Dipole Moments and Second-Order Hyperpolarizabilities of Imidazole-Based Betaines Determined from the Solvatochromic Effect

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Abstract
We report on results of measurements of the solvatochromic effect in two benzimidazole-based betaines (hereafter referred to as IB1 and IB2). The solvatochromic shifts exceed 2000 cm−1 in both molecules; however, the directions of the shift were found opposite in spite of similar chemical structures of the molecules under study. The ground-state dipole moments in both betaines exceed 13 Debye units, whereas the excited-state moments, calculated from the solvatochromic shift, differ markedly amounting to ca. 3 D in IB1 and 18 D in IB2. Quantum-chemical calculations indicate that betaines enjoy some freedom of orientation of their fragments which may influence the energies and intensities of the electronic transitions. The second-order hyperpolarizabilities, estimated from the measurements amount to ca. 20×10−40 m4/V (4.8×10−30 esu) for IB1 and 55×10−40 m4/V (13.8×10−30 esu) for IB2.