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Modern chemistry is increasingly focused on developing sustainable processes that reduce energy consumption and minimize waste. Photocatalysis, which uses light to promote chemical reactions, offers a promising alternative to more aggressive conventional methods. However, most existing photocatalysts are homogeneous—they dissolve in the reaction medium and cannot be easily recovered or reused—and they typically rely on blue or ultraviolet light, which is more energy-demanding and penetrates poorly into reaction mixtures, limiting their large-scale and biological applications.

Researchers at the Center for Research in Biological Chemistry and Molecular Materials (CiQUS) have developed an innovative, more sustainable method that uses red light—a low-energy, deeply penetrating light source—together with recyclable solid catalysts to promote chemical reactions cleanly and efficiently. The study highlights the potential of covalent organic frameworks (COFs) as red-light-active heterogeneous photocatalysts, a field that remains largely unexplored. This combination of reusable materials and mild light represents a significant step toward greener chemical methodologies.

The work is in the Journal of the American Chemical Society.

COFs are porous, crystalline materials assembled from organic building blocks, forming structures akin to "molecular LEGO." Sharing some characteristics with MOFs—which were recognized in the most recent Nobel Prize in Chemistry—COFs are fully organic and stand out for their tunability, stability and light-harvesting capabilities.

In this work, the team designed a COF incorporating a photoactive molecular fragment (based on a benzothiadiazole structure), enabling efficient red-light absorption and the generation of reactive species to trigger the transformation. As a solid material, the catalyst can be easily recovered and reused at least six times without loss of activity—a key advantage over conventional homogeneous systems.

To showcase the potential of this approach, the researchers applied the new COF to a model C–H bond sulfonylation of anilines, a reaction that yields sulfones—valuable structural motifs found in numerous pharmaceuticals and bioactive compounds. These groups enhance molecular stability and biological interactions, making the development of clean, direct and versatile synthetic methods highly relevant for both academia and industry. Using minimal amounts of catalyst and mild conditions, the system achieved efficient transformations across a broad substrate scope.

The study also illustrates the power of internal collaboration at CiQUS. One group contributed its expertise in organic synthesis and photocatalysis, while the other focused on COF design and synthesis. This synergy was supported by the CiQUS-Synergy program, which fosters cooperation between research lines within the center.

This advance paves the way for new applications of COFs in sustainable red-light photocatalysis, with potential impact in organic synthesis and beyond—including fields such as biomedicine, where red light offers clear advantages.