Cyano‐Mediated Donor‐Acceptor Engineering: A Rational Strategy to Boost Charge Transfer in Carbon Dots for NIR‐II Bioimaging

Carbon dots (CDs) have emerged as an important class of probes for biomedical imaging. However, engineering CDs with near-infrared emission, especially in the second near-infrared window (NIR-II), remains a significant challenge. Herein, we propose a cyano-mediated donor-acceptor (D-A) engineering strategy to synthesize NIR-II emissive CDs (FC-CDs), using benzene-1,2,4,5-tetracarbonitrile, a molecule rich in electron-withdrawing cyano groups, as the precursor. Meanwhile, the in situ introduction of folic acid (FA) during the solvothermal process facilitates enhanced biocompatibility and tumor-targeting ability. The cyano groups on FC-CDs enable the formation of a unique D-A configuration, in which the carbon core acts as the electron donor and the electron-withdrawing-groups-functionalized surface serves as the electron acceptor. Experimental and theoretical evidence suggests that this D-A structure facilitates efficient intramolecular charge transfer and narrows the optical bandgap, both of which are key to achieving NIR-II emission. Thanks to the good biocompatibility and NIR-II characteristics, the performance of FC-CDs in in vivo NIR-II bioimaging (e.g., angiography, hepatic ischemia-reperfusion monitoring, and tumor-targeted imaging) has been verified. This cyano-mediated D-A engineering strategy thus provides insights into designing NIR-II emissive CDs for biomedical applications.