Fluorinated metal–organic frameworks: hydrophobic nanospaces with high fluorine density and proton conductivity

Abstract

The current work revealed the relationship between the fluorine density of organic-based porous materials and water dynamics (proton conduction) in the hydrophobic nanospace. In detail, by focusing on UiO-66 with structural durability at high temperature/humidity based on strong Zr–O bonds, we prepared UiO-66-CF₃ and UiO-66-(CF₃)₂ with different fluorine densities. The activation energies for proton conduction of UiO-66-CF₃ (0.91 eV) and UiO-66-(CF₃)₂ (1.38 eV) were significantly larger than that in UiO-66 (0.47 eV) depending on their fluorine density. Introducing fluorine into organic-based porous materials allowed the hydrophobic nanospace to interact with protons, yielding a larger energy for proton conduction (activation energy). The fluorine density and activation energy were proportional. We clarified that the activation energy of proton conduction increased proportionally with the fluorine density in the nanospace. This indicated the possibility that the state of proton/water in the nanospace could be freely controlled by the fluorine density.