TD-DFT study of the excited-state potential energy surfaces of 2-(2'-hydroxyphenyl)benzimidazole and its amino derivatives.

In this study, we used TD-PBE0 calculations to investigate the first singlet excited state (S(1)) behavior of 2-(2'-hydroxyphenyl)benzimidazole (HBI) and its amino derivatives. We employed the potential energy surfaces (PESs) at the S(1) state covering the normal syn, tautomeric (S(1)-T(syn)), and intramolecular charge-transfer (S(1)-T(ICT)) states in ethanol and cyclohexane to investigate the reaction mechanisms, including excited-state intramolecular proton transfer (ESIPT) and intramolecular charge-transfer (ICT) processes. Two new S(1)-T(ICT) states, stable in ethanol and cyclohexane, were found for HBI and its amino derivatives; they are twisted and pyramidalized. The flat PES of the ICT process makes the S(1)-T(ICT) states accessible. The S(1)-T(ICT) state is effective for radiationless relaxation, which is responsible for quenching the fluorescence of the S(1)-T(syn) state. In contrast to the situation encountered conventionally, the S(1)-T(ICT) state does not possess a critically larger dipole moment than its precursor, S(1)-T(syn) state; hence, it is not particularly stable in polar solvents. On the basis of the detailed PESs, we rationalize various experimental observations complementing previous studies and provide insight to understand the excited-state reaction mechanisms of HBI and its amino derivatives.