Dimeric phenalenyl-based neutral radical molecular conductors

We report the preparation, crystallization, and solid-state characterizatio n 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 t he solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic is-dimer (interplanar separa tion of similar to3.1 Angstrom), whereas 3 is a paramagnetic pi -dimer (int erplanar separation of .similar to3.3 Angstrom), and both compounds show ph ase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transi tion from the high-temperature paramagnetic pi -dimer form to the low-tempe rature diamagnetic pi -dimer structure is accompanied by an increase in con ductivity by about 2 orders of magnitude. This behavior is unprecedented an d is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tent atively 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.