Polymorphism in New Thienothiophene-Thiazolothiazole Organic Semiconductors

Charge transport in organic semiconductors is heavily influenced by their solid‐state packing. To separate intermolecular packing effects from molecular electronic effects, we have synthesized two polymorphs of crystalline thienothiophene–thiazolothiazole trimers and studied them by DFT calculations, optical spectroscopy, single‐crystal X‐ray diffraction and charge‐transport measurements in field‐effect transistors. One polymorph was found to be a p‐type semiconductor with a hole mobility of 3.3×10−3 cm2 V−1 s−1, while the other was insulating. The different charge‐transport behavior and optical properties of the two polymorphs are strongly related to the topology of their π‐stacks. Subtle differences between the polymorphs, including S⋅⋅⋅S close contacts and bond‐length alternation are also discussed for their possible effects on charge transport. Crystal polymorphs are a girl's best friend: Polymorphs of novel thienothiophene‐thiazolothiazole trimers are synthesized and studied to elucidate structure–property relationships, including the effect of heteroatoms and packing geometry on charge transport.