HETEROCYCLIC PYRIDINIUM BETAINES, A NEW CLASS OF 2ND-ORDER NONLINEAR-OPTICAL MATERIALS - COMBINED THEORETICAL AND EXPERIMENTAL INVESTIGATION OF FIRST-ORDER HYPERPOLARIZABILITY THROUGH AB-INITIO, INDO S, AND HYPER-RAYLEIGH SCATTERING/

The first hyperpolarizability, beta, of one of the simplest heterocycl ic pyridinium betaines, (1-pyridinio)-benzimidazolate, was measured ex perimentally with the hyper-Rayleigh scattering technique and compared to the calculated values obtained by the semiempirical INDO/S and ab initio molecular orbital methods. Heterocyclic pyridinium betaines are characterized by their zwitterionic character where a negatively char ged aromatic electron-donating group and a positively charged aromatic electron-withdrawing group are linked directly. The experimentally ob tained \beta\ value, (115 +/- 25) x 10(-30) cm(5) esu(-1), was in good agreement with those derived from the LNDO/S calculations. The first excited state was found to be characterized by a strong intramolecular charge-transfer state, when the electron transition occurs from a neg atively charged aromatic electron-donating group (benzimidazole ring) to a directly linked, positively charged aromatic electron-withdrawing group (pyridinium ring). This nature of the excited state would give rise to a large difference between the dipole moments in the excited a nd ground states and would contribute to enlargement of beta values.