Lawrence Berkeley National Laboratory

Disk-shaped molecules with large aromatic π-surfaces are a class of organic semiconductors in which the charge-carrier transport properties could be greatly facilitated by preferred intermolecular stacking of the π-surfaces. The optical and electronic properties are not only determined by the core aromatic structure of these disk-shaped molecules but are also strongly dependent on the side chains, which directly impact the molecular self-assembly behavior in condensed phases. Triphenylene-tris(naphthaleneimidazole) (TP-TNI) is a recently reported n-type semiconductor featuring a large π-core and branched side chains, with an electron-transporting mobility reaching 10^-4 cm² V^-1 s^-1. To further improve material performance, a detailed study is needed to understand the dependence of carrier transport properties on both the core electronic structure and side chain. Here, we present the detailed synthesis and characterization of a TP-TNI derivative bearing linear side chains, which has demonstrated a field-effect electron-transport mobility of up to 1.3 × 10^-3 cm² V^-1 s^-1. The more than 1 order improvement in electron-transport properties over the branched side chain homologue can be correlated to ordered twisted packing in the thin film, as revealed by in situ variable temperature grazing incidence wide-angle X-ray scattering studies. In-depth theoretical understanding of the frontier orbitals, reorganization energies, and charge-transfer integrals of TP-TNI molecules has provided further insight into the relationship between the molecular stacking geometry and charge-transport properties.