Absorptiometric and fluorimetric study of solvent dependence and prototropism of 2-substituted benzimidazole derivatives

Abstract

A study of the effects of solvents on the absorption and fluorescence spectra of 2-substituted benzimidazoles has shown that the lowest-energy transition of the chloromethyl, dichloromethyl, and cyanomethyl derivatives is of π–π* character, and that of the 5-chloro-2-trichloromethyl, 2-trifluoromethyl, and 2-chloro derivatives is of charge-transfer character. In the case of the monocations and monoanions of these compounds, the lowest-energy transitions are of charge-transfer and π–π* nature respectively. The presence of the methylene group in between the benzimidazole and the heteroatom substituent reduces direct interaction; this is apparent from the study of proton-transfer reactions as well as from the spectral changes. The pK_a values for these reactions in the ground and excited states have been determined and discussed. Acetonitrile quenches the fluorescence of the chloromethyl, dichloromethyl, and trichloromethyl compounds. The mechanism could involve charge-transfer complex formation.