Manipulation of Dipole Moment and Hyperpolarizability Based on Heterocyclic Pyridinium Betaine Structures:  <i>Ab Initio</i> and INDO/S MO Calculations

This contribution explores the first electronic hyperpolarizabilities β of the heterocyclic pyridinium betaines consisting of a negatively charged aromatic electron-donating group and a positively charged aromatic electron-withdrawing group. On the basis of enhanced charge separation between the pyridinium and benzimidazole rings, the betaines can 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 is the most remarkable feature of the heterocyclic pyridinium betaines. This nature of the betaines makes it possible to manipulate the ground-state dipole moment and β. The ab initio and INDO/S molecular orbital calculations have shown that the reduction of the ground-state dipole moment, and the enlargement of the excited-state dipole moment and β would be achieved by a D--A+-π-A system, in which a traditional A is introduced through a π-conjugation path attached to A+. On the other hand, the enlargement of the ground-state dipole moment and the reduction of β were achieved by a D--A+-π-D system. It is shown that this concept for manipulation of the dipole moments and β can be used to construct more efficient second-order nonlinear optical materials based on the heterocyclic pyridinium betaine structures.