Unravelling the solvent-dependent excited-state double proton transfer in (Oxa-OH)2 system: asymmetric barrier regulation by water fraction

Excited-state intramolecular proton transfer (ESIPT) is an essential excited-state relaxation pathway in photophysics, and excited-state double proton transfer (ESDPT) systems with dual hydrogen-bonding sites attract intensive research interest. Herein, a symmetric dihydroxy fluorene-oxazoline derivative (Oxa-OH)2 was chosen as the model molecule. Using DFT/TDDFT calculations incorporating the IEFPCM implicit solvent model, we systematically explored its ESDPT mechanism in cyclohexane-water mixtures with water fractions of 0%, 30%, 60%, and 90%. Herein, the water content in cyclohexane-water binary mixtures refers to volume fraction. Geometric parameters, IR spectra, AIM, ELF, electrostatic potential, and two-dimensional excited-state potential energy surfaces (PESs) were analyzed to uncover how water content tunes ESIPT pathways and energy barriers. Photoexcitation reinforces intramolecular hydrogen bonds, and added water further stabilises the hydrogen-bonded framework. Photoexcitation accumulates negative charge on nitrogen atoms, supplying strong electrostatic driving force for proton transfer. PESs verify that (Oxa-OH)2 follows a stepwise ESDPT mechanism governed by single proton transfer across all solvent conditions, where two hydrogen bonds respond differently to solvent polarity. This work quantifies the selective modulation of water fraction over individual steps of stepwise ESDPT, offering theoretical insights into ESDPT dynamics in mixed solvents and guiding the design of fluorescent probes for aqueous sensing.