Ion-specific structural effects on excited-state proton transfer dynamics in aqueous deep eutectic solvents

Aqueous deep eutectic solvent (DES) mixtures have gained considerable attention for their emergent properties not seen in the neat components. Even small amounts of water can strongly reorganize the DES microstructure and alter its molecular environment. Herein we investigated the excited-state proton transfer (ESPT) reaction of a photoacid, 8-hydroxy-pyrene-1,3,6-trisulfonic acid (HPTS), in aqueous mixtures (30% v/v) of acetamide-salt based DES using steady-state and time-resolved fluorescence spectroscopy. Experimental results indicate that the presence of water in DESs facilitates ESPT, which is otherwise absent or negligible in neat DESs. To understand the influence of distinct microstructural morphologies on photochemical reaction rates, we systematically varied the salt cations and anions in DES compositions. Both steady-state spectra and time-resolved decay profiles show that the nature of the salt cation/anion present in aqueous DES (ADES) significantly influences the proton transfer process. TRANES (time-resolved area normalised emission spectra) analysis show that proton transfer time ranges from 2.7 to 11.8 ns in these ADES. It increases markedly when the K⁺ ion in the system is replaced by Na⁺, while the anion (SCN^-) remains the same. It also reveals that substituting the SCN^- ion with ClO₄^- significantly prolongs the reaction time when the cation (Na⁺) in the ADES remains unchanged. Moreover, the ESPT time is substantially extended when Na⁺ in the ADES is replaced by Li⁺, with ClO₄^-as the common anion. Overall, the time scale of the ESPT process increases with decreasing cation size in ADES, following the trend K⁺ < Na⁺ < Li⁺, and with anions it follows the trend SCN^- < NO₃^- < ClO₄^-, in good agreement with the steady-state fluorescence. Together, these results highlight how DES-constituting cations and anions interact specifically with the added water, thereby modulating its ability to accept protons from the excited photoacid.