Light Control of Stoichiometry and Motion in Pseudorotaxanes Comprising a Cucurbit[7]uril Wheel and an Azobenzene-Bipyridinium Axle

The Diversity of Cucurbituril Molecular Switches and Shuttles

Ring translocation switches and shuttles featuring a macrocycle (or a ring molecule) navigating between two or more stations continue to attract attention. While the vast majority of these systems are developed in organic solvents, the cucurbituril (CB) macrocycles are ideally suited to prepare such systems in water. Indeed, their stability and their relatively high affinity for relevant guest molecules are key attributes toward translating the progresses made in organic solvents, into water. This concept article summarizes the findings, key advances and multiple possibilities offered by CBs toward advanced molecular switches and shuttles in water.

Cucurbit[7]uril Complexation of Near-Infrared Fluorescent Azobenzene-Cyanine Conjugates

Two new azobenzene heptamethine cyanine conjugates exist as dispersed monomeric molecules in methanol solution and exhibit near-infrared (NIR) cyanine absorption and fluorescence. Both conjugates form non-emissive cyanine H-aggregates in water, but the addition of cucurbit[7]uril (CB7) induces dye deaggregation and a large increase in cyanine NIR fluorescence emission intensity. CB7 encapsulates the protonated azonium tautomer of the 4-(N,N-dimethylamino)azobenzene component of each azobenzene-cyanine conjugate and produces a distinctive new absorption band at 534 nm. The complex is quite hydrophilic, which suggests that CB7 can be used as a supramolecular additive to solubilize this new family of NIR azobenzene-cyanine conjugates for future biomedical applications. Since many azobenzene compounds are themselves potential drug candidates or theranostic agents, it should be possible to formulate many of them as CB7 inclusion complexes with improved solubility, stability, and pharmaceutical profile.

Multiple Azoarenes Based Systems - Photoswitching, Supramolecular Chemistry and Application Prospects

In the recent decades, the investigations on photoresponsive molecular systems with multiple azoarenes are quite popular in diverse perspectives ranging from fundamental understanding of multiple photoswitches, supramolecular chemistry, and various application prospects. In fact, several insightful and conceptual designs of such systems were investigated with architectural distinctions. In particular, the demonstration of applications such as data storage with the help of multistate or orthogonal photoswitches, light modulation of catalysis via cooperative switching, sensors using supramolecular host-guest interactions, and materials such as liquid crystals, grating, actuators, etc. are some of the milestones in this area. Herein, we cover the recent advancements in the research areas of multiazoarenes containing systems that have been classified into Type-1 {linear, non-linear, and core-based (A)}, Type-2 {tripodal C₃ -symmetric (C3)} and Type-3 {macrocyclic (M)} structural motifs.

A light-responsive molecular switch based on cucurbit[7]uril and 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium dibromide displaying white light emission

By using ¹H NMR, ESI-MS and UV spectra, a novel light-responsive molecular switch constructed using 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium (1²⁺) and cucurbit[7]uril (Q[7]) is demonstrated. The E- to Z-isomerization of the double bond in 1²⁺ results in the transition of the switching states from the 1 : 2 complex E-1²⁺@Q[7]₂ to the stable 1 : 1 complex Z-1²⁺@Q[7]. In particular, both the 1 : 2 complex and the 1 : 1 complex can emit cold white fluorescence under UV light.

Toward Light-Controlled Supramolecular Peptide Dimerization

The selective photodeprotection of the NVoc-modified FGG tripeptide yields the transformation of its 1:1 receptor-ligand complex with cucurbit[8]uril into a homoternary FGG₂@CB8 assembly. The resulting light-induced dimerization of the model peptide provides a tool for the implementation of stimuli-responsive supramolecular chemistry in biologically relevant contexts.

Photoresponsive Binding Dynamics in High-Affinity Cucurbit[8]uril-Dithienylethene Host-Guest Complexes

The use of external stimuli to control the binding kinetics in supramolecular systems is of critical importance for the development of advanced molecular machines and devices. In this work, a study focused on the kinetics of a water-soluble host-guest system based on cucurbit[8]uril and two dithienylethene (DTE) photoswitches is reported. It is shown that for the DTE guest comprising two anionic sulfonate side arms appended to pyridinium moieties, the formation/dissociation of the pseudorotaxane structures is slowed down by more than 100000-fold with respect to its bipyridinium analogue. The decrease in ingression rate leads to the emergence of a competitive metastable product with the open DTE isomer that has an important influence in the overall binding kinetics. Moreover, the host-guest dissociation kinetics is demonstrated to be approximately 100-fold slower for the closed DTE isomer (t_1/2 =107 h vs. t_1/2 =1.2 h for the open isomer) allowing control over the dissociation rate with light.

