title: Crystalline Bis-urea Nanochannel Architectures Tailored for Single-File Diffusion Studies.
abstract: Urea is a versatile building block that can be modified to self-assemble into a multitude of structures. One-dimensional nanochannels with zigzag architecture and cross-sectional dimensions of only ∼3.7 Å × 4.8 Å are formed by the columnar assembly of phenyl ether bis-urea macrocycles. Nanochannels formed by phenylethynylene bis-urea macrocycles have a round cross-section with a diameter of ∼9.0 Å. This work compares the Xe atom packing and diffusion inside the crystalline channels of these two bis-ureas using hyperpolarized Xe-129 NMR. The elliptical channel structure of the phenyl ether bis-urea macrocycle produces a Xe-129 powder pattern line shape characteristic of an asymmetric chemical shift tensor with shifts extending to well over 300 ppm with respect to the bulk gas, reflecting extreme confinement of the Xe atom. The wider channels formed by phenylethynylene bis-urea, in contrast, present an isotropic dynamically average electronic environment. Completely different diffusion dynamics are revealed in the two bis-ureas using hyperpolarized spin-tracer exchange NMR. Thus, a simple replacement of phenyl ether with phenylethynylene as the rigid linker unit results in a transition from single-file to Fickian diffusion dynamics. Self-assembled bis-urea macrocycles are found to be highly suitable materials for fundamental molecular transport studies on micrometer length scales.
authors.affiliation: †Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States.
authors.forename: Clifford R
authors.initials: CR
authors.lastname: Bowers
authors.affiliation: †Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States.
authors.forename: Muslim
authors.initials: M
authors.lastname: Dvoyashkin
authors.affiliation: ‡Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States.
authors.forename: Sahan R
authors.initials: SR
authors.lastname: Salpage
authors.affiliation: †Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States.
authors.forename: Christopher
authors.initials: C
authors.lastname: Akel
authors.affiliation: †Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States.
authors.forename: Hrishi
authors.initials: H
authors.lastname: Bhase
authors.affiliation: ‡Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Geer
authors.affiliation: ‡Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States.
authors.forename: Linda S
authors.initials: LS
authors.lastname: Shimizu
mesh_terms: D003460:Crystallization; D004058:Diffusion; D009682:Magnetic Resonance Spectroscopy; D053758:Nanoparticles / Q000737:chemistry*; D014508:Urea / Q000737:chemistry*; D014979:Xenon Isotopes
chemical_list: D014979:Xenon Isotopes; D014508:Urea
keywords: SEOP; bis-urea; hyperpolarization; nanotubes; single-file diffusion; spin-exchange optical pumping; xenon-129