Computational Exploration for Revealing Tunable ESIPT/ESDPT Behavior for TrySPy Fluorophore in Solvents with Various Polarities

The asymmetric dual-hydrogen-bond system TrySPy exhibiting ESIPT characteristics has attracted intramolecular double proton transfer (ESIDPT) mechanism under varying solvent polarities (cyclohexane, toluene, THF, acetonitrile) using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Through comparison of hydrogen bond-related parameters, we confirmed that dual hydrogen bonds of TrySPy are strengthened in the S₁ state. Furthermore, we discovered opposite responses of the two hydrogen bonds to solvent polarity in the excited state: O₁-H₂···N₃ strengthens while N₄-H₅···N₆ weakens with increasing polarity. Frontier orbital analysis demonstrates dominant HOMO-LUMO transition (>97%) driving charge redistribution from proton donors to acceptors. Potential energy surface calculations identify the stepwise I → III → IV pathway as dominant for double proton transfer, but I → II as dominant for single proton transfer with O₁-H₂···N₃ transfer exhibiting lower barriers than N₄-H₅···N₆. Solvent polarity differentially modulates emission spectra, inducing redshift in native form fluorescence and blue shift in tautomer emission. This work establishes solvent polarity as a precise regulator for proton transfer dynamics in TrySPy, an asymmetric dual-hydrogen-bond system.