Engineering Ultrastable Intrinsic Radicals: Graphene Quantum Dots With NIR-II Emission for Dynamic Deep-Tissue Bioimaging.

Stable radicals in carbon-based materials have witnessed great consideration in the field of photonic and quantum information technologies, yet their development has been hindered by inherent instability arising from high reactivity. Here, we demonstrate the stabilization of intrinsic radicals within graphene quantum dots (GQDs) synthesized via two-dimensional polymerization of perylene derivatives. Numerous radicals are stably localized at periodically arranged lattice defects in bilayer graphene-like plates, where the induced structural distortions and intralayer oscillations minimize interlayer interactions, thus enabling exceptional radical stability across a broad temperature range (100-500 K). This configuration generates an intrinsic radical defect (IRD) state with an absorption band extending beyond 780 nm. Via experimental and theoretical investigations, we reveal that the radical-related singly occupied molecular orbital (SOMO) level facilitates exciton dissociation and diffusion from π-conjugated domains to IRD domains with an efficient charge transfer process. This process yields efficient second near-infrared (NIR-II) photoluminescence with a quantum yield of 1.8%. Leveraging this property, the GQDs facilitate high-resolution NIR-II fluorescence imaging of vessels, tumors, and lymph nodes in mice. This discovery boosts the applications of carbon-based radicals in photonic technologies.