Thiacycle-fused benzo[1,2-: B:4,5- b ′]dithiophenes (BDTs): Synthesis, packing, molecular orientation and semiconducting properties

The molecular and packing structures of organic semiconductors play crucial roles in determining their charge carrier mobilities in organic field-effect transistors (OFETs). In this article, a systematic study on the functionalization of benzo[1,2-b:4,5-b′]dithiophene (BDT) with thiacycles to tune the packing, molecular orientation and semiconducting properties is reported. Among the designed BDT derivatives with six- or five-membered thiacycles containing sulphur atoms connected at the α-, β- or both α- and β-positions, the derivatives with β-sulphur atoms in the six-membered thiacycle exhibited a rubrene-like "pitched" π-stacking pattern and edge-on molecular orientation on the substrate. On the other hand, other derivatives exhibited different packing structures with a smaller intermolecular orbital overlap and end-on orientation. The thin-film OFETs based on the former molecules exhibited higher mobility than the latter, correlating the transport properties in the thin-film state with the position of the sulphur atoms and size of the thiacycles, which suggests novel molecular modification strategies in thienoacenes for the development of high performance semiconducting materials.