Visible-Light-Driven Production of Solar Fuels Catalyzed by Nanosized Porphyrin-Based Metal–Organic Frameworks and Covalent–Organic Frameworks: A Review

Abstract

Porphyrin-based materials are excellent chromophores because they strongly absorb visible light and their relatively low-energy lowest unoccupied molecular orbitals thermodynamically favor photoinduced electron transfer. They can generate charge-transfer excited states with and without cocatalyst(s) and ease energy transfer and ultrafast excitation energy migration. Combined with synthetic accessibility, these qualities make them ideal building blocks for porous metal–organic framework (MOF)- and covalent–organic framework (COF)-based photocatalysts to produce solar fuels. This review first describes the structures of the most common porphyrinic MOFs and COFs and their excited-state properties and semiconducting behavior as well as that of derived composites. The generally accepted mechanisms of formation of H2, CH4 and derivatives, and N2 are then reviewed, followed by the detailed examples of nano-MOFs and nano-COFs used for the said purpose: characteristic parameters such as rates of production, turnover numbers (TONs), turnover frequencies (TOFs), and apparent quantum efficiencies are described and compared. This shows that porphyrin-based MOFs and COFs are efficient solar-fuel-producing photocatalysts, with characteristics comparable to those of nonporphyrinic MOF and COF photocatalysts, although, on some occasions, the rates of production fall short of record values. Conversely, porphyrinic MOFs and COFs exhibit the greatest TONs and TOFs of any solar-fuel-producing MOFs or COFs but still face shortcomings concerning selectivity in CH4 production because of the many possible side products. Importantly, while the best rate of photoproduction of solar fuels has been observed from nanoscale photocatalysts, there seem to be no drastic differences in the rate (within μmol–1 h–1 and mmol g–1 h–1) between nanoscale and microscale heterogeneous photocatalysts. This observation suggests that the more active sites are mostly located at or near the surface of the particles. Overall, nanosized porphyrin-based MOFs and COFs show rich and promising photocatalytic properties for generating solar fuels but still have room for improvement.