An electronic structure investigation of excited state intramolecular proton transfer in amino-benzazole derivatives: Relative energies and electron density descriptors

Abstract

An electronic structure study of the excited-state intramolecular proton transfer (ESIPT) process of six amino-benzazole derivatives (4-AHBI, 4-AHBO, 4-AHBT, 5-AHBI, 5-AHBO, and 5-AHBT) based on Density Functional Theory (DFT) is carried out in gas phase and also in dichloromethane. The energies, geometries and electronic excitations are initially evaluated, being followed by electron density descriptor investigations done to provide a better understanding of the hydrogen bond (HB) strength of enol (N···H–O), EI, and keto (O···H–N), KI, forms of these molecules. First, one can notice that the solvent effect is fundamental to explain the ESIPT process in 5-AHBO and 5-AHBT due partially to a covalent character increment of these N–H bonds in KI, while gas phase results obtained in the first excited state indicate that the enol tautomer is instead more stable than the keto one. The time-dependent DFT (TDDFT) values achieved support a reevaluation of the experimental data available, showing that ESIPT is really observed for all these compounds in dichloromethane. The most important effect of the solvent on HBs seems to be a destabilization of O···H interactions in the KI form, which is observed in both the electronic states (S0 and S1). The electronic excitation also appears to induce a weakening of O···H bonds in the KI tautomer along with some strengthening of N···H interactions in EI. Thus, this HB force variation profile does not favor the ESIPT occurrence and one must consider instead the effect of changes in the covalent bonds on excitation to understand why the KI tautomer becomes more stable than EI in the S1 state, such as supported by evidence regarding the weakening of the O–H bonds and the strengthening of the N–H ones driven by the electronic transition.