Heteromeric Completive Self-Sorting in Coordination Cage Systems

Heteroleptic coordination cages, nonstatistically assembled from a set of matching ligands, can be obtained by mixing individual components or via cage-to-cage transformations from homoleptic precursors. Based on the latter approach, we here describe a new level of self-sorting in coordination cage systems, namely, 'heteromeric completive self-sorting'. Here, two heteroleptic assemblies of type Pd2A2B2 and Pd2A2C2, sharing one common ligand component A but differing in the other, are shown to coexist in solution. This level of self-sorting can be reached either from a statistical mixture of assemblies based on some ligands B and C or, alternatively, following a first step of integrative self-sorting giving a distinct Pd2B2C2 intermediate. While subtle enthalpic factors dictate the outcome of the self-sorting, we found that it is controllable. From a unique set of three ligands, we demonstrate the transition from strict integrative self-sorting forming a Pd2AB2C cage to heteromeric completive self-sorting to give Pd2A2B2 and Pd2A2C2 by variation of the ligand ratio. Cage-to-cage transformations were followed by NMR and MS experiments. Single crystal X-ray structures for three new heteroleptic cages were obtained, impressively highlighting the versatility of ligand A to either form a π-stacked trans-figure-of-eight arrangement in Pd2A2B2 or occupy two cis-edges in Pd2A2C2 or only a single edge in Pd2AB2C. This study paves the way toward the control of heteroleptic cage populations in a systems chemistry context with emerging features such as chemical information processing, adaptive guest selectivity, or stimuli-responsive catalytic action.

Internal Dynamics and Modular Peripheral Binding in Stimuli-Responsive 3 : 2 Host:Guest Complexes

Now that the chemistry of 1 : 1 host:guest complexes is well-established, it is surprising to note that higher stoichiometry (oligomeric) complexes, especially those with excess host, remain largely unexplored. Yet, proteins tend to oligomerize, affording new functions for cell machinery. Here, we show that cucurbit[n]uril (CB[n]) macrocycles combined with symmetric, linear di-viologens form unusual 3 : 2 host:guest complexes exhibiting remarkable dynamic properties, host self-sorting, and external ring-translocation. These results highlight the structural tunability of cucurbit[8]uril (CB[8]) based 3 : 2 host:guest complexes in water and their responsiveness toward several stimuli (chemicals, pH, redox).

Photoresponsive Metal-Organic Frameworks as Adjustable Scaffolds in Reticular Chemistry

The easy and remote switching of light makes this stimulus an ideal candidate for a large number of applications, among which the preparation of photoresponsive materials stands out. The interest of several scientists in this area in order to achieve improved functionalities has increase parallel to the growth of the structural complexity of these materials. Thus, metal-organic frameworks (MOFs) turned out to be ideal scaffolds for light-responsive ligands. This review is focused on the integration of photoresponsive organic ligands inside MOF crystalline arrays to prepare enhanced functional materials. Besides the summary of the preparation, properties and applications of these materials, an overview of the future outlook of this research area is provided.

Light driven molecular lock comprises a Ru(bpy)2(hpip) complex and cucurbit[8]uril

Here, complex 1 ([Ru(bpy)2(hpip)]2+-MV2+) and CB[8] can form a stable 1 : 1 inclusion complex in aqueous solution, resembling a U-shaped conformation. Upon light irradiation, two complex 1 were reversibly locked through the formation of a MV˙+ radical dimer that is stabilized in the cavity of CB[8] with Ru complexes as blockers, in which complex 1 was transformed from a U-shaped conformation to a interlocked complex. This study provided a feasible strategy for the fabrication of a photo-driven supramolecular machine resembling a "lock".

Influence of exo-Adamantyl Groups and endo-OH Functions on the Threading of Calix[6]arene Macrocycle

Calix[6]arenes bearing adamantyl groups at the exo-rim form pseudorotaxanes with dialkylammonium axles paired to the weakly coordinating [B(Ar^F)₄]⁻ anion. The exo-adamantyl groups give rise to a more efficient threading with respect to the exo-tert-butyl ones, leading to apparent association constants more than one order of magnitude higher. This improved stability has been ascribed to the more favorable van der Waals interactions of exo-adamantyls versus exo-tert-butyls with the cationic axle. Calix[6]arenes bearing endo-OH functions give rise to a less efficient threading with respect to the endo-OR ones, in line with what was known from the complexation of alkali metal cations.

Quantitative Supramolecular Heterodimerization for Efficient Energy Transfer

The challenge of quantitatively forming self-assembled heterodimers without other equilibrium by-products is overcome through self-sorting favored by the introduction of designed shape-complementary moieties. Such a supramolecular strategy based on cucurbit[8]uril-directed dimerization is further applied to generate hetero-chromophore dimers quantitatively, leading to efficient energy transfer (>85 %) upon photoexcitation.

