Self-Assembly of a Water-Soluble Pd16 Square Bicupola Architecture and Its Use in Aerobic Oxidation in Aqueous Medium.

Designing supramolecular architectures with uncommon geometries has always been a key goal in the field of metal-ligand coordination-driven self-assembly. It acquires added significance if functional building units are employed in constructing such architectures for fruitful applications. In this report, we address both these aspects by developing a water-soluble Pd16L8 coordination cage 1 with an unusual square orthobicupola geometry, which was used for selective aerobic oxidation of aryl sulfides. Self-assembly of a benzothiadiazole-based tetra-pyridyl donor L with a ditopic cis-[(tmeda)Pd(NO3)2] acceptor [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine] produced 1, and the geometry was determined by single-crystal X-ray diffraction study. Unlike the typically observed tri- or tetrafacial barrel, the present Pd16L8 coordination assembly features a distinctive structural topology and is a unique example of a water-soluble molecular architecture with a square orthobicupola geometry. Efficient and selective aerobic oxidation of sulfides to sulfoxides is an important challenge as conventional oxidation generally leads to the formation of sulfoxide along with toxic sulfone. Cage 1, designed with a ligand containing a benzothiadiazole moiety, demonstrates an ability to photogenerate reactive oxygen species (ROS) in water, thus enabling it to serve as a potential photocatalyst. The cage showed excellent catalytic efficiency for highly selective conversion of alkyl and aryl sulfides to their corresponding sulfoxides, therefore without the formation of toxic sulfones and other byproducts, under visible light in aqueous medium.

Enhancing Fluorescence in Both Solution and Solid States Induced by Imine Cage Formation.

Developing luminescent materials that exhibit strong emissions in both solution and solid phases is highly desirable and challenging. Herein, we report imine-bond directed formation of a rigid organic cage (TPE-cage) that was synthesized by [2+4] imine condensation of a TPE-cored tetra-aldehyde (TPE-TA) with a clip-like diamine (XA) to illustrate confinement-induced fluorescence enhancement. Compared to the non-emissive TPE-TA (ϕF =0.26 %) in the dichloromethane (DCM) solution, the TPE-cage achieved a remarkable (~520-fold) emission enhancement (ϕF =70.38 %). In contrast, a monomeric tetra-imine model compound (TPE-model) showed only a minor enhancement (ϕF =0.56 %) in emission compared to the parent tetra-aldehyde TPE-TA. The emission of TPE-cage was further enhanced by ~1.5-fold (ϕF =80.96 %) in the aggregated state owing to aggregation-induced emission enhancement (AIEE). This approach establishes the potential for synthesizing luminescent materials with high emission in both solution and solid-state by employing a single-step imine condensation reaction.

Pt(II) Tetrafacial Barrel with Aggregation-Induced Emission for Sensing.

A new tetraphenylpyrazine-based tetraimidazole ligand (L) was synthesized and used for subcomponent self-assembly with cis-(tmeda)Pd(NO3)2 and cis-Pt(PEt3)2(OTf)2, leading to the formation of two tetrafacial barrels [Pd8L4(tmeda)8](NO3)16 (1) and [Pt8L4(PEt3)16](OTf)16 (2), respectively. Although ligand L is aggregation-induced emission (AIE) active, barrel 2 showed a magnificently higher AIE activity than ligand L, while 1 failed to retain the AIE properties of the ligand. Pd(II) barrel 1, undergoing an aggregation-caused quenching (ACQ) phenomenon, nullified the AIE activity of the ligand to be used in the photophysical application. The enhanced emission in the aggregated state of Pt(II) barrel 2 was used for the recognition of picric acid (PA), which is explosive in nature and one of the groundwater contaminants in landmine areas. The recognition of picric acid was found to be selective in comparison with that of other nitroaromatic compounds (NACs), which could be attributed to ground-state complex formation and resonance energy transfer between picric acid and barrel 2. The use of new AIE-active assembly 2 for selective detection of PA with a low detection limit is noteworthy.

Coordination-Induced Emissive Poly-NHC-Derived Metallacage for Pesticide Detection.

