Cyclophane-Sustained Ultrastable Porphyrins

Ultrahigh-Affinity Molecular Recognition in Water and Biomedical Applications

Aqueous-phase molecular recognition pairs with ultrahigh binding affinity hold immense value in biotechnology and chemical applications. However, the rational design of synthetic pairs with such exceptional binding strength has long remained a significant challenge, with notable progress achieved only in recent years. In this minireview, we begin by defining the term "ultrahigh-affinity" through a comprehensive analysis of available data on aqueous-phase molecular recognition by water-soluble macrocyclic hosts. Based on this foundation, we provide a detailed overview of the latest advancements in various classes of ultrahigh-affinity receptors, extracting key design principles that drive their remarkable performance. We further highlight emerging applications of ultrahigh-affinity molecular pairs in biomedical materials, spanning bioorthogonal chemistry, biosensing, bioimaging, drug delivery, and toxin sequestration. These examples underscore the transformative potential of ultrahigh-affinity recognition in addressing real-world biomedical challenges. Finally, we offer a forward-looking perspective on the future of this rapidly evolving field, exploring potential directions for designing more diverse and functional ultrahigh-affinity molecular recognition tools. By bridging the gap between fundamental science and practical applications, this minireview aims to inspire the development of next-generation molecular recognition systems and foster deeper integration between supramolecular chemistry and biomedical materials, paving the way for innovative solutions to pressing biomedical needs.

Enhancing the Chemical Stability of P12L24 Cage: Transformation of the Chemically Labile Imine Cage into a Robust Carbamate Cage

Herein, we report enhancement in chemical stability of the imine-based porphyrinic cage P12L24 by converting it into a robust carbamate porphyrinic cage, c-P12L24, through a two-step post-synthetic modification process. First, the imine bonds in P12L24 were reduced to form an amine-based cage, r-P12L24, followed by carbamation using N,N'-carbonyldiimidazole (CDI) to yield c-P12L24. The resulting carbamate cage exhibits high stability under acidic and basic conditions (pH 1-13) and in the presence of moisture. 1H NMR, DOSY NMR, and DFT calculations revealed that reducing the imine bonds to amine increases the framework's flexibility, causing partial structural collapse, whereas the carbamate formation restores structural rigidity. The insertion of a 4.0 nm molecular ruler into the cavity of zinc-metallated c-P12L24 via metal-ligand coordination further confirmed restoration of the cavity size and geometry of the original cage. This enhancement of chemical stability through carbamate formation can pave the way to a wide range of potential applications for the gigantic porphyrinic cage.

From small changes to big gains: pyridinium-based tetralactam macrocycle for enhanced sugar recognition in water

The complex distribution of functional groups in carbohydrates, coupled with their strong solvation in water, makes them challenging targets for synthetic receptors. Despite extensive research into various molecular frameworks, most synthetic carbohydrate receptors have exhibited low affinities, and their interactions with sugars in aqueous environments remain poorly understood. In this work, we present a simple pyridinium-based hydrogen-bonding receptor derived from a subtle structural modification of a well-known tetralactam macrocycle. This small structural change resulted in a dramatic enhancement of glucose binding affinity, increasing from 56 M-1 to 3001 M-1. Remarkably, the performance of our synthetic lectin surpasses that of the natural lectin, concanavalin A, by over fivefold. X-ray crystallography of the macrocycle-glucose complex reveals a distinctive hydrogen bonding pattern, which allows for a larger surface overlap between the receptor and glucose, contributing to the enhanced affinity. Furthermore, this receptor possesses allosteric binding sites, which involve chloride binding and trigger receptor aggregation. This unique allosteric process reveals the critical role of structural flexibility in this hydrogen-bonding receptor for the effective recognition of sugars. We also demonstrate the potential of this synthetic lectin as a highly sensitive glucose sensor in aqueous solutions.

Recent advances in porous organic cages for energy applications

In recent years, the energy and environmental crises have attracted more and more attention. It is very important to develop new materials and technologies for energy storage and conversion. In particular, it is crucial to develop carriers that store energy or promote mass and electron transport. Emerging porous organic cages (POCs) are very suitable for this purpose because they have inherent advantages including structural designability, porosity, multifunction and post-synthetic modification. POC-based materials, such as pristine POCs, POC composites and POC derivatives also exhibit excellent energy-related properties. This latest perspective provides an overview of the progress of POC-based materials in energy storage and conversion applications, including photocatalysis, electrocatalysis (CO2RR, NO3RR, ORR, HER and OER), separation (gas separation and liquid separation), batteries (lithium-sulfur, lithium-ion and perovskite solar batteries) and proton conductivity, highlighting the unique advantages of POC-based materials in various forms. Finally, we summarize the current advances, challenges and further perspectives of POC-based materials in energy applications. This perspective will promote the design and synthesis of next-generation POC-based materials for energy applications.

