Isomeric Acceptor–Acceptor Polymers: Enabling Electron Transport with Strikingly Different Semiconducting Properties in n-Channel Organic Thin-Film Transistors

The “acceptor–acceptor” (A–A) backbone strategy is considered one of the most promising molecular design strategies to achieve high-performance n-type semiconducting polymers. However, developing high-mobility A–A type polymers is highly challenging because of the steric hindrance inherited in typical acceptor building blocks. On the other hand, the acceptor units with isomeric chemical structures, which can induce interesting optoelectronic properties, are rarely studied in n-type semiconducting polymers because of the great challenge in the synthesis of isomers. To deeply understand the effects of isomeric electron-accepting structures on the physicochemical properties and device performances of n-type semiconducting polymers, herein, we design and synthesize two isomeric bithiazole dicarboxylate ester derivatives, namely, 2-BTzE (2,2′-bithiazole) and 5-BTzE (5,5′-bithiazole), leading to two isomeric polymers P(BTI-2-BTzE) and P(BTI-5-BTzE), respectively. These two polymers have the same backbone and side chains but different linking positions of bithiazole. This subtle change leads to a striking difference in their polymerization reaction activity, molecular geometry, and solid-state packing. Thus, P(BTI-2-BTzE) demonstrates higher Mn, more planar backbone, and more ordered solid-state packing than those of P(BTI-5-BTzE). Thanks to the favorable optoelectronic properties and the backbone geometry, P(BTI-2-BTzE)-based organic thin-film transistors (OTFTs) yield a significantly higher electron mobility (μe) of 0.1 cm2 V–1 s–1, which is >200 times higher than that of P(BTI-5-BTzE) (μe = 4.1 × 10–4 cm2 V–1 s–1). Overall, this study demonstrates that the isomerization of acceptors is an effective strategy to solve the “steric hindrance” issue of A–A type polymers, eventually maximizing the device performance of n-channel OTFTs.