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Science 8 July 2005:
Vol. 309. no. 5732, pp. 281 - 284
DOI: 10.1126/science.1112446
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Reports

Resonating Valence-Bond Ground State in a Phenalenyl-Based Neutral Radical Conductor

S. K. Pal,1 M. E. Itkis,1 F. S. Tham,1 R. W. Reed,2 R. T. Oakley,2 R. C. Haddon1*

An organic material composed of neutral free radicals based on the spirobiphenalenyl system exhibits a room temperature conductivity of 0.3 siemens per centimeter and a high-symmetry crystal structure. It displays the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility that implies a density of states at the Fermi level of 15.5 states per electron volt per mole. Extended Hückel calculations indicate that the solid is a three-dimensional organic metal with a band width of ~0.5 electron volts. However, the compound shows activated conductivity (activation energy, 0.054 electron volts) and an optical energy gap of 0.34 electron volts. We argue that these apparently contradictory properties are best resolved in terms of the resonating valence-bond ground state originally suggested by Pauling, but with the modifications introduced by Anderson.

1 Departments of Chemistry and Chemical and Environmental Engineering, University of California, Riverside, CA 92521–0403, USA.
2 Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.

* To whom correspondence should be addressed. E-mail: haddon{at}ucr.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)