Fluorescence of pyrene and its derivatives to reveal constituent and composition dependent solvation within hydrophobic deep eutectic solvents

Abstract

Depending on their constituents and composition, deep eutectic solvents (DESs) may exhibit widely varying physicochemical properties. Based on the miscibility of water in a DES, it can broadly be classified as ‘hydrophobic’ or ‘hydrophilic’. The polarity offered by hydrophobic DESs as compared to common organic solvents in the context of solute solubilization thus becomes of the utmost importance. A versatile fluorescence probe, pyrene (Py), its aldehyde derivative, pyrene-1-carboxaldehyde (PyCHO), and a termini end-tagged dipyrenyl polydimethylsiloxane polymer (Py-PDMS-Py) are employed to assess the solvation environment afforded by DESs composed of thymol (Thy), (−)-menthol (Men), and n-decanoic acid (DA). To decipher the effect of the constituents and composition on solute solvation, DESs composed of different pairs of constituents in different molar ratios, namely Thy : Men (1 : 1 and 1 : 2), DA : Men (1 : 1 and 1 : 2), and Thy : DA (2 : 1, 1 : 1, and 1 : 2), are investigated. Pyrene, through its band 1-to-band 3 emission intensity ratio (Py I₁/I₃), displays higher cybotactic region dipolarity in Thy-containing DESs due to the phenyl ring of Thy; the sensitivity of Py I₁/I₃ towards temperature is also higher in Thy DESs. The fluorescence lifetime of pyrene and its temperature dependence are higher in Men-containing DESs in comparison. Quenching of pyrene fluorescence by nitromethane in these DESs is dynamic in nature and the recovered bimolecular quenching rate constants (k_q) reveal efficient diffusion of the fluorophore–quencher pair in these DESs as compared to other iso-viscous media. The k_q adheres to the Stokes–Einstein relation, implying inherent homogeneity associated to these DESs. PyCHO emission spectra show a high energy structured band in Thy : Men DESs, whereas the band shifts bathochromically and becomes broad in DA-containing DESs. The PyCHO cybotactic region is relatively nonpolar in Thy : Men DESs as compared to Thy : DA and Men : DA DESs. The extent of intramolecular excimer formation by Py-PDMS-Py reveals these DESs to be “good” solvents for polymer solvation where DES–polymer interactions are maximized. The microviscosity (η_μ) surrounding Py-PDMS-Py correlates with the bulk dynamic viscosity (η_bulk) within the investigated DESs, further corroborating the absence of microheterogeneity. Overall, the observations reveal the similarity of these hydrophobic DESs to common organic solvents in the context of solute solubilization.