Aqueous polyaromatic receptors for biomolecules with high selectivity

Discrimination of Perfluorinated Arenes/Alkanes by Modulable Polyaromatic Capsules

Efficient and selective binding of perfluorocarbons (PFCs), comprising only fluorine and carbon atoms, unlike hydrocarbons, remains quite difficult owing to the repulsive nature of fluorine. Here we describe that a cavity modulation strategy enables metal-linked polyaromatic capsules to quantitatively bind PFCs with excellent selectivity. From a mixture of perfluoroarene and the corresponding perfluoroalkane (i.e., perfluoronaphthalene and perfluorodecalin), a Pt(II)-linked capsule exclusively binds the arenes in water at room temperature, via effective D-A-A-D π-stacking interactions. The size-selective binding toward perfluoroarenes (i.e., perfluoronaphthalene and perfluorobenzene) is improved by using the analogous N-doped capsule from 85% to quantitative selectivity. Furthermore, unlike the Pt(II)-capsule, an isostructural Pd(II)-linked capsule displays the unusual length/shape-selective binding of linear/cyclic perfluoroalkanes (i.e., perfluoroheptane and perfluorodecalin) under similar conditions. Recognition of substituted hydrogen atoms on perfluorobiphenyl can also be accomplished using the Pt(II)-capsule. Various PFCs are thus clearly distinguished for the first time by the modulable polyaromatic capsules under ambient aqueous conditions.

Orientational Compatibility Modulation of Ligands in Low-Symmetry Multi-Cavity Discrete Coordination Cages by Neighbouring Cage Participation

Complexation of Pd(II) with a designer unsymmetrical bis-monodentate ligand (2 : 4 ratio) yielded a specific Pd2L4 type "single-cavity discrete coordination cage" (SCDCC), from a pool of 4 isomeric structures. The observed selctivity is attributed to inherent orientational preference of the ligand strands around the metal centers. Crafting a short coordinating arm at either ends of the bis-monodentate ligand (i.e the longer-arm) produced a pair of unsymmetrical isomeric tris-monodentate ligands; whereas crafting the same short-arm at both ends of the ligand gives an unsymmetrical tetrakis-monodentate ligand. Complexation of Pd(II) with either of the isomeric tris-monodenate ligands (3 : 4 ratio) resulted in corresponding low-symmetry "multi-cavity discrete coordination cage" MCDCC having two conjoined cavities, though the inherent relative orientational preference of the longer arms is not achievable in these cages. The enforced orientation is sustained by "Neighbouring Cage Participation" (NCP). However, one-pot combination of Pd(II), with a mixture of isomeric tris-monodentate ligands in 3 : 2 : 2 ratio produced an integratively self-sorted mixed-ligated MCDCC from a pool of 31 structures. Also, mixing Pd(II) with the tetrakis-monodentate ligand produced a MCDCC having three conjoined cavities. The inherent orientational preference of longer-arm of the ligand strands is retained in the mixed-ligated double-cavity and the homo-ligated triple cavity cages.

Synthetic molecular cage receptors for carbohydrate recognition

A captivating challenge in chemistry lies in achieving robust and precise binding of uncharged, hydrophilic carbohydrate entities. Although past decades have provided a variety of excellent molecular architectures tailored for carbohydrate recognition, including acyclic receptors, macrocycles and foldamers, recent advances have highlighted the potential of synthetic molecular cages. These structures are equipped with intricately designed cavities that contain bespoke noncovalent binding sites for carbohydrate interactions. Constructed with the principles of complementarity and preorganization, these cage receptors demonstrate high affinity and exquisite selectivity in carbohydrate recognition through noncovalent interactions, capitalizing on multivalency and cooperativity. This Review highlights recent advances in the design and application of molecular cages with diverse structures, interactions and binding capacities for carbohydrate recognition. In the concluding remarks, we discuss future avenues for further exploration.

