Optical Properties of Oligophenylenevinylenes

Encyclopedia of Nanoscience and Nanotechnology

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Optical Properties of Oligophenylenevinylenes Johannes Gierschner, Dieter Oelkrug University of Tübingen, Tübingen, Germany

CONTENTS 1. Introduction 2. Materials 3. Optical and Photophysical Properties in Solution 4. Model Compounds for OPV–OPV Interaction Studies 5. Optical and Photophysical Properties in the Solid State Glossary References

1. INTRODUCTION In the last 20 years, polyene-like organic molecules have attracted much attention as active components in organic light-emitting diodes (OLEDs), microlasing cavities, ultrafast photoswitches and detectors, as well as organic field effect transistors (FETs) [1]. Among the varieties of materials, polyphenylenevinylene (PPV)-based materials are widely investigated due to their use in OLEDs [2]. In order to improve the optical and photophysical properties, that is, tunable emission wavelengths and high fluorescence quantum yields, the interplay of intramolecular effects (the effective conjugation length and band shifts by chemical substitution) and intermolecular interactions (mutual geometrical orientations, electronic and vibrational coupling between the chromophores) has to be understood. Oligophenylenevinylenes (OPVs, see Fig. 1) make possible the systematic study of these parameters, since the oligomers provide well-defined conjugation lengths, variable substitution patterns, and a variety of intermolecular orientations in the condensed phases. OPVs have been studied since the 1960s as scintillators [3], laser dyes [4], and optical whiteners [5]. The photochemistry of stilbenoid compounds, especially the trans–cis ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

photo-isomerization of stilbene, has been extensively analyzed [6] and is still under investigation [7–12]. Since the early 1990s, with the synthesis of homologous series of soluble t-butyl-substituted OPVs by Schenk and co-workers [13], the oligomers have become a widely used model system for PPV. Since then, a great variety of substitution patterns has been established, allowing the tuning of the emission wavelength over the visible range [14, 15]. Quantum chemical investigations and theoretical modeling of the OPVs permit the precise prediction of the electronic transition energies and spectra of the oligomers in solution and extrapolation to the ideally conjugated polymer. Experimental and theoretical studies of intermolecular orientations in different model systems enlightened the correlation of structural and optical properties in the condensed phases and opened the opportunity to achieve full control of the photophysics of these systems. In the recent years, films, single crystals, dendrimer systems, and host–guest compounds of the OPVs have attracted growing attention for potential applications in optoelectronic devices due to their electroluminescent [16–23], lasing [24, 25], (photo) conductive [26–31], or photovoltaic properties [32, 33]. Substituted OPVs with large two-photon absorption cross sections are attractive materials for fluorescence microscopy, optical limiting, and optical data storage [34]. The article focuses on the photophysics of individual and condensed para-oligophenylenevinylenes and their correlation to intra- and intermolecular structural effects. In Section 2, an overview of the materials is given, including the variety of substitution patterns and the geometrical structures of the molecules. Section 3 discusses the features of the molecules in solution (absorption and fluorescence spectra, electronic transition energies, fluorescence quantum yields, and decay times) and the influence of mesomeric and inductive substituent effects, solvent shifts, and thermal effects. In Section 4, model compounds for the investigation of intermolecular interactions and their impact on the optical properties are discussed. In Section 5, the photophysics of condensed phases such as films, nanoparticles, and single crystals of OPVs are addressed. Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (219–238)

220

Optical Properties of Oligophenylenevinylenes

2

5 4

3

1

1′ n

Figure 1. Structure and notation of the atoms for oligophenylenevinylenes (nPVs).

2. MATERIALS 2.1. Substitution Patterns The homologous series of unsubstituted oligophenylenevinylenes, nPV, where n denotes the number of phenylenevinylene units (see Fig. 1 and Table 1), has been synthesized up to n = 7 [5, 27, 35, 36]. In order to enlarge the solubility of the oligomers, alkyl or alkoxy groups were introduced. The series of t-butyl-substituted oligomers (BnPV, see Table 2) with n = 1–6 was synthesized by Schenk et al. [13], and series of alkoxy-substituted OPVs were synthesized by Stalmach et al. (nPOPVs with n = 1–11, see Table 3) [37] and Peeters et al. (nBOPVs, with n = 1–6, see Table 4) [38]. Recently, the syntheses of further homologous series with alkyl [39–42] and alkoxy [16, 40–43] substituents was reported. Finally, homologous series of donor–acceptor (DA)-substituted OPVs were synthesized, with D,A substituents either in the terminal 5,5 positions (see Fig. 1, D,

dialkylamino, and A, cyano, carbonyl, nitro groups) [44] or in the vinylene units (1,1 positions with D, dibutylamino, A, cyano) [45]. The substitution pattern was widely varied, especially on para-distyrylbenzene (2PV), including ring substitutions (Tables 5, 6) with alkyl [13, 26, 36, 39, 46–49], alkoxy [16, 26, 36–38, 43, 49–53], cyano [36, 49, 54–57], amino [34, 49, 58], nitro [26, 49], fluoro [59–61], bromo [49], carbonyl [62], carboxyl [49], sulfonyl [53], and silyl groups [63, 64] or push– pull systems [44, 53], as well as substitutions in the vinylene unit (Tables 7, 8) with sulfonyl [50, 65, 66] or cyano groups [48, 50, 66–71]. A systematic variation of cyanosubstitution was also performed on 4PV (Table 9) [72–75]. Thus, different substitution patterns with mesomeric (±M) and/or inductive (±I) groups, but also with varying sterical requirements, are now available. Recently, general procedures of OPV syntheses were reviewed [14].

2.2. Molecular Structure According to X-ray studies on trans-stilbene (1PV) [76–79], 2PV [80], and 4PV [17], the unsubstituted OPVs are only slightly twisted, with torsional angles around the C1 –C2 single bonds (Fig. 1) of  ≈ 5 . A perfect planar structure (C2h symmetry) of 1PV is found in the gas phase at low temperatures according to jet-cooled spectra [81–84]. The C2h symmetry is also predicted by density functional (B3LYP) [8, 85–88] and semiempirical PM3 [89–92] quantum chemical

Table 1. Experimental and calculated adiabatic and vertical electronic transition energies (103 cm−1 ) of unsubstituted oligophenylenevinylenes nPV in vacuo.

n

1PV

Adiabatic transition energies 00

Experiment Calculation

a

In vacuo Ab initio

Vertical transition energies vert

Experiment e Calculation

In vacuo Ab initio

Semiempirical

TDFT Semiempirical

Fluorescence

F F / ns

2PV

3PV

4PV

Ref.

315 317 338 300

277 281 299 264

260 263 287 244

251 —b 27.4 c 23.3 d

[91] [91] [246] [97]

338 339 418 325 329 338 331 319

298 297 366 261 291 293 287 280

280 282 343 229 269 273 269 262

270 —f 33.0 g 21.1 h 25.7 i 26.2 j 25.9 k 25.2 l

[91] [91] [246] [245] [97] [91] [243] [107]

— —

[148] [148]

0036 m 009 m

Note: Fluorescence quantum yields (F ) and decay times (F ) in solution are shown. a Obtained from experiments in solution by extrapolation to a refractive index n = 1. b RCIS/6–311G∗ . c RCIS/3–21G. d AM1 CAS. e Obtained from the experimental adiabatic transition energies by addition of the equilibration energy exp . f From 00 (RCIS/6–311G∗ ) by addition of the explicitly calculated [91] equilibration energy calc . g RCIS/3–21G (direct calculation). h Time-dependent density functional B3LYP/6–31G∗ . i AM1. j ZINDO/S-CI, involving (2n + 1) occupied and unoccupied molecular levels. k ZINDO/S-CI (full configuration interaction). l PPP. m [278].

