Assessing rotation and solvation dynamics in ethaline deep eutectic solvent and its solutions with methanol

Area of Science:

• Physical Chemistry
• Photochemistry
• Materials Science

Background:

• Deep eutectic solvents (DES) like ethaline offer unique solvation properties.
• Understanding solute-solvation dynamics in mixed solvents is crucial for chemical applications.
• Coumarin dyes are widely used fluorescent probes for studying solvation.

Purpose of the Study:

• To investigate the solvation dynamics of coumarin 153 (C153) and coumarin 343 (C343) in methanol-ethaline binary solutions.
• To correlate solute rotational reorientation times and solvation dynamics with solvent viscosity and composition.
• To elucidate the role of methanol dilution versus viscosity changes on solvation.

Main Methods:

• Steady-state and time-resolved fluorescence spectroscopy.
• Time-resolved anisotropy decays for rotational reorientation times.
• Viscosity measurements across the full range of methanol mole fractions.

Main Results:

• Viscosity decreased exponentially with increasing methanol mole fraction.
• Solute rotational reorientation times showed a power-law dependence on viscosity (∼η^0.85) and approached the stick boundary limit.
• Solvation dynamics, quantified by time-resolved Stokes shifts, also followed a power-law dependence on viscosity but were not solely dictated by it.

Conclusions:

• Methanol dilution significantly impacts solute dynamics and solvation in ethaline, with effects not linearly proportional to viscosity changes.
• Solvation dynamics exhibit distinct behaviors for C153 (monoexponential) and C343 (biexponential) in methanol-rich mixtures.
• The interplay between viscosity and specific solvent-solute interactions governs solvation dynamics in these binary mixtures.