Switching the activity of a photoredox catalyst through reversible encapsulation and release

Planar Polymer Membranes Accommodate Functional Self-Assembly of Inserted Resorcinarene Nanocapsules

Solid-supported polymer membranes (SSPMs) offer great potential in material and life sciences due to their increased mechanical stability and robustness compared to solid-supported lipid membranes. However, there is still a need for expanding the functionality of SSPMs by combining them with synthetic molecular assemblies. In this study, SSPMs served as a flexible matrix for the insertion of resorcinarene monomers and their self-assembly into functional hexameric resorcinarene capsules. Resorcinarene capsules provide a large cavity with affinity specifically for cationic and polyhydroxylated molecules. While the capsules are stable in apolar organic solvents, they disassemble when placed in polar solvents, which limits their application. Here, a solvent-assisted approach was used for copolymer membrane deposition on solid support and simultaneous insertion of the resorcinarene monomers. By investigation of the molecular factors and conditions supporting the codeposition of the copolymer and resorcinarene monomers, a stable hybrid membrane was formed. The hydrophobic domain of the membrane played a crucial role by providing a sufficiently thick and apolar layer, allowing for the self-assembly of the capsules. The capsules were functional inside the membranes by encapsulating cationic guests from the aqueous environment. The amount of resorcinarene capsules in the hybrid membranes was quantified by a combination of quartz-crystal microbalance with dissipation and liquid chromatography-mass spectrometry, while the membrane topography and layer composition were analyzed by atomic force microscopy and neutron reflectometry. Functional resorcinarene capsules inside SSPMs can serve as dynamic sensors and potentially as cross-membrane transporters, thus holding great promise for the development of smart surfaces.

Remarkably Selective Binding, Behavior Modification, and Switchable Release of (Bipyridine)3Ru(II) vis-à-vis (Phenanthroline)3Ru(II) by Trimeric Cyclophanes in Water

A recurring dream of molecular recognition is to create receptors that distinguish between closely related targets with sufficient accuracy, especially in water. The more useful the targets, the more valuable the dream becomes. We now present multianionic trimeric cyclophane receptors with a remarkable ability to bind the iconic (bipyridine)3Ru(II) (with its huge range of applications) while rejecting the nearly equally iconic (phenanthroline)3Ru(II). These receptors not only selectively capture (bipyridine)3Ru(II) but also can be redox-switched to release the guest. 1D- and 2D(ROESY)-NMR spectroscopy, luminescence spectroscopy, and molecular modeling enabled this discovery. This outcome allows the control of these applications, e.g., as a photocatalyst or as a luminescent sensor, by selectively hiding or exposing (bipyridine)3Ru(II). Overall, a 3D nanometric object is selected, picked-up, and dropped-off by a discrete molecular host. The multianionic receptors protect excited states of these metal complexes from phenolate quenchers so that the initial step in photocatalytic phenolate oxidation is retarded by nearly 2 orders of magnitude. This work opens the way for (bipyridine)3Ru(II) to be manipulated in the presence of other functional nano-objects so that many of its applications can be commanded and controlled. We have a cyclophane-based toolkit that can emulate some aspects of proteins that selectively participate in cell signaling and metabolic pathways by changing shape upon environmental commands being received at a location remote from the active site.

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts

Molecular recognition of guest molecules in a confined cavity is one of the important phenomena in biological and artificial molecular systems. When the guest is trapped within an artificial nano-space, its conformation is fixed in an unusual fashion by noncovalent interactions with host frameworks, and also the guest is kept away from the bulk solvent by the steric effect of the host. Therefore, host-guest formations lead to the effective modulation of the chemical and physical properties of guests via noncovalent interactions. In contrast to the many examples of organic guests, the examples of host-guest formation using coordination complex guests have been less explored. This is simply due to the size and shape complementarity problem between small hosts and large coordination complex guests. Resorcin[4]arene-based supramolecular hosts have been shown to provide internal cavities that are large enough to fully accommodate coordination complexes within the internal spaces via effective molecular interactions. In this article, we focus on supramolecular strategies to control the chemical and physical properties of the coordination complex guests within resorcin[4]arene-based supramolecular hosts. By the careful selection of the host and guest complexes, these combinations can produce a new supramolecular system, showing unusual structures, redox, catalytic, and photophysical properties derived from the entrapped coordination complexes.

