Chemical control of the monovalent–divalent electron-transfer phase transition in biferrocenium–TCNQ salts

An ionic molecular crystal of (1',1'''-dineopentylbiferrocene)(F1TCNQ)3 exhibits a first-order phase transition from a monovalent state (D(+)A3(-)) to a divalent state (D(2+)A3(2-)) at around 120 K. The transition was successfully controlled by modulation of the redox potentials using FnTCNQ (n = 0-2) and by chemical-pressure effects.