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

Abstract

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 μm). TCNQ undergoes two consecutive one-electron reductions with comproportionation occurring between TCNQ2–, 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 TCNQ solution by the TCNQ– diffusion layer of the large Pt substrate. A comproportionation rate constant, k(comp) = 1 × 10(6) M(–1) s(–1), 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.

Investigation of the Redox and Acid‐Base properties of TCNQF and TCNQF2: Electrochemistry, Vibrational Spectroscopy, and Substituent Effects

“…The diffusion coefficients ( D ) of TCNQF and TCNQF 2 found by analysis of the scan rate dependence of 1.0 mM concentrations and use of Equation (7) were 2.15±0.13×10 −5 and 2.10±0.17×10 −5 cm s −1 , respectively. These D values are similar to those of TCNQF 4 (2.0×10 −5 cm s −1 ) and TCNQ (1.9×10 −5 cm s −1 ) ,. Similarly, D (TCNQF 1− , TCNQF 2 1− )=1.9×10 −5 cm 2 s −1 , D (TCNQF 2− , TCNQF 2 2− )=1.5×10 −5 cm 2 s −1 , D (TCNQF)=2.15×10 −5 cm 2 s −1 were determined from 1.0 mM solutions, prepared by bulk electrolysis of the neutral species (see Figure ).…”

Review—Advances in Scanning Electrochemical Microscopy (SECM)

“…This is a significant advantage in measuring kinetics, even for rapid processes, under conditions where double-layer charging and adsorption do not contribute to the observed currents. 64,65 However, transient measurements are also possible where the tip current, i T , is recorded as a function of time. The transient mode is useful in the measurement of homogeneous kinetics, for systems that are changing with time, and for determining the diffusion coefficient of a species without needing to know the solution concentration or number of electrons transferred in the electrode reaction.…”

Scanning Electrochemical Microscopy of Belousov–Zhabotinsky Reaction: How Confined Oscillations Reveal Short Lived Radicals and Auto-Catalytic Species

“…The incorporation of a heterogeneous substrate reaction was also attempted (data not shown); however, since the optimized homogeneous reaction rate was high (based on i inf ), any heterogeneous rate employed either produced a negative or positive feedback response, in good agreement with SECM theory for coupled homo/heterogeneous reactions. 35,36,38,42,48 2.4.2. Reactivity from the Envelope of the Oscillations.…”

“…SECM has been used extensively for interrogating the reactivity of chemical species in solution, at a substrate, 3,4,[35][36][37][38][39][40][41][42][43] as well as for characterizing short-lived radical species. 37,[39][40][41]43 Therefore, from both geometric and mechanistic aspects, SECM is ideal to interrogate oscillating chemical reactions in confined conditions.…”