Preparation of metallic BEDT-TTF charge transfer complex of 3,3′,5,5′-tetranitro-4,4′-biphenyldiol dianion (TNBP2−) having flexible molecular shape

Abstract

Two kinds of 3,3′,5,5′-tetranitro-4,4′-biphenyldiol dianion (TNBP2−) molecules having different dihedral angles (φ) between the two phenyl rings exist in the tetrabutylammonium salt of TNBP2− (φ = 0° and 35°), while only one kind of TNBP2− molecule exists in the tetraphenylphosphonium salt (φ = 0°). The 4∶1 cation radical salt of BEDT-TTF (ET) with TNBP2− asa counter anion is metallic down to 3 K. The ET molecules form a two-dimensional conducting layer which is sandwiched by the layers of flat TNBP2− molecules (φ = 0°). The crystal structure indicates the β″432-type stacking of tetramerized ET molecules which is caused by hydrogen bonds between the CH groups of ET and the oxygen atoms (nitro and phenol) of TNBP2−. The optical spectra in a KBr pellet are consistent with both the highly conductive and distorted structural nature. The magnetic susceptibility at room temperature is high (χ = 6.7 × 10−4 emu spin−1) compared with the conventional organic metal based on ET (χ = 3.4–5.5 × 10−4 emu spin−1) indicating strong electron correlation in the 4∶1 complex. The temperature dependence of the magnetic susceptibility suggests a semimetallic nature which is consistent with the calculated Fermi surface. The calculated band width is comparable to the estimated effective on-site Coulomb repulsive energy. A semiconductive behavior with high conductivity is observed in the nearly 2∶1 ET complex of TNBP2−. A partial charge transfer state of the ET molecules is supported by the optical spectra. The high conductivity and optical data suggest that the 2∶1 salt is not (ET1+)2(TNBP2−) but most plausibly (ET0.5+)2(HTNBP1−).