Charge-assisted hydrogen bonding in a bicyclic amide cage: an effective approach to anion recognition and catalysis in water

Hydrogen bonding is prevalent in biological systems, dictating a myriad of life-sustaining functions in aqueous environments. Leveraging hydrogen bonding for molecular recognition in water encounters significant challenges in synthetic receptors on account of the hydration of their functional groups. Herein, we introduce a water-soluble hydrogen bonding cage, synthesized via a dynamic approach, exhibiting remarkable affinities and selectivities for strongly hydrated anions, including sulfate and oxalate, in water. We illustrate the use of charge-assisted hydrogen bonding in amide-type synthetic receptors, offering a general molecular design principle that applies to a wide range of amide receptors for molecular recognition in water. This strategy not only revalidates the functions of hydrogen bonding but also facilitates the effective recognition of hydrophilic anions in water. We further demonstrate an unconventional catalytic mechanism through the encapsulation of the anionic oxalate substrate by the cationic cage, which effectively inverts the charges associated with the substrate and overcomes electrostatic repulsions to facilitate its oxidation by the anionic MnO4 -. Technical applications using this receptor are envisioned across various technical applications, including anion sensing, separation, catalysis, medical interventions, and molecular nanotechnology.

Supramolecular cage-mediated cargo transport

A steady stream of material transport based on carriers and channels in living systems plays an extremely important role in normal life activities. Inspired by nature, researchers have extensively applied supramolecular cages in cargo transport because of their unique three-dimensional structures and excellent physicochemical properties. In this review, we will focus on the development of supramolecular cages as carriers and channels for cargo transport in abiotic and biological systems over the past fifteen years. In addition, we will discuss future challenges and potential applications of supramolecular cages in substance transport.

Design and assembly of porous organic cages

Porous organic cages (POCs) represent a notable category of porous materials, showing remarkable material properties due to their inherent porosity. Unlike extended frameworks which are constructed by strong covalent or coordination bonds, POCs are composed of discrete molecular units held together by weak intermolecular forces. Their structure and chemical traits can be systematically tailored, making them suitable for a range of applications including gas storage and separation, molecular separation and recognition, catalysis, and proton and ion conduction. This review provides a comprehensive overview of POCs, covering their synthesis methods, structure and properties, computational approaches, and applications, serving as a primer for those who are new to the domain. A special emphasis is placed on the growing role of computational methods, highlighting how advanced data-driven techniques and automation are increasingly aiding the rapid exploration and understanding of POCs. We conclude by addressing the prevailing challenges and future prospects in the field.

Water-soluble organic macrocycles based on dye chromophores and their applications

Traditional water-soluble organic macrocyclic receptors generally lack photofunctionality, thus monitoring the drug delivery and the phototheranostic applications of these host-guest macrocyclic systems has been greatly restricted. To address this issue, incorporating π-conjugated dye chromophores as building blocks into macrocyclic molecules is a straightforward and promising strategy. This approach not only imparts intrinsic optical features to the macrocycles themselves but also enhances the host-guest binding ability due to the large planar structures of the dyes. In this feature article, we focus on recent advances in water-soluble macrocyclic compounds based on organic dye chromophores, such as naphthalimide (NDI), perylene diimides (PDI), azobenzene (azo), tetraphenylethylene (TPE) and anthracene, and provide an overview of their various applications including molecular recognition, drug release, biological imaging, photothermal therapy, etc. We hope that this article could be helpful and instructive for the design of water-soluble dye-based macrocycles and the further development of their biomedical applications, particularly in combination with drug therapy and phototheranostics.

