Excited-State Proton Transfer Dynamics in Betaine-Based Deep Eutectic Solvents: Is the Reaction Controlled by Solvent Relaxation?

Despite the growing interest in deep eutectic solvents (DESs), studies involving excited-state photochemical dynamics in such media are very scarce. Herein, we investigated the excited-state proton transfer (ESPT) of a photoacid (HPTS) in betaine (BET)-based DESs with systematically varied hydrogen-bond donors (HBD): ethylene glycol (EG), 1,2-propanediol (PD), and glycerol (Gly), using steady-state and time-resolved fluorescence spectroscopy. The results confirm strong ESPT in these DESs; the time-resolved fluorescence displays multiexponential decay behavior, and the initial proton-transfer and dissociation times are 0.17, 0.43, and 0.63 ns, and 0.51, 0.78, and 0.93 ns, respectively, in BET-EG, BET-PD, and BET-Gly systems. Time-resolved emission spectra show decay of the protonated state and growth of the deprotonated state without an iso-emissive point, accompanied by a continuous red shift in both emissions, indicating that ESPT and solvation occur on comparable time scales. The solvation dynamics of both states of the photoacid exhibit biphasic relaxation, with components of a few hundred picoseconds and a few nanoseconds. Similar solvation time scales, 0.10-0.30 ns and 0.94-2.36 ns for the nonproton-transferring analogue, and 0.10-0.20 ns and 0.40-0.80 ns for a standard solute (C153) demonstrate that ESPT in these DESs is governed by solvent reorganization. Temperature-dependent measurements reveal concurrent acceleration of both solvation and ESPT rate, with the strongest temperature sensitivity in BET-Gly and the weakest in BET-EG. These findings demonstrate that the photochemical reactivity of HPTS is regulated by the solvation-shell dynamics, and that tuning the HBD component of a DES provides a rational strategy for modulating proton transport in these novel media.