THE RELATIONSHIP BETWEEN THE ELECTROCHEMISTRY AND THE CRYSTALLOGRAPHYOF MICROCRYSTALS - THE CASE OF TCNQ (7,7,8,8-TETRACYANOQUINODIMETHANE)

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+, R b+ and Cs+, and the electrode potential is cycled, reversible phase tr ansformations take place between the TCNQ and its corresponding cation salts. The electrochemical responses of these reversible phase transf ormations show that, in all cases, nucleation-growth kinetics are rate -determining. Ancillary techniques, such as optical microscopy, scanni ng electron microscopy and X-ray diffractometry, elucidate the corresp onding crystal structure changes. In combination, these. techniques pr ovide deep insights into the relationship between the electrochemistry and crystallography of microcrystals. Optical microscopy reveals a co lour change from yellow to blue-green upon electrochemical reduction o f TCNQ microcrystals, and also reveals that small crystals react faste r than large crystals. Unfortunately, analysis of morphological change s in situ in real time is prevented by the limited resolution of optic al 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 th ese can be used to reconstitute the reaction pathway. Finally, X-ray d iffractometry allows the spatial coordinates of the TCNQ molecules to be determined both before and after the phase transformations with acc uracies of +/- 0.001 nm. Such data reveal, for the first time, the cha nges that occur in molecular orientation during electrochemically indu ced solid-solid phase transformations in pi-stacked organic conductors .