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

Type V deep eutectic solvents (DESs) are composed exclusively of nonionic molecular species, in contrast to type I-IV DESs, which contain at least one ionic component. In the absence of ionic constituents, the cohesive energy of type V DESs is governed primarily by hydrogen bonding and van der Waals interactions. Such an intermolecular framework imparts distinct physicochemical properties, such as increased hydrophobicity and relatively low viscosity (compared to their ionic counterparts), substantially broadening the application potential of DESs. Owing to their fundamental distinction from ionic DESs and ionic liquids (ILs), it is imperative to understand how the nonionic nature of type V DESs influences their microstructure and dynamic behavior for their rational and strategic utilization. To address this, we examined the microenvironment of a thymol/camphor-based type V DES using three molecular probes: Coumarin-153, a neutral benchmark probe widely employed for investigating dynamics in solutions, and two fluorescent molecular rotors, Thioflavin T and Auramine O, whose fluorescence quantum yield is sensitive to the microviscosity of the local environment. Absorption and steady-state fluorescence measurements reveal that the local micropolarity experienced by Coumarin-153 in the DES is comparable to that of acetonitrile. A slight red-edge excitation shift observed in the steady-state fluorescence spectra with varying excitation wavelengths points to the presence of subtle spatial heterogeneity within the solvent. Time-resolved emission spectra (TRES) were recorded over the temperature range of 278-333 K and used to construct the solvent correlation function, which describes the solvent's reorganization in the excited state following the electronic excitation of Coumarin-153. The solvation dynamics were found to be bimodal, with a dominant, markedly slow component indicative of a highly sluggish solvent response. Correlating the solvation dynamics with the shear viscosity of the solvent reveals significant decoupling ( ≈ 0.62, where is the decoupling parameter), suggesting decoupling of the solvation dynamics from the viscosity of the medium. Furthermore, a dynamic crossover was observed at approximately 308 K or 35 °C, showing temperature-dependent modulation in the structural features of the thymol/camphor-based DES. In contrast, the rotational dynamics of Coumarin-153 closely follow the Stokes-Einstein-Debye hydrodynamic model ( ≈ 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.