Developing sensitive, rapid, and convenient methods for the detection of residual toxic pesticides is immensely important to prevent irreversible damage to the human body. Luminescent metal-organic cages and macrocycles have shown great applications, and designing highly emissive supramolecular systems in dilute solution using metal-ligand coordination-driven self-assembly is demanded. In this study, we have demonstrated the development of a silver-carbene bond directed tetranuclear silver(I)-octacarbene metallacage [Ag4(L)2](PF6)4 (1) based on an aggregation-induced emissive (AIE) cored 1,1',1″,1‴-((1,4-phenylenebis(ethene-2,1,1-triyl))tetrakis(benzene-4,1-diyl))tetrakis(3-methyl-1H-imidazol-3-ium) salt (L). A 36-fold enhanced emission was observed after metallacage (1) formation when compared with the ligand (L) in dilute solution due to the restriction of intramolecular motions imparted by metal-ligand coordination. Such an increase in fluorescence made 1 a potential candidate for the detection of a broad-spectrum pesticide, 2,6-dichloro-nitroaniline (DCN). 1 was able to detect DCN efficiently by the fluorescence quenching method with a significant detection limit (1.64 ppm). A combination of static and dynamic quenching was applicable depending on the analyte concentration. The use of silver-carbene bond directed self-assembly to exploit coordination-induced emission as an alternative to AIE in dilute solution and then apply this approach to solve health and safety concerns is noteworthy and carries a lot of potential for future developments.

Molecular Cavity for Catalysis and Formation of Metal Nanoparticles for Use in Catalysis.

The employment of weak intermolecular interactions in supramolecular chemistry offers an alternative approach to project artificial chemical environments like the active sites of enzymes. Discrete molecular architectures with defined shapes and geometries have become a revolutionary field of research in recent years because of their intrinsic porosity and ease of synthesis using dynamic non-covalent/covalent interactions. Several porous molecular cages have been constructed from simple building blocks by self-assembly, which undergoes many self-correction processes to form the final architecture. These supramolecular systems have been developed to demonstrate numerous applications, such as guest stabilization, drug delivery, catalysis, smart materials, and many other related fields. In this respect, catalysis in confined nanospaces using such supramolecular cages has seen significant growth over the years. These porous discrete cages contain suitable apertures for easy intake of substrates and smooth release of products to exhibit exceptional catalytic efficacy. This review highlights recent advancements in catalytic activity influenced by the nanocavities of hydrogen-bonded cages, metal-ligand coordination cages, and dynamic or reversible covalently bonded organic cages in different solvent media. Synthetic strategies for these three types of supramolecular systems are discussed briefly and follow similar and simplistic approaches manifested by simple starting materials and benign conditions. These examples demonstrate the progress of various functionalized molecular cages for specific chemical transformations in aqueous and nonaqueous media. Finally, we discuss the enduring challenges related to porous cage compounds that need to be overcome for further developments in this field of work.

Pt(II)/Pd(II)-Based Metallosupramolecular Architectures as Light Harvesting Systems and their Applications.

The development of artificial light-harvesting systems mimicking the natural photosynthesis method is an ever-growing field of research. Numerous systems such as polymers, metal complexes, POFs, COFs, supramolecular frameworks etc. have been fabricated to accomplish more efficient energy transfer and storage. Among them, the supramolecular coordination complexes (SCCs) formed by non-covalent metal-ligand interaction, have shown the capacity to not only undergo single and multistep energy migration but also to utilize the harvested energy for a wide variety of applications such as photocatalysis, tunable emissive systems, encrypted anti-counterfeiting materials, white light emitters etc. This review sheds light on the light-harvesting behavior of both the 2D metallacycles and 3D metallacages where design ingenuity has been executed to afford energy harvesting by both donor ligands as well as metal acceptors.

Molecular Barrels as Potential Hosts: From Synthesis to Applications.

Self-assembled discrete molecular architectures that show selective molecular recognition within their internal cavities are highly desirable. Such hosts often show guest recognition through several noncovalent interactions. This emulates the activity of naturally occurring enzymes and proteins. Research in the formation of 3D cages of different shapes and sizes has progressed rapidly since the development of coordination-driven self-assembly and dynamic covalent chemistry. Such molecular cages find applications in catalysis, stabilization of metastable molecules, purification of isomeric mixtures via selective encapsulation, and even in biomedical applications. Most of these applications stem from the ability of the host cages to bind guests strongly in a selective fashion, providing a suitable environment for the guests to perform their functions. Molecular cages having closed architectures with small windows either show poor encapsulation or inhibit easy guest release while those with wide open structures fail to form stable host-guest complexes. In this context, molecular barrels obtained by dynamic metal-ligand/covalent bond formation techniques possess optimized architectures. With a hollow-walled cavity and two large openings, molecular barrels satisfy the structural requirements for many applications. In this perspective, we will discuss in detail the synthetic strategies for obtaining barrels or barrel-like architectures employing dynamic coordination and covalent interactions, their structure-based classification, and their applications in catalysis, storing transient molecules, separation of chemicals, and photoinduced antibacterial activity. We aim to highlight the structural advantages of molecular barrels over other architectures for efficiently carrying out several functions and for the development of new applications.

