Excited-state intramolecular proton transfer in 2-(2 '-acetamidophenyl)benzimidazole

An excited-slate intramolecular proton-transfer process has been studied in 2-(2'-acetamidophenyl)benzimidazole in different solvents using steady-sta te and time-resolved fluorescence spectroscopy. Semiempirical quantum mecha nical calculations have also been carried out. Dual fluorescence (normal an d tautomer fluorescence) is observed in all the solvents. On the basis of t he fluorescence excitation spectra recorded at different wavelengths and th e lifetime data, it is concluded that the normal fluorescence originates fr om the rotamer trans-II in aprotic solvents and the rotamers trans-II and t rans-IV in protic solvents. The tautomer emission originates from the tauto mer trans-m in aprotic solvents and tautomers trans-III and trans-III' in p rotic solvents. The presence of an adjacent acetyl group increases the acid ity of the N-H (amide proton) bond present in the phenyl ring. This leads t o the faster rate of proton transfer in the S-1 state. As a result of this, the quantum yield of the tautomer band increases noticeably. With the incr ease of excitation wavelengths, the ratio of tautomer to normal emission in creases in all the solvents. Semiempirical quant-um 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 sol vation energy is included. Under isolated conditions the activation energy for the interconversion of rotamers is 2.9 kJ mol-(1) in the ground state a nd 69.2 kJ mol-(1) in the first excited singlet state.