Solvatochromic Effect in a Benzimidazole-Based Betaine: Determination of the Dipole Moments and Second-Order Hyperpolarizability

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$\text{}^{-1}$; 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 × 10$\text{}^{-40}$ m$\text{}^{4}$/V (4.8 × 10$\text{}^{-30}$ esu), depending on the solvent used.