Ilha Hwang

Supramolecular assemblies built with host-stabilized charge-transfer interactions

Host-stabilized charge-transfer (CT) interactions and supramolecular assemblies built with these interactions are described. A variety of supramolecular assemblies including polyrotaxanes, molecular necklaces, and rotaxane dendrimers were synthesized through the intramolecular or intermolecular host-stabilized CT complex formation using cucurbit[8]uril (CB[8]) and D-A molecules having both electron-donor and electron-acceptor units connected by various types of linkers. Applications, including the design and synthesis of redox-driven molecular machines such as molecular loop locks, development of redox-controllable vesicles and detection of biologically important molecules, are also described.

Synthetic Molecular Machine Based on Reversible End-to-Interior and End-to-End Loop Formation Triggered by Electrochemical Stimuli

We have designed and synthesized a novel [2]pseudorotaxane-based molecular machine in which the interconversion between end-to-interior and end-to-end loop structures is reversibly controlled by electrochemical stimuli. Cucurbit[8]uril (CB[8]) and the thread molecule 3(4+) with an electron-rich hydroxynaphthalene unit and two electron-deficient viologen units form the 1:1 complex 4(4+) with an end-to-interior loop structure, which is reversibly converted into an end-to-end structure upon reduction. Large changes in shape and size of the molecule accompany the reversible redox process. The key feature of the machine-like behavior is the reversible interconversion between an intramolecular charge-transfer complex and viologen cation radical dimer inside CB[8] triggered by electrochemical stimuli.

Unconventional U-shaped conformation of a bolaamphiphile embedded in a synthetic host

The X-ray crystal structure of 1,12-dodecane diammonium (C(12)DA(2+)) encapsulated in cucurbit[8]uril reveals an unconventional U-shaped conformation of C(12)DA(2+), which is attributed to the favorable host-guest interactions to overcome the charge-charge repulsion of the two ammonium groups in close proximity and high internal strain.

Highly Stable, Water-Dispersible Metal-Nanoparticle-Decorated Polymer Nanocapsules and Their Catalytic Applications†

A facile synthesis of highly stable, water-dispersible metal-nanoparticle-decorated polymer nanocapsules (M@CB-PNs: M=Pd, Au, and Pt) was achieved by a simple two-step process employing a polymer nanocapsule (CB-PN) made of cucurbit[6]uril (CB[6]) and metal salts. The CB-PN serves as a versatile platform where various metal nanoparticles with a controlled size can be introduced on the surface and stabilized to prepare new water-dispersible nanostructures useful for many applications. The Pd nanoparticles on CB-PN exhibit high stability and dispersibility in water as well as excellent catalytic activity and recyclability in carbon-carbon and carbon-nitrogen bond-forming reactions in aqueous medium suggesting potential applications as a green catalyst.

Hollow nanotubular toroidal polymer microrings

Despite the remarkable progress made in the self-assembly of nano- and microscale architectures with well-defined sizes and shapes, a self-organization-based synthesis of hollow toroids has, so far, proved to be elusive. Here, we report the synthesis of polymer microrings made from rectangular, flat and rigid-core monomers with anisotropically predisposed alkene groups, which are crosslinked with each other by dithiol linkers using thiol-ene photopolymerization. The resulting hollow toroidal structures are shape-persistent and mechanically robust in solution. In addition, their size can be tuned by controlling the initial monomer concentrations, an observation that is supported by a theoretical analysis. These hollow microrings can encapsulate guest molecules in the intratoroidal nanospace, and their peripheries can act as templates for circular arrays of metal nanoparticles.

Electrochemically Controllable Reversible Formation of Cucurbit[8]uril-Stabilized Charge-Transfer Complex on Surface

The reversible formation of cucurbit[8]uril (CB[8])-stabilized charge-transfer (CT) complex on gold, which can be regulated by electrochemical stimuli, is reported. A viologen-based thread molecule with a thiol terminus was designed to synthesise a stable CB[8]-threaded [2]pseudorotaxane (5 2 + ). A mixed self-assembled monolayer (SAM) of 5 2 + and 3-mercaptopropionic acid (3-MPA) on gold was prepared and characterised by reflectance FT-IR spectroscopy. The formation and disruption of a CT complex on surface was investigated by cyclic voltammetry with the SAM on gold as a working electrode in a supporting electrolyte solution containing 2,6-dihydroxynaphthalene (DHNp). The cathodic peak was shifted to a more negative potential (ΔE∼40 mV) when compared with that measured without DHNp in the electrolyte, which confirmed the formation of a CT complex on gold upon immersion of the SAM of 5 2 + on gold in the supporting electrolyte containing DHNp. When the viologen unit was reduced, the CT complex was destroyed and the electron-rich guest molecule was released. Upon reoxidation of the reduced viologen unit, however, the CT complex was slowly reformed.

