Caffeine Confinement into a Series of Functionalized Porous Zirconium MOFs: A Joint Experimental/Modeling Exploration

A multitechnique approach was conducted to investigate the confinement of caffeine in a series of UiO-66(Zr)-type MOFs, functionalized with -H, -NH2, -Br, and -2OH groups. DFT calculations were first undertaken to elucidate the preferential geometries of the drug within the pores and the resulting drug/host framework interactions. It was shown that the caffeine molecules are preferentially located in the smaller cages, giving rise to only weak interactions with the function groups grafted on the organic linker. These host/guest interactions were concomitantly probed by advanced ID and 2D high-field/ultrafast MAS NMR and FTIR spectroscopies, which allowed us to not only validate the DFT predictions but also to bring complementary insight into the nature of the interacting sites of both the caffeine and the MOFs. Dielectric relaxation measurements further revealed significant modifications of the ligand dynamics upon the drug encapsulation for all UiO-66(Zr) solids. It was demonstrated that the perturbation of the ligand flip strongly depends on the nature of the grafted function. While the dynamics of the ligand is slightly enhanced in the case of the -NH2 form, it is significantly slower for the -Br analogue. Such specific behaviors were then interpreted in light of the conclusions drawn from the DFT calculations and NMR observations.