A Series of Singlet‐Triplet InVerted TADF Fluorescent Probes with High Stability, Low Molecular Weight, and Synthesis Accessibility

Abstract 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.