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

Abstract

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.

Mixed‐Valence Biferrocenium Salts of Xn–TCNQ (X = F, Cl; n = 1, 2): Correlation between Molecular Structures and Assembled Structures

“…Biferrocenium salts with TCNQ and F 4 –TCNQ mostly exhibit a D/A ratio of 1:3 and 1:1, respectively,8–10 and this study confirmed that salts with X 1 – and X 2 –TCNQ (X = F, Cl) exhibit an intermediate D/A ratio of 1:2. This result implies that the strong acceptor F 4 TCNQ accommodates an electron from one molecule while weak acceptors need to form dimers or trimers to produce a sufficient electron affinity.…”

“…The slight decrease of the magnetic moments with decreasing temperature may be ascribed to temperature‐independent paramagnetism and/or intermolecular antiferromagnetic interactions. Similar magnetic behaviors are often observed in biferrocenium salts containing other acceptors 8–10. The χ m T value for the mixed‐stack salt 5 was smaller, which may be ascribed to antiferromagnetic interactions between the acceptors.…”

“…Differential scanning calorimetry (DSC) measurements revealed that 2 undergoes a first‐order phase transition at 150.7 K (Δ H = 0.43 kJ mol –1 , Δ S = 2.9 J K –1 mol –1 ) but the structural changes at the phase transition were very small 14. Although the segregated‐stack salts containing TCNQ with a D/A ratio of 1:3 are typically semiconductors,8 these salts were insulators because of the non‐mixed‐valent nature of the acceptors.…”

“…Several biferrocenium salts with organic acceptors have also been synthesized 5a,7. Previously, we reported the structures and electronic properties of alkyl‐ and thioalkyl‐biferrocenium salts containing F n –TCNQ (TCNQ = tetracyanoquinodimethane; n = 0, 1, 2, 4)8,9a,10 and [M(mnt) 2 ] (mnt = maleonitriledithiolate; M = Ni, Pt) 9,11. It was revealed that the valence state of the cations in these salts were dependent on the donor/acceptor (D/A) ratio.…”

Magnetische Molekülverbindungen: Schaltung magnetischer Eigenschaften durch externe Stimuli

“…Die ungepaarten Spins der isolierten TTF(D 1+ )-und CA(A 1À )-Einheiten an den Domänengrenzen führen zu den experimentell beobachteten ESR-Signalen. [104,105] Bei Raumtemperatur handelt es sich um einen einwertigen Festkörper mit dem Monokation npbifc + und Abbildung 13. a) Phasenumwandlung neutral ! [97] Nochmals wird darauf hingewiesen, dass die neutrale und die ionische Phase diamagnetisch sind, sodass TTF-CA und verwandte Verbindungen weniger wegen ihrer magnetischen als vielmehr wegen ihrer dielektrischen Eigenschaften interessant sind.…”