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

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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

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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