Determination of Binding Strengths of Host-Guest Complexes in Deep Eutectic Solvents Using Spin Probe Methodology

Electron paramagnetic resonance (EPR) and spin probe methodologies have been employed to study the complexation properties of cyclodextrins (CDs) and cucurbit[n]urils (CB[n]s) in the deep eutectic solvent (DES) choline chloride-urea. In the presence of γ-CD an affinity constant very similar to that measured in water was measured in DES with benzyl-tert-butyl nitroxide (BTBN). With β-CD, complexation of BTBN is significantly depressed, although still maintained. Complexation of TEMPO radical probe by CB[7] or CB[8] was instead almost entirely cancelled in DES. In addition, this methodology enabled for the first time to measure the single rate constants for the association and dissociation processes with CDs in DES.