An unusual [4 + 2] fusion strategy to forge meso-N / O -heteroarene-fused (quinoidal) porphyrins with intense near-infrared Q-bands

Here we present a divergent synthesis of brand-new types of / -heteroarene-fused (quinoidal) porphyrins through Rh-catalyzed β-C-H activation/annulation of 5,15-dioxoporphyrins and dioxime derivatives with alkynes, in which the synthetic disconnections are difficult to access through the commonly used intramolecular cyclization strategy. Using the -methyl oxime as a traceless oxidizing directing group, the -embedded pyridine-fused -quinoidal porphyrin and pyridinium-fused cation are formed with controllable chemoselectivity and complete -selectivity. Replacing the exocyclic oxime with a carbonyl group delivers the pyran-fused porphyrin , achieving structural conversion from a quinoidal conformation to a stable porphyrin macrocycle. Further oxidation of the expanded dimer gives the oxonium , which exhibits intense near-infrared (NIR) Q-bands up to 1300 nm. Theoretical studies demonstrate that the incorporation of a heteroatom at the -position enables more effective π-extension, resulting in a 22π aromatic ( 18π aromatic) character of pyran-fused porphyrins ( / - ). Compared with the commercially available methylene blue ( ), - exhibits a better ability ( = 0.61) to sensitize singlet oxygen ( O ) when irradiated with a 680 nm laser beam, and has potential as a photodynamic therapy (PDT) photosensitizer in the body's therapeutic window (650-900 nm).