Packing structures of (trialkylsilyl)ethynyl-substituted dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophenes (DNTTs): effects of substituents on crystal structures and transport properties

Abstract

The electronic structure of an organic semiconductor that is governed by its packing structure in the solid state is critically important for the transport properties of the organic-semiconductor solid. Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophenes (DNTTs), which have been utilized as active semiconducting materials in high-performance organic field-effect transistors (OFETs), crystalized into a herringbone packing structure that affords a two-dimensional (2D) electronic structure. Here, we have investigated the effects of substituents on the packing structure of a series of DNTT derivatives modified with (trialkylsilyl)ethynyl substituents. The packing structures of the derivatives with small (trialkylsilyl)ethynyl substituents, such as (trimethylsilyl)ethynyl (TMSE) (1) and (triethylsilyl)ethynyl (TESE) (2) groups, were revealed to be the herringbone packing, whereas (triisopropylsilyl)ethynyl (TIPSE)-substituted DNTT (3) crystalized into a sandwich herringbone structure, in which face-to-face dimers of molecules are packed into a herringbone array. In-depth analyses of the packing structures by means of the functional-group symmetry-adapted perturbation theory (F-SAPT) calculations showed that the size of the TESE groups is critical to determine the packing structure; an intermolecular interaction between the sterically demanding TIPSE groups does not allow 3 to crystalize into the herringbone structure that is favorable for most of the DNTT derivatives including 1 and 2. Although the sandwich herringbone structure has not been regarded as a potential one for efficient carrier transport in the solid state, OFETs based on 1–3 showed a comparable high mobility of 0.19 to 0.87 cm² V⁻¹ s⁻¹, indicating that the sandwich herringbone structure is not necessarily quite unsuitable for carrier transport, which was also endorsed by the transfer integrals calculated based on the packing structure elucidated by single-crystal X-ray analysis. These results imply that versatile modifications on the molecular and packing structures can be applied to the DNTT core for the development of superior organic semiconductors.