3 in various solvents. (From Ref. [234] with permission of the American Chemical Society.)

molecules is far away from a point dipole, showing reduced reliability for application of this model.

N N 44

Chromophore 45 bears a thiophene moiety instead of the diphenylamino group. The thiophene moiety is known to have electron-donating properties as well. TP experiments show that this group does not have the desired impact on TPA. Data observed for 45 are significantly smaller in comparison with 43b. Although the latter bears an additional polarizable double bond, we believe that the higher d of 43b is caused by the diphenylamino group. R1

R1

N S

45

190

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Chromophore 46 can be considered a nitrobenzene bearing in the para position a substituted pyrazoline moiety. TPA of this chromophore is large and presumably caused by the strong acceptor group and the electron-donating pyrazoline unit resulting finally in a large
m01 .

N N

NO2

46

The TP chromophore 47 was investigated from a theoretical point of view [445]. The time-dependent DFT/B3LYP method resulted in the transition dipole moment M01 , excitation energy, dipole moment for the ground state, and dipole moment for the excited state needed for calculation of TPA. Furthermore, by using a finite field approach on the excitation energy, one can get a direct measure of the dipole moment change between the ground and excited state S1 (
m01 ). d was calculated from these data and the theoretical solvatochromic studies showed a dependence on the dielectric surrounding. Calculations screened an excess of electron density at the donor. This implies that 47 is able to donate an electron into the surrounding; this is essential to function as a TP photoinitiator. Furthermore, the quantum chemical calculations of optical properties were also confirmed by OPA experiments [426]. Solvatochromic studies in different solvents showed a huge change in
m01 (29 D). Nearly full charge separation over the long distance of the chromophore causes this huge change of
m01 . The fluorescence quantum yields in even polar solvents are significantly large compared with other dipolar chromophores compiled in Table 3.5. The TPA cross section was equal to 880 GM. The highly delocalized p-system and the separated charges are the main factors initiating TP polymerization by TP excitation. C4H9 N C4H9 N 47

D-A chromophores bearing triple bonds were investigated to understand the relationship between chromophore size and both nonlinear absorption [425] and nonlinear hyperpolarizablity [446]. In particular, 48 [425] comprises a chromophore bearing at one side a dialkylamino group as donor and at the opposite side

191

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

a cyano group, connected through an alternating anthryl-ynyl p-bridge. TPA data were larger compared to 41a, due to the larger size of the p-system affecting in particular the transition dipole moment M01 . Several TP excited states were reported for 48. One TP excited state is S1, exhibiting similar excitation energy as that needed for OP excitation. A further TP excited state exhibiting a larger d at higher TP excitation energy was reported for this chromophore as well.

R1 N

CN

R1 n

48

Synthesis, structure, and fluorescence properties were disclosed for a series of D-p-A compounds bearing trivalent boron as the electron-deficient group as the acceptor on one side, and distinct donor moieties on the other side of the chromophore [427, 447]. Structure 49 is one possibility. Incorporation of the dimesityl boron unit results in an increase in the strength of the electron-withdrawing part. The acceptor strength of dimesityl boron is located between a cyano group and a nitro group, which was concluded from spectroscopic measurements [448]. These organic boron compounds are stable and exhibit intense fluorescence, large TPEF cross-section action, and huge red-shift of fluorescence upon increase of solvent polarity (Fig. 3.37). The spectral shift of the emission was about 100 nm by switching the surrounding from a nonpolar (toluene) to a polar surrounding (acetonitrile). R2 R1 N R1

B R2 49

As mentioned earlier, the B(mesityl)2 group causes the strong bathochromic shift. Intramolecular charge transfer from donor to acceptor greatly enhances the dipole moment of the excited state. Thus, change of the dipole moment with respect to the ground state is large, as documented by the large slopes of the Lippert–Mataga equation (Eq. (25b)). Both 49a and 49c (Table 3.5) exhibit a slope of about 13,000 cm1 [427], which is again large for a substituted stilbene [337]. Thus, the huge
m01 obtained from solvatochromic experiments also explains the large d observed for 49a–d, showing the different strengths of the donating groups according to the relationship in Eq. (24). The Hammett substituent constant sH is a useful parameter to obtain an estimate of the electronwithdrawing ability of an acceptor group. Based on a spectroscopic method, a

192

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

0.5

580

49a

401

0.4 0.3

538

403

0.2 403

Toluene THF CH3CN

592

0.1

Absorbance or Intensity (a.u.)

0.0

0.5

476 402 404

522

0.4 574

399

0.3

49c Toluene THF CH3CN

0.2 0.1 0.0 300

400

500

600

700

800

λ/nm

Figure 3.37. Linear absorption spectra and fluorescence spectra of 49a and 49c in three solvents. (From Ref. [427] with permission of Wiley-VCH.)

value of sH ¼ 0:65 was spectroscopically determined for a dimesitylboryl group [448]. Absorption spectra obtained in this study show that the dimesityl-boron group has an acceptor strength between the cyano and the nitro group as shown in the following series: Br < CN < B(Mes)2 < NO2. Replacement of the phenyl group by a thiophene unit results in structure 50. These chromophores exhibit less TPA amplitude compared to the similar structures of 49. d of 50 decreases to about half that for comparable compounds 49, as shown by the data in Table 3.5. Thus, incorporation of thiophene in the p-bridge results in less efficient chromophores compared to corresponding compounds bearing stilbene in the main molecular skeleton. These findings agree well with the results disclosed for 45.

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

193

R2

S B

R1 N R1

R2 50

However, d again increases if a second thiophene is placed at the opposite side, resulting in 51. TPA of this chromophore is comparable to that of 49. Thus, the thiophene unit at the left side is an appropriate donor in this series. It functions similarly as the anilino group in 49. R2

S B

R2 S

51

TP chromophore 52 bears a triazine as electron-withdrawing moiety, which is also known as a strong acceptor [449, 450]. This is concluded from electrochemical measurements [451]. d is comparable to the TPA of 41f, 49a, and 49b bearing strong electron-withdrawing groups. R R

N N N

N 52

2. Ionic Donor-p-Acceptor Compounds Compounds with the general structure 53 have been well investigated with OPA. They are well known as chargeshift probes [452–458] in which the charge of the chromophore moves from one side to the opposite, resulting in a change of the dipole moment. This can be seen by a blue shift of fluorescence upon increase of solvent polarity [337], which is called negative solvatochromism [335], in contrast to the neutral fluorophores of the previous section. In other words, the ground state possesses a fairly large dipole moment, while upon excitation the dipole moment of the excited state decreases with respect to the S0 state as a result of charge shift. Photonic data of the TP chromophores 53–56 are compiled in Table 3.6 [122, 156, 160, 161, 172, 173, 176, 229, 370, 459–462]. R2

N R1 N

R3

53

194

Substituents

R1: CH3 ; R2 CH3; R3: C2H4OH

R1: CH3; NR2R3: pyrrolidinyl

R1: CH3; R2 C2H5; R3: C2H4OH R1: CH3; R2 Ph; R3: Ph X ¼ S; R1 ¼ n-butyl, R2 ¼ CH3, R3 ¼ C2H4OH

Compound

53a

53b

53c 53d 54a

43,500d

1

12

3.4

8

10

e
m01 (M cm1) (D)

1064 920 1064 1064 930 1064 1064 1064 1064

1064 1064 1064 1064

1300 1010

700 1010 47000 312 930 570 470 880 920

600 730 540 540

d lTP (GM) (nm) f

530

484

482 482

0.01

478 0.009 0.007

474

lOP max (nm)

NLT NLT

NLT NLT NLT NLT

Methodb

DMF

NLT

Epoxy matrix DMF NLT DMF NLT DMF NLT DMF NLT DMF NLT DMF NLT

DMF BzOH HEMA P-HEMA CH3CN EtOH DMF

Solventa

ps

ps ps ns ps ps ps ns ps ps

ps ns ns ns

[460]

[161] [160] [160] [160] [173] [176] [229] [460] [156] [459] [459] [229] [172] [229] [122]

Excitation Sourcec Reference

TABLE 3.6 Compilation of Extinction Coefficient (e), Change of State Dipole Moment (
l01 ), Two-Photon Absorption Cross Section (d), Maximum for Two-Photon Excitation (kTP), Maximum for One-Photon Excitation (kOP max ), Fluorescence Quantum Yield (f), and the Light Source Used for Determination of TPA Data for a Series of Dipolar Chromophores with the General Structures Taken in Various Solvents Using Different Methods

195

X ¼ S; R1 ¼ CH3, R2 ¼ C2H5, R3 ¼ C2H4OH R1: CH3; R2 C2H5; R3: C2H4OH R1: CH3; R2 Ph; R3: Ph 51,400

11 12

880 1064 1090 1064 119 790

1064

559 0.004

531

DMF DMF DMSO

DMF

NLT NLT NLT

NLT

ps ps fs

ps

[229] [229] [370]

[467]

a DMF, dimethylformamide; BzOH, benzylalkohol; HEMA, hydroxyethylmethacrylate; P-HEMA, poly(hydroxyethylmethacrylate); EtOH, ethanol; DMSO, dimethylsulfoxide; CH3CN, acetonitile. b TPEF, two-photon excited fluorescence; WLC, white light continuum; NLT, nonlinear transmission. c fs, femtosecond; ps, picosecond; ns, nanosecond. d Dependent on gegen ion.

55a 55b 56

54b

196

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Charge Shift

R2 N R3

N R1

S0

hν

* R2 N R3

N R1

R2 N R3

N R1

S1

Figure 3.38. Change of bond order upon charge shift for the probe 53.

A general sketch in Figure 3.38 shows the change of bond order upon charge shift. Thus, a single bond becomes a double bond and a double bond returns to a single bond, agreeing well with the model of bond length alternation discussed in Section II.A.4. The structure on the left side is similar to a stilbenoid pattern while the structure of S1 exhibits a chinoid-like pattern. Interestingly, trans–cis photoisomerization of 53 is of minor importance [214, 412, 463]. Thus, these chromophores are useful as fluorescent probes [452–458, 463, 464]. TPA of 53, compiled in Table 3. 6 for a series of different substituents, is larger compared to that of the neutral stilbene 41a (Table 3.5). This may be caused by the mechanism depicted in Figure 3.38, showing the acceptance of charge by an electron-withdrawing group. The pyridinium ring in 53 is a better electron acceptor than pentafluorobenzene 41a according to the higher reduction potential (Ered (pyridinium) ¼ 1.4 V [465]; Ered (pentafluorobenzene) ¼ 2.4 V [342]). Therefore, d of 53a must be significantly larger compared to that of 41a [161–163, 172, 173, 176, 459, 460]. The easier shift of the charge can be seen as one main reason for the higher TPA of 53 in comparison with neutral D-A chromophores. The boron-bearing chromophore 49 contains a strong electron-withdrawing group and the solvatochromic response is even stronger compared to 53. Hence, a large
m01 is a prerequisite to obtain large d values. This was concluded from solvatochromic studies in which 53 exhibited a smaller response [337]. Thus, the capability to shift the charge within the molecular skeleton is important to obtain large d values according to Eq. (24). This occurs presumably with higher efficiency in the charged chromophore, resulting in a high twophoton absorbing material, which is receiving increased interest for use as two-photon pumped lasing material [126, 160–163, 165–168, 171–173, 176]. Similar results were also obtained for 54 and 55, which bear the benzothiazol and quinolinium group as unsaturated moiety. Ered of these units is about the same order as that of pyridinium [466]. According to Eq. (24), d is proportional to the square of M01 and
m01 , respectively. Chromophores compiled in Table 3.6 exhibit significantly larger
m01 values that may explain the larger TPA in comparison with 41a. However, the small fluorescence quantum yield of the ionic chromophores compiled in Table 3.6 diminishes the TPA cross section action for fluorescence (Ztp f ), which

197

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

is important for imaging applications. The dependence of Ztp f on f was shown by Eq. (55). R1 N R2 N R3

N R1

R2 N R3

X

55

54

Recently, chromophore 56 was the subject of TPA studies (Table 3.6) [370]. The TPA cross section is smaller in comparison to that of 53–55 although d for 56 was not taken at the peak maximum of the TPA spectrum. The maximum for TPA is far away from the data reported [370]. Data obtained are larger compared with 43b, which was extensively investigated in TPA studies. This shows again that dipolar ionic chromophores are more appropriate for TPA applications because of the larger d. The fluorescence quantum yield is also low and comparable with that of 56. H3C

N

CH3

N

N

S N

S

56

C. Symmetric Chromophores with Large Two-Photon Absorptivities 1. Donor-p-Donor Compounds TPA of symmetric donor-p-donor compounds is significantly larger compared to chromophores having a similar size of the p-system with no donor groups at the ends. This becomes more clear by comparison of stilbene (14) with the donor substituted 4,40 -amino-stilbenes 57a and 57b (Table 3.7). Thus, the end capped amino groups result in a better spreading of excitation density from the ends to the chromophore center. This is schematically shown in Figure 3.39. Furthermore, extension of the p-system results in an increase of the transition dipole moment M01 as shown for the polyenes 57b–g (Table 3.7). TPA of all symmetric compounds discussed in this section can be described with the mechanism depicted in Figure 3.2a showing that the TP excited state is energetically higher compared to the S1 state. Thus,

198 65,000 81,000 96,000 103,000 47,000

R1 ¼ C4H9, n ¼ 1

R1 ¼ CH3, n ¼ 2

R1 ¼ CH3, n ¼ 3

R1 ¼ CH3, n ¼ 4

R1 ¼ CH3, n ¼ 5 R1 ¼ CH3, n ¼ 6 N(R1)2 ¼ N-carbazolyl, n ¼ 1 N(R1)2 ¼ N-carbazolyl, n ¼ 3 R1 ¼ C4H9, R2 ¼ H, n ¼ 1

R1 ¼ C6H13, R2 ¼ H, n ¼ 1 R1 ¼ C6H13, R2 ¼ H, n ¼ 3

R1 ¼ C4H9, R2 ¼ CH3O, n ¼ 1 R1 ¼ C4H9, R2 ¼ CH3O, n ¼ 3

57b

57c

57d

57e

57f 57g 57h 57i 58a

58b 58c

58b 58c

67,000 111,000

74,000 60,000 82,800 127,000

48,000 47,000 46,000

R1 ¼ Phenyl, n ¼1

57a

Structure Substitution 75 190 240 200 230 260 340 320 410 425 1300 190 250 950 995 900 870 1450 1230 900 1420

e d (M1cm1) (GM) 720 690 605 600 640 640 695 710 695 730 730 690 640 695 730 <740 720 810 720 730 840

lTP max (nm)

429 468

449 387 348 388 409 419 412 435

430

412

390

389 387 374

lOP max (nm)

10.9

9.4

7.7

8.1

0.88 10.5 0.65 14.2

0.89 11.3 0.92 14.7

12.4 0.023 12.8 0.87 8.1 0.67 0.68 0.88 10.7

0.42

0.76

0.8

0.78 0.87 0.9

f

10.5 TOL 11.0 TOL

TPEF, ns TPEF, ns

[132] [132]

TPEF, fs [447] TPEF, ps [132] TPEF, ps [132] TPEF, ns [132] TPEF, ps [132] TPEF, ns [132] TPEF, ps [132] TPEF, ns [132] TPEF, ps [132] TPEF, ns [132] TPEF, ns [132] TPEF, ps [132] TPEF, ps [476] TPEF, ps [476] TPEF, ns [132] TPEF, fs [469] TPEF, fsþns [474] TPEF, fsþns [474]

Method and Excitation Solventa Sourceb Reference

THF 6.7 TOL TOL 5.8 TOL TOL 5.7 TOL TOL 6.2 TOL TOL 6.0 TOL 9.2 TOL 6.8 TOL TOL TOL 12.1 TOL DMSO 11.0 TOL 15.9 TOL

M01 M12 (D) (D)

TABLE 3.7 Compilation of Extinction Coefficient (e), TPA Cross Section (d), Maximum for TP Excitation (kTP max ), Maximum for ), Fluorescence Quantum Yield ( ), Transition Dipole Moment Between States S and the S (M ), Transition Dipole OPA (kOP 01 f 0 1 max Moment Between S1 State and the Lowest TP Excited State (M12 ), and the Method Used for Determination of TPA. Data for a Series of Chromophores with the General Structures 57–79 in Different Solvents

199

R1 ¼ C8H17, R2 ¼ —CH2— CH2—, n ¼ 1 R1 ¼ C6H13,, R2 ¼ without, n ¼ 1, R1 ¼ C6H13,, R2 ¼ —CH2— CH2—, n ¼ 2 R1 ¼ C8H17, R2 ¼ — CH2 — CH2 — R1 ¼ C6H13 R1 ¼ C6H13 R1 ¼ C6H13, R2 ¼ C11H23 R1 ¼ C6H13, R2 ¼ C11H23 R1 ¼ C6H13, R2 ¼ C11H23 R1 ¼ C6H13, R2 ¼ C11H23 R1 ¼ C10H21

60b

68a

62 63 64a 64b 65 66 67

61

60c 60d

59 60a

R1 ¼ C10H21, n ¼ 1

N(R1)2: N-carbazolyl, R2 ¼ H, n¼1 R1 ¼ C4H9, R2 ¼ CH3O, n ¼ 1 R1 ¼ C4H9, R2 ¼ —CH2— CH2—, n ¼ 1

58g

58f

R1 ¼ Phenyl, R2 ¼ H, n ¼ 1 N(R1)2: —N(Ph)(m-Tolyl), R2 ¼ H, n ¼ 1 N(R1)2: —N(Ph)(m-Tolyl), R2 ¼ CH3O, n ¼ 1

58d 58e

166,000 135,000 83,000 110,000 129,000 125,000

600 600

610 610

1000 240 4100 710

740 740

700 500

411

400 406 374 381 387 411 390

422

421 410

3760

750

740

454 421

428

1300 1730

67,000

775 765

378

426

412 411

950 3310

1250 1200

690

1050

93,000

745

855

65,000

745 745

920 805

82,700 63,000

0.63 1

0.8 0.5 0.87 0.82 0.82 0.61 0.4

0.66

0.84 0.6

0.86 0.86

0.12 12.8 0.77

0.68

0.97 10.8

0.68 0.93 10.0

TPEF, fs

TPEF, fs TPEF, fs

TPEF, fs TPEF, fs

TPEF, ns TPEF, fs

TPEF, ps

TPEF, ns

TPEF, ns TPEF, ns

TOL TPEF, fs TOL TPEF, fs TOL TOL TOL TOL Hexane WLC, fs Hexane TPEF, fs CHEXON Hexane WLC, fs Hexane TPEF, fs

CHCl3

TOL TOL

TOL TOL

10.2 TOL DMSO

TOL

10.6 TOL

THF 12.0 TOL

[470] [470] [470] [470] [470] [470] [473] [473] [473] [473] [473]

[83]

[472] [83]

[469] [83]

[132] [475]

[476]

[132]

[447] [132]

200

(Continued)

45,000 64,200 73,000 50,600 52,000 42,000 92,300 108,000 108,000 66,000 104,000 192,000 238,000

R1 ¼ C10H21; n ¼ 12

R1 ¼ C2H5 R1 ¼ C2H5 R1 ¼ Ph R1 ¼ Ph

R1 ¼ C4H9

R1 ¼ p-CH3O-Ph R1 ¼ C10H21 R1 ¼ p-CH3O-Ph R1 ¼ p-Cl-Ph R1 ¼ C10H21, n ¼ 1 R1 ¼ C10H21, n ¼ 2 R1 ¼ C10H21, n ¼ 3 R1 ¼ p-CH3O-Ph R1 ¼ C6H13, n ¼ 1 R1 ¼ C6H13, n ¼ 2

R1 ¼ C6H13, n ¼ 3

69 70 71 72a

72b

73 74 75a 75b 75c 76a 76b 76c 77 78a 78b

78c

17,200 6800 1050 335 1190 1990 270 530 670 65 980 540 400 550 720 760 820 280 1410 2210 3430 4910 3890

e d (M1cm1) (GM)

68b

Structure Substitution

TABLE 3.7

600 600 810 810 810 810 796 795 795 790 712 780 822 809 800 800 780 728 720 720 820 720 830

lTP max (nm)

454

465 465 457 389 510 491 474 508 510 507 370 439 457

416

lOP max (nm)

12.4 7.2 8.4

7.1 9.5 7.8 14.6 9.1 16.2

11.4 9.7 9.4

10.4 11.3 20.7 0.75 23.3

0.77 0.54 0.93 1 0.49 0.55 0.49 0.73 0.86 0.87

0.63 0.63

0.47

1

f

M01 M12 (D) (D) Hexane Hexane TCE TCE TCE TCE TCE TOL TOL THF TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL TOL

WLC, fs [473] TPEF, fs [473] NLT, ns [468] NLT, ns [468] NLT, ns [468] NLT, ns [468] NLT, ns [468] TPEF, fs [475] TPEF, fs [469] NLA, fs [471] TPEF, fs [478] TPEF, ns [425] TPEF, fs [478] TPEF, fs [478] TPEF, ns [425] TPEF, ns [425] TPEF, ns [425] TPEF, fs [478] TPEF, fsþns [474] TPEF, fsþns [474] TPEF, fsþns [474] TPEF, fsþns [474] TPEF, fsþns [474]

Method and Excitation Solventa Sourceb Reference

201

b

a

X ¼ Cl X ¼ Nþ(CH3)3I X ¼ Cl X ¼ Nþ(CH3)3I X ¼ Cl X ¼ Nþ(CH3)3I 1290 370 2080 690 1690 700

725 725 770 750 700 700

434 435 441 431 420 410

0.92 0.04 0.92 0.52 0.95 0.42

TOL H2O TOL H2O TOL H2O

TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, fs

[477] [477] [477] [477] [477] [477]

THF, tetrahydrofurane; TOL, toluene; DMSO, dimethylsulfoxide; CHCl3, chloroform; CHEXON, cyclohexanone; TCE, tetrachloroethane. TPEF, two-photon excited fluorescence; NLT, nonlinear transmission; WLC, white light continuum; fs, femtosecond; ns, nanosecond; ps, picosecond.

79a1 79a2 79b1 79b2 79c1 79c2

202

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

D

D

D

D

Figure 3.39. Sketch for spreading of excitation density for symmetric D-p-D compounds.

Eq. (27) represents the relationship between d and the transition dipole moments M01 and M12 , respectively. In particular, M12 , representing the electronic coupling between the energetically higher TP excited state and the lowest excited singlet state, is important for the effective population of the OP photoactive S1 state. No electronic coupling would result in deactivation of the TP excited state by the release of two photons located in the red/NIR spectral range. Data of some representative D-p-D chromophores are collected in Table 3.7 [83, 132, 425, 447, 468–478]. R1 R1 N R1

N n

R1 57

Increase of the p-system is also related to an increase of the molar extinction coefficient (Table 3.7). Because the molar extinction coefficient is related to the square of M01 , d increases as long as the size of the chromophore increases, as shown for 57b–f. Increase of the p-system is one key point achieving large d values according to Eq. (27). Another important parameter tuning d is the transition dipole moment M12 , representing electronic coupling between the OP allowed S1 state and the energetically higher TP excited state, similar to 57b–g. Moreover, the lower e and M01 of 57g may be caused by a stronger distortion of the planarity of the polyene chain resulting in a decrease of e and M01 , respectively. Furthermore, carbazole was also found as a functioning donor group in 57h and 57i (Table 3.7). The TPA amplitude of both chromophores is comparable to that of 57b and 57d, respectively. Distyrylbenzenes with the general structure 58 have become one of the most widely used compounds for applications in TP photosciences. The TPA of these chromophores is higher compared to that of donor substituted polyenes 57. Thus, the (phenylene-vinylene) moiety is presumably a better p-bridge to increase d compared to the diphenylpolyene in 57. Increase of the p-bridge in 58 has an impact on M01 and M12 , respectively, resulting in an increase of d because for such chromophores d is proportional to (M01
M12 )2, Eq. (27). Furthermore, substitution of stilbene and distyrylbenzene by amino groups at the chromophore ends results in a three- to fourfold increase of M12 while

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

203

only slight changes were reported for M01. R2

R1 N R1

R1 N R1

n R2

58

By comparison, substitution by methoxy groups at the chromophore center does not significantly affect TPA as shown for the data obtained for 58a and 58c (Table 3.7). Quantum chemical calculations show that the transition dipole moments possess the largest value along the molecular axis. An increase in the size of the connecting polyene chain in 59 has a positive impact on d. The larger TPA is caused by the increased M01 in comparison with 58b (Table 3.7) while M12 shows no significant changes upon increase of the chromophore size from 58b to that of 59. R1 N

R2

R1 R1 N R1

R2 59

Additional molecular engineering studies were carried out focusing on the general pattern of 60. This chromophore possesses a biphenyl hinge in the center. The connecting bridge R2 keeps the geometry at a nearly planar level. An increase in the size of the conjugated system results in a doubling of d by comparison of 60b with 60d (Table 3.7). However, TP absorptivity approaches a limit when compared to the large p-system of 61. Thus, d approaches a certain level depending on the nature of the conjugated system, showing agreement with theoretical calculations between chromophore size and TPA [236, 479]. Although calculations clearly show that TPA is significantly below the theoretical limit, the procedure used for calculation of d allows one to draw guidelines for further molecular design strategies of TP chromophores [479]. This is supported by TDDFT calculations, demonstrating the suitability of this theoretical tool to obtain reliable calculations for TPA of large organic TP chromophores, as already shown in Figure 3.3 [236]. R2 R1 N R1

R1 N n

n

R1

60

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

R1

R1 N

N

R1

R1 61

Studying the relationship between the length and topology of the conjugated connectors resulted in the TP fluorophores shown in Figure 3.40. This allows modification of their photophysical properties. Biphenyl or fluorene was chosen as the central core. Because the fluorene moiety comprises a stiff structure, replacing it with a more flexible biphenyl connector should affect photonic properties as well. Both phenyl rings are twisted in the biphenyl hinge, resulting in distortion of planarity for the entire chromophore. However, an almost planar p-skeleton is one prerequisite to achieve large transition dipole moments. Furthermore, conjugated rods built from phenylene–ethynylene and/or

R

D

rigid or semi-rigid conjugated linker

R

conjugated core

rigid or semi-rigid conjugated linker

D

Figure 3.40. Molecular engineering of D-p-D fluorophores for TP excited fluorescence. (Adapted from Ref. [470].)

205

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

phenylene–vinylene oligomers were incorporated to obtain a chromophore with strong fluorescence, with the idea of modulating the electronic coupling between the donor group at the end and at the center of the chromophore. Thus, this study was carried out to understand the function of the core or the distinct rod moieties on the amplitude of TPA. Long alkyl chains could be added at the end groups and the central block in order to obtain highly soluble derivatives. For this purpose, 62–66 are interesting TP chromophores because variation of the core in 63 and 66 and changing the nature of the conjugated connectors by replacing the double bond by a triple bond in 62 and 63 and vice versa results in a drawback about the function of double and triple bonds in such conjugated systems. In addition, incorporation of the triple bond in both 64 and 65 and comparison of data with 60 gives information about how these structural changes affect TPA. R1

R1

N

N

R1

62 R1

R1

R1

N

N

R1

63 R1

R1

R1

N

N

R1

n

n

64 R1

R1

R1

N

N

R1

65 R1

R2

R2

R1

R1

N R1

N R2

R2

66 R1

Comparing data in Table 3.7, 62–66 exhibit intense linear absorption in the UV and blue spectral region. They are also highly transparent in the visible and NIR region. In addition, these fluorophores are highly fluorescent because f ranges between 0.50 and 0.87 (Table 3.7). Change of the biphenyl by a fluorene moiety in 65 and 66 yields little red-shift of absorption, indicating the improved electronic conjugation of those bearing fluorene as a central core due to increased planarity. However, change of the molecular pattern of the connecting

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

rods results in a change of both absorption and emission. Furthermore, incorporation of triple bonds in the conjugated rods does not result in a decrease of f. However, replacing a triple bond by a double bond affects photonic properties depending on location in the conjugated rods. A double bond does not significantly change f when placed as an adjacent unit to the central core, while a decrease of f was observed if the double bond was placed close to the end group. Solvent polarity also allows tuning of the luminescence properties. A pronounced positive solvatochromism observed for the emission supports the idea of ICT formation in these huge fluorophores 62–66 in which the core functions as a slight acceptor. Therefore, these chromophores may be considered as quadrupolar TP chromophores from a broad point of view. Such a class of chromophores is discussed in detail in Section III.C.3. TP excited fluorescence studies of 62 and 63 exhibit broad bands for TP excitation, which is comparable to fluorescein, which has become a standard in TP imaging applications. The TPA cross section is 950 and 1050 GM for the fluorophores 62 and 63 at 740 nm. These values are about 35 times larger than the TP standard fluorescein (Fig. 3.41). In addition, incorporation of triple bonds instead of double bonds improves photostability as indicated by the absence of photodegradation for 64 and 65. This is important for applications needing a broad tunable range and satisfied photostability. These derivatives are promising for future TP developments regarding the design of new molecular probes for biological nonlinear microscopy as well

Figure 3.41. Two-photon excited fluorescence spectra of 63 (&) and 62 (~) calibrated using fluorescein (12) (dotted line). (From Ref. [470] with permission of the American Chemical Society.)

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

207

as applications based on optical power limitation or upconverted blue emitting luminescent displays. The linear, symmetrical, diphenylaminofluorene-based chromophores 67 and 68 were investigated for their TPA properties. Chromophore 67 is a low molecular weight compound, while 68 can be either an oligomer or a polymer. TPA spectra were determined by nonlinear transmission using a femtosecond whitelight continuum (WLC) and two-photon excited fluorescence (TPEF). Based on TPEF, a d of 240 GM was determined for the low molecular weight compound 67. Significantly higher TPA with 710 GM at 600 nm was observed for oligomer 68b incorporating 14 aminofluorene units with respect to 68a, and having a polymerization degree of about unity (Table 3.7). Thus, TPA of 68b is about ten times larger compared to that of 68a and 30 times larger compared to the TPA of 67, which has no additional fluorene bridge at the ends. R1

R1

N

N

67 R1

R1

N

N

R1

n

R1

R1

R1

HN

NH 68

A further class of TP chromophores are 69–72 [468, 469, 475], bearing a dithienothiophene unit as the core. Such groups have been known to increase the efficiency of charge transfer. They should therefore affect TPA as well. Moreover, these materials differ regarding the nature of the connecting bridge between donor and conjugated core in the middle. By comparison, the largest TPA in this series was obtained for 72, which has a phenylene-vinylene connector on both sides. Less TPA was reported for both 69 bearing carbazole on both ends as donor and 71 containing the oxadiazole as donor at one side but phenylene-vinylene as connector on both sides. The lowest value in this series exists

208

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

for 70. Comparing the TPA of 72b and 58a shows that the unsaturated distyrylbenzene pattern in 58a is more appropriate to increase d as the unsaturated psystem in 72b (Table 3.7). S S

S

R1 N

N R1

69 S S

S O

R1 N N

N

70 S S R1

S O

N N

R1

N

71 S S R1

S N

N

R1

R1

R1 72

Low TPA was observed for 73, which has a thiophene-vinylene-thiophene core at the center and phenylene-vinylene hinge between the core and the end capped diphenylamino donor group. Thiophene is an electron-rich heterocycle and it is therefore an electron-donating group. Thus, a bigger amplitude for TPA was expected for this chromophore. Experiments, however, demonstrated a lower TPA ampliude. Therefore, incorporation of such electron-rich moieties

209

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

diminishes TPA. This agrees with data of 72b, which has a smaller TPA compared to 58a.

N

S S N

73

TP excitation spectra of the triple bond bearing chromophores 74 and 75 show large TPA in the spectral range between 700 and 960 nm (Fig. 3.42) [478]. They bear a distinct central core, which is a xylene moiety in the case of 74. Chromophore 75 bears an anthracene unit as the central core. Furthermore, 75a contains a slightly stronger donor compared to 74 and 75b–c, resulting in a slight enhancement of d in the case of 75a (Table 3.7). This is consistent with data obtained for distyrylbenzenes having amino groups at the ends. Compound 58a exhibits a slightly larger TPA compared to 58b, supporting

40 400 20

40 d / GM

600

74

e / M–1 cm–1

200

0

30

75b

250 200

20

150

10

100

0 360

400

440

350

480

400

450

λ / nm 50×103

550

600

OPA 500 TPA 400

40 e / M–1 cm–1

500

λ / nm

30

75c

300

20

200

10

100

350

400

450

500

550

d / GM

cm–1 –1

e/M

60

400 OPA TPA 350 300

50×103

d / GM

1000 OPA TPA 800

80×103

600

λ / nm

Figure 3.42. OPA and TPA of 74 and 75b–c in toluene; left axis, OPA (----); right axis, TPA (74, (; 75b, &; 75c, !). (Adapted from Ref [478].)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

the results obtained for 75a and 75b, respectively. CH3 R1 N R1

R1 N R1 H3C

74

R1 N R1

R1 N R1

75

TPA cross section of the triple bond bearing chromophore 74 is similar compared to the distyrylbenzene chromophore 58b. Although the structural pattern connecting the end capped triphenylamine structures and the central p-unit is slightly different, the similar TPA cross section observed for both chromophores questions whether BLA is an important factor controlling TPA in 74. This result addresses the question of whether or not bond length alternation mainly explains changes of d. It turns out that d of the double-bond chromophore 58b or the triple-bond compound 72 does not significantly change. Hence, excitation density can easily travel through the p-chain no matter whether double or triple bonds were incorporated in the unsaturated skeleton. If bond length alternation is important for triple bonds, TPA should be significantly different compared to 58b, because energetically higher cumulene structures are required to explain this mechanism in 74. The small hypsochromic shift of TP excitation maximum for 74 compared to 58b reflects the influence of the p-system on TP excitation energy. In contrast, both anthracene bearing chromophores 75b and 75c exhibit significantly smaller d compared to 74. This is caused by the smaller electronic coupling between the one-photon (OP) state and the two-photon (TP) excited state (Table 3.7). TPA and OPA spectra for 74 and 75b–c are shown in Figure 3.42. The OPA and TPA spectra of 75 and 75b–c show a higher excitation energy of the TP excited state compared to the OP allowed state (S1). Efficient electronic coupling between these two states explains the large transition dipole moment M12 connecting these states (Table 3.7). M12 was calculated using Eq. (28), while M01 was evaluated by Eq. (26). Both M01 and M12 exhibit large values (Table 3.7). M01 varies very little for 74 and 75b–c. Moreover, M12 , describing the electronic coupling between the OP (S1) and TP (S2) excited states, is significantly larger for 74 than for 75b. Thus, the xylene group in 74 results in a higher M12 in comparison with the anthracene centered 75b and 75c.

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

211

Furthermore, the TPA spectrum of 74 exhibits an additional small absorption at similar wavelength where one-photon (OP) resonance occurs. Direct TPA into the S1 state is forbidden by symmetry according to the symmetric substitution pattern of 74 (compare Section II.A.3). However, vibrational modes can break the symmetry of the chromophore, resulting in a geometry that increases the probability of direct TPA into the first excited singlet state [371]. Such vibronically allowed events as examined in Section II.B.3 may explain the relationship between geometry dependence and the electronic Hamiltonian for the electronic transition dipole moment. In the case of 74, the first part of Eq. (32) is close to zero because it contains the term representing pure electronic contributions for direct TPA into the S1 state, which is forbidden by symmetry for its equilibrium geometry. The second term contains contributions of vibronic coupling to the overall probability for TP excitations. Thus, the TP forbidden transition borrows some intensity from a higher vibrational mode, which is nearby the next TP excited state. Only those vibrations contribute to the summation in Eq. (32) whose symmetry is equal to the product of the energetically higher excited state and the symmetry of the contributing vibronic level. In contrast, the TPA spectra of 74b and 75c (Fig. 3.42) do not exhibit increased amplitudes at the OP resonance. Presumably, the energy difference between the S2 and S1 states is much larger for 75b (3400 cm1) and 75c (3600 cm1) than for 74 (2300 cm1), which is expected to reduce intensity borrowing as described earlier. However, the TPA spectra of 75b and 75c exhibit strong vibrational features for the TP S0 !S2 transition, which is even more pronounced than the vibrational features for the S0 !S1 transition in the case of 74. This is caused by the anthracene structure and the energy difference between each vibrational mode being in the range of inter-ring deformation. In contrast, the TP S0 !S2 transition of 74 does not exhibit any vibrational features. This may be explained by a pronounced electronic coupling as shown for the large transition dipole moment between the S2 and S1 states (Table 3.7). This was also discussed for the TPA of the ladder-type polymer 20 [386]. Some of these coupling efficiencies may be lost by inter-ring vibrations of the anthracene moiety in 75b and 75c, resulting in a decrease of M12 . Increase of the conjugated chain by an additional ethynyl-phenylene-ethynylanthryl connector results in a large increase of the molar extinction coefficient of 75b compared to 76a (n ¼ 1), but the increase of d is moderate with respect to the number of unsaturated carbons. A similar tendency was observed for 76b (n ¼ 2) and it seems that the TPA approaches a limit in the case of 76c (n ¼ 3). This agrees with theoretical studies using time-dependent density functional theory (TDDFT) calculations in which phenylene-vinylene compounds were the target [479]. Moreover, the fact that the anthryl group diminishes TPA as shown for 75b may be a reason for the less efficient TPA. Nevertheless, 74–76 are considered interesting chromophores in TP photosciences and even

212

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

their improved photostability makes them attractive for applications in which photochemistry is not desired.

R1

R1

N

N

R1

R1

n

76

The TPA spectrum of the cyclophane derivative 77 is depicted in Figure 3.43 [478]. It shows slightly higher excitation energy for the TP excited state compared to OPA of the S1 state ( about 480 cm1). The excitation mechanism can be understood within the framework of molecular excitons because complete localization of excitation is impossible in a system consisting of identical particles [64, 241, 408, 409, 480]. Hence, there is a finite probability for excitation energy transfer from one p-system over the paracyclophane moiety to the opposite p-unit of 77. The coupling of the two equal chromophore subunits (4-N,Ndianisylaminotolan) results in an excited state splitting of about 480 cm1 as shown in Figure 3.44. The higher excited state is the TP excited state, while the lower excited state is the OP excited state. Because the fluorescence excitation spectra are almost similar for both OP and TP excitation, favored deactivation of the excited state occurs by the release of upconverted fluorescence from the S1 state. Though the energy difference between OPA and TPA is small, population of the energetically higher TP excited state occurs first by TPA, and consecutive electronic coupling with the OP allowed S1 state results in deactivation. The alignment of both 4-N,N-dianisyltolan subunits by the rigid paracyclophane moiety results in optimal conditions for excitation energy transfer because OPA TPA

60 × 103

200

40

150

30

d (GM)

e (M–1 cm–1)

50

250

100 20 50

10

0 360

380

400

420

440

460

l (nm)

Figure 3.43. OPA and TPA of 77 in toluene. Data from Ref. [478].

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

213

E exciton formation

energy splitting

TP excitation

OP excitation

excited state

ground state

Figure 3.44. Energy splitting in the exciton formed in 77.

both phenyl rings are nearly perfectly aligned by the p-stacked substructures. The small exciton coupling energy V ¼ ðES2  ES1 Þ=2  240 cm1 is important for further design strategies of such photonic materials. A value of similar magnitude (150–300 cm1, depending on the choice of the center of point dipoles) can be derived from the classical point dipole–point dipole approximation supporting these interpretations [64, 409, 480]. R1 N R1

R1 N R1

77

Moreover, chromophores 77 and 78 can be considered as ‘‘twin chromophores’’ that are connected through s-bonds. While 77 represents a triplebond chromophore, 78 incorporates a distyrylbenzene moiety. In particular, the strong alignment of the p-chromophores through the cyclophane structure results in an increase of both OP and TP properties.

