Engineering the electronic and optical properties of 2D porphyrin-paddlewheel metal-organic frameworks

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.

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

“…However, quasi-complete selectivity of CH 4 (>97%) over other products has been achieved with several nonporphyrinic MOF photocatalysts. , Despite their high light-harvesting abilities in the visible region and high degrees of electron–hole separation, thus rendering electrons available for the multistep reduction of CO 2 , such stellar selectivities for CH 4 (i.e., >97%) currently elude porphyrin-based materials because in most cases CO is the major product. CH 3 OH has a CO 2 /CH 3 OH redox potential close to that of CO 2 /CH 4 , meaning that CH 3 OH might have regularly been observed during photocatalysis, but this is not the case. Indeed, CH 3 OH has only been sporadically observed before porphyrin-based MOF photocatalysts have been used , but may simply exist transiently and, like water, may react further and be consumed.…”

“…Solar fuels can be divided into three main categories: (1) H 2 from water splitting (or from a proton source, often called a sacrificial agent); (2) hydrocarbons from carbon dioxide (CO 2 ) reduction, and (3) active nitrogen-containing compounds generated by the N 2 fixation reaction, with each fuel accessible through several types of photocatalysts. − MOFs and COFs can be easily customized by choosing the metal inserted in the porphyrin macrocycle, the metal in the nodes, and the substitution of the ligands, allowing tuning of the band-gap and optical properties of the MOFs and COFs to the desired redox reactions at the interfacial site. − Engineering the structures of these photocatalysts, , including intentional defects, also helps in achieving better performances of the desired reactions. The primary photophysical processes consist of a charge-separation characteristic of semiconducting materials, as illustrated by the Graphic Art. , …”

Computational modeling guided design of metal–organic frameworks for photocatalysis – a mini review

“…In 2021, Posligua et al reported that some modifications to porphyrin-paddlewheel MOFs can enhance photocatalytic performance. 120 Through replacing paddlewheel metal Zn with Co, Cu, or Ni respectively, it was found that the energy level of empty 3d orbitals showed a continuous decreasing trend. The photocatalysis like water splitting was driven by photogenerated electrons and holes for H 2 and O 2 evolution.…”

Simulating excited states in metal organic frameworks: from light-absorption to photochemical CO2 reduction

“…Screening studies compare various functionalisations by substituting the metal, node or linker, or by introducing defects. This might involve substitution of the transition metal in the node; 89 changing the coordination environment of the node; substitution of the linker; 22 substitution of functional groups within the linker; 90 introduction of photocatalytic guests; 91 or a combination of these. The relative energies and band gaps can then be compared to reveal qualitative trends.…”