090 120

085 110

221

Optical Properties of Oligophenylenevinylenes

Table 2. Absorption maxima ( max ), oscillator strengths (f ), fluorescence subbands ( 1 2 ), fluorescence quantum yields (F ), fluorescence decay times (F ), and radiative rate constant (kF ) of t-butyl-substituted oligophenylenevinylene BnPVs in solution (dichloromethane) at T = 293 K. [175].

n–1

Absorption

Fluorescence

max /nm B1PV B2PV B3PV B4PV B5PV B6PV

316 360 387 403 410 417

f

1 2 /nm

F

F /ns

kF /109 s−1

344, 361 397, 419 428, 456 448, 478 459, 490 464, 495

010 086 082 075 071 071

— 110 106 097 073 065

— 078 077 077 097 109

053 089 121 160 180 188

Table 3. Absorption maxima ( max ), extinction coefficient (), fluorescence maxima ( 1 2 ), fluorescence quantum yields (F ), fluorescence decay times (F ), and radiative rate constant (kF ) of propyloxy-substituted oligophenylenevinylene nPOPVs in solution at T = 293 K. OC3H7

OC3H7

Absorptiona H7C3O

n

H7C3O

1POPV 2POPV 3POPV 4POPV 6POPV 8POPV 11POPV a b

Fluorescenceb

max /nm [37]

max /103 l mol−1 cm−1 [37]

1 2 /nm [283]

F [283]

F /ns [283]

kF /109 s−1 [283]

354 401 431 450 466 475 481

17 41 59 85 117 146 196

(408), 425 460, 486 500, 529 522, 555 543, 580 549, 586 551, 590

045 082 094 070 043 048 048

189 160 172 110 062 062 064

023 051 055 064 070 077 075

In chloroform. In dichloromethane.

Table 4. Absorption maxima ( max ), extinction coefficient (), fluorescence maxima ( 1 2 ), fluorescence quantum yields (F ), fluorescence decay times (F ), and radiative rate constant (kF ) of methylbutoxy-substituted oligophenylenevinylene nBOPVs in chloroform at T = 293 K [173]. O

O Me O

1BOPV 2BOPV 3BOPV 4BOPV 5BOPV 6BOPV

O

Me n

Absorption

Fluorescence

max /nm

max /103 l mol−1 cm−1

max /nm

F

F /ns

kF /109 s−1

357 407 437 454 464 475

21 44 68 88 107 142

409 466 502 523 534 541

— 062 076 049 041 025

— 170 132 073 052 045

— 037 058 067 080 055

222

Optical Properties of Oligophenylenevinylenes

Table 5. Absorption maxima ( max ), extinction coefficient (), fluorescence maxima ( max ), fluorescence quantum yields (F ), fluorescence decay tpa times (F ), two-photon excitation maximum ( max ), and two-photon absorption cross section () of substituted distyrylbenzenes in toluene at T = 293 K.

X

R N R

R

R

n-Butyl n-Butyl n-Butyl n-Butyl n-Butyl Phenyl n-Butyl a

R′

R′′

R′′

R′



R

H F OMe SO2 C3 H7 CN CN H

R N R

X

One-photon absorption



3

X

H F H H H H H

max /nm max /10 l mol

H H H H H H CN

410 430 428 439 490 475 438

−1

Fluorescence

cm

−1

74 77 67 43 66 66 54

max /nm

F

Two-photon absorption

F /ns



a

455 456 480 528 536 528 504

088 135 058 079 088 138a 031 101 069 130 087 146 0003 <0015

tpa max

/nm /10−50 cm4 /s/photon

730 760 730 816 830 830 790

955 1400 900 4100 1750 1640 890

Ref. [270, 268] [271] [268, 34] [271] [270] [270] [270]

In acetonitrile.

Table 6. Absorption maxima ( max ), fluorescence maxima ( max ), fluorescence quantum yields (F ), and fluorescence decay times (F ) of distyrylbenzene (2PV) derivates with substituents in the phenylene moieties. R′′

R

R′′

R′

R′

R′′

R

R′

R

R

H OC8 H17 OC8 H17 OC8 H17 OC8 H17 OCH3 OCH3

Absorption



R

H H Ph OPh COPh NPh2 H



H H H H H H OCH3

Fluorescence a

max /nm

max /nm

355 387 405 392 415 425 393

388, 410 440 460 444 487 480 446, 470

F

F /ns

Solvent

Ref.

0.90 0.83 0.90 0.91 0.70 0.75 0.92

1.29 1.74 1.35 1.49 1.52 1.26 1.55

Dioxane Dioxane Dioxane Dioxane Dioxane Dioxane C6 H12

[48] [48] [48] [48] [48] [48] [69]

Note: Data in solution are shown. a For structured spectra the spectral positions of the two high energetic subbands are given.

Table 7. Absorption maxima ( max ), fluorescence maxima ( max ), fluorescence quantum yields (F ), and fluorescence decay times (F ) of distyrylbenzene (2PV) derivates with substituents in the vinylene moieties.

Y

R′′′ R′ R′′

Type A B

C

X

X

R

R′′ R′ R′′′

Y

R

Absorption 

R



R



R

R

X

OC3 H7 OC8 H17 OC8 H17 OCH3

H H OPh H

H H H OCH3

H H H H

CN CN CN CN

OCH3

H

OCH3

H

C6 H13

H

H

C6 H13 C6 H13

H H

H C6 H13

Y

Fluorescence

max /nm

max /nm

H H H H

425 422 428 408

H

CN

H

CN

H C6 H13

H CN

a

F

F /ns

489 512 511 478

— 0.36 0.49 0.06

410

445, 475

0.10

H

375

435, 458

CN H

304 325

410 441

0.002 ∼05–0.8 0.001 0.003

— 290 223 03 25% 10 75% 02 12% 09 88% <001 15 — —

Note: Data in solution are shown. a For structured spectra the spectral positions of the two high energetic subbands are given.

Solvent

Ref.

CHCl3 Dioxane Dioxane C6 H12

[50] [48, 284] [48, 284] [69]

CH2 Cl2

[69]

CH2 Cl2 EPA, 77 K CH2 Cl2 CH2 Cl2

[146] [237] [146]

223

Optical Properties of Oligophenylenevinylenes

Table 8. Fluorescence properties of (substituted) distyrylbenzenes (2PV) in solution, thin films, and nanoparticles (NP): fluorescence maxima ( max ; us, unstructured), quantum yields (F ), decay times (F , ne, nonexponential), natural lifetimes (F0 ), and fluorescence anisotropy (rF ). R

m o Y p R′

R′

X

X

p

Y

R

o m

R

R

X

Y

State

max /nm

F

F /ns

F0 /ns

rF

H

H

H

H H

HexO

H

H

H

030

J aggregates

HexO

H

H

CN

000

Excimer emission

HexO

H

CN

H

HexO

o-CN

H

H

HexO

m-CN

H

H

HexO

p-CN

H

H

H

p-CN

H

H

1.2 2.5 (n.e.) 9–12 (n.e.) 1.1 0.4 1.6 21 (n.e.) 0.9 2.2 (n.e.) 2.1 30 (n.e.) 2.0 9.5 1.9 2.3 1.8 0.5–1.0 1.2 1.3 ∼0.3 (n.e.) 12 (n.e.) 20 (n.e.)

H aggregates

H

0.92 0.05–0.1 0.3–0.5 0.82 0.18 0.66 ∼0.15 — — 0.32 ∼0.3 0.65 0.03–0.06 0.72 0.04–0.08 0.81 0.7–0.8 0.80 0.2 ∼0.2 0.4–0.5 0.4

025

H

388, 409 (415), 441 (421), 448 403, 424 417, 440 446, 467 583 u.s. 497 u.s. 478 u.s. 520 u.s. 622 u.s. 487 u.s. 580 u.s. 460 u.s. 576 u.s. 500 u.s. 482, 514 409, 433 (430), 460 436, 463 560 u.s. 560 u.s.

