SOLVATOCHROMISM OF A ZWITTERIONIC BENZIMIDAZOIE-BASED PYRIDINIUM BETAINE DYE - UV-VIS SPECTROSCOPIC MEASUREMENTS AND QUANTUM-CHEMICAL CALCULATIONS

The ground- and excited-state dipole moments of a zwitterionic benzimi dazole-based pyridinium betaine dye 4,6-triphenylpyridinium-1-(1H-benz imidazol-2-ide), hereafter referred to as IB1] were determined, The gr ound-state dipole moment, calculated from measurements of the static e lectric permittivity of IB1 in 1,4-dioxane, exceeds 13 D units. The ch arge distribution in the molecule changes significantly upon excitatio n: the excited-state dipole moment, calculated from the solvatochromic shift of the low-energy UV-vis absorption band, is equal to ca. 3 D. The experimental results are supported by quantum-chemical calculation s, which indicate that both the molecular geometry and positions of th e energy levels are crucially influenced by solute-solvent interaction s. The vectorial part of the second-order hyperpolarizability of IB1 m olecules was estimated from the experimental data employing the two-st ate model: its off-resonance value amounts to ca. -15 x 10(-40) m(4)/V (-3.6 x 10(-30) esu).