Helicity control of a polyaromatic coordination capsule through stereoselective CH-π interactions

Although square-planar ML4 units are essential building blocks for coordination cages and capsules, the non-covalent control of the chirality and helicity of the resultant nanostructures is quite difficult. Here we report the helicity control of an M2L4 polyaromatic capsule, formed from metal ions with square-planar coordination geometry and bent bispyridine ligands, through stereoselective CH-π interactions with monosaccharide derivatives. Thanks to host-guest CH-π multi-interactions, one molecule of various permethylated monosaccharides is quantitatively bound by the capsule in water (K a up to >108 M-1). In the polyaromatic cavity, among them, the selective binding of a β-glucose derivative (>80 : 20 ratio) is demonstrated from a mixture of the α/β-glucoses, through the equatorial-selective recognition of the anomeric (C1) group. A similar stereoselective binding is accomplished from an α/β-galactose mixture. Interestingly, single equatorial/axial configurations on the bound monosaccharides can regulate the helical conformation of the capsule in water, confirmed by CD, NMR, and theoretical analyses. An intense capsule-based Cotton effect is exclusively observed upon encapsulation of the permethylated α-glucose (>20-fold enhancement as compared to the β-glucose derivative), via the induction of a single-handed host helicity to a large extent. Inverse capsule helicity is induced by the binding of a β-galactose derivative under the same conditions.

Stimuli-Mediated Structural Interchange Between Pd6 and Pd12 Architectures: Selective Recognition of E-Stilbene by the Pd6 Architecture and its Photoprotection

The dynamic behaviour of metal-ligand bonding cultivates stimuli-mediated structural transformations in self-assembled molecular architectures. The propensity of synthetically designed self-assembled systems to interchange between higher-order architectures is increased multi-fold when the building blocks have higher conformational degrees of freedom. Herein, we report a new ligand, (2,7-bis(di(pyridin-4-yl)amino)-9H-fluoren-9-one) (L), which, upon self-assembly with a cis-[(ethylene-1,2-diamine)Pd(NO3)2] acceptor (M), resulted in the formation of a M6L3 trifacial barrel (C1) in water. Interestingly, during crystallization, a rare M12L6 triangular orthobicupola architecture (C2) was generated along with C1. C2 could also be generated in solution via the application of several stimuli. C1 in aqueous media could stabilize one trans-stilbene (tS) or cis-stilbene (cS) molecule in its cavity, with a selectivity for the former from their mixture. Moreover, C1 acted as an effective host to prevent the otherwise facile photoisomerization of tS to cS inside its hydrophobic cavity under UV irradiation. Conversely, the visible-light-induced reverse isomerization of encapsulated cS to encapsulated tS could be achieved readily due to the better stabilization of tS within the cavity of C1 and its transparency to visible light. A multi-functional system was therefore designed, which at the same time is stimuli-responsive, shows isomer selectivity, and photo-protects trans-stilbene.

Bioinspired Binding and Conversion of Linear Monoterpenes by Polyaromatic Coordination Capsules

Linear monoterpenes, versatile reaction biosubstrates, are bound and subsequently converted to various cyclic monomers and oligomers with excellent selectivity and efficiency, only in natural enzymes. We herein report bioinspired functions of synthetic polyaromatic cavities toward linear monoterpenes in the solution and solid states. The cavities are provided by polyaromatic coordination capsules, formed by the assembly of Pt(II) ions and bent bispyridine ligands with two anthracene panels. By using the capsule cavities, the selective binding of citronellal from mixtures with other monoterpenes and its preferential vapor binding over its derivatives are demonstrated in water and in the solid state, respectively. The capsule furthermore extracts p-menthane-3,8-diol, with high product- and stereoselectivity, from a reaction mixture obtained by the acid-catalyzed cyclization of citronellal in water. Thanks to the inner and outer polyaromatic cavities, the catalytic cyclization-dimerization of vaporized citronellal efficiently proceeds in the acid-loaded capsule solid and product/stereoselectively affords p-menthane-3,8-diol citronellal acetal (∼330% yield based on the capsule) under ambient conditions. The solid capsule reactor can be reused at least 5 times with enhanced conversion. The present study opens up a new approach toward mimicking terpene biosynthesis via synthetic polyaromatic cavities.

