Radical-Stabilizing Supramolecular Metallacage for Near-Infrared Photothermal Therapy.

The advancement of photothermal therapy (PTT) hinges on developing photothermal agents that combine high efficiency with exceptional chemical and thermal stability. Naphthalene diimide (NDI) derivatives are promising candidates due to their ability to generate radical anions that absorb in the near-infrared (NIR) region. However, these materials generate intrinsically unstable radical species under ambient conditions, hindering their widespread application. In this work, we report the design and synthesis of a core-substituted NDI-based tetrahedral metallacage that exhibits satisfactory radical anion stability and outstanding photothermal conversion efficiency. Strategic incorporation of strongly electron-withdrawing cyano groups at the NDI core lowers the lowest unoccupied molecular orbital (LUMO) energy level to below -4.0 eV, enhancing the stability of the radical anions in air. Furthermore, the metallacage's 3D architecture offers a confined and protective environment that mitigates radical degradation pathways. Both in vitro and in vivo studies confirm the excellent biocompatibility and potent photothermal therapeutic efficacy of the NDI-based metallacage. This work not only introduces a robust supramolecular strategy for stabilizing NDI radical anions but also establishes a general and synthetically accessible platform for the development of new photothermal agents.