Central Dynamic Conversion of Dicyanomethyl Radicals via Intermolecular Charge Transfer for Ultrabright NIR‐II Fluorescence Imaging‐Guided Mild Photothermal Therapy

Abstract Stable organic radicals have emerged as promising candidates in biotheranostics owing to their unique electronic configurations and tunable photophysical characteristics. Among these, dicyanomethyl radical derivatives stand out as key players in dynamic covalent chemistry, demonstrating exceptional temperature‐responsive reversible polymerization‐dissociation dynamics and remarkable long‐wavelength absorption profiles despite their low molecular weights. In this work, we developed a novel radical system designated CNPJ through the strategic incorporation of julolidine into a dicyanomethyl‐functionalized phenoxazine scaffold. The stability and radical nature of CNPJ were first corroborated by nucleus‐independent chemical shift (NICS) calculations. Structural analyses revealed two critical phenomena enabled by efficient spin delocalization and intermolecular charge transfer: i) polarity‐triggered radical center redistribution and ii) thermally regulated monomer‒π dimer equilibrium. These distinctive features confer enhanced structural rigidity and an exceptional absolute photoluminescence quantum yield of 2.57%, a remarkably high value for near‐infrared emitters, accompanied by a bathochromically shifted π dimer absorption band centered at 960 nm. Capitalizing on these attributes, we engineered a theranostic platform by integrating CNPJ with the pH‐responsive amphiphilic polymer PEA through pifithrin‐μ (PES) inhibitor mediation. This system achieves ultrahigh‐brightness fluorescence imaging‐guided mild photothermal tumor therapy, yielding promising therapeutic outcomes.