Enantiopure Corral[4]BINOLs as Ultrastrong Receptors for Recognition and Differential Sensing of Steroids

The precise recognition and sensing of steroids, a type of vital biomolecules, hold immense practical value across various domains. In this study, we introduced corral[4]BINOLs (C[4]BINOLs), a pair of enantiomeric conjugated deep-cavity hosts, as novel synthetic receptors for binding steroids. Due to the strong hydrophobic effect of their deep nonpolar, chiral cavities, the two enantiomers of C[4]BINOLs demonstrated exceptionally high recognition affinities (up to 1012 M-1) for 16 important steroidal compounds as well as good enantioselectiviy (up to 15.5) in aqueous solutions, establishing them as the most potent known steroid receptors. Harnessing their ultrahigh affinity, remarkable enantioselectivity, and fluorescence emission properties, the two C[4]BINOL enantiomers were employed to compose a fluorescent sensor array which achieved discrimination and sensing of 16 structurally similar steroids at low concentrations.

Uncovering tetrazoles as building blocks for constructing discrete and polymeric assemblies

Metal-organic self-assembly with flexible moieties is a budding field of research due to the possibility of the formation of unique architectures. Tetrazole, characterised by four nitrogen atoms in a five-member ring, exhibits immense potential as a component. Tetrazole offers four coordination sites for binding to the metal centre with nine distinct binding modes, leading to various assemblies. This review highlights different polymeric and discrete tetrazole-based assemblies and their functions. The meticulous manipulation of stoichiometry, ligands, and metal ions required for constructing discrete assemblies has also been discussed. The different applications of these architectures in separation, catalysis and detection have also been accentuated. The latter section of the review consolidates tetrazole-based cage composites, highlighting their applications in cell imaging and photocatalytic applications.

Metal-Organic Cage Receptors for Encapsulation and Sensing of Bile Acids

Developing synthetic supramolecular receptors to solubilize, scavenge, recognize, encapsulate, and sense steroids is challenging. Despite a limited number of receptors having affinity with steroids, none exists to bind steroidal bile acids selectively. Herein, we report a C2-symmetric metal-organic cage [Pd6L24]12+ and an expanded version of the Fujita cage [Pd6L14]12+, built with a conformationally flexible ligand L2, accessed through coordination-driven self-assembly. We examined both cages for steroid recognition in water: both have certain shared characteristics and distinctive features. [Pd6L14]12+ binds hydrophobic bile acids and other steroids by forming a 1:1 complex. In contrast, the expanded [Pd6L24]12+ cage exhibits an affinity for amphiphilic bile acids and selective steroids to encapsulate them as dimers, promoted by cooperative interguest hydrogen bonding. [Pd6L24]12+ has a 5 times stronger solubility enhancement ability for cholic acid compared to [Pd6L14]12+. Further, the expanded [Pd6L24]12+ cage can selectively sense bile acids in nanomolar detection limits through indicator displacement assay by employing sulforhodamine 101 (SR101).

Iridium(III)-based minor groove binding complexes as DNA photocleavage agents

Transition metal complexes containing the qtpy ligand (2':4,4'':4',4'''-quaterpyridyl) are known to be DNA intercalators or minor groove binders. In this study, new tricationic iridium(III) complexes of qtpy are reported. Both [Ir(bpy)2(qtpy)]3+1 and [Ir(phen)2(qtpy)]3+2 display good water solubility as chloride salts. The complexes possess high-energy excited states, which are quenched in the presence of duplex DNA and even by the mononucleotides guanosine monophosphate and adenosine monophosphate. Further studies reveal that although the complexes bind to quadruplex DNA, they display a preference for duplex structures, which are bound with an order of magnitude higher affinities than their isostructural dicationic RuII-analogues. Detailed molecular dynamics simulations confirm that the complexes are groove binders through the insertion of, predominantly, the qtpy ligand into the minor groove. Photoirradiation of 1 in the presence of plasmid DNA confirms that this class of complexes can function as synthetic photonucleases by cleaving DNA.

