Does Viscosity Decoupling Guarantee Dynamic Heterogeneity? A Clue through an Excitation and Emission Wavelength-Dependent Time-Resolved Fluorescence Anisotropy Study

Traditionally, deviation from Stokes-Einstein-Debye (SED) p relation in terms of viscosity dependence of medium dynamics, i.e., t(x) 8( ?/T )(p) with p ? 1, is taken as a signature of dynamic heterogeneity. However, it does not guarantee medium heterogeneity, as the decoupling may also originate from the deviation of the basic assumption of SED. Here, we developed a method to find a stronger relation between viscosity decoupling (p ? 1) and dynamic heterogeneity in terms of rotational motion. Our approach exploited the fact that in heterogeneous media, a solvatochromic probe will be solvated to a different extent at different microdomains (subpopulations), and photoselection of these subpopulations can be achieved by excitation or emission wavelength-dependent measurements. We hypothesized that the dynamics of a homogeneous system might show viscosity decoupling, but the extent of decoupling at different excitations (or at different emissions) should not be different. On the other hand, in a heterogeneous medium, this extent of viscosity decoupling (p-value) should be different at different excitations (or at different emissions). As proof of concept, we investigated three versatile solvent media: squalane (viscous molecular liquid), 1-ethyle-3-methylimidazolium ethyl sulfate ionic liquid (IL), and [0.78 acetamide + 0.22 LiNO3] deep eutectic solvent (DES). We found that squalane is homogeneous, although it shows fractional viscosity dependence (p ? 1). Interestingly, mild heterogeneity in IL and significant heterogeneity in the DES were observed. Overall, we conclude that the difference in the p-value as a function of excitation (or emission) wavelength dependent might be a superior way for the detection of dynamic heterogeneity.

Automated summary

Traditionally, deviation from Stokes-Einstein-Debye (SED) p relation in terms of viscosity dependence of medium dynamics, i.e., t(x) 8( ?/T )(p) with p ? 1, is taken as a signature of dynamic heterogeneity. However, it does not guarantee medium heterogeneity, as the decoupling may also originate from the deviation of the basic assumption of SED. We developed a method to find a stronger relation between viscosity decoupling (p ? 1) and dynamic heterogeneity in terms of rotational motion, using solvatochromic probe measurements at different excitations or emissions.