title: Diffusion-Controlled Rotation of Triptycene in a Metal-Organic Framework (MOF) Sheds Light on the Viscosity of MOF-Confined Solvent.
abstract: Artificial molecular machines are expected to operate under conditions of very low Reynolds numbers with inertial forces orders of magnitude smaller than viscous forces. While these conditions are relatively well understood in bulk fluids, opportunities to assess the role of viscous forces in confined crystalline media are rare. Here we report one such example of diffusion-controlled rotation in crystals and its application as a probe for viscosity of MOF-confined solvent. We describe the preparation and characterization of three pillared paddlewheel MOFs, with 9,10-bis(4-pyridylethynyl)triptycene3as a pillar and molecular rotator, and three axially substituted dicarboxylate linkers with different lengths and steric bulk. The noncatenated structure with a bulky dicarboxylate linker (UCLA-R3) features a cavity filled by 10 molecules ofN,N-dimethylformamide (DMF). Solid-state2H NMR analysis performed between 293 and 343 K revealed a fast 3-fold rotation of the pillar triptycene group with the temperature dependence consistent with a site exchange process determined by rotator-solvent interactions. The dynamic viscosity of the MOF-confined solvent was estimated to be 13.3 N·s/m2(or Pa·s), which is 4 orders of magnitude greater than that of bulk DMF (8.2 × 10-4N·s/m2), and comparable to that of honey.
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095-1569, United States.
authors.forename: Xing
authors.initials: X
authors.lastname: Jiang
authors.affiliation: School of Environmental Science, Nanjing Xiao Zhuang University, Nanjing, Jiangsu 211171, P.R. China; Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, United States.
authors.forename: Hai-Bao
authors.initials: HB
authors.lastname: Duan
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095-1569, United States.
authors.forename: Saeed I
authors.initials: SI
authors.lastname: Khan
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095-1569, United States.
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Garcia-Garibay