Ab initio MO investigations of molecular structures in the ground and first excited states of heterocyclic pyridinium betaine

The molecular structure in the first excited state of the simplest heteorcyclic pyridinium betaine, (1-pyridinio)benzimidazolate, was investigated 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-quinoid-like structure. The dipole moment direction in the excited state and the ground state are anti-parallel, and the benzimidazole ring acts as an electron-donating group and the pyridinium ring acts as an electron-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 manipulating the molecular dipole moment and molecular first-order hyperpolarizability is proposed.