Organic Cocrystals in the NIR Window: Functional Design, Controlled Synthesis and Frontier Applications
Organic cocrystals have emerged as a distinct functional material platform for near-infrared (NIR) optoelectronics, exploiting charge-transfer (CT) interactions between donor and acceptor constituents to generate ultranarrow optical bandgaps. This review systematically consolidates design strategies, controlled synthesis routes, structure–property relationships, characterization methodologies, and frontier applications of NIR-active organic cocrystals. The central mechanistic premise is that hybridization of the donor HOMO and acceptor LUMO produces new excited states whose absorption or emission extends into the 780–2500 nm window. Unlike conventional inorganic NIR materials, which incur high cost and complex fabrication, or discrete organic small molecules, which require multistep synthesis, cocrystals offer a non-destructive assembly route with tunable photophysical output, improved biocompatibility, and environmental stability. The review surveys cocrystal systems including TCNQ- and TCNB-based architectures, carbazole-based CT complexes, and TTF-derived conductors, with emphasis on how hydrogen- and halogen-bonding motifs govern packing, ionicity, and optoelectronic response. Controlled synthesis methods—mechanochemical grinding, vapor digestion, and solution assembly—are compared with respect to polymorph selectivity and stoichiometric fidelity. Characterization approaches spanning three-dimensional electron diffraction, FT-IR, and single-crystal X-ray analysis are evaluated for their capacity to resolve CT degree and band structure. Applications in NIR photothermal conversion, organic field-effect transistors, optical waveguides, and amplified spontaneous emission are assessed against performance metrics. Remaining challenges include scalable polymorph control, quantitative prediction of CT degree, and long-term operational stability under ambient and biological conditions.