Electrochemical and X-ray diffraction study of the redox cycling of nanocrystals of 7,7,8,8-tetracyanoquinodimethane. Observation of a solid-solid phase transformation controlled by nucleation and growth

The redox cycling of nanocrystals of 7,7,8,8-tetracyanoquinodimethane (TCNQ) immobilized on the surface of a variety of electrodes has been carried out in aqueous solutions of 1:1 electrolytes containing Group 1 cations (Na⁺, K⁺, Rb⁺, Cs⁺). It is found that the overall process follows the general equation xTCNQ + ye^- + yM⁺ ⇌ (M⁺)_y(TCNQ^-)_y(TCNQ)_x-y (solid) (aq) (solid) where M⁺ is a Group 1 cation. It is also found, by a combination of voltammetric and XRD techniques, that the overall process is rate-controlled by nucleation and growth of the solid phases. Simple intercalation is ruled out by observing that significant structural rearrangement of the solid phases accompanies the redox reactions, and that x and y are integers. In the voltammograms, unusual inert zones are observed between the reduction and re-oxidation peaks, unlike anything that is seen in the conventional redox cycling of solution species. Theoretical analysis reveals that these are caused by the need to expend energy to create the solid/solid interface between the reduced and neutral forms of TCNQ in the critical nuclei of the solid phases.