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

Formation of a Pd16 Molecular Basket Architecture of Reduced Symmetry and Angular Deviation in a Fluorenone Scaffold to Govern the Host-Guest Chemistry of Pd6 Trifacial Tubes

The employment of flexible ligands with significant conformational freedom in coordination-driven self-assembly enables the formation of unique and intricate structures. In this study, the self-assembly of such a fluorenone-appended ligand (L1) with a sterically demanding acceptor, [Pd-(tmed)-(ONO2)2] (M1), generated a new and unique molecular basket architecture, (M1) 16 (L1) 8 (B), featuring a large hollow cavity. B possesses an unusual twisted architecture of low symmetry, consisting of 16 Pd-(II) centers arranged as four tetrahedra connected by eight flexible ligands, representing a structurally complex system reminiscent of biological architectures. Designing such entropically disfavored, large architectures of reduced symmetry is challenging but desirable, since they can act as ideal models to study complicated natural systems. The host-guest property of supramolecular hosts is governed by the confined cavities and noncovalent interactions, which are dictated by the angular disposition of ligand coordination sites. To explore this, the fluorenone scaffold was used to synthesize two other tetradentate ligands (L2 and L3) that differed in the spatial distributions of their coordination vectors. The self-assembly of these ligands with [Pd-(en)-(ONO2)2] (M2) resulted in the formation of water-soluble (M2) 6 (L1/L2/L3) 3 trifacial tubes of different geometries with varying internal cavity dimensions. These angular variations further altered the orientation of the fluorenone carbonyl groups within the cavities, thereby modulating their guest binding abilities and highlighting the importance of tailoring supramolecular hosts for specific guest binding.

Host-Guest Approach to Enhancing Photocatalysis via Photoinduced Energy and Electron Transfer from a Photoactive Triphenylamine-Based Metal-Organic Cage to Bound Guests

The host-guest strategy presents an ideal way to construct versatile supramolecular systems that mimic the structure and functionality of natural enzymes and, therefore, achieve efficient chemical conversions. An emissive triphenylamine-based cage-like host donor was constructed as an energy or electron donor to achieve efficient photoinduced energy or electron transfer (PEnT or PET) by encapsulating the energy or electron acceptor into the cavity of the cage. The host-guest complexes, which served as enzyme-mimicking supramolecular systems, were successfully used as photocatalysts for the selective aerobic oxidation of sulfides and the efficient photocatalytic reduction of aryl halides with high reduction potentials. This work details a promising approach for creating a host-guest system via a host-guest encapsulation strategy to enhance the efficiency of the PEnT or PET process. The resulting designed artificial supramolecular systems achieve efficient chemical conversions by mimicking the structure and functionality of natural enzymes.

Guest-Induced "Breathing-Helical" Dynamic System of a Porphyrinic Metallo-Organic Cage for Advanced Conformational Manipulation

Host-guest dynamic systems in coordination-driven metallo-organic cages have gained significant attentions since their promising applications in chiral separation, drug delivery, and catalytical fields. To maximize guest-binding affinity, hosts adopting multiple conformations are widely investigated on their structural flexibility for guest accommodation. In this study, a novel metallo-organic cage S with breathing inner cavity and freely twisted side chains was proposed. Single-crystal X-ray diffraction analyses depicted a characteristic "breathing-helical" dynamic system on the semiflexible framework, which led to an unprecedent co-crystallisation of racemic and symmetric conformations via the encapsulation locking of C70 guests. By taking advantages of the high binding affinity, selective extraction of C70 was realized. This research provides new ideas for the modification on the helicities of metallo-organic cages, which could pave a new way for advanced conformational manipulation of supramolecular host systems.

Unveiling the structural aspects of novel azo-dyes with promising anti-virulence activity against MRSA: a deep dive into the spectroscopy via integrated experimental and computational approaches

A novel series of azo dyes was successfully synthesized by combining amino benzoic acid and amino phenol on the same molecular framework via azo linkage. The structural elucidation of these dyes was carried out using various spectroscopic techniques, including UV-vis, FT-IR, NMR spectroscopy, and HRMS. Surprisingly, the aromatic proton in some dyes exhibited exchangeability in D2O, prompting a 2D NMR analysis to confirm this phenomenon. Furthermore, comprehensive density functional theory (DFT) calculations were conducted to unravel synthetic dyes' geometrical and electronic properties. Meanwhile, the reactivity of various sites was further investigated through Frontier Molecular Orbitals (FMOs) analysis and molecular electrostatic potential mapping. Besides, the experimental NMR spectra were interpreted by incorporating theoretically computed NMR spectrum and reduced density gradient (RDG) function. These computations revealed a pronounced intramolecular hydrogen bond through O-H⋯N interaction that significantly influenced the proton chemical shift. The dyes were assessed for their antimicrobial activities using agar diffusion, micro broth dilution, and biofilm inhibition assays. Interestingly, one of the synthetic dyes showed promising antibacterial effects against S. aureus (ATCC-6538) as well as against a multidrug-resistant MRSA clinical isolate with a MIC (minimum inhibitory concentration) of 78.12 μg mL-1. Moreover, that dye inhibited biofilm formation of the strong biofilm former clinical MRSA isolate with a concentration as low as 0.25 MIC (19.53 μg mL-1). Indeed, our qPCR data suggest that inhibiting the SaeS/SaeR system is another potential mechanism by which D4 exerts its antibacterial and anti-virulence effects. Altogether, this shows these synthetic azo dyes' promising antibacterial and anti-virulence activities concerning MRSA clinical infections.

