CTC Highlight: Engineering Electrical Conductivity in Stable Zirconium-Based PCN-222 MOFs with Permanent Mesoporosity

9/24/2020

Title: Engineering Electrical Conductivity in Stable Zirconium-Based PCN-222 MOFs with Permanent Mesoporosity
Date of Publication: June 29, 2020
Journal: Chemistry of Materials
CTC authors: Saied Md Pratik, Laura Gagliardi, and Chris Cramer

Metal organic frameworks (MOFs) continue to evolve as an important class of materials due to their inherent structural diversity and compositional flexibility, along with their demonstrated applicability in diverse fields, including gas storage and separation, catalysis, and chemical sensing. More recently, they are being considered for optoelectronic and electrochemical applications. MOFs that are assembled from photo- or electro-active ligands and metal nodes have the potential to be exploited for a broad range of technologically and industrially relevant applications, including batteries, supercapacitors, and fuel cells. 

 

Engineering Electrical Conductivity in Stable Zirconium-Based PCN-222 MOFs with Permanent Mesoporosity

However, advances in these applications typically depend on intrinsic electrical conductivity or fast charge delocalization within the framework itself. In practice, most MOFs are electrical insulators or poor conductors due to the intrinsically insulating nature of the organic linkers and/or the lack of energetic and orbital overlap between the linkers and metal-oxo nodes. Due to their electrically insulating nature, as well as their poor light-harvesting propensity, low efficiency of charge separation, transport, and/or the lack of a wide range of thermal and chemical stability, utilization of MOFs for the applications mentioned above is significantly limited.

In this work, the authors have demonstrated that electrical conductivity can be achieved within the MOF framework by introducing complimentary guest molecules in their pore. Specifically, the research shows that the zirconium-based porphyrinic MOFs PCN-222 and PCN-222-Zn incorporating electron-deficient molecules such as transition metal(IV) bis(dicarbollide) or C60 as a guest can address these limitations, and thus they can be exploited for many applications. 

Several existing publications guided this research. In previous work (J. Phys. Chem. C 2020, 124, 3, 1878–1887), the researchers characterized the electronic structure of porphyrin-containing MOFs incorporating C60 in the pore and showed how specific donor-acceptor CT interactions can lead to photoelectrical conductivity useful for the design of optoelectronic devices. However, the successful application of MOFs to most device applications will require that they be thermally and chemically stable under a range of conditions. Zirconium-based MOFs have shown particular promise compared to other transition metals. 

The researchers focus on PCN-222 MOFs because they feature hexa-zirconium nodes and porphyrin-based linkers. Additionally, PCN-222 possesses one-dimensional mesoporous hexagonal channels and microporous triangular channels, so they may in principle host guest molecules in the small pores while retaining mesoporosity in the channels. On the other hand, Farha et. al (J. Am. Chem. Soc. 2018, 140, 3871−3875) have engendered electrical conductivity incorporating electron-deficient nickel-(IV) bis(dicarbollide) (i.e., NiCB) in NU-1000 MOF, which has the same topology as PCN-222 but with pyrene linkers instead of porphyrins and the NiCB guests exclusively occupy the small triangular pores of NU-1000, while the mesoporous hexagonal pores remain open.

According to Pratik, the publication’s first author, the most interesting finding was that spatially infiltrated electron-deficient molecules such as transition metal(IV) bis(dicarbollide) or C60 in the microporous channels of the zirconium-based porphyrinic MOFs PCN-222 and PCN-222-Zn, generate highly stable and electrically conductive frameworks in which the mesopores remain accessible to other guests. These systems realize the key design principles of concomitant presence of permanent mesoporosity, large surface area, excellent chemical stability, tuned optical properties, and enhanced electrical conductivity, which are expected to be critical for practical MOF-based photoelectrochemical devices.