AB-INITIO MO INVESTIGATIONS OF MOLECULAR-STRUCTURES IN THE GROUND ANDFIRST EXCITED-STATES OF HETEROCYCLIC PYRIDINIUM BETAINE

The molecular structure in the first excited state of the simplest het erocyclic pyridinium betaine, (1-pyridinio)benzimidazolate, was invest igated through ab initio molecular orbital calculations. The central C -N bond shortens to a nearly double bond character in contrast to the case for the ground state, and the pyridinium ring takes a para-quinoi d-like structure. The dipole moment direction in the excited state and the ground state are anti-parallel, and the benzimidazole ring acts a s an electron-donating group and the pyridinium ring acts as an electr on-withdrawing group, and the first excited state is characterized by an intramolecular charge transfer state, Based on these characteristic features of heterocyclic pyridinium betaines, a new concept for manip ulating the molecular dipole moment and molecular first-order hyperpol arizability is proposed.