Triplet–triplet annihilation upconversion (TTA-UC) luminescence, which converts near-infrared (NIR) light—with its low scattering and strong penetration—into high-energy blue light, offers unique advantages in fields such as photocatalysis, bioimaging, photodynamic therapy, and 3D printing. In TTA-UC, the process typically proceeds in sequence as follows: excitation of the photosensitizer by low-energy photons, intersystem crossing (ISC), triplet–triplet energy transfer (TET), triplet–triplet annihilation (TTA), and emission from the annihilator molecule (high-energy photons). Traditional NIR photosensitizers, however, suffer from a large singlet–triplet energy gap (Δ E ST ), significant energy losses occur during the ISC process, making it difficult to achieve high-efficiency blue-light emission; moreover, most of these near-infrared photosensitizers contain heavy metals such as platinum, palladium, and osmium, which severely limits their further applications. Therefore, the design and synthesis of novel near-infrared organic photosensitizers that can realize high-performance TTA-UC emission from the near-infrared region to the blue region represent a key challenge in this research field.
▲Figure 1. Novel boron–nitrogen heteroaromatic near-infrared photosensitizer ( BNS ) Achieving high-performance blue-light upconversion
Recently, Wang Xiaoye, Nankai University Researchers and Huang Ling Researchers, in collaboration, have reported the first boron–nitrogen heteroaromatic near-infrared photosensitizer exhibiting multiple-resonance thermally activated delayed fluorescence (MR-TADF) properties. BNS , which exhibits strong absorption in the near-infrared region and a small Δ E
ST (0.14 eV) and a longer delayed fluorescence lifetime (115 μs). BNS The combination with a blue-light quencher enables a TTA-UC process with an anti-Stokes shift of 1.03 eV, setting a new record among metal-free, near-infrared photosensitizer–driven TTA-UC systems. Moreover, this system exhibits a low threshold power density of 125 mW cm -2 ), it boasts a high upconversion quantum yield (2.9%), requires a low concentration of annihilators (50 μM), and allows for deep excitation light penetration, making it promising for important applications in photodynamic therapy and 3D printing technologies, among others.
▲Figure 2. Near-Infrared Photosensitizer BNS Synthetic route (a) and photophysical property studies (b–e)
BNS Commercially available raw materials 1 Starting from, it is efficiently prepared via a two-step simple reaction. First, the authors, through 1 The aromatic nucleophilic substitution reaction with 1,4-dibromo-2,3,5,6-tetrafluorobenzene affords the precursor compound in 92% yield. 2 ; Subsequently, a one-pot lithium–halogen exchange–nucleophilic substitution–intramolecular hydroboration sequence was employed to afford the product in 22% yield. BNS 。
BNS Its optical bandgap is only 1.68 eV, which is the lowest value among MR-TADF materials. In addition, BNS It exhibits strong absorption in the near-infrared region and boasts a very high fluorescence quantum yield (82%) as well as a small Δ. E
ST (0.14 eV) and a relatively long delayed fluorescence lifetime (115 μs), indicating BNS It holds great potential as a near-infrared photosensitizer.
▲Figure 3. Investigation of TTA-UC properties from the near-infrared to blue light (a) BNS /Pei and BNS /TIPS-BEA Upconversion emission spectra of the composite; (b) BNS CIE coordinates for luminescence and upconversion luminescence; (c, d) BNS /Pei and BNS /TIPS-BEA Plot of the upconversion intensity of the composite as a function of excitation light power density
Due to BNS Having a smaller Δ E
ST , which significantly reduces energy loss during the TTA-UC process, in combination with blue-light annihilators such as perylene or 9,10-bis((triisopropylsilyl)ethynyl)anthracene ( TIPS-BEA ) The combination achieves TTA-UC emission spanning from the near-infrared to blue light, with anti-Stokes shifts of 1.01 eV and 1.03 eV, respectively—representing the largest values reported to date for metal-free near-infrared photosensitizers. BNS /Pei and BNS /
TIPS-BEA The composite exhibits a lower threshold power density (125 mW cm⁻², respectively). -2 With 165 mW cm -2 ) and higher upconversion quantum yields (1.5% and 2.9%, respectively, with a theoretical maximum of 50%). In addition, BNS /Pei and BNS /
TIPS-BEA The system also boasts advantages such as low annihilator concentrations (210 μM and 50 μM, respectively) and deep excitation-light penetration, thereby laying a solid foundation for the application of near-infrared–driven TTA-UC in fields like photocatalysis and 3D printing.
In summary, this work has designed and synthesized the first boron–nitrogen heteroaromatic near-infrared photosensitizer with MR-TADF properties ( BNS ), which exhibits strong near-infrared absorption and Δ E
ST It boasts advantages such as a small bandgap (0.14 eV) and a long delayed fluorescence lifetime (115 μs). BNS With the blue-light annihilator TIPS-BEA This combination achieves the largest anti-Stokes shift (1.03 eV) in a metal-free near-infrared photosensitizer–driven near-infrared-to-blue TTA-UC system. This work not only offers a novel design strategy for the development of metal-free near-infrared photosensitizers but also opens up new opportunities for the application of near-infrared–driven TTA-UC technology in future optoelectronic and biomedical fields.
This achievement was recently published in Angew
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Chemistry
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Int
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. In the above, the first author of the paper is Dr. Ji-kun Li and Ph.D. student Ming-yu Zhang from Nankai University, with Nankai University serving as the corresponding institution. Researcher Wang Xiaoye with Researcher Huang Ling This work was supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Haihe Laboratory of Tianjin Municipality, and the Fundamental Research Funds for Central Universities. Professor Pang Daiwen of Nankai University provided meticulous guidance and substantial support for this research.
