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

Linking a cyclodextrin (CD) host to a hydrophobic guest can result in two distinct conformations: an introverted form (in), in which the guest is self-included within the CD cavity, and an extraverted form (out), which enables intermolecular interactions and thus the formation of a supramolecular polymer. In this study, we demonstrate that a subtle variation of the linker enables interconversion between these two conformations, the in conformer being thermodynamically the most stable in water. At basic pH (>8) the out conformer is instantly converted into the in. In contrast, at acidic pH (<2), the out monomer can be kinetically trapped and can self-assemble into a supramolecular polymer. DFT calculations reveal that the interconversion mechanism is governed by a key hydrogen bond that locks the conformational states. Furthermore, we show that pH provides fine kinetic control over the interconversion rate and, consequently, the polymerization process. The system can then be reset toward the out conformation by using DMSO. This system stands in contrast to known transient supramolecular polymerization processes, which rely on metastable (non-assembled) monomers. Here, it is the kinetic trapping of the assembling monomer that allows control over the lifetime of the transient supramolecular polymer via a pH-responsive mechanism.