Enhanced Conductivity and Magnetic Ordering in Isostructural Heavy Atom Radicals

Synthetic methods have been developed to generate the complete series of resonance-stabilized heterocyclic thia/selenazyl radicals 1a−4a. X-ray crystallographic studies confirm that all four radicals are isostructural, belonging to the tetragonal space group P4̅2₁m. The crystal structures consist of slipped π-stack arrays of undimerized radicals packed about 4̅ centers running along the z direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular E₂---E₂′ contacts. Variable temperature conductivity (σ) measurements reveal an increase in conductivity with increasing selenium content, particularly so when selenium occupies the E₂ position, with σ(300 K) reaching a maximum (for E₁ = E₂ = Se) of 3.0 × 10⁻⁴ S cm⁻¹. Thermal activation energies E_act follow a similar profile, decreasing with increasing selenium content along the series 1a (0.43 eV), 3a (0.31 eV), 2a (0.27 eV), 4a (0.19 eV). Variable temperature magnetic susceptibility measurements indicate that all four radicals exhibit S = 1/2 Curie−Weiss behavior over the temperature range 20−300 K. At lower temperatures, the three selenium-based radicals display magnetic ordering. Radical 3a, with selenium positioned at the E₁ site, undergoes a phase transition at 14 K to a weakly spin-canted (ϕ = 0.010°) antiferromagnetic state. By contrast, radicals 2a and 4a, which both possess selenium in the E₂ position, order ferromagnetically, with Curie temperatures of T_c = 12.8 and 17.0 K, respectively. The coercive fields H_c at 2 K of 2a (250 Oe) and 4a (1370 Oe) are much larger than those seen in conventional light atom organic ferromagnets. The transport properties of the entire series 1a−4a are discussed in the light of Extended Hückel Theory band structure calculations.