Hydrogen-bonded Capsules Research Articles

(Invited) Luminescent Coordination Complexes as a Molecular Probe in Supramolecular Organic-Inorganic Assemblies

Guest molecules entrapped in a confined space can show unusual chemical and physical properties because their behaviors are manipulated by multiple noncovalent interactions from the interiors of the space. Thus, it is important to understand the local environment in the confined space and noncovalent interactions between the guest molecule and the interiors for establishing a new molecular system based on the unusual chemical and physical phenomena from the trapped molecules. Photophysical properties of coordination complexes are easily adjusted by variation of metal ions and organic ligands, leading to a fine-tuning of their excited states related to photoluminescence. In particular, a charge-transfer excited state in the coordination complexes is an environment-sensitive feature and thus they will act as a molecular probe for elucidating a local environment around the complexes. Herein we confirmed the photophysical properties of IrIII complexes within a hydrogen-bonded hexameric capsule, which are derived from their mixed states of 3MLCT/3LLCT excited states. The photophysical properties of IrIII complexes were altered by the encapsulation within the hydrogen-bonded capsule, owing to the effective modulation of a local environment around the complex by noncovalent interaction. This result also provides us a better understanding of the internal space within the hydrogen-bonded capsule. Figure 1

Shape-Selective Supramolecular Capsules for Actinide Precipitation and Separation.

Improving actinide separations is key to reducing barriers to medical and industrial actinide isotope production and to addressing the challenges associated with the reprocessing of spent nuclear fuel. Here, we report the first example of a supramolecular anion recognition process that can achieve this goal. We have designed a preorganized triamidoarene receptor that induces quantitative precipitation of the early actinides Th(IV), Np(IV), and Pu(IV) from industrially relevant conditions through the formation of self-assembled hydrogen-bonded capsules. Selectivity over the later An(III) elements is shown through modulation of the nitric acid concentration, and no precipitation of actinyl or transition-metal ions occurs. The Np, Pu, and Am precipitates were characterized structurally by single-crystal X-ray diffraction and reveal shape specificity of the internal hydrogen-bonding array for the encapsulated hexanitratometalates. This work complements ion-exchange resins for 5f-element separations and illustrates the significant potential of supramolecular separation methods that target anionic actinide species.

Self-assembly of a fluorescent hydrogen-bonded capsule based on an amino-acid functionalised tetraphenylethylene.

Herein we report the self-assembly of an amino-acid substituted tetraphenylethylene (TPE) into a hydrogen-bonded dimeric capsule. Depending on the conditions applied the presented TPE derivative exhibits bluish photoluminescence with QYs up to 24% due to the AIE effect and excimer formation.

C–H⋯S Hydrogen Bond Assisted Supramolecular Encapsulation of Fullerenes with Nanobelts

C–H⋯S Hydrogen Bond Assisted Supramolecular Encapsulation of Fullerenes with Nanobelts

Anion-Based Self-assembly of Resorcin[4]arenes and Pyrogallol[4]arenes.

Spatial sequestration of molecules is a prerequisite for the complexity of biological systems, enabling the occurrence of numerous, often non-compatible chemical reactions and processes in one cell at the same time. Inspired by this compartmentalization concept, chemists design and synthesize artificial nanocontainers (capsules and cages) and use them to mimic the biological complexity and for new applications in recognition, separation, and catalysis. Here, we report the formation of large closed-shell species by interactions of well-known polyphenolic macrocycles with anions. It has been known since many years that C-alkyl resorcin[4]arenes (R4C) and C-alkyl pyrogallol[4]arenes (P4C) narcissistically self-assemble in nonpolar solvents to form hydrogen-bonded capsules. Here, we show a new interaction model that additionally involves anions as interacting partners and leads to even larger capsular species. Diffusion-ordered spectroscopy and titration experiments indicate that the anion-sealed species have a diameter of >26 Å and suggest stoichiometry (M)6(X–)24 and tight ion pairing with cations. This self-assembly is effective in a nonpolar environment (THF and benzene but not in chloroform), however, requires initiation by mechanochemistry (dry milling) in the case of non-compatible solubility. Notably, it is common among various polyphenolic macrocycles (M) having diverse geometries and various conformational lability.

Tannic Acid-Stabilized Self-Degrading Temperature-Sensitive Poly(2-n-propyl-2-oxazoline)/Gellan Gum Capsules for Lipase Delivery.