1.3 ∼30

Hex

Solution Film/NP 293 K 15 K Solution Film Solution Film Solution Film Solution Film Solution Film Solution Film Solution Film Solution NP (methanol/H2 O) Film, fast deposited Film, annealed NP (dioxane/H2 O)

1.3 2.2 2.4 ∼140 — — 6.6 ∼100 3.1 ∼200 2.6 ∼40 2.2 ∼1 1.5 6.5 ∼15 ∼30 ∼50

Remarks

Ref. [54] [54]

— —

Excimer emission

001

Excimer emission

025

Excimer emission

020

J aggregates

030 030 — 023

H aggregates J aggregates Excimer emission Excimer emission

[54] [54] [237] [54, 57] [237] [237] [237] [237] [57] [54, 57] [57] [54, 57] [57] [54] [55] [319] [55] [55] [319]

Table 9. Fluorescence properties of (substituted) distyryl(distyrylbenzene)s (4PV) in solution, thin films, nanoparticles (NP), and single crystals: fluorescence maxima ( max ; u.s., unstructured), quantum yields (F ), and decay times (F , mono- or biexponential fits).

Y

X

R′

X

R

Y

Name

R

R

X

Y

4PV

H

H

H

H

MEH-4PV

EtHexOa

MetO

H

H

Oct-4PV

Oct

H

H

H

OctO-4PV

OctO

H

H

H

OctO-4PV-CNo

OctO

H

CN

H

OctO-4PV-CNi a

OctO

EtHexO is 2-ethyl-hexyloxy.

H

H

CN

State

max /nm

F

F /ns

Solution NP Solution Film Solution Film Solution Film, as deposited Film, annealed Single crystal Solution Film, as deposited Film, annealed Single crystal Solution Single crystal

440, 471 (483), 515 480, 510 (500), 530 455, 485 505, 533 483, 515 529 539 539, (563) 537, (570) 595, (620) 615 u.s. 630 u.s. 510 u.s. 560, 593

— ∼01 — 0.15 0.85–0.9 — 0.7–0.9 0.5 0.7 0.5 0.7–0.9 0.4 0.6 0.5 0.04 0.4

— — 0.65 1.2 — — ∼1.0 1.1 1.7 1.0 1.4 1.4, 7.0 3.4, 8.0 8.0 0.08 2.2

Remarks H aggregates

J aggregates Cofacial x,z-shifted Excimer emission Cofacial z-shifted Cofacial x,z-shifted

Ref. [91] [174] [152, 154, 155] [153, 320] [305] [303] [25, 305] [24, 306] [24, 306] [306] [25, 110, 275] [306, 110] [306] [110, 306] [25] [306]

224 calculations on 1PV. Ab initio Hartree–Fock (HF) [86, 87, 92–95] and semiempirical AM1 [89, 90, 92, 93, 96, 97] calculations yield a slightly nonplanar structure (C2 symmetry) of 1PV; however, the ring-torsional potential is very flat for  < 30 . The bond lengths and angles of the nPVs, as determined by X-ray measurements [17, 76–80], are quite well reproduced by theory, resulting in pronounced bond alterations in the vinylene units [8–10, 87–94, 98–103]. In the gas phase at higher temperatures, the average geometrical structure, according to electron diffraction [104] and photoelectron studies [105, 106] of 1PV, is nonplanar, due to the low torsional barrier for rotations around the ethenyl–phenyl (C1 –C2 ) single bonds [8, 85, 89–93, 95–97, 107]. Deviations from planarity in the solid state are observed in several substituted OPVs. According to quantum chemical calculations [69, 92, 93], the deviations from planarity depend sensitively on the kind and positions of substituents [93]. However, the torsional angles in most of the reported X-ray structures [17, 47, 60, 73, 74, 80, 108–114] do not exceed 11 –18 . Larger deviations from planarity are found for 2PV derivatives with cyanosubstituents in the vinylene moiety and additional alkyl substituents at the central phenyl ring (see Tables 7–9) [68].

Optical Properties of Oligophenylenevinylenes 'Shifted' arrangements

CMe3

1a

2a

Me3C

Ref. [118, 119, 121]

'Angled' arrangements 1b

2b

CMe3 'Crossed' arrangements Me3C

Me3C

Ref. [119, 121] Me3C

3

2.3. Bridged Oligophenylenevinylenes Oligophenylenevinylenes with defined mutual geometric alignment are represented by (i) hydrogen-bonded OPV pairs, introduced by Meijer and co-workers [115–117], and by (ii) covalently linked oligomers, which were synthesized by Schenk et al. [13], Anger et al. [11], Rau and Waldner [12], Bazan [118–129], and Song et al. [130–133]. The covalently linked OPVs cover dimeric stilbenoid cyclophanes with partial – overlap and different intermolecular orientations [13, 118–125, 134] (Fig. 2), stilbenophanes with perfect face-to-face alignment [11, 12, 135] (Fig. 2), tetrameric systems with tetrahedral OPV orientation [126–128, 136] (Fig. 3), and dendrimers [128, 136–141]. Dimeric OPVs were also prepared by Lewis and Letsinger using 1:1 mixtures of complementary oligonucleotides containing stilbene units [142–145].

2.4. Condensed Phases Films of OPVs were prepared by vapor deposition from high vacuum [20, 28, 74, 146–155], by spin or drop coating from solution [21, 23, 156–158], and by the Langmuir–Blodgett technique [131, 159–170]. Nanoparticles were obtained by fast precipitation from solutions of the oligomers by addition of a poor solvent (i.e., water) and measured in suspension [38, 54, 116, 146, 148, 170–178]. The size of the nanoparticles (20–500 nm) can be controlled by the preparation conditions (concentration, solvent, temperature, mixing ratio). Nanoparticles were also prepared by cooling saturated solutions of the OPVs [62, 179–182]. General procedures for nanoparticle preparation were reviewed recently [183, 184]. Single crystals were grown from solution [60, 61, 68, 75–78, 108–112, 163, 185], melt [186], and gas phase [31, 79, 187, 188].

CMe3

CMe3 'Face-to-Face' arrangements

Ref. [120] S

S Ref. [11, 135] S

4a

S 4b

Ref. [12]

Figure 2. Structures of stilbenoid cyclophanes.

Host–guest compounds (HGCs), where the OPVs are incorporated in an inert host material such as -cyclodextrin [189] or channel-forming perhydrotriphenylene (PHTP) [55, 190, 191], were prepared by cocrystallization from solution. The cavities of the cyclodextrin HGCs permit the inclusion of cofacially oriented stilbene pairs. PHTP HGCs open the opportunity to investigate specific “head-to-tail” interactions of the oligomers.

R R

AdR4 R R

R

CR4 R R

R R = t-butyl-2PV [126–128]

R = t-butyl-2PV [126, 128] R = alkoxy-2PV [128]

Figure 3. Structures of tetrahedral oligophenylenevinylenes.

225

Optical Properties of Oligophenylenevinylenes

3. OPTICAL AND PHOTOPHYSICAL PROPERTIES IN SOLUTION 3.1. Absorption and Fluorescence Spectra At low temperatures (T < 20 K), the S0 → S1 absorption and S1 → S0 fluorescence spectra of 1PV [192] and the longer oligomers [91] show very small 0–0 bandgaps and the spectra are almost mirror symmetrical (Fig. 4). Thus, the frequencies and nuclear displacements of the totally symmetrical ag vibrational modes which couple to the electronic transition must be very similar in S0 and S1 , in agreement with ab initio quantum chemical calculations [91]. Detailed vibrational analyses of the electronic spectra were carried out on (substituted) trans-stilbenes under collisionfree conditions [81–84, 193–196]. The fine structures of the experimental spectra are dominated by the progressions and combinations of a few prominent modes, which also becomes evident for the longer oligomers in solid solution

