Pyrene Fluorescence To Probe a Lithium Chloride-Added (Choline Chloride + Urea) Deep Eutectic Solvent

Deep eutectic solvents (DESs) have shown promise as environmentally benign and inexpensive media with superior properties. Because of their structural features, they have potential to be versatile alternatives to the conventional electrolytes in various applications in science and technology. A mixture of a common and popular DES Reline, composed of salt choline chloride and H-bond donor urea in a 1:2 mole ratio, and lithium salt LiCl is investigated in a 298.15–358.15 K temperature range using a well-known multidimensional fluorescence probe pyrene. The band 1-to-band 3 emission intensity ratio (Py I₁/I₃) at a given temperature reveals no change in the dipolarity of the pyrene cybotactic region of Reline on addition of up to 2.093 m(LiCl). Decrease in dipolarity of the medium on increasing temperature becomes less pronounced in the presence of LiCl. Excited-state intensity decay of pyrene fits best to a two-exponential decay equation irrespective of the temperature and LiCl concentration. While the shorter of the decay times does not vary with temperature, the longer one shows decrease with increasing temperature that is independent of the LiCl concentration. At a given temperature, addition of LiCl results in a slight decrease in the longer decay time because of the presence of Cl^–, which facilitates nonradiative decay pathways of excited pyrene. Decrease in the pyrene decay time by an electron/charge acceptor quenching agent nitromethane is found to obey the Stern–Volmer equation, implying the quenching to be purely dynamic in nature. The estimated bimolecular quenching rate constant (k_q) first increases as LiCl is added to Reline before decreasing monotonically on further addition of LiCl. The decrease in k_q with increasing LiCl is attributed to the exponential increase in the dynamic viscosity of Reline with LiCl addition. The initial increase is due to the stabilization of the partial positive charge that develops on excited pyrene during the electron/charge transfer to nitromethane by the added Cl^– during the quenching process. The Stokes–Einstein equation is not obeyed within LiCl-added Reline, but it is found to be followed at a given LiCl concentration. Iso-viscous LiCl-added Reline mixtures are found to have significantly different k_q values, suggesting the important role of LiCl in controlling the bimolecular quenching process within the system.