Structural Tuning of Anion-Templated Motifs with External Stimuli through Crystal-to-Crystal Transformation

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Crystallography

Background:

• Protonation of organic molecules can alter their structural and reactive properties.
• Photochemical reactions and thermal stimuli can induce significant structural changes in crystalline solids.
• Hydrogen-bonded organic frameworks (HOFs) offer tunable properties for various applications.

Purpose of the Study:

• To investigate the stimuli-responsive structural transformations of protonated trans-1,2-bis(4-pyridyl)ethylene (4,4′-bpe).
• To characterize the different crystalline adducts formed under various conditions (photochemical, thermal, chemical diffusion).
• To elucidate the mechanisms driving these crystal-to-crystal transformations.

Main Methods:

• Synthesis of protonated 4,4′-bpe adducts with sulfuric acid.
• Photochemical irradiation with UV light (365 nm) to induce [2+2] cycloaddition and molecular sliding.
• Thermal treatment at 50°C to form a 3D hydrogen-bonded organic framework (HOF).
• Chemical diffusion of iodine to induce sulfate reduction and form a 1D chain.
• Solid-state characterization using 13C CPMAS NMR, IR, DSC, and powder XRD.

Main Results:

• A protonated adduct [{4,4′-bpe⋅2H+}2{HSO4}2−{SO4}2−{H2O}2] (1) was synthesized.
• UV irradiation of 1 led to molecular sliding and formation of adduct 2, stabilized by OSO4⋅⋅⋅π interactions.
• Heating 1 induced thermal dissociation and formation of a 3D HOF (3).
• Iodine diffusion facilitated sulfate reduction, yielding a 1D hydrogen-bonded chain (4).
• Crystal-to-crystal transformations involving significant molecular movements were confirmed by solid-state analyses.

Conclusions:

• Protonated 4,4′-bpe exhibits remarkable stimuli-responsive behavior, undergoing diverse structural transformations.
• Photochemical, thermal, and chemical stimuli can precisely control the formation of different crystalline architectures.
• These findings highlight the potential of molecular design for creating dynamic and responsive materials.