2PV

a) experiment (T = 15 K)

fluorescence intensity

b) calculated in vacuo

c) experiment (T = 293 K)

d) simulated

[91, 118, 179, 197, 198]. Theoretical calculations, obtained by a Franck–Condon approach [85, 91, 99, 100, 199, 200] from ab initio quantum chemical calculations, demonstrate that the nuclear displacements of the prominent modes essentially coincide with the geometrical changes upon the electronic transition. Among these modes, the low-frequency longitudinal in-plane deformation mode ( = 204 cm−1 for 1PV [193]) strongly decreases with n, resulting in  → 0 for n →  [91], whereas the vinylene C1 –C1 stretching mode around 1650 cm−1 [201–208] decreases only very little with n [96, 209–211]. The optical spectra of the nPVs were calculated at different levels of theory for 1PV [85, 99, 100, 212] and the longer oligomers [91, 200, 213], and good agreement with the experiment was obtained at the ab initio HF/restricted configuration interaction singles (RCIS) 6– 311G∗ level, including the progressions and combinations of the complete sets of ag modes (Fig. 4). With increasing temperature, the mirror symmetry of absorption and fluorescence is lost and a 0–0 bandgap opens, which extends to 200 cm−1 at 77 K [173, 175, 214] and to 1000 cm−1 at room temperature (Fig. 4). The room temperature fluorescence spectra still exhibit distinct vibronic structure, whereas the absorption spectra are only slightly structured and asymmetrically shifted to the blue [91, 96, 146, 210, 214–219] (see Fig. 4). The differences in the spectral characteristics of fluorescence and absorption are due to the different torsional barriers of the ground and excited state structures. Compared to the S0 state, the C1 –C2 bond is rigidified in the S1 state [8, 9, 89–91, 95, 100–103, 220], leading to a steeper potential of the torsional modes [8, 90, 91, 97, 212]. The torsional barrier can be investigated in the gas phase, where the low-frequency torsional mode of 1PV was determined to  = 8 cm−1 in the S0 state and  = 48 cm−1 in the S1 state [81]. Therefore, the room temperature absorption spectra arise from superimposed transitions of different nonplanar conformers of the molecules [91, 96, 146, 215, 216, 221], and the total band shapes are well described by a combination of a Gaussian-shaped and an exponential convolution of the low temperature spectra (Fig. 4). The Gaussian part accounts for inhomogeneities of the environment, and the exponential part accounts for the increase in the torsional potential in S1 against S0 [212]. Since the potential in S1 is high against the thermal energy, the room temperature fluorescence spectra can be exclusively fitted by the Gaussian contribution to line broadening. In rigid environments, where the potential in S0 is also high against the thermal energy, the absorption spectrum can also be reasonably fitted by a Gaussian convolution [178, 222].

3.2. Electronic Transition Energies 20000

25000

30000

ν / cm–1 Figure 4. Fluorescence emission (left) and excitation (right) spectra of distyrylbenzene (2PV). (a) Experimental spectra in tetradecane at T = 15 K. (b) Calculated low temperature spectra (HF/RCIS 6–311G∗ ) [91]. (c) Experimental spectra in dioxane at T = 293 K. (d) Simulated spectra, obtained by convolution of the experimental low temperature spectra with a Gaussian and an exponential distribution [91].

The energetic positions of the absorption spectra of the OPVs are strongly affected by the conjugation length and to a smaller extent also by the solvent and the temperature (Fig. 4) [36, 223] due to changes in the polarizability of the environment [224–226]. In nonpolar solvents, the polarizability shift becomes a linear function of the Onsager relation [224], which allows the extrapolation to the spectral positions in vacuo with a refractive index of nsolv = 1 [91]. The solvent red shift amounts to R = 2500 cm−1 compared to the values in vacuo (Fig. 4); thus solvent effects

226 should be considered if experimental transition energies are compared to calculated ones. Specific solvent interactions in polar solvents are observed for OPVs with electron withdrawing substituents like cyano, nitro, triflyl (SO2 CF3 ), or fluoro groups [59, 75] and for donor–acceptor-substituted OPVs [225–227], resulting in unstructured, red-shifted fluorescence spectra and large Stokes shifts between the absorption and fluorescence maxima due to the contribution of charge separation not only in the S1 , but also in the S0 state [228]. Vertical transition energies vert for the S0 → S1 transition were calculated at different levels of theory for 1PV [85, 88, 89, 99, 100, 229–233], substituted OPVs [8, 16, 55, 60, 67, 92, 119, 232–239], and the homologues series of the nPVs [91, 97, 101, 103, 107, 218, 234, 240–251]. The latter reproduce the experimental result of an inverse chain length dependence 1/N (with N = 3n + 2) of vert [91, 96, 173, 216, 237, 247, 248]. However, the results obtained by semiempirical methods depend sensitively on the number of electron configurations considered in the calculations [89]. Time-dependent density functional (TDFT) methods tend to underestimate the vertical transition energies (Table 1) and to overestimate the slope of the 1/N dependence [245]. Reliable results were obtained by the ab initio RCIS/6– 311G∗ method, where the vertical transition energies were calculated from adiabatic transition energies 00 by addition of the equilibration energy [91] (see Table 1). In contrast, the direct calculation of the vertical transition energies by ab initio methods [246] yields unreasonably high values for vert (see Table 1). For longer oligomers (n > 4), a deviation from the linear 1/N dependence is observed both experimentally [27, 37, 173, 175] and theoretically [234, 246, 247], which results in a faster convergence for n → . The experimental transition energies can be reasonably fitted by empirical functions [44, 246], within the extended Hückel Molecular Orbital [252] and free electron gas model [253] including bond alternation, Simpson’s exciton model [254], or the classical oscillator approach by Kuhn [255]. The extrapolation to idealized PPV yields 00 ≈ 20,300 cm−1 in solution (23,400 cm−1 in vacuo). The equilibration energy for PPV was calculated to vert − 00 ≈ 1750 cm−1 [91]. The introduction of substituents in the terminal phenyl rings with +M effect (e.g., alkoxy [16, 215, 223, 256] or amino [34, 49, 58] groups), −M effect (e.g., carbonyl [62] or cyano [36, 49, 54–57] groups), or +I effect (e.g., alkyl groups [39, 96, 107, 175, 216, 218, 223, 257, 258]) shifts the S0 → S1 transitions slightly to lower energies. The additional introduction of alkoxy substituents [18, 38, 43, 48, 51, 69, 173, 234, 237] in the central phenyl rings (nPOPVs, Table 3, nBOPVs, Table 4) leads to a splitting of the low energetic absorption band with a systematic bathochromic shift of the S0 → S1 transition compared to that of the nPVs. According to quantum chemical calculations, this effect is ascribed to the interaction between carbon and oxygen  orbitals, destabilizing the highest occupied molecular orbital (HOMO) more strongly than the lowest unoccupied molecular orbital (LUMO) [238, 259]. In addition, a blue-shifted “RO band” appears, due to strong configuration interactions between the HOMO and phenyl orbital branches [234]. Extrapolation of the nBOPV spectral positions to n →  yields

Optical Properties of Oligophenylenevinylenes

00 ≈ 17,600 cm−1 for the idealized polymer (experimentally: 00 (BOPPV) = 18,100 cm−1 [260]). The fluorescence and absorption spectra of OPVs with cyanosubstituents in the vinylene unit are strongly red-shifted against the unsubstituted ones (Tables 7–9), in agreement with quantum chemical calculations [238, 259, 261]. Terminal donor–acceptor substitution of 1PV, for example, 4-(dialkylamino)-4 -nitrostilbene [122, 225–227], causes a large bathochromic shift in the transition energies due to the intramolecular charge transfer (CT) character of the transition. These results are also well reproduced by quantum chemical calculations [239, 247]. With increasing chain length n, the extent of the CT character decreases [44, 239, 247]. Thus, for some donor–acceptor-substituted homologous series, the decrease in the CT character overcompensates the bathochromic shift induced by the increase in n, thus causing a hypsochromic shift with n [44]. According to polarized absorption [262] and fluorescence studies [148, 263, 264] in stretched polyethylene films and to the rotational resolved fluorescence excitation spectrum of 1PV under collision-free conditions [84], the S0 → S1 transition moment # of the nPVs is oriented approximately parallel to the long geometrical axes of the molecules (defined as the distance between the terminal carbon atoms d% % ). The experimental results are supported by quantum chemical calculations, [9, 218, 229, 265, 266], yielding for 2PV an angle of approximately 8 between d% % and # [266]. The oscillator strengths increase roughly linearly with the chain length (Tables 2–4). The energetic positions, orientations, and oscillator strengths of the higher S0 → Sn transitions of the OPVs were assigned by polarized absorption spectroscopy on 1PV [262, 263] and fluorescence excitation measurements (1PV [265, 267], BnPVs [218]) and correlated with quantum chemical calculations [9, 218, 229, 265, 272]. Two-photon absorption studies allowed the assignment of the weak S0 → S2 transition in 1PV [267] and substituted OPVs [270, 268] (Table 5) located about 4000 cm−1 above the S0 → S1 transition, in qualitative agreement with quantum chemical calculations [270, 97, 232, 269]. Substituted nPV derivatives with donor–donor, donor–acceptor–donor, and acceptor–donor–acceptor structural motifs exhibit large two-photon absorption cross sections,  [34, 268, 270, 271], in combination with high fluorescence quantum yields (Table 5). According to quantum chemical calculations, the magnitude of  can be correlated with the degree of intramolecular charge transfer from the terminal donor groups to the  bridge [34, 268] and the calculated  values are in reasonable agreement with the experimental ones [34, 286, 271]. Triplet T1 → Tn transitions of nPVs (n = 1 [273], n = 2 [221]) alkoxy OPVs [173, 274] and differently substituted 4PVs [275] were determined by photoinduced absorption spectroscopy and by pulse radiolysis, respectively. The transition energies decrease with increasing chain length [173, 257], in agreement with quantum chemical calculations [101, 103], whereby the decrease is somewhat stronger than the one in the S0 → S1 transition [173]. The equilibration energies of the triplet transition decrease rapidly with chain length n [173] in agreement with theoretical results [101]. The triplet lifetimes at T = 100 K are in the range of 3–8 ms [173].