Cavity-Shape-Dependent Divergent Chemical Reaction inside Aqueous Pd6L4 Cages.

Chemical reactions inside the confined pockets of enzyme-mimicking hosts, such as cages and macrocycles, have been an emerging field of interest over the past decade. Although many such reactions are known, the use of such cages toward the divergent synthesis of nonisomeric products has not been well explored. Divergent synthesis is a technique of forming two or more distinct products from the same reagents by changing the catalyst or reaction conditions. Changing the shape of the cage can also change the nature and magnitude of the host-guest interactions. Thus, is it possible for such changes to cause differences in the reaction pathways leading to formation of nonisomeric products? Herein, we report a divergent chemical transformation of anthrone [anthracen-9(10H)-one] inside different water-soluble M6L4 cages. When anthrone was encapsulated inside a newly synthesized M6L4 octahedral cage 1, it dimerized to form dianthrone [9,9'-bianthracen-10,10'(9H,9'H)-dione]. In contrast, when the same chemical reaction was performed inside a M6L4 double-square shaped cage 2, it was oxidized to form anthraquinone [anthracene-9,10-dione]. Similar results were obtained with a different set of isomeric aqueous Pd6 cages 3a (octahedral cage) and 3b (double-square cage), indicating the dependence of the shape of cavity on the divergent synthesis. The present report demonstrates a unique example of different outcomes/results of a reaction depending on the shape of the molecular container, which was driven by the host-guest interactions and the preorganization of the substrates.

Rigidification-Induced Emissive Metal-Carbene Complexes for Artificial Light Harvesting.

A tetraphenylethylene (TPE)-based flexible imidazolium (L) salt was used to develop a di-nuclear silver(I)-tetracarbene (1) complex. Coordination-induced rigidity upon formation of 1 exhibited a 6-fold increase in emission intensity in acetonitrile compared to starting L. Despite TPE being a well-known aggregation-induced emissive moiety, AgI-N-heterocyclic carbene (NHC) complex 1 had a remarkably higher fluorescence emission (4-fold) in dilute solution when compared with L in its aggregated state. Finally, this enhanced emission was used to institute a new platform for an artificial light-harvesting system. 1 acted as an energy donor and efficiently transferred energy to Eosin Y (ESY) with a high saturation at a 67:1 (1/ESY) molar ratio. Use of rigidification-induced emission of the AgI-NHC complex to fabricate a light-harvesting scaffold is a new approach and can greatly impact the generation of smart materials.

Water-Soluble Pd6L3 Molecular Bowl for Separation of Phenanthrene from a Mixture of Isomeric Aromatic Hydrocarbons.

Phenanthrene is a high-value raw material in chemical industries. Separation of phenanthrene from isomeric anthracene continues to be a big challenge in the industry due to their very similar physical properties. Herein, we report the self-assembly of a water-soluble molecular bowl (TB) from a phenothiazine-based unsymmetrical terapyridyl ligand (L) and a cis-blocked 90° Pd(II) acceptor. TB featured an unusual bowl-like topology, with a wide rim diameter and a hydrophobic inner cavity fenced by the aromatic rings of the ligand. The above-mentioned features of TB allow it to bind polyaromatic hydrocarbons in its confined cavity. TB shows a higher affinity for phenanthrene over its isomer anthracene in water, which enables it to separate phenanthrene with ∼93% purity from an equimolar mixture of phenanthrene and anthracene. TB is also able to extract pyrene with around ∼90% purity from an equimolar mixture of coronene, perylene, and pyrene. Moreover, TB can be reused for several cycles without significant degradation in its performance as an extracting agent. This clean strategy of separation of phenanthrene and pyrene from a mixture of hydrophobic hydrocarbons by aqueous extraction is noteworthy.

Structural Switching of a Distorted Trigonal Metal-Organic Cage to a Tetragonal Cage and Singlet Oxygen Mediated Oxidations.

