Alkali-Metal-Intercalated Aromatic Hydrocarbon Conductors

The structural organization and the determination of an intercalant nature are necessary to understand the band structure and electronic activities of alkali-metal-intercalated aromatic molecular crystals. Here we report the effect of conjugation extension and potassium intercalation on the conductivity of polyphenyls (biphenyl, p-terphenyl, and p-quaterphenyl); metallic conductivity is observed for potassium-intercalated p-quaterphenyl. The conductivity increases with an increase in the length of the paraphenylene repeating units, while mechanistic studies uncover the nature of intercalation between alkali-metal and paraphenylene repeating units through electron and Raman spectroscopy and thermal analysis. The charge-transfer structures are described on the basis of the characteristics of the radical formation and polaron–bipolaron transitions, while the partial chemical bonds between potassium and the paraphenylene chains result from the hybridization between the s orbitals of potassium and the p orbitals of phenylene. This study improves our understanding of the roles of conjugation length in the charge transport properties of aromatic molecular crystals.