Impact of molecular and packing structure on the charge-transport properties of hetero[8]circulenes

Abstract

Hetero[8]circulenes have been shown to be potential charge transport materials in the field-effect devices. In particular, the hole mobility of the octathia[8]circulene thin-film can reach as much as 9 × 10⁻³ cm² V⁻¹ s⁻¹ with on/off current ratio of 10⁶ (E. S. Balenkova, R. M. Osuna, F. Rosei, V. G. Nenajdenko and D. F. Perepichka, Chem. Commun., 2008, 5354–5356). In the present paper we carry out a detailed computational study for sixteen crystals of hetero[8]circulenes to gain insight into the design principles of organic semiconductors based on relationships between the crystal packing and charge transport properties. The charge transport parameters and carrier mobilities for the hetero[8]circulenes are systematically explored using the Marcus–Hush electron transfer theory and the Einstein relation. The results show that the O/NH replacement and benzoannelation decreases the reorganization energy during the charge hopping process and increases the electronic coupling within the charge transport pathways, and that sequential replacements of the peripheral sulfur atoms by selenium atoms improve the charge transfer properties. Interestingly, the S/Se substitution induces a conduction inversion; i.e. octathia[8]circulene shows dominating hole transfer and is more suitable as a p-type material, while tetrathiatetraselena[8]circulene and octaselena[8]circulene demonstrate electron-dominated mobility and represent n-type materials. The transfer integrals are sensitive to the stacking organization of the molecules in the crystal, something that is especially clear for regioisomers of tetra-tert-butyl-substituted tetraoxa[8]circulene and azaoxa[8]circulenes. A general trend is that the charge transport within the π–π stacks plays a dominant role for the carrier mobility in the heterocirculene crystals due to the large transfer integrals within such stacking dimer models.