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

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.