Electron Transfer Kinetics in a Deep Eutectic Solvent.

Electron transfer (ET) kinetic rate constants ks in Ethaline (1:2 choline chloride + ethylene glycol) have been measured for two common redox couples (ferrocene/ferrocenium and ferrocyanide/ferricyanide) on a glassy carbon electrode and compared with ET kinetics in ionic liquids and classical organic solvents in the same conditions (acetonitrile and water). Particular care has been taken to treat ohmic drop in DES. For both couples, we found that ET rate constants are just a little lower than those measured in classical solvents (around 50% or less). These results contrast with ET rates in ionic liquids where electron transfers are considerably slower (100 times lower). Data are discussed as a function of the solvent relaxation time using Marcus Theory for an adiabatic electron transfer.