Localization of spin and charge in phenalenyl-based neutral radical conductors

We report the development of an experimentally based structural analysis to examine the degree of localization of the spin and charge in the phenalenyl-based neutral radical molecular conductors--the results motivate a reinterpretation of the electronic structure of a number of the radicals that we have reported over the past 10 years. The analysis is based on the well-known relationship between bond order and bond length and makes use of the experimental bond distance deviations between the molecular structure of the radical and its corresponding cation. We determined the single crystal X-ray structure of the ethyl radical (1) at 11 temperatures between 90 K and room temperature so that we could follow the evolution of the structure and the electron density distribution through the magnetic phase transition that occurs in the vicinity of 140 K. We show that the enhanced conductivity in the dimeric ethyl (1) and butyl (3) radicals at the magnetic phase transition results from the development of a complex, but highly delocalized electronic structure and not to the formation of a diamagnetic pi-dimer. We find that the monomeric radicals 4, 12, and 13 have an asymmetric electron density distribution in the crystal lattice whereas radical 11 is the only monomeric radical which remains fully delocalized. The pi-chain radicals (7, 8, 14, and 15) retain the strongly delocalized electronic structures expected for a resonating valence bond ground-state structure.