R1

R1

R1

N

N

R1

n

n n R1

N R1

n

N R1

R1 78

214

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

This was concluded by comparison of the TP excitation spectra of the reference compounds 58b and 58c with the paracyclophane containing substances 78a–b (Table 3.7). These chromophores exhibit significant increase of d in the case of 78 with increasing length (n) compared to the 1D reference compounds (Fig. 3.45). The paracyclophane compounds exhibit an approximately

Figure 3.45. Experimental and theoretical TP induced fluorescence excitation spectra (without (e) from the original Ref. [474]) for (a) compound 58b, (b) compound 78a, (c) compound 58c, (d) compound 78b, and (f) compound 78c. Experimental results: (~) femtosecond measurements using fluorescein as standard; (&) nanosecond measurements using fluorescein as standard; () nanosecond measurements using bis(methylstyryl)benzene as standard. Theoretical results: solid line. (From Ref. [474] with permission of the American Chemical Society.)

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

215

twice higher TPA cross section in comparison with the corresponding reference compound with no paracyclophane. While the shapes of the spectra for 78a–c are almost similar, there exists a remarkable increase of d with increasing length n. The broadness of the bands in the spectra of 78 result from the unresolved components of the main transition as a result of Davydov splitting [409]. This is caused by the coupling between the two chromophores, similar to that for 77 as shown in Figure 3.43. Table 3.7 shows the transition moments of the paracyclophane derivatives. Electronic coupling between OP and TP excited states expressed by M12 exhibits similar data for the given 1D molecule 58 and the corresponding dimers 78a–c considering the peaks at a similar spectral position. A remarkable increase of the OP quantity M01 was observed with increasing length of 78. This confirms experimental findings that OP properties are mostly affected by the topology of these chromophores. Each representative chromophore of 78 shows large d in a spectral range from 700 to 850 nm. This spectral region overlaps well with the tuning range of Ti:sapphire lasers, which are a preferred source for TP fluorescence imaging applications. The TPA cross section is 1410 GM for 78a, about 3430 GM for 78b, and 3890 GM for 78c. Amplitude of the TPA is significantly enhanced for 78a and 78b as a result of incorporating the paracyclophane moiety, which is absent in the corresponding reference chromophores 58b and 58c. The TPA enhancing function of the paracyclophane unit was the subject of semiempirical AM1 and ZINDO/S studies [481]. Furthermore, the broad TPA spectra with bandwidth of about 150 nm open the way for applications relying on high TPA in a large spectral range. This can be useful for applications such as TPEF, imaging materials, and optical power limiters. Recently, TPA studies of 79 were reported [477]. These compounds are similar to 78 but (1) the nature of the aryl moiety (R) differs and (2) change of the substituent X improves solubility in water. The latter is important in order to use such compounds in biological applications. The largest TPA cross section was found in the case when R represents a triphenylamino moiety. Moreover, f and d drop in the polar surrounding water, which may be caused by quenching of CT states. Thus, a decrease of d is expected, particularly if such CT states are involved in the coupling mechanism according to Eq. (27) derived from the SOS expression [56]. TPA data obtained for 79a1 agree well with those of 78a, comprising a similar chromophore (Table 3.7). 2. Acceptor-p-Acceptor Compounds This section discusses reverse molecular motifs. The molecular engineering strategy focuses on polarizable p-systems bearing acceptor groups at the ends (Fig. 3.46). Representative examples are the chromophores 80–98 [131, 176, 370, 394, 415, 447, 469, 482–486] compiled in Table 3.8. Excitation of these chromophores spreads excitation density from the p-rod to the end capped acceptor groups, resulting in a partially charged system.

216

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

X CH2

CH2 n

N

R:

N

CH2 X 6

N

CH2 X 6 (b)

(a)

(c) CH2 X

R

R

R

R 79

This is particularly enhanced by incorporating unsaturated electron-rich moieties as shown for chromophores with thiophene or carbazole moieties. This procedure can be associated with formation of the quadrupolar pattern shown in Figure 3.46, which is opposite to that of Figure 3.39. The few example chromophores 80–98 demonstrate the relationship between choice of p-system, length of p-system, and strength of acceptor group and TPA. Because the excitation mechanism depicted in Figure 3.2a appropriately describes TPA of these symmetric chromophores and d relates to Eq. (27), the microscopic parameters affected by changing of the molecular motif are the excitation energies for the S1 and the TP excited states, the transition dipole moment for excitation into the S1 state (M01 ), and the coupling between the TP excited state and the S1 state (M12 ). The same p-core was studied in the case of 60 and 80 as well as for 72 and 81. There is, however, a distinction regarding the substitution at the chromophore

A

π

A

A

π

A

Figure 3.46. Sketch for spreading of excitation density for symmetric Acceptor-pAcceptor compounds.

217

R1 ¼ SO2Me R1 ¼ CHO R1 ¼ CF3 R1 ¼ NO2 R1 ¼ H, R2 ¼ CN, R3 ¼ CN R1 ¼ CN, R2 ¼ H, R3 ¼ CN R1 ¼ CH3, R2 ¼ H, R1 ¼ CH3, R2 ¼ CH3O R1 ¼ Mesityl, n ¼ 1 R1 ¼ Mesityl, n ¼ 2 R1 ¼ Mesityl R1 ¼ CH3O, R2 ¼ C12H25O

R1 ¼ CH3O, R2 ¼ C12H25O R1 ¼ CH3O, R2 ¼ C12H25O R1 ¼ CH3OR1 ¼ H; R2 ¼ CH3 R1 ¼ CH3O—; R2 ¼ CH3

R1 ¼ C3H7—

80a 80b 81a 81b 82a 82b 83a 83b 84a 84b 85 86

87 88 89 90a 90b

91

Structure Substitution

30,000 18,000 40,000 82,000 93,200

78,000 50,000 31,000 51,000

e (M1cm1)

41 835 1340 480 620 1750 4400 363 554 848 2420 13 433

46 64

160 450 210

d (GM)

720c 730c 775c 825 940 970 975 694 720 764 571 700 588

<740 <735 735 765 <710 <710

lTP max (nm) 0.92 0.96 0.48 0.47 0.75 0.02 0.16 0.83 0.71 0.61 0.82

f

373

0.68

554 0.06 618 0.0085 397 0.85 421 0.1 467 0.01

384 397 424 481 367 364 493 464 372 397 435 513

lOP max (nm) M01 M12 (D) (D) DMSO DMSO DMSO DMSO TOL TOL EtOH EtOH THF THF THF TOL TOL TOL TOL DMSO DMSO DMSO DMSO CH2Cl2 CH2Cl2

TPEF, ns TPEF, ns

TPEF, fs TPEF, fs TPEF, fs TPEF, ns TPEF, ns TPEF, ns TPEF, ns NLT, fs NLT, fs NLT, fs

TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, fs

Method and Excitation Solventa Sourceb

[484] [484]

[469] [469] [469] [469] [486] [486] [176] [176] [447] [447] [447] [131] [131] [131] [131] [482] [482] [482]

Reference

TABLE 3.8 Compilation of Extinction Coefficient (e), TPA Cross Section (d), Maximum for TP Excitation (kTP max ), Maximum for ), Fluorescence Quantum Yield ( ), Transition Dipole Moment Between States S and S (M ), Transition Dipole OPA (kOP 01 f 0 1 max Moment for Coupling Between S1 State and the Lowest TP Excited State (M12 ), and the Method Used for Determination of TPA Data for a Series of Chromophores with the General Structures 80–98 in Different Solvents

218

(Continued)

Z: N

R1 ¼ Biphenyl, R2 ¼ Biphenyl R1 ¼ Biphenyl, R2 ¼ 2-Naphtyl R1 ¼ Biphenyl, R2 ¼ Phenyl R1 ¼ p-CH3O-Biphenyl, R2 ¼ Biphenyl R1 ¼ R2 ¼ p-CH3O-Biphenyl R1 ¼ p-CH3O-Biphenyl, R2 ¼ Phenyl R1 ¼ R2 ¼ Naphtyl R1 ¼ 2-Naphtyl, R2 ¼ Biphenyl R1 ¼ Phenyl, R2 ¼ Phenyl n¼1 n¼2 20 25 85 30 <5 490 2850 660 119 71 530 243 250,000 250,000 80,000

89,000 62,000 47,000 40,000 46,000

430 490

79,000 72,100

612 795 745 775 835

489 30 60 30 200

75,000 45,000 61,000 59,000

800 800 588 790 840 710 585 730 850 980

775 805

785 787

lTP max (nm)

d (GM)

e (M1cm1)

430

420 398 364 390 430 370 524 422

410

414 411 416 394 410

lOP max (nm)

0.74 0.14 0.65

0.40 0.35 0.20

0.85 0.85

0.72 0.50 0.35 0.70 0.75

f

M01 M12 (D) (D)

b

DMSO, dimethylsulfoxide; TOL, toluene; EtOH, ethanol; THF, tetrahydrofurane; CH2Cl2, methylene chloride. TPEF, two-photon excited fluorescence; NLT, nonlinear transmission; fs, femtosecond; ns, nanosecond. c Wavelength used for excitation; maximum is localized at other wavelength.

a

98

93g 93h 93i 94a 94b 95 96 97

93e 93f

92 93a 93b 93c 93d

Structure Substitution

TABLE 3.8

CH2Cl2 CH2Cl2 CH2Cl2 THF THF CH2Cl2 DMSO CH2Cl2

CH2Cl2 CH2Cl2

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

NLT

TPEF, fs TPEF, fs TPEF, fs NLT, ns NLT, ns TPEF, fs NLT, fs TPEF, ns, fs

TPEF, fs TPEF, fs

TPEF, ns TPEF, fs TPEF, fs TPEF, fs TPEF, fs

Method and Excitation Solventa Sourceb

[485]

[483] [483] [483] [415] [415] [394] [370] [484]

[483] [483]

[484] [483] [483] [483] [483]

Reference

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

219

ends. Electron-withdrawing groups at the end (80 and 81) result in a decrease of TPA compared to the donor-bearing compounds 60a and 72. This is a factor of about 9 for 80a bearing as s-acceptor the SO2R group and a factor of 3 for 80b having the aldehyde group as p-acceptor. There was no clear correlation between TPA and Hammett’s substituent constants [442].

R1

R1

80 S

S

S R1

R1 81

The TPA of 82 is comparable to that of 2,20 -dimethyl-distyrylbenzene (18), which is a standard for TPEF [49, 380]. The TPA cross section of 82 was not taken at the maximum due to the limited wavelength range of the excitation source chosen. Thus, the significantly smaller d of 18 (691 nm: 8.2 GM [380], 6.3 [85]; 725 nm: 6 GM [380]) was reported in comparison with 82 (710 nm: 40–60 GM [486]), which indicates that incorporation of cyano groups in 82 benefits an increase in d compared to 18. R1 R2 R2

82

Photonic properties of 83a–b were reported and compared with both OP and TP lasing performance of 53 [176]. Large Stokes shifts were observed for the emission of 83b, consistent with the observations of 53. TPA was discussed from a qualitative point of view. R2 N R1 R1 N R2

83

220

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Systematic exploration of TPA for the A-p-A-type compounds 84 and 85 [447] bearing dimesitylboryl as the strong acceptor indicate again the potential of boron to function as an appropriate electron acceptor in TP absorbing materials [447]. In contrast to trivalent nitrogen, which has a lone pair of electrons in its p-orbital, the three-coordinate boron atom has an empty p-orbital as acceptor at the ends of the chromophore. Reports regarding the function of boron in TPA materials are very few. The difference between compounds 84 and 85 can be seen at the p-bridge. Two of the phenyl rings in 84 are replaced by two thienyls, resulting in 85. R1 B R1

R1

B

n

R1 84

R1

S B

R1

S

R1

B R1 85

The TPA of 84a is slightly smaller in comparison with 82 despite the distinct substitution of the terminal benzene rings in 82. Because the dimesitylboron group has an electron-withdrawing strength located between cyano and nitro, the similar TPA data of both 82 and 84a are mainly caused by the similar strength of the acceptor groups. This may explain the similar TPA data found for both chromophores having similar length. Furthermore, increase of the p-bridge length in 84b results in a 20-fold increase of d compared to 84a by an additional phenylene-vinylene moiety. Moreover, a significant increase of TPA was observed by replacing the phenyl group in 84a by a thienyl group, resulting in 85. Figure 3.47 exhibits the spectral profiles and peak positions of 85. Both OPEF and TFEP show similar spectra. This implies that both emissions belong to the same fluorescent excited state. This agrees with the mechanism depicted in Figure 3.2a. Chromophores 86–88 bear acceptor groups of different strengths at the ends while the conjugated p-bridge is a distyrylbenzene. All three chromophores exhibit large d [131] (Table 3.8). The TPA increases from 86 to 87 and shows the largest TPA for 88 within this series. This may be caused by the different strengths of the acceptor groups. The barbituric acid moiety in 87 was the subject of NLO studies supporting the hypothesis that this substituent appropriately increases TPA if it is placed at the ends of the centrosymmetric chromophore

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

221

Figure 3.47. Normalized single-photon excitation (SPE) and two-photon excitation (TPE) spectra (left) and single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (TPEF) spectra (right) of 85 in THF. In the TPE spectrum, the wavelength data are divided by 2, considering the TPA characteristic. (From Ref. [447] with permission of the American Chemical Society.)

[380]. Further evidence for increase of the acceptor strength can be obtained from the absorption spectra shifting bathochromically from 86 to 88. Chromophore 88 bears at the ends the dicyanomethylidenedihydrothioene dioxide moiety, which must be in this series the strongest acceptor. Thus, the stronger the acceptor, the more efficient the charge separation shown in the sketch of Figure 3.46. Large TPA was reported for the bis(pyridylstyryl)-substituted diacetylene 89 that was additionally alkylated at the pyridinium nitrogen, resulting in 90. In accordance with the charged pyridinium chromophores 53, a larger TPA was also observed for 90 in comparison with 89. This example also shows that incorporation of triple bonds into the -bridge does not diminish TPA even in A-p-A   chromophores. It confirms that the triple bond of N Ph C Ph C Ph  N  C  C chromophores does not diminish d. TPA spectra were also measured for the bis-dioxaborines 91 and 92 by TPEF (Table 3.8). The dioxaborine unit exhibits an electron-withdrawing unit. The main TPA of 91 and 92 occurs into a higher excited state. This is consistent with the findings for other centrosymmetric systems. A minor peak was observed at longer wavelength in the TPA spectrum of 91, whose excitation energy is located near the OPE state.

222

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

R1

CN CN

R2 R1 R1 NC

R2 CN

R1

86

S N N

O

R1 R2 R1

O

O

R1 R2 O

N

R1

N S 87 R1

O2S CN

R2 R1

NC

CN R1 NC

R2 R1

SO2

88 R1

R1

N

N

R1

R1

89 R1

R1 R2 N

N R2

R1 90

R1

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

F

F

F B O

223

F

O B

O

O

R1

R1

91 F

R2

F B O

F

R2

F

O B

O

O

R1

R1

92

The TPA of the diaroyl(methanato)boron difluorides 93 was evaluated regarding the design of efficient fluorescent probes (Table 3.8) [487]. All derivatives of 93 exhibit large OPA (e > 50; 000 M1 cm1 ). Their strong fluorescence emission covers most parts of the visible spectrum depending on substitution with R1 and R2, respectively. The maximum for OPA bathochromically shifts with increasing conjugation length of the chromophore. In addition, solvatochromism was observed for some derivatives of 93 in the UV/Vis absorption spectra taken in several solvents. A correlation was found between OP excitation energy and the empirical parameter ET(30)-value introduced by Reichardt [488, 489]. In addition, fluorescence emission also bathochromically shifts with increasing solvent polarity. These results explain the increase of the dipole moment of the excited state with respect to the ground state. The TPA of the 93 derivatives ranges from 20 to 200 GM. Much lower d was reported in the absence of donating substituents on the aryl moieties. This supports the theory (cf. Eq. (24)) that the dipolar contribution is important, as observed for other D-A systems. The substituent effect is much more pronounced in the case of 93d, exhibiting about one order of magnitude higher TPA response. Nevertheless, data of 93 show the relationship between the nature and length of the aryl moieties and their influence on the amplitude of TPA. Increasing the donating strength of the aromatic moiety or its size results in an increase of TPA. R2

R1 O F

B

O F

93

Symmetrical chromophores with the structure 94 bear as electron-rich aromatic bridge a thiophene moiety substituted at the end with electron-withdrawing benzothiazole groups. In this A-p-A chromophore, TPA significantly

224

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

increases with incorporation of an additional thiophene moiety, seen by comparing 94a with 94b (Table 3.8). N

N

S n

S C10H21O

S

OC10H21

94

The TPA cross section of 95 slightly increases in comparison with 94a. Although thiophene possesses a higher electron density compared to fluorene, the TPA values of both 94a and 95 slightly differ. Thus, the efficiency to spread excitation density from the center is similar no matter whether the core displays a thiophene or biphenyl moiety. C10H21

C10H21 N

N

S

S

95

Chromophore 96 bears a pyrrole group at the center, which is connected over a vinylene group with a pyridinium moiety at both ends. The TPA was not taken at the maximum and it was expected that a much higher TPA cross section should exist for 96 [370]. It was also mentioned that TPA benefits from incorporation of either electron-deficient (pyridinium) or electron-rich heterocyclic moieties. This is consistent with previous studies [129, 461]. H3C

N

CH3

N

CH3

N 96

Chromophore 97 has a carbazole group as its central core, whereas 92a has a fluorenyl group. The central hinge in both 97 and 92 is stiff, while the biphenyl moiety in 91 is more flexible. Therefore, planarity of the conjugated system can easily be distorted in the latter, but this does not have the expected impact on d. There is a small TPA closely located with excitation energy similar to the S0 – S1 excitation. Because several conformers can exist in 97, the symmetry can easily break, resulting in asymmetric chromophores having a TPA mechanism similar to Figure 3.2b. In addition, a model describing the transfer of oscillator strength was introduced to explain the TPA occurring in 91, 92, and 97.

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

225

C5H13 N

F O F B

O B

O

O C3H7

C3H7

F F 97

Chromophore system 98 can be considered a double-conjugated-segment molecule [485]. This structure incorporates a D-p-A chromophore and a D-p-D chromophore that are covalently linked by an aliphatic chain. It should be possible to incorporate A-p-A structures as well. TP excitation covers the entire spectral region between 700 and 1000 nm with large TPA cross sections. This demonstrates that such systems are attractive candidates for applications based on optical limiters. A detailed explanation for these data was not given. The spectral shape shown in Figure 3.48 exhibits a composition of several components, in which the peak at about 720 nm could be assigned to the distyrylbenzene compared to 58, while the peak at about 1000 nm is caused by the D-A substituted azobenzene moiety. The peak appearing at 850 nm must be caused by interaction between both chromophores incorporated in 98. 3. Donor-Acceptor-Donor Compounds The amplitude of TPA can be increased by incorporating strong electron-withdrawing groups at the center of

Figure 3.48. TPA spectrum of 98. The inset exhibits the OPA spectrum. (From Ref. [485] with permission of the Royal Chemical Society London.)

226

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

OC 6 H13 N N O

N

Z

Z

98

NO 2

the p-rod and electron-donating groups at the ends. Some representative chromophores are 99–105 [131, 224, 287, 375, 428, 490, 491] in Table 3.9. This results in quadrupolar compounds exhibiting larger TPA in comparison with their analogous D-p-D chromophores that have no additional acceptor groups. Thus, the acceptor groups either at the center as shown for 99, 100, 102, and 103 or at the main chain as shown for 101 should increase the transfer of excitation density simultaneously from the ends to the center as demonstrated in the graphical sketch of Figure 3.39. This was particularly observed for the symmetric derivatives 100–103 while d of nonsymmetrical 99 is lower. Therefore, a systematic reduction of electron density at the central ring results in compounds with solvatochromism [337] being important for the design of fluorescent probes and large d. Substitution of all hydrogens on the central ring in 58a by fluorine results in the quadrupolar compound 103a. The donoracceptor-donor pattern of the latter causes the solvatochromism observed [224]. Consequently, the tetrafluorobenzene moiety in 103a was replaced by a 2,5-bis(alkylsulfonyl)benzene unit resulting in 102. Optical properties must be affected by this change because the alkylsulfonyl substituent has been known to have stronger electron- withdrawing properties. This was concluded from the reduction potential (Ered ) of the corresponding substituted benzenes. F

~ ~

<< F –3.4V [379] (vs. SCE)

CN

F

<< NC

F

–2.4V [342] (vs. SCE)

CN

–2.3V [341] (vs. SCE)

SO2Me ~ ~ MeO2S

–1.7V [492] –1.6V [494] (vs. SCE) (vs. NHE) –1.6V [493] (vs. SCE)

227

105c

105a 105b

104

103c

R1 ¼ C6H13, R2 ¼ H, R3 ¼ OCH3

R1 ¼ CH3 R1 ¼ Ph R1 ¼ CH3 R1 ¼ C4H9, Hal1 ¼ Hal2 ¼ F, n ¼ 1 R1 ¼ CH3, Hal1 ¼ Hal2 ¼ F, n¼2 R1 ¼ Ph, Hal1 ¼ Br, Hal2 ¼ H, n ¼ 1 R1 ¼ C6H13; Hal1 ¼ Hal2 ¼ Hal3 ¼ Hal4 ¼ F R1 ¼ C6H13, R2 ¼ H, R3 ¼ H R1 ¼ C6H13, R2 ¼ OC3H7, R3 ¼ H

101a 101b 102 103a

103b

R1 ¼ CH3 R1 ¼ CH3 R1 ¼ Ph

99 100a 100b

Structure Substitution

730 800 770 800 770 800

600 1140 1810 1900 2580 2490

800

450 39,000

780

1700

91,000

740 830 830 840 790 825 816 760

lTP max (nm)

260 1750 1640 1370 890 730 4100 1400

d (GM)

32,000 66,000 66,000 77,200 54,000 50,000 42,500 77,000

e (M1cm1)

476 465 478

427 448

386

424

453

426 490 475 473 438 440 439 430

lOP max (nm)

0.66

0.64 0.75

0.99

0.41

0.48

0.52 0.69 10.0 0.87 10.1 0.73 0.003 9.6 0.015 9.0 0.31 0.58

f

M01 (D)

12.9 13.6

15.1 15.5

TOL TOL TOL

TOL TOL

TOL

TOL

TOL

TOL TOL TOL TOL TOL TOL TOL TOL

M02 (D) Solventa

TPEF, fs TPEF, fs TPEF, fs

TPEF, fs TPEF, fs

TPEF, fs

TPEF, ns

TPEF, fs

TPEF, fs TPEF, ns TPEF, ns TPEF, fs TPEF, ns TPEF, ns TPEF, ns TPEF, fs

Method and Excitation Sourceb

[491] [491] [491]

[491] [491]

[224]

[131]

[224]

[224] [287] [287] [490] [287] [287] [224] [224]

Reference

TABLE 3.9 Compilation of Extinction Coefficient (e), TPA Cross Section (d), Maximum for TP Excitation (kTP max ), Maximum for OPA (kOP max ), Fluorescence Quantum Yield (f), Transition Dipole Moment Between States S0 and the S1 (M01 ), Transition Dipole Moment for Coupling Between S1 State and the Lowest TP Excited State (M12 ), and the Method Used for Determination of TPA Data for a Series of Chromophores with the General Structures 99–107

228 5530 3650 250 527

R1 ¼ C6H13, R2 ¼ CN, R3 ¼ H R1 ¼ C6H13, R2 ¼ CN, R3 ¼ OCH3 R1 ¼ C10H21, R2 ¼ Ph R ¼ C2H5 64,700 980 980 790 521

lTP max (nm) 0.13 0.064 0.11

436

f TOL TOL THF CHCl3

M01 M02 (D) (D) Solventa

531 575

lOP max (nm)

b

Reference

TPEF, fs [491] TPEF, fs [491] WLCþNLT, fs [375] TPEF, fs [428]

Method and Excitation Sourceb

TOL, toluene; THF, tetrahydrofurane; CHCl3, chloroform. TPEF, two-photon excited fluorescence; WLC, white light continuum; NLT, nonlinear transmission; fs, femtosecond; ns, nanosecond.

a

105d 105e 106 107

Structure Substitution

d (GM)

e (M1cm1)

TABLE 3.9 (Continued)

229

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

In addition, the cyano substituted chromophores 99 and 100 were investigated in order to change the electron density at the central ring. Because 99 bears only one cyano group, the electron density at the central ring must be significantly smaller in comparison with 100, which has two cyano groups at the central core. This must have an impact on TPA if the sketch in Figure 3.39 represents the situation of such chromophore types. Thus, TPA can be tuned by the strength of each electron-withdrawing substituent. A similar pattern was obtained for the TPA of 101, which has the cyano groups at the vinylene chain of 100. Changes of the acceptor properties correlate with Ered and are therefore also important for the efficiency of ICT formation. Moreover, 104, which has only slight electrondonating groups at the ends, was included in this approach. Compound 103 bears a dialkylaniline pattern at both ends and possesses an oxidation potential of about 0.5 V (vs. SCE) [495]. However, 1,2-dimethoxybenzene, which has an oxidation potential of 1.45 V (vs. SCE) [496], has a lower electron-donating capability compared to dialkylaniline. As a result, change of the donor pattern is also responsible for tuning of solvatochromism and TPA resulting in an increase of d with increasing donor strength of the terminal substituent. Moreover, 103 possesses an extended p-chain resulting in an increase of d as well [137, 497]. The longer and more planar the p-conjugation, the better the two-photon absorptivity. N

R1 N R1

N

R1 N R1

R1

R1 NC

99

100

NC

N

R1 N R1

N R1

CN

R1

SO2R2

R1 R1 N R1

R1

CN

R1

CN

R1 SO2R3

101

102 OR1

Hal2

R1

Hal1

Hal2

Hal1

Hal1

Hal2

OR1

N R1 N R1

n

R1

R1O

n

Hal1

Hal2 103

R1O 104

The TPA cross sections of the D-A-D distyrylbenzenes (Table 3.9) 100–103 exhibit a higher d in comparison with 58. TP excitation spectra of 99, 100, 102, and 103a are shown in Figure 3.49. Incorporation of electron-withdrawing

230

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.49. Two-photon excitation spectra in toluene: (a) 103a, (b) 99, (c) 102, (d) 100. Inset is the square dependence of the TPEF as a function of the laser power used. (Adapted from Ref. [224].)

groups results in both an increase of d and a bathochromic shift of the TP excitation maximum if the molecule is centrosymmetric (Figure 3.49a,c,d and Table 3.9). This example demonstrates the relationship between bathochromic shift of the TP excitation maximum and the electron deficiency of the central benzene ring. The influence of donor-acceptor substitution was previously described for other unsaturated centrosymmetric compounds [131, 137, 224, 226, 233, 284, 285, 287, 290, 314, 469, 475, 497–500]. As a result, d correlates with the electron affinity of the central ring. This is concluded from the reduction potential of the corresponding acceptor substituted benzenes mentioned earlier. However, the nonsymmetric distyrylbenzene 99 (Figure 3.49b) possesses the lowest TPA cross section in the distyrylbenzene series although the cyano group reduces the electron density in a similar way to the four fluorine atoms in the 1,2,4,5-tetrafluorobenzene moiety in 103a. Therefore, keeping the symmetry of a quadrupolar molecule is important to increase d because ICT can simultaneously occur from both donor moieties to the central ring with the same efficiency. Violation of this rule by symmetry break (cf. structural pattern of 99) results in lower d. Thus, such compounds are rather assigned as either ortho or meta substituted stilbenes. The insets in Figure 3.49, which are double logarithmic plots of the integrated TPEF signal as a function of the laser power P, show the nonlinearity of the

231

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

observed absorption process. The slope is approximately 2. Therefore, the data are related to a quadratic nonlinear absorption process. Consideration of threephoton excitation can be omitted because the laser beam was not tightly focused into the sample [224]. Compound 101, bearing the cyano groups at the main chain, exhibits a d half as large as that for 100, although this value can still be considered large. Decrease of the TPA cross section can be caused by distortion of planarity. Indeed, crystallographic data of 101 show a stronger distortion of the conjugated system compared to 100. This may explain the differences between 100 and 101. Interestingly, 103c bears two bromine atoms at the central benzene ring. The TPA cross section of 103c exhibits a d value one-third that of the value for tetrafluorinated compound 103a. The decrease of d may be caused by a higher electron density of the central ring in 103c in comparison with 103a due to substitution with only two electron-withdrawing bromines in 103c. Furthermore, bromine is a less vigorous electron-withdrawing substituent compared to fluorine. This may have an impact on TPA as well. Moreover, incorporation of heavy atoms into the molecular skeleton of the conjugated systems enhances quantum yield of triplet formation and gives access to triplet photochemistry by TP excitation. Hence, 103c can be applied as a singlet oxygen sensitizer. This is important for cytotoxicity and photodynamic therapy in biological tissues [170, 171, 181, 186, 203, 204, 255]. The TP chromophore 105 exhibits an interesting structural feature. It possesses a long p-rod, bears donor groups at the ends, and has electron-withdrawing groups at the anthracene moiety functioning as the central core. As expected, the TPA cross section is larger in comparison with that of 100. This molecular motif also demonstrates the increase of d as a result of cyano substitution of the central core (R2 ¼ CN), resulting in a quadrupolar molecular pattern. However, the d values (Table 3.9) are not as large as expected for such giant chromophores. Presumably, the theoretical model representing the dilution of p-chromophores [479] should be taken into account in order to understand the differences between experiment and theory [224, 491]. R1 R3 R2 R3 R3 R2 R1

N R1

R3 105

N

R1

232

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.50 Absolute TPA spectrum (solid line) and normalized OPA spectrum (dashed line) of 106 taken in THF. (From Ref. [375] with permission of the American Chemical Society.)

Measurement of the TPA spectrum for 106 was carried out by using the WLC method [375]. Results showed a hypsochromic shift of the TPA spectrum compared to the linear absorption as shown by comparison of both absorption spectra depicted in Figure 3.50. This agrees with the general finding of this chromophore class because the TP excited state possesses a higher excitation energy in comparison with the lowest OP excited state, S1. Thus, population of the TP excited state by TPA occurs by the mechanism shown in Figure 3.2a. Furthermore, the TPA spectrum in Figure 3.50 possesses the largest amplitude at around 780 nm with 1.1  1020 cm4/GW (data were taken from the graphics), which is 280 GM at this wavelength. R2 N R2

N

R2

S N

S R1

N

R1

R2 R1

106

R1

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

233

TPA chromophore 107 represents, in analogy to its dipolar analog 52, a typical quadrupolar chromophore with both increased linear and nonlinear absorption properties [428].pThus, the extinction coefficient and d are about 1.3 and 1.4 ffiffiffi times larger (about 2) in comparison with that of 52. This again shows that incorporation of the electron-deficient triazine results in an increase of d for quadrupolar compounds considering both OP and TP quantities.

R

R

R

N

N

R

N N

N CH3 107

Comparison of the energies for TPA and OPA shows a higher energy for the TP excited state compared to the lowest allowed OP excited state (S1) for all chromophores (99–107) examined in this section. Hence, the amplitude of TPA can be described by Eq. (27). There are no dipolar contributions, no matter that solvatochromism occurs for the samples 99–104. The influence of the quadrupolar pattern is reflected in M12 . Application of models of the symmetry break are not likely to provide clarification of the relation between TPA properties and the distinct quadrupolar substitution of the chromophores 99–107.

D. Octupolar/‘‘Propeller Shaped’’ Chromophores with Large Two-Photon Absorptivity An additional design strategy for TP materials with large TPA is based on octupolar chromophores [288, 425, 428, 462, 490, 501–513]. Some representative chromophores are 108–116 and 120–121 (Table 3.10) [288, 425, 428, 490, 501-511]. These compounds are sometimes called propeller-shaped chromophores [505, 512]. It is not required to include dipolar moieties to obtain a propeller-shape chromophore as shown for 117–119 and 122 comprising donor 2

m201 groups at the center and at the ends. d is generally proportional to M01 if the propeller-shape chromophore comprises several dipolar arms. Thus, octupolar compounds combine three dipole arms, resulting in high symmetry. Figure 3.51 shows the general structure of an octupolar chromophore with a (D-p)3-A pattern on the left side, and a (A-p)3-D motif on the right side, emphasizing the direction of partial charge transfer in each individual arm. Thus, each individual arm can be associated with a low-lying excited charge transfer (CT) state characterized by M01 and
m01 . Furthermore, the excited state

234 75,000 74,000 67,000 122,000 114,100

R1 ¼ C6H13, N(R2)2 ¼ N(Me)EtOH N(R1)2 ¼ N(Me)EtOH, R2 ¼ C6H13, R1 ¼ Ph, R2 ¼ C6H13

R ¼ C2H5 R1 ¼ Ph, R2 ¼ C10H21 R1 ¼ Ph, R2 ¼ 3,7-dimethyloctyl

R1 ¼ Ph, R2 ¼ allyl N(R1)2 ¼ N(Ph)(p-CH3-Ph) R1 ¼ Ph R ¼ C6H13 R1 ¼ C6H13

R ¼ C6H13 R1 ¼ C4H9, R2 ¼ C9H19

109a 109b 109c 110

111 112a 112b

112c 113 114 115 116

117 118 119a 119b

131,000 131,000 115,000 244,000 110,000 200,000

762 673 3215 3010 5395 840 820 671 137 127

R1 ¼ CH3

108b

510 407 30 3660 1300 2700

112 2300 120

1980

R1 ¼ CH3

d(GM)

108a

Structure Substitution

1

e (M cm1)

790 825 740 532 740 755 740 740 680 694

751 1200 840 880 990 990 990 780 990 800 790 790

lTP max (nm) f

384 384 385 428 412 417

414 414 424 373

438 415

498 485 490 499

0.52

0.36 0.36 0.77 0.74

0.1 0.47 0.48 0.42 0.47 0.06 0.08

0.10 0.08 0.08 0.46

636 0.031

596 0.018

lOP max (nm)

THF THF THF TOL TOL TOL TOL CH2Cl2 CH2Cl2

CHCl3 THF THF THF

Glycerol Glycerol Glycerol Glycerol CHCl3 CHCl3 CHCl3 TOL

TPEF, ns TPEF, ns DFWM, ps TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, fs

TPEF, ns NLT, fs NLT, fs

TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, ns TPEF, ns TPEF, ns TPEF, ns

[428] [508] [508] [509] [508] [503] [503] [504] [501] [502] [505] [506] [517, 518] [517, 518, 520]

[288] [288] [288] [288] [507] [507] [507] [425]

Method and Solventa Excitation Sourceb Reference

TABLE 3.10 Compilation of Extinction Coefficient (e), Two-Photon Absorption Cross Section (d), Maximum for Two-Photon Excitation OP (kTP max ), Maximum for One-Photon Excitation (lmax ), Fluorescence Quantum Yield (f), and the Method Used for Determination of TPA Data for a Series of Electron-Donating Substituted Chromophores with the General Structures 108–120 in Different Solvents

235

b

a

n ¼ 0, R ¼ SO2C8H17 n ¼ 0, R ¼ SO2CF3 n ¼ 1, R ¼ SO2C8H17 n ¼ 1, R ¼ SO2CF3 R1 ¼ Ph R1 ¼ OCH3, R2 ¼ OCH3, R3 ¼ OCH3 R1 ¼ CH3, R2 ¼ OCH3, R3 ¼ OCH3 R1 ¼ H, R2 ¼ OCH3, R3 ¼ OCH3 R1 ¼ Cl, R2 ¼ OCH3, R3 ¼ OCH3 1772

675

81,000 60,000 135,000 60,000 239,000

160 740 495 740 1065 740 1080 740 5030 840 267 810 760 358 (440) 1278 740 740 348 (444)

388 405 397 408 495 388 0.76 0.60 0.22

0.77 0.78 0.79 0.72 0.67 0.68

TOL TOL TOL TOL TOL Acetone Acetone Acetone Acetone

TPEF, fs TPEF, fs TPEF, fs TPEF, fs TPEF, ns TPEF, fs TPEF, fs TPEF, fs TPEF, fs

[505] [505] [505] [505] [490] [511] [511] [511] [511]

CHCl3, chloroform; TOL, toluene; THF, tetrahydrofurane; CH2Cl2, methylenechloride. TPEF, two-photon excited fluorescence; NLT, nonlinear transmission; DFWM, degenerate for wave mixing; fs, femtosecond; ns ¼ nanosecond; ps, picosecond.

120a 120b 120c 120d 122 123a 123b 123c 123d

236

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

(D-π)3-A

D

(A-π)3-D

A q0

q0

π

π

q0

A

π

D

D

∆µ01, M01

π

π

A

∆µ01, M01

π q0

q0

D

q0

A

Figure 3.51. Schematic presentation of CT in the excited state of a C3h chromophore along the three axes favored for CT in octupolar chromophores; q0 corresponds to the amount of charge transferred in each individual arm [left: (D-p)3-A chromophore; right: (A-p)3-D chromophore].

wavefunction of the individual arms form a reproducible representation of the C3h symmetry group described by the basis (a*, b*, c*). Thus, the use of group and Gram–Schmidt orthogonalization results in new symmetry-adapted wavefunctions ca1–ca3, defined in Eqs. (56)–(58). More details were summarized in the original reference [288]. ca1 ¼ ða þ b þ c Þ 1 ca2 ¼ pffiffiffi ð2a  b  c Þ 6 1  ca3 ¼ pffiffiffi ðb  c Þ 2

ð56Þ ð57Þ ð58Þ

In the C3h [C3] point group, ca1 belongs to the A0 [A] representation. The residual functions ca2 and ca3 belong to doubly degenerate E0 [E] representations, respectively. Both, A0 and E0 are TP allowed while OP excitation results only in population of E0 . Because the three dipolar arms are not isolated, an additional interaction energy term must be included describing the interactions of the dipolar arms in one molecule. This is the coupling energy Ecoup between the excited states localized on the separated arms. Thus, the 2A0 state becomes destabilized by 2Ecoup (increase of excitation energy) while the 1E0 stabilizes by Ecoup (decrease of excitation energy). As a result, the TPA cross sections can be represented by Eqs. (59) and (60) for TP excitation into the 1E0 and 2A0 , respectively [288], by taking the correct sign of Ecoup in both equations. More details can be

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

237

found in the original reference [288]. A similar approach was described in Ref. [514]. 2 3 M01
m201 d1 E 0 /   8 E01  Ecoup 2

2

ð59Þ 

2 6 M01
m201 d 2 A0 /  2 8 E01  2Ecoup  2

ð60Þ

Consideration of both equations shows higher d values for TP excitation into the 2A0 state in comparison with TP excitation into the 1E0 state. This is caused by the more favored detuning factor (E01 – Ecoup Þ=2, resulting in an increase for the ratio d2 A0 =d2E0 . This corresponds to a factor of about 5 if E01 is about 2.5 eV and Ecoup  0.3 eV. In contrast to quadrupolar chromophores, excitation into the energetically higher 2A0 state compared to the 1E0 state is controlled under resonant conditions by the amount of photoinduced transferred charge q0 . Thus, change of the state dipole moment
m01 of each individual arm contributes to TPA although it is an octupolar molecule. Maximizing M01 and
m01 can therefore lead to the engineering of optimal TPA. These are the molecular parameters mainly affecting TPA according to the Eqs. (59) and (60) [288]. This is different from the quadrupolar chromophores discussed in Section II.C.3. Experimental data show that change of electronic properties of substituents affects mainly M01 and M12 [224]. Comparison of TPA data for the triphenylmethane dyes Crystal Violet (108a) and Brilliant Green (108b) demonstrates the validity of these relations for octupolar molecules. Both chromophores differ regarding the substitution with dipolar arms. Compound 108a can be assigned to a (D-p)3-A pattern as shown on the left side of Figure 3.51. Replacing one dialkylamino substituent in 108a by a phenyl moiety results in 108b, which changes d (Table 3.10). The nature and strength of the coupling energy strongly depend on the structure of the octupolar chromophore. Consequently, the TPA cross section of 108a is greater than that of 108b because of the higher dimensionality with dipolar arms in 108a. The TPA cross section of 108b is 762 GM and 673 GM for excitation at 840 nm and 880 nm, respectively. This matches well with the second absorption band being slightly OP allowed due to formation of conformers with lowered symmetry. This may be caused by the twin structure of 108b, favoring coupling with different vibrational modes. However, 108a exhibits a significantly larger d with 1980 GM excited at 752 nm. Furthermore, a vibrational feature exists in the TP excitation spectrum of 108a. The low TPA at 1200 nm was not quantified because an appropriate standard was not available at 1200 nm. In addition, the measured values agree well with calculations using the semiempirical

238

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

INDO-MRDCI/SOS method. This example demonstrates the tuning effect on d by the number of dipolar arms. This is larger in 108a in comparison with 108b. R1

R1

N

C

C R1

N

N

R1

R1

R1

R1

N

N

R1

R1

R1

108b

108a

Large TPA was also reported for chromophore 109 [507], which bears an electron-deficient 1,3,5-triscyanobenzene in its central core. Intramolecular charge transfer occurs with increased efficiency as a result of substituting the benzene with three strong electron-withdrawing groups; it is easier to reduce red red ¼ 1:35 V [449]) this moiety in comparison with benzene (E1=2 ¼ 3.4 (E1=2 V [379]). However, the triphenylmethane dyes 108a–b have a significantly stronger acceptor in the core, which was concluded from the electrochemical red : 108a ¼ 0.7 V, data of a series of triphenylmethane compounds (E1=2 þ (C—(Ph)3) ¼ 0.19 V) [515]. The larger TPA cross section compared to 108a can be caused by the larger number of unsaturated carbons incorporated in 109, affecting M01 more than
m01 , and the higher planarity compared to the triphenylmethane dyes. The TPE spectrum exhibits an additional band at 800 nm, which is about twice the wavelength where OP resonance occurs. This supports the coupling mechanism disclosed above in which TPE splits into d1E0 (Eq. (59)) and d2A0 (Eq. (60)) according to the substitution pattern. R2 R2

N

R2 N

CN

CN

NC

R1

N

109

R1

R2

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

239

Compound 110 [425] exhibits less TPA, although it bears a triscyanobenzene central core. The TPE maximum resides at about 990 nm. This is the doubled wavelength of OPA but the main peak for TP excitation is at 780 nm. This agrees again with the model of dipolar coupling introduced in this section. The TPA cross section is smaller compared to the smaller conjugated system 109. Thus, the incorporation of anthracene moieties is not appropriate to increase TPA, showing the agreement of the results obtained for other triple-bond chromophores disclosed in Section III.C.1 [478]. Increase of the p-system by incorporation of an additional ethynyl-anthryl moiety has therefore only a minor impact on d [425].

