Design, synthesis and characterization of neutral radical organic conductors: The spiro-bis (1,9-disubstituted phenalenyl) boron system

Novel electronic devices (LED, FET, etc.) and room-temperature superconductivity are the most challenging fields in material science. Progress in these fields relies on the design and synthesis of new materials that have novel electronic properties in the solid state. Organic conductors and superconductors have focused on charge-transfer salts for more than 30 years. Although progress has been made, the transition temperatures of the majority of organic superconductors are far less than those of the ceramic-based superconductors.; In 1975 Haddon predicted in a Nature article that neutral radicals, especially the phenalenyl system (PLY), could be used to make organic conductors and superconductors. Spiro-biphenalenyl boron radicals were proposed to be promising candidates in that they potentially form high-dimensional solids with a quarter-filled energy bands, which are thought to be important in stabilizing the metallic state. The synthesis of these radical crystals includes the preparation of various ligands and then their boron complexes, the purification of the precursors, the search for suitable reducing reagents and the crystallization of the radicals.; Hexyl radical (27) is monomeric in the solid state, and shows Curie behavior throughout the temperature ranges 10–400 K. At room-temperature its conductivity σ reaches 0.05 S/cm, the highest yet for a neutral organic molecular solid. Ethyl radical (23) has parallel PLY planes in the solid state, which form diamagnetic π dimers below 150K, while butyl radical (25) forms a diamagnetic dimer at 340K. Surprisingly the conductivity increases sharply by two orders of magnitude at the transition to the dimerized state in both compounds. Interestingly, propyl radical (24) has a conductivity four orders of magnitude smaller than those of all other compounds mentioned.; The location of the electrons in the radicals was studied. Several conductivity models were proposed based on the experimental results. By a careful study of the crystal packing, possible conducting pathways were delineated.