Metal-organic frameworks (MOFs) based on Zr6(μ3-O)8 oxometallic clusters are attracting attention as potential proton conductors due to their high surface area, ease of further substitution, and exceptional chemical stability. We hereby present an examination of two Zr(IV)-MOFs with a tetrakis(4-carboxyphenyl)porphyrin (TCPP4-) linker, PCN-222 and PCN-224, as proton conductors. It was found that, in spite of their excellent stability in aqueous suspensions, in the environment of elevated air humidity, serious changes in their bonding system occur, mainly involving breakage of the carboxylate coordination bonds and hydration of the Zr6(μ3-O)8 clusters, which leads to gradual amorphization and loss of porous character. The stability of the structures can be improved by postsynthetic modification with diphenylphosphinic acid (DPPA) to some extent. Inclusion of host imidazole molecules facilitates proton mobility in the pore system of the MOFs, further accelerating the structural degradation. Even though the original structures of the MOFs collapse under the conditions of proton conductivity measurement, the resulting amorphous solids still reveal a proton conductivity up to 6.7 × 10-6 S·cm-1 at ambient temperature and a 92% relative humidity, which is comparable to that of other Zr(IV)-MOFs with well-preserved structures. The presented study demonstrates an important phenomenon that has to be considered with any investigation using MOFs as proton conductors.

Center of Materials and Nanotechnologies Faculty of Chemical Technology University of Pardubice Studentská 95 53210 Pardubice Czech Republic

Department of Chemical Physics and Optics Faculty of Mathematics and Physics Charles University Ke Karlovu 3 12116 Prague Czech Republic

Institute of Inorganic Chemistry of the Czech Academy of Sciences Husinec Řež 1001 25068 Řež Czech Republic

Institute of Macromolecular Chemistry of the Czech Academy of Sciences Heyrovského nám 2 162 06 Prague Czech Republic

Polymer Institute Slovak Academy of Sciences Dúbravská Cesta 9 84541 Bratislava Slovakia

School of Chemistry and Forensic Science University of Kent CT2 7NH Canterbury U K

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