227

Optical Properties of Oligophenylenevinylenes

3.3. Fluorescence Quantum Yields and Decay Times

4.1. Arrangements with Weak Dipolar Coupling

The fluorescence quantum yields F of the parent nPVs and their derivatives with substituents at the phenylene moieties are fairly high (Tables 2–6) [16, 36, 48, 54, 59, 60, 66, 69, 146, 175, 237, 276, 277]. An exception is 1PV (trans-stilbene) due to the crossing of the S1 potential surface with the S2 state in the twisted geometry, leading to efficient nonradiative decay and trans–cis photoisomerization [6, 278]. Consequently, F (1PV) strongly increases in highly viscous [279] or rigid environments [190], at low temperatures [279–281], and in stiffed trans-stilbenes [7, 279, 282], where the flexibility of 1PV is strongly reduced. The radiative rate constants kr for the S1 deactivation of the OPVs in solution (Tables 2–4) increase with increasing conjugation length n, saturating at kr = 1 ± 03109 s−1 [146, 173]. The increase in the nonradiative rate constant with n is even stronger, due to the increase in torsional phonon state density with decreasing S1 → S0 transition energy, leading to an overall decrease in F with n (Tables 2–4) [16, 173, 175, 283]. Compared to the alkyl- and alkoxy-substituted OPVs, the fluorescence quantum yields are significantly lowered for OPVs with cyanosubstituents in the vinylene unit. The reduction is a sensitive function of the additional substitution patterns (Table 7). Medium quantum yields (F = 03–0.5) are observed if the central phenyl ring of 2PV is additionally substituted with alkoxy groups (molecules of Type A, see Table 7) [48, 50, 284]. Further alkoxy substitution at the terminal phenyl rings reduces F to 5–10% (Type B) [69, 146]. Finally, the oligomers become practically nonfluorescent if the central phenyl ring is substituted with alkyl groups (Type C) [146, 237]. Fluorescence quenching of these compounds is mainly a sterical effect. While molecules of Type A are almost planar [74], the structures of Type C molecules are far from planarity in the S0 state [68], thus enhancing torsional induced nonradiative deactivation [146]. Rigid environments enforce planarization of the C-type molecules in the S1 state; thus F increases by 2 orders of magnitude in EPA glass at 77 K [146] (see Table 7).

In order to investigate specific head-to-tail interactions, HGCs were prepared, where OPVs and their p,p substituted derivatives are incorporated in the channels of the pseudo-hexagonal host lattice of PHTP [55, 190, 191]. The intrachannel distances of the oligomers are given by the long axis dimensions and the van der Waals distances. The interchannel distances are in the order of ra b ≈ 15 nm [190, 286], so that side-by-side interactions between the oligomers are suppressed. The fine structures and spectral positions of the fluorescence spectra are very similar to those observed in solution [55, 190, 191], thus indicating only small dipolar OPV–OPV interactions. The observed red shift of the spectra R ≈ 700 cm−1 against solution is partly due to the high polarizability of the host lattice, and only a portion of R ≈ 300 cm−1 can be attributed to OPV–OPV interactions [55, 191]. The intermolecular excitation transfer of energy (ET) in the HGCs was investigated by time-resolved fluorescence on doped systems [191]. The results were interpreted according to the Förster-type ET mechanism, considering homo- and heterotransfer as well as intra- and interchannel transfer steps [191, 287]. The spectral features of the OPV tetramers in Figure 3 are also comparable with those of solutions of the monomers [126–128]. The spectra remain structured and are red-shifted by R ≈ 350 cm−1 against the monomers. This also indicates that in these compounds the interactions between the OPV units are only of weak dipolar character. However, very fast ET is observed. The monoexponential fluorescence anisotropy decay of AdR4 can be reasonably fitted assuming a F¨orster-type energy transfer, again indicating a weak coupling of the OPV units [128]. For CR4 a biexponential decay was observed, which may suggest the presence of short-range exchange (Dexter)-type interaction [128].

4. MODEL COMPOUNDS FOR OPV–OPV INTERACTION STUDIES Intermolecular interactions between OPV chromophores and their impact on optical and photophysical properties can be studied on model compounds, which are formed of a small number of oligomers with defined intermolecular geometrical alignment. Simple tetrameric structures were obtained with 2PV derivatives, where the oligomers are covalently linked via a central sp3 carbon atom or via adamantan (Fig. 3) [126–128]. One-dimensional head-to-tail alignments could be realized in channel-forming host–guest compounds [55, 190, 191, 285]. Since in these structures the OPV–OPV distances are large, the interactions between the oligomers are only of weak dipolar character. An increase in interactions is expected for arrangements with – overlap, represented by stilbenoid cyclophane compounds [11–13, 118–124, 134] and 1:1 mixtures of complementary oligonucleotides containing stilbene units [142–144].

4.2. Arrangements with
–
Overlap Stronger interactions are expected in OPV pairs with partial – electron overlap of the OPV units, as in cyclophanes [118–125]. For the “shifted” and “angled” arrangements of the 2PV cyclophanes (structures 2a, 2b in Fig. 2), a red shift of R = 1100 cm−1 against the spectra of the parent monomer unit (2c in Fig. 5) is observed [119, 121]. The intermolecular coupling is still of dipolar character, since the vibronic features of the room temperature spectra as well as the fluorescence quantum yields and decay times [119] are very similar to those of the monomer. A monomer-like emitting state was also predicted by theoretical calculations on the basis of the collective electronic oscillator (CEO) method [119]. However, differences between 2a and the monomer become apparent at low temperatures. In contrast to the monomer, well-resolved phonon lines could not be observed in the fluorescence spectrum of 2a, probably due to an increase in the Franck–Condon activity in low-frequency chain deformation modes [118]. Significant changes in the optical properties are observed if the relative extent of – overlap between the oligomers is enhanced. Thus, the shifted and angled arrangements of 1PV cyclophanes [119, 134] (structures 1a, 1b in Fig. 2)

228

Optical Properties of Oligophenylenevinylenes

0.3

1 108

0.2 5 107 0.1

400

300

200

100

0

0 2a 2b 2c

0.8

0

1.5 109

0.6 1 108 0.4 5 107

0.2 0

Emission Intensity (c.p.s.)

Normalized Absorbance (a.u.)

1.5 109

Absorbance rel. Intensities

0.4

500

Emission Intensity (c.p.s.)

Normalized Absorbance (a.u.)

