The Chemistry of Organic Contrast Agents in the NIR‐II Window

Optical imaging, especially fluorescence and photoacoustic imaging, is a non-invasive imaging approach with high spatial and temporal resolution and high sensitivity, compared to positron emission tomography (PET) or magnetic resonance imaging (MRI). Due to the merits of imaging using the second near-infrared (NIR-II) window, like deeper penetration depth, high signal-to-noise ratio, high resolution, and low tissue damage, researchers have devoted great effort to developing contrast agents with NIR-II absorption or emission. In this Review, we summarize recently developed organic luminescent and photoacoustic materials, ranging from small molecules to conjugated polymers. Then, we systematically introduce engineering strategies and describe the imaging performance classified by the skeleton cores. Finally, we elucidate the challenges and prospects of these NIR-II organic dyes for potential clinical applications. We expect our summary can inspire researchers to expand the spectrum of NIR-II contrast agents for diverse bioapplications.