Heterocyclic Pyridinium Betaines, A New Class of Second-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, β, of one of the simplest heterocyclic pyridinium betaines, (1-pyridinio)benzimidazolate, was measured experimentally 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 charged aromatic electron-donating group and a positively charged aromatic electron-withdrawing group are linked directly. The experimentally obtained |β| value, (115 ± 25) × 10-30 cm5 esu-1, was in good agreement with those derived from the INDO/S calculations. The first excited state was found to be characterized by a strong intramolecular charge-transfer state, where the electron transition occurs from a negatively 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 and ground states and would contribute to enlargement of β values.