A Chiral Emissive Conjugated Macrocycle for High-Affinity and Highly Enantioselective Recognition in Water

Accurately distinguishing between enantiomeric molecules is a fundamental challenge in the field of chemistry. However, there is still significant room for improvement in both the enantiomeric selectivity (KR(S) /KS(R) ) and binding strength of most reported macrocyclic chiral receptors to meet the demands of practical application scenarios. Herein, we synthesized a water-soluble conjugated tubular host-namely, corral[4]BINOL-using a chiral 1,1'-bi-2-naphthol (BINOL) derivative as the repeating unit. The conjugated chiral backbone endows corral[4]BINOL with good fluorescent emission (QY=34 % ) and circularly polarized luminescence (|glum | up to 1.4×10-3 ) in water. Notably, corral[4]BINOL exhibits high recognition affinity up to 8.6×1010  M-1 towards achiral guests in water, and manifested excellent enantioselectivity up to 18.7 towards chiral substrates, both of which represent the highest values observed among chiral macrocycles in aqueous solution. The ultrastrong binding strength, outstanding enantioselectivity, and facile accessibility, together with the superior fluorescent and chiroptical properties, endow corral[4]BINOL with great potential for a wide range of applications.

A Closed Cavity Strategy for Selective Dipeptide Binding by a Polyaromatic Receptor in Water

Precise recognition of peptides is a daunting task owing to the substantial number of available amino acids and their combination into various oligo/polymeric structures in addition to the high hydration of their flexible frameworks. Here, we report the selective recognition of a dipeptide through a closed cavity strategy, in contrast to previous synthetic receptors with open cavities. A polyaromatic receptor with a virtually isolated, hydrophobic cavity exclusively binds one molecule of phenylalanine dipeptide from a mixture with its amino acid and tripeptide in water via multiple CH-π and hydrogen-bonding interactions in the complementary cavity. The binding selectivity persists even in the presence of other dipeptides, such as leucine-leucine, leucine-phenylalanine, tyrosine-phenylalanine, tryptophan-tryptophan, and aspartame, revealed by NMR/MS-based competitive binding experiments. ITC studies reveal that the selective binding of the phenylalanine dipeptide is relatively strong (Ka = 1.1 × 105 M-1) and an enthalpically and entropically favorable process (ΔH = -11.7 kJ mol-1 and TΔS = 17.0 kJ mol-1). In addition, the present receptor can be used for the emission detection of the dipeptide through a combination with a fluorescent dye in water.

A Redox-Responsive Ferrocene-Based Capsule Displaying Unusual Encapsulation-Induced Charge-Transfer Interactions

A ferrocene-based capsule is spontaneously and quantitatively formed in water by the assembly of bent amphiphiles carrying two ferrocene units. The disassembly and assembly of the new organometallic capsule, with a well-defined and highly condensed ferrocene core, are demonstrated by chemical redox stimuli in a fully reversible fashion under ambient conditions. In contrast to previously reported multiferrocene assemblies, only the present capsule efficiently encapsulates typical organic/inorganic dyes as well as electron-accepting molecules in water. As a result, unusual host-guest charge-transfer (CT) interactions, displaying relatively wide absorption bands in the visible to near-infrared region (λ=650-1350 nm), are observed upon the encapsulation of acceptors (i.e., chloranil and TCNQ). The resultant encapsulation-induced CT interactions can be released by a redox stimulus through the disassembly of the capsule.