Mutualistic benefit in the self-sorted co-aggregates of peri-naphthoindigo and a 4-amino-1,8-naphthalimide derivative

Photoluminescence enhancement for all the members of a self-sorted co-aggregate was observed for the first time by successfully amalgamating AIEE and social self-sorting. Intermolecular H-bonding and π-π stacking were utilised to prepare several co-aggregates of peri-naphthoindigo (PNI) and a 4-amino-1,8-naphthalimide derivative dye, NH₂-NMI. In the heteromeric aggregates, photoluminescence intensities were increased by 28% for the imide and more than 400% for PNI. Due to spectral overlap between the emission of the imide and the absorption of PNI, energy transfer took place from the former to the latter. The heteromeric aggregates are dual emissive and the relative intensities of the emissions can easily be tuned by varying the stoichiometry of the dyes.

Controllable and large-scale supramolecular vesicle aggregation: orthogonal light-responsive host–guest and metal–ligand interactions

On the basis of the host–guest molecular recognition interaction between β-cyclodextrin and azobenzene, two kinds of supramolecular self-assemblies (Py-CD⊃Azo-C and Py-CD⊃Azo-C3) were constructed.

Supramolecular self-assemblies for bacterial cell agglutination driven by directional charge-transfer interactions

Two supramolecular amphiphiles are fabricated through directional charge-transfer interactions, which self-assemble into nanofibers and nanoribbons. Due to the existence of galactose on their surface, these self-assemblies act as a cell glue to agglutinate E. coli, benefiting from multivalent interactions.

Narcissistic chiral self-sorting of molecular face-rotating polyhedra

Narcissistic chiral self-sorting prevailed in the assembly of molecular face-rotating polyhedra from a C_3h building block 5,5,10,10,15,15-hexabutyl-truxene-2,7,12-tricarbaldehyde and racemic mixtures of 1,2-diamines.

Five-Component Self-Assembly of Cucurbituril-Based Hetero-pseudorotaxanes

[5]Pseudorotaxanes can be obtained by self-sorting using heteroditopic guests and various cucurbituril homologues as hosts. The assembly and chemically induced disassembly of the pseudorotaxanes can be monitored by measuring the fluorescence of the anthracene guest in solution. Mass spectral evidence for the supramolecular assemblies is obtained in the gas phase. The disassembly in the gas phase can be achieved by collision-induced dissociation leading to the corresponding [2]- and [3]pseudorotaxanes.

Contrasting pK a Shifts in Cucurbit[7]uril Host-Guest Complexes Governed by an Interplay of Hydrophobic Effects and Electrostatic Interactions

Cucurbit[7]uril inclusion complexes with guests bearing dimethylamino groups show the expected upward pK a shifts, whereas their diethylamino counterparts display a decrease in pK a due to the preferential stabilization of the unprotonated form. These results identify the diethylamino group as the substituent of choice to avoid receptor-assisted protonation of guest molecules and present new evidence for the role of the hydrophobic effect as a driving force in cucurbituril complexation.

Orthogonal switching of self-sorting processes in a stimuli-responsive library of cucurbit[8]uril complexes

The self-sorting processes in dynamic libraries of cucurbit[8]uril complexes can be switched orthogonally by pH and redox stimuli.

pH-Driven self-sorting in a four component host–guest system

A four component host–guest system was found to self-sort only under equimolar and acidic conditions.

A Kinetic Self-Sorting Approach to Heterocircuit [3]Rotaxanes

In this proof-of-concept study, an active-template coupling is used to demonstrate a novel kinetic self-sorting process. This process iteratively increases the yield of the target heterocircuit [3]rotaxane product at the expense of other threaded species.

A cationic water-soluble biphen[3]arene: synthesis, host–guest complexation and fabrication of a supra-amphiphile

The first cationic water-soluble biphen[3]arene was synthesized, which was used to change the aggregation of the amphiphilic guest in water.

Controllable supramolecular polymerization through self-sorting of aliphatic and aromatic motifs

Self-sorting is one of the effective strategies to realize control over supramolecular polymerization.

The synthesis, structure, and molecular recognition properties of a [2]calix[1]biphenyl-type hybrid[3]arene

The first [2]calix[1]biphenyl-type hybrid[3]arene was synthesized. Its molecular recognition was investigated by using 1-dihexylammonium hexafluorophosphate as a guest.

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.

Gas-phase chemistry of molecular containers

The use of mass spectrometry for the investigation of supramolecular capsules and containers in solution and the gas-phase is reviewed.

Integrative self-sorting: a versatile strategy for the construction of complex supramolecular architecture

Integrative self-sorting expands the concepts of self-assembly and allows the programming of supramolecular architecture on a higher level of complexity.

