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 4kF charge-density waves as an origin of the low-temperature insulating
state.