Lewis Acid-Catalyzed Condensation for Facile Synthesis of Quinoidal 2-(5-Methylenethiazol-2(5H)-ylidene)malononitrile End-Capped Shortwave Infrared Organic Semiconductors

The facile synthesis of ultranarrow-bandgap (<1 eV) small-molecule semiconductors remains a persistent challenge for advancing shortwave infrared (SWIR) organic optoelectronic technologies. Donor–acceptor (D-A)-type molecules featuring quinoidal 2-(5-methylenethiazol-2(5H)-ylidene)malononitrile (TM) end-groups are promising for achieving this goal. However, despite extensive studies of TM-based asymmetric merocyanine dyes, their optoelectronic performances remain far from satisfactory, and the synthetic methodologies for symmetric TM-end-capped molecules remain largely unexplored. Here, we present a Lewis acid (LA)-catalyzed condensation strategy for synthesizing symmetrical TM-based molecules, overcoming the high activation energy and side reactions plaguing the conventional method. This strategy enables room-temperature construction of ADA and ADA′DA structures with high isolated yields up to 82%. Multiple SWIR absorbing materials with tunable energy levels, bandgaps, and fluorescence characteristics are constructed through systematic TM's substituent engineering. By leveraging noncovalent O–H or N–H conformational locks provided by heteroaryl substituents on TM, ultranarrow-bandgap TM-based materials down to 0.83 eV in the film state are obtained. Bulk-heterojunction (BHJ) photodetectors are fabricated using an ADA′DA-type molecule as an electron acceptor. The devices exhibit broadband response (400–1100 nm), high external quantum efficiency (EQE) (>40%), and noise-limited detectivity (D*) exceeding 1 × 1013 Jones at 1020 nm, alongside a large linear dynamic range (LDR) (118 dB) and fast response (26 kHz at −3 dB). This work establishes TM-based materials as competitive candidates for silicon-rivaled photodetection and highlights the untapped potential of quinoidal TM moieties in designing next-generation organic materials for SWIR-based technologies.