Redox electrocatalysis by tetracyanoquinodimethane in phospholipid monolayers adsorbed at a liquid/liquid interface

Abstract

Electron-transfer reactions at the water/1,2-dichloroethane interface with an adsorbed phospholipid monolayer have been studied for three organic redox couples: TCNQ, Ru(TPP)(py)₂ and Lu(PC)₂. The presence of a monolayer at the immiscible interface inhibited electron-transfer reactions to increasing extents on going from TCNQ to Ru(TPP)(py)₂ to Lu(PC)₂. This effect is attributed to the different Gibbs energies of transport across the monolayer of each species resulting from size effects, which prevent the approach of the redox centres present in the adjoining phase. Electron transfer mediated by TCNQ in a redox electrocatalytic cycle has been observed between the ruthenium(II) prophyrin complex, Ru(TPP)(py)₂, and aqueous Fe(CN)^3–/4–₆. The differences in rates of electron transfer for the three compounds are discussed in terms of their structures and of the distances of closest approach between redox centres present in the different phases.