A Comprehensive Investigation into the Mechanism Governing the ESDPT Behavior of Bis-3,6-(2-benzoxazolyl)pyrocatechol Regulated by Solvent Effects and Chalcogen Substitutions

In this study, we investigated bis-3,6-(2-benzoxazolyl)pyrocatechol (BBPC) derivatives with symmetric structures and their intramolecular hydrogen bonds. We systematically explored the regulation mechanism of solvent polarity and the electronegativity of chalcogen elements on the excited-state double proton transfer (ESDPT) behavior of BBPC derivatives (BBPC-O, BBPC-S, and BBPC-Se). By selecting hexane, chloroform, and acetonitrile as surrounding solvents, it was found that polar solvents can significantly enhance the intramolecular O₁-H₂···N₃ and O₄-H₅···N₆ double hydrogen bond interactions of BBPC, providing a key driving force for the ESDPT process. Regarding the substitution effect of chalcogen elements (O, S, and Se), the study combined multiple characterization methods and found that as the atomic electronegativity decreases, the intramolecular hydrogen bond interaction in the S₁ state gradually increases. This conclusion was consistently verified through the calculation of geometric parameters, infrared (IR) vibration frequencies, and charge distributions. Frontier molecular orbital (MO) analysis showed that the HOMO/LUMO distribution of BBPC derivatives systematically shifts with the change in the electronegativity of chalcogen elements, providing an electronic structure-level driving force for the ESDPT process. The results of the potential energy surface construction and reaction path comparison indicated that the ESDPT process is dominated by stepwise proton transfer (I → II → III), and a decrease in the electronegativity of chalcogen elements results in a systematic drop in the energy barriers of the stepwise reactions.