One-Step Synthesis of Cyanated Quinoxaline Regioisomers Enables n-Type Polymers with Record Ambient-Processed Thermoelectric Performance

Abstract

Advancements in organic electronics critically depend on the development of high-performance n-type π-conjugated polymers. However, electron-depleted polymers of this kind exhibiting facile synthesis, excellent processability and good air stability remain rare due to limitations in accessing building blocks meeting these requirements. Here we develop two novel, structurally simple and yet π-extended and highly electron-deficient cyanated building blocks: 2,3-quinoxalinedicarbonitrile (QxCN) and 6,7-quinoxalinedicarbonitrile (CNQx). These units are synthesized in a single-step from low-cost precursors via regioselective cyanation of the weak acceptor quinoxaline. Copolymerization with thiophene-flanked diketopyrrolopyrrole affords two acceptor–acceptor type copolymers TDPP-QxCN and TDPP-CNQx, respectively, exhibiting significantly deeper LUMO levels (∼−3.9 eV) compared to the noncyanated polymer analogue TDPP-Qx (−3.35 eV). In-depth spectroscopic, theoretical, and microstructural analyses reveal that cyanation at the 6,7-positions in TDPP-CNQx suppresses minority carrier effects associated with intramolecular charge transfer in TDPP-QxCN, while promoting excellent conformational adaptability that drives efficient self-assembly into a highly crystalline film with a favorable balance of ordered and disordered domains. This microstructural organization ensures rapid electron transport and efficient dopant diffusion, leading to large field-effect mobilitiesfor TDPP-CNQx (up to 1.3 cm² V^–1 s^–1) and unprecedented n-doping efficiency even under air-processed conditions. Consequently, n-doped TDPP-CNQx films exhibit a high n-type electrical conductivity (44.50 S cm^–1), excellent power factor (65.16 μW m^–1 K^–2), and ultralow thermal conductivity (0.10 W m^–1 K^–1), collectively culminating in a record thermoelectric figure of merit (ZT = 0.20) for solution/ambient-processed films. This performance also surpasses nearly all vacuum-processed counterparts, positioning CNQx as a highly promising building block for next-generation organic electronics.