Pressure Enhanced Conductivity in Bis-1,2,3-Thiaselenazolyl Dimers

A synthetic sequence to salts of N-alkylated pyridine-bridged 1,2,3-thiaselenazolo-1,2,3-thiaselenazolylium cations [2]+ (R1 = Me, Et; R2 = H) is described. The corresponding radicals 2 (R1 =
Me, Et; R2 = H) can be generated from the cations by chemical or electrochemical reduction. Crystals of
the two radicals are isostructural and consist of interpenetrating π-stacked arrays of closed-shell Se−Se
σ-bonded dimers [2]2 laced together with numerous short intermolecular Se- - -Se, Se- - -S, and Se- - -N
contacts. Variable-temperature magnetic, conductivity, and near-infrared optical measurements indicate
that the bulk materials behave as small band gap semiconductors with room-temperature conductivities
σRT near 10-6 S cm-1 and thermal activation energies Ea of 0.32 eV (R1 = Me) and 0.36 eV (R1 = Et).
LMTO band structure calculations on both compounds are consistent with this interpretation. The application
of external pressure leads to dramatic increases in conductivity; at 4 GPa σRT reaches a value near 10-1
S cm-1 for R1 = Me and 10-2 S/cm for R1 = Et. The conductivity remains activated for both compounds,
but for R1 = Me the activation energy Ea is reduced to near 0.03 eV at 5 GPa, suggestive of a weakly
metallic state.