An Elastic Hydrogen-Bonded Cross-Linked Organic Framework for Effective Iodine Capture in Water.

A crystalline microporous hydrogen-bonded cross-linked organic framework has been developed through covalent photo-cross-linking of molecular monomers that are assembled in a crystalline state. The elastic framework expands its void space to adsorb iodine rapidly with a high uptake capacity in an aqueous environment as well as recovering its crystalline form after the release of iodine.

Cooperative capture synthesis: yet another playground for copper-free click chemistry.

Click chemistry describes a family of modular, efficient, versatile and reliable reactions which have acquired a pivotal role as one of the most useful synthetic tools with a potentially broad range of applications. While copper(i)-catalysed alkyne-azide cycloaddition is the most widely adopted click reaction in the family, the fact that it is cytotoxic restricts its practice in certain situations, e.g., bioconjugation. Consequently, researchers have been exploring the development of copper-free click reactions, the most popular example so far being strain-promoted alkyne-azide cycloadditions. An early example of copper-free click reactions that is rarely mentioned in the literature is the cucurbit[6]uril (CB6) catalysed alkyne-azide cycloaddition (CB-AAC). Despite the unique ability of CB-AAC to generate mechanically interlocked molecules (MIMs) - in particular, rotaxanes - its slow reaction rate and narrow substrate acceptance limit its scope. In this Tutorial Review, we describe our efforts of late in developing the fundamental principles and practical applications of a new copper-free click reaction - namely, cooperative capture synthesis, whereby introducing a cyclodextrin (CD) as an accelerator in CB-AAC, hydrogen bonding networks are formed between the rims of CD and CB6 in a manner that is positively cooperative, giving rise to a high level of pre-organisation during efficient and quick rotaxane formation. For example, [4]rotaxanes can be prepared nearly quantitatively within a minute in water. Furthermore, we have demonstrated that CB-AAC can accommodate a wider substrate tolerance by introducing pillararenes as promoters. To date, we have put cooperative capture synthesis into practice by (i) preparing polyrotaxanes containing up to 200 rings in nearly quantitative yields, (ii) trapping conformational isomers of polymacrocycles as rings in rotaxanes, (iii) demonstrating solid-state fluorescence and Förster resonance energy transfer (FRET) processes by fixing the fluorophores in a rotaxane and (iv) establishing the principle of supramolecular encryption in the preparation of dynamically and reversibly tunable fluorescent security inks.

Ring Shuttling Controls Macroscopic Motion in a Three-Dimensional Printed Polyrotaxane Monolith.

Amplification of molecular motions into the macroscopic world has great potential in the development of smart materials. Demonstrated here is an approach that integrates mechanically interlocked molecules into complex three-dimensional (3D) architectures by direct-write 3D printing. The design and synthesis of polypseudorotaxane hydrogels, which are composed of α-cyclodextrins and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, and their subsequent fabrication into polyrotaxane-based lattice cubes by 3D printing followed by post-printing polymerization are reported. By switching the motion of the α-cyclodextrin rings between random shuttling and stationary states through solvent exchange, the polyrotaxane monolith not only exhibits macroscopic shape-memory properties but is also capable of converting the chemical energy input into mechanical work by lifting objects against gravity.

Concurrent Covalent and Supramolecular Polymerization.

Covalent and supramolecular polymerizations, both of which offer their own unique advantages, have emerged as popular strategies for making artificial materials. Herein, we describe a concurrent covalent and supramolecular polymerization strategy-namely, one which utilizes 1) a bis-azide-functionalized diazaperopyrenium dication that undergoes polymeriation covalently with a bis-alkyne-functionalized biphenyl derivative in one dimension as a result of a rapid and efficient ß-cyclodextrin(CD)-accelerated, cucurbit[6]uril(CB)-templated azide-alkyne cycloaddition, while 2) the aromatic core of the dication is able to dimerize in a criss-cross fashion by dint of π-π interactions, enabling simultaneous supramolecular assembly, resulting in an extended polymer network in an orthogonal dimension.

Pillar[5]arene as a co-factor in templating rotaxane formation.

After the manner in which coenzymes often participate in the binding of substrates in the active sites of enzymes, pillar[5]arene, a macrocycle containing five hydroquinone rings linked through their para positions by methylene bridges, modifies the binding properties of cucurbit[6]uril, such that the latter templates azide-alkyne cycloadditions that do not occur in the presence of only the cucurbit[6]uril, a macrocycle composed of six glycoluril residues doubly linked through their nitrogen atoms to each other by methylene groups. Here, we describe how a combination of pillar[5]arene and cucurbit[6]uril interacts cooperatively with bipyridinium dications substituted on their nitrogen atoms with 2-azidoethyl- to 5-azidopentyl moieties to afford, as a result of orthogonal templation, two [4]rotaxanes and one [5]rotaxane in >90% yields inside 2 h at 55 °C in acetonitrile. Since the hydroxyl groups on pillar[5]arene and the carbonyl groups on cucurbit[6]uril form hydrogen bonds readily, these two macrocycles work together in a cooperative fashion to the extent that the four conformational isomers of pillar[5]arene can be trapped on the dumbbell components of the [4]rotaxanes. In the case of the [5]rotaxane, it is possible to isolate a compound containing two pillar[5]arene rings with local C5 symmetries. In addition to fixing the stereochemistries of the pillar[5]arene rings, the regiochemistries associated with the 1,3-dipolar cycloadditions have been extended in their constitutional scope. Under mild conditions, orthogonal recognition motifs have been shown to lead to templation with positive cooperativity that is fast and all but quantitative, as well as being green and efficient.

Tunable solid-state fluorescent materials for supramolecular encryption.

Tunable solid-state fluorescent materials are ideal for applications in security printing technologies. A document possesses a high level of security if its encrypted information can be authenticated without being decoded, while also being resistant to counterfeiting. Herein, we describe a heterorotaxane with tunable solid-state fluorescent emissions enabled through reversible manipulation of its aggregation by supramolecular encapsulation. The dynamic nature of this fluorescent material is based on a complex set of equilibria, whose fluorescence output depends non-linearly on the chemical inputs and the composition of the paper. By applying this system in fluorescent security inks, the information encoded in polychromic images can be protected in such a way that it is close to impossible to reverse engineer, as well as being easy to verify. This system constitutes a unique application of responsive complex equilibria in the form of a cryptographic algorithm that protects valuable information printed using tunable solid-state fluorescent materials.

Activation-Enabled Syntheses of Functionalized Pillar[5]arene Derivatives.

A series of regioselective di- and trifunctionalized pillar[5]arene derivatives have been synthesized by a deprotection-followed-by-activation strategy, and their constitutions have been established as a result of having access to their solid-state structures. De-O-methylation occurs in a stepwise manner at lower temperatures under kinetic control, affording the desired oligo-substituted pillar[5]arene derivatives. In addition, the regioisomers of these derivatives can be isolated by installing triflate groups on the free hydroxyl groups.

Efficient syntheses of pillar[6]arene-based hetero[4]rotaxanes using a cooperative capture strategy.

While a single pillar[6]arene ring, nestling between two cucurbit[6]uril rings in a series of three hetero[4]rotaxanes, is conformationally mobile in solution, it adopts the energetically most favourable conformation with local C3V symmetry in the solid state.