Supramolecular Architectures Bearing Half-Sandwich Iridium- or Rhodium-Based Carboranes: Design, Synthesis, and Applications

The utilization of carboranes in supramolecular chemistry has attracted considerable attention. The unique spatial configuration and weak interaction forces of carboranes can help to explore the properties of supramolecular complexes, particularly via host-guest chemistry. Additionally, certain difficulties encountered in carborane development─such as controlled B-H bond activation─can be overcome by judiciously selecting metal centers and their adjacent ligands. However, few studies are being conducted in this nascent research area. With advances in this field, novel carborane-based supramolecular complexes will likely be prepared, structurally characterized, and intrinsically investigated. To expedite these efforts, we present major findings from recent studies, including π-π interactions, host-guest associations, and steric effects, which have been leveraged to implement a regioselective process for activating B(2,9)-, B(2,8)-, and B(2,7)-H bonds of para-carboranes and B(4,7)-H bonds of ortho-carboranes. Future studies should clarify the unique weak interactions of carboranes and their potential for enhancing the utility of supramolecular complexes. Although carboranes exhibit several unique weak interactions (such as dihydrogen-bond [Bδ+-Hδ-···Hδ+-Cδ-], Bδ+-Hδ-···M+, and Bδ+-Hδ-···π interactions), the manner in which they can be utilized remains unclear. Supramolecular complexes, particularly those based on host-guest chemistry, can be utilized as a platform for demonstrating potential applications of these weak interactions. Owing to the importance of alkane separation, applications related to the recognition and separation of alkane isomers via dihydrogen-bond interactions are primarily summarized. Advances in the research of unique weak interactions in carboranes will certainly lead to more possibilities for supramolecular chemistry.

Remarkably Selective Binding, Behavior Modification, and Switchable Release of (Bipyridine)3Ru(II) vis-à-vis (Phenanthroline)3Ru(II) by Trimeric Cyclophanes in Water

A recurring dream of molecular recognition is to create receptors that distinguish between closely related targets with sufficient accuracy, especially in water. The more useful the targets, the more valuable the dream becomes. We now present multianionic trimeric cyclophane receptors with a remarkable ability to bind the iconic (bipyridine)3Ru(II) (with its huge range of applications) while rejecting the nearly equally iconic (phenanthroline)3Ru(II). These receptors not only selectively capture (bipyridine)3Ru(II) but also can be redox-switched to release the guest. 1D- and 2D(ROESY)-NMR spectroscopy, luminescence spectroscopy, and molecular modeling enabled this discovery. This outcome allows the control of these applications, e.g., as a photocatalyst or as a luminescent sensor, by selectively hiding or exposing (bipyridine)3Ru(II). Overall, a 3D nanometric object is selected, picked-up, and dropped-off by a discrete molecular host. The multianionic receptors protect excited states of these metal complexes from phenolate quenchers so that the initial step in photocatalytic phenolate oxidation is retarded by nearly 2 orders of magnitude. This work opens the way for (bipyridine)3Ru(II) to be manipulated in the presence of other functional nano-objects so that many of its applications can be commanded and controlled. We have a cyclophane-based toolkit that can emulate some aspects of proteins that selectively participate in cell signaling and metabolic pathways by changing shape upon environmental commands being received at a location remote from the active site.

Crystalline Nanoparticles of Water-Soluble Covalent Basket Cages (CBCs) for Encapsulation of Anticancer Drugs

We herein describe the preparation, assembly, recognition characteristics, and biocompatibility of novel covalent basket cage CBC-11, composed of four molecular baskets linked to four trivalent aromatic amines through amide groups. The cage is tetrahedral in shape and similar in size to small proteins (Mw =8637 g/mol) with a spacious nonpolar interior for accommodating multiple guests. While 24 carboxylates at the outer surface of CBC-11 render it soluble in aqueous phosphate buffer (PBS) at pH=7.0, the amphiphilic nature prompts its assembly into nanoparticles (d=250 nm, DLS). Cryo-TEM examination of nanoparticles revealed their crystalline nature with wafer-like shapes and hexagonally arranged cages. Nanoparticulate CBC-11 traps anticancer drugs irinotecan and doxorubicin, with each cage binding up to four drug molecules in a non-cooperative manner. The inclusion complexation resulted in nanoparticles growing in size and precipitating. In media containing mammalian cells (HCT 116, human colon carcinoma), the IC50 value of CBC-11 was above 100 μM. While this work presents the first example of a large covalent organic cage operating in water at the physiological pH and forming crystalline nanoparticles, it also demonstrates its biocompatibility and potential to act as a polyvalent binder of drugs for their sequestration or delivery.

Porous Organic Cages

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

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.

