Construction of an Artificial Light-Harvesting System with Efficient Photocatalytic Activity in an Aqueous Solution Based on a FRET-Featuring Metallacage

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.

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.

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.

Carbazole-based artificial light-harvesting system for photocatalytic cross-coupling dehydrogenation reaction

A carbazole-based artificial light-harvesting system (LHS) was successfully fabricated based on the supramolecular assembly of AIE-enhanced donor (CTD), water-soluble phosphate-pillar[5]arene (WPP5), and eosin Y (ESY) acceptor. The formed WPP5-CTD possessed remarkable AIE emission, featuring an ideal energy donor for light harvesting. After encapsulation of ESY, the energy of WPP5-CTD was efficiently transferred to ESY in WPP5-CTD-ESY, and the antenna effect was 38.5, which was much higher than that of recently reported LHSs. Notably, WPP5-CTD-ESY was successfully utilized as a photocatalyst to realize the cross-coupling dehydrogenation reaction of diphenylphosphine oxide and benzothiazole derivatives, suggesting great potential for aqueous photocatalytic applications of this LHS.

Three-step cascaded artificial light-harvesting systems with tunable efficiency based on metallacycles

It is still challenging to develop multi-step cascaded artificial light-harvesting systems (ALHSs) with tunable efficiency. Here, we designed novel cascaded ALHSs with AIE-active metallacycles as the light-harvesting antenna, Eosin Y (ESY) and sulforhodamine 101 (SR101) as conveyors, near-infrared emissive chlorin-e6 (Ce6) as the final acceptor. The close contact and fair spectral overlap between donor and acceptor molecules at each level ensured the efficient sequential three-step energy transfer. The excited energy was sequentially and efficiently funneled to Ce6 along the cascaded line MTPEPt1 → ESY → SR101 → Ce6. Additionally, a unique strategy for regulating the efficiency of ALHS was illustrated by adjusting hydrophilic and hydrophobic interactions.

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.

An artificial light-harvesting system constructed from a water-soluble metal-organic barrel for photocatalytic aerobic reactions in aqueous media

An artificial light-harvesting system constructed from a water-soluble host-guest complex can be regarded as a high-level conceptual model of its biological counterpart and can convert solar energy into chemical energy in an aqueous environment. Herein, a water-soluble metal-organic barrel Ga-tpe with twelve sulfonic acid units was obtained by subcomponent self-assembly between Ga3+ ions and tetra-topic ligands with tetraphenylethylene (TPE) cores. By taking advantage of host-guest interactions, cationic dye rhodamine B (RB) was constrained in the pocket of Ga-tpe to promote the Förster resonance energy transfer (FRET) process for efficient photocatalytic aerobic oxidation of sulfides and cross-dehydrogenative coupling (CDC) reaction in aqueous media.

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, Ag^I-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 Ag^I-NHC complex to fabricate a light-harvesting scaffold is a new approach and can greatly impact the generation of smart materials.