Epitaxial Growth of γ-Cyclodextrin-Containing Metal-Organic Frameworks Based on a Host-Guest Strategy.

A class of metal-organic frameworks (MOFs)-namely CD-MOFs-obtained from natural products has been grown in an epitaxial fashion as films on the surfaces of glass substrates, which are modified with self-assembled monolayers (SAMs) of γ-cyclodextrin (γ-CD) molecules. The SAMs are created by host-guest complexation of γ-CD molecules with surface-functionalized pyrene units. The CD-MOF films have continuous polycrystalline morphology with a structurally out-of-plane ( c-axial) orientation, covering an area of several square millimeters, with a thickness of ∼2 μm.

Radically promoted formation of a molecular lasso.

Two potential viologen-based molecular lasso precursors-both composed of a 4,4'-bipyridinium (BIPY) unit as part of a rope appended to a cyclobis(paraquat--phenylene) (CBPQT) loop-that have been designed to mimic the threading/unthreading motion of lasso peptides, have been synthesised and characterised. Solution and solid-state experiments reveal that, when the BIPY unit in the rope and the CBPQT loop are connected by a bulky linker, no lasso-like conformational transformation is observed between the different redox states on account of steric effects. In sharp contrast, when the linker size is small, the molecule can be switched between (i) a free rope-like conformation in its fully oxidised state and (ii) a self-entangled lasso-like conformation under reducing conditions employing either chemical or electrochemical stimuli: the BIPY˙ unit in the rope resides inside the cavity of the CBPQT˙ loop, forming a pseudo[1]rotaxane.

Encapsulation of Ibuprofen in CD-MOF and Related Bioavailability Studies.

Although ibuprofen is one of the most widely used nonsteroidal anti-inflammatory drugs (NSAIDs), it exhibits poor solubility in aqueous and physiological environments as a free acid. In order to improve its oral bioavailability and rate of uptake, extensive research into the development of new formulations of ibuprofen has been undertaken, including the use of excipients as well as ibuprofen salts, such as ibuprofen lysinate and ibuprofen, sodium salt. The ultimate goals of these studies are to reduce the time required for maximum uptake of ibuprofen, as this period of time is directly proportional to the rate of onset of analgesic/anti-inflammatory effects, and to increase the half-life of the drug within the body; that is, the duration of action of the effects of the drug.

Symbiotic Control in Mechanical Bond Formation.

Since the advent of mechanically interlocked molecules (MIMs), many approaches to templating their formation using various different noncovalent bonding interactions have been introduced and explored. In particular, employing radical-pairing interactions between BIPY(.+) units, the radical cationic state of 4,4'-bipyridinium (BIPY(2+) ) units, in syntheses is not only a convenient but also an attractive source of templation because of the unique properties residing in the resulting catenanes and rotaxanes.

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 β-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.

Oligorotaxane Radicals under Orders.

A strategy for creating foldameric oligorotaxanes composed of only positively charged components is reported. Threadlike components-namely oligoviologens-in which different numbers of 4,4'-bipyridinium (BIPY(2+)) subunits are linked by p-xylylene bridges, are shown to be capable of being threaded by cyclobis(paraquat-p-phenylene) (CBPQT(4+)) rings following the introduction of radical-pairing interactions under reducing conditions. UV/vis/NIR spectroscopic and electrochemical investigations suggest that the reduced oligopseudorotaxanes fold into highly ordered secondary structures as a result of the formation of BIPY(•+) radical cation pairs.

CD-MOF: A Versatile Separation Medium.

Porous metal-organic frameworks (MOFs) have been studied in the context of a wide variety of applications, particularly in relation to molecular storage and separation sciences. Recently, we reported a green, renewable framework material composed of γ-cyclodextrin (γ-CD) and alkali metal salts--namely, CD-MOF. This porous material has been shown to facilitate the separation of mixtures of alkylaromatic compounds, including the BTEX mixture (benzene, toluene, ethylbenzene, and the regioisomers of xylene), into their pure components, in both the liquid and gas phases, in an energy-efficient manner which could have implications for the petrochemical industry.

