Transient Supramolecular Polymers by pH-Gated Conformational Control of a Self-Assembling Cyclodextrin

Area of Science:

• Supramolecular Chemistry
• Polymer Science
• Physical Chemistry

Background:

• Cyclodextrin (CD) host-guest systems can adopt introverted (self-included) or extraverted (intermolecularly interacting) conformations.
• The extraverted form enables supramolecular polymer formation, but controlling this assembly transiently remains a challenge.

Purpose of the Study:

• To demonstrate a pH-switchable system for controlling interconversion between CD host-guest conformations.
• To achieve kinetic trapping of an extraverted monomer for transient supramolecular polymerization.
• To investigate the mechanism governing conformational changes and polymerization.

Main Methods:

• Synthesis of a cyclodextrin-guest conjugate with a linker sensitive to pH.
• pH-dependent spectroscopic studies to monitor conformational changes.
• Analysis of supramolecular polymer formation and dissociation kinetics.
• Density Functional Theory (DFT) calculations to elucidate the mechanism.

Main Results:

• A subtle linker modification allows switching between introverted and extraverted conformations.
• The introverted form is thermodynamically favored in water.
• Basic pH (>8) rapidly converts the extraverted form to the introverted form.
• Acidic pH (<2) kinetically traps the extraverted monomer, enabling self-assembly into supramolecular polymers.
• DFT calculations identified a key hydrogen bond controlling conformational locking.
• pH provides fine kinetic control over interconversion and polymerization rates.
• Disassembly to the extraverted form is achieved using DMSO.

Conclusions:

• This work presents a novel pH-responsive system for dynamic control over supramolecular polymerization.
• Kinetic trapping of the assembling monomer, rather than reliance on metastable monomers, allows precise control over the transient polymer lifetime.
• The system offers a new paradigm for designing responsive and tunable supramolecular materials.