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.