Host-Guest Interactions Induced Enhancement in Oxidase-Like Activity of a Benzothiadiazole Dye Inside an Aqueous Pd8L4 Barrel

The effect of host-guest interactions on the chemistry of encapsulated molecules is a fascinating field of research that has gained momentum in recent years. Much of the work in this field has been focused on the effect of such interactions on catalysis and photoluminescence of encapsulated dyes. However, the effect of such interactions on related photoinduced processes, such as photoregulated oxidase-mimicking activity, has not been explored much. Herein, we report a unique example of enhancement of oxidase-like activity of a benzothiadiazole dye (G1) in water through encapsulation within a M8L4 molecular barrel (1). Favorable host-guest interactions helped the encapsulated guest G1 to have better photoinduced electron transfer to molecular oxygen leading to increased production of superoxide radical anions and oxidase-like activity. Furthermore, encapsulation inside 1 also caused a change in the redox potentials of the guest (G1) which after photoinduced electron transfer produced a better oxidizing agent than free G1. These phenomena combined to enhance the oxidase-like activity of dye G1 upon encapsulation inside cage 1. The present report demonstrates a unique effect of host-guest chemistry on photoregulated processes.

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 cages for gas adsorption and separation

The unique high surface area and tunable cavity size endow metal-organic cages (MOCs) with superior performance and broad application in gas adsorption and separation. Over the past three decades, for instance, numerous MOCs have been widely explored in adsorbing diverse types of gas including energy gases, greenhouse gases, toxic gases, noble gases, etc. To gain a better understanding of the structure-performance relationships, great endeavors have been devoted to ligand design, metal node regulation, active metal site construction, cavity size adjustment, and function-oriented ligand modification, thus opening up routes toward rationally designed MOCs with enhanced capabilities. Focusing on the unveiled structure-performance relationships of MOCs towards target gas molecules, this review consists of two parts, gas adsorption and gas separation, which are discussed separately. Each part discusses the cage assembly process, gas adsorption strategies, host-guest chemistry, and adsorption properties. Finally, we briefly overviewed the challenges and future directions in the rational development of MOC-based sorbents for application in challenging gas adsorption and separation, including the development of high adsorption capacity MOCs oriented by adsorbability and the development of highly selective adsorption MOCs oriented by separation performance.

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.

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.

Highly efficient Förster resonance energy transfer between an emissive tetraphenylethylene-based metal-organic cage and the encapsulated dye guest

The host-guest strategy presents an ideal way to achieve efficient Förster resonance energy transfer (FRET) by forcing close proximity between an energy donor and acceptor. Herein, by encapsulating the negatively charged acceptor dyes eosin Y (EY) or sulforhodamine 101 (SR101) in the cationic tetraphenylethene-based emissive cage-like host donor Zn-1, host-guest complexes were formed that exhibit highly efficient FRET. The energy transfer efficiency of Zn-1⊃EY reached 82.4%. To better verify the occurrence of the FRET process and make full use of the harvested energy, Zn-1⊃EY was successfully used as a photochemical catalyst for the dehalogenation of α-bromoacetophenone. Furthermore, the emission color of the host-guest system Zn-1⊃SR101 could be adjusted to exhibit bright white-light emission with the CIE coordinates (0.32, 0.33). This work details a promising approach to enhance the efficiency of the FRET process by the creation of a host-guest system between the cage-like host and dye acceptor, thus serving as a versatile platform for mimicking natural light-harvesting systems.

A Porous Polyaromatic Solid for Vapor Adsorption of Xylene with High Efficiency, Selectivity, and Reusability

Development of porous materials capable of capturing volatile organic compounds (VOCs), such as benzene and its derivatives, with high efficiency, selectivity, and reusability is highly demanded. Here we report unusual vapor adsorption behavior toward VOCs by a new porous solid, composed of a polyaromatic capsule bearing a spherical nanocavity with subnano-sized windows. Without prior crystallization and high-temperature vacuum drying, the porous polyaromatic solid exhibits the following five features: vapor adsorption of benzene over cyclohexane with 90 % selectivity, high affinity toward o-xylene over benzene and toluene with >80 % selectivity, ortho-selective adsorption ability (>50 %) from mixed xylene isomers, tight VOCs storage even under high temperature and vacuum conditions, and at least 5 times reusability for xylene adsorption. The observed adsorption abilities are accomplished at ambient temperature and pressure within 1 h, which has not been demonstrated by organic/inorganic porous materials reported previously.

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.