Regulating excited-state intramolecular proton transfer behavior and fluorescent property of 3-thiolflavone by electron-donating ability: A time-dependent density functional theory study

Background: Since the excited-state intramolecular proton transfer (ESIPT) process involving sulfur is extremely fast, its mechanism is still unclear, and its derivatives are rarely studied, we have performed a theoretical study the ESIPT process of 3-thiolflavone (3TF) and its derivatives.Objective: The ESIPT mechanism and photophysicalproperties of 3-thiolflavone (3TF) and its derivatives 3NTF, 3ATF and 3OTF modified by -N(C2H5)(2), -NH2 and -OCH3 groups were studied theoretically.Methods: Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) with PBE0 functional and 6-31G(d, p) basis set.Results: For 3TF, 3NTF, 3ATF and 3OTF, the ESIPT processes are barrier-less, and the barrier of ground-state intramolecular proton transfer (GSIPT) reduces in the order of 3TF > 3OTF > 3ATF > 3NTF, as the electron-donating ability of the substituent increases. When the electron-donating groups are introduced, the absorption and fluorescence wavelengths of 3TF red-shift and blue-shift, respectively.Conclusion: Substituents with electron-donating ability can regulate the ground-state intramolecular proton transfer process and photophysical properties of 3TF.