Highly water-tolerant ESIPT fluorophore BP(OH)₂DCEt₂: A theoretical study in DMSO-water binary solvents

Excited state intramolecular proton transfer (ESIPT) fluorescent molecules have demonstrated broad application prospects in fields of fluorescent probes, biological imaging and optoelectronic devices due to their unique advantages such as large Stokes shifts and negligible self-absorption properties. However, most ESIPT molecules are prone to fluorescence quenching in aqueous environments due to the disruption of intramolecular hydrogen bonds, which has severely limited their practical applications. Herein, the high-performance ESDPT fluorescent molecule diethyl 3,3'-dihydroxy-[2,2'-bipyridine]-5,5'-dicarboxylate (BP(OH)₂DCEt₂) is systematically studied. The S₀ and S₁ properties of the molecule in DMSO-water binary mixtures with water contents ranging from 0% to 90% are investigated using DFT and TDDFT methods in conjunction with the IEFPCM implicit solvent model. Theoretical calculations reveal that the two equivalent intramolecular O-H···N hydrogen bonds of 3,3'-dihydroxy-[2,2'-bipyridine]-5,5'-dicarboxylate are strengthened in S₁ state, providing a driving force for proton transfer. All geometric and electronic structure parameters show high stability over the broad range of water fractions examined. The ESDPT reaction follows a stepwise mechanism, characterized by an exceptionally low energy barrier (<0.4 kcal/mol) for the initial proton transfer step, which occurs essentially instantaneously, and this guarantees the high efficiency of the ESIPT process. Furthermore, with increasing water content, all proton transfer pathway barriers exhibit a downward trend, which confers a modest promotional influence on the reaction kinetics. The present study systematically elucidates the underlying mechanism of the excellent water-resistant ESIPT performance of 3,3'-dihydroxy-[2,2'-bipyridine]-5,5'-dicarboxylate, and provides a solid theoretical foundation for its practical applications in aqueous environments.