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 is-dimer (interplanar separation of similar to3.1 Angstrom), whereas 3 is a paramagnetic pi -dimer (interplanar separation of .similar to3.3 Angstrom), 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.
作者:X. Chi、M. E. Itkis、K. Kirschbaum、A. A. Pinkerton、R. T. Oakley、A. W. Cordes、R. C. Haddon
DOI:10.1021/ja0039785
日期:2001.5.1
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 is-dimer (interplanar separation of similar to3.1 Angstrom), whereas 3 is a paramagnetic pi -dimer (interplanar separation of .similar to3.3 Angstrom), 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.
作者:Sushanta K. Pal、Mikhail E. Itkis、Fook S. Tham、Robert W. Reed、Richard T. Oakley、Robert C. Haddon
DOI:10.1021/ja077771o
日期:2008.3.1
We report the preparation, crystallization, and solid-state characterization of the first member of a new family of tris(1,9-disubstituted phenalenyl)silicon neutral radicals. In the solid state, the radical packs as weak partial pi-dimers with intermolecular carbon...carbon contacts that fall at the van der Waals atomic separation. Magnetic susceptibility measurements indicate approximately 0.7 Curie
我们报告了三(1,9-二取代苯基)硅中性自由基新家族第一个成员的制备、结晶和固态表征。在固态下,自由基以弱的部分 pi-二聚体形式与分子间碳......碳接触,落在范德华原子分离处。磁化率测量表明,从室温到 50 K,每个分子大约有 0.7 个居里自旋,低于这个温度,反铁磁耦合变得明显;该化合物的室温单晶电导率为 sigmaRT = 2.4 x 10(-6) S cm(-1)。