Molecular Principles of the Excited-State Intramolecular Thiol Proton Transfers in 3-thiolflavone Derivatives

The excited-state intramolecular proton transfer (ESIPT) effect has attracted considerable attention due to its potential applications in photoluminescent materials. However, only a few theoretical reports have investigated the ESIPT process involving sulfur-hydrogen bonds. Herein, we systematically investigated the ESIPT effect of three 3-thiolflavone derivatives containing sulfur-hydrogen bonds with M06-2X functional combined Def2-TZVP basis set. The intramolecular sulfur-hydrogen bonds were confirmed in the ground and excited states via analyzing the bond lengths, interaction energies, and infrared vibrational spectra. Besides, we demonstrated that the electron-withdrawing group led to a more stable tautomer compared to the electron-donating group. Conversely, the electron-donating group played a crucial role in reducing the energy barrier of the ESIPT reaction due to the strengthening of the hydrogen bond in the excited state. Interestingly, the substituent group can determine the excited-state electronic properties of keto tautomers. Specifically, the electron-withdrawing group caused significant improvement in the nπ* transition configuration, significantly reducing the radiation rate. The electron-donating group increased the proportion of the ππ* transition configuration, thus, the keto tautomer had bright emission. We expect these findings to open new avenues for designing potential luminescent materials.