Two-Dimensional π‑Expanded Quinoidal Terthiophenes Terminated with Dicyanomethylenes as n‑Type Semiconductors for High-Performance Organic Thin-Film Transistors

Quinoidal oligothiophenes (QOT), as classical n-type semiconductors, have been well-known for a long time but with non-optimal semiconducting properties. We report here the design and selective synthesis of new two-dimensional (2D) π-expanded quinoidal terthiophenes, 2DQTTs, with proximal (2DQTT-i) and distal (2DQTT-o) regiochemistry for high-performance n-channel organic thin-film transistors (n-OTFTs) featuring high electron mobility, solution processability, and ambient stability. The elegant combination of thieno­[3,4-b]­thiophene [TT, donor (D)] and 5-alkyl-4H-thieno­[3,4-c]­pyrrole-4,6­(5H)-dione [TPD, acceptor (A)] units with relatively large π-surface endows these 2DQTTs with distinctive 2D structural characteristics and flat configuration stabilized by weak intramolecular S–O/S weak interactions. Furthermore, the A–D–A–D–A electronic structure maintains an adequately low LUMO energy level. These 2DQTTs are shown to exhibit outstanding semiconducting properties with electron mobilities of up to 3.0 cm2 V–1 s–1 and on/off ratios of up to 106 (2DQTT-o) in ambient- and solution-processed OTFTs. Investigations on thin-film morphology reveal that the microstructure of 2DQTTs is highly dependent on the orientation of the fused thiophene subunits, leading to differences in electron mobilities of 1 order of magnitude. X-ray diffraction studies in particular reveal increased crystallinity, crystalline coherence, and orientational order in 2DQTT-o compared to 2DQTT-i, which accounts for the superior electron transport property of 2DQTT-o.