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

Abstract 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 A 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.