2 108 1a 1b 1c

0.5

0 250

300

350

400

450

500

Wavelength (nm) Figure 5. Absorption and fluorescence spectra of stilbenoid cyclophanes. (Top) Spectra of compounds 1a, 1b (see Fig. 2) and of the parent monomer unit (1c). (Bottom) Spectra of compounds 2a, 2b (see Fig. 2) and of the parent monomer unit (2c). Reprinted with permission from [121], W. J. Oldham Jr. et al., J. Am. Chem. Soc. 120, 419 (1998). © 1998, American Chemical Society.

both show structureless, strongly red-shifted emission bands (Fig. 5) with low radiative rate constants [119], reminiscent of excimer emission. According to CEO calculations the emission of the 1PV cyclophanes occurs from a low-lying state with a significant contribution of the through-space delocalized paracyclophane core [119]. The main absorption band of 1b shows a splitting of about 9700 cm−1 , in good agreement with CEO calculations, due to the “Davydovlike” splitting of the S0 → S1 transitions of the monomer units in the angled arrangement [119]. An equivalent explanation accounts for the properties of the “crossed” 2PV cyclophane [120, 134] (structure 3 in Fig. 2), where the – overlap of the monomer units is increased against 2a and 2b, respectively. Perfect face-to-face alignment in 1PV cyclophane dimers was realized in the dimers 4a and 4b of Figure 2. In contrast to the stilbenophane 4a, which can form ZZ, ZE, and EE isomer pairs [11] due to the flexibility of the ethyl linkers, 4b is unable to undergo E–Z isomerization [12]. The main absorption band of 4b shows two components, an intense one at 1 ≈ 300 nm, slightly blue-shifted against the monomer peak at = 308 nm, and a red-shifted weak one at 2 ≈ 350 nm (Fig. 6). Due to the strong – overlap of the stilbene units, the fluorescence mimics excimer emission with an even stronger red-shifted emission spectrum than 1a and 1b and a fluorescence lifetime of F = 11 ns [12]. Similar

300

400

500

600

Wavelength/nm

Figure 6. Absorption (dots), fluorescence excitation (solid line), and emission (dashed line) spectra of stilbenophane 4b. Reprinted with permission from [12], H. Rau and I. Waldner, Phys. Chem. Chem. Phys. 4, 1776 (2002). © 2002, Royal Society of Chemistry.

features are also observed for 1:1 mixtures of complementary oligonucleotides containing stilbene units [142–144] and ternary complexes of 1PV/ -cyclodextrin/cyclohexane [189]. The electronic properties of cofacial nPV dimers were theoretically treated by Cornil [242, 288–290] and by Tretiak and Mukamel [291] and Tretiak et al. [292] as a function of the interchain distance. For large interchain separations (d
5 Å), a symmetrical splitting of the first single chain transition into a dipole allowed higher energetic components and a forbidden lower one was calculated (H-type interaction), in agreement with Kasha’s molecular exciton model [293, 294]. The excitation energy is still localized on a single chain. Below a critical distance of d ≈ 5 Å the splitting becomes unsymmetrical due to second-order perturbations, inducing an additional stabilization of the both states [242]. The lower state is delocalized over both chromophores, whereas the upper state has small interchromophore coherence [292]. The splitting tends to decrease nonlinearly with the chain length of the oligomers [242, 288, 295–297], in contrast to the predictions of the molecular exciton model, which states that there is an increase with chain length [293]. The splitting for a cofacial 1PV dimer [242] at a distance of d = 35 Å, which roughly corresponds to the average interchromophore distance in 4b [298], is  = 5800 cm−1 at the INDO/S-CI level, in reasonable agreement with experiments (see Fig. 6). Analogous to considerations on 2,2 paracyclophane [299], the symmetrically forbidden lower excited electronic Bg state in 4b may borrow intensity from the higher Bu state via an au vibrational mode. The weakly allowed Bg state is responsible for the low radiative rate constant of 4b. The large red shift of the fluorescence spectra of 1a and 1b and particularly 4b is mainly due to strong intermolecular vibronic coupling between the chromophore units: Quantum chemical calculations of 2,2 -paracyclophane [300] and 4b [298] predict a strong decrease in the interring distance after S0 → S1 excitation, which results in extremely high Franck–Condon activity of interring breathing modes [298].

229

Optical Properties of Oligophenylenevinylenes

5. OPTICAL AND PHOTOPHYSICAL PROPERTIES IN THE SOLID STATE The optical and photophysical properties of OPVs in the solid state are extensively studied on vapor-deposited (VD) films. Many experimental data are available on the absorption and fluorescence spectra [20–22, 28, 51, 54, 55, 67, 110, 146–149, 153, 158, 237, 258, 301–304], fluorescence quantum yields [22, 54, 55, 237], and fluorescence decay kinetics [22, 54, 55, 110, 158, 237, 258]. However, in only a few cases were the geometrical arrangements of the oligomers in the films determined by atomic force microscopy [149] or by polarized absorption studies [148, 159, 169, 301]; thus structure–property correlations are often missing. On the other hand, the structures of several single crystals have been determined by X-ray measurements [17, 25, 47, 60, 61, 68, 73–80, 108–114, 163, 185, 305, 306], and in some samples the fluorescence properties were also investigated [80, 110, 163, 185–188, 306]. VD films, formed from the same molecules as the corresponding single crystals, are expected to form similar structures, but substrate-induced orientation effects and the formation of meta-stable phases cannot be excluded. The model compounds of Section 4 demonstrated how the optical and photophysical properties of the individual molecules are changed by specific intermolecular electronic interactions and vibrational coupling. Similar interactions are expected in the solid state structures, but the extension to three dimensions will induce an increase in the total intermolecular coupling strength. Moreover, the photophysical properties in the solid state are influenced by additional effects, such as exciton diffusion, structural defects, and chemical impurities. However, most of the spectral characteristics in OPV condensed phases can be explained by (short-range) molecular ordering. A schematic matrix representation of nearest neighbor arrangements is given in Figure 7.

5.1. Unsubstituted Oligomers 5.1.1. Structure In 4PV single crystals the molecules are oriented with their long axes parallel to each other, where the short axes of adjacent molecules are arranged in a T-shaped manner (Fig. 7.21) with a short-axis inclination angle of ( ≈ 71 [17]. The T-shaped short-axis arrangement of adjacent molecules is also found for 2PV [80]. The corresponding “herringbone” arrangement is also predicted for 7PV by force field calculations [307]. A different packing was observed for 1PV single crystals [76–79]. The unit cell contains two molecules that are oriented with their long axes almost perpendicular to each other. Nearest neighbor molecules are cofacially arranged with a slight shift in the x,z plane (Fig. 7.23). On the other hand, T-shaped arrangements are also assumed for small aggregates of phospholipids with 1PV substituents [130–133]. The “unit” aggregate consists of a chiral tetramer “pinwheel,” as determined by induced circular dichroism spectra [131, 133]. Analogous structures are assumed in self-organized nanoparticles [146, 148, 174–179] or VD films [20, 21, 28, 51, 146–148, 301] of unsubstituted nPVs (n = 2–4) as well as in Langmuir–Blodgett (LB) films

x,y-rotated

coplanar

x-shifted

11)

12)

13)

21)

22)

23)

z-shifted

α

parallel

31)

d

32)

x,z-rotated

z y x

Figure 7. Schematic matrix representation of simple unit cells of oligophenylenevinylenes in the solid state.

of terminal-substituted nPVs (n = 1 [131, 159–164], 2 [165– 169]), since the spectral and photophysical features are very similar for all of these systems. Especially, both LB [159, 169] and VD films [148, 301] exhibit strong optical linear dichroism, indicating a preferential parallel alignment of the transition dipole moments.

