Theoretical insights into fluorescent properties and ESIPT behavior of novel flavone-based fluorophore and its thiol and thione derivatives.

For the typical ESIPT process, the proton transfer process is often completed via the intramolecular hydrogen bond (IHB) with oxygen or nitrogen as proton donor or proton acceptor. In recent years, the ESIPT process for sulfur-containing hydrogen bonds has received more and more attention, but it has been rarely reported. We systematically studied the ESIPT processes and photophysical properties of 2-(benzothiophene-2-yl)-3-hydroxy-4H-chromen-4-one (BTOH), 2-(benzothiophene-2-yl)-3-mercapto-4H-chromen-4-one (BTSH) and 2-(benzothiophen-2-yl)-3-hydroxy-4H-chromene-4-thione (BTS) at the HISSbPBE/6-31+G(d,p) and TD-HISSbPBE/6-31+G(d,p) computational level. The IHBs were investigated by analyzing structural parameters, infrared (IR) spectra and electron densities. All the results showed that the IHBs become stronger in the excited state. Among these three molecules, the ESIPT energy barrier of BTSH is the lowest, while that of BTS is the highest. By calculating the natural population analysis (NPA) charge, we found that SH group as a proton donor is easier to provide protons than OH group, and the S group as a proton acceptor is more difficult to obtain protons than O group. The simulated electronic spectra showed that the absorption and fluorescence wavelengths of BTSH and BTS have more or less red-shift compared with BTOH.