Viologen Covalent Organic Frameworks Integrating Synergistic Singlet and Triplet Excitation for Efficient Photosynthesis of Benzazole Pharmaceuticals.

Viologen and its derivatives serve as an ideal platform for constructing radical-type photocatalysts, but their reductive nature often causes stacking-induced quenching and over-reduction to the neutral state, substantially suppressing their photocatalytic performance. Herein, we intentionally introduced a 3,3'-viologen skeleton in covalent organic frameworks via a facile cyclization ionization strategy, and the resulting PyBpy-3,3'-ion features unique cationic and diradical characteristics in its neutral state, thereby promoting the simultaneous generation of superoxide radical and singlet oxygen through synergistic singlet (S₁) or triplet (T₁) excitation. Meanwhile, the diradicals in the catalyst exhibit a strong repulsive interaction toward oxygen, ensuring its long-term stability and activity under aerobic conditions. As a result, PyBpy-3,3'-ion exhibits exceptional photocatalytic efficiency (>90%, 1 h) for the synthesis of benzimidazole-[thiazole/oxazole] (up to 40 examples) under ambient conditions, outperforming most of the state-of-the-art catalytic systems. More impressively, PyBpy-3,3'-ion serves as an outstanding photocatalyst for the facile and highly efficient syntheses of over 10 key benzazole pharmaceuticals, such as Ridinilazole, PMX-610, Tafamidis, and so on, as well as the gram-scale preparation of Thiabendazole and Pimobendan under mild conditions. Our work establishes a rational design protocol to achieve the synergistic S₁ and T₁ excitation in viologen for highly efficient photocatalytic synthesis of high-value pharmaceuticals.