Structural Transformations of Metal-Organic Cages through Tetrazine-Alkene Reactivity.

The assembly of metal-organic cages is governed by metal ion coordination preferences and the geometries of the typically rigid and planar precursor ligands. PdnL2n cages are among the most structurally diverse, with subtle differences in the metal-ligand coordination vectors resulting in drastically different assemblies, however almost all rely on rigid aromatic linkers to avoid the formation of intractable mixtures. Here we exploit the inverse electron-demand Diels-Alder (IEDDA) reaction between tetrazine linker groups and alkene reagents to trigger structural changes induced by post-assembly modification. The structure of the 1,4-dihydropyridazine produced by IEDDA (often an afterthought in click chemistry) is crucial; its two sp3 centers increase flexibility and nonplanarity, drastically changing the range of accessible coordination vectors. This triggers an initial Pd4L8 tetrahedral cage to transform into different Pd2L4 lantern cages, with both the transformation extent (thermodynamics) and rate (kinetics) dependent on the alkene dienophile selected. With cyclopentene, the unsymmetrical 1,4-dihydropyridazine ligands undergo integrative sorting in the solid state, with both head-to-tail orientation and enantiomer selection, leading to a single isomer from the 39 possible. This preference is rationalized through entropy, symmetry, and hydrogen bonding. Subsequent oxidation of the 1,4-dihydropyridazine to the aromatic pyridazine rigidifies the ligands, restoring planarity. The oxidized ligands no longer fit in the lantern structure, inducing further structural transformations into Pd4L8 tetrahedra and Pd3L6 double-walled triangles. The concept of controllable addition of limited additional flexibility and then its removal through well-defined reactivity we envisage being of great interest for structural transformations of any class of supramolecular architecture.

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.

Multi Stimuli Responsive Dual Aggregation-Induced Emission and Photochromic Behavior of a Tetraphenyl Substituted Triphenylamine Derivative and its Application as Anti-counterfeiting Agent.

A multi-stimuli responsive tetraphenyl substituted tripehnylamine-based aggregation induced emissive (AIE) material coupled with spiropyran was prepared. Owing to the presence of AIE and photochromic moiety, the molecule exhibits emissive aggregates, photochromism, and acidochromism. The multiple stimuli sensitive behavior of the molecule was explored for anti-counterfeiting behavior on TLC plate and commercial banknotes. The fluorogenic and photogenic response under UV and visible light established the potential of the candidate as a new generation encryption material.

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.

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.

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.

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.

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.

Self-Assembled Pd12 Coordination Cage as Photoregulated Oxidase-Like Nanozyme.

Designing supramolecular architectures with uncommon geometries embedded with functional building units is of immense importance in contemporary research. In this report, we present a new water-soluble Pd12L6 supramolecular coordination nanocage (1) that was synthesized via self-assembly of a tetradentate donor (L) with ditopic acceptor cis-[(en)Pd(NO3)2] [en = ethylenediamine]. Self-assembly of a tetratopic donor with a cis-blocked 90° acceptor commonly produces tri/tetra- or hexagonal barrel-type structures. However, the resulting cage 1 has an uncommon geometry consisting of two triangular cupolas conjoined through an irregular common hexagonal base. Incorporation of the benzothiadiazole unit in the structure helped in the photogeneration of reactive oxygen species (ROS) in water. Many nanomaterials have shown to have the ability to mimic the catalytic activity of natural enzymes (nanozymes). Majority of such nanozymes are water insoluble metal/metal-oxide nanoparticles or extended metal organic frameworks (MOFs)/metal-carbon composites, etc. The present water-soluble Pd12 nanocage 1 has shown excellent oxidase-like activity upon irradiation with white light. The enzymatic activity of 1 is photoregulated which offers other obvious advantages, such as external control of enzymatic activity and noninvasiveness. The oxidase-like activity and exogenous ROS generation have been further exploited in photocatalytic antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) bacterial strain.

Self-Assembled PtII8 Metallosupramolecular Tubular Cage as Dual Warhead Antibacterial Agent in Water.

Development of photosensitizer-based self-assembled metallosupramolecular architectures with important applications is an emerging trend in supramolecular chemistry. In this study, we report a new benzothiadiazole-based tetra-pyridyl ligand (L), which upon self-assembly with a cis-block 90° Pt(II) acceptor generated an unprecedented tetrafacial Pt(II)8 photoactive tubular molecular cage (PMB1). This cage could bring an extraordinary photosensitizer, benzothiadiazole, into water which is otherwise insoluble. PMB1 is fluorescent and shows photogeneration of singlet oxygen in an aqueous medium. These features make PMB1 a potent antimicrobial agent in water in both the presence and absence of light. In comparison to its building blocks and water-soluble alkylated charged ligand ([LMe4][4NO3]), the cage shows much enhanced photoinduced antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) as a representative of Gram-positive bacteria and Pseudomonas aeruginosa (PA) as a representative of Gram-negative bacteria. PMB1 is successful at inactivating the bacterial growth via both photoactivation of molecular oxygen and membrane depolarization mechanisms, thus proving to be a dual warhead. Inactivation of bacteria in water using such a supramolecular architecture is noteworthy and can shed light on the generation of new antimicrobial supramolecular systems.

