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Magnetic Ordering and Conductivity in Heavy Atom and Multi-Orbital Radicals

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 [1]. 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 [2]. We have also demonstrated that heavy atom radicals are effective in the design of organic ferromagnets [3], 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 [4]. Ferromagnetic exchange interactions are also favored by Hund’s rule coupling [5]. 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.

[1] N. H. McCoy and W. C. Moore. J. Am. Chem. Soc. 33, 273 (1911).
[2] A. A. Leitch, K. Lekin, S. M. Winter, L. E. Downie, H. Tsuruda, J. S. Tse, M. Mito, S. Desgreniers, P. A. Dube, S. Zhang, Q. Liu, C. Jin, Y. Ohishi and R. T. Oakley. J. Am. Chem. Soc. 133, 6051 (2011).
[3] S. M. Winter, S. Hill and R. T. Oakley. J. Am. Chem. Soc. 137, 3720 (2015).
[4] D. Tian, S. M. Winter, A. Mailman, J. W. L. Wong, W. Yong, H. Yamaguchi, Y. Jia, J. S. Tse, S. Desgreniers, R. A. Secco, S. R. Julian, C. Jin, M. Mito, Y. Ohishi and R. T. Oakley. J. Am. Chem. Soc. 137, 14136 (2015).
[5] A. Mailman, S. M. Winter, J. W. L. Wong, C. M. Robertson, A. Assoud, P. A. Dube and R. T.