Long-range proton transfer mechanism and fluorescence properties of HQBT chromophore: TD-DFT/CASSCF study

Abstract

Herein, the intramolecular long-range proton transfer reaction mechanism of the HQBT chromophore in different solvents is investigated employing density functional theory and complete active space self-consistent field methods. The results show that HQBT successfully undergoes the first excited state intramolecular proton transfer (ESIPT) under photoexcitation. Subsequently, through the first cis–trans isomerization process of dihedral angle torsion, a minimum energy conical intersection (MECI) is formed between the ground state and the first excited state. The MECI continues the second cis–trans isomerization to generate the trans-keto structure. At this point, a new intramolecular hydrogen bond is formed and experiences the second ESIPT, thus achieving long-range transport of hydrogen protons. This careful theoretical research has significant guiding significance for the design of intelligent and efficient tautomeric molecular switches in the future.