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.