Excited state proton transfer (ESPT) reaction of a photoacid has been performed in tetraethylammonium bromide (TEAB) based deep eutectic solvents (DESs) confined in polyether sulfone membranes with a pore size of ~ 350 nm and in bulk media. For this purpose we have used 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) photoacid and three DESs constituted of TEAB with various hydrogen bond donors (HBD), ethylene glycol, glycerol, and 1,3-propanediol. The photoprotolytic process has been studied using steady-state and time-resolved fluorescence spectroscopy. We found that both protonated and deprotonated forms of the photoacid exhibit fluorescence in the confined environment of DESs, however, only the protonated form shows fluorescence in bulk media. Moreover, the fluorescence intensity decay of the photoacid fits with a bi-exponential function of time (~ 1.5 and 4 ns) in the confined DESs whereas it fits with a single-exponential function of time (~ 3 ns) in bulk liquids. Therefore, both steady-state and time-resolved fluorescence studies confirm that ESPT is a favorable process in the confined DESs although the pores of the confinement are large but it is an unfavorable process in the bulk environments. We have also carried out an excitation wavelength-dependent fluorescence study of a solute, coumarin 153, in the confined as well as in bulk DESs and observed a significant red-edge excitation shift (~600 cm<sup>-1</sup>) in the confined environments, while a negligible shift (&lt; 100 cm<sup>-1</sup>) has been seen in bulk media. This indicates that the DESs confined in the polyether sulfone membranes are more heterogeneous than in the bulk environments and suggests that the extent of microheterogeneity plays an important role in controlling the rate of ESPT in confined DESs. We have also found that the rate of proton transfer is modulated when the hydrogen bond donor of DES was changed from ethylene glycol to glycerol, and then to 1,3-propanediol.

Excited state proton transfer processes in bulk and confined deep eutectic solvents

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