In principle, the unpaired electron furnished by an organic radical should be capable of serving as a carrier of charge, just like the valence electron in an elemental metal like sodium . In practice, however, charge transport has been difficult to achieve in radical-based materials, because of a large onsite Coulomb barrier ( U ) to charge transfer coupled with weak intermolecular interactions and hence a low bandwidth ( W ). The development of radicals which do not dimerize in the solid state, and yet display a sufficiently large bandwidth to overcome U has been a huge synthetic challenge. For many years our approach focused on the use of sulfur and selenium containing heterocycles. The combination of heavy (soft) heteroatoms and a highly delocalized spin distribution decreases U , while the enhanced intermolecular interactions afforded by the spatially extensive 3p/4p valence orbitals of S/Se lead to increased bandwidth. This chemical control, coupled with the aid of physical pressure to amplify W , produced the first organic radical metals . We have also demonstrated that heavy atom radicals are effective in the design of organic ferromagnets , and shown that the accompanying spin-orbit coupling effects give rise to magnetic anisotropies normally associated with d-block metals.
More recently we have found that both conductivity and magnetic properties in radical-based materials can be dramatically improved by the use of multi-orbital systems, that is, radicals with low lying pi -acceptor levels which afford additional degrees of freedom for charge transport and magnetic exchange. In the solid state, such radicals enjoy enhanced conductivity by virtue of an intrinsically lower U and a larger effective bandwidth . Ferromagnetic exchange interactions are also favored by Hund’s rule coupling . In this presentation the structures, magnetic and charge transport properties of key examples of heavy atom and multiband radicals, at ambient and elevated pressures, will be described.
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