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Thermally activated delayed fluorescence (TADF)‐based fluorescent biological probes embedded with near infrared (NIR) emission have some unique properties, such as minimal photodamage, low light scattering, and deep tissue penetration. Typical TADF materials are based on electron donors and acceptors, all of them have broad emission and high molecular weight, which is not ideal for biological applications. The cycl[3,3,3]azine is a multiple resonance effect (MRE)‐like TADF fluorescent compound, with narrow NIR emission, inverted singlet‐triplet gaps, and low molecular weight, which is an ideal platform for biological applications. However, the low stability of cycl[3,3,3]azine and its anti‐Kasha's rule nature limit its biological applications. In this work, the authors have rationally investigated the decomposition process of cycl[3,3,3]azine, and a series of NIR and MRE‐like molecules are designed. These molecules would be stable in the biological environment and NIR fluorescence can also be emitted from the first excited state, the synthesis route has also been proposed. In a word, this work provides a series of biological probes with inverted singlet‐triplet gaps, good stability, and low molecular weight for further study. [ABSTRACT FROM AUTHOR]

The development of efficient non‐doped organic light‐emitting diodes (OLEDs) is highly desired but very challenging because of a severe aggregation‐caused quenching effect. Herein, we present a heptagonal diimide acceptor (BPI), which can restrict excessive intramolecular rotation and inhibit close intermolecular π–π stacking due to well‐balanced rigidity and rotatability of heptagonal structure. The BPI‐based luminogen (DMAC‐BPI) shows significant aggregation‐induced delayed florescence with an extremely high photoluminescence quantum yield (95.8 %) of the neat film, and the corresponding non‐doped OLEDs exhibit outstanding electroluminescence performance with maximum external quantum efficiency as high as 24.7 % and remarkably low efficiency roll‐off as low as 1.0 % at 1000 cd m−2, which represents the state‐of‐the‐art performance for non‐doped OLEDs. In addition, the synthetic route to DMAC‐BPI is greatly streamlined and simplified through oxidative Ar−H/Ar−H homo‐coupling reaction. [ABSTRACT FROM AUTHOR]