Fluorescence quenching of polycyclic aromatic hydrocarbons within deep eutectic solvents and their aqueous mixtures

Two common and popular deep eutectic solvents (DESs) composed of the salt choline chloride and H-bond donors glycerol and urea in 1:2 mol ratio named glyceline and reline, respectively, are investigated for the analysis of polycyclic aromatic hydrocarbons (PAHs) using quenching of both steady-state and time-resolved fluorescence of ten different PAHs by nitromethane at 30 degrees C. Based on their quenching efficiencies, the PAHs are divided into two groups group 1 is constituted of the five PAHs whose fluorescence are quenched less effectively by nitromethane whereas the other five exhibiting high quenching efficiency are associated to group 2. Quenching of steady-state fluorescence of group 1 PAHs by nitromethane, albeit not very significant, follow a simple Stern-Volmer behavior. The excited-state emission intensity decay of these PAHs, in both absence and presence of nitromethane, fit best to a single exponential model with small but monotonic decrease in lifetimes. The decrease in lifetime also follows Stern-Volmer behavior, however, the quenching constants (K-D) are lower than those obtained from steady-state fluorescence (K-SV). This is ascribed to the possible formation of charge-transfer complex between the PAH and the nitromethane. Steady-state fluorescence quenching of group 2 PAHs exhibit distinct upward curvature from linear Stern-Volmer behavior implying highly efficient quenching. The intensity decay fits best to a double exponential decay model with longer of the decay times following simple Stern-Volmer behavior. Formation of a complex or the presence of nitromethane within the quenching sphere of action of the PAH having short decay time is proposed. Quenching behavior was found to be similar irrespective of the identity of the DES. A representative group 2 PAH, pyrene, is employed to investigate diffusion dynamics within aqueous mixtures of the two DESs. The bimolecular quenching rate constant (k(q)) is found to increase linearly with the mole fraction of water (x(w)) in aqueous mixtures of both the DESs. Stokes-Einstein relationship is not obeyed within the DES aqueous mixtures and is attributed to be due to the inherent heterogeneity of the mixtures. Diffusion dynamics of pyrene-nitromethane pair is compared with that of a tracer fluorescein isothiocyanate obtained using fluorescence correlation spectroscopy. Good correlation between the two highlights generality of diffusion dynamics with aqueous DES mixtures as far as solutes are concerned. (C) 2016 Elsevier B.V. All rights reserved.