Chaperone Mimetic Strategy for Achieving Organic Room-Temperature Phosphorescence based on Confined Supramolecular Assembly.

The development of organic materials that deliver room-temperature phosphorescence (RTP) is highly interesting for potential applications such as anticounterfeiting, optoelectronic devices, and bioimaging. Herein, a molecular chaperone strategy for controlling isolated chromophores to achieve high-performance RTP is demonstrated. Systematic experiments coupled with theoretical evidence reveal that the host plays a similar role as a molecular chaperone that anchors the chromophores for limited nonradiative decay and directs the proper conformation of guests for enhanced intersystem crossing through noncovalent interactions. For deduction of structure-property relationships, various structure-related descriptors that correlate with the RTP performance are identified, thus offering the possibility to quantitatively design and predict the phosphorescent behaviors of these systems. Furthermore, application in thermal printing is well realized for these RTP materials. The present work discloses an effective strategy for efficient construction of organic RTP materials, delivering a modular model which is expected to help expand the diversity of desirable RTP systems.

Pillar[5]arene Derivatives Embedded with Aggregation-Induced Emission Luminogens and Their Fluorescence Regulation.

Through McMurry coupling reaction, three meso-position functionalized pillar[5]arene derivatives (H-1, H-2, and H-3) have been successfully prepared by embedding aggregation-induced emission luminogens (AIEgens, diphenyldibenzofulvene (DPDBF) and tetraphenylethylene (TPE)) into the skeleton of supramolecular macrocycles. H-1, bearing [15 ]paracyclophane ([15 ]PCP) and DPDBF moiety, exhibits yellow emission and demonstrates obvious AIE effect. In order to further improve the host-guest properties of this type of structure, H-2 and H-3 are prepared by replacing the [15 ]PCP moiety with pillar[5]arene backbone, both of which show significant AIE effect and excellent host-guest complexation properties with pyrazine salt guest G-1 and 1,4-dicyanobutane G-2. Our findings indicate that G-1 can decrease the fluorescence intensity of the AIE macrocycles, while G-2 can increase their fluorescence intensity in solution.

Tetraphenylethylene-embedded pillar[5]arene-based orthogonal self-assembly for efficient photocatalysis in water.

Herein, we have designed and fabricated a simple and efficient supramolecular self-assembled nanosystem based on host-guest interactions between water-soluble tetraphenylethylene-embedded pillar[5]arene ( m -TPEWP5) and ammonium benzoyl-ʟ-alaninate (G) in an aqueous medium. The obtained assembly of m -TPEWP5 and G showed aggregation-induced emission (AIE) via the blocking of intramolecular phenyl-ring rotations and functioned as an ideal donor. After the loading of eosin Y (EsY) as acceptor on the surface of the assembly of m -TPEWP5 and G, the worm-like nanostructures changed into nanorods, which facilitates a Förster resonance energy transfer (FRET) from the m -TPEWP5 and G assembled donor to the EsY acceptor present in the nanorod assembly. The system comprising m -TPEWP5, G and EsY displayed moderate FRET efficiency (31%) at a 2:1 molar ratio of donor-to-acceptor. Moreover, the obtained supramolecular nanorod assembly could act as a nanoreactor mimicking natural photosynthesis and exhibited a high catalytic efficiency for the photocatalytic dehalogenation reaction of various bromoketone derivatives with good yields in short reaction time in water.

Role of Functionalized Pillararene Architectures in Supramolecular Catalysis.

The many useful features possessed by pillararenes (PAs; e.g. rigid, capacious, and hydrophobic cavities, as well as exposed functional groups) have led to a tremendous increase in their popularity since their first discovery in 2008. In this Minireview, we emphasize the use of functionalized PAs and their assembled supramolecular materials in the field of catalysis. We aim to provide a fundamental understanding and mechanism of the role PAs play in catalytic process. The topics are subdivided into catalysis promoted by the PA rim/cavity, PA-based nanomaterials, and PA-based polymeric materials. To the best of our knowledge, this is the first overview on PA-based catalysis. This Minireview not only summarizes the fabrications and applications of PAs in catalysis but also anticipates future research efforts in applying supramolecular hosts in catalysis.

Insight into functionalized-macrocycles-guided supramolecular photocatalysis.

Due to the unique characteristics of macrocycles (e.g., the ease of modification, hydrophobic cavities, and specific guest recognition), they can provide a suitable environment to realize photocatalysis via noncovalent interactions with different substrates. In this minireview, we emphasized the photochemical transformation and catalytic reactivity of different guests based on the binding with various macrocyclic hosts as well as on the role of macrocyclic-hosts-assisted hybrid materials in energy transfer. To keep the clarity of this review, the macrocycles are categorized into the most commonly used supramolecular hosts, including crown ethers, cyclodextrins, cucurbiturils, calixarenes, and pillararenes. This minireview not only summarizes the role that macrocycles play in photocatalytic reactions but also clarifies the photocatalytic mechanisms. Finally, the future research efforts and new pathways to apply macrocycles and supramolecular hybrid materials in photocatalysis are also discussed.

Supramolecular Strategies for Controlling Reactivity within Confined Nanospaces.

