Porphyrin Covalent Organic Frameworks: A Duet in Photocatalysis
Porphyrins, nature's molecular workhorses, operate at the core of photosynthesis and cytochrome P450 catalysis, offering a blueprint for sustainable energy and environmental systems. Their rigid, conjugated macrocycles provide broad solar-spectrum absorption, long-lived excited states, and efficient charge transfer, making them ideal building blocks (knots) for covalent organic frameworks (COFs). Since 2011, porphyrin-based COFs have been synthesized via boronate ester and imine linkages, with imine-linked variants proving stable for photocatalysis. A critical design challenge is the strategic selection of linker molecules that bridge porphyrin knots and allow functionalization. Benzothiadiazole (BT) and its dimethoxy derivative (BT(OMe)2) serve as electron acceptors, forming donor-acceptor COFs with porphyrin as the donor. Jiang et al. recently reported H2P-BT-COF and H2P-BT(OMe)2-COF, which exhibit strong electronic coupling, short interlayer distances, and extensive hydrogen-bond networks. In H2P-BT(OMe)2-COF, methoxy groups elevate frontier orbital energies, narrow the bandgap, and redistribute frontier orbital density, while hydrogen bonding strengthens interlayer interactions and facilitates ambipolar charge transfer through segregated π-columns. This dual mechanism suppresses charge recombination and enhances overall charge transfer. Notably, these COFs synergistically utilize both electron transfer (ET) and energy transfer (EnT) pathways: H2P and BT units act as independent oxidation/reduction centers for ET, while π-arrays of H2P serve as active sites for EnT. Methoxy groups increase thermodynamic driving force for superoxide radical formation and establish hydrogen-bond networks that promote singlet oxygen generation, cooperatively supporting both pathways. The polar methoxy groups also create one-dimensional channels for efficient reactant delivery. Consequently, H2P-BT(OMe)2-COF demonstrates outstanding performance in selective organic transformations using O2 as oxidant, including oxidative coupling of benzylamine and oxidative condensation of o-phenylenediamine.