Type I Photosensitization with Strong Hydroxyl Radical Generation in NIR Dye Boosted by Vigorous Intramolecular Motions for Synergistic Therapy

Abstract Development of type I photosensitizers (PSs) with strong hydroxyl radical (^·OH) formation is particularly important in the anaerobic tumor treatment. On the other hand, it is challenging to obtain an efficient solid‐state intramolecular motion to promote the development of molecular machine and molecular motor. However, the relationship between them is never revealed. In this work, a pyrazine‐based near‐infrared type I PS with remarkable donor–acceptor effect is developed. Notably, the intramolecular motions are almost maximized by the combination of intramolecular and intermolecular engineering to simultaneously introduce the unlimited bond stretching vibration and boost the group rotation. The photothermal conversion caused by the intramolecular motions is realized with efficiency as high as 86.8%. The D–A conformation of PS can also induce a very small singlet‐triplet splitting of 0.07 eV, which is crucial to promote the intersystem crossing for the triplet sensitization. Interestingly, its photosensitization is closely related to the intramolecular motions, and a vigorous motion may give rise to a strong ^·OH generation. In view of its excellent photosensitization and photothermal behavior, the biocompatible PS exhibits a superior imaging‐guided cancer synergistic therapy. This work stimulates the development of advanced PS for the biomedical application and solid‐state intramolecular motions.