C10H21

C10H21 N

C10H21 N C10H21

CN

NC

C10H21

CN

N

C10H21

110

Replacement of 1,3,5-triscyanobenzene in 109 by 1,3,5-triazine results in 111 [428]. This TPA chromophore also exhibits large TPA. 1,3,5-Triazine is an electron-deficient moiety. It has a reduction potential of about 2 V [516], showing that reduction needs less energy in 1,4-cyanobenzene (Ered ¼ 1:6V½493) but more in 1,2,4,5-tetrafluorobenzene (Ered ¼ 2:4 V [342]). Thus, 111 exhibits an additional chromophore with three dipolar arms, in which each arm contributes

240

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

to the photonic properties. The dipolar properties result in formation of an intramolecular charge transfer (ICT), which is experimentally evidenced by the solvatochromism observed upon change of solvent polarity (lmax fluorescence: 474 nm in benzene, 564 nm in CH3CN). The slopes obtained from the Lippert–Mataga equations (Eqs. (25a,b)) are comparable for many compounds known as fluorescent probes [337]. Compared to the twin compound 107 and the dipolar chromophore 52, 111 possesses the largest slope in the Lippert–Mataga equation. TPA cross section is 671 GM and was recorded at 800 nm. Although no scan of the wavelength was made in this contribution [428], one can assume that d is located in the region of the 2A0 transition. Compared to that of 109, the TPA cross section is smaller, mainly as a result of the lower electron-withdrawing property of the triazine.

R

R

R

N

N

R

N N

N

R

N

R

111

Change of the arylene-ethylene pattern in 111 by a fluorene moiety results in 112. This TP chromophore also shows large d with 137 GM at 790 nm, which was close to the maximum [508]. Data were taken using the WLC method, and the TPA spectra obtained for 112a–c are depicted in Figure 3.52. Spectra exhibit one peak at 779 nm and a second appears at 820–830 nm. The latter is about twice the wavelength at which OPA occurs [509]. This example shows the coupling mechanism in octupolar compounds, appropriately describing the TPA of such chromophores. In addition, ICT formation was also discussed as a result of both stationary and time-resolved experiments carried out in solvents of different polarity. The bathochromic shift of fluorescence observed upon increase of solvent polarity shows ICT formation (lmax fluorescence: hexane ¼ 432 nm, THF ¼ 496 nm). The octupolar pattern consisting of three dipolar arms in 112 was believed to be one reason for the surrounding dependent fluorescence [509]. In addition, the photochemistry of this chromophore occurs mainly

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

241

Figure 3.52. Two-photon absorption spectra for 112a (AF450), 112b (AF455), and 112c (AF457). (From Ref. [508] with permission of the American Chemical Society.)

from the excited singlet state, which was concluded from the low quantum yields for intersystem crossing isc. These considerations show the change in d caused by incorporation of 1,3,5triazine as the central core. In addition, increase of the electron density of the p-system, particularly at positions connected by an electron-donating group, significantly increases d. This was concluded by comparison of the TPA of

242

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

R1

N

R1

R2 R2

N

N N

R1

R2

N R1

R2

R2

R2

N R1 R1

112

113 and 112. Chromophore 113 peaks at 825 nm with an amplitude of 2300 GM. This is 20 times larger than that of 112. The importance of the triazine moiety as the electron-withdrawing group becomes more clear when we compare the d of 113 and that of 114. The latter bears a benzene moiety as the central core. This unit is a weak acceptor and therefore this compound is called a (Donor-p)3-p chromophore. Consequently, d decreases for 114 to 120 GM, peaking at 740 nm [504]. This decrease of d is caused mainly by the decreased dipolar strength of each individual arm in 114, which is important in order to disclose TPA according to the Eqs. (59) and (60). Presumably, electronic interaction between each individual arm in 114 maintains TPA in a region in which one can still assign a ‘‘large’’ TPA for this compound. Similar results were obtained for the p-branched TPA chromophore 115 [504]. The (D-p)3-p chromophore 116 exhibits d ¼ 510 GM, a larger value compared to that for 114. This can be caused by the larger p-system. Chromophore 116 shows broad TPE spectra, which makes these materials interesting for applications requiring a large tunable wavelength region. Synthesis of octupolar chromophores also results in an increase of OP quantities; that is, the molar extinction coefficient e is significantly enhanced. This becomes clearer in a synthetic study disclosing photonic properties of D-p-D (60c), (D-p)3-p (116), and a higher branched chromophore. The molar extinction coefficient (e) significantly increases with increase of branches at the central core [501, 502], while d behaves in the opposite direction—that is, it slightly decreases with increase of branches at the central core. This may be caused by the lowered dipolar

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

R1

R2

R2

N

N

S

243

R1

S N N

N

R1

R2

R2

N

N

S

R1

S

S

S

R1

N

R1

R2

N

R2

114

113 R R

R

S

S

R

R

S

S R

R

R R S R R S R 115

contribution in each individual arm in 116. Presumably, the mechanism explained by Eqs. (59) and (60) is not applicable for such chromophores. (Donor-p)3-Donor chromophores are another group of propeller-shaped chromophores. TPA studies of 117 indicated only a small d of 30 GM at 740 nm. The shape of the excitation spectrum showed that the maximum must be located at l < 740 nm [505]. Chromophore 117 has an amino nitrogen at the central core as donor, which combines three unsaturated arms with end capped amino

244

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

R1

R1 N

N

R1

N

R1

R1

R1 116

nitrogen. However, 100-fold larger d was determined for 118 [506], which belongs to the same group of chromophore but the rigid p-core between the amino nitrogens is larger and distinct compared to 117. While in the latter the  excitation density can be spread over a Ph—C C—  C—Ph rod, the Ph—C Ph—Ph—C C—Ph rod in 118 functions presumably more efficiently in the excitation mechanism. This is shown by the molar extinction coefficient, which is about twice as large in 118 as it is in 117 [293, 310]. Furthermore, one may consider 118 as a chromophore in which three D-p-D molecules are ‘‘stapled’’ at the centered nitrogen. This is concluded from the TPA of 60a. This chromophore possesses only one-third of the TPA with respect to 118, although the p-rod between the nitrogens is similar compared to 60a. Missing solvatochromism in 118 may show that the dipolar mechanism as explained in Eqs. (59) and (60) has only minor importance. An apprpropriate explanation for the enhanced d in (D-p)3-D chromophores was concluded in studies of the general structure 119 [517–520]. Structure 119a (Table 3.10) incorporates three arms of 57a, resulting in a fourfold increase of d. Structure 119a can also be considered as a chromophore comprising several moieties of triphenylamine, which are connected over vinylene bridges. Additional increase of d was reported if more branches were incorporated, resulting in 119b [517–520]. The TPA cross section increases in the ratio 57b:119a:119b 1:4:8.4. A model of strong electronic cooperative enhancement of TPA was discussed to explain this ratio. It was expanded to dendritic structures. Thus, the ratio found is close to d / N 2 ðN ¼ number of chromophores incorporated),

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

R1

N

245

R1

N

R1 N

N R1

R1

R1

117

R1

N

R1

R2 R2

N

R2

R2

R2 R1

R2

N

N

R1

R1 118

R1

246

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

scaling about 1:4:9 for 57a, 119a, and 119b, respectively. The chromophore in 119 comprises triphenylamine. Increase of N up to 30, in which each triphenylamine is connected by a vinylene bridge, results in a d of 11,000 GM [517]. Therefore, d increases with the square of N, indicating the impact of incorporated chromophores on d. On the other side, the OPA cross section scales linearly with N. The TPA absorption mechanism of (D-p)3-D chromophores occurring in 119 is quite different from octupolar chromophores. The TPA cross section depends on the transition dipole moments M01 and M12 as shown for 

M01 M12 d/
E02 E01  E02 =2

2

Thus, increase of size for the conjugated system has an impact on both M01 and M12 . R1

R1 N

N

TPA-H N R2

R2 N

R2

N

a: R1, R2 = H

b: R2 = H; R1 = TPA-H

R2

119

This is again different in the octupolar (A-p)3-D chromophores 120 [505] and 121 [508] for which the relationships in Eqs. (59) and (60) can be applied. The molecular pattern is depicted on the right side of Figure 3.51. The sulfonyl substituent put at the end of each arm in 120 is a strong electron-withdrawing group.

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

247

Adding a perfluorinated alkylene group can enhance the electron deficiency at this position. The data in Table 3.10 also show an increase of d with increasing size of the chromophore. TPA cross section of 120 is less compared to 119. This was concluded from a systematic study [505] and is confirmed by similar structures [514]. Therefore, systematic expansion of the general pattern of 119 is more successful to increase the TPA cross section as shown for some dendrimers based on 119 [517, 518, 520] as the incorporation of dipolar branches in 120 [505]. (D-p-A)3-D chromophores exhibit large TPA, as shown for chromophore 122 [490]. This compound can be considered a propeller-shaped molecule consisting of three quadrupolar arms. This study demonstrates that addition of each individual arm results in a twofold increase of d as shown for the TPA of 122 (d ¼ 5030 GM) in comparison with the quadrupolar chromophore 100c (d ¼ 1370 GM). The latter comprises one arm of 122. Substitution of one arm by a methyl group results in a twin chromophore exhibiting a d of 3130 GM. Thus, one can conclude that TPA depends on the number of arms (na), na  2, incorporated in both (D-p-A)na-D and (D-p)na-D chromophores. The structural pattern of 122 comprises three quadrupolar arms. Thus, Eqs. (59) and (60) cannot be applied to describe TPA of 122 because
m01  0 due to the quadrupolar pattern of each arm. Therefore, the enhacement of d is better explained by the increase of the transition dipole moments M01 and M12 because  d/

M01 M12
E02 E01  E02 =2

2

Presumably, cooperativity effects disclosed for 119a–c may explain the large d observed for 122. Pyryllium chromophores (123) have also become the target of TPA studies [169, 511, 521, 522]. Data were taken using the z-scan technique [522]. The structure of 123 can be assigned as a (p)3-A chromophore in which the unsaturated system bears, at some positions, slight electron-donating groups. Slight variation of the substitution strength from a slight donor (MeO) to an acceptor (Cl) significantly increases d. Thus, change of the structural pattern has an important impact on TPA. According to the model in Figure 3.51 (right side), one would expect the largest d for 123a because this substitution should favor the more efficient TPA of 123a in comparison with that of 123c or 123d. The latter bear only two dipole arms, and according to the model in Figure 3.51, they should exhibit lower TPA. The experiments showed the opposite behavior. A lot of work has been done to synthesize TPA chromophores with higher branches [502, 503, 507, 514, 517–520, 523]. Most of these chromophores possess both large OPA and TPA coefficients. A critical comparison of TPA

248

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

R

n

N

120 n

R n

S

N

N

121 N

N S

S

R

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

R1 N R1

NC

CN

CN N R1 N R1

NC

NC

CN

122

N R1 R1

R2

R1

O

123

R3

249

250

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

between D-p-D, and D-A-D molecules and the propeller-shaped chromophores discussed in this section shows that large d values can still be accomplished with a D-p-D pattern although the quadrupolar strength in D-A-D chromophores also has an impact on d.

E. Polymers with Nonlinear Absorbing Chromophores Polymers bearing nonlinear absorbing moieties are the subject of studies to determine the third-order nonlinear hyperpolarizability [70, 71, 73, 129, 170, 272, 273, 318, 398]. Such materials are particularly interesting for device manufacturing if a transparent film is a prerequisite. Therefore, a polymer or copolymer bearing the chromophore groups in the side chain displays one representative example (Figure 3.53a). Moreover, the chromophore may also be a part of the main chain. Thus, this moiety can be alternately linked with groups, increasing the solubility in common coating solvents (Figure 3.53b). The TPA cross section can be determined from the molecular weight of each repeating unit. It is therefore not necessary to determine the effective molecular weight as described in Section III.A.2, which is necessary for conjugated polymers. One representative example of a covalently bound chromophore is the polymer 124. This material was subjected to nonlinear absorption measurements using the neat film. The TPA coefficient was 7 cm/GW [524], which can be considered large. Furthermore, this polymer shows a reduction of the photon-number noise of 0.1 dB (4.6%) during nonlinear transmission. This is important in order to minimize the optical loss at the laser wavelength.

(a)

(b)

hn : π - system

: aliphatic segment/chain

n

n

hn

Figure 3.53. Graphical sketch for location of the chromophore moiety (a) in the side chain and (b) in the main chain.

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

x

O

y

251

z

NHCOCH3

SO3Na NH

R O

124

Azobenzenes are one of the most intensively investigated chromophores in nonlinear optics [129]. One dye is Disperse Red 19 (DR19), which has large nonlinear optical hyperpolarizability and therefore a large TPA cross section according to the dipolar substitution pattern [430]. Functionalization of the OH groups by polyaddition of a diisocyanate results in the polymer 125 [525]. This material exhibits a large first hyperpolarizability. Therefore, the TPA must be large according to the dipolar nature of the chromophore explained by Eq. (24). The first hyperpolarizability does not significantly differ using either the low molecular dye DR19 or the polymer material 125. O HN

NH O

O

O

N

O O

n

N

N

NO2 125

Large TPA was reported for the side chain polymer 126 [507]. This material bears the structure of the low molecular weight chromophore 109 in the side chain. Compared to 109, 126 exhibits similar d if one repetitive unit is taken for the calculation of this quantity. In other words, dividing the d values reported in this reference by the average number of repeating units determined by GPC results in a d that is comparable with that of 109. The d reported for this polymer is one of the largest values found for a side chain polymer. Similar results were

252

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

obtained for 127, which contains the chromophore in the main chain [507]. This polymer was made by polyaddition of a diisocyanate and a chromophore with two OH groups. These results demonstrate that manufacture of a polymer with a chromophore is possible without significant change of the optical properties by (1) radical polymerization/copolymerization of a double bond monomer resulting in the general structure depicted in Figure 3.53a and (2) polyaddition of a chromophore bearing two functional groups and having the capability to react with another reactant resulting in the general structure shown in Figure 3.53b. CH3 C CH2 C O

n

O N

O N

NC

O

O O

CN

N H

n

N

CN NC

N H

CN

CN C6H13

N

N

C6H13

C6H13

C6H13 126

C6H13

N

C6H13 127

Similar results were obtained for the polymers 128–130 [414, 526]. These materials were mainly investigated using OPA. The excellent film properties [414] make them attractive for device applications [218, 526]. Polymer 128 contains a modified distyrylbenzene moiety with a quadrupolar pattern, similar to 104, and an aliphatic spacer. It was made by polyreaction of a dialdehyde and a bisphosphonate [414, 526]. This polymer results in excellent film by spin coating. However, coating of the low molecular weight distyrylbenzene 104 results in a nontransparent crystalline film. TPA studies carried out in solution showed a similar d in comparison with the low molecular weight material 104 [527]. Using a similar synthetic approach—that is, reaction of a dialdehyde connected by an aliphatic spacer with bifunctional reactant—one obtains 129a–c and 130 [528]. These materials comprise an A-p-A chromophore. Replacing the methyl groups in 129b by alkoxy groups results in 129c, which differ in the length of the alkoxy substituent at the central core. Optical properties are summarized in Table 3.11.

253

CHROMOPHORE DESIGN AND OPTIMIZATION OF TWO-PHOTON ABSORPTION

TABLE 3.11 Compilation of Maximum for One-Photon Excitation (kOP max ), Maximum for Two-Photon Excitation (kTP max ), Two-Photon Absorption Cross Section (d), and Fluorescence Quantum Yield (f) Applying 150 fs Excitation and TPEF for the General Structures 128–130 in Different Solvents lOP max (nm)

Material

378 363 363 422 423 374

128a 128b 129a 129b 129c 130 a

lTP max (nm)

d(GM)

714 705 738 738 738 738

625 152 185 37 28 251

f

Solventa

Reference

0.52 0.21 0.01 0.26 0.22 0.06

CHCl3 CHCl3 CH3CN CH3CN CH3CN CH3CN

[414, 527] [414, 527] [528] [528] [528] [528]

CHCl3, chloroform; CH3CN, acetonitrile.

O O

n

O R F

O

F O

R

R: a: CH3O ; b: CH3

R F

F 128a–b

R

n R N 2 BF4

N R R: a = H; b = CH3; c = CH3O 129a–c

n N 2 BF4

N 130

254

1.0

500

0.6

400 300

0.4

200

0.0

0

0.0

440

100 80

0.2

400 λ / nm

120

0.4

100 360

140

0.6

0.2

320

(b)

0.8

60 40

δ /GM

0.8

absorption (a.u.)

600

(a)

δ / GM

absorption (a.u.)

1.0

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

20 0 320

360

400 λ / nm

440

Figure 3.54. TPA ( with data shown at the right y-axis) and OPA (solid line with data shown at the left y-axis) of 128a (a) and 128b (b) measured in toluene for TPA and in chloroform for OPA using a Ti:sapphire laser with 150 fs pulses for excitation [527].

OPA and TPA spectra of 128a–b are depicted in Figure 3.54. Compound 128a exhibits a significantly larger d (Figure 3.54a) in comparison with 128b (Figure 3.54b). The alkoxy groups placed at the distyrylbenzene result in a stronger quadrupolar strength compared to 128b, thus explaining the larger d of 128a. The influence of enhancing d as a result of quadruplar substitution on d was discussed in Section III.C [224]. Interestingly, both materials exhibit an additional absorption in the TPA spectrum where the S0 ! S1 transition occurs. The latter is TP forbidden in 128 according to the symmetry pattern of the chromophore. The progression of the additional absorption band can therefore be caused by vibronic induced transitions borrowing intensity from a higher TP allowed excited state, resulting finally in an enhancement of the optical transition. Such events are discussed in Section II.B.3 and were theoretically explained based on the Herzberg–Teller theorem [12, 19, 231, 235, 308, 309, 343–345, 371, 372]. The excitation amplitude of the TP excited state is larger compared to the S1 state, agreeing well with the results disclosed for C2h chromophores. In general, the TPA cross section is in the same order compared to the corresponding low molecular weight chromophores; that is, one repetition unit considering the p-system. The central benzene ring in 129b–c possesses a slightly increased electron density resulting in a bathochromic shift of OPA. The TPA of these compounds is smaller in comparison with 128. This is consistent with the results obtained for the low molecular weight materials 83, which are considered A-p-A type chromophores. The TPA of these chromophores is less efficient in comparison with chromophores bearing a D-p-D pattern. The larger TPA of 130 can be caused by the larger conjugation pattern of the connecting core. Film formation

TWO-PHOTON EXCITED FLUORESCENCE

255

of 128–130 occurs with satisfied quality, which makes these materials interesting for applications requiring a transparent film of the neat materials.

IV.

TWO-PHOTON EXCITED FLUORESCENCE

TP chromophores emitting strong fluorescence are of interest for TP microscopy [78–82, 84–112, 170, 171, 181, 192–195, 199–201, 208, 209, 211, 212, 242, 275–277, 356, 363, 366–368, 373, 374, 452, 529–540], known as fluorescence imaging. TP microscopy has received increased attention because of its use in biological applications [90, 91, 109, 171, 181, 192–195, 199–201, 275–277, 540]. This includes screening of the mechanism of action in cells, which are the subject of cancer research in chemotherapy [90, 91, 186, 193, 195, 199]. The mechanism of action occurring in these cells is not well understood on a molecular level. Application of TP excitation has the benefit of better light penetration into the sample if the linear absorption coefficient vanishes at the excitation wavelength. TP excited fluorescent materials can therefore overcome reabsorption of the material in which they are embedded. This is important for biological applications because absorption of collagen vanishes at l > 670 nm. Thus, TP chromophores bound to drug delivery materials can be very important in this field. The chromophore bound at the receptor is able to confirm entry of the drug into the cytoplasm and subsequently into the nucleus. The study of these processes is important to understand the mechanism of action in cells. Furthermore, research is also focused on the intrinsic TP fluorescence of the biological comprising structures (amino acids, proteins) that are able to fluoresce. The latter possesses only a small TPA cross section and higher laser energy would be required to image the fluorescence. This can result in destruction of the target material. Thus, the matters of choice are fluorescent probes. Their cross sections can be tuned according to the molecular guidelines discussed in this chapter. Fluorescent probes are often employed to obtain a sufficient signal-to-noise ratio in the fluorescence microscope. Embedding of fluorescent probes in biological material requires a certain solubility in an aqueous surrounding, which is the case of biological matter. From this point of view, most of the chromophores compiled in Tables 3.1–3.10 would not fit these criteria, and thus the number of prospective TP fluorophores is mostly limited by solubility problems. Thus, research started with the study of dyes showing a sufficient solubility in protic solvents [85, 86, 366–368, 373]. This research activity focused on building up a reliable database of TP absorbing materials for quantitative nonlinear microscopic studies. Fluorescein (12) was found as a reasonable TP standard because it covers a broad spectral range of the Ti:sapphire laser (Figure 3.55).

256

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.55. Plots of the TPE action cross sections (filled circles) for dextran – fluorescein at a concentration of 123 mM in H2O at pH11 (solid line). (From Ref. [373] with permission of the Optical Society of America.)

The following additional chromophores were investigated: Rhodamine B (131), Indo-1 (132), Coumarin 307 (133), Cascade Blue (134), Lucifer Yellow (135), Dansyl Hydrazine (136), DAPI (137), and Bodipy (138) [85]. Data are compiled in Table 3.12 [85]. In this series, Rhodamine B (131) exhibits the largest d. Surprisingly, the dansyl chromophore 136, a surroundingdependent fluorophore studied with OP photon excitation [541–543], exhibits TABLE 3.12

Collection of TPA Data of Chromophores Investigated as Bioprobes

Name Fluorescein Rhodamine B Indo-1 Coumarin 307 Cascade Blue Lucifer Yellow Dansyl hydrazine DAPI Bodipy

Number lTP (nm) d(GM) 12 131 132 133 134 135 135 137 138 139 140 141

782 840 700 776 750 860 700 700 920 810 810 710

df(GM)

38 210 12

1800 2150 180

19 2.1 0.95 0.72 0.16 17 180 430

Solvent Water, pH ¼ 11 Methanol Water Methanol Water Water Methanol Water Water Acetonitrile Acetonitrile Acetonitrile

Reference [85] [85] [85] [85] [85] [85] [85] [85] [85] [226] [226] [219, 544]

257

TWO-PHOTON EXCITED FLUORESCENCE

O O

O

N

O

O

N

O O

N

O

N

O

O

O

NH

CF3

CO2

HN C2H5

O O 131

132

H2N O3S

OR

O3S

SO3

O O

Li+

133

NH NH

N

O

O3S

134

O

NH2

O S O NHNH2

SO3 Li+

135

137

NH HN NH2

N H 137

NH2 F

B

N F

138

only a small TPA. In fluorescence imaging applications, the fluorescence crosssection action is important. According to Eq. (55), this is the product of f and d. A large TPA cross-section action is especially desirable when TP techniques are used to image molecular activities in living biological preparations. From this point of view, the chromophores discussed in Sections III.B–D form a reasonable starting point for future work in this field. Most of them possess a larger TPA cross-section action as shown for fluorescence by the compounds 12 and 131–138 in Table 3.12. Incorporation of substituents improving the solubility in either water or biological materials would definitively bring more effort in this research field. Chromophores 139 and 140 were investigated to show the feasibility of screening first and second main group metal ions [226]. The chromophore

O

258

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

skeletons of both compounds are comparable with the quadrupolar chromophore 100 [287]. The incorporated crown ether group can complex metal ions [545, 546]. This has widened the scope of research to develop fluorophores having the capability of detecting metal ions [547–553]. Thus, 139 and 140 are applicable fluorescent TP metal ion sensor materials, particularly for the second main group metal ions [226]. O

CN

O

N C4H9

O

O

N NC

C4H9

139 O

CN O

O

N O

O

N

O

O

NC

O

140

O O O F

F O

O

O

O F

F

O O O 141 2þ

The presence of Mg strongly affects the amplitude of TPA for 139 and 140 in the wavelength range between 720 and 860 nm. In the case of 139, d strongly decreases from its maximum at 800 nm by a factor of nearly 7 in the presence of 2 mM Mg2þ. This is more pronounced for 140. Addition of magnesium ions (2 mM) results in a decrease of fd excited at 810 nm by a factor of 15 (Figure 3.56). The determined binding constants show that 87% of 139 complexes the Mg2þ, resulting in a 1:1 complex. In the case of 140, 95% of the chromophore complexes the metal ion, yielding 61% of a 1:1 complex and 34% of a fully complexed 1:2 form. The drop of d upon addition of metal ions (Fig. 3.56)

TWO-PHOTON EXCITED FLUORESCENCE

259

Figure 3.56. Two-photon action spectra of 140 (open squares are data for the dye in the absence of magnesium; filled squares are data obtained with the fluorophore in the presence of 2 mM magnesium). (From Ref. [226] with permission of the American Chemical Society.)

assigns these fluorophores as on–off switchable systems; that is, fluorescence turns off in the presence of metal ions. The decrease of TPA in the presence of metal ions can be explained by the lowered donor strength of the nitrogen by the complexed metal ion, resulting in a decrease of the quadrupolar strength. The decrease of d upon addition of metal ions was additionally confirmed by 141 (Fig. 3.57) [544]. This compound was also investigated with OP excitation [219]. It exhibits a quadrupolar pattern resulting in surrounding-dependent fluorescence [219]. Similar to 139, complexation by metal ions results in a decrease of the electron-donating strength of the crown ether moiety and therefore a decrease of the intramolecular charge transfer. As a result, the TPA cross section decreases. Complexation of lithium ions by the 15-5 crown occurs in higher quantity compared to calcium ions. Consequently, the decrease of TPA at 710 nm is significantly larger for lithium ions compared to calcium ions. The broadest impact of TP excited fluorescent probes can be seen in the medical sector [70, 90, 91, 109, 171, 186, 192–195, 199–201]. TP chromophores have the potential to probe the entry of chemotherapeutic agents into the cell cytoplasm, which can be screened by TP microscopy. This technique has the capability to image small volumes down to the nanoscale. Incorporation of TP chromophores in biological materials requires a covalent binding of the dye at the drug delivery agent. This was successfully accomplished with the D-A stilbene C625 (41i), whose covalent linking by the glutaric acid at

260

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.57. TP excitation spectra of 141 in acetonitrile in the absence (upper curve) and in the presence of calcium ions (middle curve) and lithium ions (lower curve) [544].

the chemotherapeutic agent resulted in 142 showing no loss of fluorescence efficiency even after binding to AN152 [90]. The covalent linkage to the drug delivery agent can overcome the insufficient solubility of the stilbene C625. This treatment allowed study of the action of the therapeutic agent in human breast cancer cells at 37˚ C. The TP chromophore C625 in 142 exhibits a large d of 744 GM at 800 nm. This study has opened the possibility of screening processes in cells at the nanoscale. [D-Lys 6]LH-RH O

O O

O

O

O

S

O

O

O

HO

O

O

HN HO H3C

OH O

HO

N

O H3CO

AN 152

C625 142

ORGANIC REACTIONS UPON TWO-PHOTON EXCITATION

261

V. ORGANIC REACTIONS UPON TWO-PHOTON EXCITATION A. Isomerization Pulsed infrared multiphoton excitation can induce isomerization of (E)-2-nalkenes, crotonitrile, crotonic esters, 1,3-pentadienes, and 1,4-pentadienes resulting in Z isomers and distinct yields of fragmentation products [48]. Two or more isomerization processes are discussed for the case of laser-induced infrared multiphoton isomerization of hexadienes occurring within or shortly after a single laser pulse [28]. The ratio of isomerization/fragmentation depends on the structure of the compound and the laser intensity. The ratio of isomerization/fragmentation observed at constant laser fluence and reactant pressure decreases with increasing chain length ((E)-2-butene (3.0) > (E)-2-pentene (1.9) > (E)-2-hexene (0.44)). Increase of the laser intensity results in a higher ratio of yield/pulse for both isomerization and fragmentation products. However, the ratio of isomerization/fragmentation decreases with increasing laser intensity. In addition to fragmentation via C—C cleavage, loss of hydrogen is another reaction in the case of (E)-2-butene resulting in 1,3-butadiene formation, although this reaction is of minor importance. Pulsed infrared multiphoton excitation of crotonitrile results in quantitative isomerization to the Z isomer that is not accompanied by fragmentation [48]. Therefore, crotonitrile has a lower threshold for isomerization than (E)-2-alkenes. Isomerization of substituted butadienes, such as 2,3-dimethyl-1,3-butadiene and 2-methyl-1,3-butadiene, was observed with a yield of 25% without significant decomposition if samples were irradiated for a few hundred pulses [34]. A strong dependence of the average yield per pulse was observed for substituted butadienes and substituted cyclobutenes [40]. A linear dependence of the average number of photons absorbed per molecule on laser intensity indicates a constant excitation cross section with respect to the pulse energy for the substituted butadienes and cyclobutenes investigated, respectively. Product ratios obtained for single-pulse excitation of hexadienes exhibit an absorption of all isomers with trans or terminal double bonds [28]. However, cis,cis-2,4-hexadiene does not absorb for a laser source. Isomerization of cis,cis-2,4-hexadiene occurs by irradiation using the P(38) laser line of an infrared laser [28]. The use of several hexadiene isomers for irradiation experiments resulted in formation of multiple products even after exposure at 1.06 mm of a single hexadiene isomer with a single laser pulse. This had an intensity of about 3.5 J/cm2 or 2.5 J/cm2 in the case of a P(38) laser [44]. Two-photon induced isomerization of 1,2-dicyano-1,2-bis-(2,4,5-trimethyl-3thienyl)ethane (143) in films of poly(methyl-methacrylate) depends on the TPA

262

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

cross section of the isomers and the quantum yield of the photoisomerization [223]. The absorbance change is given by Eq. (61), where the steady-state absorbance of isomer B of 143 at 532 nm is called Abs1 532 . The TPA cross sections of 775 the A and B isomers of 143 at 775 nm are expressed by d775 A and dB in GM units, respectively. The quantum yields of the TP induced A(143) ! B(143) and B(143) ! A(143) reactions (Eq. (62)) at 775 nm are called 775 AB and , and the extinction coefficient of the B isomer of 143 at 532 nm is e532 775 BA B 1 1 (in M cm ), l (given in cm) is the film thickness, and c0 (in M) is the isomers’ total concentration in Eq. (61). Abs1 532 ¼

e532 B l c0 d775 775 1 þ B775 BA dA 775 AB

ð61Þ

The slope of the logarithmic plot of absorbance change versus the logarithm of the pump intensity is 1.84, indicating the occurrence of a nonlinear absorption process. The TPA cross section of isomer A is about 0.76 GM and an increased d was observed for isomer B (d ¼ 6 GM) due to the closed ring form and higher planarity. One-photon and two-photon induced isomerization of 143 shown in Eq. (62) results in similar spectra and in the same isosbestic points observed at 329 nm, 377 nm, and 429 nm. These results clearly indicate participation of the same excited state applying either OP or TP excitation. Quantum yields are close to unity. A NC

B CN

CH3

H3C H3C

CH3 S

S

NC hν

CN

H3C

CH3

ð62Þ

hν'

H3C

S

S

CH3

143

Photo-orientation of the chromophore was observed by TP induced isomerization. Such events can be accomplished by exposure to polarized light as shown for OP induced orientation and polarized light [554]. However, the molecular order induced by TP excitation is about half that of OP induced photo-orientation. Furthermore, the sign of the anisotropy induced by TP excitation is opposite that of OP excitation (Fig. 3.58). Two-photon induced isomerization occurs in case of 3-[1-(1,2-dimethyl-1 H-indol-3-yl)ethylidene]-4-isopropylidene-dihydrofuran-2,5-dione (144) in toluene excited at 775 nm using a 137-nJ pulse at a 1-kHz repetition rate. A new

263

ORGANIC REACTIONS UPON TWO-PHOTON EXCITATION

Figure 3.58. Absorbance of diarylethene embedded in PMMA films as a function of the angle between the infrared irradiation and the probe beam polarizations. The infrared polarization is either parallel (k) or perpendicular (?). The normalized absorbance is expressed by the ratio of ðAbs  Absmin Þ=ðAbsmax  Absmin Þ, where Absmin is the minimum absorbance and Absmax corresponds to the maximum absorbance. (From Ref. [223] with permission of Elsevier.)

absorption band appeared at 590 nm (Eq. (63)). The TPA cross section of 144a is 1030 GM at 775 nm [116]. O O O

N

O

hν1

hν2

O N

ð63Þ

O λ max = 385 nm

λ max = 590 nm

144a

144b

Two consecutive laser pulses are necessary to form cis-stilbene after irradiation of trans-stilbene in a 3:2 mixture of methylcyclohexane/2-methylpentane at 90 K [39] (Fig. 3.59a). Although [2 þ 2] intermolecular cycloaddition is the major pathway for OP excitation of trans-stilbene, this reaction is remarkably suppressed during nonresonant TP excitation in an excess amount of tetramethylethylene (Fig. 3.59a) [217]. Thus, the trans–cis isomerization of trans-stilbene is the main reaction path during nonresonant TP excitation [215]. Furthermore, nonresonant TP excitation of cis-stilbene using 532-nm laser pulses results in

264

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

main reaction

(a)

intermolecular [2 + 2] cycloaddition suppressed

TP excitation (532 nm)

(b)

final product OP excitation (266 nm)

O2

Figure 3.59. Sketch for photoreactions of trans-stilbene (a) and diphenylbutadiene (b) applying TP and OP excitation.

trans-stilbene in hexane, and no cyclization to dihydrophenanthrene was found, which was the major side reaction product in the case of OP excitation. These effects are caused by the difference in the selection between the OP and the nonresonant TP excitation. A [2 þ 2] cycloaddition reaction occurs from the lowest singlet 1Bu state of stilbene, which is OP allowed, while isomerization occurs from the TP allowed 2Ag state [217]. Excitation into the 3Ag state is followed by direct internal conversion to the nonfluorescent double-bond twisted state. Coupling with a torsional vibrational mode results in trans–cis photoisomerization without going through any potential barrier [32]. The efficiency of the nonresonant TP isomerization of stilbenes strongly depends on the wavelength of the laser pulses [216]. A similar effect was also found for nonresonant TP excitation of 1,4-diphenyl-1,3-butadienes with 532-nm laser pulses in a hexane solution. Selective formation of cis,trans-1,4-diphenyl-1,3-butadiene occurred, while OP excitation at 266 nm resulted in cis,trans-1,4-diphenyl-1,3-butadiene and

ORGANIC REACTIONS UPON TWO-PHOTON EXCITATION

265

1-phenylnaphthalene (145). The latter was formed by cyclization of cis,trans1,4-diphenyl-1,3-butadiene by consecutive oxidation [216] (Fig. 3.59b). Direct excitation of retinal (146) to the 31 A g state was studied by nonresonant TP excitation at 490 nm using laser pulses with an energy of 8.5 mJ and 5 ns pulse duration, resulting in formation of the 13-cis isomer as the main component and the 11,13-bis cis-isomer as by-product [222]. The relative speed of TP induced isomerization depends on the geometry of the isomer (all-trans > 7-cis  9-cis  11-cis  13-cis > 11,13-bis cis). This was different from OP excitation. O H 2

146

B. Cycloaddition Intermolecular cycloaddition reactions are suppressed during TP excitation, and isomerization reactions are favored under these irradiation conditions [217]. For example, [2 þ 2] intermolecular cycloaddition of trans-stilbene is remarkably suppressed during nonresonant TP excitation in an excess amount of tetramethylethylene, although this reaction is the major pathway during OP excitation [217]. Excimer laser photolyses of 1,2 bis-[(phenylthio)methylbenzene] (147) in acetonitrile results in generation of o-quinodimethane via a TPA event [555]. Photolysis of 147 in the presence of a dienophile (e.g., maleic anhydride (148)), results in a cycloaddition product of the o-quinodimethane and 148 (Fig. 3.60). Therefore, the cyclization observed in the case of TP excitation exhibits a consecutive reaction starting from the intermediate that is an excited state. The most important factor for efficient cycloaddition product formation (up to 48% yield) is the photon density, which can be increased by using either a higher laser intensity or a decreased optical path. A decrease of the optical path from 10 mm to 1 mm resulted in an almost twofold increase in the yield of the cycloaddition product. A similar increase in the yield of the cycloaddition product was obtained if the laser intensity was increased by a factor of about 25. 5,7-Dimethoxycoumarin (149) resulted in syn head-to-tail photodimer products excited either by simultaneous TPA (650 nm excitation, pulse width 120 fs, repetition rate 1 kHz) or by common OP broadband UV excitation [487]. Syn head-to-head and syn head-to-tail photodimers (Fig. 3.61) were formed in 1:3 and 1:2.8 ratios under OP (broadband UV) and TP (650 nm, 120 fs) irradiation in anisole. Furthermore, a tenfold increase of the quantum yield was observed under TP excitation (102) compared to broadband UV irradiation (103).

