Insights into the excited state intramolecular proton transfer process and mechanism of the novel 3-hydroxythioflavone system: A theoretical study

It is well known that the molecular excited state dynamical process plays important roles in designing and developing novel applications. In this work, based on density functional theory and time-dependent density functional theory methods, we theoretically explored a novel 3-hydroxythioflavone (3HTF). Through calculating the electrostatic potential surface of the 3HTF structure, we confirm the formation of intramolecular hydrogen bonding O2-H3 center dot center dot center dot O4. Our theoretically obtained dominating bond lengths and bond angles involved in hydrogen bonds demonstrate that the intramolecular hydrogen bonds should be strengthened in the S-1 state. Coupling with the simulated infrared vibrational spectra, we further verify the enhanced hydrogen bonding O2-H3 center dot center dot center dot O4 in the S-1 state. Upon photoexcitation, we found that the charge transfer characteristics around hydrogen bonding moieties play important roles in facilitating the excited state intramolecular proton transfer (ESIPT) process. Via constructing potential energy curves in both S-0 and S-1 states, we confirm the almost nonbarrier ESIPT reaction should be an ultrafast process that further explains the previous experimental phenomenon. At last, we search the S-1-state transition state (TS) structure along with ESIPT path, based on which we simulate the intrinsic reaction coordinate path that further confirms the ESIPT mechanism. We hope that our theoretical work could guide novel applications based on the 3HTF system in future.