Structure and Electronic Properties of Biferrocene–TCNQ Charge-Transfer Complexes: Effects of Acceptors and Crystal Environment on the Mixed-Valence States

Correlations between the valence states, crystal structure, and electronic states in biferrocenium charge-transfer complexes with tetracyanoquinodimethane (TCNQ) acceptors have been revealed. Structural determination of (1′,1″′-dipropyl-1,1″-biferrocenium)+(TCNQ)3− (1) and (1′,1″′-dibutyl-1,1″-biferrocenium)+(TCNQ)3− (2) showed that these complexes form a 1:3 segregated stack structure, and are semiconductors. Stabilization of the averaged valence states in the biferrocenium cations in 1 and 2 is attributed to the symmetrical environment around the cations and to the shielding of the dipole–dipole interactions by the TCNQ columns. Structural determination of (biferrocenium)+(F4TCNQ)− (3) and (1′,1″′-diethyl-1,1″-biferrocenium)+(F4TCNQ)− (4) revealed that 3 has a segregated structure, while 4 exhibits a 2:2 mixed-stack structure, represented as ...D+D+A−A−D+D+A−A−... Valence localization in the biferrocenium cations in these complexes can be rationalized by electrostatic interactions between the cations and the anions in addition to the low crystal symmetry. All the complexes show paramagnetic susceptibilities, which mainly arise from the contribution of the donor molecules.