266

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

SPh

SPh excimer laser - S-Ph

SPh

147

excimer laser

- S-Ph

O +

148

O O H

O O

H

O

Figure 3.60. Sketch of TP excited excimer laser photolyses of 1,2 bis-[(phenylthio)methylbenzene] (147) in acetonitrile resulting in generation of o-quinodimethane, which reacts with maleic anhydride (148).

C. Singlet Oxygen Photosensitized generation of singlet oxygen 1O2 (a31
g) is being examined in different disciplines ranging from photobiology to polymer science [191]. Although ground state oxygen (X3 g) can quench the S1 state of some sensitizers, singlet oxygen is rather more efficiently generated from the triplet state (T1). The latter is generated by an intersystem crossing (ISC) from the S1 to the T1 state. From this point of view, TP chromophores are also suitable for generating singlet oxygen by a TPA process since they can populate the T1 state. Using a symmetric chromophore, TPA results in population of a higher excited state, which relaxes into the S1 state by electronic coupling as shown in

267

ORGANIC REACTIONS UPON TWO-PHOTON EXCITATION

O

O O H H

O

O

O O H H O

O O

O

O hν

O

O H H

O

O

syn-Head-to Head (minor)

H H O

O

syn-Head-to Tail (major)

O O

O

O

O O H H

149

O O H H

O

O O

O

O O

OH H O

anti-Head-to Head (minor)

H H O

O

anti-Head-to Tail (minor)

Figure 3.61. Photocycloaddition products formed by TP excitation of 5,7-dimethoxycoumarin (149) in anisole using a Ti:sapphire femtosecond laser system [487].

Figure 3.2a. This state is photoactive and it generates the T1 state in a consecutive step (Fig. 3.62). Generation of singlet oxygen by TPA may also have an impact for prospective applications. TPE occurs mainly at the focus of the collimating lens. Excitation of the target sample can occur even in thick samples E

Sn

electronic copuling ISC

T1

S1 + O2(X3Σg- )

S0

O2(a31∆g)

S0

Figure 3.62. Mechanism for photosensitized generation of singlet oxygen using TP excitation.

268

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

because penetration of the excitation light inside the sample occurs with no significant attenuation as long as the linear absorption process vanishes at the excitation wavelength. This may be important for photodynamic therapy if an excitation length being used is greater than the absorption of collagen (>670 nm), for example, where human skin is the biological target. From this point of view, chromophores with large d are required to maintain the excitation power as low as necessary in order to avoid material damage caused by the excitation beam. The experimental setup for quantitative determination of singlet oxygen is depicted in Figure 3.63 [179]. Either a ns laser or fs laser is used for excitation. The femtosecond experiments use a regenerative amplified Ti:sapphire system that operates at a repetition rate of 1 kHz. Samples are irradiated at 802 nm with 90-fs pulses. The nanosecond experiments use a Nd:YAG pumped optical parametric oscillator (OPO) as the irradiation source (pulse FWHM 5 ns, 10-Hz repetition rate). The laser intensity is varied over the range using neutral density filters placed in the unfocused beam. For a dual-beam experiment, a prism splits the beam into two parts. The polarization of the beam in one path is then chosen to be either parallel or perpendicular to the other beam path using a zero-order l/2 phase retardation plate. The beams are then focused into the sample using the same 35-mm focal length lens. The respective beams intersect at the same point in the sample. The 1270-nm singlet oxygen emission is detected through a silicon window and an interference filter. Singlet oxygen phosphorescence is monitored with a Ge detector operated at 77 K. TP chromophores investigated for sensitized generation of singlet oxygen are difuranonaphthalenes (150) [179, 180], the distyrylbenzene 103 [179, 180],

Figure 3.63. Experimental setup for detection of singlet oxygen generated by TP sensitizers. (From Ref. [179] with permission of the American Chemical Society.)

ORGANIC REACTIONS UPON TWO-PHOTON EXCITATION

269

TABLE 3.13 Compilation of Selected Data of TP Chromophores Applied for TP Sensitized Manufacture of Singlet Oxygen Chromophore

lTP(nm) d(GM) 


150a1, R ¼ H 150a2, R ¼ CN 150a3, R ¼ CHO 150a4, R ¼ Br 150b, R ¼ H 150c1, R1 ¼ H, R2 ¼ CO-Ph, R3 ¼ C2H5 150c2, R1 ¼ H, R2 ¼ H, R3 ¼ C8H17 150c3, R1 ¼ CN, R2 ¼ H, R3 ¼ C8H17 150c4, R1 ¼ Br, R2 ¼ H, R3 ¼ C8H17 150c5, R1 ¼ CHO, R2 ¼ H, R3 ¼ C8H17 150d 103c 100b 151a 151b 151c

618 618 618 618

8 139 205 7

618 618 618 618 618 618 835

70 8 139 7 205 780 450

0.36 0.36 0.49 0.42 0.37 0.67 0.36 0.36 0.42 0.49 0.13 0.46 0.08 0.46 0.08 0.09

Solvent Reference Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene Toluene

[80] [180] [180] [180] [180] [180] [179] [179] [179] [179] [180] [179, 180] [183] [183] [183] [183]

Note: lTP ¼ wavelength related to TPA cross section measured, d ¼ TP absorption cross section, 
¼ quantum yield of singlet oxygen formation.

triple-bond chromophores [183], and phthalocyanines [181, 190, 203, 204, 556]. A large compilation of quantum yields for sensitized singlet oxygen generation can be found in Ref. [182]. Data are compiled in Table 3.13. The difuranonaphthalenes 150a–b and 150d possess large quantum yields for the generation of singlet oxygen (
). The TPA cross section is the largest in this series for 150d. It decreases for 150a depending on the nature of the substituent R. The larger d of 150d is caused by the larger p-system, while the twist of the terminal phenyl rings distorts the geometry and the system cannot maintain its planarity. The terminal benzoyl structures in 150c presumably favor an efficient intersystem crossing, which explains the large 
according to the mechanism in Figure 3.62 if singlet oxygen is formed by the triplet state of the sensitizer. The role of the triplet in the photosensitized generation of singlet oxygen is supported by the data of 103c, which has two heavy atoms promoting the ISC. Although 103c possesses a fairly large fluorescence quantum yield, the large quantum yield for singlet oxygen formation leads to the conclusion that the triplet of this chromophore must be involved in singlet oxygen formation. The distyrylbenzene 100b has cyano groups instead of bromine. This compound also strongly fluoresces but the quantum yield for singlet oxygen drops tremendously

270

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

C2H5 O

O R

R

R

R O

O

C2H5 150a

150b R3 O R1

R2 R2

R1 O R3

C2H5

150c

O

CHO

O

OHC C2H5

150d

as a result of less efficient ISC. In addition, the triple-bond bearing chromophores 151a–c [183] were investigated in order to demonstrate the relationship between the structural pattern and quantum yield of singlet oxygen formation (
). Compound 151a bears heavy atoms at the central cores favoring ISC while the central cores of 151b and 151c do not have heavy atoms. Consequently, 151a possesses a fairly large quantum yield for triplet formation, supporting the mechanism shown in Figure 3.62 involving the triplet state. The structure of 151b is comparable to 74. The latter is highly fluorescent and should therefore have a low fluorescence quantum yield for ISC. The results in Table 3.13 demonstrate that both high fluorescent chromophores 153b–c possess a low 
. Interestingly, the absorbance of 151a remains nearly constant during the sensitized manufacture of singlet oxygen, while the distyrylbenzene 103c exhibits significant bleaching. This is caused by the double bond having a larger reactivity with singlet oxygen compared to the triple bond. This example shows that triple-bond chromophores are interesting compounds in this part of photosciences according to the better photostability in presence of singlet oxygen, and their attractivity is going to increase within the next few years because of the

271

ORGANIC REACTIONS UPON TWO-PHOTON EXCITATION

efforts in organic chemistry that have opened new synthetic routes for the manufacture of such interesting structures.

R2

a: R1 = Br; R2 = H b: R1 = R2 = H c: R1 = R2 = F

R1

N

N R1

R2 151

Singlet oxygen studies were also carried out with the chromophore 152 [190]. This compound bears at the center a covalently bound porphyrin with additional eight chromophore moieties of 43e. A large TPA was reported for 152 [186, 190]. These studies demonstrate the fluorescence resonance energy transfer (FRET) from the TP chromophore 43e to the porphyrin core [556]. The emission of 152 overlaps with that of the isolated porphyrin structure of 152 (i.e., R ¼ CH3), which is a prerequisite for FRET. The efficiency of energy transfer was determined with 97%. Thus, the emission of singlet oxygen generated by TPA was about 17 times stronger compared to 152 (R ¼ methyl), applying 780-nm excitation. This example demonstrates the suitability of incorporation of a dendrite array with TP chromophores on a porphyrin resulting in the enhancement of singlet oxygen formation, in addition to the feasibility of transfering the excitation energy from a TP excited chromophore with low photochemical reactivity (singlet oxygen formation) to a chromophore with low TP absorptivity but higher photochemical reactivity.

R R: O

R

O

N

R

R N NH

HN N R

R

S N R

R 152

272

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

VI.

SIMULTANEOUS TWO-PHOTON INITIATED POLYMERIZATION AND CROSSLINKING

Two-photon initiated crosslinking polymerization has gained considerable importance for three-dimensional microfabrication and nanofabrication, for manufacturing three-dimensional data storage systems requiring high precision, such as optical band-gap materials [74–76, 114, 133–136, 139–142, 146, 147, 149, 150, 251, 263, 265–269, 362, 426, 445, 525, 526, 557–562]. Despite the increasing practical importance of TP initiated crosslinking polymerization, basic research has particularly concentrated on the synthesis and analysis of new TP active chromophores, which have proved useful as OP sensitizers or OP photoinitiators. Furthermore, conventional photoinitiators have been tested in TP initiated polymerization as well. However, the efficiency of such conventional photoinitiators is low in TP initiated polymerization because of their low TPA cross sections compared to chromophores, as shown in the Tables 3.1–3.10. Conventional photoinitiators such as Irgacure 184 (153), Irgacure 261 (154), Irgacure 369 (155), Irgacure 651 (156), Irgacure 754 (157), Irgacure 819 (158), Irgacure 907 (159), Doracure TPO (160), Doracure MBF (161), Doracure 1173 (162), CD 1012 (163), ITX (164), and Irgacure OXE01 (165) exhibit TPA cross sections (d) lower than 30 GM [376]. Only Irgacure OXE01 (lmax ¼ 328 nm) has a d value of 38 GM obtained by the WLC method, which is also not sufficient [376]. Therefore, new photoinitiator systems have been developed exhibiting large d values. Such TP sensitizers must undergo efficient TP initiated reactions resulting in sensitization of polymerization initiating species. TP sensitizers with large TPA cross sections (d) are important in the use of low-cost microlasers [263]. A wide variety of photoinitiators have been investigated for polymerization of different monomers, such as acrylates, epoxides, vinyl ethers, and thiol-ene monomers. From this point of view, the D-p-D or A-p-A chromophores are favored sensitizers if they are combined with a coinitiator [269, 563]. The sensitizer excited by TPA can be either oxidized (route A) or reduced (route B) by the coinitiator, depending on the chemical structure of the coinitiator (Fig. 3.64). Coinitiators according to route A are electron-deficient materials. Representative examples are onium compounds and triazines [269, 563]. The free energy of photoinduced electron transfer (
Gel) between a photosensitizer and a coin1=2 itiator is described by Eq. (64), in which Eox is the half-wave oxidation poten1=2 1=2 tial (route A: Eox is representative for the sensitizer; route B: Eox stands for the 1=2 1=2 coinitiator), Ered is the half-wave reduction potential (route A: Ered is represen1=2 tative for the coinitiator; route B: Ered stands for the sensitizer), and E00 is the excitation energy of the sensitizing chromophore. 1=2

1=2  Ered  E00
Gel ¼ Eox

ð64Þ

SIMULTANEOUS TWO-PHOTON INITIATED POLYMERIZATION AND CROSSLINKING

N Fe

PF6 O

OH

N O

O 153 (λ max = 246 nm)

154 (λ max = 242 nm)

155 (λ max = 324 nm)

O

O

H3CO

O OCH3

O

O

O

O

O 156 (λmax = 254 nm)

O

157 (λmax = 253 nm)

O

O

O

O

O

P

P

N S O 158 (λ max = 295 nm)

159 (λ max = 306 nm

O

160 (λ max = 299 nm)

O OCH3

I OH

O

OH

161 (λ max = 255 nm)

162 (λ max = 244 nm)

SbF6

163 (λ max = 247 nm) O

O

S

N

O

O S 164 (λ max = 382 nm)

165 Irgacure OXE01 (λ max = 328 nm)

273

274

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

route A

Sensitizer

Coinitiator

route B

[Sensitizer]*

Coinitiator

Sensitizer Coinitiator

hn

Coinitiator

Sensitizer

Figure 3.64. Sketch of two possible reactions of a sensitizer in combination with a coinitiator relying on redox photochemistry with the possibility to oxidize the sensitizer (route A) or to reduce the sensitizer (route B).

In some cases it is necessary to add a second coinitiator to the composition in order to make the back electron transfer irreversible.

A. Radical Two-Photon Initiated Polymerization Several TP initiators containing the general D-p-D pattern have been tested in TP initiated polymerization of acrylates [133, 134, 150, 263, 265, 268], methacrylates [135, 139, 264, 557], and acrylamide [562] (Table 3.14). These TP initiators possess large d values. They also have a chemical functionality that can be activated by excitation of the chromophore, that is, the capability for electron transfer resulting in an efficient initiation of the polymerization. This was shown for 47, which has a solvent-dependent d [445]. Quantum chemical calculations indicated a complete charge separation in the S1 state. Thus, the negative charge formed at the acceptor part may be able to initiate the polymerization of methacrylates. Excitation of the chromophore by simultaneous absorption of two photons and electron transfer reaction with the coinitiator (Fig. 3.65) results in the initiating species (I
). The latter initiates polymerization of the monomer, followed by chain propagation and termination (Fig. 3.65). The TP initiators summarized in Table 3.14 exhibit a higher photosensitivity in comparison with the conventional ultraviolet-absorbing photoinitiators. Furthermore, (E,E,E,E,E,E)-1,13-bis-[4-(diethylamino)phenyl]-tri-deca-1,3,5,6,8,10,12-hexaen-7-one (166) shows an absorption maximum at 509 nm with a strong molar extinction coefficient of 79,500 M1cm1. TPA of this compound peaks at 950 nm, exhibiting a d of 200 GM using rhodamine B as reference (Fig. 3.66) [263]. The threshold-absorbed energy density was determined as 5500 J
cm3, and the threshold incident exposure dose was 2050 J
cm2. Additional TP photoinitiators are the chromophores 167–172 [135, 139, 268, 557].

SIMULTANEOUS TWO-PHOTON INITIATED POLYMERIZATION AND CROSSLINKING

275

TABLE 3.14 Two-Photon Initiators for Radical Polymerization of Acrylates and TP Methacrylates: kOP max ¼ One-Photon Absorption Maxima, kmax ¼ Two-Photon 1=2 Excitation Maxima, d ¼ Two-Photon Absorption Cross Section, Eox ¼ Oxidation potential Two-Photon Initiator lOP max (nm) 57b 57c 57f 58a 58d 58e 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176

lTP max (nm) d(GM)

374 390

600 645

410 425 448 246 242 324 254 253 295 306 299 255 244 247 382 328 509 349 374

730 730 775 500 500 636 500 500 600 600 600 500 500 546 754 660 950 532 532 800

381 401

800 800 800 800 800

210 260 1300 995 900 1250 <20 <20 27 <20 10 <5 <5 <5 <20 <20 14 4 38 200 30 80 30

1=2

EOX (mV)

Reference

a

35 80a

[134] [134] [225] [134] [35] [134] [376] [376] [376] [376] [376] [376] [376] [376] [376] [376] [376] [376] [376] [263] [268] [268] [135] [139] [557] [557] [562] [562] [562] [558]

þ90a 10a 45a

380b 320b 0.2 63c 156c

10 10 10 3146

Versus ferrocenium/ferrocene in tetrahydrofurane/ 0.1 M [Bu4Nþ][PF6]. Relative to Ag/Agþ. c Relative to phenothiazinium/phenothiazin. a b

O

C2H5

N

N C2H5

C2H5

C2H5 166

276

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.65. Mechanism of radical polymerization of an acrylate after excitation of a TP initiator.

Figure 3.66. Two-photon absorption spectrum of 166 in chloroform obtained by upconversion fluorescence measurements using a pumped Nd:YAG laser supplying 2.6-ns pulses in the spectral range from 780 to 1120 nm with 10-Hz repetition rate. (From Ref. [263] with permission of Elsevier.)

277

SIMULTANEOUS TWO-PHOTON INITIATED POLYMERIZATION AND CROSSLINKING

O

O

N

N

N

N R2

R1 O

167

O

168

N I

N H N

O

O

O

O

OBu

I 169

170 C4H9 N

C4H9

S N C2H5

C4H9

171

C4H9

C4H9

N

N

Cl

C4H9

S

COO–

Cl

I

I N C2H5

O

O I

172

COO–

COO– Br

I

I O

O 174

Br

O

O–

O Br

I

I

173

O N

175

N

176

O– Br

O– I

278

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Efficient TP excitation requires a high peak laser power. This can be adjusted locally with high precision, resulting in TPA particularly at the focal point in a small volume of microscopic size. This is the biggest advantage compared to OP excitation. However, the TP initiator must have a high TPA cross section at the operation wavelength of the laser. Furthermore, a triethanolamine coinitiator was used for sensitized TP polymerization of acrylamide initiated by the xanthene dyes 173–175 [562]. Another example for a TP sensitizer is 2,7bis[[4-(dimethylamino)-phenyl] methylene]-cycloheptanone 176, which was applied together with 1,10 -2,20 -bis(o-chlorophenyl)-4,40 ,5,50 -tetraphenyl-bisimidazole as coinitiator, and 3-mercapto-4-methyl-4H-1,2,4-triazole as a chain transfer agent in the polymerization of 2-phenoxyethyl acrylate in the presence of cellulose acetate butyrate as the binder polymer [558]. The crucial point was to quantify the sensitivity of TP initiated polymerizing systems. These reactions have been studied mostly in solutions in which the solvent comprised a crosslinkable monomer. The information regarding monomer conversion must be obtained from a small reaction volume (i.e., the focal point). Standard analytics do not work for this purpose. Thus, investigations focused on the irradiated volumes that changed from liquid to solid. SEM was found as to be a versatile tool to determine the necessary exposure energy of a TP photoinitiated system. Figure 3.67 shows the relation between exposure time and pulse energy [225].

Figure 3.67. SEM image of some of the columnar structures produced by irradiating a solution comprising a TP photoinitiator and a crosslinkable monomer applying a dosearray measurement. The width of the smallest feature is 60 mm. Across the array, the pulse energy increases from left to right, and the exposure time increases from front to back. For the four visible columns, the pulse energies used are 0.8, 1.0, 1.2, and 1.5 mJ. (From Ref. [225] with permission of Elsevier.)

SIMULTANEOUS TWO-PHOTON INITIATED POLYMERIZATION AND CROSSLINKING

279

One can easily see that polymer formation, as shown by the rod-like structures, depends on both parameters. There is still a need for methods allowing quantification of processes occurring in the focal volume of a TP initiated process. Kinetic analysis of TP induced radical polymerization was done on the basis of the general reaction mechanism of radical polymerization depicted in Figure 3.65 [136]. According to this mechanism, a radical is formed by TP excitation of a sensitizer resulting in an electron transfer to either the coinitiator or the monomer. The radical formed reacts with the monomer in the initiation reaction, forming a new radical species. This new radical species reacts with further monomer molecules, forming a polymer radical in the propagation reaction. Termination reaction occurs by disproportionation or recombination. The rate of monomer conversion (d[M]/dt) is considered equal to the rate of propagation given in Eq. (65), where kp is the rate constant for propagation, [M] is the monomer concentration, and [P
] is the total concentration of propagating polymer radicals. Crosslinked TP initiated polymerization has not been considered for first-order termination and termination by primary radicals. This was disclosed as common competitive termination for OP initiated polymerization of multifunctional (meth)acrylates in polymeric binders [564–566]. 

d½M ¼ kp ½P
½M dt

ð65Þ

The time-dependent concentration of propagating radicals is given by Eq. (66), where kt is the rate constant for second-order termination, and Ri represents the initiation rate. d½M
 ¼ Ri  2kt ½M
2 dt

ð66Þ

For simplicity, only second-order termination is considered in the Eqs. (66)–(69). Considering the rate for generation of excited photoinitiator molecules by the TPA process and the efficiency for formation of polymer radicals results in an expression for the average initiation rate in Eq. (67), where R is the quantum yield for generation of polymer radical, dTPA is the two-photon absorption cross section of the TP initiator with the concentration [I], ho is the photon energy, Ð 2 I dt is the square of the light intensity integrated over one pulse, and f is the pulse repetition rate of the laser. 0 Ri ¼  R

dTPA ½Sensitizer B @ 2ð hoÞ2

ð pulse

1 C I 2 dtAf

ð67Þ

280

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

The concentration of polymer radicals increases during exposure until a steadystate polymer radical concentration is reached. The steady-state concentration of the polymer radical ½P
s is given by Eq. (68):  1=2 Ri ð68Þ ½P
s ¼ 2kt For the short exposure time used in TPA experiments (e.g., about 10 ms), the rate of polymerization given by Eq. (69) is influenced by laser parameters and material parameters including the rate constants for propagation (kp) and termination (kt) as well as the properties of the photoinitiator. kp 1 1=2 1=2 ¼ kp ½M
s ¼ pffiffiffiffiffiffi Ki Kl tp 2kt

ð69Þ

where tp ¼ lifetime of a propagating polymer radical. The term Ki is expressed by Eq. (70) while Eq. (71) displays a relation for the intensity. The laser parameters include the intensity of the laser light, the pulse duration, and the repetition rate. Ki ¼ R dTPA ½Sensitizer   Ð 2 I dt fl Kl ¼

ð70Þ

pulse

ð71Þ

2ðhoÞ2

Using a TPA initiator with a high TPA cross section, for example, 167, the speed of polymerization was approximately the same for either a nanosecond microlaser or a femtosecond laser [136]. Mixtures of a polyester acrylate, 1,6-dihexanediole diacrylate and a dendrimer containing acrylamide end groups doped with 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyrane (177), and 4,40 bis(N,N-dimethylamino)benzophenone (178) as photoinitiator were applied for TP initiated crosslinking polymerization using a Nd:YAG laser (532 nm, 8-ns pulse width) [559, 560]. The dendrimer used in this study encapsulates the laser dye and functions as a limiter for energy transfer during photochemical processes, resulting in formation of grating structures by photopolymerization. Dendrimers can also contain a chemically bound TP active group [567]. NC

O

CN O N

N 177

N 178

SIMULTANEOUS TWO-PHOTON INITIATED POLYMERIZATION AND CROSSLINKING

281

Furthermore, the time dependence for the density of radicals (r) obtained by irradiation with femtosecond laser pulses can be described by Eq. (72) [149]: @r ¼ ðr0  rÞ deff F 2 @t

ð72Þ

The primary initiator particle density (r0 ), the effective TPA cross section (deff), and the photon flux (F) relate to the radical density. The effective TPA cross section deff is defined as the product of the ordinary TPA cross section (d) and the efficiency of the initiation process (Zi) in Eq. (73): deff ¼ d Zi

ð73Þ

The photon flux is considered to be constant during the laser pulse, and the losses of radicals are neglected between the laser pulses. Using these assumptions, the pixel diameter (d) obtained by radical polymerization is obtained by using Eq. (74), where tl is the pulse length. The term C is given by Eq. (75), where r0 is the primary initiator particle density, and rth stands for the minimum of primary initiator particle density at the threshold energy. The pixel length (l) was estimated by Eq. (76), where zR is the Rayleigh length [149]. The photon flux Nfl is considered as constant. 

1=2 lnðdeff Nfl2 n tL Þ dðN0 ; tÞ ¼ r0 C   r0 C ¼ ln ðr0  rth Þ " 1=2 #1=2 deff Nfl2 n tL lðN0 ; tÞ ¼ 2zR 1 C

ð74Þ ð75Þ

ð76Þ

B. Cationic Two-Photon Initiated Polymerization TP initiated cationic photopolymerization is an additional research target in TP photosciences [225]. The crosslinking of higher functional epoxies benefits from the lower shrinkage in comparison with acrylates [269]. In particular, those TP active systems with the capability to transfer an electron according to route A in

282

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.64 result finally in generation of Hþ/Lewis acids that can initiate polymerization of oxiranes, oxetanes, or vinylethers. Parameters tuning the efficiency of the electron transfer between the TP sensitizer and the onium compound are 1=2 1=2 Eox , Ered , and the excitation energy of the sensitizer according to Eq. (64). Nevertheless, this bimolecular electron transfer reaction is diffusion controlled as long as
G < 0. Thus, increase of viscosity, which has been a regular phenomenon of crosslinking polymerizations, can result in a decrease of the rate constant for the bimolecular diffusion controlled electron transfer reaction. In addition, such reactions may be frozen if no diffusion occurs between both reactants, which is the case when the system begins to vitrify. Diffusion controlled processes for Hþ/Lewis acid generation upon both OP and TP excitation can be of minor importance if the onium compound comprises a part of the TP chromophore. Compound 179 is such a chromophore. This compound combines a distyrylbenzene with end capped amino groups. The latter bears a sulfonium group in m-position. OPA of 179 occurs at 392 nm (e ¼ 5:5  104 M1cm1). TPA was investigated between 705 and 850 nm. The TPA cross section of 179 is 690 GM (Fig. 3.68a). Furthermore, the quantum yield of OP initiated generation of Hþ is about 0.5, while the quantum yield for fluorescence is significantly less (f ¼ 0:013) [137, 225]. TP initiated generation of Hþ/Lewis acid of 179 occurs with both OP and TP excitation. Figure 3.68b shows the dependence of [Hþ] on the square of the excitation power, indicating generation of Hþ by a TPA process. This efficient TP initiated generation of Hþ was successfully applied in amplified TP positive resists [137, 264]. However, it is not clear from the original references whether Hþ/Lewis acid formation occurs according to a heterolytic mechanism, as demonstrated for many other sulfonium compounds applying standard OP excitation [269, 563, 568–572], or by intramolecular electron transfer according to

6

800

(a)

–4.0

(b)

700

5

600 500

3

400 300

2

200 1 0 250

–4.5

log([H +] / M)

4

–5.0

Slop = 2.3 –5.5

100 300

350

400

Wavelength / nm

450

0 500

–6.0 –1.7 –1.6 –1.5 –1.4 –1.3 –1.2 –1.1 –1.0 –0.9

log(Excitation power /mW)

Figure 3.68. Acid generated under TP excitation of 179 [225] at 745 nm in acetonitrile. (From Ref. [225] with permission of Elsevier.)

283

APPLICATIONS

C 4 H9

S N N 2 SbF 6

C 4H9 179

S

the mechanism in Figure 3.64 (route A). The latter is reliable for many onium compounds and thus for sulfonium salts as well.

VII. APPLICATIONS Two-photon absorption (TPA) processes have been successfully applied to initiate crosslinking photopolymerization, resulting in high local confinement of 3D structures in microdevices [74–76, 88, 113–115, 133–135, 138, 139, 141, 142, 146, 147, 150, 251, 263, 265, 269, 426, 559, 561, 573]. Attenuation of excitation light in the sample occurring in a OPA event and the fact that TP initiated processes vanish outside the focus have become important aspects favoring TP excitation for applications needing an accurate resolution. This can be significantly lower, up to the diffraction limit of the excitation light. Thus, TP initiated processes exhibit a spatial confinement of >120 nm, keeping again in mind that most examples were excited with red/NIR light. Scanning the focal laser spot according to preprogrammed patterns results in solidified points, lines, and volumes. A solid skeleton can remain after developing the nonreacted resin. The spatial resolution of this microfabrication technology is defined in dimensions of the smallest achievable volume in which the photochemical reactions occur, while the surrounding region is not modified. The spatial resolution can be improved by reducing the beam–sample interaction area, for example, by near-field optical microscopy (NFOM). Photonic crystals, or photonic bandgap materials, have received a lot of attention because they may be useful in optical waveguides, optical gratings, bits for 3D optical memory, and fabrication of other optical and photonic devices. Fabrication of photonic band-gap materials using laser microfabrication by TPA photopolymerization results in lattice structures with improved spatial confinement compared to OP technologies [150]. The 3D photonic band-gap materials can be manufactured in a single step, resulting in a periodic error of about 50 nm [151].

284

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

A. Three-Dimensional Micro- and Nanofabrication Two-photon three-dimensional (3D) micro- and nanofabrication using a femtosecond laser have been used to create various types of 3D micro- and submicrometer structures [70, 74, 260, 265, 574]. A microscope with axial (z) and lateral (r) resolutions given by Eqs. (77) and (78) was used for laser microfabrication [574]. Wavelength of irradiation light (l), refractive index of the material (n), and numerical aperture of the objective lens (NA) influence the resolution in axial and lateral directions. z¼2

nl NA2

r ¼ 0:61

l NA

ð77Þ ð78Þ

Usually, oil-immersion objectives with a numerical aperture higher than 1 are used to obtain a high spatial resolution. The size of the focal spot containing 84% of the light energy is given by Eq. (79). For l ¼ 0:8 mm and a numerical aperture of 1.3 the size of the focal spot is 0.75 mm. 1:22 l ¼ 0:75 mm NA

ðl ¼ 0:8 mm and NA ¼ 1:3Þ

ð79Þ

Assuming Gaussian beams, a stronger localization of local excitation confinement is expected at the focal point. Further factors of influence are thermal conductivity, heat capacity, melting temperature, boiling point, overheating at the focal point, and sound velocity of the material. The diffusion related processes are dependent on the pulse duration. Subpicosecond pulses can result in optical damage of multiphoton absorption processes for a high power laser (>1014 W/cm2) because of electron tunneling during the period of light oscillation. The different mechanisms depicted in Figure 3.69 disclose dissipation of the light pulse energy. Thermalization of free carriers with the lattice is not finished during the pulse and the temperature of free carriers is higher than that of the lattice. Therefore, processes induced by subpicosecond pulses are considered nonthermal. Furthermore, free-carrier absorption can result in ionization. Additional events, such as multiphoton absorption and ionization can generate free carriers. Moreover, Auger recombination may contribute to the overall carrier generation–recombination process. The use of TP initiated polymerization for 3D microfabrication has several advantages over OP initiated polymerization. A 3D resolution can be achieved with lateral and depth resolutions of 0.2 mm and 0.28 mm. This is fabrication at a

APPLICATIONS

285

Figure 3.69. Energy relaxation processes in TPA processes of solid state materials. (Adapted from Ref. [574].)

submicron level, applying a scanning speed of 50 mm/s. This results in freely movable microstructures without supporting columns during the manufacturing process [575]. Near-field scanning optical microscopy (NSOM) and apertureless NSOM make it possible to produce extremely localized positions with resolution below 100 nm in diameter. Nevertheless, the fabricated nanostructures are slightly larger than the idealized focal point. Light scattering is an important factor to determine the local light-intensity distribution at the focus accompanied by a change of the refractive index. The first irradiation pulse results in partially solidified material. The high spatial resolution brings new applications into the nanoworld [576]. Three-dimensional lithographic microfabrication, often abbreviated as 3DLM, includes the manufacture of complex 3D microstructures in a single TP excitation step, which are difficult and/or time consuming to fabricate on the micrometer or submicrometer scale as compared with any other technique. TP initiated polymerization (see Chapter 6) has mostly been used to crosslink a TP sensitive composition in the focus of a laser. This gives the pattern, which was translated into the focus of an intense laser beam. A focused spot with high photon density and constant total number of photons at every cross section is formed locally (Fig. 3.70). The integrated light intensity at a given point is constant because the laser beam is scanned two dimensionally in the focused plane (Fig. 3.70), resulting in an enhanced time-integrated material response at a focused position if the material response is proportional to the square of

286

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

integration of laser beam intensity

optical axis

integration of squared intensity of laser beam

position

position focused laser beam (a)

(b)

(c)

Figure 3.70. Formation of a focused laser beam (a) and its influence on OPA (b) and TPA (c). (Adapted from Ref. [76].)

the photon density. However, OPA based on a linear response of the material to the light intensity does not have optical sectioning capability (Fig. 3.70). The quadratic dependence of the TPA rate on light intensity confines absorption to the highly localized area of the focal point. The TP sensitive composition consists of a polymeric binder, a crosslinkable monomer, and a D-p-D chromophore as TP photoinitiator [133, 134, 137, 225, 271]. Furthermore, siloxanes functionalized by methacrylic groups have been investigated using TP polymerization initiated by femtosecond laser pulses as possible materials for waveguide and photonic band-gap applications because of their low optical losses (e.g., less than 0.2 dB/cm at 1310 nm and 0.55 dB/cm at 1550 nm), tunable refractive index, and high thermal and mechanical stability [262]. Monomers used were acrylates [134, 152, 576, 577], methacrylates [264], urethane acrylates [146, 575], and epoxy resins [225, 575]. Complex 3D microstructures were obtained by dissolving the unexposed material. The 3D microstructures such as microcoils and microtubes were fabricated with a resolution of 0.62 mm using a femtosecond-pulsed near-infrared laser (titanium:sapphire laser) with a pulse length of 100 fs and a peak power of several tens of kilowatts [76]. An example for an optical system used for 3D microfabrication is given in Figure 3.71 [76]. In this example, a mode-locked titanium:sapphire laser was used as well. The laser beam is introduced into the mirror–scanner system, and its direction is deflected in two dimensions. Then it is focused with an objective lens. Any 3D structure

287

APPLICATIONS

photopolymerizable resin

focused beam

monitor

CCD camera lens

z-scan stage objective lens

pump laser

Ti:sapphire laser

lens

attenuator shutter

x-scan mirror

y-scan mirror

Figure 3.71. Optical system used for 3D microfabrication using TP initiated polymerization of a photopolymerizable composition. The numerical aperture of the objective lens is 0.85 (magnification of 40), the accuracy of the galvano-scanner set and the dc motor scanner were 0.3 and 0.5 mm, the beam power at peak in the photocrosslinkable composition is about 3 kW, with a repetition rate of 76 MHz and a pulse width of 130 fs at a wavelength of 770 nm [76].

can be formed if the beam is laterally scanned in the photocrosslinkable composition and the cell with the photocrosslinkable composition is vertically staged. The noncrosslinked material is removed using a solvent and the crosslinked material remains because it is insoluble. The lateral resolution and depth resolution were 0.62 mm and 2.2 mm, respectively, using the optical system described in Figure 3.71. However, the lateral resolution and depth resolution depend on the average power of the laser beam [575] and the exposure time [146]. Although the linewidth in the lateral position does not exceed 1 mm up to a

288

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Depth and width of solidified polymer thread (µm)

3 Depth

2

1 Width

0 0

20 40 60 80 Average power of laser beam (mW)

100

Figure 3.72. Dependence of lateral resolution and depth resolution on average power of a mode-locked Ti:sapphire laser (wavelength 763 nm, 82 MHz repetition, 130 fs pulse width) during TP initiated polymerization of a urethane acrylate resin. (From Ref. [575] with permission of SPIE—The International Society for Optical Engineering.)

power of 80 mW, the linewidth at depth is between 1 and 3 mm in the same power range (Fig. 3.72) [575]. The voxel (i.e., volume sized pixel) size (S) depends on the diffraction limit of a laser focal spot (ldiff), the constant reflecting characteristics (a) of TP materials and exposure schemes, the real (actually applied) laser power (Ere), and the threshold laser power (Eth). Equation (80), with n ¼ 2 for the TP process, was used for quantification of the voxel size, which depends on the laser power and the exposure time [146]. rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi lnðEre =Eth Þ ð80Þ S ¼ ldiff a 4n ln 2 Excitation of the TP active chromophore with the first irradiation pulse initiates polymerization accompanied by a partial solidification of the resin resulting in a local increase in the refractive index. The next incoming laser pulse can be scattered and finally gives the observed drop-like shape [576]. Furthermore, volume of the polymerized voxels is influenced by the scan speed and the laser power as well. As depicted in Figure 3.73, the volume of the voxels increases with the inverse of the scan speed, which is proportional to the exposure time of a single voxel with increasing laser power [133]. The exposure conditions including laser power and exposure time influence the structure of the crosslinked microstructures [146], and therefore their properties as well [133]. Three-dimensional lithographic microfabrication can be used for production of 3D periodic structures, which are useful for photonic band-gap materials with

APPLICATIONS

289

Figure 3.73. Volume size of voxels assuming ellipsoid structure as a function of the inverse of the scan speed. The voxels were obtained by TP initiated crosslinking radical polymerization of acrylates in the presence of poly(styrene-co-acrylonitrile) as binder and an amino-substituted distyrylbenzene as TP active initiator using a pulsed laser (150-fs pulses at a 76-MHz repetition rate or 85-fs pulses at a repetition rate of 82 MHz). (From Ref. [133] with permission of the Technical Association of Photopolymers, Japan.)

unique optical properties, for tapered optical waveguides, and for microelectromechanical systems (Fig. 3.74) [134]. Furthermore, TP microstereolithography and two-photon 3D microfabrication can be used to prepare optically driven micromanipulators with submicron probe tips [578]. Among the microstructures useful for fabrication of micromachines are microtubes [76] and photonic bandgap materials [134, 150, 152, 579]. Most of these microstructures are wire-frame objects; some are not wire-frame components. For example, probe tips of microtweezers that measure only 1.8 mm in length and 250 nm in diameter were fabricated by point-by-point exposure with 30-mW laser power, where each exposure lasted 100 ms [578]. An enhanced field generated at the apex of a gold tip can be used as excitation source as well. Generation of an enhanced field at the apex of a sharp metal probe in the focus of a laser beam is called apertureless near-field scanning optical microscopy (NSOM) [262]. Using the NSOM technique, a spatial resolution of 20 nm can be obtained, demonstrating again that TP excitation has the capability to bring applications into the nanoworld [89]. The array of gold tips fabricated on a glass substrate using nanosphere lithography results in an increased polymerization rate in the vicinity of the gold tips, using a reactive trifunctional acrylate monomer together with

290

TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.74. Three-dimensional microstructures (photonic band-gap structure (a), magnified top view of the photonic band-gap material (b), tapered waveguide structure (c), cantilevers (d)) obtained by TP initiated polymerization. (From Ref. [134] with permission of Macmillan Magazines.)

poly(styrene-co-acrylonitrile) and the distyrylbenzene 66e as the TP absorbing dye, although no polymerization was observed above the gold tips [362]. Positive tone resins based on a photoacid generator and either a random copolymer of tetrahydropyranyl methacrylate and methyl methacrylate [137] or random copolymers of tetrahydropyranyl methacrylate, methyl methacrylate, and t-butyl methacrylate (180) were used for fabrication of 3D microchannel structures [264]. Simple placed channels connected to cubic or prismatic trenches, which are open to the surface as well as optical grating structures, comprise a set of several parallel channels placed about 10 mm below the surface with connecting reservoirs on both ends. They were fabricated by positive TP microlithography (Fig. 3.75) [264]. CH3 CH2 O

C

x

O

CH3 CH2

C

O

CH3 CH2

y

O

CH3

O O

180

C

z

O

H3C C CH3 CH3

APPLICATIONS

291

Figure 3.75. Three-dimensional microstructures: (a) cubic trenches (top: fully developed; bottom: not fully developed), (b) prismatic trenches (top: fully developed; bottom: not fully developed), and (c) prismatic cavities of 100 mm width, 20 mm length, and 20 mm depth, which are connected by twelve channels of 50 mm length and 4 mm  4 mm cross section bearing a periodicity of 8 mm and lying 10 mm below the surface. (From Ref. [264] with permission of Wiley-VCH.)

