One-Shot Carbon Insertion Unlocks Polycyclic Frameworks and Near-Infrared Photocatalysts.

One-shot single-atom insertion represents an elegant yet largely unexplored strategy for rapidly generating molecular complexity. While heteroatom-based variants involving nitrogen and boron are well established, carbon insertion has remained unrealized despite carbon's central role in defining molecular architecture. Here we report a one-shot carbon-insertion annulation that directly converts diarylaminobenzenes into fused polycyclic carbocations through a cascade of electrophilic aromatic substitutions using the Vilsmeier reagent as a single-carbon source. The resulting π-conjugated cationic frameworks, inaccessible by stepwise synthesis, exhibit tunable red-to-near-infrared absorption and function as efficient near-infrared-responsive photocatalysts. More broadly, this work establishes carbon insertion annulation as a direct link between single-atom molecular construction and emergent photonic function.