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.