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

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Date and time Oct 26, 2016
from 12:10 PM to 01:00 PM
Location 60 St. George Street, MP 606
Host Stephen Julian

Richard Oakley

Department of Chemistry, University of Waterloo

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.

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