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 H-2-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 (10(3)-10(6) s(-l)) to values consistent with confined viscosity values (ca. 10(0)-10(3) pa s) that are up to 10000 greater than those in the bulk. Magic angle spinning assisted H-1 T-2 measurements of included solvents revealed relaxation times of approximately 100-1000 ms over the explored temperature ranges, and MAS-assisted H-1 T-1 measurements of included solvents suggested a much lower activation energy for rotational dynamics as compared to those measured by the rotating pillars using H-2 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.

Automated summary

Artificial molecular machines are expected to operate in environments where solvent behaviors dramatically change upon confinement into limited spaces, impacting molecules significantly. In this study, an amphidynamic metal-organic framework with barrierless rotators was demonstrated to operate as NMR sensors for solvent viscosity, showing that confined solvents have significantly higher viscosity values compared to bulk solvents. Relaxation times and activation energies for rotational dynamics of solvents in MOFs were also explored, revealing different behaviors compared to those in bulk solvents. Translational diffusion measurements of solvents in MOFs showed intermediate dynamics for the translational motion.