Similar microchannel structures are manufactured using one-step exposure, post-baking, and dissolving of the exposed material [137]. The resist resolution is influenced by dark reactions. This can affect ester cleavage reactions and the development process. Resolution is not sufficient if the 3D microstructures are either not fully developed or overdeveloped (Fig. 3.75). The limiting resolution in 3D microlithography is determined by the dimensions of the volume element, called volume pixel (voxel), which is generated by TP exposure. Size of a voxel depends on the focal spot size, processing conditions such as laser power, scan speed, chemical reactions, and physical processes occurring during TP initiated polymerization, and the developing process [74, 146, 580]. Manufacturing a series of voxels using identical exposure time and laser pulse energy for each voxel by increasing the focusing level above the substrate step by step (called the ascending method) (Fig. 3.76) can provide information about both the lateral size and the longitudinal size of the voxels [580]. If the laser is closely focused on the substrate surface (called truncation), the voxels are only partly recorded images of the focal spot (voxels A, B, and C in Fig. 3.76b). On the other hand, focusing the laser far from the substrate surface resulted in voxels floating away during the washing process (example G in Fig. 3.76). If the laser was well focused on the substrate surface, voxels were produced with reasonable adhesion to the substrate surface (voxels D, E, and F in Fig. 3.76b). Only these voxels give both lateral and longitudinal information of their sizes [580]. Fully developed 3D microstructures are depicted in Figure 3.77. However, edges of the positive-resist structures are rounded in Figure 3.75. This is mainly caused by proton diffusion during the postexposure step [264]. The prismatic cavities shown in Figure 3.75c may be a possible approach for switchable grating devices if they are filled with a low molecular weight liquid crystal.

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Figure 3.76. Voxels manufactured by TP initiated polymerization using different focusing height level (a) and scanning electronic microscopic images of the produced voxels (b). (From Ref. [580] with permission of the American Institute of Physics.)

O

O O

O O

O

O

O

n

O O

O

(a)

m

O

179

(b)

(c)

(d)

Figure 3.77. Three-dimensional microstructures fabricated using TPA: (a) microgearwheel, (b) microchain, (c) microbull (from Ref. [265] with permission of Elsevier), and (d) microcapsules (from Ref. [149] with permission of the Optical Society of America).

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293

Figure 3.78. Scanning electron microscope images of a coin manufactured by TP initiated polymerization of tris(2-hydroxyethyl)isocyanurate triacrylate in the presence of poly(styrene-co-acrylonitrile) as binder and 167 as TP initiator using a frequency-doubled Nd:YAG microlaser (0.5-ns pulses, 6.5-kHz repetition rate, wavelength 532 nm, average power 1.2 mW, 1.8-mm focal spot): (a) overview and (b) part of the coin with larger magnification. (From Ref. [136] with permission of the Optical Society of America.)

In contrast to the positive-tone systems, where the irradiated parts of the material become soluble using a developer, the exposed parts become insoluble in negative-tone systems such as epoxide-based resins. In the case of the negative-tone systems, the final structure is the complement of the exposure pattern [225]. Columnar structures and ‘‘stack-of-logs’’ photonic band-gap structures have been manufactured using epoxy resin SU-8 (179) as the TP active initiator [225]. Furthermore, 3D microstructures fabricated using TPA are a microgearwheel, a microchain, a microbull (Figure 3.77) [74, 265], and microcapsules [149]. Fine structuring of the microstructure is seen in the coin in Figure 3.78, manufactured by TP initiated polymerization of tris(2-hydroxyethyl) isocyanurate triacrylate in the presence of poly(styrene-co-acrylonitrile) as binder and 167 as TP initiator using a frequency-doubled Nd:YAG microlaser with 0.5-ns pulses with a 6.5-kHz repetition rate at a wavelength of 532 nm, and an average power of 1.2 mW focusing into a 1.8-mm focal spot [136]. By applying fluorescent dyedoping in TP initiated polymerization, an internal diagnosis of the 3D microstructures is possible, as shown for cubic cages, microtubes, and other geometric structures [142].

B. Three-Dimensional Data Storage Three-dimensional (3D) data storage forms a necessity for further computer development. Three-dimensional optical memory with high storage density is a promising technology for increasing the capacity of computer data storage.

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Among the physical principles used for 3D optical data storage, TP chemistry is growing rapidly [244]. The capacity of a 3D memory based on TPA to write and read is limited by the memory volume divided by the optical spatial resolution, resulting in an upper bound on data storage density of about 6:5  1012 bits/cm3, whereas in a common 2D memory the storage density is only 3:5  108 bits/cm3 [119, 581]. The higher information densities using TPA processes are caused by the deeper penetration of the excitation light into the material, which absorbs particularly at the focal region, and a reduction of Rayleigh scattering at longer wavelengths [134]. However, high intensities (e.g., on the order of hundreds of MW/cm2) are necessary for TP induced processes to achieve reasonably efficient recording as compared to OP processes. An upper limit of the writing power should not be reached in order to avoid saturation effects or permanent damage of the storage material [144]. Pulsed femtosecond lasers are the most efficient light source [246, 259], whereas cw exposure has been used in rewritable data storage [249]. A 3D optical memory device can be based on different principles. Among them are the write once and read many times principle, based on TP initiated photopolymerization resulting in a photopolymer structure [242] or photobleaching of a fluorescent material [250]. The rewritable principle is based mainly on TP induced molecular change. This TP induced molecular change can be an isomerization reaction in the writing process and OP induced fluorescence in the reading process [119, 144, 247, 258]. Photopolymerization and isomerization processes result in a change of refractive index [249]. For example, the index of refraction changes from 1.541 to 1.554 [242], and from 1.527 to 1.55 [266] in the case of photopolymerized materials. Doping poly(methylmethacrylate) with 7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl-diphenylamine (45f2) results in a data storage medium working with the TP writing and OP readout principle [245]. In the write-once, readmany technique (WORM) [242], films were gelled before irradiation started with 100-fs laser light pulses in order to prevent shrinkage and flow. The level of the focus is controlled by a computer during irradiation with a pulsed laser. Well-defined spots with micrometer or submicrometer diameter are produced by a second harmonic generation (SHG) of a Ti:sapphire femtosecond laser [246, 259]. Three-dimensional stacks of data are written by moving the sample in the axial or z-direction. Differential interference contrast (DIC) optics and a 488-nm Arþ-ion laser are used to read the stored information. The interference modulated transmitted light is detected through the photomultiplier of the laser scanning microscope [242]. Another kind of WORM material contains 43e2 as TP absorbing fluorescent dye embedded in a polymer such as polystyrene or a copolymer manufactured of diethyl vinylbenzylphosphonate and methylmethacrylate in a 1:3 molar ratio [118]. The dye possesses functional groups with different basicity that are pro-

295

APPLICATIONS C10H21

C10H21

43e2 N

N

C10H21 H

+

C10H21

H N

N S

λmax (absorption) = 390 nm λmax (emission) = 490 nm

S

λmax (absorption) = 500 nm λmax (emission) = 625 nm

C10H21 H

+

H N

C10H21

H N S

λmax (absorption) = 370 nm λmax (emission) = 445 nm

Figure 3.79. Protonation of 43e2 caused by irradiation of triarylsulfonium hexafluoroantimonate as photoinduced acid generator [118].

tonated upon exposure of triarylsulfonium hexafluoroantimonate as photoinduced acid generator (Fig. 3.79). Selective protonation is observed due to basicity differences between the nitrogen-containing benzothiazolyl (pKb ¼ 13) and triarylamino groups (pKb ¼ 19), where the benzothiazolyl nitrogen is protonated at first and then the triarylamino nitrogen. Therefore, UV irradiation of 43e2 in the presence of a photoacid generator results in three species: the nonprotonated dye, the monoprotonated product, and the diprotonated product, which have distinct absorption and fluorescence properties. The exposed 2–3 mm thick single layer or multilayer polymer films show TP induced dual-channel fluorescence image formation after exposure to UV light through a number of different masks (Fig. 3.80). A slow rate of acid generation and, therefore, a slow protonation rate are observed in polymer films, resulting in formation and stabilization of the monoprotonated fluorene. Furthermore, the emission maxima differ for the nonprotonated and the protonated products. The nonprotonated species show an

Figure 3.80. Two-photon induced dual-channel fluorescence image formation within a photosensitive polymer film containing 43e2 and triarylsulfonium hexafluoroantimonate as photoinduced acid generator. (From Ref. [118] with permission of the American Chemical Society.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.81. Two-photon fluorescent images recorded by channel 1 (a) or channel 2 (b) that is formed via 350-nm broadband exposure of photosensitive polymer films containing 43e2 and triarylsulfonium hexafluoroantimonate as photoinduced acid generator using a light intensity of 4.4 mW/cm2. (From Ref. [118] with permission of the American Chemical Society.)

emission at 490 nm for excitation at 390 nm; the monoprotonated species results in an emission at a longer wavelength (625 nm) upon excitation at 500 nm, whereas the diprotonated species emits at a shorter wavelength (445 nm). The differences in emission of these species facilitate a two-channel fluorescence imaging. Fluorescence quenching at about 490 nm and fluorescence enhancement at longer wavelength cause the contrast (Fig. 3.81). An example for a read only memory (ROM) storage system comprises rhodamine B dispersed in a poly(methylmethacrylate) matrix [257]. The lactone of rhodamine B (183b) exists as a colorless base form. In the presence of nitroso acid (182) formed by TP induced rearrangement of 1-nitro-2-naphthaldehyde (181), the lactone 183a reacts to the strong colored fluorescing rhodamine B (183b). The proton required for the ring opening of 183a resulting in 183b is released by dissociation of the photoproduct 182 into the anion of the carboxylic acid and Hþ (Fig. 3.82). TP induced fluorescence of rhodamine B is used in the reading process. The size of the voxels is influenced by exposure conditions. Theoretical calculation of the near-focus results in a complicated square of the light intensity contour as depicted in Figure 3.83 [580]. A tubular structure of the high light intensity was found, which is of considerable importance for data storage, microfabrication, and imaging. Because of diffraction at circular apertures, the intensity distribution is rotationally symmetrical about the optical axis in the neighborhood of the focus. The voxel obtained by focused laser irradiation is a spinning ellipsoid (Figure 3.84) with axis lengths of 3.4 mm and 1.4 mm, resulting in an elongation factor of 2.4 [265, 580]. Furthermore, the longitudinal size

297

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NO2

NO2

O

O

OH

182

181

NO2 O

+ H+

O

C2H5

C2H5 N

C2H5 N C2H5

O

H+

C2H5

C 2H 5 N

O

C2H5 N C2H5

O O

183a

COOH

183b

read out the fluorescence by one photon

Figure 3.82. TP photochemical rearrangement of o-nitrobenzaldehyde (181) into o-nitroso benzoic acid and consecutive dissociation into the corresponding anion and Hþ. Hþ opens the lactone ring in 183a, resulting in rhodamine B (183b).

Figure 3.83. Theoretically calculated square of the light intensity isophotes of a focused laser beam in normalized axial (u) and lateral (v) coordinates. (From Ref. [580] with permission of the American Institute of Physics.)

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Figure 3.84. Voxel obtained by irradiation of a mixed urethane acrylate oligomer and a urethane acrylate monomer in the presence of a mixture of benzoyl cyclohexanol and morpholino phenyl amino ketones at 780 nm using a 150-fs pulsed Ti:sapphire modelocked laser operating at 76 MHz, where light was focused by a 1.4 numerical aperture objective lens: (a) scanning electronic microscopic images of the voxel and (b) longitudinal and lateral voxel size as function of the exposure time. (From Ref. [580] with permission of the American Institute of Physics.)

and lateral size of the voxel differ considerably. This is influenced by the exposure time (Fig. 3.84). From this it can be concluded that the lateral and longitudinal spatial resolutions are important for characterizing the 3D spatial resolution of the voxel. The information storage process of a 3D erasable and rewritable memory includes writing, reading, and erasing of information based on a refractive index change caused by TPA [143]. An ultrashort laser pulse is focused by an oilimmersion objective onto a TP active film. Spherical aberration caused by the mismatch of the refractive indices between the recording material and its immersion medium can be reduced by using an oil-immersion objective. The reading wavelength selected is outside the absorption band of the recording material. In this case, the recorded data bits are not erased during the reading process. Photodestruction, reverse photoreaction, light absorption by other levels of molecules, and photoreaction quantum yield change because the inhomogeneity of photoactive molecules embedded in a polymer matrix strongly influences the achievable number of record–erase cycles [258]. To avoid complications possibly caused by fluorescence during the writing process, TP active materials exhibiting

299

APPLICATIONS

only slight fluorescence should be chosen for such applications. Only those structures should fluoresce that are in the written form. The high reading sensitivity is based on the high sensitivity of the fluorescence process. Erasing of selected bits is done by light. However, erasing of the entire memory is done by an increase of temperature [119]. A rewritable optical data storage material obtained by chemical binding of the benzophenoxazin dye 184 showing an intensive fluorescence and the photochrome N-(4-aminophenyl)fulgimide 185 was developed in order to separate the absorption bands of the write, read, and erase forms [582]. The emission quantum yield of the fluorescing dye is influenced by the surrounding polarity and depends on the structure of 185. Therefore, 185 is called the driver. Both photoisomeric forms of the N-(4-aminophenyl)fulgimide are temperature stable. Only excitation with light results in a transformation from one isomer of 185 to the other (Fig. 3.85). The photorefractive effect used in rewritable 3D optical data storage is based on nonuniform space-charge distribution produced by interfering beams 'Dye'

'Driver'

HO O NH

N

N N

O

Cl

O

184

C2H5 N

185

C2H5

'Write' form

400 nm

530 nm

HO O NH

N

N N

O

Cl

O

C2H5 N C2H5

'Read' form

Figure 3.85. Chemical structure of a molecular memory based on chemically bonded benzophenoxazin dye to the photochrome N-(4-aminophenyl)fulgimide and its photoisomerization [582].

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.86. Photorefractive mechanism discussed by using an objective. (From Ref. [247] with permission of Wiley-VCH.)

(Fig. 3.86). The beam, which is focused by an objective, generates a nonuniform charge distribution corresponding to an array pattern. After diffusion and recombination of the free charges, the position-dependent internal space-charge field results in a modulation of the refractive index [247]. The written bits are read by fluorescence. The TP two-beam picosecond ‘‘page-by-page’’ writing and fluorescent ‘‘page-by-page’’ reading technology has been successfully developed. The nonlinearity of TP writing and the thin photosensitive layers provide the location of the volume pixel (voxel) inside the storage medium. A voxel of the memory is addressed by two beams. One way of addressing is called orthogonal addressing and the other pulse collision [581]. For orthogonal addressing, the volume of overlap is defined by the overlap of the beam cross-sectional area. The minimum resolvable spot and the volume determine the data storage density and the total capacity. The spot size resolution is limited by diffraction. The spot size is lim-

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301

Figure 3.87. Principle of pulse collision addressing in a 3D memory. (From Ref. [581] with permission of SPIE—The International Society for Optical Engineering.)

ited by Gaussian beam propagation [581]. For pulse collision addressing, the volume of overlap of two short laser pulses is determined by both the overlap of the beam cross sections and the spatial thickness of the pulse. The principle of pulse collision addressing is depicted in Figure 3.87 [581]. Pulse collision addressing provides high data transfer because of more parallel data channels. A further advantage is the incorporation of the storage material into a waveguide array. Therefore, a dynamic focusing lens can be eliminated [581]. Polymers occupy a key function in materials useful for TP induced isomerization because they can act as matrix for TP active molecules. The TP active structures can also be chemically bound as side chains to the polymer backbone. Furthermore, they can be incorporated into the main chain of the polymer [213]. Among the polymers, thermally crosslinked and photocrosslinked polymers offer an excellent opportunity for 3D data storage because these systems offer a stable orientation of the structures formed in time and during temperature change. Incorporation of TP active structures in soluble polymers or in crosslinking systems results in monomers containing both the TP active structure and a reactive functional group. Primary amino groups reacting with epoxy groups and methacrylate groups undergoing polymerization have been tested as reactive functional groups [213]. Polymer systems have advantages over traditional inorganic photorefractive crystals: low cost, easy manufacturing, and their tailor-made physical-optical properties, such as photorefractive effect. On the other hand, the application field of polymers is limited because of the strong absorption of polymeric materials and melting of the polymer matrix upon warming up during exposure [247]. Transparent multilayer polymer photochromic

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.88. Composition of a multilayer optical data storage medium: photosensitive layer of about 1-mm thickness (1), nonphotoactive polymer as separating layer of 30-mm thickness (2), and glass substrate (3) [244].

matrices are manufactured by photosensitive layers (1-mm layer thickness), which are separated by nonphotosensitive polymer layers (30-mm layer thickness) as depicted in Figure 3.88 [244, 258]. Poly(methylmethacrylate) or copolymers made of methylmethacrylate and styrene or acrylonitrile are useful hosts for two-photon active compounds, such as naphthacene chromophores (186) [258]. Poly (ethylene terephthalate) was used as the separating layer. Polymer materials containing photosensitive layers separated by nonphotosensitive layers give 2.5D optical memory systems [258]. Further examples for solid solutions of TP active layers are poly(methylmethacrylate) doped with cis-1,2-dicyano-1,2bis(2,4,5-trimethyl-3-thienyl)ethane (187) [144] and copolymers of methylmethacrylate and a phosphorylated styrene doped with 3-[1-(1,2-dimethyl-1Hindol-3-yl)-ethylidene]-4-isopropylidene-dihydrofuran-2,5-dione (188) [116]. N S

OPh O N

N

O O 186

S 187

O

O 188

The copolymers doped with 188 can also be used for holographic recording in a TP process because of TP induced photoisomerization of the fulgide [116]. However, trans–cis photoisomerization causes a slow index grating writing [213]. A further disadvantage of holographic data storage systems consists of erasing of the recorded information [250]. Furthermore, isomerizing systems

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303

Figure 3.89. Normalized TP fluorescence intensity of polymer dispersed liquid crystals as a function of the polarization angle of the reading beam, which is defined as the angle between the writing and reading polarization states. Inset top: Schematic alignment of liquid crystal directors in exposed (dots) and unexposed regions. Inset bottom: Two fluorescing data bits obtained by irradiation at a polarization angle of 90 . (From Ref. [248] with permission of the American Institute of Physics.)

suffer from the back reaction that occurs upon long storage in the dark (i.e., the information can be lost upon storage). Furthermore, polymer dispersed liquid crystals result in a fluorescence intensity at the illuminated focal area that is dependent on the polarization state of the reading beam, as depicted in Figure 3.89. Therefore, the use of polymer dispersed liquid crystals opens further possibilities in 3D optical data storage media because of their alignment-based fluorescence depending on the polarization state of the excitation field [248]. As in the case of other 3D optical data storage media, data stored in polymer dispersed liquid crystals can be erased by heating the sample above the glass transition temperature of the polymer. Furthermore, such data stored in polymer dispersed liquid crystals can be erased either by irradiation with nonpolarized light or by illumination by laser light with a polarization state perpendicular to that of the writing beam. The last erasing mechanism opens the possibility for single-bit erasure of recorded data. Advantages of holography can be seen in single-step processing of complex structures and formation of small periods (<1 mm) with high aspect ratios (>50 mm). Morphology of the structures formed is controlled by balance between polymerization and phase separation. The ultrafast TP initiated polymerization process drives the phase separation during fabrication of complex permanent gratings. Bis(diphenylamino)diphenyl hexatriene (general structure 57 with n ¼ 3 and R ¼ phenyl) induces a localized phase separation in holographic TP initiated polymerization of dipentaerythritol pentaacrylate in the presence of N-vinyl pyrrolidinone as the

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

diluent embedded in polymer dispersed nematic liquid crystals. The delineation of polymer-rich and LC-rich regions with a grating spacing of 3.0 mm is sharper in comparison with a similar system manufactured using conventional OP holography [251]. Three-dimensional data storage using TPA could be the data storage system of the future [250]. Important factors of influence are the size of the storage medium and its capacity. Optimization of 3D optical memory based on TPA includes minimization of data writing time with minimum intensities of light waves and an increase in the TPA cross section [243]. If the intensity of the second-harmonic beam is set at a small value, the time required for data writing increases. However, the time of data writing can be reduced by increasing d. Therefore, the TPA cross section is a key parameter for improving the speed of an optical memory device. As depicted in Figure 3.90, the integral absorption coefficient strongly depends on the angle between the polarization vectors of light waves [243]. In addition, the polarization dependence of d provides a further opportunity to increase the efficiency of optical memory devices without changing the energy parameters of the laser part of the device and without synthesis of new TP active compounds.

Figure 3.90. Dependence of the integrated absorption coefficient (%) on the angle between polarization vectors of light waves of a mode-locked Nd:YAG laser generating 35-ps light pulses at the wavelength of 1.06 mm (dots correspond to experimental data and the solid line represents the result of computer simulation). (From Ref. [243] with permission of The Japan Society of Applied Physics.)

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305

C. Optical Band-Gap Materials Two-photon initiated polymerization results in three-dimensional (3D) periodic structures with high spatial resolution. The volume pixels exhibit well-defined shapes. They are spatially organized, similar to atoms arrayed in crystals. The arrangement of volume pixels in 3D dot patterns results in a periodic modulation of light, similar to the way in which the Coulomb potential behaves for electrons in actual crystals. The volume pixels are also called ‘‘photonic atoms’’ to express the similarity to crystal structures. The 3D periodic structures cause a Bragg-like diffraction. Therefore, the 3D structures can create a photonic band gap, which is a region of the frequency spectrum where propagating modes are forbidden. Important parameters for a photonic band-gap structure include dielectric contrast, crystal structure, and filling ratio [150, 153]. The filling ratio is the volume percentage of holes that is roughly estimated according to Eq. (81), where r is the solidified-rod diameter and d is the in-plane rod spacing [150]. f ¼

pr 4d

ð81Þ

The Bragg equation for a fcc crystal as assumed in the photonic crystal based on polystyrene nanoparticles [154] is expressed in Eq. (82). The spacing between (111) planes in the fcc crystal is related to d111 . The effective refractive index (neff) was obtained by considering the long-wavelength limit of the photondispersion relation o(k), Eq. (83), where c is the speed of light. l111 ¼ 2 neff d111   ck neff ¼ lim k!0 o

ð82Þ ð83Þ

The lattice constant is strongly influenced by the size of the volume pixels. It mainly determines the wavelength of the band-gap center. The exposure dose generally controls the size of the crosslinked material. As depicted in Figure 3.91, the size of the crosslinked rod increases with the laser pulse energy. Therefore, it is possible in principle to tune the lattice constant and the filling ratio [150]. However, photonic band gaps of most lattices manufactured by two-photon initiated crosslinking polymerization have not been satisfied yet because the transmission does not vanish in the gap region. This may be caused by defects and the relatively low refractive index contrast [150, 155]. Different fabrication techniques have been used for manufacturing photonic band-gap structures. Among them are layer-by-layer structure formation [150] and crystallization of polystyrene nanoparticles. The latter possess a diameter of 200 nm. They are infiltrated with coumarin-503. This dye exhibits strong

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.91. Dependence of the crosslinked rod size of a TP polymerized acrylic acid ester on the laser pulse energy of a Ti:sapphire femtosecond laser with a pulse width of 130 fs, a repetition rate of 5 kHz, and a wavelength of 800 nm that was obtained by doubling of 400 nm. (From Ref. [150] with permission of the American Institute of Physics.)

fluorescence with f ¼ 0:84 [154] and is acceptable for TP holographic photopolymerization [114, 252, 558, 561, 583]. An example of a lattice obtained by layer-by-layer structure formed by TP initiated crosslinking polymerization of an acrylic acid ester is given in Figure 3.92 [150]. The refractive index of the polymerized acrylic acid ester [150] as well as of the polystyrene nanoparticles [154] is only 1.6. This limits the maximum of dielectric contrast, and therefore use as a photonic band-gap material. A photonic crystal-type ‘‘stack of logs’’ was obtained by TP initiated crosslinking radical polymerization of acrylates in the presence of poly(styreneco-acrylonitrile) as binder and an amino substituted distyrylbenzene (58) as two-photon active initiator (Fig. 3.93) [133]. This photonic crystal-type microstructure has an average periodicity of 1 mm, a base area of 60 mm  60 mm, and a height of 8 mm. The lines are about 200 nm wide, which is considerably smaller than the fabrication wavelength (Fig. 3.93) [133]. Two-photon initiated polymerization initiated by bis(diphenylamino)diphenyl hexatriene (57j) was found to work well for holographic recording in the presence of dipentaerythritol pentaacrylate, a nematic liquid crystal (NLC), N-vinyl pyrrolidinone as binder, and octanoic acid. A reflection-type holographic setup containing counterpropagating beams resulted in electrically switchable reflection gratings, showing a relatively low change in morphology and diffraction efficiency caused by saturation effects [561]. Two-photon initiated holographic

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307

Figure 3.92. Lattice obtained by layer-by-layer structure formation using an acrylic acid ester containing a radical photoinitiator (Nopcocure 800 resin from San Nopco) that was placed on a quartz glass plate on a 3D computer-controlled piezoelectric translator and scanned with a preprogrammed computer-aided design pattern using a Ti:sapphire femtosecond laser pulse generation (800-nm wavelength, 5-kHz repetition rate, approximately 130-fs pulse width), a chirp pulse amplification system, an inverted microscope with an objective lens (100; NA ¼ 1:35) for irradiation induced crosslinking polymerization and washed after polymerization to remove nonpolymerized material from unexposed regions. Left: Top view of the layer-by-layer structure. Right: Cross section of the layer-by-layer structure. (From Ref. [155] with permission of Elsevier.)

polymerization of trimethylolpropane triacrylate and dipentaerythritol pentaacrylate in the presence of a polycationic peptide, which was derived from the C. fusiformis silaffin-1 protein, results in a hologram having the capability to act as matrix for formation of hybrid organic/inorganic ordered nanostructures of silica spheres from silica acid [252]. Silica nanospheres of an average diameter of 452 nm (81 nm) were formed within minutes from the silicic acid in the presence of the short 19-amino-acid R5 peptide unit (SSKKSGSYSG SKGSKRRIL) of the silaffin-1 precursor polypeptide at ambient temperature. The periodicity of the hologram is 1.33 mm in the case of the untreated grating. The hybrid hologram has a periodicity of 1.6 mm owing to the added mechanical strength of the silica spheres. The diffraction efficiency increases from 0.02% for the untreated grating to 0.95% for the grating with the silica spheres. This means a 47-fold increase in diffraction efficiency contributed to the nanospere structures formed [252]. Furthermore, the multifunctional epoxy resin SU8 (179) was used for manufacturing photonic crystal structures by TP initiated polymerization [583].

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.93. Scanning electron micrograph image of a photonic band-gap crystal obtained by TP induced crosslinking radical polymerization of acrylates in the presence of poly(styrene-co-acrylonitrile) as binder and an amino substituted distyrylbenzene as TP active initiator using a pulsed laser (730-nm excitation wavelength, 0.45-mW laser power, 50-mm/s scan speed, writing 9 layers of 1-mm spaced parallel rods with a layer spacing of 1 mm). (From Ref. [133] with permission of the Technical Association of Photopolymers, Japan.)

D.

Waveguide Materials

Waveguides are an important component for all-optical processing because of their light guiding function [53, 129, 399, 584]. Waveguides consist of a substrate, a guiding layer, and an overlayer in the case of three-layer waveguides. The light propagates with high power density in a medium as long as the refractive index of the guiding layer is higher compared to those of the substrate and the top layer. Furthermore, the waveguide geometry including waveguide thickness influences the propagation of light. The efficiency can be improved by increasing the optical field overlap between the propagating waveguide modes as seen in four-layer and five-layer waveguides [129]. Two-photon absorption

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309

is of increasing importance as well for manufacturing waveguides by micro- and nanofabrication as well as for waveguide function. Monomode waveguides with thicknesses between 100 and 200 nm and a refractive index of 2.0 or higher are useful in achieving ultralow detection limits in the femtomolar range in the case of visible light. Strong excitation fields are available along the entire surface of the waveguide in which the light is guided and are not localized in only a microscopic focus. Interference from the bulk medium can be differentiated with high efficiency. Besides inorganic materials [250, 585], nonlinear optical polymer materials are useful alternatives as waveguide materials as well. Periodic poling of the waveguide material is necessary to fabricate quasi-phase-matching waveguides. Furthermore, half-ellipsoid voxels and half-cylindrical rod shapes were fabricated by TP polymerization where a single voxel was obtained by a laser power of 50 mW and an exposure time of 0.5 s [135]. The half-cylindrical rod shapes based on poly(methylmethacrylate) possess a higher reflective index compared to air of about 0.5. Hence, this material can be used as waveguide material. The channel waveguide fabricated using a scanning speed of 40 mm/s and an input laser power of 500 mW of a femtosecond laser beam with a 0.65 NA microscope objective exhibits a width of 2 mm. In a similar way, a grating was made with 10 lines per 50 mm [135]. Optical circuits have been manufactured in bulk from an epoxy resin containing urethane acrylate oligomers, a thermal curing agent, and (6-benzothiazol-2-yl-(2-naphthyl)diphenylamine as the TP fluorophore using a Ti:sapphire laser (800-nm wavelength, about 100-fs pulse width, 100-mW average power). The beam was focused using a microscope objective lens (20, numerical aperture about 0.4 or 60, numerical aperture about 0.85). A scan rate of about 20 mm/s was applied [266]. The use of the 60 objective (numerical aperture of about 0.85) during channel writing results in a reduction of the asymmetry in channel dimensions (about 1.5 mm in the lateral direction and about 2.5 mm in the axial direction). The photopolymer structure obtained depends, for example, on laser fluency, scan rate, and dye concentration. It therefore influences the refractive index change of the exposed regions that is important for the efficiency of the written channels, such as straight channels, curved channels, and 1  2 and 2  2 splitter channels [266]. The contrast in refractive index between the resin and the crosslinked channels in the bulk is about 0.023. This is sufficient for waveguides. Photobleaching and inhomogeneous polymerization of the resin restrict formation of smooth channels for wave guiding. Poly[1,4-phenylene-1,2-di(phenoxyphenyl)vinylene] (189a) and poly[1,4phenylene-1,2-diphenylvinylene-co-2,7-fluorenylene-1,2-di-phenylvinylene] (189b) possess in the wavelength region between 880 and 990 nm a nonlinear refractive index of 1  10 14 cm2/W and 2  10 14 cm2/W, respectively, and low linear

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

losses (<1.5 dB/cm). Therefore, these poly[phenylenevinylenes] are promising for manufacturing waveguide elements [229, 586]. O

b n

a n

O 189a

189b

Two-photon absorption can reduce the device throughput and spoil switching even without linear loss because of TP induced degradation in time [584, 587]. The upper limit for the TPA coefficient in waveguides based on polymers containing the 4-dialkylamino-40 -nitro-stilbene chromophore in the side chain was determined to be 0.08 cm/GW at an operating wavelength of 1319 nm [584]. Furthermore, the influence of TPA is larger for the guided wave throughput than for the in-coupling process [587]. A further factor of influence on throughput is the intensity dependence of the refractive index. A channel waveguide, Tshaped waveguide, and directional coupler can be manufactured by TP initiated polymerization resulting in microfabrication (Fig. 3.94).

Figure 3.94. Waveguide structures manufactured by TP initiated polymerization of methyl methacrylate in the presence of a coumarin derivative and diphenyliodonium hexafluorophosphate as TP initiator using a 500-mW Ti:sapphire femtosecond laser at 800 nm, equipped with a 0.65 NA microscope objective and a scanning speed of 40 mm/s. (a) Channel waveguide (2 mm), (b) T-shaped waveguide (signal input: A; signal output: B; probe signal: C), and (c) directional coupler (coupling length is about 30 mm). (From Ref. [135] with permission of the Institute of Physics.)

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E. Fluorescence Imaging Fluorescence imaging is a useful technique for tissue investigation in biological and medical applications and for material characterization in modern materials science. In any case, the excitation light and the emitted photons have to travel through the material where they may interact with other molecules resulting in absorption or scattering. The fluorescence light emitted by a fluorophore may be diffracted, reflected, and refracted by any specimen on its way to the objective lens, resulting in scattered light, which does not mirror the real location of the emitting fluorophore. The use of TP excitation in microscopy results in less sensitivity to scattering, and therefore in an increase in resolution, and in deeper penetration compared with traditional optical microscopy, which uses a OPA process. Furthermore, the emitted photons have higher energies than the excitation light, which is in the near-infrared region (700–1000 nm), and the signal (S) dependence on the light intensity (I) is S / I 2 . However, an expensive pulsed laser system is necessary to achieve sufficient excitation rates. The excitation laser and the detection pathway mainly distinguish a two-photon excitation microscope (TPEM) setup from a laser scanning confocal microscope (LSCM) setup but both have a lot of similarities [78, 80, 81, 86, 87, 89, 92– 102, 105–109, 111, 118, 142, 195, 201, 208, 212, 275–277, 532–536, 588]. The principles of LSCM and TPEM are presented in Figure 3.95. A confocal

Figure 3.95. Fundamental principles of the LSCM (a) and the TPEM (b). In the LSCM, OP excitation laser light is condensed in a focal plane (endothelial layer) by an objective lens; the light also excites upper (smooth muscle layer) and lower planes of the focal plane. However, fluorescence emission exclusively from the focal plane is detected through the pinhole. (From Ref. [105] with permission of The Japanese Pharmacological Society.)

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microscope scans the image by illuminating one region after another until the entire field is illuminated using light focused to the smallest possible spot (scanning the illumination) and simultaneously masks the illuminated region from return light to the detector (scanning the detection). A pinhole aperture placed in front of the detector removes light from out-of-focus sources before it can reach the detector. The pinhole aperture and the illumination spot are simultaneously focused at the same spot. In the case of the LSCM, the excitation light is condensed in a focal plane by an objective lens. In addition, light intensity inversely decreases as the square of the distance from the focal plane in the z-axis direction. Fluorescence emission originating from the focal plane passes through a small pinhole to the photomultiplier detector. Thus, fluorescence that is not located at the focus cannot be detected by the system. Therefore, the diameter of the pinhole is a critical factor with respect to optical sectioning capability of the LSCM. On the other hand, the TPEM detects fluorescence only at the focal point. The photon density must be sufficient to excite the chromophore by a TPA process. The intensity of TPA is inversely proportional to the biquadrate of the distance from the focal plane in the z-axis direction. This is because TPA depends on the square of the excitation light intensity. Therefore, the fluorophore is excited only in the focal volume. As a result, excitation efficiency of the fluorophore decreases as the fourth power of the distance from the focal plane. The emitted photons originate exclusively from the focal plane, resulting in an improved spatial resolution seen visually by the image. This is particularly important to image Ca2þ in biological cells in situ using ion sensitive chromophores [105]. Application of TPE in biological targets benefits furthermore from a deeper penetration of light into the material. For example, TPE with wavelengths >670 nm would open the possibility to investigate materials comprised of collagen. The spatial resolution can be improved by applying near-field fluorescence imaging [70, 89, 112]. This method uses a metal tip, which is illuminated with a femtosecond light source. The strongly enhanced electric field at the metal tip (about 15-nm end diameter) localizes the excitation source for fluorescence. Thus, the spatial resolution is improved compared to the conventional aperture technique. The latter was applied to image fragments at the nanoscale (20 nm) and single molecule level [89, 112]. Targets can be photosynthetic membranes, as well as molecular aggregates. Figure 3.96 shows two general principles. Light can travel through an optical fiber coated with metal (Fig. 3.96a). An apertureless setup is shown in Figure 3.96b. A sharp metallic tip scatters the radiation. The enhanced electromagnetic field around the metallic tip is strongly confined. The most interesting aspect of near-field excitation can be seen in the strong confinement of light, which is significantly lower than its wavelength. Thus, NIR light of 800 nm can be squeezed down to 50–100 nm [70].

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(b)

(a)

Metal tip

metal film

Optical near-field

} Optical near-field

aperture

illumination

signal

Figure 3.96. General scheme for near-field optics showing aperture controlled near-field optics using a metal-coated fiber (a) and an apertureless setup based on scattering of the excitation light on the metal side (b). (Adapted from Ref. [70].)

The general setup of the TP microscope used to image the fluorescence of single dye molecules is shown in Figure 3.97. The single molecule patterns have both a high signal-to-background ratio (>30:1) and a high spatial resolution (< l/3). TP excitation was performed by fs pulses from a mode-locked

Argon Ion Laser

Mode-Locked Ti-Sapphire 840 nm, 100 fs, 76 Mhz

Dispersion Prisms

Neutral Density Wheel Sample XY Piezo Scanbed RG780 Shutter filter CCD BG-39 Cutoff Filter

Intensity Modulator

Nik on Diaphot 300 Inverted Microscope APD

111

Disc.

Counter 2001 1 1 1 1

Counting Timing Electronics

Digital Instruments Nanoscope IIIA Controller XY Scanbed Controller

To XY Scanbed

Figure 3.97. Schematic setup of the combination of the mode-locked Ti:sapphire laser, the inverted fluorescence microscope, and the detection system. (From Ref. [112] with permission of the American Chemical Society.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Ti:sapphire laser providing 100-fs pulses at a repetition rate of 76 MHz and a tunable range from 790 to 900 nm. The beam was sent into an inverted fluorescence microscope and was reflected by a dichroic beam splitter into a microscope objective. A prism pair was applied to precompensate the group velocity dispersion in the objective lens, resulting in pulse widths that are approximately 180 fs at the sample. The average excitation power was 0.2–3 mW at the diffraction-limited focal spot. Fluorescence was collected by the same objective lens, filtered by the combination, and detected by a photon counting system. A modified Nanoscope IIIA controller controlled the scan bed and image acquisition. An image was composed of 256  256 pixels with a line scan rate of 1 Hz. After obtaining the image, an immobilized molecule can be placed onto the focus. Hence, measurements can be completed before the sample becomes photobleached. Figure 3.98 displays a 5 mm  5 mm fluorescence image of single rhodamine B molecules (183b) on a glass substrate taken with the apparatus shown in Figure 3.97. Each peak in Figure 3.98 is due to a single molecule. The distinct

Figure 3.98. Fluorescence image of single immobilized rhodamine B in a 5 mm  5 mm field taken with TP excitation (5-min acquisition time). Each peak was due to a single molecule with the FWHM being 250 nm (
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intensities of the molecules are caused by different molecular orientations, resulting in distinct amplitudes of the absorption spectra of the immobilized emitter.

F.