5.1.2. Absorption Spectra The absorption characteristics can be well studied in systems with low optical density, such as VD films (film thickness <50 nm), small nanoparticles ( < 100 nm), or LB films. In thick films [21, 28, 51, 301, 308] or micro [51, 174, 309] or single crystals [80] the increase in the optical density masks most of the spectral characteristics by saturation effects. The absorption spectra of thin VD films and nanoparticles of unsubstituted nPVs consist of some weak bands (A1 , A2 ) followed by the main maximum (H band, e.g., for 2PV [175] see Fig. 8), which is blue-shifted by  ≈ 6000 cm−1 against the monomer spectrum. The blue shift is a consequence of exciton coupling of the transition dipole moments (H-type aggregation). According to the theoretical treatment by Spano [266, 310–314], the weak low energy peaks (A1 , A2 ) of the spectrum are ascribed to vibrationally dressed excitons in pinwheels [266, 312] which may be present in the inhomogeneous distribution of different aggregate sizes [266]. The “noninteracting domains model” [313], which accounts for the distribution of molecular transition frequencies caused by disorder, reproduces the fine structure of the spectrum. The deviation from perfect parallel alignment of the transition dipoles was determined from the decrease in the timeresolved fluorescence anisotropy from the initial value rF 0 = 04 to the steady-state value of rF = 02–0.3 [146, 148, 175– 178], which yields a mean deviation angle of ) ≈ 30 between the emitting species and the originally excited absorber. The forbidden lower exciton state (J band) is seen as a very weak shoulder at the low energy side of the spectrum [146, 313],

230

Optical Properties of Oligophenylenevinylenes

F2

2PV F3

H F1 A1

A2

fluorescence intensity

B2PV

Hex-2PV

pCN-2PV

20000

30000

40000

ν /cm – 1 Figure 8. Fluorescence emission (left) and excitation (right) spectra of (substituted) distyrylbenzene (2PV) nanoparticles (solid lines) and spectra in solution (dashed lines).

where the energetic position in 2PV nanoparticles coincides well with the electronic origin (00 = 23 900 cm−1 ) observed in single crystals of 2PV [188]. The 00 position is red shifted against solution by R = 1800 cm−1 and against vacuum by R = 3800 cm−1 due to the high optical polarizability of the solid samples in the direction of the long molecular axes. The anisotropic polarizability of 2PV nanoparticles was demonstrated by polarized light scattering measurements [176] and the spectral positions of the sensitized fluorescence of dopants [178].

5.1.3. Fluorescence Properties The room temperature steady-state fluorescence spectra of the unsubstituted nPVs are very similar for VD films, LB films, and nanoparticles, whereas in thick films and microcrystals the high energy side of the emission spectra is suppressed by reabsorption [80, 146, 187]. The spectra reveal some vibronic structure (progression Fi , see Fig. 8) where the energy spacing of the subbands resembles the one in solution. Cooling of the films and nanoparticles does not significantly enhance the vibrational resolution of the spectra, which is due to the spatial inhomogeneity of these systems. Rich vibronic fine structure of the spectra was observed in

2PV single crystals [188] at low temperatures, clearly demonstrating that the vibronic progressions and intercombinations are very similar to those observed in the spectra of the isolated molecules. Thus the contribution of intermolecular vibrational modes is small. However, the electronic origins as well as the F1 features (Fig. 8) are only of very weak intensity, in significant contrast to the solution spectra. According to the theoretical investigations by Spano [310– 314] and Meskers et al. [158], this effect is ascribed to electronic interactions in the medium coupling regime. In the nanoparticles, the F1 band shows varying intensities [174], which are caused by distributions of zero-order excited state energies [158] and/or distributions of interchain interactions [158, 310–314] due to (point) defects [312, 313]. The fluorescence lifetimes (2PV: F ≈ 25 ns, Table 8) are considerably longer than those in solution (2PV: F = 12 ns), whereas the fluorescence quantum yields are reduced from F = 09 in solution to F = 005–0.1 in the films and nanoparticles. Thus the radiative rate constant kF decreases against solution by 1 order of magnitude. At T = 20 K, F increases by a factor of 4–5 [54, 309], similar to PPV [258, 309], reaching the room temperature value of the 2PV single crystal (F = 065 [80]). The increase in F is accompanied by an increase in F of the same order [54, 146, 177]; thus in a first approximation kF is independent of temperature. Energy migration of the S1 excitation energy in nPV films is due to a diffusion-enhanced F¨orster transfer process, as revealed from the bimolecular rate constant of S1 –S1 annihilation [308]. The exciton diffusion (diffusion constant D ≈ 8 · 10−4 cm2 s−1 ) [308] allows the deactivation via intrinsic or extrinsic traps. For 6BOPV (Table 4), which also forms H aggregates [38, 158, 173], a distribution of disorder-induced trap sites was deduced from time-resolved and site-selective fluorescence spectra [158]. With increasing excitation wavelength, the fluorescence decay becomes exponential on the time scale of the first nanoseconds [158]. However, on longer time scales, up to the ms regime [315], the fluorescence decay traces of the nPVs are strongly nonexponential [146, 148, 177]. This may be due to the recombination of bounded polaron pairs, whose generation was observed in a photoinduced absorption (PIA) study [308]. In this study evidence for fission of vibrationally hot S1 states into triplet pairs was also found, a process which additionally reduces F . PIA studies on alkyloxy-substituted 4PV films (MeH4PV, see Table 9) [152–157, 316, 317] show that at low excitation densities, singlet excitons are formed as the only photoexcitations. At higher excitation densities, the singlet excitons are subject to bimolecular annihilation. In contrast to unsubstituted OPV, no evidence of exciton diffusion is found [153]. An additional contribution to the PIA at high excitation densities is ascribed to biexciton states that are stable in solution but dissociate into charged pairs in thin films [154]. The dissociation of singlet excitons into charged states is found to be a common process in conjugated polymers [157].

5.1.4. Doping with Acceptor Molecules for Electronic Excitation Transfer Doping of the nPVs with suitable acceptor molecules, such as longer OPVs [146, 177], oligothiophenes [178], or acenes [186], leads to efficient energy transfer to these extrinsic

231

Optical Properties of Oligophenylenevinylenes

traps, resulting in sensitized fluorescence of the dopants. At doping ratios of xdop > 5 · 10−4 the emission is almost completely dominated by the acceptor emission [146, 177, 178]. The appropriate choice of the acceptors allows tuning of the emission wavelength over the visible range. The acceptor molecules possess the same preferential orientation as the surrounding nPV molecules, as revealed from the constant fluorescence anisotropy over the whole emission range [177, 178]. The deactivation kinetics depend strongly on the choice of the dopant. OPVs as acceptors enhance the overall fluorescence quantum yield of doped 2PV nanoparticles up to F tot = 06 [146, 177]. Doping with oligothiophenes does not change F tot , which is probably a consequence of the competition between energy transfer and photoinduced electron transfer [178] from the HOMO of quinquethiophene to the HOMO of 2PV, which is approximately 1 eV lower in energy [150, 318]. The strong decrease in donor emission is not accompanied by a comparable decrease in fluorescence decay, thus indicating spatial inhomogeneities in these systems [146, 178].

5.2. Substituted Oligomers 5.2.1. t-Butyl-Substituted OPVs Space-consuming substituents like tertiary butyl groups (BnPVs, see Table 2) hinder the coplanar or side-by-side alignments of adjacent molecules (Fig. 7.21, 7.22), since now these structures are energetically less favored than, for example, pairs with x,z-rotated axes (Fig. 7.31, 7.32). The lack of short-range parallel intermolecular orientation was demonstrated by the very fast complete fluorescence depolarization, occurring in the first ps after excitation [148, 176, 177]. Due to the different orientations of neighboring transition dipoles, the absorption spectra are very similar to those in solution [148, 176, 177] (see Fig. 8) and no indication of a blue-shifted H band can be observed. Also the energy spacings and relative intensities of the Fi bands in the fluorescence spectra are almost identical to the ones in solution. The small red shift of R ≈ 400 cm−1 against solution is due to the high, approximately isotropic polarizability of the condensed material. The polarizability in the nonabsorbing region of B2PV nanoparticles was determined as isotropic by polarized light scattering measurements [176, 178]. The calculated isotropic value of the refractive index is in good agreement with the averaged value obtained from the polarizabilities of the constituting atoms and the one derived from the spectral positions of the sensitized fluorescence spectra of dopants [178]. The fluorescence quantum yields are low at room temperature (e.g., B2PV: F = 012) [148], increasing by a factor of three at T = 20 K [258].

