Determination of Heterogeneous Electron Transfer and Homogeneous Comproportionation Rate Constants of Tetracyanoquinodimethane Using Scanning Electrochemical Microscopy

We report the use of scanning electrochemical microscopy (SECM) in determining the heterogeneous electron transfer and homogeneous comproportionation kinetics of tetracyanoquinodimethane (TCNQ) in acetonitrile at Pt tip UMEs (radius 12.5-1 mu m). TCNQ undergoes two consecutive one-electron reductions with comproportionation occurring between TCNQ(2-), the product of the second reduction, and bulk TCNQ to produce TCNQ(-). A standard rate constant, k(1)(0) = 2.9 +/- 0.5 cm/s, for the first reduction was determined by tip voltammetry with total positive feedback and a large Pt substrate. A comparatively smaller rate constant, k(2)(0) = 0.44 +/- 0.05 cm/s, for the second reduction was determined in the absence of comproportionation by tip voltammetry with the tip shielded from the bulk TCNQsolution by the TCNQ(-) diffusion layer of the large Pt substrate. A comproportionation rate constant, k(comp) = 1 x 10(6) was determined by tip pulse chronoamperometry at coaxially aligned tip and substrate UMEs of the same radius. Diffusion coefficients of the TCNQ species and standard potentials for the reductions were also determined. Experimental results were compared with 2D transient axisymmetric simulations and reported analytical equations.