Two-Photon Medical Applications

For biomedical applications, TP excitation has been used in imaging cells in therapeutic processes [91, 170, 171, 186, 189, 192, 194, 199, 202, 255], screening drug delivery [109, 187, 194], and photodynamic therapy (PDT) [170, 181, 184–189, 195–198, 202–204, 206, 207, 210, 253–256]. This requires unification of several functionalities in the therapeutic material. Most important functions are incorporation of groups that (1) target tumor cells and deliver the therapeutic material in particular to those cells, (2) combine the therapeutic function resulting finally in cell death of the tumor cells after completing the therapy, (3) sensor/image the therapeutic process, and (4) possess a sufficient solubility in biological cells. Development of nanosciences has resulted in the development of nanoclinics, which excellently fulfill these requirements [70, 192, 194, 195, 200]. These are nanoparticles with a size of 20–30 nm having various optical diagnostic tools and therapeutic agents in a silica bubble. Chemical modification of the surface results in coupling of appropriate carriers or biotargeting groups. The carriers are selective to interact with the receptors available in the biological target material, resulting in a direct transfer of the nanoclinic to a specific compartment of a biological cell, a specific cancer cell, or a diseased tissue. The silica shell of the nanoclinic can also be chemically modified to obtain either a hydrophobic or a hydrophilic shell with the goal of maintaining the compatibility of the nanoclinic and the biological nanomaterial. Furthermore, the porosity of the surface can be varied in order to optimize the transfer process of nanoclinic components into the surrounding cell material. Thus, optical probes can be embedded in the nanoclinics, functioning as fluorophores inside the nanoclinic for applications in bioimaging, biosensing, and real-time monitoring of intracellular physiological activities. This can be a TP chromophore with large TPA and reasonable emission behavior. Moreover, therapeutic agents can be either immobilized on the surface (as for gene delivery) or encapsulated inside the nanoclinic. Light can trigger the release of therapeutic agents. A prototypic nanoclinic was made of magnetic iron oxide (Fe2O3) core, a TP optical probe, and a silica shell (Figure 3.99) [192]. Covalently coupling the leutinizing hormone–releasing hormone (LH-RH) to the shell results in transfer to the receptor available on the surface of cells in some types of cancer, including breast, prostate, and ovarian. Nanoclinics containing the magnetic Fe2O3 nanoparticles offer a new targeted therapeutic effect: the selective destruction of

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.99. Schematic presentation of multifunctional nanoclinics. (a) The TP dye ASPI-SH coated on Fe2O3 is encapsulated in a silica nanobubble. On the surface, a spacer molecule and the targeting agent (LH-RH) are attached. (b) High-resolution transmission electron microscopy and electron diffraction prove the structure of the nanoclinic. The bar represents 5 nm. (From Ref. [192] with permission of Kluwer Academic Publishers.)

targeted cancer cells in a DC magnetic field (magnetocytolysis), known as the hyperthermal effect. The problem is to find the appropriate dose to complete the destruction process. The use of TP excitation as an optical technique allowing excitation in even deeper spheres of the tissue offers better control of excitation location. Compared to OP excitation, TP excitation has the advantages of less collateral tissue damage and a highly localized focus, thus avoiding out-of-focus

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Figure 3.100. TPLSM demonstrates the uptake of nanoclinics by cancer cells. The particles are incubated with adherent MCF-7 cells (breast carcinoma) at room temperature. (a) Particle accumulation on and inside the cell versus incubation time (images recorded after 3, 15, and 27 minutes incubation). (b, c) Single optical sections of cells after 15 minutes of incubation. The arrow shows accumulation of nanoclinics on and inside the cell (b: transmission, c: fluorescence). (From Ref. [192] with permission of Kluwer Academic Publishers.)

fluorescence. The use of a NIR pulsed laser minimizes autofluorescence of the tissue as well as cell damage. Autofluorescence of the tissue can become a problem because collagen absorbs up to 670 nm, so the use of an excitation source emitting at a wavelength greater than 670 nm tremendously diminishes the inner filter effect of collagen. The mechanism of action of the nanoclinic is shown in Figure 3.100. TP laser scanning microscopy (TPLSM) demonstrates the uptake of nanoclinics by cancer cells [192]. The particles are incubated with adherent MCF-7 cells (breast carcinoma) at room temperature. Figure 3.100a shows particle accumulation on and inside the cell versus incubation time. Images were recorded after 3, 15, and 27 minutes incubation. One can see the accumulation of TP dye after distinct times as shown in Figure 3.100b–c. The arrow in Figure 3.100c shows accumulation of nanoclinics on and inside the cell. This example shows the power of this method. TP excitation controls the consumption of the particles

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

in the cells and if consumption is completed treatment of the cells in a DC magnetic field can occur, resulting in destruction of the cancer cells by the magnetic Fe2O3. The goal is to find the time when consumption of the particles is completed, which is individually different for each biological target. From this point of view, TP excitation comprises a versatile tool to control the consumption of drugs/medication in cancer treatment. Thus, application of this method in magnetocytolysis allows one to examine the time for completion of accumulation of nanoclinics in the tumor cells. Magnetocytolysis results in complete destruction of the cells incorporating the nanoclinics in a DC magnetic field (7 T). Cell destruction occurs selectively without significant effect on healthy cells. Chemotherapy exhibits an additional effective method in tumor therapy. Because chemotherapeutic agents are toxic, the development of diagnostic tools that allow one to screen the action of chemotherapeutic agents would help in discovering the correct dose for each patient and thus minimize toxic side effects. Thus, AN-152 coupled with the TP chromophore C625 resulted in 142. It facilitates study of the uptake of the AN-152 into the cancer cell [90, 91]. AN-152 is more potent against LH-RH receptor-bearing cancers and produces less peripheral toxicity than other agents used for this therapy. The entry of AN-152:C625 (142) was studied by TPLSM. The first TP fluorescence of the cell appeared in about 5 min. It was followed by entry of the drug into the cytoplasm. Increase of the drug treatment time results in an increase of TP fluorescence as shown by comparing the images in Figure 3.101a–d. After 20 min, the drug enters into the nucleus of most of the cells as observed by TPLSM. These observations are necessary to understand in more detail the mechanism of action of this drug. Binding a TP probe to other biologically active molecules can extend this approach. These are proteins, peptides, nucleic acids, and others. Thus, optical recording using the TP fluorescent probe occurs with minimal photodamage by applying either in vitro or in vivo real-time tracking. Photodynamic therapy is an alternative treatment of tumors and age-related degeneration. Near-infrared light is used for exposure of tissue because it is

Figure 3.101. Fluorescence images of a single MCF-7 cell (LH-RH receptor-positive), taken (a) 4 min, (b) 20 min, (c) 40 min, and (d) 50 min after treatment with AN-152:C625 (0.6 mM). The staining of the cell occurred within 5 min, and entry into the nucleus occurred after 20 min, with an increase of the fluorescence intensity over time. (From Ref. [90] with permission of Kluwer Academic Publishers.)

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within the so-called tissue transparency window (l  750–1000 nm); that is, above absorption of collagen (l > 670 nm). Therefore, near-infrared light can penetrate deeper into the body (up to 3 cm) without significant scattering or absorption by normal tissue. Porphyrines are often applied as photoactive compounds because they have the ability to accumulate in pathologic cells, absorb the photon energy, and produce singlet oxygen, resulting in cell death of the tumor cells [46, 170, 181, 184–190, 195–198, 202–207, 210, 253–256, 589– 592]. The singlet oxygen wipes out the surrounding tumor tissue. Therefore, porphyrines with large TPA cross section are needed for efficient photodynamic therapy. This was recently shown for a group of porphyrins [181, 203, 556, 593, 594]. An interesting approach is shown Figure 3.102. The main system combines several functionalities and comprises [203, 204]:  The TP photodynamic chromophore (A), resulting in efficient generation of singlet oxygen; the singlet oxygen attacks the diseased cells that are finally wiped out after treatment with light.  The OP imaging chromophore (B) selected from cyanines.  The molecular targeting agent (C) selected from a representative sequence of amino acids, resulting in uptake of the triade system particular in the tumor cells. (A), (B), and (C) are covalently linked by a linker group comprising ester groups. Classical photodynamic therapy has several disadvantages. First, the time for accumulation of the photodynamic therapeutic agent is either not known or not easy to determine. The time factor is important, because exposure must mainly impact diseased cells while healthy cells should not be affected during therapy. According to Figure 3.102, determination of the necessary time for almost complete accumulation of the triade can be realized by the imaging dye (B) using a low NIR exposure dose, which does not affect the TP sensitizer for singlet oxygen production (A). Furthermore, it is difficult to discriminate between healthy cells and tumor cells. The new system in Figure 3.102 shows that a special selection of amino acid sequences (C) selectively targets the tumor and delivers the PDT agent to the tumor cells. In addition, the OP image dye (B) also monitors the time of disease aborting after PDT, which was more difficult in the traditional PDT. Thus, the system in Figure 3.102 combines two photonic functions of the triade. The TP sensitizer (A) results in generation of singlet oxygen that selectively destroys the tumor cells after selective targeting by (A) while the OP imaging dye (B) monitors the stage of delivery to the tumor and the destruction of the tumor after exposure. The deeper penetration of TP excitation light into the tissue allows an even more accurate destruction of tumor cells. This can be a distance up to 3 cm and was carried out with living mouse models [203].

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

O

R

+ (b) N N +

CH2 4 SO3

n

CH2 4 SO3

N

dPhen Cys Phe dTrp Lys Thr Cys Thr OH

COO

+ (c)

O Cl

Cl NH N

N

N

N HN Cl

Cl

O OH (a) N

+ Linker OP imaging for time tracking

(B) TP sensitizer for singlet oxygen generation

(A)

Linker (C)

Targeting

Figure 3.102. Components of TP PDT triade comprising the TP sensitier for singlet oxygen generation (a), the OP imaging dye (b), and the targeting agent (c). The linker unifies these three components in the triade by an appropriate esterification. (Adapted from Refs. [203] and [204].)

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The TP dye in Figure 3.102 comprises at the core a porphyrin, which is covalently bound by an unsaturated hinge to a p-moiety showing large TPA. Such structures were already discussed in Section III. Thus, a large d between 500 and 1000 GM was reported for this moiety in the 800–900 nm region (optically open for TP PDT) using 100-fs pulses for excitation. Cooperativity effects [203] explain the enhancement of d compared to porphyrins [181, 556].

G. Upconverted Lasing TP excitation can be used for the setup of lasing devices. This is known as upconverted lasing. It is an active area of research [126, 156–168, 170–177]. The main advantages of upconverted lasing are (1) elimination of the phasematching requirement, (2) the possibility of using semiconductor lasers as the pump source, and (3) the capability of adapting waveguide and fiber configurations. However, the lifetime of such upconverted lasers is significantly shorter compared to inorganic SHG material due to photoprocesses that result in the photodestruction of the chromophore. This was reported for the lasing dye ASPT (53a) in the mid-1990s [156]. The lasing lifetime of 53a is about 40,000 pulses if one considers a 15% decrease in the initial value using 8-ns pulses and a repetition frequency of 2 Hz. No cooling was necessary. Lasing efficiency and lifetime can be considerably increased by further improvements of the optical quality of the rod and optimization of the system design. The TP lasing dye 53a is embedded in either a Sol-Gel glass or a polymer [158, 173, 176] in order to restrict nonradiative deactivation. This is more effective in solution [337, 463, 464]. The cavity consists of a 100% reflecting back mirror and a 70% reflecting dielectric output coupler. The lasing efficiency of 53a in polymerized epoxy resin is about 3.5% [158]. Figure 3.103 shows the emission spectra of 53a doped in a 7-mm long rod using TP excitation at 1.06 mm. A broad fluorescence spectrum is obtained if the excitation energy is less than the threshold of 53a. Above the threshold, the spectrum exhibits a significantly smaller bandwidth depending on the pump energy. Cavity lasing occurs at the central region of the TPEF band. The bandwidth is about 8 nm. This is much smaller compared to the OPEF bandwidth. Furthermore, conjugated polymers were found to function as outstanding TP lasing materials [174]. These materials can easily be processed by spin-coating and some of them possess a large TPA cross section (cf. Section III.A.2). Among the large number of investigated conjugated polymers for NLO purposes, the ladder-type polymer 20 exhibits a large TPA amplitude—one of the largest reported for an organic material [382]. The extraordinarily large d recorded for this polymer is one prerequisite for a TP upconverted lasing material. The following conditions must also be fulfilled for a film made of 20 that shows

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

Figure 3.103. Fluorescence spectra of a 7-mm long and 10-mm diameter ASPT doped poly-HEMA (HEMA ¼ hydroxyethylmethacrylate) rod excited with 1.06-mm radiation. (a) Excitation energy is below the lasing threshold and (b) the spectra were recorded above the lasing threshold energy. Solid line: 0.4 mJ; dotted line: 0.5 mJ; long dashed: 0.8 mJ. (From Ref. [158] with permission of the American Institute of Physics.)

lasing upon TP excitation: (1) rapid relaxation from the TP excited state into the lowest OP allowed state (S1) prior to subsequent stimulated emission; (2) population inversion between the S1 and ground state; and (3) a resonator matching the Bragg condition within the region of the optical gain providing optical feedback. More details were previously reported [178, 595]. The average diameter of the grating was 100 mm. The thickness of the films was about 130 nm. Samples were exposed by a regenerative Ti:sapphire amplifier system providing a pulse repetition rate of 1 kHz and a pulse width of 150 fs. The lasing eigenmodes observed according to this two-dimensional structure are second-order Bragg modes. A typical example for TP lasing of 20 is shown in Figure 3.104. Lasing starts above a certain threshold level as shown by narrowing of the emission spectrum. A broad spectrum was observed using low excitation energy. Figure 3.105 shows the emission output by comparison of OP and TP excitation. A clear threshold behavior and linear increase of the intensity with increasing pump power can be seen in both cases. However, the threshold for TP excitation upconverted lasing is about 300 times larger compared to the threshold for OP excitation. Despite these superior properties one still has to keep in mind that the optical feedback has to be properly sealed in order to avoid photodamage during the excitation cycles.

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Figure 3.104. Emission of the circular grating structure below (dotted line) and above (solid line) the lasing threshold excited at 1.64 eV. (From Ref. [174] with permission of Wiley-VCH.)

H. Optical Power Limiting Lasers have become increasingly important in various applications. They can be found in laser printers, CD/DVD writers and players, scanners, laser pointers, control instruments, and scientific equipment. Furthermore, various kinds of

Figure 3.105. Emission intensity as a function of pump power for OP (&) and TP () excitation. (From Ref. [174] with permission of Wiley-VCH.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

cw and pulsed lasers are used for numerous applications. The use of lasers requires protection of human eyes, sensors, and detectors because they can blind humans or dazzle them for varying time periods, and they can destroy sensors and detectors of equipment. Further applications of optical power limiting materials are laser mode locking and optical pulse shaping. Because of a broad variety of different lasers emitting at different wavelengths with distinct pulse energy, there is a substantial need to design optical power limiting materials operating on a short time scale. Optical power limiters need to reduce laser fluency at the operating wavelength; they should have a fast temporal response time and a rapid recovery time. Optical power limiters should have a sufficient transmission of light at low intensities but they must suppress high pulse energy. Distinct mechanisms are used for optical power limiter constructions because of the broad variety of different laser types. These are reverse saturable absorption, multiphoton absorption, nonlinear refraction of materials such as carbon disulfide or liquid crystals, linear scattering of materials, nonlinear scattering using carbon black suspensions or fullerenes, self-focusing and defocusing, free-carrier absorption, intermolecular charge transfer absorption, photochromics, liquid crystals, and photonic crystals [53, 124, 125, 127–129, 156]. A combination of various methods results in an increase in the efficiency of optical power limiters. An example is a combination of TPA and nonlinear scattering using single-wall carbon nanotubes that are suspended in a solution containing the TP absorbing chromophore and a surfactant avoiding aggregation of the chromophores and the nanotubes [596]. A further example is a combination of a reverse saturable absorber with a TP active chromophore within the same molecule such as structure 57a that is called bi mechanistic optical power limiter [127]. This bi mechanistic optical power limiter shows reverse saturable absorbance at the lower wavelengths (400–600 nm) of visible light and TPA at the higher wavelengths (500–700 nm) of visible light. Therefore, bi mechanistic optical power limiters can cover the entire visible light range. Optical power limiting [597] behavior may be caused, for example, by reverse saturable absorption, TPA, or nonlinear refraction. This includes beam-induced refractive-index changes and optically induced scattering [121, 123–125, 171, 173, 177, 299, 459, 529, 573, 597–599]. TPA active materials show optical power limiting effects in liquid solvents, in solid matrices, in the pure liquid state, and in the crystalline state. Although handling of optical power limiters in a solid form is favored over the liquid form, it should be noted that repeated pulses at the same spot can result in a higher amount of degradation in the case of solid matrices than in liquids. Optical limiting by TP processes does not require linear loss and shows a short response time (1012 s or less) [127, 129], although a relatively large energy is necessary in the case of long pulses. Strong TPA during irradiation with ultrashort visible laser pulses causes optical power limiting behavior of solutions of the organic compounds depicted in Figure 3.106 [529]. These chromo-

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phores exhibit strong OPA from the UV to the violet spectral region but they do not have OPA in either the red or the NIR region. Distinct working mechanisms and different designs of optical power limiters make evaluation of their efficiency difficult and requires comparable experimental 2.0 I 1.5

S

S S

N

1.0

H21C10O

N OC10H21

0.5 (a) 0.0 200 2.0

400

600

800 N

II

N

S

1.5

1000

H

S S

1.0

CH3

H3C

Absorbance

0.5 (b) 0.0 200 2.0

400

600

800

III

H11C10O

1.5 1.0

OC10H11

0.5 0.0 200 2.0

1000

(c) 400

600

800

1000 IV

H11C10

1.5 1.0

C10H11

0.5 0.0 200

(d) 400

600

800

1000

Wavelength (nm)

Figure 3.106. Chemical structure and UV absorption spectra of organic compounds showing optical limiting properties in solution. (From Ref. [529] with permission of the American Institute of Physics.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

data. Transmission is a characteristic size to compare optical limiters. The TPA induced decrease of transmission (T) of a solution can be described as the ratio of the intensity after transmission (I) and the incident intensity (I0) according to Eq. (84) by neglecting the linear attenuation of the medium and assuming a uniform rectangular transverse intensity distribution within the medium. The transmission is influenced by the thickness of the solution sample (L) and the TPA coefficient (b). The TPA coefficient (b) given in units of cm/GW is determined by Eq. (85), where NA is the Avogadro constant, d is the TPA cross section, and [c] is the concentration of this compound in solution [529]. TðI0 Þ ¼

IðLÞ 1 ¼ I0 ð1 þ I0 L bÞ

b ¼ d NA ½c  103

ð84Þ ð85Þ

If the beam has a Gausian transverse distribution in the medium, the TPA induced transmissivity change is described by Eq. (86): TðI0 Þ ¼

IðLÞ lnð1 þ I0 L bÞ ¼ I0 I0 L b

ð86Þ

The transmissivity is also a function of the solute concentration [c] for a given input intensity level I0 (Eq. (87)): TðI0 ; dÞ ¼

lnð1 þ I0 L d NA ½c  103 Þ I0 L d NA ½c  103

ð87Þ

The transmissivity decreases with the input intensity (Fig. 3.107) and dopant concentration (Fig. 3.108) [529]. The good agreement between experimental data and the theoretical predictions given by Eqs. (86) and (87) demonstrates the dominant role of TPA in causing the observed optical limiting properties of compound 94a [529]. Further parameters that are useful to compare the performance of different optical power limiters are the transmission contrast ratio (TCR) Eq. (88), the transmission dynamic range (TDR) Eq. (89), and the transmission cutoff (TCO) Eq. (90). The values Tmax and Tmin are the maximum of transmission and the minimum of transmission, and I90% and I10% are the intensity of the 90% and 10% transmission points, respectively [600]. Tmax  Tmin  100% Tmax þ Tmin Tmax TDR ¼ Tmin I90%  I10% TCO ¼ I90% þ I10% TCR ¼

ð88Þ ð89Þ ð90Þ

327

APPLICATIONS

1.0 L = 1 cm d = 0.051 M/I b = 0.8 cm/GW

Transmissivity

0.9 0.8 0.7 0.6 0.5 0.4 0.0

0.5

1.0

1.5

2.0

2.5

Input Intensity (GW / cm2)

Figure 3.107. Transmissivity of a solution of 94a in tetrahydrofurane (5:1  10 2 M/L, L ¼ 1 cm, d ¼ 0:051 M/L, b ¼ 0:8 cm/GW) as a function of the input laser beam intensity: (o) experimental data, solid line is calculated using Eq. (86) and a fitting parameter of b ¼ 0:8 cm/GW. (From Ref. [529] with permission of the American Institute of Physics.)

1.0 L = 1 cm I0 = 2.21 (GW / cm2)

Transmissivity

0.8

0.6

0.4

0.2

0.0 0.00

0.05

0.10

0.15

0.20

Dopant Concentration (M / I)

Figure 3.108. Transmissivity of a solution of 94a in tetrahydrofurane (5:1  102 M/L, L ¼ 1 cm, d ¼ 0:051 M/L, b ¼ 0:8 cm/GW) as a function of the solute concentration: triangle, experimental data; solid line is calculated using Eq. (87), an input intensity I0 ¼ 2:21 GW/cm2, and a fitting parameter of s2 ¼ 2:6  1020 cm4/GW. (From Ref. [529] with permission of the American Institute of Physics.)

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TWO-PHOTON PHYSICAL, ORGANIC, AND POLYMER CHEMISTRY

In an ideal system, the transmission contrast ratio is 100%, the transmission dynamic range goes to an infinite value, and the transmission cutoff is zero. Two-photon optical limiting can be used successfully to convert a Gaussian beam into a flat beam. This is useful for multifocal multiphoton microscopy [246]. The Gaussian laser beam profile has a higher light intensity at the center compared to the corners and results in a heterogeneous image from a uniform plane. Therefore, the information stored may not be recorded or read out properly. Two-photon optical limiting can be used to change the laser intensity distribution of an intense fs laser (intensity of about 0.5 GW/cm2) spatially using compounds with appropriate TPA coefficients (50 cm/GW or higher). Under these circumstances, the Gaussian profile of the laser beam is converted into a flat beam [246]. Organic molecular crystals not only show an increase of the transmitted intensity with the incident light intensity, but also show a small increase of the transmitted intensity with the polarization of natural crystal faces. Only tiny TPA cross sections were measured for the case of perpendicular alignment of the TP transition dipole moments to natural faces or the light polarization perpendicular to these transition dipole moments. They can even vanish because polarization of light has a strong impact on TPA [597].

VIII.

OUTLOOK FOR TWO-PHOTON PHOTOSCIENCES

Although many TP contributions have focused on theoretical design strategies, synthesis, and characterization of TP chromophores, TPA in photosciences is still in its infancy. The main concern is whether one really needs TP excitation because many applications in biology, medicine, and material science are successfully based on OP excitation. In order to favor TP excitation, one has to list the advantages of TPA.  Deeper penetration of excitation light into the sample as long as the linear absorption coefficient vanishes at the excitation wavelength. Both medical and biological applications benefit from this advantage because attenuation of excitation light by self - absorption of the target material can be diminished in the case of TP excitation (e.g., collagen absorbs below 670 nm).  Improved spatial resolution (120 nm) compared to OP excitation, which is lexc=2 (lexc ¼ excitation wavelength). The use of special optical techniques results in improvement of the spatial resolution (<100 nm) and brings the TP scientist into the real nanoworld, although NIR light (e.g., 800 nm) is used for excitation.

OUTLOOK FOR TWO-PHOTON PHOTOSCIENCES

329

Figure 3.109. Schematic sketch showing the interdisciplinary relationships between basic research and prospective applications.

Figure 3.109 shows a general scheme combining distinct disciplines in TP photosciences. It demonstrates the interdisciplinary cooperation needed to become an accepted scientific field in both academic and industrial areas. It demonstrates the workflow, starting from basic research including theory, synthesis, and chromophore characterization. Development of TP chromophores, materials needed for TP application, and methods and equipment required in TP photosciences will require interdisplinary work by theoretical scientists, organic chemists, polymer chemists, physical chemists, and physicists. Large TPA cross section and optimal matching between the TPE maximum and the wavelength of the emitting laser source are some prerequisites. Furthermore, development of methods and light sources is necessary to accomplish TP excitation in a short time with a spatial resolution in the nano region. This will require a lot of effort compared to OP excitation, which is limited to lexc =2. Development of microlaser sources emitting cw light would revolutionize TP

330

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photosciences because these less expensive cw laser sources can be substituted for expensive pulsed laser sources and will be indispensable for industrial applications. Efforts in chromophore design and characterization as well as the development of less expensive laser sources will demonstrate how efficiently TPA can be applied in medicine (screening of tumor therapy, photodynamic therapy), biology (imaging of cellular processes), nanofabrication in material science and engineering (information recording materials, upconverted lasing), and photonics (photonic band-gap materials, waveguide materials, optical power limiting materials).

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INDEX

Absorbance, of diarylethene, 263 Absorbance change, pump intensity versus, 262 Absorption. See also Excited state absorption (ESA); Femtosecond transient absorption measurements; Linear absorption coefficient; Multiphoton absorption; Nonlinear absorption coefficient (b); Nonlinear absorption data; One-photon absorption (OPA); Steady state absorption spectra; Three-photon absorption; Transient absorption; Two-photon absorption (TPA) linear, 205 nonlinear, 190–191 Absorption cross sections, 118. See also OPA cross section; TPA cross section (d) Absorption spectra, 82, 83, 86, 88–89, 97, 101, 102, 103, 181. See also TPA spectra of anthracene, 160 of conjugated polymers, 170 of excited-state xanthone ketyl radicals, 90 of irradiated stilbene, 58–61 of 1-methoxynaphthalene, 76–77 of methyl-substituted ladder-type poly( p-phenylene), 163–165 of 1-NpCH2–OBP, 73 of oligothiophenes, 84 steady state, 14–15, 38 transient, 37–45

Acceptor groups, in symmetric donor-acceptordonor chromophores, 226 (Acceptor-p)3-donor chromophores, 246–247, 248 Acceptor-p-acceptor (A-p-A) compounds, symmetric, 215–225 Acetonitrile, 58 Acetylene mesomer, of polydiacetylene, 168–169 Acousto-optic measurements, 145 Acrylamide, TP initiated polymerization of, 274, 278 Acrylate monomer, in three-dimensional microfabrication, 289–290 Acrylates, TP initiated polymerization of, 274, 275, 276, 278, 286, 289 Acrylonitrile, in 3D data storage, 302 Addressing, in 3D data storage, 300–301 Alkenes, infrared multiphoton excitation isomerization of, 260 Alkoxy groups, in nonlinear absorbing chromophores, 252, 253, 254 Alkylene groups, in octupolar chromophores, 247, 248 Alkylhalides, pulse radiolysis in, 58 Alkylsulfonyl substituent, in symmetric donor-acceptor-donor chromophores, 226 AM1 approach, 125 Amino acids, TPEF and, 255

Advances in Photochemistry, Volume 29 Edited by Douglas C. Neckers, William S. Jenks, and Thomas Wolff # 2007 John Wiley & Sons, Inc.

355

356

INDEX

Amino nitrogen, in octupolar chromophores, 243–244, 245 N-(4-Aminophenyl)fulgimide, as 3D data storage material, 299 4,40 -Amino-stilbenes, 197, 202 Amino-substituted distyrylbenzene, 289, 290 Amplitude, chromophore number and, 18 AN152, medical applications of, 260, 318 Angle dependence, between monomer units, 180 Anisole (ANS), 58–60, 61 Anisotropy decay times, 18, 19, 20–21 Anisotropy decay traces, 19, 21–22 Annihilation processes, 44, 47. See also Singlet–singlet annihilation Anthracene energy gap law and, 70–71 TPA spectra of, 160 Anthracene moieties in octupolar chromophores, 239 in symmetric donor-acceptor-donor chromophores, 231 in symmetric donor-p-donor chromophores, 209–210, 210 Anthraquinones, 64 Anti-Stokes–Raman scattering, 121 Apertureless NSOM, in three-dimensional microfabrication, 285, 289 Aromatic hydrocarbons (AHs) energy gap law and, 70–71 multiphoton absorption by, 157 Aromatic resonance, 182 Aromatic rings, TPA data for, 158, 159 Arylene-ethylene pattern, in octupolar chromophores, 240 ASPT lasing dye, in upconverted lasing, 321, 322 Autofluorescence of tissue, 317 Average initiation rate, of radical polymerization, 279 Average molecular weight, 178 Azaxanthone (AX), excited-state ketyl radicals of, 88–89 Azobenzenes donor–acceptor substituted, 187 in nonlinear absorbing chromophores, 251 B3LYP geometries, for two-photon absorption, 126–128 Band formation, 179–180. See also Optical band-gap materials

Barbituric acid moiety, in symmetric acceptor-p-acceptor chromophores, 220–221, 222 Bathochromic shifts, 126–128 of nonlinear absorbing chromophores, 254 of octupolar chromophores, 240 of symmetric donor-acceptor-donor chromophores, 230 Bay-functionalized perylene bisimide, 7–8 Beam pulse shapes, 297 for quantifying TPA cross section, 145–146, 147, 149–150, 151, 152 in three-dimensional microfabrication, 284, 287 Beam–sample interaction area, 283 Benzene(s) TPA data for, 158, 159 TP transition in, 139 Benzene moiety, in octupolar chromophores, 242, 243 Benzene ring, in nonlinear absorbing chromophores, 254–255 Benzocyclobutene, 94–95 Benzophenone (BP), 4–5 bond dissociation from Tn states in, 72–74 excited-state ketyl radicals of, 85–88, 89–90 naphthalene Tn states and, 66–67 substituent effects on Tn state lifetimes of, 71–72 Benzophenone derivatives (BPD), excited-state ketyl radicals of, 85–88, 89 Benzophenoxazin dye, as 3D data storage material, 299 6-Benzothiazol-2-yl-(2-naphthyl) diphenylamine, in optical circuits, 309 7-Benzothiazol-2-yl-9,9-diethyl-fluoren-2-yldiphenylamine, in 3D data storage, 294 Benzothiazole groups, in symmetric acceptor-pacceptor chromophores, 223–224 Benzothiazolyl group, 295 1-[(4-Benzoylphenoxy)methyl]naphthalene (1-NpCH2–OBP), bond dissociation from Tn states in, 72–74 2-[(4-Benzoylphenoxy)methyl]naphthalene (2-NpCH2–OBP), 100 bond dissociation from Tn states in, 73–74 Biexcitonic states, 171 Bifluorenyl model dimer, 174 Bi mechanistic optical power limiter, 324 Biological structures, TPEF of, 255

INDEX

Biomedical applications, of two-photon absorption, 315–321 Bioprobes, fluorescent, 255, 256, 259–260 Biphenyl, 63, 64, 204, 205 Biphenyl hinge, 203, 204 Biphenyl moiety, in symmetric acceptor-pacceptor chromophores, 224 [(1,8-Bis[(4-benzoylphenoxy) methyl]naphthalene (1,8-(BPO-CH2)2Np), stepwise bond photocleavage in, 100–103 (E,E,E,E,E,E)-1,13-Bis-[4(diethylamino)phenyl-tri-deca1,3,5,6,8,10,12-hexaen-7-one, 274, 275, 276 2,7-Bis[[4-(dimethylamino)phenyl] methylene]cycloheptanone, 277, 278 2,5-Bis(alkylsulfonyl)benzene unit, in symmetric donor-acceptor-donor chromophores, 226 Bis-dioxaborine units, in symmetric acceptor-p-acceptor chromophores, 221, 223 Bis(diphenylamino)diphenyl hexatriene in holographic recording, 306–307 in 3D optical data storage, 303–304 4,40 -Bis(N,N-dimethylamino)benzophenone, 280 1,10 -2,20 -Bis(o-chlorophenyl)-4,40 ,5,50 tetraphenyl-bisimidazole, 278 p-Bis(o-methylstyryl)benzene, as TPA measurement standard, 159 1,2-Bis-[(phenylthio)methylbenzene], laser photolysis of, 265, 266 Bis(pyridylstyryl)-substituted diacetylene unit in symmetric acceptor-p-acceptor chromophores, 221, 222 Bit erasing, in 3D data storage, 299, 303 B(mesityl)2 group, in neutral donor-p-acceptor chromophores, 191–192 Bond dissociation, from Tn states, 72–75 Bond length alternation (BLA), 174–175, 182 in symmetric donor-p-donor chromophores, 210 two-photon absorption and, 130–132 Born–Oppenheimer approximation, two-photon excitation and, 139–140 Boron in ionic donor-p-acceptor chromophores, 196 in neutral donor-p-acceptor chromophores, 191

357

in symmetric acceptor-p-acceptor chromophores, 220, 223 Bragg diffraction in optical band-gap materials, 305 in upconverted lasing, 322 Brilliant Green, 237–238 Bromine, 269 Butadienes, infrared multiphoton excitation isomerization of, 260 Butatriene mesomer, of polydiacetylene, 168–169 t-Butyl methacrylate, 290 C625, medical applications of, 259–260 Cancer photodynamic therapy for, 91–94, 318–321 TP applications in treating, 315, 316, 318 Carbazole, 202 Carbazole moieties, in symmetric acceptor-pacceptor chromophores, 216, 224–225 Carbon nanotubes, in limiting laser optical power, 324 Carbon–silicon (C–Si) bond, bond dissociation of, 74–75 Carbon tetrachloride (CCl4) electron transfer to, 76–80 naphthalene Tn states and, 67–70 Cationic polymerization, two-photon initiated, 281–283 Cavity lasing, 321 C–C cleavage, in infrared multiphoton excitation isomerization, 260 Cell imaging, 315, 317–318 Centrosymmetric chromophores, three-level TPA model for, 136–139 CEO-INDO/S procedure, 13. See also ZINDO/S package Change of state dipole moment (
m01) in ionic donor-p-acceptor chromophores, 194–195, 196–197 in neutral donor-p-acceptor chromophores, 183–185, 186 in octupolar chromophores, 233, 237 in symmetric donor-p-donor chromophores, 198–201 Change of transmission. See Nonlinear transmission methods Channel structures, 290, 291 Charge separated resonance structure, bond length alternation and, 132 Charge-shift probes, 193

358

INDEX

Charge transfer (CT), 3 in octupolar chromophores, 233, 236 Chemical reactions, multibeam-irradiationinduced, 55–57 Chemotherapy, 255, 259–260 TP fluorescence in, 318 Chinoid pattern, 196 Chirp, 152 Chromophore conjugation, 134 Chromophore design, two-photon absorption optimization and, 156–255 Chromophore-labeled dendrimers, 5–6 Chromophores. See also Chromophore design; Multichromophoric systems; TP chromophores bond length alternation in, 130 calculating distances between, 22 diphenylaminofluorene-based, 207 dipolar, 182–197 dithienothiophene-based, 207–208 future applications of, 329, 330 with large p-systems, 157–182 in nanoclinics, 315 neutral donor-p-acceptor, 182–193 in nonlinear TPA measurements, 154–155 in numerical TPA simulations, 156 octupolar, 233–250 in photodynamic therapy, 319 photo-orientation of, 262 polymers with nonlinear absorbing, 250–255 ‘‘propeller shaped,’’ 233–250 quadrupolar, 136, 237 quantifying TPA cross section in, 144, 149–150, 152–154 singlet oxygen and, 266–267, 268 spectra and, 14–15 in structural isomers, 12–13 surface plasmon effects in, 140–143 symmetric acceptor-p-acceptor, 215–225 symmetric donor-acceptor-donor, 225–233 symmetric donor-p-donor, 197–215 in two-level TPA model, 134 two-photon absorbing, 6 two-photon excitation of, 116, 118, 122, 123–125 two-photon excited fluorescence of, 255–260 in two-photon initiated cationic polymerization, 281–283 ultrafast energy hopping between, 21 in upconverted lasing, 321–323

Chromophoric backbones, 2, 3 Chrysene (CHR) direct observation of Tn states in, 85 energy gap law and, 70–71 Circular grating structure, in upconverted lasing, 322, 323 cis–trans isomerization, of stilbene, 58–61, 64. See also trans–cis photoisomerization Coinitiators, 272–274, 278 Collection efficiency (Zcol), 152 Collimated light, in nonlinear TPA measurements, 154–155 Complex metal ions, fluorescent chromophores and, 258 Complex refractive index, 120 Computer applications. See Three-dimensional data storage Concentration of propagating radicals, in radical polymerization, 279–280 Concerted mechanism, of dissociative ELT, 80–81 Condensed aromatic rings, TPA data for, 158 Conjugated oligomers, two-photon absorption by, 160–182 Conjugated polymers bearing PPV pattern, 169–170 TPA data from, 181–182 two-photon absorption by, 160–182 Conjugating rods, in symmetric donor-p-donor chromophores, 204–205, 205–206. See also Connecting rods; Rod shapes Conjugation chain, in symmetric donor-p-donor chromophores, 211–212 Conjugation length, 169 effective, 175–177, 178 of phenyl-substituted ladder-type poly( p-phenylene), 166–167 Connecting rods. See also Conjugating rods in (donor-p)3-donor chromophores, 244 in symmetric donor-acceptor-donor chromophores, 231 in symmetric donor-p-donor chromophores, 204–205, 205–206 Continuum beam, in nonlinear TPA measurements, 154–155 Coulomb interactions, 11–12 Coumarin-2 dyes, 5–6 Coumarin-305, in fabricating optical band-gap materials, 305–306 Coumarin-343 dye, 6

INDEX

Coupling energy (Ecoup), in octupolar chromophores, 236 Coupling mechanism, of octupolar chromophores, 240 Covalent bonding, two-photon absorption and, 173–174 Crosslinked polymers, in 3D data storage, 301 Crosslinked TP initiated polymerization, 279 Crosslinking simultaneous two-photon initiated, 272–283 in two-photon initiated cationic polymerization, 281 Crotonic esters, infrared multiphoton excitation isomerization of, 260 Crotonitrile, infrared multiphoton excitation isomerization of, 260 Crystallization, fabricating optical band-gap materials via, 305 Crystals. See also Liquid crystals (LCs); Photonic crystals as optical band-gap materials, 305 organic molecular, 328 Crystal Violet, 237–238 Cubic trenches, 291 Cyano groups, 269–270 in neutral donor-p-acceptor chromophores, 190–191 in symmetric donor-acceptor-donor chromophores, 229, 230, 231 Cycloaddition of o-quinodimethane, 94–96 TP-excitation initiated, 115 two-photon induced, 265–266, 267 Cyclobutenes, infrared multiphoton excitation isomerization of, 260 Cyclophane derivatives, of symmetric donor-pdonor chromophores, 212–214 Damping factors, 123, 126 Data storage holographic, 302–303 three-dimensional, 115, 293–304 Davydov splitting, 215 Decay times, 34 Degenerate four-wave mixing, 145 Degenerate TPA, 121 d values. See TPA cross section (d) Dendrimers, 2 chromophore-labeled, 5–6

359

energy transfer among, 2–10 first generation, 10 metal-containing, 3–4 naphthalene, 4–5 phenylacetylene, 2–3 phenylacetylene-based, 9–10 polyacetylene, 3 polyphenylene, 2, 8–9 poly(propylene-imine), 7 in radical polymerization, 280 second generation, 10, 11 spectra of first generation, 14–15 Dendrimer systems distyrylbenzene, 7 triarylamine, 7 Dendritic structures, ensemble photophysics of rigid polyphenylene-based, 1–51 Density functional theory (DFT), for two-photon absorption, 126–128 Density of radicals (r), in radical polymerization, 281 Depolarization, fast, 20, 21 Depolarization time constant, 19 De Schryver, F. C., 1 Dexter interactions, 3 Dialdehyde unit, in nonlinear absorbing chromophores, 252, 253 Dialkylamino group in neutral donor-p-acceptor chromophores, 190–191 in octupolar chromophores, 237 Dialkylaniline pattern, of symmetric donoracceptor-donor chromophores, 229 Diaroyl(methanato)boron difluorides, 223 Diarylethene, absorbance of, 263 Dibenzanthracene (DBA), energy gap law and, 70–71 Dichlorobenzene (DCB), naphthalene Tn states and, 68–70 1,2-Dichloroethane, 58, 59 1,2-Dicyano-1,2-bis-(2,4,5-trimethyl-3thienyl)ethane, two-photon induced isomerization of, 260–261 cis-1,2-Dicyano-1,2-bis(2,4,5-trimethyl-3thienyl)ethane, 302 Dicyanoanthracene, 64 Dicyanobenzene (DCNB), naphthalene Tn states and, 68–70 4-(Dicyanomethylene)-2-methyl-6-(4dimethylaminostyryl)4H-pyrane, 280