A pseudorotaxane formed from a cucurbit[7]uril wheel and a bioinspired molecular axle with pH, light and redox-responsive properties

A pH-, light- and redox-responsive flavylium-bipyridinium molecular dyad (bioinspired in natural anthocyanins) was synthesized and employed to devise a pseudorotaxane with the macrocycle cucurbit[7]uril (CB7) in aqueous solution. The inclusion complex was characterized by UV-Vis absorption, fluorescence emission, NMR and electrochemical techniques which demonstrate formation of a stable binary complex between the dyad and CB7 both under acidic and neutral conditions. It is noteworthy that the flavylium-bipyridinium tricationic dyad is only stable in highly acidic media, undergoing a reversible hydration reaction at slightly acidic or neutral pH to give a trans-chalcone-bipyridinium dication. 1H NMR experiments showed that in this last species the CB7 binds to the bipyridinium unit while in the tricationic species the macrocycle is positioned between the flavylium and the bipyridinium moieties. The different location of the CB7 wheel in the two dyad states allows control of the shuttling movement using light and pH stimuli that trigger the interconversion between these two species.

Reversing Chemoselectivity: Simultaneous Positive and Negative Catalysis by Chemically Equivalent Rims of a Cucurbit[7]uril Host

Enzyme catalysis has always been an inspiration and an unattainable goal for chemists due to features such as high specificity, selectivity, and efficiency. Here, we disclose a feature neither common in enzymes nor ever described for enzyme mimics, but one that could prove crucial for the catalytic performance of the latter, namely the ability to catalyze and inhibit two different reactions at the same time. Remarkably, this can be realized by two identical, spatially resolved catalytic sites. In the future, such a synchronized catalyst action could be used not only for controlling chemoselectivity, as in the present case, but also for regulating other types of chemical reactivity.

Threading-gated photochromism in [2]pseudorotaxanes

Rigid, Y-shaped imidazole compounds containing the bis(thienyl)ethene moiety were designed and synthesized. The 4,5-bis(benzothienyl)-2-phenylimidazolium cations were then used as axles for [2]pseudorotaxane formation with 24-membered crown ether wheels. It was demonstrated using 1H NMR spectroscopy, UV-Vis absorption and emission spectroscopies that this host-guest interaction results in significant changes in the photochromic properties of the imidazolium axles. This is a rare example of gated photochromism, which exploits the recognition event of an interpenetrated molecular system to tune the photochromic properties in one of the components.

Optically-controlled supramolecular self-assembly of an antibiotic for antibacterial regulation

A new supramolecular photo-responsive antibiotic (azobenzene-norfloxacin/αCD) exhibited a higher “on–off” ratio of antibacterial ability than Azo-Nor alone.

The chaotropic effect as an orthogonal assembly motif for multi-responsive dodecaborate-cucurbituril supramolecular networks

In aqueous solution, host-guest complexation is frequently driven by the hydrophobic effect, which also constitutes a popular approach in the design of supramolecular assemblies. Herein, we report an orthogonal assembly motif, the chaotropic effect, which exploits the tendency of chaotropic anions to associate with hydrophobic surfaces.

pH-Gated photoresponsive shuttling in a water-soluble pseudorotaxane

Phototriggered ring shuttling in a water-soluble fluorescent pseudorotaxane can be enabled and disabled at different pH values.

Photochemical investigation of cyanoazobenzene derivatives as components of artificial supramolecular pumps

The threading kinetics of a self-assembled molecular pump is increased upon functionalization of the azobenzene moiety with a cyano group.

Supramolecular complexes formed by dimethoxypillar[5]arenes and imidazolium salts: a joint experimental and computational investigation

The stability of host–guest complexes formed by dimethoxypillar[5]arenes and imidazolium salts has been analyzed as a function of different structural features of the guest, using a combined approach of different techniques.

Light-induced piston nanoengines: ultrafast shuttling of a styryl dye inside cucurbit[7]uril

A proof of principle for an ultrafast molecular shuttle based on the light-operated movement of a styryl dye inside cucurbit[7]uril was described.

Calix-Like Metal-Organic Complex for High-Sensitivity X-Ray-Induced Photochromism

Metal-organic complexes (MOCs) as promising candidates for directly visual X-ray detection at room temperature are rare and discovered unexpectedly, even though every crystalline material needs X-ray diffraction studies. Here, we report a rational strategy of mimicking host-guest system for developing high-sensitive X-ray-induced photochromic materials. Two resulting calix-like metal-organic complexes (cMOC-1 and cMOC-2) were prepared by encapsulating the electron-capturing "guest" into the cavity of calix-like electron-donating "host." One of them (cMOC-1) achieves instantaneous X-ray-induced photochromism and easy recovery by synergizing the aprotic matrix of MOC and similar host-guest interaction. Their strikingly different response to X-ray irradiation resulting from the structural difference demonstrates the feasibility and acceptability of our design strategy. This strategy may open new perspectives for developing high-performance photo-responsive functional materials.

