Biaxially Extended Conjugated Polymers with Thieno[3,2‑b]thiophene Building Block for High Performance Field-Effect Transistor Applications

Biaxially thiophene side chain extended thieno­[3,2-b]­thiophene (TT2T)-based polymers, PTTT2T, P2TTT2T, PTTTT2T, and PTVTTT2T, were synthesized by Stille coupling polymerization with different conjugated moieties of thiophene (T), bithiophene (2T), thieno­[3,2-b]­thiophene (TT), and thiophene–vinylene–thiophene (TVT), respectively. The electronic properties of the prepared polymers could be effectively tuned because the variant π-conjugated building block affected the backbone conformation and the resulted morphology. The morphology of the thin films characterized by atomic force microscopy and grazing incidence X-ray diffraction showed that P2TTT2T and PTVTTT2T thin films possessed a better molecular packing with a nanofiber structure owing to their coplanar backbone. The average field-effect mobilities of PTTT2T, P2TTT2T, PTTTT2T, and PTVTTT2T were 6.7 × 10^–6, 0.36, 2.2 × 10^–3, and 0.64 cm² V^–1 s^–1 (maximum 0.71), respectively, attributed to the coplanarity of polymer skeleton. In addition, the fabricated FET devices showed a high on/off ratio over 10⁷ under ambient for over 3 months, suggesting the excellent environmental stability. The above results demonstrated that the biaxially extended fused thiophene based conjugated polymers could serve as a potential candidate for organic electronic device applications.