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Multicentre bonding of semiquinone radicals under variable temperature and high pressure conditions

The dominant intermolecular interaction in crystals of organic radicals is π-stacking, which involves some of the closest interplanar and intermolecular C-C contacts (< 3 Å). [1-2] Quantum chemical models indicate a considerable covalent component in the total interaction, and this type of weak multicentre covalent bonding has been dubbed ‘pancake bonding’. [3] Physical properties of crystals, such as magnetism and electric conductivity, are defined by stacking, so its in-depth study is important for understanding and designing radical-based molecular materials with targeted properties. Three types of stacks of planar radicals are known: (1) stacks of 'pancake-bonded' dimers, separated by weaker stacking contacts, (2) stacks of equidistant radicals with long-range ordering and (3) stacks of 'pancake-bonded' trimers of partially charged radicals. [1-2] The first combined variable-temperature (90 – 400 K) and high-pressure (0.1 MPa – 6.0 GPa) single-crystal X-ray diffraction study of multicentre bonding (‘pancake bonding’) between planar semiquinone radicals revealed their behavior under a broad range of conditions. The dimension of a unit-cell, volume and distances between ring planes increased with increasing temperature. High pressure caused the phase transitions and decreased interplanar separations, indicating increased covalent contribution as compared to multicentre bonding at ambient pressure.

multicentre bonding ; semiquinone radicals, variable-temperature ; high-pressure

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