Exploring Local Disorder within CAU-1 Frameworks Using Hyperpolarized ¹²⁹Xe NMR Spectroscopy.

Area of Science:

• Materials Science
• Chemistry
• Nanotechnology

Background:

• Functionalized metal-organic frameworks (MOFs) offer tunable sorption properties for diverse applications.
• Incorporating functional side chains often introduces structural disorder due to synthesis limitations.
• Isoreticular CAU-1 frameworks with side-chain disorder serve as model systems for studying these effects.

Purpose of the Study:

• To investigate the influence of side-chain disorder on Ar sorption and 129Xe nuclear magnetic resonance (NMR) spectroscopy in functionalized CAU-1 MOFs.
• To evaluate the accuracy of predictive models for pore dimensions and distributions in disordered MOFs.
• To establish 129Xe NMR as a sensitive tool for characterizing porosity and disorder in MOFs.

Main Methods:

• Synthesis of isoreticular CAU-1 frameworks with amino, acetamide, and methyl urea functionalities.
• Density functional theory (DFT) calculations for structure optimization.
• Void analyses and Monte Carlo (MC) force field simulations for Ar sorption.
• Hyperpolarized 129Xe NMR spectroscopy for porosity and disorder analysis.

Main Results:

• Standard Ar sorption models (NLDFT) do not accurately reproduce pore dimensions and distributions affected by side-chain disorder.
• 129Xe NMR chemical shift analysis at high temperatures provides a detailed fingerprint of porosity and side-chain disorder.
• DFT-calibrated 129Xe NMR shifts correlate with reduced accessible pore space and reveal side-chain preferences for octahedral voids.

Conclusions:

• Side-chain disorder significantly influences MOF porosity, challenging traditional characterization methods.
• Hyperpolarized 129Xe NMR is a powerful technique for probing complex porosity and disorder in functionalized MOFs.
• The presented strategy offers a versatile approach for analyzing disorder phenomena in MOFs.