MANIPULATION OF DIPOLE-MOMENT AND HYPERPOLARIZABILITY BASED ON HETEROCYCLIC PYRIDINIUM BETAINE STRUCTURES - AB-INITIO AND INDO S MO CALCULATIONS/

This contribution explores the first electronic hyperpolarizabilities beta of the heterocyclic pyridinium betaines consisting of a negativel y charged aromatic electron-donating group and a positively charged ar omatic electron-withdrawing group. On the basis of enhanced charge sep aration between the pyridinium and benzimidazole rings, the betaines c an be represented as a D-(donor)-A(+) (acceptor) system having a very large ground-state dipole moment in the direction from D- to A(+). The inversion of the dipole moment direction upon electronic excitation i s the most remarkable feature of the heterocyclic pyridinium betaines. This nature of the betaines makes it possible to manipulate the groun d-state dipole moment and P. The nb initio and INDO/S molecular orbita l calculations have shown that the reduction of the ground-state dipol e moment, and the enlargement of the excited-state dipole moment and b eta would be achieved by a D--A(+)-pi-A system, in which a traditional A is introduced through a pi-conjugation path attached to A(+). On th e other hand, the enlargement of the groundstate dipole moment and the reduction of beta were achieved by a D--A(+)-pi-D system. It is shown that this concept for manipulation of the dipole moments and beta can be used to construct more efficient second-order nonlinear optical ma terials based on the heterocyclic pyridinium betaine structures.