One-dimensional metallic conducting pathway of cyclohexyl-substituted spiro-biphenalenyl neutral radical molecular crystal

The unprecedented metallic character of the cyclohexyl-substituted spiro-biphenalenyl neutral radical molecular crystal (5) suggested by its Pauli paramagnetism [Science 2005, 309, 281] is contradicted by the thermally activated conduction measured along the needle axis of crystal 5 and by an optical gap of E_g = 0.34 eV. Herein we provide the first high quality ab initio electronic structure calculations using density functional theory to reconcile these properties. The calculations point toward 5 being a quasi one-dimensional (1-D) material, with a 1-D conducting pathway along the [101] π-chain direction. Along any directions other than the π-chain, conduction is impeded by the small interchain overlap. 5 has a quarter-filled band structure with a density of states of N(E_f) = 7.5 states eV^-1 at the Fermi level, leading to a metallic character along the π-chain.