Pressure Induced Phase Transitions and Metallization of a Neutral Radical Conductor

The crystal structure and charge transport properties of the prototypal oxobenzene-bridged 1,2,3-bisdithiazolyl radical conductor <b>3a</b> are strongly dependent on pressure. Compression of the as-crystallized α-phase, space group <i>Fdd</i>2, to 3–4 GPa leads to its conversion into a second or β-phase, in which <i>F</i>-centering is lost. The space group symmetry is lowered to <i>Pbn</i>2₁, and there is concomitant halving of the <i>a</i> and <i>b</i> axes. A third or γ-phase, also space group <i>Pbn</i>2₁, is generated by further compression to 8 GPa. The changes in packing that accompany both phase transitions are associated with an “ironing out” of the ruffled ribbon-like architecture of the α-phase, so that consecutive radicals along the ribbons are rendered more nearly coplanar. In the β-phase the planar ribbons are propagated along the <i>b</i>-glides, while in the γ-phase they follow the <i>n</i>-glides. At ambient pressure <b>3a</b> is a Mott insulator, displaying high but activated conductivity, with σ­(300 K) = 6 × 10^–3 S cm^–1 and <i>E</i>_act = 0.16 eV. With compression beyond 4 GPa, its conductivity is increased by 3 orders of magnitude, and the thermal activation energy is reduced to zero, heralding the formation of a metallic state. High pressure infrared absorption and reflectivity measurements are consistent with closure of the Mott–Hubbard gap near 4–5 GPa. The results are discussed in the light of DFT calculations on the molecular and band electronic structure of <b>3a</b>. The presence of a low-lying LUMO in <b>3a</b> gives rise to high electron affinity which, in turn, creates an electronically much softer radical with a low onsite Coulomb potential <i>U</i>. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs upon pressurization, so that a metallic state is readily achieved at relatively low applied pressure