Key Takeaways & Executive Findings
- •• • Carbazole-based cocrystals with tunable CT degree achieve NIR photothermal conversion, with performance directly correlated to CT strength (CrystEngComm, 2022, 24: 4622–4628); this enables precise thermal-dose engineering for clinical hyperthermia and industrial NIR curing. • • TCNQ/TCNB cocrystals based on an orthocetamol backbone were solved by three-dimensional electron diffraction (Cryst Growth Des, 2022, 22: 1155–1163), providing an analytical route for nanocrystalline phases that cannot be resolved by conventional single-crystal XRD, critical for quality control of micro- and nanoscale cocrystal formulations. • • TTF–CA black polymorph synthesis is solvent-dependent, with mechanochemical and vapor digestion routes yielding distinct polymorphs confirmed by FT-IR and crystal packing analysis (Cryst Growth Des, 2014, 14: 91–100); this establishes solvent and processing history as first-order variables in conductivity and optical performance. • • Internal molecular motions in charge-transfer cocrystals govern photothermal conversion efficiency (Chem Mater, 2023, 35: 10009–10017), indicating that molecular dynamics—not static CT energetics alone—must be engineered to optimize heat generation for NIR-triggered applications.
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Abstract
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.
1. Introduction
Near-infrared (NIR) functional materials have historically been dominated by inorganic semiconductors and rare-earth-doped systems, which deliver robust absorption and emission but impose high raw-material cost, complex vacuum fabrication, and limited biocompatibility. Organic small-molecule alternatives reduce cost and toxicity but typically require multistep covalent synthesis, and their optical bandgaps rarely extend into the NIR without elaborate molecular engineering. The result is a persistent gap between the demand for low-cost, solution-processable NIR materials and the available commercial palette, particularly for biomedical and large-area optoelectronic applications.
Organic cocrystals address this bottleneck through a non-destructive supramolecular assembly strategy: donor and acceptor molecules are combined in a stoichiometric ratio, and HOMO–LUMO hybridization generates new excited states with ultranarrow bandgaps that push absorption or emission into the 780–2500 nm window. This review consolidates the design rules, controlled synthesis methods, structural characterization techniques, and frontier applications of NIR-active organic cocrystals, with emphasis on charge-transfer modulation, hydrogen- and halogen-bonding control, and the translation of these materials into photothermal, transistor, and waveguide platforms.
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SHI Rui, HONG Wanglong, ZHU Shengli, CUI Zhenduo, LI Zhaoyang, WU Shuilin, XU Wence, GAO Zhonghui, LIANG Yanqin, JIANG Hui (2025). Organic Cocrystals in the NIR Window: Functional Design, Controlled Synthesis and Frontier Applications. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3546-5
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Frequently Asked Questions
What is the primary failure mechanism of organic NIR cocrystals under continuous NIR irradiation, and how does it limit clinical photothermal applications?
The dominant degradation pathway is photoinduced molecular motion and sublimation of the weaker-bound component, particularly in CT cocrystals where internal molecular motions govern photothermal conversion (Chem Mater, 2023, 35: 10009–10017). Under sustained NIR flux, these motions can drive polymorphic transitions or component segregation, reducing CT efficiency and shifting the absorption band. For clinical hyperthermia, this translates to a loss of thermal dose predictability and potential toxicity from free acceptor molecules. Mitigation requires either covalent tethering of the donor–acceptor pair or encapsulation in a rigid matrix to suppress molecular mobility.
Can organic NIR cocrystals achieve cost parity with inorganic NIR materials such as rare-earth-doped nanoparticles for biomedical imaging?
Cost parity is plausible at the material level because cocrystals are assembled from commercially available organic donors and acceptors via mechanochemical or solution routes, avoiding rare-earth precursors and high-vacuum processing. However, the hidden cost lies in polymorph control and purification: solvent-dependent polymorphs such as TTF–CA black (Cryst Growth Des, 2014, 14: 91–100) require stringent process control, and batch-to-batch reproducibility remains a scalability bottleneck. For clinical translation, the cost advantage is offset by the need for extensive toxicity and stability testing, which inorganic systems have already partially cleared.
How does the degree of charge transfer quantitatively affect NIR photothermal conversion efficiency in carbazole-based cocrystals?
Carbazole-based cocrystals with different degrees of charge transfer show a direct correlation between CT strength and NIR photothermal conversion (CrystEngComm, 2022, 24: 4622–4628). Higher CT degree narrows the optical bandgap, red-shifting absorption deeper into the NIR and increasing the fraction of absorbed photons converted to heat. However, excessive CT can also increase non-radiative decay pathways that compete with radiative emission, which is detrimental for imaging but beneficial for photothermal therapy. The practical implication is that CT degree must be tuned to the specific application: moderate CT for NIR emission, high CT for photothermal heating.
What scalability bottlenecks prevent the industrial production of TCNQ- and TCNB-based NIR cocrystals?
The principal bottleneck is polymorph and stoichiometry control during scale-up. TCNQ/TCNB cocrystals with an orthocetamol backbone required three-dimensional electron diffraction to resolve their structure (Cryst Growth Des, 2022, 22: 1155–1163), indicating that conventional XRD is insufficient for nanocrystalline batches. Mechanochemical synthesis can produce phase-pure material at gram scale, but heat generation during milling can induce polymorphic transitions. Vapor digestion offers better polymorph selectivity but is slow and difficult to scale. Without in-line process analytical technology capable of resolving CT degree and polymorph in real time, industrial production will struggle to meet the batch-to-batch consistency required for optoelectronic device integration.
What are the operational stability thresholds for organic cocrystal-based organic field-effect transistors (OFETs) under ambient conditions?
Organic donor–acceptor complexes used as active elements in OFETs (J Mater Chem C, 2018, 6: 3485–3498) face ambient instability driven by oxygen and moisture doping of the charge-transfer interface. The operational threshold is typically defined by the on/off ratio and threshold voltage shift over time; cocrystals with strong CT interactions and dense packing via hydrogen or halogen bonding show improved stability, but quantitative lifetime data under continuous bias stress remain sparse. For commercial viability, devices must maintain less than 10% threshold voltage drift over 1000 hours of ambient operation, a target that current cocrystal OFETs have not yet demonstrably met.
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