Benzothienobenzothiophene-Based Molecular Conductors: High Conductivity, Large Thermoelectric Power Factor, and One-Dimensional Instability

On the basis of an excellent transistor material, [1]­benzothieno­[3,2-b]­[1]­benzothiophene (BTBT), a series of highly conductive organic metals with the composition of (BTBT)₂XF₆ (X = P, As, Sb, and Ta) are prepared and the structural and physical properties are investigated. The room-temperature conductivity amounts to 4100 S cm^–1 in the AsF₆ salt, corresponding to the drift mobility of 16 cm² V^–1 s^–1. Owing to the high conductivity, this salt shows a thermoelectric power factor of 55–88 μW K^–2 m^–1, which is a large value when this compound is regarded as an organic thermoelectric material. The thermoelectric power and the reflectance spectrum indicate a large bandwidth of 1.4 eV. These salts exhibit an abrupt resistivity jump under 200 K, which turns to an insulating state below 60 K. The paramagnetic spin susceptibility, and the Raman and the IR spectra suggest 4k_F charge-density waves as an origin of the low-temperature insulating state.