ARTICLE Squeezing xenon into phenylether bis-urea nanochannels Clifford R. Bowers, Muslim Dvoyashkin, Sahan R. Salpage, Christopher Akel, Hrishi Bhase, Michael F. Geer, and Linda S. Shimizu Abstract: One-dimensional nanochannels, hundreds of microns in persistence length but with elliptical cross-sectional dimen- sions of only 3.7 Å × 4.8 Å, are formed by the columnar assembly of phenylether bis-urea macrocycles. Hyperpolarized Xe-129 NMR is utilized to investigate the Xe atom packing and Xe diffusion inside the needle shaped crystals. The elliptical channel structure produces a Xe-129 powder pattern characteristic of an asymmetric chemical shift tensor extending to well over 300 ppm with respect to the gas phase, reflecting the highly anisotropic electronic environment and extreme confinement of the atom. Consistent with the simple geometrical criterion, hyperpolarized tracer exchange NMR data reveals single-file diffusion in the bis-urea nanochannels. Key words: xenon-129, nanotubes, bis-urea, single-file diffusion, hyperpolarization, SEOP, spin exchange optical pumping. Résumé : Des nanocanaux unidimensionnels d’une longueur de persistance de centaines de microns, mais de dimensions de section elliptique de seulement 3.7 Å × 4.8 Å, se forment par l’assemblage colonnaire de macrocycles de bis-urée de phényléther. Nous avons utilisé la RMN du Xe-129 hyperpolarisé pour étudier l’empilement des atomes de Xe et la diffusion du Xe a ` l’intérieur des cristaux en forme d’aiguille. La structure elliptique des canaux produit un signal typique d’une de poudre de Xe-129, caractérisé par un tenseur de déplacement chimique asymétrique s’étendant bien au-dela ` de 300 ppm, par comparaison a ` la phase gazeuse. Ce comportement témoigne d’un environnement électronique fortement anisotrope et d’un confinement extrême des atomes. En accord avec le critère géométrique simple, les données RMN du traceur hyperpolarisé révèlent une dynamique de diffusion en file indienne dans les nanocanaux de bis-urée. [Traduit par la Rédaction] Mots-clés : xénon-129, nanotubes, bis-urée, diffusion en file indienne, hyperpolarisation, SEOP, pompage optique par échange de spin. Introduction The exploration of crystalline materials consisting of one- dimensional channel structures with sub-nm cross-sectional dimen- sions has proven to be one of the most intriguing uses of 129 Xe NMR. 1–15 The 129 Xe peak for the adsorbed phase of Xe in sub-nm diameter channels is typically well-resolved from the bulk gas peak, and the 129 Xe magnetic shielding depends on channel architecture, cross-sectional dimensions, chemical composition, loading, dynam- ics, and channel orientation in the static magnetic field. Distortions from spherical symmetry resulting from intermolecular interactions induce a paramagnetic deshielding of the 129 Xe nucleus relative to the free atom. Hence, the chemical shift anisotropy inside one- dimensional channels depends on both Xe-channel and Xe–Xe inter- actions. 16,17 In channels too narrow for Xe atoms to pass one another, single-file packing and diffusion effects can be seen by 129 Xe NMR with a clarity and detail unparalleled by any other technique. These effects have been reported in various types of porous solids with one-dimensional channels, including TPP, 5,6 dipeptides (L-alanyl-L- valine, L-valyl-L-alanine), 7,9,11,13,14 molecular sieves 17 (e.g., ALPO-11, 1,4 ALPO-5, 2 SSZ-24 18 ), silicalite single crystal, 19 and transition metal mo- lecular wheels. 12 In many cases, the NMR studies were facilitated by 129 Xe hyperpolarization (HP) signal enhancement afforded by Rb-Xe spin exchange optical pumping (SEOP). 18,20–27 The polar urea functional group readily forms intermolecular interactions, which can be used to direct supramolecular assem- bly and generate sheets, fibers, columns, and other diverse struc- tures. 28–31 In the phenylether bis-urea macrocycle ( 1) shown in Fig. 1a, two rigid C-shaped phenylether spacers pre-organize two urea groups approximately perpendicular to the plane of the macro- cycle. This geometry affords columnar self-assembly with high fidelity. 32–36 In the columns, bis-urea macrocycles are held to- gether by three centered urea hydrogen bonds that set a repeat distance of about 4.6 Å. 33 Alternating edge to face aryl stacking interactions further stabilize the columnar assembly and give the walls of the cavity a slight curvature that oscillates back and forth along its length, which is highlighted in brown in Fig. 1c. The cavity is seen to be roughly elliptical with cross-sectional dimensions of 4.8 Å × 3.7 Å. Heating removes the crystallization solvent, leav- ing behind guest-accessible channels with uniform one-dimensional channel structures. The cross-section of 1 is substantially smaller than the collision diameter of the Xe atom of 4.4 Å. Hence, adsorption of Xe atoms into these nanochannels requires a large distortion of its electron density. Since the channel walls are not smooth on the atomic scale, Xe–wall interactions vary periodically along the channel axis but will be dynamically averaged on the NMR time scale if Xe diffusion is sufficiently fast. As can be seen in the space-filling model in Fig. 1b, the cavity of 1 is too narrow to accommodate more than one Xe atom. Therefore, Xe atoms adsorbed into the one-dimensional channels are expected to exhibit molecular packing and transport characteristic of a single-file system. Here we report the results of HP 129 Xe studies that have been Received 26 March 2015. Accepted 30 April 2015. C.R. Bowers, M. Dvoyashkin,* C. Akel, and H. Bhase. Department of Chemistry, University of Florida, Gainesville, FL 32611, USA. S.R. Salpage, M.F. Geer, and L.S. Shimizu. Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA. Corresponding authors: Clifford R. Bowers (e-mail: bowers@chem.ufl.edu) and Linda S. Shimizu (e-mail: shimizls@mailbox.sc.edu). *Present address: Institute of Chemical Technology, Universität Leipzig, 04103 Leipzig, Germany. This article is part of a Special Issue in honour of Dr. John Ripmeester and his outstanding contributions to science. Pagination not final (cite DOI) / Pagination provisoire (citer le DOI) 1 Can. J. Chem. 93: 1–4 (2015) dx.doi.org/10.1139/cjc-2015-0152 Published at www.nrcresearchpress.com/cjc on 1 May 2015. Can. J. Chem. Downloaded from www.nrcresearchpress.com by UNIVERSITY OF SOUTH CAROLINA on 06/23/15 For personal use only.