Electron transfer controlled by solvent and counter-anion dynamics in electrochemistry of viologen-type ionic liquid

Abstract Temperature-dependent diffusion coefficients ( D O ) and heterogeneous electron transfer rate constants ( k 0 ) were measured for the viologen dication (V 2+ ) in 1-butyl-1′-heptyl-4,4′-bipyridinium bis[(trifluoromethane)sulfonyl]imide [C 4 VC 7 ][Tf 2 N] 2 redox-active ionic liquid (RAIL) and its solution. The measurements were made in three systems: (1) 10 mM of [C 4 VC 7 ][Tf 2 N] 2 dissolved in 0.1 M K[Tf 2 N]/acetonitrile (an organic electrolyte), (2) 10 mM [C 4 VC 7 ][Tf 2 N] 2 dissolved in a 1- n -hexyl-3-methylimidazolium tris(pentafluoroethyl)-trifluorophosphate, [HMIm][FEP] ionic liquid (IL) (an ionic liquid (IL) electrolyte), and (3) the undiluted RAIL of 1.68 M [C 4 VC 7 ][Tf 2 N] 2 with no additional solvents or supporting electrolytes. At 25 °C, the D O values in systems (1), (2), and (3) were 8.2 × 10 −6 , 6.7 × 10 −8 , and 1.3 × 10 −9  cm 2  s −1 , and the k 0 values were 1.1 × 10 −2 , 3.6 × 10 −3 , and 2.0 × 10 −5  cm s −1 , respectively. Temperature-dependent D O and k 0 values were further interrelated to investigate heterogeneous interfacial electron transfer processes in systems (2) and (3). Solvent dynamics and counter-ion relaxation processes were used to describe the results. In system (3), k 0 values were proportional to both D O and counter-ion diffusion coefficients ( D CIon ) but inversely proportional to the viscosity ( η ). Moreover, almost the same magnitude of thermal activation barrier (∼64 kJ mol −1 ) was found for k 0 , D O , D CIon , and η −1 in the system (3). In system (2), k 0 values showed a non-linear relationship with D O , D CIon and η −1 , and a thermal activation barrier was found to be 22.2 kJ mol −1 . These results enabled us to conclude that the V + /V 2+ heterogeneous electron transfer in system (3) is controlled concurrently by solvent dynamics (segmental motion of alkyl chains attached to viologen centers) and electro-inactive counter anion motion. This work reveals an inherent dependency of heterogeneous electron transfer kinetics on the solvent and counter-ion dynamics in viologen IL systems.