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) proper-ties of (dimethylamino)stilba zolium cation (DAS(+)) by semiempirical quantum chemical calculation. Two d ifferent 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 othe r 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 An gstrom were considered. The geometry and electronic structure of DAS(+) hav e 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 DA S(+) are influenced by the counter anion in a subtle way in the case of typ e A. The signs of first and second hyperpolarizabilities (beta and gamma) o f DAS(+) are changed when the counter anion approaches DAS(+), and the maxi mum absolute values of beta and gamma are larger than those of DAS(+) witho ut the counter anion. These results provide a guideline for molecular desig n of stilbazolium-based zwitterions with large NLO properties. (C) 2000 Els evier Science B.V. All rights reserved.