Hierarchical self-assembly and controlled disassembly of a cavitand-based host-guest supramolecular polymer

There is considerable interest in dynamic materials featuring modular components with nano-scale dimensions and controlled responsiveness to external stimuli. Supramolecular polymers are a class of materials that fulfil all these conditions well. Here, we present a family of host-guest supramolecular polymers that combine the outstanding complexing properties of tetraphosphonate cavitands toward N-methylpyridinium guests with molecular switching. The designed monomer is a cavitand featuring four inward facing PO groups at the upper rim and a single N-methylpyridinium unit at the lower rim, forming instantaneously a polymeric species in solution, thanks to the high complexation constants measured for these host-guest interactions. This system has been analyzed by NMR spectroscopy and electrochemical techniques. In order to interpret the results of diffusion-sensitive experiments, we took advantage of the X-ray crystal structure obtained for the polymeric species and developed an original treatment for the PGSE data by non-linear fitting. The analysis of the experimental data identified an isodesmic polymerization model at a monomer concentration below 20 mM, driven by intrachain host-guest interactions, and an additional level of tetrameric bundle aggregation above 20 mM, due to interchain dipolar and quadrupolar interactions. Two orthogonal disassembly procedures have been implemented: electrochemical reduction for the linear chains and solvent-driven dissolution for the bundles.