Solvatochromism of a Zwitterionic Benzimidazole-Based Pyridinium Betaine Dye: UV−Vis Spectroscopic Measurements and Quantum-Chemical Calculations

Abstract

The ground- and excited-state dipole moments of a zwitterionic benzimidazole-based pyridinium betaine dye [2,4,6-triphenylpyridinium-1-(1H-benzimidazol-2-ide), hereafter referred to as IB1] were determined. The ground-state dipole moment, calculated from measurements of the static electric permittivity of IB1 in 1,4-dioxane, exceeds 13 D units. The charge distribution in the molecule changes significantly upon excitation: 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 calculations, which indicate that both the molecular geometry and positions of the energy levels are crucially influenced by solute−solvent interactions. The vectorial part of the second-order hyperpolarizability of IB1 molecules was estimated from the experimental data employing the two-state model: its off-resonance value amounts to ca. −15 × 10-40 m4/V (−3.6 × 10-30 esu).