Electrooptical Chromophores for Nonlinear Optical and Photorefractive Applications

Abstract

Electrooptical (EO) materials are able to change their refractive index under the influence of an externally applied electric field, an effect that is of great technological importance as a means to control the phase of laser light. [1] In recent years, a variety of cheap and easily processable organic nonlinear optical (NLO) materials [2] were designed for this purpose. Poled polymers for high-frequency modulation of optical signals were developed, and photorefractive (PR) polymers [3] emerged as promising materials for widespread holographic optical applications. In both types of material, the EO response is provided by chromophores that are usually incorporated into the material as guests or as side-groups in functionalized polymers. In this communication, we analyze the microscopic mechanism leading to the refractive index modulation and define appropriate molecular figures-of-merit (FOMs) for EO chromophores, F 0 Pockels and F 0 Kerr . In order to investigate optimization strategies for both FOMs, we synthesized a series of dyes based on quinonoid carbocyclic and heterocyclic acceptor units, which exhibit significantly higher acceptor strength and better thermal stability than common open-chain acceptor groups.All chromophores investigated in this study are displayed in Figure 1. The synthetic procedures for the new EO dyes 4±11 are shown in Schemes 1 and 2. Accordingly, EO dyes 4 and 10 were prepared by Knoevenagel condensations of p-N,N-dimethylamino cinnamaldehyde (13a) with benzylidene malononitrile 15 [4] and 1-naphthylmalononitrile (16) [5] in acetic acid anhydride in yields of 72 and 43 %, respectively (Scheme 1). Similarly, dye 3 (DCM, a commercial laser dye) was obtained according to the literature from 14. [6] For dye 11, the polyenic chain of 13b was extended via a Wittig oxopropenylation [7] using the protected formylmethylenetriphenylphosphorane 17 to give pentadienal 18 in 60 % yield. The following condensation of 18 with 16 gave 11 in 10 % yield. The EO dyes 5±9, bearing the new powerful thiazolidenemalononitrile acceptor unit, were synthesized according to Scheme 2. Reaction of stoichiometric amounts of a-thiocyanatoketones 19a and b with malononitrile afforded 2-dicyanomethylthiazoles 20a and b in almost quantitative yields. [8] Because of an equilibrium between various tautomeric forms, these heterocycles may react easily in condensation reactions with aldehydes 13a and b, 21, and 22 to give dyes 5, 6, and 9a and b in yields of 60±80 %. The corresponding aldehydes 21 and 22 were obtained from N,N-dibutylaniline and N-alkylated 3,3-dimethyl-2-methyleneindoline by Vilsmeier formylations. For the synthesis of EO dyes 7 and 8 the reaction conditions had to be changed because of the high basicity of the methylene bases of 23 and 24. In a one-pot sequence in acetic acid anhydride, 3ethyl-2-methylbenzoxazolium iodide (23) and 1-hexyl-4methylpyridinium bromide (24) were first converted to the anils by N,N-diphenylformamidine (DPFA). They were then in-situ N-acetylated at 150 C, [9] before being ...

Molecular design of photorefractive polymers

“…The dipole moment in the ground state and the difference of the dipole moments are calculated using electro-optical absorption measurements. 86 Polarizability anisotropy Δα, the ground-state dipole moment μ and the first hyperpolarizability β are significantly affected by structural modifications due to changes in the electron-donating and -accepting substituent groups. The two terms of FOM of the NLO chromophores with various π-conjugated structures with different electron-donating and -accepting substituent groups were individually evaluated as a function of the resonance parameter c 2 for two basic resonance states of push-pull chromophores, a neutral polyene limit (D-A, c 2 = 0), cyanine limit with c 2 = 0.5, zwitterionic limit (betaine type, D + -A − , c 2 = 1) 26,86,87 and as a function of π-bond-order alternations (BOAs).…”

“…86 Polarizability anisotropy Δα, the ground-state dipole moment μ and the first hyperpolarizability β are significantly affected by structural modifications due to changes in the electron-donating and -accepting substituent groups. The two terms of FOM of the NLO chromophores with various π-conjugated structures with different electron-donating and -accepting substituent groups were individually evaluated as a function of the resonance parameter c 2 for two basic resonance states of push-pull chromophores, a neutral polyene limit (D-A, c 2 = 0), cyanine limit with c 2 = 0.5, zwitterionic limit (betaine type, D + -A − , c 2 = 1) 26,86,87 and as a function of π-bond-order alternations (BOAs). 27,88,89 For π-conjugated molecules with single-double bond series, bondlength alternation (BLA) is defined in terms of the difference between the average lengths of single bonds and that of double bonds, which is significantly related to the optimized geometries.…”

“…The same types of plots were reported by Würthner's group. 86,87 Kippelen et al 88 also plotted the two terms of FOM and FOM as a function of BOA. The product of μβ was shown to have two extrema with a positive maximum (at 0oc 2 o0.5, − 0.65oBOAo0), a negative maximum (at 0.5oc 2 o1, 0oBOAo0.65) and zero at the cyanine limit (c 2 = 0.5, BOA = 0).…”

“…Namely, the contribution of the hyperpolarizability term due to the electro-optic effect (Pockels effect) was far outweighed by the contribution of the linear polarization term due to the birefringence of the NLO molecule by the space-charge field (molecular orientation enhancement). 26,27,[86][87][88][89] Beyond the cyanine limit (c 2 40.5, BOA40), some problems have been noted: 26,87 high-melting crystalline solids are generated, which lead to difficulties of dissolving the materials in rather non-polar polymeric matrices. Then, the molecular design around cyanine limits offers better NLO chromophores for enhanced PR performance.…”

Optimized solution-processed merocyanine:PCBM organic bulk heterojunction solar cell

“…The origin behind this apparent discrepancy between a decent EQE and a rather poor carrier mobility could be the strong polarity of the MD304 (MC dyes, in general), the molecular dipole moment being ∼14 D. 26 Organic materials commonly feature rather low dielectric constants (ε r = 2-3), and as a result, the Coulomb interaction between charges is strong, leading to losses due to recombination. A more polar environment would stabilize charges and thus reduce recombination effects.…”