Preparative, structural, and electrical studies of the 1 : 1 and 2 : 3 tetracyanoquinodimethane salts of 4,4′-bithiopyranylidene (BTP)

Abstract

The preparation, single crystal growth, and single crystal conductivity of the 1 : 1 and 2 : 3 7,7,8,8-tetracyanoquinodimethane (TCNQ) salts of 4,4′-bithiopyranylidene are reported. The crystal structure of the 2 : 3 salt, the first structure of a 2 : 3 TCNQ salt from a neutral planar closed-shell donor, has been determined by X-ray diffraction at room temperature. (BTP)₂(TCNQ)₃ crystallizes in the triclinic space group P, a= 8.351(5), b= 17.445(4), c= 8.020(4), α= 97.35(3), β= 103.10(4), γ= 88.15(4), U= 1 128.6(8)ų, Z= 1, D_calc.= 1.467 g cm^–3, D_obs.= 1.46(1) g cm^–3. The structure was solved by Patterson and Fourier methods to a final R of 0.046 for 1 490 unique reflections with F₀ > 3σ(F₀). (BTP)₂(TCNQ)₃ has a mixed stack crystal structure which alternates a pair of BTP cation-radicals and three TCNQ molecular species. There are two crystallographically distinct types of TCNQ : a centrosymmetric (c-TCNQ) and a non-centrosymmetric (n-TCNQ) form in the ratio 1 : 2. The bond lengths of centrosymmetric TCNQ closely resemble those of neutral TCNQ while those of non-centrosymmetric TCNQ are similar to those of TCNQ^–1, a situation found in other 2 : 3 TCNQ salts. The bridging double bond of the BTP cation-radical is quite long, 1.429(6)Å, ca. 0.05 Å longer than that of a neutral BTP derivative. As found in the 1 : 1 heterafulvalene–TCNQ segregated stack conductors, there are short contact distances between the sulphur atoms on the BTP cation and the cyano nitrogens on TCNQ. However, in the present structure this mode of interaction is qualitatively different in that each BTP sulphur atoms contacts (‘plugs into’) two cyano groups, while in the heterafulvalene–TCNQ salts, each chalcogen contacts only one cyano nitrogen. While BTP is iso-π-electronic to tetrathiafulvalene (TTF), which interacts with TCNQ to give a metallic 1 : 1 salt, BTP–TCNQ is a semiconductor with a room temperature conductivity (σ) of 30 ohm^–1 cm^–1. The temperature dependence of σ for BTP–TCNQ between room temperature and 80 K is well described by σ(T)=AT^–αexp(–Δ/T), an expression representing the product of an activated charge carrier concentration with a system specific activation energy, Δ and a temperature dependent mobility, with α 2.64 and Δ 1 480 K (0.13 eV). As expected from its mixed stack crystal structure, (BTP)₂(TCNQ)₃ is a semiconductor with a room temperature single crystal conductivity of 0.10, ohm^–1 cm^–1 and an activation energy of 0.16 and 0.18 eV measured from conductivity and thermoelectric power respectively.