Theoretical Investigation on Proton Transfer Directionality and Dynamics Behavior of 3-(Benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde with Two Asymmetric Proton Acceptors

A detailed theoretical investigation on the excited state intramolecular proton transfer (ESIPT) directionality and dynamics behavior of 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) with two unsymmetric proton acceptors (N and O₂) has been performed. The hydrogen bond O₁–H···N in BTHMB-a formed by the O₁–H group with the N atom or O₁–H···O₂ in BTHMB-b formed by the O₁–H group with the O₂ atom is enhanced upon photoexcitation, and the strength of the O₁–H···N bond is stronger, which will drive the O₁–H proton to the N atom. Potential energy curves further confirm that ESIPT occurs in the N atom because of the smaller energy barrier (0.39 kcal/mol). Results of dynamics simulations manifest that no surface hopping exists between the S₀ and S₁ states within 300 fs, and ESIPT time constants of BTHMB-a and BTHMB-b are 48 and 151 fs, respectively. While the reverse ESIPT is observed in BTHMB-b at 294 fs, implying that the O₁–H proton is transferred to the N atom instead of the O₂ atom. The consistency of the calculated absorption (390 nm) and fluorescence spectra (443 and 602 nm) of BTHMB-a with the experimental values (390, 410, and 605 nm) confirms this conclusion again. The charge distribution analysis shows that the charge on the proton acceptors increases, and the O₂ atom has higher electronegativity because it has more negative charges. The minimum surface electrostatic potential on the N atom in BTHMB-b correlating with the pKb value is −47.38 kcal/mol, indicating that the N atom has strong basicity. Therefore, the basicity of the N atom dominates the ESIPT process rather than the electronegativity of the O₂ atom.