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.

Modulating the reactivity of chromone and its derivatives through encapsulation in a self-assembled phenylethynylene bis-urea host

“…3a). Previous work demonstrated that the $0.9 nm diameter nanochannels of the self-assembled phenylethynylene bis-urea host are accessible to gases including Xe and CO 2 [27,30]. The channels can also accommodate a range of organic guests including coumarins, acenaphthylene and stilbenes [21,27].…”

Hyperpolarized Laplace NMR

“…Guillot et al utilized hyperpolarized 3 He and CPMG with gradients to investigate diffusion in aerogel. Hyperpolarized xenon has also been exploited in the investigation of single‐file diffusion effects in small channels, in which adsorbed Xe atoms cannot pass each other . Chen and Hilty used DNP and small flip‐angle PGSTE technique to study suppression of flow motion after the injection of hyperpolarized substance.…”

Fluorescence Polarization Measurements to Probe Alignment of a Bithiophene Dye in One-Dimensional Channels of Self-Assembled Phenylethynylene Bis-Urea Macrocycle Crystals

“…Phenylethynylene bis-urea macrocycle 1 was synthesized following a previously reported four-step procedure (Scheme 2.1; sample 1 H and 13 This process was monitored using TGA, which shows a two-step desorption curve between 30 and 130 °C with a weight loss of 18.38% ( Figure 2.3) corresponding to the removal of DMSO.…”