Construction of Covalent Organic Cages with Aggregation-Induced Emission Characteristics from Metallacages for Mimicking Light-Harvesting Antenna

A series of covalent organic cages built from fluorophores capable of aggregation-induced emission (AIE) were elegantly prepared through the reduction of preorganized M2 (LA )3 (LB )2 -type metallacages, simultaneously taking advantage of the synthetic accessibility and well-defined shapes and sizes of metallacages, the good chemical stability of the covalent cages as well as the bright emission of AIE fluorophores. Moreover, the covalent cages could be further post-synthetically modified into an amide-functionalized cage with a higher quantum yield. Furthermore, these presented covalent cages proved to be good energy donors and were used to construct light-harvesting systems employing Nile Red as an energy acceptor. These light-harvesting systems displayed efficient energy transfer and relatively high antenna effect, which enabled their use as efficient photocatalysts for a dehalogenation reaction. This research provides a new avenue for the development of luminescent covalent cages for light-harvesting and photocatalysis.

Exploiting reduced-symmetry ligands with pyridyl and imidazole donors to construct a second-generation stimuli-responsive heterobimetallic [PdPtL4]4+ cage

A new sequential metalation strategy that enables the assembly of a new more robust reduced symmetry heterobimetallic [PdPtL4]4+ cage C is reported. By exploiting a low-symmetry ditopic ligand (L) that features imidazole and pyridine donor units we were able to selectively form a [Pt(L)4]2+ "open-cage" complex. When this was treated with Pd(ii) ions the cage C assembled. 1H and DOSY nuclear magnetic resonance (NMR) spectroscopy and electrospray ionisation mass spectrometry (ESIMS) data were consistent with the quantitative formation of the cage and the heterobimetallic structure was confirmed by single crystal X-ray crystallography. The cage C was shown to bind anionic guest molecules. NMR studies suggested that these guests interacted with the cavity of the cage in a specific orientation and this was confirmed for the mesylate ion (MsO-) : C host-guest adduct using X-ray crystallography. In addition, the system was shown to be stimulus-responsive and could be opened and closed on demand when treated with appropriate stimuli. If a guest molecule was bound within the cage, the opening and closing was accompanied by the release and re-uptake of the guest molecule.

Anion-Driven Programmable Chiral Self-Sorting in Metal-Organic Cages and Structural Transformations between Heterochiral and Homochiral Cages

When a racemic mixture of chiral building blocks self-assembles to form discrete molecular or supramolecular cages, the system can adopt either social or narcissistic chiral self-sorting. However, control over such chiral self-sorting is hard to achieve with a desired choice of outcome. Herein, we report anion templated high-fidelity chiral self-sorting during the coordination-driven self-assembly of [Pd₂ L₄ ] metal-organic cages, with a racemic mixture of an axially chiral ligand. Upon varying the counter-anions, the outcome of the choice of chiral self-sorting, whether social or narcissistic, leading to kinetically favored heterochiral or thermodynamically favored homochiral cages, can be controlled through specific anion encapsulation. Non-encapsulating anion afforded a mixture of all possible diastereomers. Anion exchange enabled structural transformations between the diastereomers and the conversion of the mixture of diastereomers into homochiral diastereomers.

Aromatic micelles: toward a third-generation of micelles

Micelles are useful and widely applied molecular assemblies, formed from amphiphilic molecules, in water. The majority of amphiphiles possess an alkyl chain as the hydrophobic part. Amphiphiles bearing hydrophilic and hydrophobic polymer chains generate so-called polymeric micelles in water. This review focuses on the recent progress of "aromatic micelles", formed from bent polyaromatic/aromatic amphiphiles, for the development of third-generation micelles. Thanks to multiple host-guest interactions, e.g., the hydrophobic effect and π-π/CH-π interactions, the present micelles display wide-ranging uptake abilities toward various hydrophobic compounds in water. In addition to such host functions, new stimuli-responsive aromatic micelles with pH, light, and redox switches, aromatic oligomer micelles, saccharide-coated aromatic micelles, and related cycloalkane-based micelles were recently developed by our group.

