Redox and Emission Characteristics of Eu3+ in Deep Eutectic Solvent: Unraveling the Hidden Potential of DES as Luminescent Media

Abstract Pertaining to the importance associated with designing of novel and green solvent for fundamental research as well as technological applications, this work is a step towards similar direction. Herein we have explored the electrochemical and photophysical investigation of europium nitrate in acidic deep eutectic solvent (DES) with Choline chloride (ChCl) as hydrogen bond acceptor and para toluene sulfonic acid (PTSA) as hydrogen bond donor. Fourier transformed infrared spectroscopy suggested formation of hydrogen bonding in ChCl-PTSA acidic DES. Cyclic Voltammetry (CV) suggested reduction of Eu(III) to Eu(II) is quasi-reversible and is controlled by both diffusion of Eu(III) ion in DES and kinetics of the charge transfer reaction occurring at the electrode-electrolyte inter phase. Temperature dependent CV suggested that diffusion coefficient increases with increase in the temperature owing to reduction in viscosity of the medium. Thermodynamic measurements postulated that standard entropy change for the redox behavior of Eu(III)/Eu(II) couple in PTSA:ChCl DES is positive which indicates that the reaction happens at the expense of more disorderness in the system. Photoluminescence spectroscopy suggested that europium ion local symmetry is highly distorted with high asymmetry. Lifetime spectroscopy suggested formation of moderate complex between DES and europium ion with presence of four molecules of water. This results in lower internal quantum efficiency of europium ion in DES. But the positive implication is its multi-wavelength excitability and higher color purity of 5D0→7F2 red emission. We envision that this Eu-DES complex work can be propagated to other lanthanides and system needs to be optimized to achieve higher internal quantum efficiency for it to fetch more commercial attentions.