Effect of the substituents on the solvatochromic behaviour of benzimidazole-based betaine dyes

The solvatochromic effect was studied in the family of benzimidazole-based zwitterionic betaine dye molecules. The ground-state dipole moments of the molecules were determined from dielectric measurements; depending on the substituents, they ranged from ca. 6 to over 13 Debye units. A pronounced shift of the lowest-lying absorption band was found depending on the polarity of solvents: the betaine dyes substituted with the dimethylamine group in the terminal phenyl ring exhibit a bathochromic shift whereas the dye unsubstituted or substituted with the nitro group exhibit a hypsochromic shift. The comparison of the experimental results with the quantum-chemical calculations seems to indicate that the reason for this difference rests in enhanced probability of specific interactions in the former betaines (possible formation of hydrogen bond to the imidazole nitrogen).