Crystalline Bis-urea Nanochannel Architectures Tailored for Single-File Diffusion Studies

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Chemical Physics

Background:

• Urea derivatives are versatile building blocks for self-assembly.
• Macrocyclic bis-ureas can form one-dimensional nanochannels.
• Understanding molecular transport in confined environments is crucial.

Purpose of the Study:

• To compare xenon (Xe) atom packing and diffusion in two distinct bis-urea nanochannel systems.
• To investigate the impact of nanochannel geometry on guest molecule dynamics.
• To explore the potential of self-assembled bis-ureas for molecular transport studies.

Main Methods:

• Synthesis and characterization of phenyl ether bis-urea and phenylethynylene bis-urea macrocycles.
• Utilizing hyperpolarized Xenon-129 Nuclear Magnetic Resonance (NMR) spectroscopy.
• Employing hyperpolarized spin-tracer exchange NMR for diffusion dynamics analysis.

Main Results:

• Phenyl ether bis-urea forms zigzag nanochannels (∼3.7 Å × 4.8 Å) causing extreme Xe confinement and asymmetric chemical shift tensors.
• Phenylethynylene bis-urea forms wider, round nanochannels (∼9.0 Å) with an isotropic, dynamically averaged electronic environment for Xe.
• A structural change in the linker unit (phenyl ether to phenylethynylene) induces a transition from single-file to Fickian diffusion dynamics for Xe atoms.

Conclusions:

• Self-assembled bis-urea macrocycles provide tunable platforms for creating nanochannels with controlled dimensions and architectures.
• Nanochannel geometry significantly dictates the confinement effects and diffusion mechanisms of guest molecules like Xe.
• These bis-urea materials are highly suitable for fundamental investigations of molecular transport phenomena at the nanoscale.