N-oxide-Functionalized Bipyridines as Strong Electron-Deficient Units to Construct High-Performance n-Type Conjugated Polymers

Developing low-cost unipolar n-type organic thin-film transistors (OTFTs) is necessary for logic circuits. To achieve this objective, the usage of new electron-deficient building blocks with simple structure and easy synthetic route is desirable. Among all electron-deficient building units, N-oxide-functionalized bipyridines can be prepared through a simple oxidized transformation of bipyridines. However, employing N-oxide-functionalized bipyridines as the building unit to construct efficient N-type polymers has been overlooked. This gap strongly encourages us to design and synthesize two new N-oxide building blocks, 5,5'-dibromo-[2,2'-bipyridine] 1-oxide (BPyO) and 5,5'-dibromo-[2,2'-bipyridine] 1,1'-dioxide (BPyDO), through the oxidation of sp²-N in 2,2'-bipyridine. The single-crystal X-ray diffraction shows that BPyO and BPyDO possess planar structure with strong π-stacking, which is beneficial for charge transport. Incorporation of these building blocks into acceptor-acceptor backbones leads to two new polymers, namely P(DPP-BPyO) and P(DPP-BPyDO). Both P(DPP-BPyO) and P(DPP-BPyDO) possess lower frontier molecular orbital energy levels than the non-oxide polymer P(DPP-BPy). Consequently, the transition from P(DPP-BPy) (without oxide group) to P(DPP-BPyO) (mono-oxide group) and then to P(DPP-BPyDO) (dioxide group) can decrease hole-transport performance and gradually switch the transport nature from p-type to n-type via ambipolar. These results prove that the introduction of sp²-N oxide groups in building units would be a promising strategy to approach high-performance n-type polymers.