360

INDEX

Dicyanomethylidenedihydrothioene moiety, in symmetric acceptor-p-acceptor chromophores, 221, 222 1,4-Dicyanonaphthalene (DCN), 91 Diels–Alder cycloaddition, 12, 45 Diethyl vinylbenzylphosphonate, in 3D data storage, 294 Differential interference contrast (DIC) optics, in 3D data storage, 294 Difuranonapthalenes, singlet oxygen and, 268–269, 270 Diisocyanate moiety, in nonlinear absorbing chromophores, 252 Dimesitylboryl group, in symmetric acceptor-pacceptor chromophores, 220 1,2-Dimethoxybenzene, in symmetric donor-acceptor-donor chromophores, 229 4,40 -Dimethoxybenzophenone, excited-state ketyl radicals of, 87–88, 89 5,7-Dimethoxycoumarin, cycloaddition of, 265, 267 3-[1-(1,2-Dimethyl-1H-indol-3-yl)-ethylidene]4-isopropylidene-dihydrofuran-2,5-dione in 3D data storage, 302 two-photon induced isomerization of, 262–263 2,20 -Dimethyl-distyrylbenzene, 219 N,N-Dimethylformamide, 64 Dimethylsulfoxide (DMSO), 146, 147 Dioxaborine unit, in symmetric acceptor-pacceptor chromophores, 221, 223 Dipentaerythritol pentaacrylate, in 3D optical data storage, 303–304 Diphenylaminofluorene-based chromophores, 207 Diphenylamino group in neutral donor-p-acceptor chromophores, 189 for symmetric donor-p-donor chromophores, 208–209 Diphenylanthracene, 169–170, 171 Diphenylbutadiene, trans–cis photoisomerization of, 264–265 1,2-Diphenylcyclobutene (CB), 61 Diphenylpolyenes with more than two ethylene moieties, 171 TP excitation spectra of, 171 trans-Diphenylpolyenes, TPA cross section in, 157–159 Di( p-methoxyphenyl)methyl chloride (An2CHCl), three-color three-laser photochemistry of, 99–100

Dipolar chromophores, 182–197 OPA and TPA of, 186 Dipolar chromophore structure bond length alternation and, 132 in two-level TPA model, 133 Dipole, transition, 178, 180, 186 Dipole moment (M), 123. See also Transition dipole moment (Mnm) induced, 120 of ionic donor-p-acceptor compounds, 193 transition, 175–177, 180–181, 190 Dipole moment changes (
m), 123 in two-level TPA model, 133 Direct excitation, of retinal, 265 Disperse Red, 251 Distyrylbenzene, amino-substituted, 290 Distyrylbenzene dendrimer systems, 7 Distyrylbenzene derivatives in singlet oxygen generation, 269–270 as symmetric donor-acceptor-donor chromophores, 229–230 as symmetric donor-p-donor chromophores, 202–203 TPA data for, 159 Distyrylbenzene moiety in nonlinear absorbing chromophores, 252, 253, 254 in symmetric acceptor-p-acceptor chromophores, 220 in symmetric donor-p-donor chromophores, 213 in two-photon initiated cationic polymerization, 282 Dithienothiophene-based chromophores, 207–208 DNA cleavage method, 98 DNA damaging, two-color two-laser, 91–94 Donor–acceptor distances, 21 Donor-acceptor-donor (D-A-D) compounds, symmetric, 225–233 (Donor-p)3-donor chromophores, 243–244, 245 (Donor-p)3-p chromophores, 242–243 (Donor-p-acceptor)3-donor chromophores, 247, 249 Donor-p-acceptor (D-p-A) compounds ionic, 193–197 neutral, 182–193 Donor-p-donor (D-p-D) compounds symmetric, 197–215 in three-dimensional microfabrication, 286 as TP initiators, 274

INDEX

Doracure photoinitiators, 272, 273 Double-conjugated-segment molecules, 225 Doublet states, excited, 55 Driving force (-
GELT), in electron transfer, 77–78 Drug delivery screening, 315 Effective conjugation length (nc), 175–177, 178 Effective emitter concentration, 178 Effective molecular weight, 177–178 Efficiency (E), 20 Electron affinity, of symmetric donor-acceptordonor chromophores, 230 Electron beams, from linear accelerator, 54 Electron density, in symmetric donor-acceptordonor chromophores, 229, 230 Electron ejection, from NDI, 93 Electronic coupling, 124, 169 in polyacetylene dendrimers, 3 singlet oxygen and, 266–267 Electronic excitation transfer, 10–14 Electronic state parameters, 175 Electron-rich moieties, in symmetric donor-pdonor chromophores, 208–209 Electrons, in surface plasmon effects, 141 Electron transfer (ELT), 68. See also ELT entries; Marcus ELT theory; Photoinduced ELT; Selective ELT quenching in dendritic macromolecules, 2–10 photoinduced, 272–274 secondary, 58 ‘‘sticky’’ dissociative, 79 in TiO2 photocatalytic reactions, 96–98 from Tn states, 75–81, 82 in two-photon initiated cationic polymerization, 281–282 Electron-withdrawing groups, 219, 225–226 in octupolar chromophores, 246–247, 248 ELT pathways, of excited-state benzophenone ketyl radicals, 88–89. See also Electron transfer (ELT) ELT quenching, 68 Emission maxima, 295–296 Emission spectra, steady state, 14–15 Emission wavelength, chromophore number and, 18 Energy exchange rate, 121 Energy gap law, Tn states and, 70–71 Energy hopping, 23–24 singlet–singlet annihilation versus, 44–45, 46

361

Energy relaxation processes, 285 Energy transfer (ENT) process, 66, 68–70, 75. See also ENT quenching in dendritic macromolecules, 2–10 Ensemble photophysics, of rigid polyphenylenebased dendritic structures, 1–51 ENT quenching, 68–70, 91. See also Energy transfer (ENT) process Epoxy resins in holographic recording, 307 in optical circuits, 309 TP initiated polymerization of, 286 Erasing, in 3D data storage, 299, 303 1-Ethylnaphthalene, electron transfer from Tn states in, 77, 79 Ethynyl-anthryl moiety, in octupolar chromophores, 239 Ethynyl-phenylene-ethynyl connector, in symmetric donor-p-donor chromophores, 211–212 ‘‘Excimer-like’’ entity, 18 Excitation, multistep, 54. See also TPE entries; TP excitation maxima; TP excited states; Two-photon excitation (TPE) Excitation energy, 123, 126, 127, 169 in multichromophoric compounds, 25–37 Excitation intensity, TPEF and, 165, 166 Excited ketyl radicals, studied via two-color two-flash photolysis, 85–91 Excited radical anions, pulse radiolysis of, 64–65 Excited radical cations fluorescence from, 61–64 pulse radiolysis of, 58–61 studied via two-color two-flash photolysis, 91 Excited state absorption (ESA), 38 in two-photon absorption, 117, 123–124 Excited state processes, in multichromophoric systems, 2–10 Excited states, 55. See also TP excited states Exciton coupling energy, in symmetric donor-pdonor chromophores, 213 Excitonic states, 171 Exciton model of linear aggregates, 175 Extinction coefficient (e), 134 in ionic donor-p-acceptor chromophores, 194–195 in neutral donor-p-acceptor chromophores, 183–185 of nonlinear absorbing chromophores, 253 in octupolar chromophores, 234–235

362

INDEX

Extinction coefficient (e) (Continued) in symmetric acceptor-p-acceptor chromophores, 217–218 in symmetric donor-acceptor-donor chromophores, 227–228 in symmetric donor-p-donor chromophores, 198–201, 202 Fabrication techniques, for optical band-gap materials, 305–306, 307 Face-centered cubic (fcc) crystals, in optical band-gap materials, 305 Fast depolarization, 20, 21 Femtosecond fluorescence upconversion measurements, 25–37, 44 Femtosecond (fs) lasers, 54, 98 multiphoton excitation via, 113–114 quantifying TPA cross section via, 144, 147–148 singlet oxygen production and, 268 for 3D optical memory systems, 294 TPA data taken with, 156–157 waveguides and, 309 Femtosecond transient absorption measurements of, 37–45 by methyl-substituted ladder-type poly( p-phenylene), 163 Fermi Golden Rule, 10–11 Field intensity/strength, with two-photon absorption, 120–121 First generation dendrimers, 10 Fluorene(s), 174, 175, 204, 205–206 donor–acceptor substituted, 187–188 TPA spectra of, 176 Fluorene moiety, 240–241 in symmetric acceptor-p-acceptor chromophores, 224 Fluorescein, 255 as TPA measurement standard, 152–154, 155 Fluorescence, 123, 124. See also Two-photon excited fluorescence (TPEF) of excited di( p-methoxyphenyl)methyl radical, 99–100 of excited radical cations, 61–64 by ionic donor-p-acceptor compounds, 193 of liquid crystals, 303 in neutral donor-p-acceptor chromophores, 188–189 of octupolar chromophores, 240–241 quantifying TPA cross section via, 144

solvent-dependent, 188–189 surface-plasmon-enhanced, 143 in symmetric donor-p-donor chromophores, 205, 206 in 3D data storage, 298–299, 300 Fluorescence cross-section action, in fluorescence imaging applications, 257 Fluorescence decay times, 177 m-C1Px, 17 Fluorescence excitation spectra, 214 Fluorescence imaging, 311–315 in 3D data storage, 295–296 Fluorescence imaging applications, fluorescence cross-section action in, 257 Fluorescence quantum yield (f), 177 from ionic donor-p-acceptor chromophores, 194–195, 196–197 from neutral donor-p-acceptor chromophores, 183–185, 186 from nonlinear absorbing chromophores, 253 from octupolar chromophores, 234–235 from symmetric acceptor-p-acceptor chromophores, 217–218 from symmetric donor-acceptor-donor chromophores, 227–228 from symmetric donor-p-donor chromophores, 198–201 Fluorescence resonance energy transfer (FRET), 271 Fluorescence spectra, 172 of excited radical cations, 91 of excited-state azaxanthone ketyl radicals, 88–89 of excited-state xanthone ketyl radicals, 88, 90 femtosecond fluorescence upconversion measurements and, 25–37 femtosecond transient absorption measurements and, 39–42 magic angle polarization and, 15–18 by methyl-substituted ladder-type poly( p-phenylene), 165 steady state, 14–15 in time-resolved fluorescence polarization measurements, 18–25 Fluorescent bioprobes, 255, 256 medical applications of, 259–260 Fluorescent dye, in 3D data storage, 294–295 Fluorinated moieties, 186–187 Fluorinated phenyl rings, 186–187 Fluorinated polyenes, 134

INDEX

Forbidden transitions, 168 Fo¨ rster interactions, 3 Fo¨ rster theory, 10–11, 14, 20, 21–22, 23, 24, 37, 46, 47 excited radical cations in, 91 Fragmentation, in infrared multiphoton excitation isomerization, 260 Franck–Condon factors, 140 Fresnel reflectance, 147 Fujitsuka, Mamoru, 53 Fulgide, in holographic data storage, 302 Gaussian beam propagation limiting laser optical power and, 326, 328 in 3D data storage, 300–301 Gaussian pulse, 145–146, 147, 150, 284 Generation number, femtosecond transient absorption measurements and, 44 Gold nanoparticles (AuNps), 89–91 surface plasmon effects and, 142–143 Gold tips, in three-dimensional microfabrication, 289–290 Go¨ ppert-Mayer, Maria, 113, 118 Go¨ ppert-Mayer (GM), 118 Green band, 139 Group delay dispersion (GDD), 152 Guanine (G), in two-color two-laser DNA damaging studies, 93–94 Hammett constants, 71–72, 188, 191–192, 216 Hartree–Fock geometries, 128 Heisenberg relationship, 118 Heptathiophene dye, 5–6 Herzberg–Teller expansion, 139, 254 Heteroaromatics, TPA data for, 159 Heterocyclic moieties, in neutral donor-pacceptor chromophores, 187–188 Heterodyned Kerr effect, 145 Hexadienes, infrared multiphoton excitation isomerization of, 260 Higher triplet excited (Tn) states, energy transfer from, 66–70 Highest occupied molecular orbital (HOMO), 182 symmetry and, 129 Hofkens, J., 1 Hole transfer, 63 Holographic data storage systems, 302–303 Holographic recording, 302–303 optical band-gap materials for, 306–307 Holography, 115

363

Hopping, rate constant of, 20–21, 22. See also Energy hopping Hybrid organic/onorganic ordered nanostructures, in holographic recording, 307 Hydrogen ions (Hþ), in two-photon initiated cationic polymerization, 282 Hydrogen loss, in infrared multiphoton excitation isomerization, 260 Hyperbolic secant. See sech2-shaped pulse Hyperpolarizability (g), 160 bond length alternation and, 130–132 nonlinear, 190–191, 250, 251 second, 182 Hyperthermal effect, 316 Hypsochromic shift of TP excitation maximum, by symmetric donor-p-donor chromophores, 210 Image potential states, surface plasmon effects and, 141 Index of refraction in 3D data storage, 294, 298 in waveguide materials, 310 Indirect population, 124 INDO-MRDCI/SOS method, 238. See also ZINDO/S package Induced dipole moment, 120 Induced polarization, 120 Infrared multiphoton excitation, isomerization via, 260. See also Near infrared (NIR) light Initiation, of radical polymerization, 276 Intensity of light, in radical polymerization, 280–281 Interchromophoric distances, 22 Interdisciplinary cooperation, in TPA research, 329 Intermediates, three-laser control of, 99–100 Intermolecular charge transfer, TP-excitation initiated, 115 Intermolecular ELT, 75 Internal conversion (IC), 116 Intersystem crossing (ISC), 66, 116 quantum yield for, 241 singlet oxygen and, 266 Intramolecular charge transfer (ICT) in neutral donor-p-acceptor chromophores, 182, 186 in octupolar chromophores, 240 of symmetric donor-acceptor-donor chromophores, 230

364

INDEX

Intramolecular ELT, 75 Ionic donor-p-acceptor (D-p-A) compounds, 193–197 Irgacure photoinitiators, 272, 273 Iron oxide (Fe2O3), in nanoclinics, 315, 316 Isomerization. See also Photoisomerization in 3D data storage, 294, 301 TP-excitation initiated, 115 two-photon-induced, 260–265 J aggregates, 31 Keggin-type polyoxometalates (POMs), in TiO2 photocatalytic reactions, 96–98 Ketyl radicals, studied via two-color two-flash photolysis, 85–91 Kinetic analysis, of TP induced radical polymerization, 279 Lactone, rhodamine B, 296 Ladder-type polymers, TPA cross section for, 163 Lambert–Beer law, 117, 177 Laser calorimetry, 144, 148 Laser flash photolysis. See also Two-color two-laser flash photolysis combined with pulse radiolysis, 57, 58 cycloaddition and, 265, 266, 267 of stilbene, 58–61 Laser pulse, ultrashort, 298 Lasers, 54 in efficient TPE, 278 femtosecond, 54, 98, 113–114 in fluorescence imaging, 313–314 in radical polymerization, 280–281 limiting optical power of, 323–328 nanoclinics and, 317–318 nanosecond, 57, 85 picosecond, 81, 85 power dependence of, 165 pulsed, 65–66 studying Tn states via, 66 in three-dimensional microfabrication, 284, 285, 286, 287, 288, 291, 293 TPEF with, 150–154 upconverting, 321–323 Laser scanning confocal microscope (LSCM), 311–312 Lattice constant, in optical band-gap materials, 305

Lattice structures, in TPA microfabrication, 283 Layer-by-layer structure formation, fabricating optical band-gap materials via, 305–306, 307 Lewis acids, in two-photon initiated cationic polymerization, 282 Light. See also Near infrared (NIR) light; Ultraviolet entries in near-field fluorescence imaging, 312, 313 in nonlinear TPA measurements, 154–155 optical band-gap materials and, 305 optical power limiters and, 324 in radical polymerization, 280–281 in 3D data storage, 295 in 3D microfabrication, 284, 285, 286, 287, 288 two-photon absorption and, 118, 120–121 in waveguide materials, 308–309 Light intensity isophotes, laser beam, 297 Limiting anisotropy, 18, 19–20 Linear absorption, in symmetric donor-p-donor chromophores, 205 Linear absorption coefficient, 144 Linear accelerator, electron beam from, 54 Linear coupling model, vibrational contributions in, 140 Linker groups, 319 Lippert–Mataga equations, 133, 191, 240 Liquid crystals (LCs), in 3D optical data storage, 303–304 Lor, M., 1 Lorenz field correction, 122 Lowest unoccupied molecular orbital (LUMO), 182 Magic angle polarization, time-resolved fluorescence measurements under, 15–18 Magnesium ions, fluorescent chromophores and, 258, 259 Magnetic fields, nanoclinics and, 316 Magnetic iron oxide (Fe2O3), in nanoclinics, 315, 316 ‘‘Magnetic nanoclinic,’’ 114–115 Magnetocytolysis, 316, 318 Majima, Tetsuro, 53 Marcus ELT theory, 80–81 Materials, two-photon-absorbing, 144–155 m-C1P1, femtosecond transient absorption measurements of, 37–39, 42 m-C1P3, femtosecond transient absorption measurements of, 39–42

INDEX

m-C1Px. See also Meta-substituted first generation carbon core dendrimers femtosecond fluorescence upconversion measurements of, 27, 28, 33 fit parameters of, 19 fluorescence decay times of, 17 MCF-7 cells, fluorescence imaging of, 317–318 Mechanism of action, 255 Mechanistic TPA models, 132–143 Medical applications of fluorescent bioprobes, 259–260 of two-photon absorption, 315–321 Memory, optical, 293–294 3-Mercapto-4-methyl-4H-1,2,4-triazole, 278 Mesomers, of polydiacetylene, 168–169 Metal-containing dendrimers, 3–4 Metal ions, fluorescent chromophores and, 257–258 Metal particles, surface plasmon effects in, 141–142 Meta-substituted compounds, spectra of, 14–15 Meta-substituted first generation carbon core dendrimers. See also m-C1Px para-substituted carbon core dendrimers versus, 18 single-photon timing measurements of, 17–18 time-resolved fluorescence polarization measurements of, 18–19 Methacrylates, TP initiated polymerization of, 274, 275, 286 p-Methoxymethylbenzophenone (BPCh2OCH3), bond dissociation from Tn states in, 74 1-Methoxynaphthalene, electron transfer from Tn states in, 76–77 Methyl groups, in nonlinear absorbing chromophores, 252, 253 Methylmethacrylate, in 3D data storage, 294, 302 1-Methylnaphthalene, electron transfer from Tn states in, 77 Methyl-substituted ladder-type poly( p-phenylene), 163–165 Microbull, 292, 293 Microcapsules, 292, 293 Microchain, 292, 293 Microchannel structures, 290, 291 Microfabrication three-dimensional, 284–293 three-dimensional lithographic, 285–293

365

two-photon initiated crosslinking polymerization in, 272, 283 Microgearwheel, 292, 293 Microscopes, in three-dimensional microfabrication, 284, 285 Microscopy, fluorescence imaging and, 311 Microtweezers, 289 Mirror–scanner system, in three-dimensional microfabrication, 286–287 Mode-locked Ti:sapphire laser, 150, 151, 152 in fluorescence imaging, 313–314 in nonlinear TPA measurements, 154–155 in three-dimensional microfabrication, 286–288 Mode-locked Nd:YAG laser, 304 Molar extinction coefficient, 134 Molecular engineering, 128 of TP fluorophores, 204–205 Molecular memory, 56 Molecular orbital (MO) methods, 125 Molecular orbitals, symmetry and, 129 Molecular parameters, with two-photon absorption, 119–120, 123, 125 Molecular switching, 57 Molecular weight average, 178 effective, 177–178 Monomer units, angle dependence between, 180 Monomode waveguides, 309 Morse potential curve, 78 Multibeam irradiation applications of, 55–57 photochemistry of short-lived species using, 53–109 Multichromophoric systems. See also Chromophores excited state processes in, 2–10 femtosecond fluorescence upconversion measurements of, 25–37, 46 femtosecond transient absorption measurements of, 37–45 time-resolved fluorescence polarization measurements of, 20–25 Multiphoton absorption by aromatic hydrocarbons, 157 surface plasmon effects and, 142 Multiphoton excitation isomerization via, 260 in photochemistry, 113 Multiple phenyl rings, TPA data for, 158

366

INDEX

Multireference double excitation configuration interaction (MRDCI) scheme, 125, 128 with three-level TPA model, 136, 138 Multistep excitation, 54 Multivalence-bond state approach, for two-photon absorption, 128 Nanoclinics, 315–318 Nanoengineered particles, surface plasmon effects in, 141–142 Nanofabrication three-dimensional, 284–293 two-photon initiated crosslinking polymerization in, 272, 283 Nanometer-sized metal particles, surface plasmon effects in, 141–142 Nanoparticles, in fabricating optical band-gap materials, 305. See also Nanoclinics Nanoscope IIIA controller, 314 Nanosecond (ns) lasers, 57 direct measurement of Tn state lifetimes via, 85 ketyl radicals in the excited state studied via, 85 singlet oxygen production and, 268 studying Tn states via, 66 TPA data taken with, 156–157 Nanosecond optical parametric oscillator, 174 Naphthacenepyridone, in 3D data storage, 302 Naphthaldiimide (NDI), in two-color two-laser DNA damaging studies, 92–94 Naphthalene (Np) energy gap law and, 70–71 Tn states of, 66–70 Naphthalene dendrimers, 4–5 Naphthalene derivatives (NpD) electron transfer from Tn states in, 75–81, 82 substituent effects on Tn state lifetimes of, 71–72 Naphthylmethylhalides, 72 Naphthylmethyl radical, bond dissociation from Tn states in, 72–74 NDI-conjugated oligodeoxynucleotide (NDI-ODN), in two-color two-laser DNA damaging studies, 92–94 Nd:YAG laser, 304 in three-dimensional microfabrication, 293 Nd:YAG pumped optical parametric oscillator, singlet oxygen production and, 268 Near-field fluorescence imaging, 312–313

Near-field optical microscopy (NFOM), 283 Near-field scanning optical microscopy (NSOM), in three-dimensional microfabrication, 285, 289 Near infrared (NIR) light, in photodynamic therapy, 318–319. See also Infrared multiphoton excitation; NIR pulsed lasers Neutral donor-p-acceptor (D-p-A) compounds, 182–193 Neutral resonance structure, bond length alternation and, 132 NIR pulsed lasers, nanoclinics and, 317. See also Near infrared (NIR) light o-Nitrobenzaldehyde, TP photochemical rearrangement of, 297 Nitrobenzene, in neutral donor-p-acceptor chromophores, 190 Nitrogen in octupolar chromophores, 243–244, 245 in 3D data storage materials, 295 Nonlinear absorbing chromophores, polymers with, 250–255 Nonlinear absorption, 190–191 Nonlinear absorption coefficient (b), 144, 146, 147, 181 limiting laser optical power and, 326 Nonlinear absorption data, of conjugated polymers and oligomers, 160–163, 181–182 Nonlinear hyperpolarizability, 190–191 in nonlinear absorbing chromophores, 250, 251 Nonlinear optical parameters, 182 two-photon absorption and, 116–122 Nonlinear optical responses, surface plasmon effects and, 142 Nonlinear refraction, in limiting laser optical power, 324 Nonlinear refractive index, 181 Nonlinear scattering, in limiting laser optical power, 324 Nonlinear TPA in absolute evaluation of TPA materials, 154–155 experimental techniques in, 144–148 Nonlinear transmission methods, 188 for quantifying TPA cross section, 144, 145–147 Nonprotonated species, in 3D data storage, 295–296 Normalized TP excitation spectra, 170–171

INDEX

Normalized transmittance (TN), 146–147 Numerical TPA simulations, 156 Octupolar (acceptor-p)3-donor chromophores, 246–247, 248 Octupolar chromophores, with large two-photon absorbtivity, 233–250 Octupolar (donor-p-acceptor)3-donor chromophores, 247, 249 Oligomer length (OL) TPA cross section and, 173 Oligomers, 177 in TP fluorophore moelcular engineering, 204–205 two-photon absorption by conjugated, 160–182 urethane acrylate, 309 Oligo( p-phenylenevinylene)s (OPVn), direct observation of Tn states in, 84 Oligothiophenes, 173 direct observation of Tn states in, 81–84 One-photon absorption (OPA), 134. See also OPA entries fluorescence imaging and, 311 with ionic donor-p-acceptor compounds, 193 in nonlinear absorbing chromophores, 252, 253 by phenyl-substituted ladder-type poly( p-phenylene), 165–167 Strickler–Berg relationship and, 177 in three-dimensional microfabrication, 286 two-photon absorption and, 117–118 in two-photon initiated cationic polymerization, 282 One-photon (OP) excitation, 113, 115, 134–135. See also OP excitation entries of conjugated polymers, 163 of rhodamine B, 256–257 symmetry and, 128–129 in three-level TPA model, 138–139 One-photon resonance, in symmetric donor-pdonor chromophores, 211 Onium compounds, as coinitiators, 272 Onsager TPA model, 133, 188–189 OPA cross section, in octupolar chromophores, 246 OPA maxima in neutral donor-p-acceptor chromophores, 183–185 in symmetric acceptor-p-acceptor chromophores, 217–218, 223

367

in symmetric donor-acceptor-donor chromophores, 227–228, 232 in symmetric donor-p-donor chromophores, 198–201, 202, 212 in TP initiators, 275 OPA spectra, 181 of conjugated polymers, 170 of nonlinear absorbing chromophores, 254 of octupolar chromophores, 239, 241 of symmetric donor-acceptor-donor chromophores, 232 of symmetric donor-p-donor chromophores, 210 OP broadband UV excitation, cycloaddition via, 265. See also One-photon entries OP excitation maxima in ionic donor-p-acceptor chromophores, 194–195 of nonlinear absorbing chromophores, 253 in octupolar chromophores, 234–235, 236 OP excitation spectra, 181 OP excited fluorescence (OPEF), 151, 152. See also Single-photon excited fluorescence (SPEF) spectra OP excited states energetic ordering of, 157 of symmetric donor-p-donor chromophores, 212 OP initiated polymerization, 279 in three-dimensional microfabrication, 284–285 Optical band-gap materials, 305–308. See also Photonic band-gap materials Optical circuits, 309 Optical elements, in absolute evaluation of TPA materials, 149 Optical feedback, in upconverted lasing, 322 Optical memory, three-dimensional, 293–294, 304 Optical parameters, nonlinear, 116–122, 182 Optical parametric oscillator (OPO), singlet oxygen production and, 268 Optical power limiters, 114, 323–328 Optical probes, in nanoclinics, 315 Optical susceptibility, 181–182 Organic boron compounds, 191 Organic chromophores, surface plasmon effects and, 142–143 Organic compounds, UV absorption spectra of, 325

368

INDEX

Organic molecular crystals, limiting laser optical power and, 328 Orthogonal addressing, in 3D data storage, 300–301 Oxidation potential, of TP initiators, 275 Oxygen. See Singlet oxygen ‘‘Page-by-page’’ writing, in 3D data storage, 300–301 Paracyclophane, in symmetric donor-p-donor chromophores, 214–215 Para-substituted carbon core dendrimers. See also p-C entries meta-substituted carbon core dendrimers versus, 18 single-photon timing measurements of, 15–17 Para-substituted compounds, spectra of, 14–15 Para-substituted first generation carbon core dendrimers, time-resolved fluorescence polarization measurements of, 19–25 Parity, in two-photon absorption, 128–129 Parity forbidden transitions, 113 p-C1P1 femtosecond fluorescence upconversion measurements of, 25–30, 31–33 femtosecond transient absorption measurements of, 37–39, 40 molecular structure of, 12–13 p-C1P2, molecular structure of, 12–13 p-C1P3, femtosecond transient absorption measurements of, 39–42 p-C1P4 femtosecond fluorescence upconversion measurements of, 26–37 molecular structure of, 12–13 time-resolved fluorescence decays of, 16 p-C1P4A, molecular structure of, 13–14 p-C1P4B, molecular structure of, 13–14 p-C1Px, femtosecond fluorescence upconversion measurements of, 27, 28, 29, 30 p-C2P1, femtosecond transient absorption measurements of, 42–45 p-C2P4 femtosecond transient absorption measurements of, 42–45 isomeric structures of, 25 molecular structure of, 10, 11 p-C2Px energy hopping in, 23–24

femtosecond fluorescence upconversion measurements of, 26, 27, 34–37 fit parameters of, 17 Pentadienes, infrared multiphoton excitation isomerization of, 260 Pentafluorobenzene moiety, 186 in ionic donor-p-acceptor chromophores, 196 Perturbation theory, two-photon absorption and, 118–119 Peryleneimide (PI), 29, 36–37, 42, 43 Peryleneimide chromophores, 8–9, 10, 14, 45–47 Perylene units, 7–8 Phenazine, 64 2-Phenoxyethyl acrylate, polymerization of, 278 p-Phenoxymethylbenzophenone (BPCh2OPh), bond dissociation from Tn states in, 74 Phenylacetylene-based dendrimers, 9–10 Phenylacetylene dendrimers, energy transfer in, 2–3 Phenylene–ethynylene oligomers, 204–205 Phenylene-vinylene hinge, in symmetric donorp-donor chromophores, 208–209 Phenylene–vinylene oligomers, 204–205 Phenyl moiety in neutral donor-p-acceptor chromophores, 192 in octupolar chromophores, 237 Phenyl rings fluorinated, 186–187 in symmetric acceptor-p-acceptor chromophores, 220 in symmetric donor-p-donor chromophores, 213 TPA data for, 158, 159 Phenyl-substituted ladder-type poly( p-phenylene), 165–167 Phosphorescence, of singlet oxygen, 268 Photobiology, singlet oxygen studies in, 266 Photobleaching, 142, 148 Photocatalytic reactions, two-color laser control of, 96–98 Photochemical reactions, TP-excitation initiated, 115 Photochemistry of di( p-methoxyphenyl)methyl chloride, 99–100 of short-lived species, 53–109 two-photon absorption in, 113–115 Photochromic matrices, in 3D data storage, 301–302 Photocrosslinked polymers, in 3D data storage, 301

INDEX

Photocycloaddition, TP-excitation initiated, 115 Photodynamic therapy (PDT), 56, 91–94, 315, 318–321 multiphoton excitation in, 114, 115 singlet oxygen in, 268 Photoinduced acid generator, 295 Photoinduced electron transfer (ELT), 75–81, 82, 272–274 DNA damage and, 92 Photoinduced processes, 55–56 Photoinitiators, 272, 273. See also TP initiators Photoisomerization, of ionic donor-p-acceptor chromophores, 196. See also Isomerization; trans–cis photoisomerization Photoluminescence intensity, 166 Photoluminescence spectra, by methylsubstituted ladder-type poly( p-phenylene), 165 Photolysis, cycloaddition and, 265, 266, 267. See also Pulse radiolysis–laser flash photolysis; Three-color three-laser flash photolysis; Two-color two-laser flash photolysis Photon flux (F ) in radical polymerization, 281 temporal fluctuations in, 149–150 ‘‘Photonic atoms,’’ 305 Photonic band-gap materials, 283. See also Optical band-gap materials three-dimensional microfabrication of, 288–289, 290, 293 Photonic crystals, 283 as optical band-gap materials, 305, 306, 308 Photonic properties, of symmetric acceptor-pacceptor chromophores, 219 Photon number noise, in nonlinear absorbing chromophores, 250 Photo-orientation, of chromophores, 262 Photophysics, of rigid polyphenylene-based dendritic structures, 1–51 Photopolymerization in 3D data storage, 294 TPA processes in, 283 two-photon initiated cationic, 281–283 Photorefractive effect, in 3D data storage, 299–300 Photosynthesis, 97 Picosecond lasers, 81 direct measurement of Tn state lifetimes via, 85

369

ketyl radicals in the excited state studied via, 85 Picosecond relaxation process, 39 p-electron density in polyenes, 130–132 surface plasmon effects and, 141 in two-level TPA model, 133 Pigment–protein complexes, 31 Pinhole aperture, 312 p-structures, 157 p-systems (-systems) chromophores with large, 157–182 polarizable, 215, 216, 219 Planarity, 186 Point dipole–point dipole approximation, for symmetric donor-p-donor chromophores, 213 Polarity, solvent, 206 Polarizability, second-order nonlinear, 120 Polarizable -systems, 215, 216, 219 Polarization, magic angle, 15–18 Polarization ratio, two-photon excited fluorescence and, 148 Poly[1,4-phenylene-1,2di(phenoxyphenyl)vinylene], in waveguide materials, 309–310 Poly[1,4-phenylene-1,2-diphenylvinylene-co2,7-fluorenylene-1,2-di-phenylvinylene], in waveguide materials, 309–310 Poly[1,6-bis(3,6-dihexadecyl-Ncarbazolyl)2,4-hexadiyne], TPA cross section of, 167–168 Poly(2,20 -bipyridine-5,50 -diylethynylene[2,5di(2-ethylhexyl)oxy-1,4phenylene]ethynylene), 178 Polyacetylene dendrimers, electronic coupling in, 3 Polyaddition, of nonlinear absorbing chromophores, 252 Poly(arylenevinylene)s, 177 Polychlorobenzenes, electron transfer to, 80–81, 82 Poly(diacetylenes), 171 mesomers of, 168–169 Polyene chain, 203 Polyenes, 197 bond length alternation in, 130 donor–acceptor substituted, 186–187 fluorinated, 134 TPA data for, 159

370

INDEX

Poly(ethylene terephthalate), in 3D data storage, 302 Polyfluorene(s), 175 TP excitation of, 167 Poly-HEMA (polyhydroxyethylmethacrylate) rod, as lasing material, 322 Polymer dispersed liquid crystals, in 3D optical data storage, 303–304 Polymerization simultaneous two-photon initiated, 272–283 TPA processes in, 283 two-photon initiated cationic, 281–283 Polymer photochromic matrices, in 3D data storage, 301–302 Polymers as lasing materials, 321–322 with nonlinear absorbing chromophores, 250–255 in 3D data storage, 301–304 two-photon absorption by conjugated, 160–182 Polymer science, singlet oxygen studies in, 266 Poly(methylmethacrylate) in 3D data storage, 294, 302 in waveguide materials, 309 Polyoxometalates (POMs), in TiO2 photocatalytic reactions, 96–98 Polyphenylene-based dendritic structures, ensemble photophysics of, 1–51 Polyphenylene dendrimers, 8–9 as chromophoric backbones, 2–10 Poly(phenylene vinylene) (PPV), 163, 169–170 Polypiridine ligands, 3–4 Poly( p-phenylene) methyl-substituted ladder-type, 163–165 phenyl-substituted ladder-type, 165–167 Poly( p-phenylene acetylene), 163 Poly(propylene-imine) dendrimers, 7 Poly(styrene-co-acrylonitrile), 289, 290 in fabricating optical band-gap materials, 306, 308 Polystyrene nanoparticles, in fabricating optical band-gap materials, 305 Poly(thiophene), 163 Poly(vinyl alcohol) (PVA) film, 89 Porphyrin arrays, 7 Porphyrins, in photodynamic therapy, 319 Porphyrin unit, singlet oxygen generation and, 271 Positive-resist structures, 291

Positive tone resins, in three-dimensional microfabrication, 290 Positive TP microlithography, 290 Primary initiator particle density (r0), in radical polymerization, 281 Prismatic cavities, 291–293 Prismatic trenches, 291 Product analysis, rate constant determination from, 94–96 Propagation, of radical polymerization, 276 ‘‘Propeller shaped’’ chromophores, with large two-photon absorbtivity, 233–250 Proteins, TPEF and, 255 Protonation, in 3D data storage, 295–296 Proton diffusion, in three-dimensional microfabrication, 291 Pulse collision addressing, in 3D data storage, 300–301 Pulsed femtosecond lasers, for 3D optical memory systems, 294 Pulsed infrared multiphoton excitation, isomerization via, 260 Pulsed lasers, 65–66 Pulsed laser systems, fluorescence imaging and, 311 Pulse propagation, in TPA equipment design, 156 Pulse radiolysis, 54 Pulse radiolysis–laser flash photolysis, 57–65 of excited radical anions, 64–65 of excited radical cations, 58–61 of NDI-ODN, 92–94 Pulse shapes, 145–146, 147, 149–150, 151, 152 in three-dimensional microfabrication, 284 Pulse width, 152 Pump and probe method, 54, 81, 98 Pumped optical parametric oscillator, singlet oxygen production and, 268 Pump intensity, absorbance change versus, 262 Pump power, 166 Pyrazoline moiety, in neutral donor-p-acceptor chromophores, 190 Pyrene–perylene bisimide system, 7–8 Pyrene units, 7–8 Pyridinium moiety, in symmetric acceptor-pacceptor chromophores, 224 Pyridinium nitrogen, in symmetric acceptor-pacceptor chromophores, 221, 222 Pyridinium ring, in ionic donor-p-acceptor chromophores, 196

INDEX

Pyrrole group, in symmetric acceptor-p-acceptor chromophores, 224 Pyryllium chromophores, 247, 249 Quadrupolar chromophores, 136 octupolar chromophores versus, 237 Quadrupolar pattern in fluorescent chromophore/metal ion complexes, 259 in symmetric acceptor-p-acceptor chromophores, 216 in symmetric donor-acceptor-donor chromophores, 226, 231, 232 Quantum yield, 262 fluorescence, 177, 183–185, 186, 194–195, 196–197, 198–201, 217–218, 227–228, 234–235, 253 for singlet oxygen generation, 269, 270 in 3D data storage, 298 two-photon, 178 Quantum yield for intersystem crossing (isc), 241 Quenching, 63, 64, 65, 68–70 intermolecular ELT, 75 by singlet oxygen, 266 o-Quinodimethane, cycloaddition of, 94–96 Quinolinium group, in ionic donor-p-acceptor chromophores, 196 Radical cations fluorescence from excited, 61–64 pulse radiolysis of excited, 58–61 studied via two-color two-flash photolysis, 91 Radical concentration, in radical polymerization, 279–280 Radical formation, in radical polymerization, 276 Radical ions, 58. See also Excited radical entries Radical polymerization/copolymerization of nonlinear absorbing chromophores, 252 two-photon initiated, 274–281 Raman spectroscopy, of phenyl-substituted ladder-type poly( p-phenylene), 166 Random hopping, 24 Rate constants laser photolysis in determination of, 94–96 two-photon absorption and, 119 Reactive intermediates, studied via two-color two-flash photolysis, 91–98 Read only memory (ROM) storage system, in 3D data storage, 296

371

Refractive index complex, 120 nonlinear, 181 in 3D data storage, 294, 298 in waveguide materials, 310 Refractive-index changes, in limiting laser optical power, 324 Relaxation process, 39 Research, in TP applications, 328–330 Resolution improving, 328 in near-field fluorescence imaging, 312 Resonance(s). See also Fluorescence resonance energy transfer (FRET) aromatic, 182 bond length alternation and, 130–132 one-photon, 211 two-photon absorption and, 119, 123 Resonant two-photon ionization (RTPI), 99–100 Response theory, for two-photon absorption, 125–126, 139 Retinal, direct excitation of, 265 Reverse molecular motifs, 215 Reverse saturable absorption, in limiting laser optical power, 324 Rewritable 3D optical data storage materials for, 299 photorefractive effect in, 299–300 Rewritable technique, for 3D optical memory systems, 294 Rhodamine B, 256–257 Rhodamine B lactone, in 3D data storage, 296 Rhodamine B molecules, fluorescence imaging of, 314–315 Rigid polyphenylene-based dendritic structures, ensemble photophysics of, 1–51 Rod shapes, in waveguide materials, 309. See also Conjugating rods; Connecting rods Ruthenium complexes, 3–4 S0 state in symmetric donor-p-donor chromophores, 211, 212 in three-level TPA model, 137–138 S1 state, 116, 124 in symmetric donor-p-donor chromophores, 211 in three-level TPA model, 137–139

