Lewis Acid-Catalyzed Condensation for Facile Synthesis of Quinoidal 2-(5-Methylenethiazol-2(5<i>H</i>)-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(5<i>H</i>)-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 (<i>D</i>*) 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.