Nanoarchitectonics for Dynamic Functional Materials from Atomic-/Molecular-Level Manipulation to Macroscopic Action

Objects in all dimensions are subject to translational dynamism and dynamic mutual interactions, and the ability to exert control over these events is one of the keys to the synthesis of functional materials. For the development of materials with truly dynamic functionalities, a paradigm shift from "nanotechnology" to "nanoarchitectonics" is proposed, with the aim of design and preparation of functional materials through dynamic harmonization of atomic-/molecular-level manipulation and control, chemical nanofabrication, self-organization, and field-controlled organization. Here, various examples of dynamic functional materials are presented from the atom/molecular-level to macroscopic dimensions. These systems, including atomic switches, molecular machines, molecular shuttles, motional crystals, metal-organic frameworks, layered assemblies, gels, supramolecular assemblies of biomaterials, DNA origami, hollow silica capsules, and mesoporous materials, are described according to their various dynamic functions, which include short-term plasticity, long-term potentiation, molecular manipulation, switchable catalysis, self-healing properties, supramolecular chirality, morphological control, drug storage and release, light-harvesting, mechanochemical transduction, molecular tuning molecular recognition, hand-operated nanotechnology.

Spatiotemporal control over the co-conformational switching in pH-responsive flavylium-based multistate pseudorotaxanes

A multistate molecular dyad containing flavylium and viologen units was synthesized and the pH dependent thermodynamics of the network completely characterized by a variety of spectroscopic techniques such as NMR, UV-vis and stopped-flow. The flavylium cation is only stable at acidic pH values. Above pH ≈ 5 the hydration of the flavylium leads to the formation of the hemiketal followed by ring-opening tautomerization to give the cis-chalcone. Finally, this last species isomerizes to give the trans-chalcone. For the present system only the flavylium cation and the trans-chalcone species could be detected as being thermodynamically stable. The hemiketal and the cis-chalcone are kinetic intermediates with negligible concentrations at the equilibrium. All stable species of the network were found to form 1 : 1 and 2 : 1 host : guest complexes with cucurbit[7]uril (CB7) with association constants in the ranges 10(5)-10(8) M(-1) and 10(3)-10(4) M(-1), respectively. The 1 : 1 complexes were particularly interesting to devise pH responsive bistable pseudorotaxanes: at basic pH values (≈12) the flavylium cation interconverts into the deprotonated trans-chalcone in a few minutes and under these conditions the CB7 wheel was found to be located around the viologen unit. A decrease in pH to values around 1 regenerates the flavylium cation in seconds and the macrocycle is translocated to the middle of the axle. On the other hand, if the pH is decreased to 6, the deprotonated trans-chalcone is neutralized to give a metastable species that evolves to the thermodynamically stable flavylium cation in ca. 20 hours. By taking advantage of the pH-dependent kinetics of the trans-chalcone/flavylium interconversion, spatiotemporal control of the molecular organization in pseudorotaxane systems can be achieved.

Construction of porous cationic frameworks by crosslinking polyhedral oligomeric silsesquioxane units with N-heterocyclic linkers

In fields of materials science and chemistry, ionic-type porous materials attract increasing attention due to significant ion-exchanging capacity for accessing diversified applications. Facing the fact that porous cationic materials with robust and stable frameworks are very rare, novel tactics that can create new type members are highly desired. Here we report the first family of polyhedral oligomeric silsesquioxane (POSS) based porous cationic frameworks (PCIF-n) with enriched poly(ionic liquid)-like cationic structures, tunable mesoporosities, high surface areas (up to 1,025 m(2) g(-1)) and large pore volumes (up to 0.90 cm(3) g(-1)). Our strategy is designing the new rigid POSS unit of octakis(chloromethyl)silsesquioxane and reacting it with the rigid N-heterocyclic cross-linkers (typically 4,4'-bipyridine) for preparing the desired porous cationic frameworks. The PCIF-n materials possess large surface area, hydrophobic and special anion-exchanging property, and thus are used as the supports for loading guest species PMo10V2O40(5-); the resultant hybrid behaves as an efficient heterogeneous catalyst for aerobic oxidation of benzene and H2O2-mediated oxidation of cyclohexane.