Construction of metal-organic cages with unique architecture and guest binding abilities is highly desirable. Herein, we report the synthesis of a distorted trigonal cage (1) from a twisted tetratopic ligand (L) and a PdII acceptor. Surprisingly, 1 exhibited a complete structural reorganization of its building units in the presence of C70 and C60 to form guest-encapsulated tetragonal cages, (C70 )2 @2 and (C60 )2 @2, respectively. These guest-bound cages were found to be potential 1 O2 generators, with the former effectively catalyzing two different varieties of 1 O2 -mediated oxidation reactions.

An Adaptable Water-Soluble Molecular Boat for Selective Separation of Phenanthrene from Isomeric Anthracene.

Anthracene crude oil is a common source of phenanthrene for its industrial use. The isolation of phenanthrene from this source is a challenging task due to very similar physical properties to its isomer anthracene. We report here a water-soluble Pd(II) molecular boat (MB1) with unusual structural topology that was obtained by assembling a flexible tetrapyridyl donor (L) with a cis-Pd(II) acceptor. The flexible backbone of the boat enabled it to breathe in the presence of a guest optimizing the fit within the cavity. The boat binds phenanthrene more strongly than anthracene, which enabled separation of phenanthrene with an >98% purity from an equimolar mixture of the two isomers using MB1 as an extracting agent. MB1 represents a unique example of a coordination receptor suitable for selective aqueous extraction of phenanthrene from anthracene with reusability of several cycles.

Post-Synthesis Conversion of an Unstable Imine Cage to a Stable Cage with Amide Moieties Towards Selective Receptor for Fluoride.

Synthesis of robust covalent macrocycles/cages via multiple amide-bond forming reaction is highly challenging and generally it needs multistep reactions. One-pot reaction of appropriate di-/tri-acyl chloride with a diamine generally results polymers or oligomers instead of discrete architectures. To overcome this limitation, a strategy is reported here using dynamic imine chemistry for facile construction of imine-based macrocycle and cage upon treatment of a diamine with di- and tri-aldehydes respectively, followed by post-synthesis one-step conversion of imine bonds to amides to form the desired robust macrocycle and cage containing multiple amide bonds. While the macrocycle was found to form aggregates in DMSO, the cage was intact without any aggregation. Six amide groups in the confined pocket of the cage made it an ideal receptor for selective binding of fluoride with very high selectivity (∼3 × ${\times }$ 103 fold) over chloride, and it was silent towards other halides, phosphate, and other oxyanions.

Conformation-Selective Self-Assembly of Pd6 Trifacial Molecular Barrels Using a Tetrapyridyl Ligand.

A conformationally flexible tetrapyridyl ligand L was assembled separately with three cis-blocked 90° PdII acceptors (M1, M2, and M3) containing different blocking diamines. Surprisingly, different conformations of the donor L were arrested by the acceptors depending on the nature of the blocking amine, leading to the formation of isomeric Pd6 barrels (B1, B2, and B3). B2 and B3 with larger windows have been used to encapsulate polyaromatic hydrocarbons.

Solvent and Counteranion Assisted Dynamic Self-Assembly of Molecular Triangles and Tetrahedral Cages.

Self-assembly of naked PdII ions separately with newly designed bis(3-pyridyl)benzothiadiazole (L1) and bis(3-pyridyl)thiazolo[5,4-d]thiazole (L2) donors separately, under varying experimental conditions, yielded Pd4L8 (L= L1 or L2) tetrahedral cages and their homologous Pd3L6 (L= L1 or L2) double-walled triangular macrocycles. The resulting assemblies exhibited solvent, temperature, and counteranion induced dynamic equilibrium. Treatment of L1 with Pd(BF4)2 in acetonitrile (ACN) resulted in selective formation of a tetrahedral cage [Pd4(L1)8](BF4)8 (1a), which is in dynamic equilibrium with its homologue triangle [Pd3(L1)6](BF4)6 (2a) in dimethyl sulfoxide (DMSO). On the other hand, similar self-assembly using L2 instead of L1 yielded an equilibrium mixture of tetrahedral cage [Pd4(L2)8](BF4)8 (3a) and triangle [Pd3(L2)6](BF4)6 (4a) forms in both ACN and DMSO. The assembles were characterized by multinuclear NMR and ESI-MS while the structure of the tetrahedral cage (1a) was determined by single crystal X-ray diffraction. Existence of a dynamic equilibrium between the assemblies in solution has been investigated via variable temperature 1H NMR. The equilibrium constant K = ([Pd4L8]3/[Pd3L6]4) was calculated at each experimental temperature and fitted with the Van't Hoff equation to determine the standard enthalpy (ΔH°) and entropy (ΔS°) associated with the interconversion of the double-walled triangle to tetrahedral cage. The thermodynamic feasibility of structural interconversion was analyzed from the change in ΔG°, which suggests favorable conversion of Pd3L6 triangle to Pd4L8 cage at elevated temperature for L1 in DMSO and L2 in ACN. Interestingly, similar self-assembly reactions of L1 and L2 with Pd(NO3)2 instead of Pd(BF4)2 resulted in selective formation of a tetrahedral cage [Pd4(L1)8](NO3)8 (1b) and double-walled triangle [Pd3(L2)6](NO3)6 (4b), respectively.

