NIR‐II Emissive Persistent Neutral π‐Radical with Rapid Doublet Internal Conversion for Efficient Cancer Photothermal Theranostics

Abstract Organic radicals are considered prospective materials for near‐infrared (NIR) photothermal applications, however, sustainability remains the major obstacle of recently reported ionic radical photothermal agents. This work achieved robust sustainability on a series of neutral π‐radicals through rational design donor (D)–acceptor (A). With efficient doublet internal conversion, 10H‐spiro(acridine‐9,9′‐fluorene) (SFA)‐BTM presented strong NIR absorption extended to 1000 nm and efficient non‐radiative relaxation. SFA‐BTM nanoparticles (NPs) realized comparable NIR‐I photothermal conversion efficiency (PCE) and photoacoustic sensitivity. Also, the π‐radical NPs displayed NIR‐II emission and achieved high‐resolution whole‐body angiography for the first time by the NIR‐II bioimaging. Ultimately, the photothermal capabilities are confirmed in an orthotopic bone tumor model by effective ablation of cancer cells in vitro and inhibition of the deterioration of tumor in vivo. This research offers a new horizon in the conception and development of sustainable organic radicals for effective NIR‐II imaging and theranostics applications.