Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Cho line Chloride plus Urea) Melts

Dynamic fluorescence anisotropy and Stokes shift measurements of [f choline chloride + (1 - f) urea)] deep eutectic solvents at f = 0.33 and 0.40 have been carried out using a dipolar solute, coumarin 153 (C153), in the temperature range 298 <= T <= 333 K. Subsequently, measured time-dependent solvent response is utilized to investigate the dynamic solvent control on the measured rates of photoexcited intramolecular charge transfer (ICT) reactions of two molecules, 4-(1-azetidinyl)benzonitrile (P4C) and 4-(1-pyrrolidinyl)benzonitrile (P5C), occurring in these media. Measured average reaction time scales ()) exhibit the following dependence on average solvation times scales ()): ) alpha (a) with a = 0.5 and 0.35 for P4C and P5C, respectively. Such a strong dynamic solvent control of ?trxn?, particularly for P4C, is different from earlier observations with these ICT molecules in conventional molecular solvents. Excitation wavelength-dependent fluorescence emissions of C153 and trans-2-[4-(dimethylamino)styryl]-benzothiazole (DMASBT), which differ widely in average fluorescence lifetimes (), suggest the presence of substantial spatial heterogeneity in these systems. Dynamic heterogeneity is reflected via the following fractional viscosity (eta) dependences of and ( being solutes average rotation time): alpha (eta/T)(p) with 0.7 <= p <= 0.9. Different correlations between and emerge at different temperature regimes, indicating variable frictional coupling at low and high temperatures. Estimated dynamic Stokes shifts in these media vary between similar to 1200 and similar to 1600 cm(-1), more than 50% of which possess a time scale much faster than the temporal resolution (similar to 75 ps) employed in these measurements. Estimated activation energy for eta is closer to that for than that for , suggesting being more decoupled from eta than .