Multifarious Polymorphism of a Multiblock Amphiphilic Macrocycle Bearing Thermally Responsive Polyether Segment

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Crystallography

Background:

• Amphiphilic macrocycles are versatile building blocks in supramolecular chemistry.
• Controlling the solid-state phases of such molecules is crucial for their applications.
• Understanding phase transitions is key to designing materials with tailored properties.

Purpose of the Study:

• To investigate the formation of multiple crystalline phases in a multiblock amphiphilic macrocycle, designated AT2B.
• To explore the conditions that induce transitions between different solid states.
• To elucidate the driving forces behind these observed phase transitions.

Main Methods:

• Single crystal X-ray diffraction for phase characterization.
• Differential Scanning Calorimetry (DSC) to study thermal transitions.
• Controlled cooling rate experiments from the melt.
• Application of mechanical stress to induce solid-state transitions.

Main Results:

• AT2B forms a single crystal phase (Cr-α) via vapor diffusion.
• Reversible single-crystal-to-single-crystal transitions between Cr-α and Cr-β phases were observed upon temperature changes.
• Cooling AT2B from its melt at different rates (1.0 K min⁻¹ vs. 2.0 K min⁻¹) yielded distinct crystalline (Cr-γ, Cr-δ) and amorphous phases.
• Mechanical stress induced a crystal-to-amorphous transition.
• The amorphous phase recrystallized into a fifth crystalline phase (Cr-γ) upon heating.

Conclusions:

• The multiblock amphiphilic macrocycle AT2B demonstrates remarkable polymorphism.
• Phase transitions are triggered by thermal stimuli (temperature changes, cooling rates) and mechanical stress.
• Tetraethylene glycol chains are suggested to be the primary drivers of these conformational changes and subsequent phase transitions.