Self-sorting behavior of a four-component host–guest system and its incorporation into a linear supramolecular alternating copolymer

Self-sorting behavior of a four-component host–guest system was found, which could be utilized to construct a linear supramolecular alternating copolymer.

Determination of the kinetics underlying the pKa shift for the 2-aminoanthracenium cation binding with cucurbit[7]uril

The binding dynamics of the 2-aminoanthracenium cation (AH⁺) and 2-aminoanthracene (A) with cucurbit[7]uril (CB[7]) was studied using stopped-flow experiments. The kinetics was followed by measuring the fluorescence changes over time for AH⁺ and A, which emit at different wavelengths. The studies at various pH values showed different mechanisms for the formation of the AH⁺@CB[7] complex, with this complex formed either by the binding of AH⁺ or by the initial binding of A followed by protonation. In the latter case, it was possible to determine the protonation ((1.5 ± 0.4) × 10⁹ M⁻¹ s⁻¹) and deprotonation (89 ± 7 s⁻¹) rate constants for complexed A/AH⁺, which showed that the pK_a shift of +3.1 for A/AH⁺ in the complex is mainly due to a lower deprotonation rate constant.

Synthesis of a novel water-soluble cylindrical macrotricyclic host and its complexation with N-methylquinolinium and N-methylisoquinolinium salts: formation of 1 : 2 complexes in water

A water-soluble cylindrical macrotricyclic host could form 1 : 2 complexes with N-methylquinolinium or N-methylisoquinolinium salts in water solution and in the solid state.

Supramolecular reactivity in the gas phase: investigating the intrinsic properties of non-covalent complexes

The high vacuum inside a mass spectrometer offers unique conditions to broaden our view on the reactivity of supramolecules. Because dynamic exchange processes between complexes are efficiently suppressed, the intrinsic and intramolecular reactivity of the complexes of interest is observed. Besides this, the significantly higher strength of non-covalent interactions in the absence of competing solvent allows processes to occur that are unable to compete in solution. The present review highlights a series of examples illustrating different aspects of supramolecular gas-phase reactivity ranging from the dissociation and formation of covalent bonds in non-covalent complexes through the reactivity in the restricted inner phase of container molecules and step-by-step mechanistic studies of organocatalytic reaction cycles to cage contraction reactions, processes induced by electron capture, and finally dynamic molecular motion within non-covalent complexes as unravelled by hydrogen-deuterium exchange processes performed in the gas phase.

Dual self-sorting by cucurbit[8]uril to transform a mixed micelle to vesicle

A systematic study on the cucurbit[8]uril (CB[8]) assisted transformation of a mixed micellar system of CTAB and a viologen surfactant to vesicles is depicted. The micelle to vesicle transformation is assisted by a charge transfer complex mediated ternary complexation between the viologen group of the surfactant, CB[8], and 2,6-dihydroxynaphthalene. In the presence of CB[8], both the surfactants formed U-shaped binary inclusion complexes inside the CB[8] cavity, and no selective binding is observed. Upon addition of DHN, CB[8] showed two different self-sorting mechanisms. The U-shaped binary complex with CTAB breaks down, and CB[8] moves toward the viologen headgroup of the other surfactant to form a stable ternary complex. In the case of the viologen surfactant, CB[8] moved toward the headgroup leaving the hydrophobic tail free in order to form the ternary complex. The mechanistic detail of this micelle to vesicle transformation is revealed through methodical studies using (1)H and DOSY NMR, ESI-MS, ITC, and other instrumental techniques.

Hydrogen-bond-driven controlled molecular marriage in covalent cages

A supramolecular approach that uses hydrogen-bonding interaction as a driving force to accomplish exceptional self-sorting in the formation of imine-based covalent organic cages is discussed. Utilizing the dynamic covalent chemistry approach from three geometrically similar dialdehydes (A, B, and D) and the flexible triamine tris(2-aminoethyl)amine (X), three new [3+2] self-assembled nanoscopic organic cages have been synthesized and fully characterized by various techniques. When a complex mixture of the dialdehydes and triamine X was subjected to reaction, it was found that only dialdehyde B (which has OH groups for H-bonding) reacted to form the corresponding cage B3X2 selectively. Surprisingly, the same reaction in the absence of aldehyde B yielded a mixture of products. Theoretical and experimental investigations are in complete agreement that the presence of the hydroxyl moiety adjacent to the aldehyde functionality in B is responsible for the selective formation of cage B3X2 from a complex reaction mixture. This spectacular selection was further analyzed by transforming a nonpreferred (non-hydroxy) cage into a preferred (hydroxy) cage B3X2 by treating the former with aldehyde B. The role of the H-bond in partner selection in a mixture of two dialdehydes and two amines has also been established. Moreover, an example of unconventional imine bond metathesis in organic cage-to-cage transformation is reported.