Imine-Amine Tautomerism vs Keto-Enol Tautomerism: Acceptor Basicity Dominates Over Acceptor Electronegativity in the ESIPT Process through a Six-Membered Intramolecular H-Bonded Network.

Photophysical properties of a synthesized asymmetric two-way proton transfer molecule 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) were studied in detail. BTHMB could undergo excited-state intramolecular proton transfer (ESIPT) involving a 6-membered H-bonded network toward either the N or the O atom present in the molecule as proton acceptors. From tedious spectroscopic dissection, however, it was established that ESIPT was driven exclusively toward the N center over the O center in the solid state as well as in the solution phase. The aforementioned deduction was based on spectral comparison with judicially designed molecules 2-hydroxy-5-methoxybenzaldehyde (HMB) and 2-(benzo[d]thiazol-2-yl)-4-methoxyphenol (BTMP). In solution, the coexistence of the anionic and neutral forms of BTHMB in basic solvents dimethylsulfoxide (DMSO) and dimethylformamide (DMF) enables BTHMB to behave as a white light emitter. Thus, apart from establishment of directionality of ESIPT in double ESIPT probes, the current work deserves special merit as BTHMB can be considered as a standard in future designs involving red light and solvent-specific white light emitters.