5.2.2. OPVs with Alkyl or Alkoxy Side Chains Alkyl or alkoxy side chains (e.g., nPOPVs, nBOPVs, Tables 3, 4) destabilize T-shaped arrangements (Fig. 7.21) and favor partial – overlap. Thus in general cofacial arrangements, displaced in the x,z plane (Fig. 7.12, 7.13, 7.23), are observed. The 4PV oligomer with octyloxy side chains in the central phenyl ring (OctO-4PV, see Table 9) [24, 25, 74, 114, 305, 306] may serve as an example: The unit cell of the single crystal contains eight molecules, where

the conjugated backbones lie parallel to each other. Neighboring molecules are oriented in a cofacial arrangement, shifted in the x,z plane [305]. The highly anisotropic character of the cofacial arrangement becomes evident from the high degree of fluorescence polarization [114]. The crystal shift of the fluorescence spectrum against solution yields R ≈ 900 cm−1 (Table 9). This value is significantly higher than that in isotropic solids (e.g., BnPVs) but much smaller than that in the herringbone-arranged unsubstituted 4PV sample (R ≈ 1800 cm−1 , see Table 9). The smaller shift is due to the displacement of the OctO-4PV molecules in the x,z plane, but also to the decrease in the packing density by 11% compared to 4PV [17, 306]. The displacement gives rise to J aggregation with a well-structured, red-shifted absorption spectrum and high fluorescence quantum yields and decay rates [24, 306] (see Table 9). Similar considerations also apply for Oct-4PV [303] (Table 9) and Hex-2PV [54] (Fig. 8, Table 8), which all display well-structured fluorescence spectra similar to the ones in solution. No evidence for contributions of intermolecular vibrational modes to the fluorescence spectra is found for these systems. On the other hand, microcrystals of hexyloxy-substituted 2PV (HexO-2PV, Table 8) show excimer-like emission [54], thus indicating a strong – overlap of the emitting adjacent molecules. The fluorescence is completely depolarized, which may be due to a random distribution of small -stacked aggregates or to self-trapping of the excitation energy in excimer-like states [54].

5.2.3. Cyanosubstituted OPVs The tendency to arrange in a -stacked structure increases by introduction of cyanosubstituents. However, the crystal structure of cyanoderivatives depends sensitively on the substitution position. Thus cyanosubstitution at the inner position of the central vinyl moieties of octyloxy-4PV (OctO4PV-CNi , see Table 9) results in a cofacial x,z-shifted arrangement of neighboring molecules (Fig. 7.13) [73], where the displacement between the molecules is too large to allow intermolecular vibrational coupling. Concomitantly the fluorescence spectrum is structured and no excimer emission is observed [306]. On the other hand, cyanosubstitution at the outer positions of the central vinyl linkages (OctO-4PV-CNo , see Table 9) results in a z shift of neighboring molecules (Fig. 7.12) by one phenylene unit [306]. The – overlap is now large and gives rise to high Franck–Condon activity of intermolecular vibrational modes, leading to excimer emission with a long fluorescence decay time of 8 ns, with a considerably high quantum yield of F = 05 [306]. Excimerlike emission spectra are also observed for most of the cyanosubstituted 2PVs with additional hexyloxy substituents in the central phenyl ring (Table 8) [54, 57, 237]. However, it should be emphasized that the preparation conditions play a crucial role in the resulting arrangement of the molecules and hence, in the optical and photophysical properties. An illustrative example is p,p -dicyanosubstituted 2PV (pCN2PV). Depending on the preparation conditions (Table 8), H aggregates [319], J aggregates with well-structured emission spectra, or arrangements with excimer-like emission spectra (Fig. 8) [55] are observed (Table 8). In all these arrangements the molecules show a preferential parallel orientation

232 of the long molecular axes, as revealed from the high values of the fluorescence anisotropy, rF (see Table 8). The differences in the absorption and fluorescence spectra of the systems indicate that small changes in the geometrical alignment may induce strong changes in the optical and photophysical properties of the materials. Extended – stacks of adjacent molecules were observed for single crystals of various fluorosubstituted distyrylbenzenes [60] and binary cocrystals, containing pairs of unsubstituted and fluorinated nPVs, repectively [61, 109]. Nanoparticles and VD films of these materials exhibit H-type absorption spectra and excimer-like emission [321]. Excimer emission is also observed for single crystals of 3,5dichloro-trans-stilbene (Fig. 1), which also crystallizes in a cofacial arrangement [185].

NOTE Since the submission of the review chapter a number of new papers in the field of oligophenylenevinylenes have been published [322–363], which could not be included in the text, but should be brought to the attention of the reader. Further studies on the trans–cis photoisomerization of trans-stilbene (1PV) were published by several authors [322–324]. Room temperature Raman studies on 1PV suggest a distorted structure in solution [325]. In a theoretical paper, MøllerPlesset (MP2, MP3) calculations suggest a strictly planar geometry of 1PV in its absolute energy minimum [326]. Several papers described the preparation and spectroscopic characterization of new phenyl [327], alkyl [328], alkoxy [329–331], alkoxysilyl [332], amino [333, 334], bromo [335], and push-pull [336, 337] substituted OPVs. The effects of conformational [338, 339] and environmental disorder [338] on the optical properties were investigated both experimentally [338] and theoretically [338, 339]. A new method for the correlation of molecular shape and polarized luminescence, examined on OPVs, was introduced in Ref. [340]. The electronic transition energies of (substituted) OPVs were investigated at different quantum-chemical levels of theory [341–345], which inter alia allow the extrapolation for several ideally conjugated substituted PPV chains [341]. Two-photon properties of substituted OPVs were determined both experimentally [346] and theoretically [347, 348]. Molecular interactions in model compounds and their impact on the optical and photophysical properties were studied in OPV dimers with biphenyl linkage center [349], complementary oligonucleotides containing 1PV units [350], tetrahedral OPV structures [351], polystyrene bearing stilbenoid side chains [352], and supramolecular OPV ensembles [353]. Reference [354] shows that excimer formation of substituted OPVs might be used as strain sensors in polymer blends. Photo- [355– 358] and electroluminescence [359–362] of films formed by substituted OPVs were investigated by a number of groups, addressing the influence of substituents [362], crystal structure [355] and morphology of the sample [356, 360]. OPVs have also been used as active material for energy transfer in gelation-assisted light harvesting [363] and for photoinduced electron transfer in TiO2 hybrid materials [364]. Silver nanoparticles with OPV coating may act as ultrabright twophoton fluorescent nanobeacons [365].

Optical Properties of Oligophenylenevinylenes

GLOSSARY Bridged oligomers Model systems of a defined number of chromophores whose conformational degrees of freedom are reduced by hydrogen or covalent bonds between the chromophores. These systems can be used to understand intermolecular interactions of adjacent molecules in the solid state and to build up supramolecular architectures, for example, dendrimers and helical self-organized structures. Nanochannel-forming host–guest compounds Host–guest compounds (HGCs), in which chromophores are incorporated into the channels of inorganic (e.g., zeolites, nanoporous silica) or organic host materials (e.g., perhydrotriphenylene). HGCs provide ideal systems for studying weak dipolar interactions of chromophores and open up the opportunity to overcome the drawbacks of conventional condensed phase device structures for organic optoelectronic applications: Improved chemical stability by exclusion of oxygen, high fluorescence quantum yields by avoiding intermolecular aggregation, intrinsic polarization, high NLO efficiencies, and spatially directed energy transfer. Nanoparticles Nanoparticles of organic chromophores are obtained by precipitation from solutions by addition of a poor solvent. Nanoparticles allow the investigation of substrate-free self-organized systems, easy color tuning by chemical doping with appropriate acceptor molecules, and preparation of optoelectronic devices. Oligomer approach Oligomeric -conjugated organic molecules provide well-defined conjugation lengths compared to polymers, variable substitution patterns, and defined intermolecular orientations in the condensed phase. Hence oligomers with variable conjugation lengths and substituents make possible the systematic study of the interplay of intramolecular (optical) properties and intermolecular interactions as well as the understanding of the properties of the polymers. These well-defined systems permit theoretical modeling of solid state properties for molecular engineering of optoelectronic devices.

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