DFT-TDDFT investigation of excited-state intramolecular proton transfer in 2-(2′-hydroxyphenyl)benzimidazole derivatives: Effects of electron acceptor and donor groups

The excited-state intramolecular proton transfer (ESIPT) reactions of 2-(2′-hydroxyphenyl)benzimidazole (HBI) derivatives were investigated using time-dependent density functional theory (TD-DFT) method at B3LYP/TZVP theoretical level. The geometric parameters, infrared (IR) vibrational spectra, frontier molecular orbitals (MOs), Mulliken charge distribution analysis, natural bond orbital (NBO) analysis and potential energy curves were calculated to provide the direct information about the effect of electron acceptor and donor groups on the ESIPT reactions. The intramolecular hydrogen bonds are significantly strengthened and the electronic density is redistributed after a vertical excitation to S1 state. The proton transfer (PT) reactions are unlikely occur in S0 state through the analysis of potential energy curves and the Hartree-Fock energy of the stable structures. The presence of the electron acceptor NO2 group can facilitate the ESIPT reaction, while the presence of electron donor NH2 group can hinder the ESIPT reaction. In general, the electron acceptor NO2 group and electron donor NH2 group can influence the ESIPT reactions in a completely different way.