Abstract
Metal-organic frameworks (MOFs) are promising photocatalytic materials due to their high surface area and tuneability of their electronic structure. We discuss here how to engineer the band structures and optical properties of a family of two-dimensional porphyrin-based MOFs, consisting of M-tetrakis(4-carboxyphenyl)porphyrin structures (M-TCPP, where M = Zn or Co) and metal (Co, Ni, Cu or Zn) paddlewheel clusters, with the aim of optimising their photocatalytic behaviour in solar fuel synthesis reactions (water-splitting and/or CO2 reduction). Based on density functional theory (DFT) and time-dependent DFT simulations with a hybrid functional, we studied three types of composition/structural modifications: (a) varying the metal centre at the paddlewheel or at the porphyrin centre to modify the band alignment; (b) partially reducing the porphyrin unit to chlorin, which leads to stronger absorption of visible light; and (c) substituting the benzene bridging between the porphyrin and paddlewheel, by ethyne or butadiyne bridges, with the aim of modifying the linker to metal charge transfer behaviour. Our work offers new insights on how to improve the photocatalytic behaviour of porphyrin- and paddlewheel-based MOFs.
Highlights
Metal-organic frameworks (MOFs) are materials where metal atoms or clusters are connected via organic linkers to form rigid frameworks, often with a porous structure [1]
Based on density functional theory (DFT) and time-dependent DFT simulations with a hybrid functional, we studied three types of composition/structural modifications: (a) varying the metal centre at the paddlewheel or at the porphyrin centre to modify the band alignment; (b) partially reducing the porphyrin unit to chlorin, which leads to stronger absorption of visible light; and (c) substituting the benzene bridging between the porphyrin and paddlewheel, by ethyne or butadiyne bridges, with the aim of modifying the linker to metal charge transfer behaviour
For Co, the preferred spin state of each metal cation is HS, which involves a local magnetic moment μ = 3 μB per Co(II) cation. This result agrees with the experimental determination by Pakula and Berry, who showed that Co(II) species are in HS state in Co paddlewheel units [59], they found that the local magnetic moments were aligned antiferromagnetically within the paddlewheel, whereas in our calculation we found that the FM alignment is more stable
Summary
Metal-organic frameworks (MOFs) are materials where metal atoms or clusters are connected via organic linkers to form rigid frameworks, often with a porous structure [1] They have found applications in gas (e.g. CO2, H2) storage and separation [2,3,4,5,6]. Leng et al [25] reported an indium-based porphyrinic MOF, USTC-8(In), where one-dimension In−oxo chains are connected by the porphyrin units, with excellent photocatalytic H2 production under visible light. In this system, the out-of-plane In3+ ions detach from the porphyrin ligands under excitation, avoiding the fast back electron transfer and electron–hole separation is improved. The proximity of the Ru cluster to the porphyrin (∼11 Å) was found to facilitate the electron transfer from the photoexcited porphyrins to the metal clusters
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