Photochemical cycloaddition and temperature-dependent breathing in pillared-layer metal-organic frameworks

Area of Science:

• Materials Science
• Crystallography
• Supramolecular Chemistry

Background:

• Single crystals of metal-organic frameworks (MOFs) are crucial for understanding their dynamic behaviors at the atomic level.
• Pillared-layer MOFs offer a versatile platform for studying structural transformations.
• Photochemical reactions and temperature variations are key stimuli for MOF flexibility.

Purpose of the Study:

• To investigate structural transformations in pillared-layer MOFs induced by photochemical cycloaddition and temperature changes.
• To characterize the resulting MOF structures and their responsive behaviors.
• To demonstrate a well-defined system with controlled flexibility in the single crystalline state.

Main Methods:

• Synthesis of pillared-layer MOFs with pyrazolecarboxylate layers and bipyridyl pillars containing ethylenic double bonds.
• UV irradiation to induce photochemical [2+2] cycloaddition in the pillar linkers.
• Single crystal X-ray diffraction to analyze structural changes.
• Variable temperature studies (298 K to 173 K) to observe breathing phenomena.

Main Results:

• Quantitative and stereoselective photochemical [2+2] cycloaddition of ethylenic double bonds in pillars upon UV irradiation, forming cyclobutane-based structures.
• Reversible breathing behavior observed, with a 10.8% change in the c-axis unit cell parameter between 298 K and 173 K.
• Structural analysis revealed layer flattening upon cooling as the origin of the breathing effect.

Conclusions:

• Pillared-layer MOFs can undergo distinct structural transformations triggered by both inherent photochemical reactions and external temperature stimuli.
• These transformations occur in the single crystalline state, offering precise control over MOF flexibility.
• The study presents a robust system for exploring controlled dynamic behaviors in MOFs.