Symmetry breaking localisation of the unpaired electron in spiro-bis(1,9-disubstituted-phenalenyl)boron radicals

We report the crystal structure of a spiro-bis(1,9-disubstituted-phenalenyl)boron cation (2(+)) and by comparison with the structures of a series of radicals we conclude that the changes in bond lengths in the radicals compared to those of the corresponding cation can be qualitatively predicted by the sign of the partial bond order of the singly occupied molecular orbital (SOMO), which contains the additional electron. By use of this information, we can analyse the structures of the pi-dimer radicals 1 and 2 and ascertain the location of the unpaired electron in these compounds. Where these radicals exist as paramagnetic pi-dimers (high-temperature regime), the unpaired electrons reside mainly in the phenalenyl units not directly involved in the pi-dimer interaction. At low temperatures, where the diamagnetic pi-dimers exist, the electrons localise in the phenalenyl units directly involved in the pi-dimer, but there still exists some degree of delocalisation of the electrons to the other halves of the molecules. We postulate that the increase in delocalisation of the unpaired electrons is related to the increase of the conductivity by about two orders of magnitude as the electrons spin pair. (C) 2002 Elsevier Science B.V. All rights reserved.