Structure, dielectric relaxation and electrical conductivity of 2,3,7,8-tetramethoxychalcogenanthrene–2,3-dichloro-5,6-dicyano-l,4-benzoquinone 1 : 1 charge-transfer complexes

Abstract

2,3,7,8-Tetramethoxychalcogenanthrenes (5,10-chalcogena-cyclo-diveratrylenes, ‘Vn₂E₂’, E = S, Se) form isotypical 1 : 1 charge-transfer (CT) complexes with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ). X-ray analysis of Vn₂S₂·DDQ shows the compound to have a columnar structure with segregated stacks of donors and acceptors. The donors are virtually planar in accordance with a formulation of [Vn₂E₂]⁺[DDQ]^–. Donor cations and acceptor anions are equidistant in their respective stacks, but in each case they inclined to the stacking axis, nevertheless guaranteeing an optimum overlap of the half-filled frontier orbitals which are of π-type character according to MNDO calculations. Dielectric ac measurements of permittivity ε′ and loss factor ε″ clearly reveal two processes, a dielectric one at low temperatures and a conductive one at high temperatures. The dielectric process can be described by the Havriliak–Negami (HN) and the Kohlrausch–Williams–Watts (KWW) model, and the conductive process by a Debye-type plot. Using these methods, the relevant parameters are evaluated. The dc conductivities of polycrystalline samples moulded at 10⁸ Pa show a temperature dependence in the plots of ln σ vs. T^–1, which is typical of semiconductors. Two slopes are found; that in the low-temperature region (< 285 K) is explained by an easy-path model (intragrain conductivity with low activation energies), whereas in the high-temperature region conduction across the grain boundaries (with higher activation energies) is becoming predominant. The activation energies for the intrinsic conductivities obtained by the ac and dc measurements are similar. Despite the columnar structure with segregated stacks, due to stoichiometric oxidation states of the components, the absolute values of conductivity are low (ca. 10 ^–6 S cm^–1 at 293 K), though higher (by a factor of ca. 10³) than those of compounds like Vn₂E2₂·TCNQ with stacks in which donor and acceptor molecules alternate.