The relationship between the electrochemistry and the crystallography of microcrystals. The case of TCNQ (7,7,8,8-tetracyanoquinodimethane)†‡

Abstract

Microcrystals of TCNQ, in the size range 100–2000 nm, may be attached to the surfaces of graphite, glassy carbon, gold, platinum and RAM^TM electrodes by a process of dry abrasion. When the resulting surfaces are placed in aqueous solutions of Group I cations, such as Na⁺, K⁺, Rb⁺ and Cs⁺, and the electrode potential is cycled, reversible phase transformations take place between the TCNQ and its corresponding cation salts. The electrochemical responses of these reversible phase transformations show that, in all cases, nucleation–growth kinetics are rate-determining. Ancillary techniques, such as optical microscopy, scanning electron microscopy and X-ray diffractometry, elucidate the corresponding crystal structure changes. In combination, these techniques provide deep insights into the relationship between the electrochemistry and crystallography of microcrystals.

Optical microscopy reveals a colour change from yellow to blue–green upon electrochemical reduction of TCNQ microcrystals, and also reveals that small crystals react faster than large crystals. Unfortunately, analysis of morphological changes in situ in real time is prevented by the limited resolution of optical techniques (500 nm). However, scanning electron microscopy is able to provide ex situ ‘snapshots’ of the microcrystal morphologies before and after the phase transformations with a resolution of 1 nm, and these can be used to reconstitute the reaction pathway. Finally, X-ray diffractometry allows the spatial coordinates of the TCNQ molecules to be determined both before and after the phase transformations with accuracies of ± 0.001 nm. Such data reveal, for the first time, the changes that occur in molecular orientation during electrochemically induced solid–solid phase transformations in pi-stacked organic conductors.