Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Choline Chloride + Urea) Melts

Abstract

Dynamic fluorescence anisotropy and Stokes shift measurements of [f choline chloride + (1 - f) urea)] deep eutectic solvents at f = 0.33 and 0.40 have been carried out using a dipolar solute, coumarin 153 (C153), in the temperature range 298 ≤ T ≤ 333 K. Subsequently, measured time-dependent solvent response is utilized to investigate the dynamic solvent control on the measured rates of photoexcited intramolecular charge transfer (ICT) reactions of two molecules, 4-(1-azetidinyl)benzonitrile (P4C) and 4-(1-pyrrolidinyl)benzonitrile (P5C), occurring in these media. Measured average reaction time scales (⟨τ(rxn)⟩) exhibit the following dependence on average solvation times scales (⟨τ(s)⟩): ⟨τ(rxn)⟩ ∝ ⟨τ(s)⟩(α) with α = 0.5 and 0.35 for P4C and P5C, respectively. Such a strong dynamic solvent control of ⟨τ(rxn)⟩, particularly for P4C, is different from earlier observations with these ICT molecules in conventional molecular solvents. Excitation wavelength-dependent fluorescence emissions of C153 and trans-2-[4-(dimethylamino)styryl]-benzothiazole (DMASBT), which differ widely in average fluorescence lifetimes (⟨τ(life)⟩), suggest the presence of substantial spatial heterogeneity in these systems. Dynamic heterogeneity is reflected via the following fractional viscosity (η) dependences of ⟨τ(s)⟩ and ⟨τ(r)⟩ (⟨τ(r)⟩ being solute's average rotation time): ⟨τx⟩ ∝ (η/T)(p) with 0.7 ≤ p ≤ 0.9. Different correlations between ⟨τ(s)⟩ and ⟨τ(r)⟩ emerge at different temperature regimes, indicating variable frictional coupling at low and high temperatures. Estimated dynamic Stokes shifts in these media vary between ∼1200 and ∼1600 cm(-1), more than 50% of which possess a time scale much faster than the temporal resolution (∼75 ps) employed in these measurements. Estimated activation energy for η is closer to that for ⟨τ(r)⟩ than that for ⟨τ(s)⟩, suggesting ⟨τ(s)⟩ being more decoupled from η than ⟨τ(r)⟩.

Deep Eutectic Solvents for Pretreatment, Extraction, and Catalysis of Biomass and Food Waste

“…DESs, which were introduced at the beginning of this century [5], are prepared by simply mixing two or three components at appropriate molar ratios to form eutectic mixtures with greatly depressed freezing points relative to their components [4,5,6]. DESs make attractive candidates for green solvents, due to properties like short preparation time, low costs, potentially good biodegradability, and low toxicity [4,5,7]. The cost of producing a DES has been estimated to be 20% of that of an IL [8].…”

Are water-xylitol mixtures heterogeneous? An investigation employing composition and temperature dependent dielectric relaxation and time-resolved fluorescence measurements

“…Temperature-dependent viscosity coefficient and the refractive index of water-xylitol mixtures were measured by using AMVn automated micro-viscometer from Anton Paar (falling ball method) and automated temperature controlled refractometer (RUDOLPH, J357), respectively. 43,45,46…”

“…TRF measurements of non-reactive solution dynamics and DR is intimately related, and a combination of them has been employed to explore dynamics and interaction in many different systems. 11,[41][42][43][44] For fluorescence measurements, we have used non-reactive hydrophilic coumarin 343 (C343) and hydrophobic coumarin 153 (C153) as external probes to profile the medium frictional response on a dissolved solute. We refrained from measuring the solvation dynamics (via dynamic Stokes shift measurements) of these probes in these mixtures as water response is too fast to be detected by the present setup (Section 2).…”

Are N-methyl groups of Tetramethylurea (TMU) Hydrophobic? A composition and temperature-dependent fluorescence spectroscopic investigation of TMU/water binary mixtures

“…Excitation wavelength dependent emission study is informative where local heterogeneity develops even at ambient condition due to competing interactions in a variety of systems, for example, deep eutectic solvents, ionic liquids, etc. [46][47][48][49][50][51][52][53][54] In viscous media, emission spectrum shifts to lower frequency on moving towards longer excitation wavelengths. This phenomenon is known as red-edge-effect (REE).…”