Influence of counter anion on geometries, electronic structures and nonlinear optical properties of dimethylaminostilbazolium cation: An investigation by semiempirical calculation

We investigated the influence of counter anion on the geometry, electronic structure, and nonlinear optical (NLO) properties of (dimethylamino)stilbazolium cation (DAS⁺) by semiempirical quantum chemical calculation. Two different positions were considered for DAS⁺ and the counter anion in our calculations. In the first case, the counter anion is placed at the side of the dimethylamino group of the phenyl ring of DAS⁺ (type A). In the other case, the counter anion is placed at the side of pyridinium ring of DAS⁺ (type B). In both the cases, for bond distances (r(a) and r(b)) 1.5-50 Å were considered. The geometry and electronic structure of DAS⁺ have found to be changed due to the interaction between the counter anion and DAS⁺, and these changes depend on r(a) and r(b). The NLO properties of DAS⁺ are influenced by the counter anion in a subtle way in the case of type A. The signs of first and second hyperpolarizabilities (β and γ) of DAS⁺ are changed when the counter anion approaches DAS⁺, and the maximum absolute values of β and γ are larger than those of DAS⁺ without the counter anion. These results provide a guideline for molecular design of stilbazolium-based zwitterions with large NLO properties. (C) 2000 Elsevier Science B.V.