In recent years, stable hydrogen-bonded stimuli-responsive polymer capsules have been receiving great interest for the encapsulation and release of sensitive molecules such as lipase enzymes. Compartmental capsules having a liquid gel core stabilized with temperature-responsive hydrogen-bonded multilayers are advantageous over other conventional systems because of their ability to maintain hydrophilic lipase and other hydrophobic compounds in compatible protected molecular vehicle environments and prolong their native properties, e.g., in the body. In this work, we report a methodology to stabilize an aqueous liquid gellan gum (GG) core in a capsule using neutral and nontoxic building blocks, namely, poly(2-n-propyl-2-oxazoline) (PnPrOx) and tannic acid (TA), to fabricate temperature-responsive capsules, comprising both lipase and hydrophobic oil droplets. The capsules were fabricated by adding GG droplets to a PnPrOx suspension at a temperature (T) higher than its cloud point temperature (TCP). Notably, the formed capsules were not stable in water without TA stabilization via hydrogen bonding. Scanning electron microscopy (SEM) investigations of the GG/building block interphase revealed that the collapsed PnPrOx globules that are present above the TCP stabilized the GG interphase as a Pickering emulsion, while undergoing a configurational transformation into its linear form by interacting with TA in the next step of capsule formation resulting in a smooth PnPrOx/TA capsule wall. The encapsulation efficiencies of the capsules for model fluorescent molecules were found to be 52, 54, and 24% for FITC-dextran, rhodamine, and Nile red, respectively. The stability experiments exhibited swelling and shell thinning at certain locations followed by complete rupture of the capsules at 37 °C, while the capsules were stable for several weeks at temperatures below the TCP of PnPrOx. The capsules were found to be stable in stimulated gastric fluid (SGF) for several hours at 37 °C while successfully releasing the encapsulated lipase and Nile red (model hydrophobic compound) in stimulated intestinal fluid (SIF). The released lipase was found to retain almost 100% of its activity. The reported capsules have high potential for use as carriers for encapsulation and release of a variety of payloads ranging from proteins and vitamin supplements to enzymes and probiotics through the oral route of administration.

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.

Precise molecular shape control of linear and branched strips with chirality-assisted synthesis

ABSTRACT This account describes the latest developments in the field of chirality-assisted synthesis (CAS). CAS has emerged as a versatile method to control the shapes of π-conjugated macromolecules and supramolecular assemblies. With CAS, the sequence and chirality of the monomeric building blocks dictates the shape and functions of the resulting macromolecules in a programmable fashion. Supramolecular materials with special functions, e.g. the ability to encapsulate macrocycles, can be created with CAS. CAS has been utilized to synthesize a variety of shape-defined nanoscale structures, including polyaromatic strips, helices, molecular claws, and very large hydrogen-bonded closed-shell capsules. Applications, challenges, and future directions of the growing CAS field are discussed in light of the unique shape-control abilities CAS has to offer.

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

Controlling the encapsulation equilibrium is a key strategy to affect host-guest associations. Ruthenium(ii) polypyridyl complex salts suspended in a chloroform solution of resorcin[4]arene afforded a host-guest complex which showed structured emission spectra even in the solution state. In contrast, a host-guest complex obtained through homogeneous encapsulation conditions by using soluble ruthenium(ii) polypyridyl complex salts showed broadened emission spectra which strongly depended on the amount of the host owing to the encapsulation equilibrium. These results demonstrate that a simple modulation of the encapsulation technique is indeed promising and a facile approach to control the photophysical properties of supramolecular complexes.

Chirality-Assisted Synthesis of a Very Large Octameric Hydrogen-Bonded Capsule.

The strategy of chirality-assisted synthesis, which makes use of enantiomerically pure building blocks that are designed to associate in a single geometric orientation, was applied to synthesize an octameric hydrogen-bonded capsule with a cavity volume of 2300 Å3 . This cube-shaped capsule forms even host-guest complexes with tetraalkylammonium ions, and accommodates the large tetrahexadecylammonium cation in its cavity. The use of an enantiopure building block was shown to be highly beneficial for capsule formation, whereas its racemate also generates a large amount of ill-defined aggregates in solution and crystallizes as a hydrogen-bonded network.

Subphthalocyanines hydrogen bonded capsules featuring norbornadiene tethers: Promising fullerene receptors

We have employed density functional theory to study new subphthalocyanine based receptors which feature the 1,4-dithiino and norbornadiene linkages. The latter linkage significantly improves the host binding capabilities with respect to subphthalocyanine based receptors. Moreover, the interaction energy of the new subphthalocyanine–norbornadiene receptor is larger than that computed for the C60H28 buckycatcher. The dimerization of the new receptor forms a hydrogen-bonded capsule. Through a combination of non-bonded interactions, this capsule can bind C60 with an unprecedented affinity. Due to the exceptional stability of this capsule, it is our hope that it can be used for other guests besides fullerenes.

Structurally Flexible C₃-Symmetric Receptors for Molecular Recognition and Their Self-Assembly Properties.

The bioinspired design and synthesis of building blocks and their assemblies by the supramolecular approach has ever fascinated scientists to utilize such artificial systems for numerous purposes. Flexibility is a basic feature of natural systems. However, in artificial systems this is difficult to control, especially if there is no preorganization of the component(s) of a system. We have designed and synthesized a series of C3 -symmetric N-bridged flexible receptors and successfully utilized them to selectively entrap the notorious and toxic nitrate anion in aqueous medium. This was the first report of highest binding affinity for the nitrate anion in aqueous medium. An impressive self-sorting phenomenon of reversibly formed hydrogen-bonded capsules, which self-assembled from flexible tripodal receptors having branches of similar size and bearing the same amide functionality, has been disclosed. Encapsulated nitrate anion has been further utilized for the photochemical [2+2] cycloaddition reaction for the synthesis of strained four-membered ring structures through dynamic self-assembly. In this Personal Account, we summarize these results showing the utility of naturally inspired flexibility in artificial systems.

