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

Abstract

Quinoidal oligothiophenes (<b>QOT</b>), 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, <b>2DQTT</b>s, with proximal (<b>2DQTT-i</b>) and distal (<b>2DQTT-o</b>) 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-<i>b</i>]­thiophene [TT, donor (D)] and 5-alkyl-4<i>H</i>-thieno­[3,4-<i>c</i>]­pyrrole-4,6­(5<i>H</i>)-dione [TPD, acceptor (A)] units with relatively large π-surface endows these <b>2DQTT</b>s 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 <b>2DQTT</b>s 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 (<b>2DQTT-o</b>) in ambient- and solution-processed OTFTs. Investigations on thin-film morphology reveal that the microstructure of <b>2DQTT</b>s 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 <b>2DQTT-o</b> compared to <b>2DQTT-i</b>, which accounts for the superior electron transport property of <b>2DQTT-o</b>