Esterase- and pH-responsive poly(β-amino ester)-capped mesoporous silica nanoparticles for drug delivery.

Gating of mesoporous silica nanoparticles (MSNs) with the stimuli-responsive poly(β-amino ester) has been achieved. This hybrid nanocarrier releases doxorubicin (DOX) under acidic conditions or in the presence of porcine liver esterase. The DOX loaded poly(β-amino ester)-capped MSNs reduce cell viability when tested on MDA-MB-231 human breast cancer cells.

Carbohydrate-mediated purification of petrochemicals.

Metal-organic frameworks (MOFs) are known to facilitate energy-efficient separations of important industrial chemical feedstocks. Here, we report how a class of green MOFs-namely CD-MOFs-exhibits high shape selectivity toward aromatic hydrocarbons. CD-MOFs, which consist of an extended porous network of γ-cyclodextrins (γ-CDs) and alkali metal cations, can separate a wide range of benzenoid compounds as a result of their relative orientation and packing within the transverse channels formed from linking (γ-CD)6 body-centered cuboids in three dimensions.

Semiconducting single crystals comprising segregated arrays of complexes of C60.

Although pristine C60 prefers to adopt a face-centered cubic packing arrangement in the solid state, it has been demonstrated that noncovalent-bonding interactions with a variety of molecular receptors lead to the complexation of C60 molecules, albeit usually with little or no control over their long-range order. Herein, an extended viologen-based cyclophane—ExBox2(4+)—has been employed as a molecular receptor which, not only binds C60 one-on-one, but also results in the columnar self-assembly of the 1:1 inclusion complexes under ambient conditions. These one-dimensional arrays of fullerenes stack along the long axis of needle-like single crystals as a consequence of multiple noncovalent-bonding interactions between each of the inclusion complexes.

Anticancer activity expressed by a library of 2,9-diazaperopyrenium dications.

Polyaromatic compounds are well-known to intercalate DNA. Numerous anticancer chemotherapeutics have been developed upon the basis of this recognition motif. The compounds have been designed such that they interfere with the role of the topoisomerases, which control the topology of DNA during the cell-division cycle.

Folding of oligoviologens induced by radical-radical interactions.

We report the synthesis of a series of homologous oligoviologens in which different numbers of 4,4'-bipyridinium (BIPY(2+)) subunits are linked by p-xylylene bridges, as a prelude to investigating how their radical cationic forms self-assemble both in solution and in the solid state. The strong radical-radical interactions between the radical cationic forms of the BIPY(2+) units-namely, BIPY(•+)-in these oligoviologens induce intra- or intermolecular folding of these homologues. UV/Vis/NIR spectroscopic studies and DFT quantum mechanics indicate that the folding of the shorter oligoviologens is dominated by intermolecular radical-radical interactions.

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.

Relative unidirectional translation in an artificial molecular assembly fueled by light.

Motor molecules present in nature convert energy inputs, such as a chemical fuel or incident photons of light, into directed motion and force biochemical systems away from thermal equilibrium. The ability not only to control relative movements of components in molecules but also to drive their components preferentially in one direction relative to each other using versatile stimuli is one of the keys to future technological applications. Herein, we describe a wholly synthetic small-molecule system that, under the influence of chemical reagents, electrical potential, or visible light, undergoes unidirectional relative translational motion.

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.

Ex(2)Box: interdependent modes of binding in a two-nanometer-long synthetic receptor.

