Slip/Stick Viscosity Models of Nanoconfined Liquids: Solvent-Dependent Rotation in Metal–Organic Frameworks

Artificial molecular machines are expected to operate in environments where viscous forces impact molecules significantly. With that, it is well-known that solvent behaviors dramatically change upon confinement into limited spaces as compared to bulk solvents. In this study, we demonstrate the utility of an amphidynamic metal–organic framework with pillars consisting of 2H-labeled dialkynyltriptycene and dialkynylphenylene barrierless rotators that operate as NMR sensors for solvent viscosity. Using line-shape analysis of quadrupolar spin echo spectra we showed that solvents such as dimethylformamide, diethylformamide, 2-octanone, bromobenzene, o-dichlorobenzene, and benzonitrile slow down their Brownian rotational motion (103–106 s–1) to values consistent with confined viscosity values (ca. 100–103 pa s) that are up to 10000 greater than those in the bulk. Magic angle spinning assisted 1H T2 measurements of included solvents revealed relaxation times of approximately 100–1000 ms over the explored temperature ranges, and MAS-assisted 1H T1 measurements of included solvents suggested a much lower activation energy for rotational dynamics as compared to those measured by the rotating pillars using 2H measurements. Finally, translational diffusion measurements of DMF using pulsed-field gradient methods revealed intermediate dynamics for the translational motion of the solvent molecules in MOFs.