372

INDEX

S2 state, 124 in symmetric donor-p-donor chromophores, 211 in three-level TPA model, 137–139 Scanning electron microscopy (SEM), 278–279 of photonic band-gap crystal, 308 in three-dimensional microfabrication, 293 Schweitzer, G., 1 Screening metal ions, fluorescent chromophores and, 257–258 sech2-shaped pulse, 145–146, 147, 150 Secondary electron transfer (ELT), 58 Second generation dendrimers, 10, 11 Second harmonic generation (SHG), 294 Second hyperpolarizability, 182 Selective ELT quenching, 64 Selective protonation, in 3D data storage, 295–296 Semiclassical perturbation theory, two-photon absorption and, 118–119 Sensitizers, coinitiators and, 272–274, 278 Short-lived species, multibeam irradiation photochemistry of, 53–109 s-bonds, in symmetric donor-p-donor chromophores, 213 Silaffin-1 protein, in holographic recording, 307 Silica nanospheres, in holographic recording, 307 Silica shell, in nanoclinics, 315, 316 Silver nanoparticles, surface plasmon effects and, 142, 143 Simultaneous TPA. See Two-photon absorption (TPA) Single-photon excited fluorescence (SPEF) spectra, of symmetric acceptor-p-acceptor chromophores, 221, 222. See also OP excited fluorescence (OPEF) Single-photon timing (SPT) measurements, 15–25, 37 Singlet oxygen, 114 applications of, 267–268 in photodynamic therapy, 319 photogeneration of, 115, 266–271 Singlet–singlet annihilation, 31, 32–34, 40–42, 43 energy hopping versus, 44–45, 46 Singlet–singlet excitation hopping, 23 ‘‘Site-selective’’ processes, 56 Solvation shell, 29 Solvatochromism, 123 with ionic donor-p-acceptor compounds, 193

in neutral donor-p-acceptor chromophores, 182, 186–187 in octupolar chromophores, 244 in symmetric donor-acceptor-donor chromophores, 226, 229 in symmetric donor-p-donor chromophores, 206 Solvent-dependent fluorescence, 188–189 Solvent polarity in oligothiophenes, 84 symmetric donor-p-donor chromophores and, 206 Spherical porphyrin arrays, 7 ‘‘Stack-of-logs’’ structure, in fabricating optical band-gap materials, 306 Steady-state absorbance, 262 Steady-state absorption spectra, 38 Steady-state radical concentration, in radical polymerization, 280 Steady-state spectra, 14–15 Stepwise bond cleavage, 100–103 Stepwise mechanism, of dissociative ELT, 80–81 Stern–Volmer plots, 69 ‘‘Sticky’’ dissociative ELT model, 79 Stilbene (St), 197, 202 cis–trans isomerization of, 58–61, 64 cycloaddition of, 265 medical applications of chromophores with, 259–260 as TPA measurement standard, 159 trans–cis photoisomerization of, 263–264 Stilbene derivatives in three-level TPA model, 136–139 TPA cross sections of, 174–175 Stilbenoid pattern, 196 Stokes–Raman scattering, 121 Stokes shifts, 86, 87, 88, 219 Storage density, of 3D optical memory, 294 Streak camera detection, 98 Strehmel, Bernd, 111 Strehmel, Veronika, 111 Strickler–Berg relationship, 177 Structural isomers, chromophores in, 12–13 Styrene, in 3D data storage, 302 Substituent effects, on Tn state lifetimes, 71–72 Substituted benzenes, TPA data for, 158 Substituted benzophenone (BPD), substituent effects on Tn state lifetimes of, 71–72 Substituted butadienes, infrared multiphoton excitation isomerization of, 260

INDEX

Substituted cyclobutenes, infrared multiphoton excitation isomerization of, 260 Sulfonyl substituents, in octupolar chromophores, 246–247, 248 Sum over states (SOS) method, 122–123, 125–126 Surface plasmons, in two-photon absorption, 140–143 Susceptibility, third-order nonlinear, 122 Switching, in waveguide materials, 310 Symmetric acceptor-p-acceptor (A-p-A) compounds, 215–225 Symmetric chromophores, 197–233 singlet oxygen and, 266–267 Symmetric donor-acceptor-donor (D-A-D) compounds, 225–233 Symmetric donor-p-donor (D-p-D) compounds, 197–215 Symmetry. See also Centrosymmetric chromophores of octupolar chromophores, 233, 236 two-photon absorption and, 128–130 T1 state, 116. See also Tn states singlet oxygen and, 267 TDDFT calculations, 203. See also Timedependent density functional theory (TDDFT) Temperature, in quantifying TPA cross section, 148. See also Thermal entries Temporal coherence (gc, gp), 149–150, 151 Termination, of radical polymerization, 276, 279 Terrylenediimide core, 8–9 Tetrabutylammonium chloride, 63 1,2,4,5-Tetrafluorobenzene, 134 Tetrafluorobenzene moiety, in symmetric donoracceptor-donor chromophores, 226 cis-1,2,3,4-Tetrahydro-2,3naphthalenedicarboxylic anhydride, 94–95 Tetrahydrofurane solution, transmissivity of, 327 Thermal blooming, 148 Thermal lensing, 144 Thermally crosslinked polymers, in 3D data storage, 301 Thick samples, nonlinear absorption measurements of, 145–146, 149, 152 Thienyl rings, in symmetric acceptor-p-acceptor chromophores, 220 Thin samples, nonlinear absorption measurements of, 146, 182

373

Thiophene moieties in neutral donor-p-acceptor chromophores, 189, 192–193 in symmetric acceptor-p-acceptor chromophores, 216, 224 Thiophene-vinylene-thiophene core, in symmetric donor-p-donor chromophores, 208–209 Third-order nonlinear susceptibility, 122 Three-color three-laser flash photolysis, 99–103 stepwise bond cleavage of C–O bonds studied via, 100–103 three-laser control of intermediate populations studied via, 99–100 Three-dimensional data storage, 115, 293–304 Three-dimensional data storage systems, two-photon initiated crosslinking polymerization in, 272 Three-dimensional lithographic microfabrication (3DLM), 285–293 Three-dimensional micro-/nanofabrication, 284–293 two-photon initiated crosslinking polymerization in, 272, 283 Three-dimensional object manufacture, 115, 284–293 Three-dimensional periodic structures, fabrication of, 288–289 Three-level TPA model, 136–139 Three-photon absorption, 165 Three-photon excitation, simultaneous, 179 Time constants, 29, 36 Time-dependent density functional theory (TDDFT), for two-photon absorption, 126, 127. See also TDDFT calculations Time-dependent DFT/B3LYP method, 190 Time-resolved fluorescence measurements, under magic angle polarization, 15–18 Time-resolved fluorescence polarization measurements, 18–25 ‘‘Time-selective’’ processes, 55–56 Ti:sapphire laser, 150, 151, 152, 255 in fluorescence imaging, 313–314 in nonlinear TPA measurements, 154–155 in optical circuits, 309 singlet oxygen production and, 268 in 3D data storage, 294 in 3D microfabrication, 286–288 Tissue autofluorescence, 317

374

INDEX

Titanium dioxide (TiO2), photocatalytic reactions on surface of, 96–98 Tn states. See also T1 state bond dissociation from, 72–75 direct observation of, 81–85 electron transfer from, 75–81, 82 energy gap law and, 70–71 energy transfer from, 66–70 stepwise bond cleavage via, 100–103 substituent effects on lifetimes of, 71–72 TPA coefficient, in waveguide materials, 310. See also Two-photon absorption (TPA) TPA cross section (d), 120, 121–122, 123, 125, 179–180, 180–181, 262 of bioprobe chromophores, 256–257, 258–259 decreasing, 174–175 effective conjugation length and, 176–177 of fluorescent chromophore/metal ion complexes, 258–259 in ionic donor-p-acceptor chromophores, 194–195, 196–197 limiting laser optical power and, 326 in neutral donor-p-acceptor chromophores, 183–185, 186 in nonlinear absorbing chromophores, 250, 251–252, 253, 254 in numerical TPA simulations, 156 in octupolar chromophores, 233–247, 250 oligomer length and, 173 of phenyl-substituted ladder-type poly( p-phenylene), 166 of poly [1,6-bis(3,6-dihexadecyl-Ncarbazolyl)2,4-hexadiyne], 167–168 of polyfluorene, 175 for polymers, 163 quantifying, 144–145, 148, 149, 150, 151, 152 in radical polymerization, 281 in relative measurement of, 155 surface plasmon effects and, 143 in symmetric acceptor-p-acceptor chromophores, 216, 217–218, 219, 223 in symmetric donor-acceptor-donor chromophores, 226, 227–228, 229, 230, 231 in symmetric donor-p-donor chromophores, 197, 198–201, 202, 203, 210 3D optical memory and, 304 in three-level TPA model, 136–137 in TP initiators, 272, 275

in two-level TPA model, 133, 134 in unsaturated conjugated systems, 157–160 vibronic coupling and, 168 TPA cross section action for fluorescence, 186 TPA data, 156–157 on bioprobe chromophores, 256, 257 from conjugated polymers/oligomers, 181–182 for ionic donor-p-acceptor chromophores, 194–195 for neutral donor-p-acceptor chromophores, 182, 183–185 for nonlinear absorbing chromophores, 253 for octupolar chromophores, 233, 234–235 on singlet oxygen chromophores, 269 for symmetric acceptor-p-acceptor chromophores, 216–225 for symmetric donor-acceptor-donor chromophores, 227–228 for symmetric donor-p-donor chromophores, 197–202 for TP initiators, 275 for triphenylmethane dyes, 237–238 TPA equipment design, 156 TPA microfabrication, 283 TPA spectra, 181 of conjugated polymers, 170 of nonlinear absorbing chromophores, 254 of octupolar chromophores, 240, 241 of radical polymer, 276 of symmetric donor-acceptor-donor chromophores, 232 of symmetric donor-p-donor chromophores, 210–211 TP chromophores, applications of, 114–115 TPE action cross sections, 256. See also Two-photon excitation (TPE) TPE fluorescence spectra, 206 TPEF materials, 255. See also Two-photon excited fluorescence (TPEF) TPE spectra, 181. See also Two-photon excitation spectra of fluorescent chromophores, 260 normalized, 170–171 of octupolar chromophores, 238 TP excitation maxima. See also TPE entries in ionic donor-p-acceptor chromophores, 194–195 in neutral donor-p-acceptor chromophores, 183–185

INDEX

in nonlinear absorbing chromophores, 253, 254 in octupolar chromophores, 234–235, 236–237, 239 in symmetric acceptor-p-acceptor chromophores, 216, 217–218 in symmetric donor-acceptor-donor chromophores, 227–228, 232 in symmetric donor-p-donor chromophores, 197, 198–201, 202, 210, 212 in TP initiators, 275 TP excited states, 123–124 energetic ordering of, 157–159 of methyl-substituted ladder-type poly( p-phenylene), 163–165 of symmetric donor-p-donor chromophores, 212 in three-level TPA model, 136–139 TP fluorescence excitation spectra, 172 TP fluorophores, 255 molecular engineering of, 204 TP induced dual-channel fluorescence image formation, in 3D data storage, 295–296 TP induced isomerization, in 3D data storage, 301 TP initiated polymerization in holographic recording, 306–307 optical band-gap materials and, 305 in 3D microfabrication, 284–285 in 3D optical data storage, 303–304 waveguide materials and, 308–310 TP initiators, 274, 275, 276, 277, 278. See also Photoinitiators TP laser scanning microscopy (TPLSM), nanoclinics and, 317–318 TP matrix elements, 125–126 TP microlithography, positive, 290 TP microscopy, 255, 259 TP microstereolithography, 289 TP PDT triade, 319–321 TP photoinitiator, 190 TP quantum yield, 178 TP sensitizers, detecting singlet oxygen via, 268 TP transitions, two-photon absorption and, 139–140 TP tunneling microscope, 143 TP two-beam picosecond ‘‘page-by-page’’ writing, in 3D data storage, 300–301 TP writing and OP readout principle, in 3D data storage, 294

375

trans–cis photoisomerization. See also Photoisomerization of diphenylbutadiene, 264–265 in holographic data storage, 302–303 of ionic donor-p-acceptor chromophores, 196 of retinal, 265 of stilbene, 263–264 Transient absorption, 82, 83, 86, 88–89 femtosecond measurements of, 37–45 in laser flash photolysis, 101, 102, 103 in 1-methoxynaphthalene, 76–77 in 1-NpCH2–OBP, 73 in oligothiophenes, 84 in stilbene, 58–61 in TiO2 photocatalytic reactions, 97 Transition densities, in three-level TPA model, 137–138 Transition dipole, 178, 180, 186. See also Dipole moment (M) Transition dipole moment (Mnm), 128–129, 140, 175–177, 180–181, 190, 216, 217–218 in octupolar chromophores, 233, 234–235, 237 in symmetric donor-acceptor-donor chromophores, 227–228 in symmetric donor-p-donor chromophores, 197, 198–201, 202–203, 210, 211 in three-level TPA model, 136–137 in two-level TPA model, 133–134 Transitions, forbidden, 168 Transmission contrast ratio (TCR), limiting laser optical power and, 326 Transmission dynamic range (TDR), limiting laser optical power and, 326 Transmissivity, limiting laser optical power and, 326, 327 Transparent multilayer polymer photochromic matrices, in 3D data storage, 301–302 Trenches, 291 Triarylamine dendrimer systems, 7 Triarylamino group, 295 Triarylsulfonium hexafluoroantimonate, in 3D data storage, 295 Triazine(s) as coinitiators, 272 in neutral donor-p-acceptor chromophores, 193 1,3,5-Triazine, in octupolar chromophores, 239–240, 241–242, 243 Triethanolamine coinitiator, 278

376

INDEX

Trifunctional acrylate monomer, in threedimensional microfabrication, 289–290 1,3,5-Trimethoxybenzyl (TMB) radical cation, transient fluorescence spectrum of, 62–64, 91 Trimethylolpropane triacrylate, in holographic recording, 307 p-Trimethylsilylmethylacetophenone, bond dissociation from Tn states in, 75 p-Trimethylsilylmethylbenzophenone, bond dissociation from Tn states in, 75 Triphenylamine units, in octupolar chromophores, 244–246 Triphenylmethane dyes, TPA data for, 237–238 Triple bonds in neutral donor-p-acceptor chromophores, 190–191 singlet oxygen generation and, 270–271 in symmetric acceptor-p-acceptor chromophores, 221, 222 in symmetric donor-p-donor chromophores, 205–206, 209–210, 213 Triplet excited (Tn) states, 55. See also Tn states energy transfer from, 66–70 Triplets, in singlet oxygen generation, 269–270, 271 Tris(2-hydroxyethyl)isocyanurate triacrylate, 293 1,3,5-Triscyanobenzene core, in octupolar chromophores, 238 Trivalent boron, in neutral donor-p-acceptor chromophores, 191 Truncation, 291 Tumors. See Cancer Tunneling microscope, 143 ‘‘Twin chromophores,’’ 213–214 Two-color experiment, 116 Two-color two-laser DNA damaging, 91–94 Two-color two-laser flash photolysis, 65–98 control of photocatalytic reactions via, 96–98 excited radical cations studied via, 91 higher triplet excited states studied via, 66–85 ketyl radicals in the excited state studied via, 85–91 in rate constant determination, 94–96 of reactive intermediates, 91–98 Two-level TPA model, 132–135 Two-photon absorbing chromophores, 6 Two-photon absorption (TPA), 116–156. See also TPA entries

absolute evaluation of materials for, 148–155 applications of, 283–328 bond length alternation and, 130–132 chromophore design and optimization of, 156–255 covalent bonding and, 173–174 evaluating materials for, 144–155 experimental techniques in nonlinear, 144–148 fluorescence imaging and, 311–315 in ionic donor-p-acceptor chromophores, 197 in limiting laser optical power, 323–328 mechanistic models of, 132–143 medical applications of, 315–321 by methyl-substituted ladder-type poly( p-phenylene), 163–165 in nonlinear absorbing chromophores, 250–255 nonlinear optical parameters and, 116–122 optical band-gap materials and, 305–308 outlook for, 328–330 by phenyl-substituted ladder-type poly( p-phenylene), 165–167 in photochemistry, 113–115 in photodynamic therapy, 318–321 by polyfluorenes, 167 prediction and discovery of, 113 pulse propagation in, 156 relative evaluation of materials for, 155 singlet oxygen via, 266–271 surface plasmons and, 140–143 symmetric chromophores with large, 197–233, 233–250 symmetry and, 128–130 theoretical background for, 116–132 theoretical methods for describing, 122–128 in 3D data storage, 293–304 three-level model of, 136–139 two-level model of, 132–135 in two-photon initiated cationic polymerization, 281–283 in unsaturated conjugated systems, 157–160 in upconverted lasing, 321–323 vibrational contributions to, 139–140 waveguide materials and, 308–310 Two-photon active compounds, in 3D data storage, 302. See also TP entries Two-photon excitation (TPE), 116, 118–119, 121, 134–135. See also TPE entries

INDEX

377

applications of, 114–115 efficient, 278 fluorescence imaging and, 311 nanoclinics and, 316–317 organic reactions upon, 260–271 outlook for, 328–330 in photochemistry, 113–115 singlet oxygen via, 267 in three-level TPA model, 138–139 Two-photon excitation microscope (TPEM), 311–312 Two-photon excitation spectra, 178–179. See also TPE spectra of conjugated polymers, 171 of fluorene dimer, 176 of stilbene derivatives, 174 Two-photon excited fluorescence (TPEF), 174–175, 255–260 in absolute evaluation of TPA materials, 148–154 of diphenylaminofluorene-based chromophores, 207 excitation intensity and, 165, 166 in neutral donor-p-acceptor chromophores, 186 quantifying TPA cross section via, 144, 147–148 of symmetric acceptor-p-acceptor chromophores, 221, 222 of symmetric donor-acceptor-donor chromophores, 230–231 Two-photon initiated cationic polymerization, 281–283 Two-photon initiated crosslinking polymerization, 272–283 Two-photon initiated polymerization, 274–281 Two-photon photosciences, outlook for, 328–330

Urethane acrylates, TP initiated polymerization of, 286

Ultrafast energy hopping, 21. See also Femtosecond entries Ultrashort laser pulse, 298 Ultraviolet (UV) absorption spectra, 325 Ultraviolet excitation, cycloaddition via, 265 Ultraviolet light, in 3D data storage, 295 Unsaturated conjugated systems, TPA data for, 157–160 Upconverted lasing, 321–323 Urethane acrylate oligomers, in optical circuits, 309

Zero differential overlap (ZDO) transition densities, 138 ZINDO/S package, 125, 128. See also CEOINDO/S procedure; INDO-MRDCI/SOS method with three-level TPA model, 136, 138 z-scan method, quantifying TPA cross section via, 146, 147, 182, 247 ‘‘Z-scheme’’ of photosynthesis, 97 Zwitterionic structures, bond length alternation and, 130–132

van der Auweraer, M., 1 VB-CT model, for two-photon absorption, 128 Vibrational contributions, to two-photon absorption, 139–140 Vibronic coupling, TPA cross section and, 168 Vibronic maxima, 15 Vinylene bridges, in octupolar chromophores, 244–246 Virtual states, in two-photon excitation, 118–119 Volume pixels. See Voxel entries Voxels addressing, 300–301 in optical band-gap materials, 305 in waveguide materials, 309 Voxel size (S), 288, 289, 291, 292 in 3D data storage, 296–298 Wavefunctions, 118–119 in octupolar chromophores, 236 symmetry and, 129 Waveguide materials, 308–310 White-light continuum (WLC) fluorescence, 240 of diphenylaminofluorene-based chromophores, 207 Wire-frame objects, 289 Write once, read many (WORM) technique, for 3D optical memory systems, 294 Xanthene dyes, 277, 278 Xanthone (Xn), excited-state ketyl radicals of, 88 Xylene moiety, in symmetric donor-p-donor chromophores, 209–210, 210 Yield. See Quantum yield entries

CUMULATIVE INDEX VOLUMES 1–29

Addition of Atoms to Olefins, in Gas Phase (Cvetanovic) . . . . . . . . . . . Advances in the Measurement of Correlation in Photoproduct Motion (Morgan, Drabbels, and Wodtke) . . . . . . . . . . . . . . . . . . . . . . . . . . AFM and STM in Photochemistry Including Photon Tunneling (Kaupp) Alcohols, Ethers, and Amines, Photolysis of Saturated (von Sonntag and Schuchmann) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alkanes and Alkyl Radicals, Unimolecular Decomposition and Isotope Effects of (Rabinovitch and Setser) . . . . . . . . . . . . . . . . . . . . . . . . Alkyl Nitrites, Decomposition of and the Reactions of Alkoxyl Radicals (Heicklen) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alternative Halocarbons, Atmospheric Photochemistry of (Francisco and Maricq) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anthracenes, Excited State Reactivity and Molecular Topology Relationships in Chromophorically Substituted (Becker) . . . . . . . . . . . . . . . Anti-Stokes Fluorescence, Cooling of a Dye Solution by (Zander and Drexhage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aromatic Hydrocarbon Solutions, Photochemistry of (Bower) . . . . . . . . Asymmetric Photoreactions of Conjugated Enones and Esters (Pete) . . . Atmospheric Reactions Involving Hydrocarbons, FTIR Studies of (Niki and Maker). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VOL. PAGE 1 115 23 19

279 119

10

59

3

1

14

177

20

79

15

139

20 1 21

59 23 135

15

69

Benzene, Excitation and Deexcitation of (Cundall, Robinson, and Pereira) Biocatalysis and Biomimetic Systems, Artificial Photosynthetic Transformations Through (Willner and Willner) . . . . . . . . . . . . . . . . . . . Biochromophoric Systems, Excited State Behavior of Some (De Schryver, Boens, and Put) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10

147

20

217

10

359

Cancer Treatment, Photochemistry in (Dougherty) . . . . . . . . . . . . . . . . Carbonyl Compounds, The Photocycloaddition of, to Unsaturated Systems: The Syntheses of Oxetanes (Arnold). . . . . . . . . . . . . . . . . . .

17

275

6

301

Advances in Photochemistry, Volume 29 Edited by Douglas C. Neckers, William S. Jenks, and Thomas Wolff # 2007 John Wiley & Sons, Inc.

379

380

CUMULATIVE INDEX VOLUMES 1–29

Catalysis of Photoinduced Electron Transfer Reactions (Fukuzumi and Itoh) Cobalt (III) and Chromium (III) Complexes, the Photochemistry of, in Solution (Valentine, Jr.). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Complexes, Photoinitiated Reactions in Weakly Bonded (Shin, Chen, Nickolaisen, Sharpe, Beaudet, and Wittig) . . . . . . . . . . . . . . . . . . . Cyclic Ketones, Photochemistry of (Srinivasan) . . . . . . . . . . . . . . . . . . 1,2-Cycloaddition Reaction of Carbonyl Compounds and Pentaatomic Heterocyclic Compounds (D’Auria, Emanuele, and Racioppi). . . . . Cyclobutanones, Solution Phase Photochemistry of (Morton and Turro) . Cyclometallated Complexes, Photochemistry and Luminescence of (Maestri, Balzani, Deuschel-Cornioley, and von Zelewsky) . . . . . . .

VOL. PAGE 25 107 6

123

16 1

249 83

28 9

81 197

17

1

8

77

14

1

16 3 8

1 241 1

9

1

13 6

237 425

22

197

29

1

20 11

1 489

16

119

Flash Photolysis with Time-Resolved Mass Spectrometry (Carr) . . . . . . Free Radical and Molecule Reaction in Gas Phase, Problems of Structure and Reactivity (Benson) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FTIR Emission Studies, Time Resolved, of Photochemical Reactions (Hancock and Heard) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Functional Molecular Glasses: Building Blocks for Future Optoelectronics (Fuhrmann and Salbeck) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

1

2

1

18

1

27

83

Gas Phase, Addition of Atoms of Olefins in (Cvetanovic) . . . . . . . . . . . Gas Phase Reaction, Photochemical, in Hydrogen-Oxygen System (Volman) Gas Phase Reactions, Involving Hydroxyl and Oxygen Atoms, Mechanisms and Rate Constants of (Avramenko and Kolesnika) . . . . . . .

1 1

115 43

2

25

2

137

19

235

a-Dicarbonyl Compounds, The Photochemistry of (Monroe) . . . . . . . . . Diffusion-Controlled Reactions, Spin-Satistical Factors in (Saltiel and Atwater). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron and Energy Transfer, Mimicking of Photosynthetic (Gust and Moore) Electron Energy Transfer between Organic Molecules in Solution (Wilkinson) Electronically Excited Halogen Atoms (Hussain and Donovan) . . . . . . . Electron Spin Resonance Spectroscopy. Appliction of to Photochemistry (Wan). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron Transfer, Photoinduced in Organic Systems, Control of Back Electron Transfer of (Fox). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron Transfer Luminescence in Solution (Zweig) . . . . . . . . . . . . . . Elementary Photoprocesses in Designed Chromophore Sequence on aHelical Polypeptides (Sisido). . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ensemble Photophysics of Rigid Polyphenylene Based Dendritic Structures (Lor, Schweitzer, van der Anweraer, Hofkens, and De Schryver). . . . Ethylenic Bonds, Present Status of the Photoisomerization Abut (Arai and Tokumaru) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Excimers, What’s New in (Yakhot, Cohen, and Ludmer) . . . . . . . . . . . . Excited Electronic States, Electronic and Resonance Raman Spectroscopy Determination of Molecular Distortions in (Zink and Shin). . . . . . .

Halogenated Compounds, Photochemical Processes in (Major and Simons) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heterogeneous Catalysts, the Question of Artificial Photosynthesis of Ammonia on (Davies, Boucher, and Edwards) . . . . . . . . . . . . . . . .

381

CUMULATIVE INDEX VOLUMES 1–29

VOL. PAGE Hydrogen-Oxygen Systems, Photochemical Gas Phase Reactions in (Volman) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydroxyl and Oxygen Atoms, Mechanisms and Rate Constants of Elemen tary Gas Phase Reactions Involving (Avramenko and Kolesnikova) Hydroxyl Radical with Organic Compounds in the Gas Phase, Kinetics and Mechanisms of the Reactions of (Atkinson, Darnall, Winter, Lloyd, and Pitts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hypericin and Its Perylene Quinone Analogs: Probing Structure, Dynamics, and Interactions with the Environment (Das, Halder, Chowdhury, Park, Alexeev, Gordon, and Petrich) . . . . . . . . . . . . . . Hypophalites, Developments in Photochemistry of (Akhtar) . . . . . . . . . Imaging Systems, Organic Photochemical (Delzenne) . . . . . . . . . . . . . . Intramolecular Proton Transfer in Electronically Excited Molecules (Klo¨ pffer). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Invention of Dylux1 Instant-Access Imaging Materials and the Development of Habi Chemistry—A Personal History (Dessauer). . Ionic States, in Solid Saturated Hydrocarbons, Chemistry of (Kevan and Libby) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isotopic Effects in Mercury Photosensitization (Gunning and Strausz) . . Ketone Photochemistry, a Unified View of (Formosinho and Arnaut) . . . Lanthanide Complexes of Encapsulating Ligands at Luminescent Devices (Sabbatini, Guardigli, and Manet) . . . . . . . . . . . . . . . . . . . . . . . . . Laser Trapping-Spectroscopy-Electrochemistry of Individual Microdroplets in Solution (Nakatoni, Chikami, and Kitamura) . . . . . . . . . . . .

1

43

2

25

11

375

28 2

1 263

11

1

10

311

28

129

2 1

183 209

16

67

23

213

25

173

1

209

1

183

1 8

209 227

Mechanism of Energy Transfer, in Mercury Photosensitization (Gunning and Strausz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanistic Organic Photochemistry, A New Approach to (Zimmerman) Mercury Photosensitization, Isotopic Effects and the Mechanism of Energy Transfer in (Gunning and Strausz) . . . . . . . . . . . . . . . . . . . Metallocenes, Photochemistry in the (Bozak) . . . . . . . . . . . . . . . . . . . . Methylene, Preparation, Properties, and Reactivities of (De More and Benson) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular Distrortions in Excited Electronic States, Electronic and Resonance Raman Spectroscopy Determination of (Zink and Shin)

2

219

16

119

Neutral Oxides and Sulfides of Carbon, Vapor Phase Photochemistry of the (Fileeth) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitric Oxide, Role in Photochemistry (Heicklen and Cohen) . . . . . . . . . Noyes, W., A., Jr., a Tribute (Heicklen) . . . . . . . . . . . . . . . . . . . . . . . . Nucleic Acid Derivatives, Advances in the Photochemistry of (Burr) . . .

10 5 13 6

1 157 vii 193

14

135

17 24 1

313 1 323

Olefins, Photolysis of Simple, Chemistry of Electronic Excited States or Hot Ground States? (Colling) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Onium Salts, Photochemistry and Photophysics of (DeVoe, Olofson, and Sahyun) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical Single-Molecule Detection at Room Temperature (Meixner) . . . Organic Molecules, Photochemical Rearrangements of (Chapman) . . . .

382

CUMULATIVE INDEX VOLUMES 1–29

VOL. PAGE Organic Molecules in Adsorbed or Other Perturbing Polar Environments, Photochemical and Spectroscopic Properties of (Nichollas and Leermakers) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Molecules in their Triplet States, Properties and Reactions of (Wagner and Hammond) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organic Nitrites, Developments in Photochemistry of (Akhtar) . . . . . . . Organic Photochemical Refractive-Index Image Recording Systems (Tomlinson and Chandross) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Organized Media on Photochemical Reactions, A Model for the Influence of (Ramamurthy, Weiss, and Hammond) . . . . . . . . . . . . . . . . . . . . Organo-Transition Metal Compounds, Primary Photoprocesses of (Bock and von Gustorf) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Perhalocarbons, Gas Phase Oxidation of (Heicklen) . . . . . . . . . . . . . . . Phenyl Azide, Photochemistry of (Schuster and Platz). . . . . . . . . . . . . . Phosphorescence and Delayed Fluorescence from Solutions (Parker) . . . Phosphorescence-Microwave Multiple Resonance Spectroscopy (El-Sayed) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoassociation in Aromatic Systems (Stevens) . . . . . . . . . . . . . . . . . . Photochemical Mechanisms, Highly Complex (Johnston and Cramarossa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemical Oxidation of Aldehydes by Molecular Oxygen, Kinetics and Mechanism of (Niclause, Lemaire, and Letort) . . . . . . . . . . . . Photochemical Reactivity, Reflections on (Hammond) . . . . . . . . . . . . . . Photochemical Rearrangements of Conjugated Cyclic Ketones: The Present State of Investigations (Schaffner) . . . . . . . . . . . . . . . . . . . Photochemical Transformations of Polyenic Compounds (Mousseron) . . Photochemically Induced Dynamic Nuclear Polarization (Goez) . . . . . . Photochemistry in Cyclodextrin Cavities (Bortolus and Monti) . . . . . . . Photochemistry of Triarylmethane Dye Leuconitriles (Jarikov and Neckers) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemistry of Conjugated Dienes and Trienes (Srinivasan) . . . . . . . Photochemistry of Rhodopsins, The (Ottolenghi) . . . . . . . . . . . . . . . . . Photochemistry of Short-Lived Species Using Multibeam Irradiation (Fujitsuka and Majima) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemistry of Simple Aldehydes and Ketones in the Gas Phase (Lee and Lewis) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemistry of the Troposphere (Levy) . . . . . . . . . . . . . . . . . . . . . . Photochemistry of Vitamin D and Its Isomers and of Simple Trienes (Jacobs and Havinga) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photochemistry, Vocabulary of (Pitts, Wilkinson, Hammond). . . . . . . . . Photochromism (Dessauer and Paris) . . . . . . . . . . . . . . . . . . . . . . . . . . Photodissociation Dynamics of Hydride Molecules: H Atom Photofragment Translational Spectroscopy (Ashfold, Mordaunt, and Wilson) Photo-Fries Rearrangement and Related Photochemical (l.j) Shifts of (j ¼ 3,5,7) of Carbonyl and Sulfonyl Groups (Bellus) . . . . . . . . . . . Photography, Silver Halide, Chemical Sensitization, Spectral Sensitization, Latent Image Formation (James) . . . . . . . . . . . . . . . . . . . . . . Photo-Induced and Spontaneous Proton Tunneling in Molecular Solids (Trommsdorff) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

315

5 2

21 263

12

201

18

67

10

221

7 17 2

57 69 305

9 8

311 161

4

1

4 7

25 373

4 4 23 21

81 195 63 1

26 4 12

1 113 97

29

53

12 9

1 369

11 1 1

305 1 275

21

217

8

109

13

329

24

147

383

CUMULATIVE INDEX VOLUMES 1–29

VOL. PAGE Photoionization and Photodissociation of Aromatic Molecules, by Ultra violet Radiation (Terenin and Vilessov) . . . . . . . . . . . . . . . . . . . . . Photoluminescence Methods in Polymer Science (Beavan, Hargreaves, and Phillips) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photolysis of the Diazirines (Frey) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photooxidation Reactions, Gaseous (Hoare and Pearson) . . . . . . . . . . . . Photooxygenation Reactions, Type II, in Solution (Gollnick) . . . . . . . . . Photophysical Probes of DNA Sequence-Directed Structure and Dynamics (Murphy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photophysics of Gaseous Aromatic Molecules: Excess Vibrational Energy Dependence of Radiationless Processes (Lim) . . . . . . . . . . . . . . . . Photopolymerization, Dye Sensitized (Eaton) . . . . . . . . . . . . . . . . . . . . Photoreactive Organic Thin Films in the Light of Bound Electromagnetic Waves (Sekkat and Knoll) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photosensitized Reactions, Complications in (Engel and Monroe) . . . . . Photosynthetic Electron and Energy Transfer, Mimicking of (Gust and Moore). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photothermal Studies of Photophysical and Photochemical Processes by the Transient Grating Method (Terazima). . . . . . . . . . . . . . . . . . . . Phytochrome, Photophysics and Photochemistry of (Schaffner, Branslavsky, and Holzwarth) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polymers, Photochemistry and Molecular Motion in Solid Amorphous (Guillet) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pressure-Tuning Photochemistry of Metal Complexes in Solution (Eldik and Ford) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proton-Transfer Reactions in Benzophenone/N,N-Dimethylaniline Photochemistry (Peters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Primary Processes and Energy Transfer: Consistent Terms and Definitions (Porter, Balzani and Moggi) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

385

11 4 3 6

207 225 83 1

26

145

23 13

165 427

22 8

117 245

16

1

24

255

15

229

14

91

24

61

27

51

9

147

Quantized Matter, Photochemistry and Photoelectrochemistry of: Properties of Semiconductor Nanoparticles in Solution and ThinFilm Electrodes (Weller and Eychmu¨ ller). . . . . . . . . . . . . . . . . . . . Quantum Theory of Polyatomic Photodissociation (Kreslin and Lester)

20 13

165 95

Radiationless Transitions, Isomerization as a Route for (Phillips, Lamaire, Burton, and Noyes, Jr.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiationless Transitions in Photochemistry (Jortner and Rice) . . . . . . .

5 7

329 149

19 26

179 93

17

145

15 7 17 11

279 311 217 105

Semiconductor Nanoclusters, Photophysical and Photochemical Processes of (Wang). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Semiconductor Photocatalysis for Organic Synthesis (Kisch) . . . . . . . . . Silver Halides, Photochemistry and Photophysics of (Marchetti and Eachus) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single Crystals, Photochemical Mechanism in: FTIR Studies of Diacyl Peroxides (Hollingsworth and McBride) . . . . . . . . . . . . . . . . . . . . Singlet Molecular Oxygen (Wayne) . . . . . . . . . . . . . . . . . . . . . . . . . . . Singlet Molecular Oxygen, Bimolecular Reactivity of (Gorman) . . . . . . Singlet Molecular Oxygen, Physical Quenchers of (Bellus) . . . . . . . . . .

384

CUMULATIVE INDEX VOLUMES 1–29

VOL. PAGE Singlet and Triple States: Benzene and Simple Aromatic Compounds (Noyes and Unger) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Small Molecules, Photodissociation of (Jackson and Okabe) . . . . . . . . . Solid Saturated Hydrocarbons, Chemistry of Ionic States in (Kevan and Libby) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solvation, Ultrafast Photochemical Intramolecular Charge Transfer and Excited State (Barbara and Jarzeba). . . . . . . . . . . . . . . . . . . . . . . . Spectroscopy and (Photochemistry of Polyatomic Alkaline Earth Containing Molecules (Bernath) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spin Conservation (Matsen and Klein) . . . . . . . . . . . . . . . . . . . . . . . . . Stilbenes, Bimolecular Photochemical Reactions of (Lewis) . . . . . . . . . Stilbenes and Stilbene-Like Molecules, Cis-Trans Photoisomerization of (Go¨ rner and Kuhn) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure and Reactivity of Organic Intermediates as Revealed by TimeResolved Infrared Spectroscopy (Toscano) . . . . . . . . . . . . . . . . . . . Sulfur Atoms, Reactions of (Gunning and Strausz) . . . . . . . . . . . . . . . . Sulfur and Nitrogen Heteroatomic Organic Compounds, Photochemical Reaction of (Mustafa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supramolecularly Organized Luminescent Dye Molecules in the Channels of Zeolite L (Calzaferri, Maas, Pauchard, Pfenniger, Megelski, Devaux) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surfactant Solutions, Photochemistry in (von Bu¨ nau and Wolff) . . . . . . The EPR Spectroscopic D Parameter of Localized Triplet Diradicals as Probe for Electronic Effects in Benzyl-type Monoradicals (Adam, Harrer, Kita, and Nau) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Photochemistry of Indoles (Weedon) . . . . . . . . . . . . . . . . . . . . . . . Theory and Applications of Chemically Induced Magnetic Polarization in Photochemistry (Wan). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thiophosgene: A Tailor-Made Molecule for Photochemical and Photophysical Studies (Moule, Fujiwara, and Lim). . . . . . . . . . . . . Transition Metal Complexes, Primary Processes in (Forster) . . . . . . . . . Triatomic Free Radicals, Spectra and Structures of (Herzberg). . . . . . . . Two-Photon Physical, Organic, and Polymer Chemistry: Theory, Techniques, Chromophore Design, and Applications (Strehmel and Strehmel) . . . .

4 13

49 1

2

183

15

1

23 7 13

1 1 165

19

1

26 4

41 143

2

63

27 14

1 273

24 22

205 229

12

283

28 16 5

27 215 1

29

111

3

157

22

1

Ultraviolet Photochemistry, Vacuum (McNesby and Okabe) . . . . . . . . . Ultraviolet Photodissociation Studies of Organosulfur Molecules and Radicals: Energetics, Structure Identification, and Internal State Distribution (Cheuk-Yiu Ng) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ultraviolet Radiation, Photoionizational and Photodissociation of Aromatic Molecules by (Terenin and Vilessov) . . . . . . . . . . . . . . . Up-Scaling Photochemical Reactions (Braun, Jakob, Oliveros, Oller do Nascimento) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

385

18

235

Velocity Mapping of UV Multiphoton Excited Molecules (Chandler and Parker) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

59

Weakly Bonded Complexes, Photoinitiated Reactions in (Shin, Chen, Nickolaisen, Sharpe, Beaudet, and Wittig) . . . . . . . . . . . . . . . . . . .

16

249

Xanthine Dyes, Photochemistry of the (Neckers and Valdes-Aguilera) . .

18

315