Abstract
The design and synthesis of new stimuli-responsive hydrogen-bonding monomers that display a diversity of self-assembly pathways is of central importance in supramolecular chemistry. Here we describe the aggregation properties of a simple, intrinsically C2-symmetric enantiopure bicyclic cavity compound bearing a terminally unsubstituted ureidopyrimidinone fragment fused with a pyrrole moiety in different solvents and in the absence and presence of C60 and C70 guests. The tetrameric cyclic aggregate is selectively obtained in chlorinated solvents, where only part of the available hydrogen bonding sites are utilized, whereas in toluene or upon addition of C70 guests, further aggregation into tubular supramolecular polymers is achieved. The open-end cyclic assemblies rearrange into a closed-shell capsule upon introduction of C60 with an accompanied symmetry breaking of the monomer. Our study demonstrates that a C60 switch can be used to simultaneously control the topology and occupancy of tubular assemblies resulting from the aggregation of small monomers.
Highlights
The design and synthesis of new stimuli-responsive hydrogen-bonding monomers that display a diversity of self-assembly pathways is of central importance in supramolecular chemistry
The latter H-bonding site was envisioned to be utilized for the connection of the bicyclic V-shaped monomers into cyclic tetrameric assemblies, the former unsubstituted urea will be required for linking the resulting cyclic tetramers into a 2H-bonded polymeric tube (Fig. 1d)
This is most likely to be due to the steric crowding arising from the bulky solubilizing groups situated on the core[20]. These groups were necessary to ensure solubility of the monomer. We reasoned that these complications could be overcome by placing the H-bonding sites required for tubular polymerization further away from the solubilizing groups using urea functional group as in monomer 1
Summary
The design and synthesis of new stimuli-responsive hydrogen-bonding monomers that display a diversity of self-assembly pathways is of central importance in supramolecular chemistry. This is reasonable taking into account the entropic cost needed to assemble a large number of small monomers and the increase of the number of alternative aggregation pathways when several less preorganized monomers are used All these challenges make it of highest interest to develop small and available monomers that selfassemble into open-end cavity aggregates capable of responding to external stimuli and switching between well-defined open, closed or polymeric forms, all possessing a tubular structure. The successful design of such responsive monomers could lead to switchable self-assembling transmembrane channels for drug delivery, shape-selective catalysts or insulator materials for molecular wires, among many other conceivable possibilities To accomplish this task, H-bonds are preferred over coordination bonds due to the more expressed dynamic nature of the former, especially when different conformers and tautomeric forms of the H-bonding unit are engaged in the assembly process. The system described represents a unique case of a sophisticated stimulus-responsive, dynamic, one-component supramolecular system
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