Coordination-Assisted Reversible Photoswitching of Spiropyran-Based Platinum Macrocycles.

Control over the stimuli-responsive behavior of smart molecular systems can influence their capability to execute complex functionalities. Herein, we report the development of a suite of spiropyran-based multi-stimuli-responsive self-assembled platinum(II) macrocycles (5-7), rendering coordination-assisted enhanced photochromism relative to the corresponding ligands. 5 showed shrinking and swelling during photoreversal, while 6 and 7 are fast and fatigue-free supramolecular photoswitches. 6 turns out to be a better fatigue-resistant photoswitch and can retain an intact photoswitching ability of up to 20 reversible cycles. The switching behavior of the macrocycles can also be precisely controlled by tuning the pH of the medium. Our present strategy for the construction of rapid stimuli-responsive supramolecular architectures via coordination-driven self-assembly represents an efficient route for the development of smart molecular switches.

Unusual Behavior of Donor-Acceptor Stenhouse Adducts in Confined Space of a Water-Soluble PdII8 Molecular Vessel.

Donor-acceptor Stenhouse adducts (DASA) are new-generation photochromic compounds discovered recently. DASA exist normally in open form (blue/violet) and readily convert to cyclic (light yellow/colorless) zwitterionic form reversibly in the presence of green light in toluene/dioxane. In aqueous medium, the open form is not stable and converts to the cyclic zwitterionic form irreversibly. We report here a new self-assembled Pd8 molecular vessel (MV) that can stabilize and store the open form of DASA even in aqueous medium. Reaction of the 90° acceptor cis-(tmeda)Pd(NO3)2 (M) [tmeda = N, N, N', N'-tetramethylethane-1,2-diamine] with a symmetric tetraimidazole donor (L, 3,3',5,5'-tetra(1 H-imidazol-1-yl)-1,1'-biphenyl) in a 2:1 molar ratio yielded a water-soluble [8+4] self-assembled M8L4 molecular barrel (MV). This barrel (MV) is found to be a potential molecular vessel to store and stabilize the open forms of DASA in aqueous medium over the more stable zwitterionic cyclic form, while in the absence of the barrel the same DASA exist in cyclic zwitterionic form in aqueous medium. The hydrophobic interaction between the cavity and the open form of DASA molecules benefits reaching an out-of-equilibrium or reverse equilibrium state in aqueous medium. The presence of excess MV could even drive the conversion of the stable cyclic form to the open form in aqueous medium. The host-guest complex is stable upon irradiating with green light. To the best of our knowledge, this is the first successful attempt to stabilize the open form of DASA molecules in aqueous medium and the first report on the fate of DASA in a confined space discrete molecular architecture. Furthermore, the molecular vessel has been utilized for catalytic Michael addition reactions of a series of nitrostyrene derivatives with 1,3-indandione in aqueous medium.

Self-Assembled Fluorescent Pt(II) Metallacycles as Artificial Light-Harvesting Systems.

Light-harvesting is one of the key steps in photosynthesis, but developing artificial light-harvesting systems (LHSs) with high energy transfer efficiencies has been a challenging task. Here we report fluorescent hexagonal Pt(II) metallacycles as a new platform to fabricate artificial LHSs. The metallacycles (4 and 5) are easily accessible by coordination-driven self-assembly of a triphenylamine-based ditopic ligand 1 with di-platinum acceptors 2 and 3, respectively. They possess good fluorescence properties both in solution and in the solid state. Notably, the metallacycles show aggregation-induced emission enhancement (AIEE) characteristics in a DMSO-H2O solvent system. In the presence of the fluorescent dye Eosin Y (ESY), the emission intensities of the metallacycles decrease but the emission intensity of ESY increases. The absorption spectrum of ESY and the emission spectra of the metallacycles show a considerable overlap, suggesting the possibility of energy transfer from the metallacycles to ESY, with an energy transfer efficiency as high as 65% in the 4a+ESY system.

Multifunctional Self-Assembled Macrocycles with Enhanced Emission and Reversible Photochromic Behavior.