Porphyrin COF and its mechanical pressing-prepared carbon fiber hybrid membrane for ratiometric detection, removal and enrichment of Cd2

A nitrogen (N), oxygen (O)-rich porphyrin-based covalent organic framework (COF), in which interlayer porphyrin molecules are vertically stacked, is prepared and characterized. As-prepared N,O-rich TpTph COF shows a high adsorption capacity for Cd²⁺ due to the abundant coordination sites. More interesting, it is found that the formation of COF enlarges the porphyrin ring center space, thus facilitating the Cd²⁺coordination, and the resulting optical signal changes make the ratiometric detection of Cd²⁺ possible. Furthermore, using carbon fiber (CF) filaments, which are obtained from low cost and easy-to-obtain actived carbon mask, as support, porphyrin COF-based CF@TpTph membrane is prepared through in-situ growth of COF on the support followed by simple mechanical pressing. The CF@TpTph membrane is demonstrated to work well for both Cd²⁺ removal and enrichment from soil and water samples, and shows the advantages of ease of handling, robust stability, reduced secondary pollution risk to samples, and good reusability. This work provides a powerful tool for Cd²⁺ removal and enrichment, exhibits that preparing porphyrin-based COFs is a feasible way to promote the interactions between porphyrin ring and Cd²⁺, and demonstrates that mechanical pressing is a promising strategy for the design of COF-based monolithic materials to promote the practical applications of COFs.

Recent trends in organic cage synthesis: push towards water-soluble organic cages

Research on organic cages has blossomed over the past few years into a mature field of study which can contribute to solving some of the challenging problems. In this review we aim to showcase the recent trends in synthesis of organic cages including a brief discussion on their use in catalysis, gas sorption, host-guest chemistry and energy transfer. Among the organic cages, water-soluble analogues are a special class of compounds which have gained renewed attention in recent times. Due to their advantage of being compatible with water, such cages have the potential of showing biomimetic activities and can find use in drug delivery and also as hosts for catalysis in aqueous medium. Hence, the synthetic strategies for the formation of water-soluble organic cages shall be discussed along with their potential applications.

Tuning the aromatic backbone twist in dipyrrolonaphthyridinediones

This communication describes the photophysical behavior of three analogs of cyclophane bearing the dipyrrolonaphthyridinedione (DPND) core. In these molecules, intersystem crossing (ISC) can be successfully induced by distinct changes in the deviation from planarity within the DPND core, allowing at the same time the emission maximum to shift from the green to red region of the visible spectrum without any synthetic modifications of the chromophore structure. This finding may build the foundation for a new paradigm for inducing ISC-type transitions within other centrosymmetric and planar cross-conjugated chromophores.

Photomodulation of ultrastable host–guest complexes in water and their application in light-controlled steroid release

Host–guest complexation of dithienylethene photoswitches with cucurbit[8]uril leads to photoresponsive binding pairs with picomolar affinity in water.

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.

Fine-Tuning Macrocycle Cavity to Selectively Bind Guests in Water for Near-Infrared Photothermal Conversion

The rational and specific synthesis of the required organic macrocycles to bind the size-matched targeted guests without undesired macrocyclic byproducts remains a great challenge. Herein, based on a new naphthalimide...

Synthesis, Characterization, and Encapsulation Properties of Rigid and Flexible Porphyrin Cages Assembled from N-Heterocyclic Carbene-Metal Bonds

Four porphyrins equipped with imidazolium rings on the para positions of their meso aryl groups were prepared and used as tetrakis(N-heterocyclic carbene) (NHC) precursors for the synthesis of porphyrin cages assembled from eight NHC-M bonds (M = Ag⁺ or Au⁺). The conformation of the obtained porphyrin cages in solution and their encapsulation properties strongly depend on the structure of the spacer -(CH₂)_n- (n = 0 or 1) between meso aryl groups and peripheral NHC ligands. In the absence of methylene groups (n = 0), porphyrin cages are rather rigid and the short porphyrin-porphyrin distance prevents the encapsulation of guest molecules like 1,4-diazabicyclo[2.2.2]octane (DABCO). By contrast, the presence of methylene functions (n = 1) between meso aryl groups and peripheral NHCs offers additional flexibility to the system, allowing the inner space between the two porphyrins to expand enough to encapsulate guest molecules like water molecules or DABCO. The peripheral NHC-wingtip groups also play a significant role in the encapsulation properties of the porphyrin cages.

