Proton transfer reactions of benzimidazole in ionic micelles in the excited singlet state

Abstract

The absorption and fluorescence spectra of benzimidazole (BI), benzimidazole cation (BI +) and benzimidazole monoanion (BI −) were studied in ionic detergents. The results indicate that the spectral characteristics of BI are hardly affected in sodium dodecylsulphate (SDS) and are slightly red shifted in cetyl trimethylammonium bromide (CTAB). The absorption band maxima of BI + and BI − are red shifted and the fluorescence band maxima of BI + and BI − are blue shifted in SDS and CTAB respectively. The fluorescence quantum yields of BI + and BI − increase in SDS and CTAB respectively compared with the values in aqueous medium. The fluorescence quenching of BI in cetyl trimethylammonium chloride (CTAC) and CTAB clearly shows that BI is bound at the micellar interface and that the effective concentration of chloride and bromide ions is fairly high at the interface. The distribution of BI in SDS was determined from the fluorescence quenching induced by bromide and iodide ions. The binding constants of the different species to the micelles were calculated. The p K a values for the protonation of BI in SDS and deprotonation of BI in CTAB were evaluated in the ground and excited singlet states.