Taming Tris(bipyridine)ruthenium(II) and Its Reactions in Water by Capture/Release with Shape-Switchable Symmetry-Matched Cyclophanes

Electron/proton transfers in water proceeding from ground/excited states are the elementary reactions of chemistry. These reactions of an iconic class of molecules—polypyridineRu(II)—are now controlled by capturing or releasing three of them with hosts that are shape-switchable. Reversible erection or collapse of the host walls allows such switchability. Some reaction rates are suppressed by factors of up to 120 by inclusive binding of the metal complexes. This puts nanometric coordination chemistry in a box that can be open or shut as necessary. Such second-sphere complexation can allow considerable control to be exerted on photocatalysis, electrocatalysis, and luminescent sensing involving polypyridineRu(II) compounds. The capturing states of hosts are symmetry-matched to guests for selective binding and display submicromolar affinities. A perching complex, which is an intermediate state between capturing and releasing states, is also demonstrated.

Highly Crystalline Ag-based Coordination Polymers for Efficient Photocatalytic Oxidation of Sulfides

Coordination polymers (CPs) display great potential for the development of highly active photocatalysts. Herein, we report the fabrication of a highly crystalline CP, [Ag₂ BTT]_n (BTT=benzene-1,2,4,5-tetrathiol). The crystal structure of [Ag₂ BTT]_n was resolved and its performance for photocatalytic oxidation of thioanisole was explored. [Ag₂ BTT]_n is highly active and selective for the photo-oxidation of sulfides to sulfoxides under mild conditions, that is, in air, at room temperature and in the absence of a sacrificial reagent, co-catalyst or redox mediator.

Ruthenium(II) complexes with phosphonate-substituted 1,10-phenanthroline ligands: synthesis, characterization and use in organic photocatalysis

Ru(II) complexes with polypyridyl ligands play a central role in the development of photocatalytic organic reactions. This work is aimed at the structural modification of such complexes to increase their...

Multiple Molecular Logic Gate Arrays in One System of (2-(2’-Pyridyl)imidazole)Ru(II) Complexes and Trimeric Cyclophanes in Water

Shape-switchable cyclophane hosts allow the controlled capture and release of reactive polypyridineRu(ii) complexes in water. This gives rise to a network of host–guest binding, acid–base reactions in ground and excited states, and chemical redox interconversions. In the case of (2-(2′-pyridyl)imidazole)Ru(ii) complexes, several molecular logic gate arrays of varying complexity emerge as a result. Cyclophane-induced ‘off–on’ switching of luminescence in neutral solution is found to originate from two features of these aromatic hosts: enhancement of radiative decay by the polarizable host and the suppression of nonradiative decay involving deprotonation by reducing the water content within the deep host cavity. These are examples of nanometric coordination chemistry/physics being controlled by inclusion in an open box. The aromatic units of the macrocycle are also responsible for the shape-switching mechanism of wall collapse/erection.

Transactions of a polypyridineRu(ii) complex with photons, protons and shape-switchable hosts show several types of molecular logic.

Tuneable Control of Organocatalytic Activity through Host-Guest Chemistry

Dynamic regulation of chemical reactivity is important in many complex chemical reaction networks, such as cascade reactions and signal transduction processes. Signal responsive catalysts could play a crucial role in regulating these reaction pathways. Recently, supramolecular encapsulation was reported to regulate the activities of artificial catalysts. We present a host-guest chemistry strategy to modulate the activity of commercially available synthetic organocatalysts. The molecular container cucurbit[7]uril was successfully applied to change the activity of four different organocatalysts and one initiator, enabling up- or down-regulation of the reaction rates of four different classes of chemical reactions. In most cases CB[7] encapsulation results in catalyst inhibition, however in one case catalyst activation by binding to CB[7] was observed. The mechanism behind this unexpected behavior was explored by NMR binding studies and pKa measurements. The catalytic activity can be instantaneously switched during operation, by addition of either supramolecular host or competitive binding molecules, and the reaction rate can be predicted with a kinetic model. Overall, this signal responsive system proves a promising tool to control catalytic activity.

