Linear and Nonlinear Optical Properties of Novel Ionic Chromophores
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INTRODUCTION π-Conjugated organic compounds have been widely studied as nonlinear optical (NLO) materials, which is expected to play a major role in various applications of NLO devices [1-2]. The optimization of NLO properties of organic materials is strongly related with both molecular and crystal engineering. In molecular level, stable molecules with large first hyperpolarizabilities (β) are requested to be designed and synthesized. These molecules should be aligned in optimized orientation so that the large β of a molecule is summed up to be large macroscopic second-order susceptibility, χ(2). For secondorder NLO materials, some guidelines of molecular design have been established. For example, those molecules with one π-conjugated system and donor and acceptor groups at the two side ends of the πconjugated system (DA molecules) are useful to obtain large β values. Unfortunately, roughly 75% of all organic molecules crystallize in centrosymmetric space groups, leading to materials with vanishing χ(2), e.g. second harmonic generation (SHG) [3]. To overcome this problem, using ionic π-conjugated chromophores could be useful to obtain noncentrosymmetric crystal structures [4]. It is because that ionic interactions could conquer dipole interactions, and numerous derivatives with different crystal structures could be prepared simply by changing the counter ions without any modification to the molecular structures of ionic π-conjugated chromophores. Along this line, many SHG active crystals of ionic chromophores including one series of stilbazolium cationic chromophores, i.e. 4-(2-(4(dimethylamino)phenyl)ethenyl)pyridinium p-toluenesulfonate (DAST), have been found to exhibit significant second-order nonlinear optical susceptibilities (χ(2)) [4-8]. However, the β values of these cationic chromophores had not been evaluated until the hyper Rayleigh scattering (HRS) method was established [9]. In order to obtain the guideline of molecular design for ionic species with large β values, it was necessary to investigate the relationship between the molecular structures and β values. In our previous studies [10-12], the β values of some organic ions have been evaluated using HRS method and semiempirical quantum chemical calculation. It was figured out that stilbazolium cations have several time larger β values than their corresponding electrically neutral molecules. The large χ(2) of
stilbazolium salts is due to the large β of stilbazolium cations. The significant electron withdrawing ability of pyridinium ring plays a major role in the intramolecular charge transfer of stilbazolium cations [12]. On the other hand, carbazole derivatives have been reported to have photoconductive, nonlinear optical and photorefractive properties [13-15]. Several types of crystals and polymers containing carbazole moieties have been obtained and showed reasonable second-order NLO properties [14-15]. Recently, we designed a new series of ionic chromophores, which was obtained by combining pyridinium ring and carbazole moiety using a double bond.
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