Precision Nanocluster-Based Toroidal and Supertoroidal Frameworks Using Photocycloaddition-Assisted Dynamic Covalent Chemistry.

Atomically precise nanoclusters (NCs) have recently emerged as ideal building
blocks for constructing self-assembled multifunctional superstructures. The
existing structures are based on various non-covalent interactions of the ligands
on the NC surface, resulting in inter-NC interactions. Despite recent demon�strations on light-induced reversible self-assembly, long-range reversible self�assembly based on dynamic covalent chemistry on the NC surface has yet to be
investigated. Here, it is shown that Au25 NCs containing thiolated umbelliferone
(7-hydroxycoumarin) ligands allow [2+2] photocycloaddition reaction-induced
self-assembly into colloidal-level toroids. The toroids upon further irradiation
undergo inter-toroidal reaction resulting in macroscopic supertoroidal honey�comb frameworks. Systematic investigation using electron microscopy, atomic
force microscopy (AFM), and electron tomography (ET) suggest that the NCs
initially form spherical aggregates. The spherical structures further undergo
fusion resulting in toroid formation. Finally, the toroids fuse into macroscopic
honeycomb frameworks. As a proof-of-concept, a cross-photocycloaddition
reaction between coumarin-tethered NCs and an anticancer drug (5-fluoro�uracil) is demonstrated as a model photo-controlled drug release system. The
model system allows systematic loading and unloading of the drug during the
assembly and disassembly under two different wavelengths. The results suggest
that the dynamic covalent chemistry on the NC surface offers a facile route for
hierarchical multifunctional frameworks and photocontrolled drug release.