A series of self-assembled functional Pt(II) molecular hexagons (M1-M3) is reported. Hexagons M1 and M2 were designed employing aggregation induced emissive and photochromic building blocks, respectively, while macrocycle M3 is a bifunctional, containing both the kinds of building units. Hexagons M1 and M3 were found to inherit the enhanced emission with aggregate formation which was explored using UV-vis and fluorescence spectroscopy. The enhanced emission of macrocycle M3 compared to that of its building units was driven both by metal-ligand coordination and formation of nanoaggregates as evident from SEM, DLS and TEM analyses. Two of the macrocycles (M2 and M3) were also found to be photochromic due to the presence of spiropyran in the molecular backbone. Due to the virtue of protonation-deprotonation equilibrium of the spiropyran, these macrocycles (M2 and M3) showed reversible acidochromic behavior. Macrocycle M3 represents the first example of a self-assembled Pt(II) architecture which is multifunctional with aggregation-induced emission (AIE), photochromic, and acidochromic properties. This new generation macrocycle (M3) also showed coordination-driven enhanced emission and light-induced color change behavior compared to the starting building blocks. Our present approach of incorporating multiple functions into a single self-assembled structure with enhanced functionality compared to the starting building blocks via coordination self-assembly is noteworthy and has huge potential for the development of multifunctional materials.

Synthesis of an Adaptable Molecular Barrel and Guest Mediated Stabilization of Its Metastable Higher Homologue.

Structural and functional modulation of three-dimensional artificial macromolecular systems is of immense importance. Designing supramolecular cages that can show stimuli mediated reversible switching between higher-order structures is quite challenging. We report here construction of a Pd6 trifacial barrel (1) by coordination self-assembly. Surprisingly, barrel 1 was found to exhibit guest-responsive behavior. In presence of fullerenes C60 and C70, 1 unprecedentedly transformed to its metastable higher homologue Pd8 tetrafacial barrel (2), forming stable host-guest complexes (C60)3⊂2 and (C70)2⊂2, respectively. Again, encapsulated fullerenes could be extracted from the cavity of 2 using 1,2-dichlorobenzene, leading to its facile conversion to the parent trifacial barrel 1. Such reversible structural interconversion between an adaptable molecular barrel and its guest stabilized higher homologue is an uncommon observation.

Fluorescence enhancement via structural rigidification inside a self-assembled Pd4 molecular vessel.

Restriction of intramolecular motion (RIM) is fundamental for the high emission of aggregation-induced emission (AIE)-active molecules in aggregates or the solid-state. However, they are weakly emissive in dilute solution, which limits their application in dilute solutions. A Pd4 molecular vessel (MP1) was constructed by assembling [cis-(en)Pd(NO3)2] (M) with a tetradentate donor (L) in a 2 : 1 molar ratio. The active intramolecular motions of an AIE active molecule SG are restricted in the narrow cavity of MP1 upon encapsulation. As a result, SG displayed significant enhancement in its emission in dilute solution upon addition of MP1. This strategy of achieving high emission of AIE active compounds in dilute solution by confinement driven RIM might have potential in designing materials for high emission in the aggregated state as well as in dilute solution.

Self-Assembly of Octanuclear PtII/PdII Coordination Barrels and Uncommon Structural Isomerization of a Photochromic Guest in Molecular Space.

Two tetragonal molecular barrels TB1 and TB2 were successfully synthesized by coordination-driven self-assembly of a tetrapyridyl donor (L) of the thiazolo[5,4-d]thiazole backbone with cis-blocked 90° Pd(II) and Pt(II) acceptors, respectively. The single-crystal structure analysis of TB1 revealed the formation of a two-face opened tetragonal Pd8 molecular barrel architecture. In contrast, the isostructural Pt(II) barrel (TB2) is water-soluble. The large confined hydrophobic molecular cavity including wide open windows and good water solubility of the barrel TB2 made it a potential molecular container for the encapsulation of guests with different sizes and properties. This has been exploited to encapsulate and stabilize the open form of a photochromic molecule (G2) in water, while the same photochromic molecule exists exclusively in a cyclic zwitterionic form in aqueous medium in the absence of the barrel TB2. This cyclic form is very stable in water and does not go back to its parent open form under common external stimuli. Surprisingly, reverse switching of the cyclic form to a colored hydrophobic open form was also possible instantly in water upon addition of the solid barrel TB2 into an aqueous solution of G2. Such a fast reverse isomerization of an irreversible process in aqueous medium by utilizing host-guest interaction of the barrel TB2 and the guest G2 is interesting. The barrel TB2 was also capable of encapsulating the water-insoluble radical initiator G1 in aqueous medium.

Linkage induced enhancement of fluorescence in metal-carbene bond directed metallacycles and metallacages.

Two new 1,4-dihydropyrrolo[3,2-b]pyrrole based aggregation induced emission (AIE)-inactive di- and tetra-imidazolium salts were employed with Ag(i) for the synthesis of a Ag-carbene bond directed metallacycle (1) and metallacage (2), respectively. Transmetalation of these complexes allowed their facile conversion to their respective Au(i)-metallacycle (3) and metallacage (4). The final assemblies exhibit linkage induced enhancement of fluorescence (LIEF). The free ligands are almost non-fluorescent (ΦF = 3.2, 3.4) in comparison to their metal-carbene counterparts (ΦF up to 32.0). Thus, without using any AIEgen, obtaining high emission efficiencies in complexes AgI-CNHC (1, 2) via linkage is a nice approach towards turn-on fluorescence.