Molecularly Aligned Hexa-peri-hexabenzocoronene Films by Brush-Coating and Their Application in Thin-Film Transistors

The facile Chinese brush-coating method was used to prepare oriented thin films of hexa-peri-hexabenzocoronene (HBC) derivatives on the silicon substrate. As a result of the directional solution-coating, the D3h-symmetry (HBC-1,3,5-Ph-C12) and the C1-symmetry (HBC-1,2,4-Ph-C12) derivatives displayed an anisotropic alignment, with mostly edge-on orientation on SiO2 surfaces modified with various silane-based monolayers. On these silane-modified surfaces, the higher symmetry molecule HBC-1,3,5-Ph-C12 developed a hexagonally packed superstructure, which provided greater π orbital overlap and presumably the electronic coupling between neighboring molecules. In particular, the use of an octyltrichlorosilane (OTS)-modified surface enabled brush-coated thin films to have higher anisotropic orientation, crystallinity, and favorable molecular arrangement. In contrast, the growth of the hexagonal packing of low-symmetry derivative HBC-1,2,4-Ph-C12 was only achieved on the phenyltrichlorosilane and OTS surfaces. Thin-film transistors based on these brush-coated films gave a maximum mobility of 0.1 and 0.056 cm2 V–1 s–1, which are 2 orders of magnitude improvement over the devices with unoriented films prepared by spin-coating. The results indicate that the molecular packing of discotic liquid crystals on the silane-modified surface is sensitively influenced by the molecular symmetry, which affects intermolecular interactions as well as molecule/surface interactions. This study provides a simple way to fabricate aligned films for HBC derivatives for transistor application.