In this Study, we used TD-PBEO calculations to investigate the first singlet excited state (SI) behavior of 2-(2'-hydroxyphenyl)benzimidazole (HBI) and its amino derivatives. We employed the potential energy surfaces (PESs) at the S, 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 radiation less relaxation, which is responsible for quenching the fluorescence of the S(1)-T(ICT) 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) stated 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.
