Mechanistic insights into charge-transfer quenching and low-barrier ESIPT in an H2S-responsive fluorescent probe: a DFT/TD-DFT study.

Hydrogen sulfide (H2S)-responsive fluorescent probes are valuable tools for biological sensing, but the molecular origin of their fluorescence switching is often difficult to determine experimentally. Herein, the "OFF-to-ON" mechanism of the ESIPT-based H2S probe DTBH and its product DTB was investigated using DFT and TD-DFT calculations. Absorption and fluorescence properties were simulated in acetonitrile, tetrahydrofuran, and toluene, and the excited-state processes were analyzed using frontier molecular orbitals, hole-electron distributions, IGMH analysis, potential-energy curves, transition-state optimization, IRC calculations, and kinetic estimation. DTBH is nearly non-emissive because photoexcitation generates an excited state with pronounced charge-transfer character and structural relaxation, thereby suppressing radiative decay. After H2S-triggered deprotection, DTB restores an ESIPT-active scaffold and emits predominantly from the keto excited-state tautomer, with Keto* emission maxima differing by less than 4 nm across the three solvents. A possible anti-Kasha S2 → S0 emission pathway was also identified for Keto*-DTB in toluene. Geometric parameters, IR red shifts, and IGMH analyses reveal strengthened O-H···N hydrogen bonding in S1, while PEC, TS, and IRC results confirm a low-barrier ESIPT pathway. These findings clarify the photophysical basis of DTBH fluorescence turn-on.