Mechanistic aspects of the electrochemical reduction of 7,7,8,8-tetracyanoquinodimethane in the presence of Mg²⁺ or Ba²⁺

The effect of aggregation of Mg²⁺ or Ba²⁺ (M²⁺) and electrochemically generated reduced forms of 7,7,8,8-tetracyanoquinodimethane (TCNQ) in acetonitrile solution was studied by voltammetric and chronoamperometric techniques, the electrochemical quartz crystal microbalance (EQCM), ESR spectroscopy, and atomic force microscopy (AFM). Although the monoreduced form, TCNQ^.-, appears not to interact strongly with either metal cation on the voltammetric time scale, the dianion TCNQ^2- was found to interact and to form a thin layer deposit of the type M(TCNQ). The potential at which this occurs was less negative than the potemial required for the formation of the free dianion in solution, reflecting the binding with the metal cation. Unusually, the initial deposition of M(TCNQ) triggered the deposition of a material possibly M(TCNQ)₂ as evidenced by, first, a characteristic voltammetric response, second, a strong ESR signal, and third, the gravimetrically determined deposited mass per charge passed. Both types of deposit gave distinct voltammetric signatures, and an overall scheme for the process is proposed.