Stimulus-Induced Dynamic Behavior Regulation of Flexible Crystals through the Tuning of Module Rigidity

Area of Science:

• Materials Science
• Crystallography
• Supramolecular Chemistry

Background:

• Flexible porous crystals, such as metal-organic frameworks (MOFs), are created by incorporating dynamic behavior into periodic structures.
• Controlling the collective dynamics of these frameworks through molecular design is crucial for achieving stimulus-induced responses.

Purpose of the Study:

• To construct two isostructural flexible MOFs with varying linker rigidity to investigate their dynamic behaviors.
• To understand how subtle changes in linker structure influence framework flexibility and gas sorption properties.

Main Methods:

• Synthesis of two isostructural MOFs using ditopic pillar linkers with different C-C bond characteristics (single vs. double bond).
• Single-crystal-to-single-crystal transformations were studied upon guest molecule removal.
• Gas sorption isotherms (C3H6/C3H8) and in situ powder X-ray diffraction (PXRD) were employed to analyze dynamic behavior.
• Molecular modeling was used to investigate the sorption mechanism and correlate microscopic flexibility with macroscopic elasticity.

Main Results:

• Two MOFs, Flex-Cd-MOF-1a and Flex-Cd-MOF-2a, were successfully synthesized, exhibiting reversible single-crystal-to-single-crystal transformations driven by linker cis-trans conformational changes.
• Flex-Cd-MOF-2a demonstrated significantly enhanced gas-induced dynamic behavior compared to Flex-Cd-MOF-1a.
• A direct correlation between the macroscopic elasticity and microscopic flexibility of Flex-Cd-MOF-2 was observed.

Conclusions:

• Varying linker rigidity in MOFs is an effective strategy to tune their dynamic responses and gas sorption properties.
• Flex-Cd-MOF-2a serves as a model for MOFs with desirable chemomechanical functions.
• This study provides fundamental insights into the origins of flexibility in MOFs, paving the way for designing advanced functional materials.