Revelation of ESIPT mechanism for the novel fluorescent system HNIBT in toluene and methanol solvents: A TDDFT study

In this work, we devote to explore excited‐state intramolecular proton transfer (ESIPT) behavior for a novel fluorescent molecule naphthalimide‐based 2‐(2‐hydroxyphenyl)‐benzothiazole (HNIBT) New J. Chem. 2019, 43, 9152.] in toluene and methanol (MeOH) solvents. Exploring weak interactions, stable HNIBT‐enol, and HNIBT‐MeOH‐enol complex can be found in S₀ state via TDDFT/B3LYP/6‐311+G(d,p) level. Given photoexcitation, intramolecular hydrogen bond O1? H2···N3 of HNIBT‐enol and HNIBT‐MeOH‐enol is dramatically enhanced, which offers impetus for facilitates ESIPT reaction. After repeated comparisons, we verify the unavailability of intermolecular hydrogen bonding effects between HNIBT‐enol and MeOH molecules. In view of excitation, HOMO (π) → LUMO (π*) transition and the changes of electronical densities indeed impulse ESIPT tendency. Via constructing potential energy curves (PECs), for both HNIBT‐enol and HNIBT‐MeOH‐enol complex, the ESIPT could only occur along with intramolecular hydrogen bond O1? H2···N3. Through comparison, the potential barrier falls from 4.124 kcal/mol (HNIBT‐enol) to 2.132 kcal/mol (HNIBT‐MeOH‐enol). Therefore, we confirm that the ESIPT of the HNIBT system happens more easily in the MeOH solvent compared with the toluene solvent.