Quantum Chemical Evaluation of Ionic Nonlinear Optical Chromophores and Crystals Considering the Counteranion Effects

The direction and the magnitude of the first hyperpolarizability are investigated by the finite field (FF) method for organic nonlinear optical ionic chromophores and crystals with an isotropic counteranion, either a point charge or hexafluorophosphate. Simple model compound 4-aminopyridinium and DAPSH (N,N-dimethylamino-N′-phenyl-4-stilbazolium hexafluorophosphate) crystal with the highest quadratic optical nonlinearities measured to date in organic crystals, as well as stilbazolium derivatives having heteroaromatic and/or bulky aromatic rings, are investigated. Calculations indicate that the extrinsic isotropic point-charges or anions almost have no effect on the direction of the intrinsic first hyperpolarizability of the cation chromophores, and induce a limited increase of the magnitude smaller than ∼15% due to their asymmetric distribution with respect to the pyridinium nitrogen atom. This finding enables one to simply approximate the first hyperpolarizability of ionic crystalline systems having 1D intramolecular charge transfer character by considering only the cation. In addition, the heteroaromatic or bulky aromatic substituents at the end of the pyridinium acceptor intrinsically do not affect the direction and only moderately affect the magnitude of the first hyperpolarizability of stilbazolium salts. These results are potentially useful for molecular design and crystal engineering of ionic organic materials, where the magnitude and the direction of first hyperpolarizability as well as the shape of the ionic chromophore are of great importance.