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 3a are strongly dependent on pressure. Compression of the as-crystallized
α-phase, space group Fdd2, to 3–4 GPa
leads to its conversion into a second or β-phase, in which F-centering is lost. The space group symmetry is lowered
to Pbn21, and there is concomitant halving
of the a and b axes. A third or
γ-phase, also space group Pbn21,
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 b-glides, while in the γ-phase they follow
the n-glides. At ambient pressure 3a is a Mott insulator, displaying high but activated conductivity,
with σ­(300 K) = 6 × 10–3 S cm–1 and Eact = 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 3a. The presence of a low-lying LUMO in 3a gives
rise to high electron affinity which, in turn, creates an electronically
much softer radical with a low onsite Coulomb potential U. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs
upon pressurization, so that a metallic state is readily achieved
at relatively low applied pressure.