Visible-light irradiation of porphyrin and metalloporphyrin dyes in the presence of molecular oxygen can result in the photocatalytic generation of singlet oxygen (1O2). This type II reactive oxygen species (ROS) finds many applications where the dye, also called the photosensitizer, is dissolved (i.e., homogeneous phase) along with the substrate to be oxidized. In contrast, metal–organic frameworks (MOFs) are insoluble (or will disassemble) when placed in a solvent. When stable as a suspension, MOFs adsorb a large amount of O2 and photocatalytically generate 1O2 in a heterogeneous process efficiently. Considering the immense surface area and great capacity for gas adsorption of MOFs, they seem ideal candidates for this application. Very recently, covalent–organic frameworks (COFs), variants where reticulation relies on covalent rather than coordination bonds, have emerged as efficient photosensitizers. This comprehensive mini review describes recent developments in the use of porphyrin-based or porphyrin-containing MOFs and COFs, including nanosized versions, as heterogeneous photosensitizers of singlet oxygen toward antimicrobial applications.

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 H 2 , CH 4 and derivatives, and N 2 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-fuelproducing 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 CH 4 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.

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A hexagonal three-dimensional (3D) metal−organic coordination network (MOCN) [(ZnTPyP)•0.75 DMSO] n (3D Helix) and a one-dimensional (1D) coordination polymer [(ZnTPyP)•DMF] n (1D Ladder) (ZnTPyP = 5,10,15,porphyrinatozinc(II)) are prepared and their photophysical properties are investigated to assess their ability to promote efficient exciton energy migration through singlet−singlet annihilation processes and to photosensitize singlet oxygen ( 1 O 2(g) ). The presence and absence of annihilation in 3D Helix and 1D Ladder, respectively, are indicative of their ability to efficiently promote exciton migration. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were used to demonstrate the presence of interporphyrin couplings in the 3D Helix structure. Using Forster's theory of energy transfer, the relative efficiencies of exciton migration across the bulk were correlated with the structural parameter κ 2 / r 6 , indicative of the relative orientations and distance between the donor and acceptor. Concurrently, based on the magnitude of 1 O 2(g) phosphorescence (1280 nm), it was noted that 3D Helix photosensitizes 1 O 2(g) more efficiently than 1D Ladder (by roughly 1 order of magnitude). During this study, a new two-dimensional (2D)-MOCN was prepared, 2D Grid [(ZnTPyP)•4CHCl 3 ] n , but weak Zn•••N interactions and evaporation of CHCl 3 transformed 2D Grid into a multiphasic mixture (ZnTPyP Morph) containing both 3D Helix and other 1D ladder-like species.