Hysteretic Spin Crossover between a Bisdithiazolyl Radical and Its Hypervalent σ-Dimer LekinKristina WinterStephen M. DownieLaura E. BaoXuezhao TseJohn S. DesgreniersSerge SeccoRichard A. DubePaul A. OakleyRichard T. 2010 The bisdithiazolyl radical <b>1a</b> is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group <i>P</i>4̅2<sub>1</sub><i>m</i> and consists of π-stacked radicals, tightly clustered about 4̅ points and running parallel to <i>c</i>. The β-phase belongs to the monoclinic space group <i>P</i>2<sub>1</sub>/<i>c</i> and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S−S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-<b>1a</b> behaves as a Curie−Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM <i>S</i> = <sup>1</sup>/<sub>2</sub> centers. The dimeric phase β-<b>1a</b> is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (<i>T</i>↑= 380 K, <i>T</i>↓ = 375 K) increase in χ and χ<i>T</i>. Powder X-ray diffraction analysis of β-<b>1a</b> at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S−S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-<b>1a</b> behaves as a Mott insulator, but the ambient-temperature conductivity σ<sub>RT</sub> increases from near 10<sup>−7</sup> S cm<sup>−1</sup> at 0.5 GPa to near 10<sup>−4</sup> S cm<sup>−1</sup> at 5 GPa. The value of σ<sub>RT</sub> for β-<b>1a</b> (near 10<sup>−4</sup> S cm<sup>−1</sup> at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ<sub>RT</sub> increases in a manner which parallels the behavior of α-<b>1a</b>. These changes in conductivity of β-<b>1a</b> are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-<b>1a</b> confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S−S···S σ-bond in β-<b>1a</b> to heat and pressure is compared to that of related dimers possessing S···Se−Se···S σ-bonds.