Preparation of metallic BEDT-TTF charge transfer complex of 3,3',5,5'- tetranitro-4,4'-biphenyldiol dianion (TNBP^2-) having flexible molecular shape

Two kinds of 3,3',5,5'-tetranitro-4,4'-biphenyldiol dianion (TNBP^2-) molecules having different dihedral angles (φ) between the two phenyl rings exist in the tetrabutylammonium salt of TNBP^2- (φ = 0°and 35°), while only one kind of TNBP^2- molecule exists in the tetraphenylphosphonium salt (φ = 0°). The 4: 1 cation radical salt of BEDT-TTF (ET) with TNBP^2- as a 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 TNBP^2- 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 TNBP^2-. 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 (X= 6.7 x 10^-4 emu spin^-1) compared with the conventional organic metal based on ET (X=3.4-5.5 x 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 TNBP^2-. 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(TNBP^2-) but most plausibly (ET^0.5 +)₂(HTNBP^1-).