Temperature-Dependent Ultrafast Solvation Response and Solute Diffusion in Acetamide–Urea Deep Eutectic Solvent

Abstract

In the present paper, we have studied the temperature dependence of translational diffusion and solvation dynamics of a dissolved dipolar dye in the nonionic acetamide–urea deep eutectic solvent (DES), to characterize the viscosity coupling of the measured relaxation times and verify the dynamical heterogeneity aspect of this medium. Three different time-resolved experimental techniques have been employed for this purpose: fluorescence correlation spectroscopy, transient absorption (TA) spectroscopy, and optical Kerr effect (OKE) spectroscopy. The first method provides the proof that the translational diffusion time of a solute in acetamide–urea DES [fCH3CONH2 + (1 – f)­CO­(NH2)2, f = 0.6] exhibits a fractional viscosity dependence, with exponent 0.758, which, when compared with the viscosity–diffusion relationship for the same solute in common molecular solvents, suggests moderate deviation from the Stokes–Einstein relation. Stokes shift dynamics of a solvatochromic dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran in this DES, followed via femtosecond TA measurements, have been found to be triexponential in nature and dominated by a ∼100 fs component. The other two components, which contribute to a total dynamic Stokes shift magnitude of ∼2500 cm–1, are characterized by time constants in the ∼5 and ∼50 ps regimes. Subsequent comparison with the femtosecond OKE measurements suggests that the relatively slower picosecond solvation components originate from the rapid reorientation of the solvent molecules, while the subpicosecond solvation response arises from the participation of the collective low-frequency solvent modes (such as intermolecular vibrations and librations). We find that the rotational diffusion lifetimes also exhibit fractional power dependence on medium viscosity and thus deviate from the Stokes–Einstein–Debye pprediction. All of these results therefore suggest that the nonionic acetamide–urea DES is a moderately heterogeneous medium.

On the coupling between ionic conduction and dipolar relaxation in deep eutectic solvents: Influence of hydration and glassy dynamics

“…The rotational and translational dynamics of fluorescent dyes have been 4 studied in reline, 16 ethaline, 17 and acetamide-urea DESs. 18 They exhibit fractional viscosity dependence and deviations from the hydrodynamic predictions based on Stokes-Einstein and Stokes-Einstein-Debye theories. These results suggest that the molecular motions of the fluorescent dyes are sensitive to the existence of dynamic heterogeneity in DESs.…”

Probing the structural features and the micro-heterogeneity of various deep eutectic solvents and their water dilutions by the photophysical behaviour of two fluorophores

“…[51] As for the fastest transients describing the decay, they can be assigned to solvation processes, that is the relaxation of solvent molecules around the excited state of the solute. As a rule, solvation can be distinguished in ultrafast components due to small amplitude motions (librations and vibrations), known as inertial solvation, and slower components associated to larger motions (rotations and translations), known as diffusive solvation, which can also include hydrogen bond stretching when dealing with hydrogen-bonded systems [44,72] Common solvents are characterized by typical times for the solvation processes: in the case of water, the whole solvation relaxation happens within 1 ps, with inertial solvation being faster than 50 fs and diffusive solvation decaying in hundreds of femtoseconds [73,74].…”

“…In many of these papers, the inhomogeneity of the microscopic structural features is underlined, that is also, as mentioned before, the reason for the liquid formation. Fluorescent probe studies were also used in these approaches in order to investigate the structural features of these systems [40][41][42][43][44][45]. In particular, a great deal of effort has recently been put in trying and understanding the difference between spatial and temporal heterogeneity in various DESs.…”

Heterogeneous Orientational Relaxations and Translation–Rotation Decoupling in (Choline Chloride + Urea) Deep Eutectic Solvents: Investigation through Molecular Dynamics Simulations and Dielectric Relaxation Measurements

“…These systems are also rich and diverse in basic scientific aspects because of the “metastability” arising from being in the liquid phase at temperatures lower than the melting temperatures of the individual components. The signature of metastability has probably been reflected via the pronounced fractional viscosity dependence of relaxation rates at temperatures ∼50–150 K above the respective thermodynamic glass transition temperatures for a variety of DESs. − ,− This is striking because such a strong viscosity decoupling of relaxation rates is known to occur for deeply supercooled liquids near glass transition, where spatial inhomogeneity is believed to characterize the liquid structure at such low temperatures.…”