Interaction and Dynamics in a Fully Biodegradable Glucose-Containing Naturally Abundant Deep Eutectic Solvent: Temperature-Dependent Time-Resolved Fluorescence Measurements

Abstract

A new room-temperature deep eutectic solvent (DES) composed of glucose, urea, and water has been prepared and its relaxation dynamics explored via temperature-dependent time-resolved fluorescence measurements employing hydrophilic and hydrophobic solute probes. Differential scanning calorimetry measurements indicate a glass transition temperature (T g) of ∼236 K. Measured viscosity coefficients (η) vary from ∼600 to ∼100 cP in the temperature range 318 ≤ T/K ≤ 343 and exhibit Arrhenius-type temperature dependence with an activation energy of ∼65 kJ mol–1. Interestingly, this DES forms a stable liquid at ∼300 K but is too viscous to be accurately measured by us below 318 K. Temperature-dependent dynamic fluorescence anisotropy measurements using hydrophobic and hydrophilic solutes of similar sizes reveal bi-exponential kinetics and Arrhenius-type temperature dependence for solute rotation times (⟨τr⟩) but with significantly decreased activation energies, ∼31 kJ mol–1 (hydrophobic) and ∼21 kJ mol–1 (hydrophilic). Deviation from hydrodynamics is further reflected in the strong fractional viscosity dependence of ⟨τr⟩: ⟨τr⟩ ∝ (η/T) p with p ≈ 0.3–0.5, indicating pronounced temporal heterogeneity in the relaxation dynamics. Dynamic fluorescence Stokes shift measurements (temporal resolution ∼85 ps) produce dynamic shifts of ∼500–700 cm–1, bi-exponential solvation energy relaxation with time constants in the range ∼0.2 ns and ∼4 ns, and estimated missing amplitudes of ∼65–75%. Impact of the density difference between a nonpolar solvent and this DES on the estimated missing amplitudes is explored via measuring the temperature-dependent densities and refractive indices of this DES. Lifetime measurements suggest considerable temperature dependence for the hydrophobic solute but no such dependence for the hydrophilic one. Excitation energy dependence of fluorescence emission of various solutes with widely different lifetimes indicates mild spatial heterogeneity for this DES.

Complete Solvation Dynamics of Coumarin 153 in Tetraalkylammonium Bromide-Based Deep Eutectic Solvents

“…32,33,35,36 This is not surprising as these TDFFS studies were based on the TCSPC technique, where a large portion of the early part of the total dynamics was missed due to limited time resolution (typically 25−50 ps). 32,33,35,36 It is important to note that the reported magnitude of the total shifts of C153 in the earlier studies of solvation dynamics in common ILs, 48 which captured the complete dynamics, are comparable to the present values. This indicates that we have been able to capture almost the entire solvation dynamics in the present case.…”

“…Notwithstanding their applications, ,− the structure and dynamics of the DESs are not very well understood from the studies made so far on the diffusion dynamics of constituents of the DESs ,− and translational and rotational diffusion dynamics of organic probe molecules in these media. − While Abbott and co-workers investigated molecular and ionic diffusion of the constituents in these solvents using pulsed field gradient NMR spectroscopy, ,, Biswas and co-workers studied rotational diffusion of several molecular systems in a number of DESs to understand the dynamics of these liquids. − We have recently investigated the translational and rotational diffusion dynamics of some probe molecules employing fluorescence correlation spectroscopy and time-resolved fluorescence anisotropy measurements. − These studies have revealed significant to moderate spatial and/or dynamic heterogeneity of these DESs, depending on the constituents.…”

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.…”

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

“…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.…”