Covalent Organic Frameworks as Porous Pigments for Photocatalytic Metal-Free C-H Borylation

Covalent organic frameworks (COFs) are highly promising as heterogeneous photocatalysts due to their tunable structures and optoelectronic properties. Though COFs have been used as heterogeneous photocatalysts, they have mainly been employed in water splitting, carbon dioxide reduction, and hydrogen evolution reactions. A few examples in organic synthesis using metal-anchored COF photocatalysts were reported. Herein, we report highly stable beta-keto-enamine-based COFs as photo catalysts for metal-free C-B bond formation reactions. Three different COFs have been availed for this purpose. Their photocatalysis performances have been monitored for 12 different substrates, like quinolines, pyridines, and pyrimidines. All the COFs showcase moderate-to-high yields (up to 96%) depending upon the substrate's molecular functionality. High crystallinity, a large surface area, a low band gap, and a suitable band position result in the highest catalytic activity of TpAzo COF. The thorough mechanistic investigation further highlights the crucial role of light-harvesting capacity, charge separation efficiency, and current density during catalysis. The light absorbance capacity of the COF plays a critical role during catalysis as yields are maximized near the COF's absorption maxima. The high photostability of the as-synthesized COFs offers their reusability for several (>5) catalytic cycles.

Automated summary

In this study, highly stable beta-keto-enamine-based COFs were reported as photo catalysts for metal-free C-B bond formation reactions. Three different COFs were used and their photocatalysis performances were monitored for 12 different substrates. The COFs showed moderate-to-high yields (up to 96%) depending on the substrate's molecular functionality. TpAzo COF exhibited the highest catalytic activity due to its high crystallinity, large surface area, low band gap, and suitable band position. The light absorbance capacity of the COFs played a crucial role during catalysis, as yields were maximized near the COF's absorption maxima. The synthesized COFs showed high photostability and could be reused for multiple (>5) catalytic cycles.