Silver(I)-Carbene Bond-Directed Rigidification-Induced Emissive Metallacage for Picric Acid Detection.

A new triphenylamine-based tetraimidazolium salt L was developed for silver(I)-carbene bond-directed synthesis of tetranuclear silver(I) octacarbene ([Ag4(L)2](PF6)4) metallacage 1. Interestingly, after assembly formation, metallacage 1 showed a nine-fold emission enhancement in dilute solution while ligand L was weakly fluorescent. This is attributed to the rigidity induced to the system by metal-carbene bond formation where the metal center acts as a rigidification unit. The enhanced emission intensity in dilute solution and the presence of the triphenylamine core made 1 a potential candidate for recognition of picric acid (PA). This recognition can be ascribed to the dual effect of ground-state charge-transfer complex formation and resonance energy transfer between the picrate and metallacage 1. For metallacage 1, a considerable detection limit toward PA was observed. The use of such metal-carbene bond-directed rigidification-induced enhanced emission for PA sensing is noteworthy.

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≡ PtMCT) and a square ([C18H24N10O6S1Pt1]4≡ PtMCS). 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' ⊂ PtMCT) in water (G1' = hydrated form of G1). The guest-encapsulated metallacycle (G1' ⊂ PtMCT) 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' ⊂ PtMCT.

Emissive Platinum(II) Macrocycles as Tunable Cascade Energy Transfer Scaffolds.

Developing artificial light-harvesting scaffolds with a cascade energy transfer process is significant for better understanding of photosynthesis. Here, we report [3+3] self-assembled PtII fluorescent macrocycles (3 a and 3 b) as light-harvesting platforms with cascade energy transfer. The PtII macrocycles aggregate into nanospheres and show emission-enhancement characteristics upon increasing water content in acetone medium. These aggregates (3aa and 3ba ) serve as energy donors when mixed with the hydrophobic dye Eosin-Y (ESY). In the presence of a second dye, Nile Red (NiR), an unusual sequential two-step energy transfer takes place from the macrocycles to NiR. In this case, ESY acts as a bridge in the relay mode. Additionally, a unique strategy to control such an energy transfer process by tuning the chain length of the alkyl group attached to the periphery of the macrocycles is demonstrated.

Special Issue: Self-Assembled Molecules/Materials.

This special issue containing one review and nine personal accounts discusses the various aspects and challenges in design, synthesis, and functions of self-assembled molecules and materials.

A Self-Assembled Palladium(II) Barrel for Binding of Fullerenes and Photosensitization Ability of the Fullerene-Encapsulated Barrel.

Fullerene extracts obtained from fullerene soot lack their real application due to their poor solubility in common solvents and difficulty in purification. Encapsulation of these extracts in a suitable host is an important approach to address these issues. We present a new Pd6 barrel (1), which is composed of three 1,4-dihydropyrrolo[3,2-b]pyrrole panels, clipped through six cis-PdII acceptors. Large open windows and cavity make it an efficient host for a large guest. Favorable interactions between the ligand and fullerene (C60 and C70 ) allows the barrel to encapsulate fullerene efficiently. Thorough investigation reveals that barrel 1 has a stronger binding affinity towards C70 over C60 , resulting in the predominant extraction of C70 from a mixture of the two. Finally, the fullerene encapsulated barrels C60 ⊂1 and C70 ⊂1 were found to be efficient for visible-light-induced singlet oxygen generation. Such preferential binding of C70 and photosensitizing ability of C60 ⊂1 and C70 ⊂1 are noteworthy.