Supramolecular dendrimer self-assemblies by cooperative binding
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Cooperativity is a general principle that governs multivalent bindings in the supramolecular assemblies in nature, which is essential for a wide variety of biological structures and functions. Various model systems have been studied to reveal the energetics and mechanisms that underline cooperative bindings. However, not much effort has been made to maximize and utilize cooperative bindings towards constructing functional supramolecular assemblies. Dendrimers are nanometer-sized macromolecules with three-dimensional, highly-branched architectures, and can provide multivalent binding sites at the periphery. Here, we exploit the unique multivalency feature of dendrimers and their ability to maximize cooperative binding as a novel modular self-assembly approach to construct functional supramolecular structures. First of all, we studied the self-assembly of spherical dendrimers via periphery cooperative salt-bridging. Dendrimers with carboxyl peripheral groups showed to form capsules in aqueous solutions with the addition of divalent metal ions. These capsules were tunable in size and thickness, and controllable in disassembly. To further exploit the cooperative feature of dendrimers, we revealed the ability of cooperative H-bonding in regulating supramolecular self-assemblies in highly-competitive solvents, e.g. water. These H-bonded dendrimer capsules have the advantage of the thermo-responsiveness to trigger controlled disassembly and release of encapsulated materials. Further more, emulsions were utilized to template the cooperative self-assembly of dendrimers into a wide size range of capsules, which provides more versatility in the encapsulation of various materials. To further advance the functionality of supramolecular dendrimer capsules, we explored the feasibility of combining π-π stacking at dendrimers’ core with the periphery cooperative bindings. Dendrimers with an arene ring as the core self-assembled into capsules with dendrimers stack vertically in the membrane. This work provides the mechanistic foundation for incorporating functional micro-cycles into dendrimers core to generate capsules with sophisticated functionalities. To explore the effect of shape symmetry in regulating supramolecular self-assemblies, the spatial symmetry of the dendrimer molecules are broken by coupling two different-sized but chemically identical dendritic fragments together. Asymmetric dendrimers assembled into cylindrical superstructures, either as parallel fibrillar bundles or supercoiled double-helices. This study provides new insights into supramolecular asymmetry, and offers a new systematic design principle for constructing novel asymmetrical supramolecular structures.