Structure of an organic charge-transfer salt derived from dibenzotetrathiafulvalene and tetrafluorotetracyanoquinodimethane (DBTTF–TCNQF4). Observation of a high-temperature phase transition

Abstract

The crystal structure of the organic charge-transfer salt DBTTF–TCNQF4 is reported. The structure has been determined by single-crystal x-ray diffraction techniques and has been shown to consist of segregated stacks of donor and acceptor molecules. Within both the donor and acceptor stacks, significant dimerization is observed at room temperature. From the geometries of the DBTTF and TCNQF4 molecules, it is concluded that the charge transfer is complete. The electrostatic contribution to the crystal cohesion for DBTTF–TCNQF4 has been calculated and compares well to those for similar salts of unit charge transfer. It is suggested that the room-temperature structure may well be representative of the low-temperature phase of a system affected by a Peierls instability. Based on diffraction data, a phase transition near 390 K is reported. The driving force for the transition is likely a spin–phonon instability. Crystal data for DBTTF–TCNQF4 are triclinic, space group P1̄; a = 13.159(3) Å, b = 13.703(4) Å, c = 7.533(2) Å, α = 102.89(2)°, β = 105.98(2)°, γ = 63.47(2)°, V = 1160 Å3; with a final R value for the refinement at 298 K of 0.063 based on the 2797 data with F0≳3σ(Fo).