PCage: Fluorescent Molecular Temples for Binding Sugars in Water

The development of synthetic receptors that recognize carbohydrates in water with high selectivity and specificity is challenging on account of their structural complexity and strong hydrophilicity. Here, we report on the design and synthesis of two pyrene-based, temple-shaped receptors for the recognition of a range of common sugars in water. These receptors rely on the use of two parallel pyrene panels, which serve as roofs and floors, capable of forming multiple [C-H···π] interactions with the axially oriented C-H bonds on glycopyranosyl rings in the carbohydrate-based substrates. In addition, eight polarized pyridinium C-H bonds, projecting from the roofs and floors of the temple receptors toward the binding cavities, form [C-H···O] hydrogen bonds, with the equatorially oriented OH groups on the sugars located inside the hydrophobic cavities. Four para-xylylene pillars play a crucial role in controlling the distance between the roof and floor. These temple receptors are highly selective for the binding of glucose and its derivatives. Furthermore, they show enhanced fluorescence upon binding with glucose in water, a property which is useful for glucose-sensing in aqueous solution.

Cucurbit[10]uril-Encapsulated Cationic Porphyrins with Enhanced Fluorescence Emission and Photostability for Cell Imaging

Porphyrins are widely applied for imaging, diagnosis, and treatment of diseases because of their excellent photophysical properties. However, porphyrins easily tend to aggregate driven by hydrophobic interaction and π-π stacking in an aqueous medium, which causes fluorescence quenching of the porphyrins as well as limitation of cell uptake and intracellular accumulation. Herein, cucurbit[10]uril (CB[10]) was used to fully encapsulate cationic porphyrin (CPor) in the large cavity with strong binding affinity in aqueous solutions, and the CPor aggregates were efficient disassembled, companying remarkable enhancing its fluorescence intensity. The CB[10]-based host-guest complex provided excellent protection to CPor, resulting in less susceptibility to oxidation and imparting higher photostability to CPor for cell imaging. In addition, by complexation with CB[10], it was found that the fluorescence signals and photostability of CPor were also effectively improved in cells with different reactive oxygen species levels. It is highly anticipated that the large macrocyclic host cavity-triggered large-guest encapsulation strategy in this work will provide a convenient and efficient method for designing supramolecular porphyrin dyes, thus broadening the diagnosis and imaging application in cells and microorganisms.

Design and Applications of Water-Soluble Coordination Cages

Compartmentalization of the aqueous space within a cell is necessary for life. In similar fashion to the nanometer-scale compartments in living systems, synthetic water-soluble coordination cages (WSCCs) can isolate guest molecules and host chemical transformations. Such cages thus show promise in biological, medical, environmental, and industrial domains. This review highlights examples of three-dimensional synthetic WSCCs, offering perspectives so as to enhance their design and applications. Strategies are presented that address key challenges for the preparation of coordination cages that are soluble and stable in water. The peculiarities of guest binding in aqueous media are examined, highlighting amplified binding in water, changing guest properties, and the recognition of specific molecular targets. The properties of WSCC hosts associated with biomedical applications, and their use as vessels to carry out chemical reactions in water, are also presented. These examples sketch a blueprint for the preparation of new metal-organic containers for use in aqueous solution, as well as guidelines for the engineering of new applications in water.

Control of Porphyrin Planarity and Aggregation by Covalent Capping: Bissilyloxy Porphyrin Silanes

Porphyrins are cornerstone functional materials that are useful in a wide variety of settings, ranging from molecular electronics to biology and medicine. Their applications are often hindered, however, by poor solubilities that result from their extended, solvophobic aromatic surfaces. Attempts to counteract this problem by functionalizing their peripheries have been met with only limited success. Here, we demonstrate a versatile strategy to tune the physical and electronic properties of porphyrins using an axial functionalization approach. Porphyrin silanes (PorSils) and bissilyloxy PorSils (SOPS) are prepared from porphyrins by operationally simple κ⁴N-silylation protocols, introducing bulky silyloxy "caps" that are central and perpendicular to the planar porphyrin. While porphyrins typically form either J- or H-aggregates, SOPS do not self-associate in the same manner: the silyloxy axial substituents dramatically improve the solubility by inhibiting aggregation. Moreover, axial porphyrin functionalization offers convenient handles through which optical, electronic, and structural properties of the porphyrin core can be modulated. We observe that the identity of the silyloxy substituent impacts the degree of planarity of the porphyrin in the solid state as well as the redox potentials.