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).

Results of measurements of the solvent-dependent shift of the low-energy absorption band solvatochromic effect in a benzimidazole-based betaine are reported in the paper. Measurements of absorption spectra in several solvents of different polarities were performed. The solvatochromic shift of the low-lying absorption band was found to exceed 3000 cm -1 ; the results obtained were then employed to calculate the ground-and excited-state dipole moment of the molecule. The spectroscopic measurements were supplemented by measurements of the ground-state dipole moment. A remarkable change in the charge distribution was found to occur upon the electronic excitation: both the experimental results and the calculations indicate that the ground-state dipole moment is close to 13 D, whereas in the excited state it amounts to ca. 3 D. The second-order hyperpolarizability of the molecule was calculated from the measurements; its off-resonance value amounts to ca. 20 x 10 -40 m4 /V (4.8 x 10 -30 esu), depending on the solvent used.

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