Theoretical Investigation on Proton Transfer Directionality\nand Dynamics Behavior of 3‑(Benzo[<i>d</i>]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde\nwith Two Asymmetric Proton Acceptors

A detailed\ntheoretical investigation on the excited state intramolecular\nproton transfer (ESIPT) directionality and dynamics behavior of 3-(benzo[<i>d</i>]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB)\nwith two unsymmetric proton acceptors (N and O<sub>2</sub>) has been\nperformed. The hydrogen bond O<sub>1</sub>–H···N\nin BTHMB-a formed by the O<sub>1</sub>–H group with the N atom\nor O<sub>1</sub>–H···O<sub>2</sub> in BTHMB-b\nformed by the O<sub>1</sub>–H group with the O<sub>2</sub> atom\nis enhanced upon photoexcitation, and the strength of the O<sub>1</sub>–H···N bond is stronger, which will drive the\nO<sub>1</sub>–H proton to the N atom. Potential energy curves\nfurther confirm that ESIPT occurs in the N atom because of the smaller\nenergy barrier (0.39 kcal/mol). Results of dynamics simulations manifest\nthat no surface hopping exists between the S<sub>0</sub> and S<sub>1</sub> states within 300 fs, and ESIPT time constants of BTHMB-a\nand BTHMB-b are 48 and 151 fs, respectively. While the reverse ESIPT\nis observed in BTHMB-b at 294 fs, implying that the O<sub>1</sub>–H\nproton is transferred to the N atom instead of the O<sub>2</sub> atom.\nThe consistency of the calculated absorption (390 nm) and fluorescence\nspectra (443 and 602 nm) of BTHMB-a with the experimental values (390,\n410, and 605 nm) confirms this conclusion again. The charge distribution\nanalysis shows that the charge on the proton acceptors increases,\nand the O<sub>2</sub> atom has higher electronegativity because it\nhas more negative charges. The minimum surface electrostatic potential\non the N atom in BTHMB-b correlating with the pKb value is −47.38\nkcal/mol, indicating that the N atom has strong basicity. Therefore,\nthe basicity of the N atom dominates the ESIPT process rather than\nthe electronegativity of the O<sub>2</sub> atom.