Revealing Temperature-Dependent Dynamic Crossover of Molecular Solutes in Thymol–Camphor-Based Type V Deep Eutectic Solvent via Optical Spectroscopy

≈ 0.8), indicating that despite heterogeneity, Coumarin-153 experiences an effectively homogeneous local environment. In the thymol/camphor-based DES, the temperature-dependent evolution of fluorescence quantum yield for both Thioflavin T and Auramine O reveals a nonuniform response across the studied range, manifesting as two distinct friction regimes separated by a well-defined crossover at 308 K. Microviscosity determination of both probes reveals decoupling of the torsional relaxation from the bulk viscosity of the solvent, which is complemented by the disparity in the activation energy values of viscous flow and nonradiative transition obtained by utilizing Arrhenius-type curves. This observation suggests that the photophysical behavior of molecular rotors like Thioflavin T and Auramine O is not governed solely by the bulk viscosity of the DES; rather, the local microstructure of the solvent plays a dominant role. These findings underscore how variations in local polarity and specific solute-solvent interactions drive preferential solvation, revealing the presence of microscopically heterogeneous domains within the nonionic, hydrogen-bonded framework of the DES.