pH-Dependent excited-state proton transfer characteristics in 2-acetyl benzimidazole and 2-benzoyl benzimidazole in aqueous and non-aqueous media

Interesting pH-dependent excited-state proton transfer reactions in the forms of ionic species and open conformer of 2-acetyl benzimidazole (2ABI) and 2-benzoyl benzimidazole (2BBI) in hydrocarbon, aqueous and alcoholic solutions at room temperature and 77 K have been reported. Increase of acidity and basicity of solution, respectively, results in increment and decrement of emission of ionic species with shrinking and intensification of zwitterionic species. A cationic species, protonated at the benzimidazole moiety could be observed under acidic conditions in all solvents considered. Mono- and di-cation species in the excited-state were found to form in 2ABI with progressive addition of acid in methanol solution as shown by formation of a new band initially and its sharp intensification thereafter. Complete extinction of anion band along with formation of a blue-shifted band in 2BBI evince that protonation rate seems to be larger in 2BBI than in 2ABI in acidic aqueous solution. Phosphorescence for both the molecules at 77 K originates from the open conformers. It is interesting to note that less conjugation of phenyl ring with benzimidazole ring in 2BBI facilitates the benzoyl group rotation to form the open conformer compared to 2ABI.