Insight into solvation-regulated emission: Dissecting the switchable ESIPT/ESPT mechanisms in HNT molecule

As a crucial physical and chemical process, excited-state proton transfer has garnered significant interest among researchers due to its broad applications in photobiology. Recently, Wu et al. synthesized 3-(benzo[d]thiazol-2-yl) naphthalene-2-ol (HNT) molecule that exhibits excited-state intramolecular proton transfer (ESIPT) characteristic in THF while undergoes excited-state intermolecular proton transfer (ESPT) in aqueous solvent, which induce its solvent-regulated emission nature [Dyes and Pigments 182 (2020) 108665]. However, the dynamic interactions between the proton transfer mechanisms and the emission properties of the HNT molecule are yet to be fully elucidated. Specifically, the role of solvation effects in regulating inter- and intramolecular proton transfer, which leads to modifications in fluorescence properties, should be in-depth considered. In the present work, we delved into the ESIPT dynamics of the HNT molecule in aprotic THF, and examined the ESPT in protic aqueous solvent. Our findings indicate that the ESIPT process promotes the intersystem crossing process, which diminishes Keto* emission in THF solvent. Furthermore, the ESPT process within the HNT-water cluster accounts for the observed dual-fluorescence in aqueous solvent. This research illuminates the role of solute–solvent interactions in affecting the ESIPT/ESPT pathways, thereby clarifying the nature of HNT emission and providing an advantageous methodology for engineering novel dyes with customizable fluorescence characteristics.