title: Dimeric phenalenyl-based neutral radical molecular conductors.
abstract: We report the preparation, crystallization, and solid-state characterization of ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals. Both of these compounds are found to be conducting face-to-face pi-dimers in the solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic pi-dimer (interplanar separation of approximately 3.1 A), whereas 3 is a paramagnetic pi-dimer (interplanar separation of approximately 3.3 A), and both compounds show phase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transition from the high-temperature paramagnetic pi-dimer form to the low-temperature diamagnetic pi-dimer structure is accompanied by an increase in conductivity by about 2 orders of magnitude. This behavior is unprecedented and is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tentatively assign the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation.
authors.affiliation: Department of Chemistry, Advanced Carbon Materials Center, University of Kentucky, Lexington, Kentucky 40506-0055, USA.
authors.forename: X
authors.initials: X
authors.lastname: Chi
authors.forename: M E
authors.initials: ME
authors.lastname: Itkis
authors.forename: K
authors.initials: K
authors.lastname: Kirschbaum
authors.forename: A A
authors.initials: AA
authors.lastname: Pinkerton
authors.forename: R T
authors.initials: RT
authors.lastname: Oakley
authors.forename: A W
authors.initials: AW
authors.lastname: Cordes
authors.forename: R C
authors.initials: RC
authors.lastname: Haddon