Excited-State Intramolecular Proton Transfer in 2-(2‘-Acetamidophenyl)benzimidazole

Abstract

An excited-state intramolecular proton-transfer process has been studied in 2-(2‘-acetamidophenyl)benzimidazole in different solvents using steady-state and time-resolved fluorescence spectroscopy. Semiempirical quantum mechanical calculations have also been carried out. Dual fluorescence (normal and tautomer fluorescence) is observed in all the solvents. On the basis of the fluorescence excitation spectra recorded at different wavelengths and the lifetime data, it is concluded that the normal fluorescence originates from the rotamer trans-II in aprotic solvents and the rotamers trans-II and trans-IV in protic solvents. The tautomer emission originates from the tautomer trans-III in aprotic solvents and tautomers trans-III and trans-III‘ in protic solvents. The presence of an adjacent acetyl group increases the acidity of the N−H (amide proton) bond present in the phenyl ring. This leads to the faster rate of proton transfer in the S1 state. As a result of this, the quantum yield of the tautomer band increases noticeably. With the increase of excitation wavelengths, the ratio of tautomer to normal emission increases in all the solvents. Semiempirical quantum mechanical calculations have shown that the rotamer trans-I is more stable than the rotamer trans-II in the ground state, both under isolated conditions and when dipolar solvation energy is included. Under isolated conditions the activation energy for the interconversion of rotamers is 2.9 kJ mol-1 in the ground state and 69.2 kJ mol-1 in the first excited singlet state.