Reversible Morphological Transformation between Polymer Nanocapsules and Thin Films through Dynamic Covalent Self‐Assembly

A facile method has been developed for synthesizing polymer nanocapsules and thin films using multiple in-plane stitching of monomers by the formation of reversible disulfide linkages. Owing to the reversibility of the disulfide linkages, the nanostructured materials readily transform their structures in response to environmental changes at room temperature. For example, in reducing environments, the polymer nanocapsules release loaded cargo molecules. Moreover, reversible morphological transformations between these structures can be achieved by simple solvent exchanges. This work is a novel approach for the formation of robust nano/microstructured materials that dynamically respond to environmental stimuli.

Molecular Pop‐up Toy: A Molecular Machine Based on Folding/Unfolding Motion of Alkyl Chains Bound to a Host

We have designed and synthesized a new type of molecular machine based on the folding/unfolding motion of an alkyl chain bound to a host, triggered by a redox stimulus. A guest molecule containing a viologen unit with a long alkyl chain 1(2+) and its one-electron reduced species 1 + . form very stable 1:1 host-guest complexes 2(2+) and 2 + ., respectively, with cucurbit[8]uril (CB[8]), where the long alkyl chain of the guests is in a folded conformation inside the host cavity. Upon addition of 2,6-dihydroxynaphthalene as an electron donor, the binary complex 2(2+) turns into a ternary complex 3(2+) through host-stabilized charge-transfer complex formation with the alkyl chain extended into the solution outside the host cavity. The ternary complex behaves like a molecular machine reminiscent of a pop-up toy, as it shows reversible folding/unfolding motion of the alkyl chain of the guest in response to a redox stimulus. For example, one-electron reduction of 3(2+) results in the rapid generation of the 2 + . complex, accompanied by a dramatic conformational change of the alkyl chain from an extended to a folded conformation, and the process can be reversed by oxidation.

A new cucurbit[6]uril-based ion-selective electrode for acetylcholine with high selectivity over choline and related quaternary ammonium ions

Detection of acetylcholine (ACh+) with high sensitivity and selectivity in the presence of interfering ions including choline (Ch+) poses a significant analytical challenge. A cucurbit[6]uril derivative, (allyloxy)12CB[6], forms a stable 1:1 inclusion complex with ACh+ but does not interact significantly with Ch+ in solution. Based on this result, we prepared ion-selective electrodes (ISEs) for ACh+ using (allyloxy)12CB[6] as an ionophore. The (allyloxy)12CB[6]-based ISEs detect ACh+ with unprecedentedly high selectivity over Ch+ as well as other interfering ions such as dopamine, NH4 +, and .

Confined Nucleation and Growth of PdO Nanocrystals in a Seed-Free Solution inside Hollow Nanoreactor

This paper reports a novel and adaptable hollow nanoreactor system containing a solution of cucurbituril (CB) inside a silica nanoparticle (CB@h-SiO2) which enables the nucleation and formation of nanocrystals (NCs) to be confined at the seed-free interior solution inside the cavity. The above nanospace confinement strategy restricted the volume of medium available for NC formation to the solution inside the cavity to a few tens of nanometers in size and allowed homogeneous NC nucleation to be examined. Harboring of CB@h-SiO2 in a Pd2+ complex solution confined the nucleation and formation of PdO NCs to the well-isolated nanosized cavity protected by the silica nanoshell, allowing the convoluted formation of clustered PdO NCs to be thoroughly examined. The corresponding temporal investigation indicated that PdO NC clusters evolved via a distinct pathway combining dendritic growth on early nucleated seed NCs and attachment of small intermediate clusters. In addition, the explored strategy was used to fabricate a recyclable nanocatalyst system for selective catalytic oxidation of cinammyl alcohols, featuring a cavity-included Fe3O4/PdO nanocomposite.