Fluorescence dynamics in supercooled (acetamide+calcium nitrate) molten mixtures

Medium decoupling of dynamics at temperatures ∼100 K above glass-transition temperature: A case study with (acetamide + lithium bromide/nitrate) melts

“…However, (t) reaches a plateau as solvation progresses with time, an aspect also observed earlier for polar solvation energy relaxation in neat solvents 76 and molten mixtures. 31,32 This and the fact that even the maximum variation of (t) being substantially smaller than the observed dynamic shift further support that the observed time-evolution of the fluorescence spectrum occurs mainly due to the medium reorganization around the perturbed solute in these melts. Table I summarizes the composition dependent observed ( ν t obs. )…”

“…31 This is a result different from earlier suggestion 3, 27 that (acetamide + electrolyte) melts containing cation other than Na + is relatively less inhomogeneous. As observed in our previous experiments, 31,32 UV-VIS absorption and fluorescence emission characteristics of C153 indicate an average polarity similar to that of liquid formamide at ambient condition, which is much smaller than what was indicated 3, 12, 13, 26 for this kind of melts. Both τ s and τ r in the present measurements increase with Br − concentration.…”

“…The closeness between these melts and dipolar ionic liquids is further evidenced when application of a semi-molecular theory that successfully described the Stokes shift dynamics in ILs and (ionic liquid + polar solvent) binary mixtures [58][59][60][61][62] could also semi-quantitatively predict the solvation time scales and dynamic Stokes shift in (CH 3 CONH 2 + Na/KSCN) and (CH 3 CONH 2 + Ca(NO 3 ) 2 ) melts. 31,32 It should be recognized here that the agreement between theory and experiments appears due mainly to the domination of the collective solvent polarization fluctuation in polar solvation energy relaxation where details of solvent structure around a dissolved dipolar solute assumes a secondary importance. [63][64][65] The striking difference, however, is while fluorescence studies with (CH 3 CONH 2 + Na/KSCN) mixtures have revealed strong deviation from the SE and SED predictions, ILs show near hydrodynamic description 66 even though both of these different systems are characterized by heterogeneity and similar interactions.…”

“…31,32 Average solvation and rotation times, obtained from dynamic Stokes shift and fluorescence anisotropy measurements, 31 have indicated strong solution heterogeneity as these quantities ( τ s and τ r ) exhibited a fractional solution viscosity dependence, τ x α (η/T) p , with p much less than unity but larger for τ r than for τ s . The fractional viscosity dependence of these quantities and the consequent departure from the conventional Stokes-Einstein (SE) 33 and Stokes-Einstein-Debye (SED) 34 predictions for these melts are reminiscent of environmental as well as translation-rotation decoupling observed in deeply supercooled liquids.…”

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

“…Clearly, the faster decay at higher temperature is due to the decrease in solution viscosity (see Table S2). Bi-exponential fit parameters required to describe the temperature dependent S(t) decays at f= 0.33 and 0.4 are summarized in Table 2 eutectics, [14][15][16]25 and resurrection from fraction to near unity value for p upon replacement of electrolytes by urea to these ionic acetamide deep eutectics. 17 We have also estimated the activation energies, ) ( E s a τ , associated with solvation process in these media and this is shown in Fig.…”