CH-π Multi-Interaction-Driven Recognition and Isolation of Planar Compounds in a Spheroidal Polyaromatic Cavity

π-π Interactions are established as a powerful supramolecular tool, whereas the usability of CH-π interactions has been rather limited so far. Here we present (i) selective binding of planar polyaromatics and (ii) effective isolation of planar metal complexes by a polyaromatic capsule, utilizing multiple CH-π interactions. In the spheroidal cavity, one molecule of large and medium-sized polyaromatic molecules (i. e., coronene and pyrene) is exclusively bound from mixtures bearing the same number of aromatic CH groups. Theoretical studies reveal that multiple host-guest CH-π interactions (up to 32 interactions) are the predominant driving force for the observed selectivity. In addition, one molecule of planar metal complexes (i. e., porphine and bis(acetylacetonato) Cu(II) complexes) is quantitatively bound by the capsule through aromatic and aliphatic CH-π multi-interactions, respectively. The ESR and theoretical studies demonstrate the isolation capability of the capsular framework and an unusual polar environment in the polyaromatic cavity.

Shifting the Triangle-Square Equilibrium of Self-Assembled Metallocycles by Guest Binding with Enhanced Photosensitization

Shifting a triangle-square equilibrium in one direction is an important problem in supramolecular self-assembly. Reaction of a benzothiadiazole-based diimidazole donor with a cis-Pt(II) acceptor yielded an equilibrium mixture of a triangle ([C₁₈H₂₄N₁₀O₆S₁Pt₁]₃≡ PtMC^T) and a square ([C₁₈H₂₄N₁₀O₆S₁Pt₁]₄≡ PtMC^S). We report here the shifting of such equilibrium toward a triangle using a guest (pyrene aldehyde, G1). While both benzothiadiazole and pyrene aldehyde can form reactive oxygen species (ROS) in organic solvents, their therapeutic use in water is restricted due to aqueous insolubility. The enhanced water solubility of the benzothiadiazole unit and G1 by macrocycle formation and host-guest complexation, respectively, enabled enhanced ROS generation by the host-guest complex (G1' ⊂ PtMC^T) in water (G1' = hydrated form of G1). The guest-encapsulated metallacycle (G1' ⊂ PtMC^T) has shown synergistic antibacterial activity compared to the mixture of macrocycles upon white-light irradiation due to enhanced ROS generation. The mechanism for such enhanced activity was established by measuring the oxidative stress and relative internalization of PtMCs and G1' ⊂ PtMC^T.

Water-Soluble Self-Assembled Cage with Triangular Metal-Metal-Bonded Units Enabling the Sequential Selective Separation of Alkanes and Isomeric Molecules

The selective separation of structurally similar aliphatic/aromatic hydrocarbons is an essential goal in industrial processes. In this study, we report the synthesis of a water-soluble (Tr₂M₃)₄L₄ (Tr = cycloheptatrienyl ring; M = metal; L = organosulfur ligand) molecular cage (1) via self-assembly of the water-soluble acceptor tripalladium sandwich species [(Tr₂Pd₃)(CH₃CN)][NO₃]₂ and the attachment onto L of solubilizing methoxyethoxy appendants to be utilized in an energy-friendly alternative approach to the separation of structurally similar molecules under ambient conditions. Cage 1, comprising a hydrophobic inner cavity, exhibited good solubility and stability in aqueous media. It also demonstrated excellent performance in the sequential separation of alkanes (C6-C9), xylene, and other disubstituted benzene isomers and cis/trans-decalin.

Caged bulky organic dyes in a polyaromatic framework and their spectroscopic peculiarities

Here we report that the open large cavity of an M2L4 polyaromatic cage can efficiently bind sterically demanding organic dyes with coumarin, perylene bisimide, and porphyrin cores in aqueous solution. The spectroscopic properties of the caged dyes are largely modulated in the cavity.