Mapping of Multirotor Dynamics in Metal-Organic Frameworks

Area of Science:

• Materials Science
• Solid-State Chemistry
• Nanotechnology

Background:

• Metal-organic frameworks (MOFs) exhibit amphidynamic behavior due to mobile rotors on organic linkers.
• Understanding these dynamics is vital for MOF applications in gas storage, separation, and molecular machinery.
• Current characterization methods lack the versatility to resolve multiple rotor motions, hindering quantitative data acquisition and structure-property correlations.

Purpose of the Study:

• To develop a versatile, label-free characterization technique for quantifying multirotor dynamics in MOFs.
• To establish a quantitative method for mapping complex motional landscapes within MOF structures.
• To correlate MOF dynamics with structural features and sorption properties.

Main Methods:

• Development of a high-resolution solid-state 13C MAS NMR method utilizing transverse relaxation theory.
• Validation of the NMR method on Zn4O(COO)6-based MOFs with varying structural complexity.
• Complementary analysis using Density Functional Theory (DFT) calculations and existing 2H NMR data.

Main Results:

• Simultaneous determination of rotor-specific phenylene motion for multiple linkers in MOFs.
• Quantification of distinct activation energies and motion frequencies (102-107 Hz) for different rotors.
• Correlation of enhanced MOF porosity and weaker π-conjugation with increased rotational mobility.

Conclusions:

• The established 13C MAS NMR method provides quantitative site-specific insights into MOF dynamics.
• Dynamics maps reveal how structural features influence rotor mobility.
• Enhanced rotor mobility improves deliverable methane capacity by reducing unusable low-pressure uptake.