Revised the excited-state intramolecular proton transfer direction of the BTHMB molecule: A theoretical study

The spectroscopic properties of 3-(benzo[d]-thiazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde molecule were investigated [J. Phys. Chem. A 2019, 123, 10246-10253]. The result shows that the excited-state intramolecular proton transfer (ESIPT) was driven toward the N center over the O center. In this research, the density functional theory and time-dependent density functional theory method were used to calculate molecule structures. Through our calculations, the ESIPT process toward N atom is proved to be feasible. Moreover, the emission peak we obtained of ESIPT process from the OH proton to aldehyde O atom is located at 564 nm, which is attributed to 500 nm in previous research. From the potential energy curves, the 0.35 kcal/mol energy barrier indicates the ESIPT process could occur when excited to S1 state from the OH proton to aldehyde O atom. In addition, the frontier molecular orbitals analysis and IR spectrum were also calculated. Finally, we revise the direction of BTHMB molecule, the two directions of ESIPT are both feasible when excited to S₁ state.