A Promising Approach for Controlling the Second Coordination Sphere of Biomimetic Metal Complexes: Encapsulation in a Dynamic Hydrogen-Bonded Capsule

The design of biomimetic models of metalloenzymes needs to take into account many factors and is therefore a challenging task. We propose in this work an original strategy to control the second coordination sphere of a metal centre and its distal environment. A biomimetic complex, reproducing the first coordination sphere, is encapsulated in a self-assembled hydrogen-bonded capsule. The cationic complex is co-encapsulated with its counter-anion or with solvent molecules. The capsule is dynamic, allowing a fast in/out exchange of the co-encapsulated species. It also provides both a hydrogen-bonding site in the second coordination sphere and a source of proton as it can be deprotonated in the presence of the complex, providing a globally neutral host-guest assembly. This simple and broad scope strategy is unprecedented in biomimetic studies. The approach appears to be a very promising method for the stabilisation of reactive species and for the study of their reactivity.

Emissive Supramolecular Systems Based on Reversible Bond Formation and Noncovalent Interactions

Noncovalent interactions and reversible bond formations are widely seen in natural systems for the construction of sophisticated molecular systems that perform various biological processes. Inspired by the natural systems, luminescent supramolecular systems constructed by coordination-driven self-assembly and homometallic metal-metal interations have been studied increasingly. These supramolecular systems show fascinating luminescent behaviors that are not observed from single components. This review summarizes our progress in the development of two types of unique luminescent supramolecular systems. The mononuclear Pt(II) complex units can sandwich coinage metal ions to form heteropolynuclear complexes involving heterometallic metal-metal interactions. A close proximity of the two or three different metal ions by the noncovalent forces lead to orbital overlapping among the coinage metal ions and the Pt(II) complex units, showing emission color change accompanied with structural transformation and reversible metal binding behaviors. Emissive host-guest systems consisting of mononuclear metal complexes and a hydrogen-bonded capsule are also developed, that show a unique encapsulation-induced emission enhancement (EIEE) behavior.

Anion-mediated encapsulation-induced emission enhancement of an IrIII complex within a resorcin[4]arene hexameric capsule

The anions of the Ir complex salts control the thermodynamic stability and photoluminescence properties of the host–guest complex.

Thermodynamically driven self-assembly of pyridinearene to hexameric capsules

Pyridinearene macrocycles have previously shown unique host-guest properties in their capsular dimers including endo complexation of neutral molecules and exo complexation of anions. Here, we demonstrate for the first time the formation of hydrogen bonded hexamer of tetraisobutyl-octahydroxypyridinearene in all three states of matter - gas phase, solution and solid-state. Cationic tris(bipyridine)ruthenium(ii) template was found to stabilize the hexamer in gas phase, whereas solvent molecules do this in condensed phases. In solution, the capsular hexamer was found to be the thermodynamically favoured self-assembly product and transition from dimer to hexamer occurred in course of time. The crystal structure of hexamer revealed 24 N-HO direct intermolecular hydrogen bonds between the six pyridinearene macrocycles without any bridging solvent molecules. Hydrogen bond patterns correlate well with DFT computed structures. Thus, all structural chemistry methods (IM-MS, DOSY NMR, DFT, X-ray crystallography) support the same structure of the hexameric capsule that has a diameter of ca. 3 nm and volume of 1160 Å3.

Visible-light photocatalytic aerobic oxidation of sulfides to sulfoxides with a perylene diimide photocatalyst

Photosensitized oxygenation has been recognised as a modern method of incorporating oxygen into a substrate, as it offers environmentally benign alternatives to several conventional synthetic procedures. A metal-free aerobic selective sulfoxidation photosensitized by a perylene diimide photocatalyst has been developed. The reaction utilizes visible light as the driving force and molecular oxygen as the oxidant. The advantages of the developed method include high efficiency and selectivity, extremely simple operation and work-up procedure, mild reaction conditions, and practical application in late-stage functionalization.

