Metallic crystals with a three-dimensional narrow band based on an aromatic hydrocarbon derivative†

The properties and crystal structures of molecular conductors depend on the intermolecular interactions between their planar parts each comprising of a conjugated molecule. Such interactions frequently produce low-dimensional band structures that are susceptible to perturbation and to lose the electrical conductivity. To extend the intra- and intercolumnar interactions, heteroatoms such as chalcogen atoms are often introduced at the periphery of the constituent molecules. Yet this strategy inevitably encounters difficulties, as such molecules would be more and more difficult to synthesize with increasing the number of heteroatoms and the molecular weights, while they would still produce highly anisotropic conductors. Herein, we report an extended intermolecular interaction pattern in a newly synthesized charge-transfer complex (EtHAC)₂I₃, which includes ten ethyl groups around the periphery of its fused aromatic rings HAC. The (EtHAC)₂I₃ crystal contains columns of stable EtHAC radical cations with intercolumnar interactions by Et–Et contacts. Theoretical calculations indicate that intramolecular Et-HAC and intermolecular Et–Et interactions enable formation of three-dimensional (3D) EtHAC network. Additionally, charge-transfer interactions via Et–I contacts lead to carrier doping into the EtHAC network. These effects are combined to produce a stable 3D metallic ground state, accounting for their high 3D electrical conductivity (ca. 10–2500 S cm⁻¹) down to ∼2 K. The unique metallic properties of (EtHAC)₂I₃ are further corroborated by the calculated band structures and polarized reflectance spectra, both indicating 3D metallic characteristics. The electron spin resonance spectra of (EtHAC)₂I₃ suggest that the highly mobile unpaired electrons behave as if they are free electrons with long relaxation times. These findings add a different strategy to develop the molecular conductors and magnets.