The influence of imperfect walls on the guest binding properties of hydrogen-bonded capsules.

Three classes of hydrogen-bonded capsules with imperfect walls have been prepared and characterized. The defects reduce the symmetry of the capsules, leading to rich isomerism. The missing hydrogen bonds provide additional flexibility to the capsules and exert an influence on their guest binding properties in different assemblies.

Hydrogen-Bonded Capsules in Water

Hydrogen-bonded capsules constrain molecules into small spaces, where they exhibit behavior that is inaccessible in bulk solution. Water competes with the formation of hydrogen bonds, and other forces for assembly, such as metal/ligand interactions or hydrophobic effects, have been applied. Here we report the reversible assembly of a water-soluble cavitand to a robust capsule host in the presence of suitable hydrophobic guests. The complexes are characterized by conventional NMR methods. Selectivity for guest length and fluorescence quenching of a stilbene guest are used as evidence for hydrogen bonding in the capsule.

Guest-Induced, Selective Formation of Isomeric Capsules with Imperfect Walls

The majority of building blocks in self-assembled capsules feature high symmetry. Reducing this symmetry inevitably leads to expanded possibilities for isomerism. Here, we report a deep cavitand host with one short and three long walls. Its dimerization to hydrogen-bonded capsules in the presence of suitable guest molecules can lead to two constitutional isomers. A given guest induces the predominant formation of only one isomer. The unexpected selectivity is interpreted in terms of the different hydrogen-bonding patterns of the capsules and their effects on the size, shape, and dynamics of the capsules' spaces.

Encapsulation of Ion Pairs in Extended, Self-Assembled Structures

Encapsulation of ion pairs in small spaces that are isolated from the medium is expected to result in amplified interactions between the ions. Yet, sequestration of ion pairs in self-assembled capsules is complicated by competition of the acids and bases for binding directly to the assembly components. We describe here a hydrogen-bonded capsule 1.2(8).1 that accommodates two γ-picolines and two acids as ion pairs. The supramolecular structure of the discrete 14-component assembly is characterized by NMR spectroscopy. The structure reveals the acids in the tapered ends of the capsule and γ-picoliniums near the glycoluril spacers in the capsule's center. Similar acid-base ion pairs are also obtained with 4-ethylpyridine, γ-picoline with difluoroacetic acid, and γ-picoline with trifluoromethanesulfonic acid. The (1)H NMR spectrum of the γ-picoline/trifluoroacetic acid ion pair shows a signal at δ = 18.7 ppm, indicating the acidic proton is in contact with both the picoline nitrogen and the trifluoroacetate oxygen. Further details about the unusual structures of ion pairs in small spaces are reported.

Formation of Heterodimeric Resorcin[4]arene Capsules on Surfaces: An X-Ray Photoelectron Spectroscopy Investigation

A perfect fit: Hydrogen bond formation of heterodimeric molecular capsules assembled from complementary functionalized resorcinarene cavitands is of considerable interest from the viewpoint of multicomponent molecular assemblies. The formation of hydrogen-bonded heterodimeric capsules AB (see picture) based on a monolayer of tetrapyridyl resorcinarenes B is verified by means of X-ray photoelectron spectroscopy analyzing N 1s signatures.

Selective dimerisation of tetraurea calix[4]arenes

The formation of hydrogen-bonded dimeric capsules from tetraurea calix[4]arenes is controlled by loops, connecting adjacent urea functions, and more or less bulky substituents. The dimerisation is only possible if loops are not overlapping and if the respective residues can pass the loops. A sorting scheme based on small and bulky residues and one to four loops allows reducing the number of possible dimers from 35 to 6 in a stoichiometric mixture of 11 ureas. With three different loop sizes (O–(CH2) n –O chains with n = 10, 14, 20 connecting adjacent phenylurea functions via their meta positions), it is possible to distinguish four urea residues of different sizes (small, medium, bulky and giant) ranging from tolyl to 4-[tris-(4-t-butylphenyl)methyl]-phenyl. While the smallest residue can pass all loops, the largest is excluded by all loops.

Serendipity in the Crystallization of a Series of C-Alkylcalix[4]resorcinarenes from Alcoholic Solvents

The crystal structures of a series of C-alkylcalix[4]resorcinarenes (alkyl = methyl, propyl, butyl, pentyl, hexyl, heptyl, and undecyl), each crystallized from tert-butanol, are reported. In all seven of these structures, eight tert-butanol molecules link two resorcinarenes with 16 hydrogen bonds, thereby facilitating the formation of simple dimeric hydrogen-bonded capsules, each of which encapsulates one (disordered) tert-butanol molecule as guest. These structures are compared with, and contrasted to, those obtained from the same series of resorcinarenes recrystallized both from mixed solvents containing tert-butanol and from methanol alone.