Incorporation of two biphenylene-bridged 4,4'-bipyridinium extended viologen units into a para-phenylene-based cyclophane results in a synthetic receptor that is ~2 nm long and adopts a box-like geometry. This cyclophane, Ex(2)Box(4+), possesses the ability to form binary and ternary complexes with a myriad of guest molecules ranging from long π-electron-rich polycyclic aromatic hydrocarbons, such as tetracene, tetraphene, and chrysene, to π-electron-poor 2,6-dinitrotoluene, 1,2,4-trichlorobenzene, and both the 9,10- and 1,4-anthraquinone molecules. Moreover, Ex(2)Box(4+) is capable of forming one-to-one complexes with polyether macrocycles that consist of two π-electron-rich dioxynaphthalene units, namely, 1,5-dinaphtho[38]crown-10.

Photoexpulsion of surface-grafted ruthenium complexes and subsequent release of cytotoxic cargos to cancer cells from mesoporous silica nanoparticles.

Ruthenium(II) polypyridyl complexes have emerged both as promising probes of DNA structure and as anticancer agents because of their unique photophysical and cytotoxic properties. A key consideration in the administration of those therapeutic agents is the optimization of their chemical reactivities to allow facile attack on the target sites, yet avoid unwanted side effects. Here, we present a drug delivery platform technology, obtained by grafting the surface of mesoporous silica nanoparticles (MSNPs) with ruthenium(II) dipyridophenazine (dppz) complexes.

Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin.

Gold recovery using environmentally benign chemistry is imperative from an environmental perspective. Here we report the spontaneous assembly of a one-dimensional supramolecular complex with an extended {[K(OH₂)₆][AuBr₄](α-cyclodextrin)₂}n chain superstructure formed during the rapid co-precipitation of α-cyclodextrin and KAuBr₄ in water. This phase change is selective for this gold salt, even in the presence of other square-planar palladium and platinum complexes.

γ-Cyclodextrin cuprate sandwich-type complexes.

Three structures, based on γ-cyclodextrin (γ-CD) and metal ions (Cu(2+), Li(+), Na(+), and Rb(+)), have been prepared in aqueous and alkaline media and characterized structurally by single-crystal X-ray diffraction. Their dimeric assemblies adopt cylindrical channels along the c axes in the crystals. Coordinative and hydrogen bonding between the cylinders and the solvent molecules lead to the formation of two-dimensional sheets, with the identity of the alkali-metal ion strongly influencing the precise nature of the solid-state structures.

A radically configurable six-state compound.

Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods.

Molecular gauge blocks for building on the nanoscale.

Molecular gauge blocks, based on 1-7, 9-11 paraxylene rings, have been synthesized as part of a homologous series of oligoparaxylenes (OPXs) with a view to providing a molecular tool box for the construction of nano architectures-such as spheres, cages, capsules, metal-organic frameworks (MOFs), metal-organic polyhedrons (MOPs) and covalent-organic frameworks (COFs), to name but a few-of well-defined sizes and shapes. Twisting between the planes of contiguous paraxylene rings is generated by the steric hindrance associated with the methyl groups and leads to the existence of soluble molecular gauge blocks without the need, at least in the case of the lower homologues, to introduce long aliphatic side chains onto the phenylene rings in the molecules. Although soluble molecular gauge blocks with up to seven consecutive benzenoid rings have been prepared employing repeating paraxylene units, in the case of the higher homologues it becomes necessary to introduce hexyl groups instead of methyl groups onto selected phenylene rings to maintain solubility.

Radically enhanced molecular switches.

The mechanism governing the redox-stimulated switching behavior of a tristable [2]rotaxane consisting of a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4'-bipyridinium (BIPY(2+)) unit, is described. The BIPY(2+) unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT(4+) ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable [2]rotaxanes and one bistable [2]catenane composed of CBPQT(4+) rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY(2+) unit with either a TTF or DNP unit, is investigated.

Rapid thermally assisted donor-acceptor catenation.

Charged donor-acceptor [2]catenanes containing cyclobis(paraquat-p-phenylene) as the ring component can be synthesised in yields of up to 88% in under one hour by heating two precursors in the presence of macrocyclic polyether templates in N,N-dimethylformamide at 80 °C.