Encapsulation condition dependent photophysical properties of polypyridyl Ru(ii) complexes within a hydrogen-bonded capsule

A simple modulation for encapsulation technique is a facile method to control the luminescent properties of supramolecular complexes.

A Switchable Gold Catalyst by Encapsulation in a Self-Assembled Cage

Dinuclear gold complexes have the ability to interact with one or more substrates in a dual-activation mode, leading to different reactivity and selectivity than their mononuclear relatives. In this contribution, this difference was used to control the catalytic properties of a gold-based catalytic system by site-isolation of mononuclear gold complexes by selective encapsulation. The typical dual-activation mode is prohibited by this catalyst encapsulation, leading to typical behavior as a result of mononuclear activation. This strategy can be used as a switch (on/off) for a catalytic reaction and also permits reversible control over the product distribution during the course of a reaction.

Efficient epoxide isomerization within a self-assembled hexameric organic capsule

The supramolecular organic nano-capsule formed by the self-assembly of six resorcin[4]arene units efficiently promotes the isomerization of epoxides (Meinwald isomerization) to the corresponding carbonyl compounds.

Hydration of aromatic alkynes catalyzed by a self-assembled hexameric organic capsule

The combination of a Brønsted acid catalyst and a supramolecular organic capsule formed by the self-assembly of six resorcin[4]arene units efficiently promotes the mild hydration of aromatic alkynes to their corresponding ketones.

Tertiary amine mediated aerobic oxidation of sulfides into sulfoxides by visible-light photoredox catalysis on TiO2

The selective oxidation of sulfides into sulfoxides receives much attention due to industrial and biological applications. However, the realization of this reaction with molecular oxygen at room temperature, which is of importance towards green and sustainable chemistry, remains challenging. Herein, we develop a strategy to achieve the aerobic oxidation of sulfides into sulfoxides by exploring the synergy between a tertiary amine and titanium dioxide via visible-light photoredox catalysis. Specifically, titanium dioxide can interact with triethylamine (TEA) to form a visible-light harvesting surface complex, preluding the ensuing selective redox reaction. Moreover, TEA, whose stability was demonstrated by a turnover number of 32, plays a critical role as a redox mediator by shuttling electrons during the oxidation of sulfide. This work suggests that the addition of a redox mediator is highly functional in establishing visible-light-induced reactions via heterogeneous photoredox catalysis.

Applications of supramolecular capsules derived from resorcin[4]arenes, calix[n]arenes and metallo-ligands: from biology to catalysis

This review summarizes supramolecular capsules based on resorcin[4]arenes, calix[n]arenes and metal–ligands, having concrete applications in biomedical field, catalysis and material science.

Giant electroactive M4L6 tetrahedral host self-assembled with Fe(II) vertices and perylene bisimide dye edges

Self-assembly of octahedral Fe(II) ions and linear perylene bisimide (PBI) dyes with 2,2'-bipyridine groups covalently attached at the imide positions quantitatively yields an Fe4(PBI)6 tetrahedron by the directional bonding approach. With an edge length of 3.9 nm and estimated internal volume >950 Å(3), tetrahedron T is one of the largest M4L6 tetrahedra ever reported. Importantly, many of the desirable photo- and electroactive properties of the PBI ligands are transferred to the nanoscale metallosupramolecule. Tetrahedron T absorbs strongly across the visible spectrum out to 650 nm and exhibits a total of 7 highly reversible electrochemical oxidation and reduction waves spanning a 3.0 V range. This facile cycling of 34 electrons between +18 and -16 charged species is likely enabled due to the porous nature of the tetrahedron that allows the necessary counterions to freely flow in and out of the host. Host-guest encapsulation of C60 by T in acetonitrile was studied by (13)C NMR spectroscopy, UV-vis spectroscopy, and ESI-MS, confirming that the tetrahedron is a suitable host for large, functional guest molecules.