Solvent polarity-tuned excited-state double proton transfer of the centrosymmetric HDJ fluorophore: a DFT/TD-DFT computational exploration

Solvent- modulated excited- state double proton transfer (ESDPT) of the centrosymmetric HDJ fluorophore was systematically explored via DFT and TD- DFT calculations within cyclohexane, dichloromethane and acetonitrile. Geometric alterations, electron- localisation- function- derived CVB indices, QTAIM- BCP electronic densities, simulated IR profiles and excited- state potential- energy surfaces (PES) collectively verified solvent- dependent intramolecular hydrogen- bond reinforcement upon photoexcitation. Raised solvent polarity shortens proton- transfer barriers and stabilises single-proton- transfer (SPT) and double- proton- transfer (DPT) tautomers. Frontier orbital and vertical excitation analyses uncovered polarity- governed electronic redistribution underlying stepwise ESDPT pathways. This work delivers atomistic understanding for tuning multi- tautomer photophysics of ESDPT- active π- extended fluorophores.