EFFECT OF THE SUBSTITUENTS ON THE SOLVATOCHROMIC BEHAVIOR OF BENZIMIDAZOLE-BASED BETAINE DYES

The solvatochromic effect was studied in the family of benzimidazole-b ased zwitrerionic betaine dye molecules. The groundstate dipole moment s of the molecules were determined from dielectric measurements; depen ding on the substituents, they ranged from ca. 6 to over 13 Debye unit s. A pronounced shift of the lowest-lying absorption band was found de pending on the polarity of solvents: the betaine dyes substituted with the dimethylamine group in the terminal phenyl ring exhibit a bathoch romic shift whereas the dye unsubstituted or substituted with the nitr o group exhibit a hypsochromic shift. The comparison of the experiment al results with the quantum-chemical calculations seems to indicate th at the reason for this difference rests in enhanced probability of spe cific interactions in the former betaines (possible formation of hydro gen bond to the imidazole nitrogen). (C) 1998 Elsevier Science B.V. Al l rights reserved.