Nanospaces are ubiquitous in the realm of biological systems and are of significant interest among supramolecular chemists. Understanding chemical behavior within nanospaces offers new perspectives on biological phenomena in nature and opens the way to highly unusual and selective forms of catalysis. Supramolecular chemistry exploits weak, yet effective, intermolecular interactions such as hydrogen bonding, metal-ligand coordination, and the hydrophobic effect to assemble nano-sized molecular architectures, providing reactions with remarkable rate acceleration, substrate specificity, and product selectivity. In this minireview, the focus is on the strategies that supramolecular chemists use to emulate the efficiency of biological processes, and elucidating how chemical reactivity is efficiently controlled within well-defined nanospaces. Approaches such as orientation and proximity of substrate, transition-state stabilization, and active-site incorporation will be discussed.

Electrostatic Control of Macrocyclization Reactions within Nanospaces.

The intrinsic structural complexity of proteins makes it hard to identify the contributions of each noncovalent interaction behind the remarkable rate accelerations of enzymes. Coulombic forces are evidently primary, but despite developments in artificial nanoreactor design, a picture of the extent to which these can contribute has not been forthcoming. Here we report on two supramolecular capsules that possess structurally identical inner-spaces that differ in the electrostatic potential (EP) field that envelops them: one positive and one negative. This architecture means that only changes in the EP field influence the chemical properties of encapsulated species. We quantify these influences via acidity and rates of cyclization measurements for encapsulated guests, and we confirm the primary role of Coulombic forces with a simple mathematical model approximating the capsules as Born spheres within a continuum dielectric. These results reveal the reaction rate accelerations possible under Coulombic control and highlight important design criteria for nanoreactors.

Mapping the Binding Motifs of Deprotonated Monounsaturated Fatty Acids and Their Corresponding Methyl Esters within Supramolecular Capsules.

A suite of NMR techniques revealed that a cavitand (1) formed 2:1 host-guest complexes with a range of monounsaturated fatty carboxylates and their corresponding methyl esters. All of the carboxylates bound to the capsule in a J-shaped motif with the carboxylate at the equatorial region of the dimeric capsule, and the reverse turn of the chain and the methyl terminal in each polar region of the host. Guest exchange was slow on the NMR time scale, while tumbling was slow or close to the NMR time scale depending on the position and stereochemistry of the double bond. In contrast, the methyl esters were found to bind in three motifs depending on the position and stereochemistry of the double bond. Thus, the esters were observed to bind in a J-shaped, U-shaped (the turn in the guest occupying a polar region and the two termini competing for occupancy of the other pole), or a reverse J-shaped motif (ester moiety and turn each occupying a pole and the methyl terminal located near the equator). Relative binding constant (Krel) determinations revealed that the affinity for the capsule was dependent on the position and stereochemistry of the double bond.

ITC and NMR Analysis of the Encapsulation of Fatty Acids within a Water-Soluble Cavitand and its Dimeric Capsule.

We report here NMR and ITC studies of the binding of ionizable guests (carboxylate acids) to a deep-cavity cavitand. These studies reveal that the shortest guests favored 1:1 complex formation, but the longer the alkyl chain the more the 2:1 host-guest capsule is favored. For intermediate-sized guests, the equilibrium between these two states is controlled by pH; at low values the capsule containing the carboxylic acid guest is favored, whereas as the pH is raised deprotonation of the guest favors the 1:1 complex. Interestingly, for one host-guest pair the energy required to de-cap the 2:1 capsular complex and form the 1:1 complex is sufficient to shift the pKa of the guest by ~ 3-4 orders of magnitude (4.1-5.4 kcal mol-1). The two largest guests examined form stable 2:1 capsules, with in both cases the guest adopting a relatively high energy J-shaped motif. Furthermore, these 2:1 complexes are sufficiently stable that at high pH guest deprotonation occurs without de-capping of the capsule.

TPE-embedded butterfly bis-crown ether with controllable conformation and supramolecular chiroptical property.

Understanding how subtle structural differences between macrocyclic conformational isomers impact their properties and separation has garnered increasing attention in the field of supramolecular synthetic chemistry. In this work, a series of tetraphenylene (TPE)-embedded butterfly bis-crown ether macrocycles (BCE[n], n = 4-7), comprising two crown ether side rings and a TPE core, are synthesized through intramolecular McMurry coupling. Unexpectedly, the presence of flexible oligoethylene chains with varying lengths are found to influence molecular conformation via multiple intramolecular interactions, resulting in the formation of two stabilized conformers with specific semi-rigid symmetric/asymmetric structures (sym-BCE[n] and asym-BCE[n], n = 5, 6). Moreover, it is noteworthy that neither symmetric nor asymmetric conformers are present in the more rigid BCE[4] or the more flexible BCE[7]. Interestingly, these conformers display distinct fluorescence properties and host-guest binding abilities, and only sym-BCE[5] can serve as a host for chiral polymer binding, resulting in the formation of chiral supramolecular assemblies through host-guest interaction induced chirality. Moreover, both circular dichroism and circularly polarized luminescence signals of the obtained assemblies can be switched off by the addition of sodium ion, suggesting potential applications in the field of dynamic chiral materials.

A cavitand-based supramolecular artificial light-harvesting system with sequential energy transfer for photocatalysis.

A novel artificial light-harvesting system, featuring sequential energy transfer processes, has been successfully constructed, which demonstrated white light emission through a precise adjustment of the donor-acceptor ratio. To better mimic natural photosynthesis, the system is employed as a nanoreactor for the photocatalysis of a cross-dehydrogenative coupling (CDC) reaction in aqueous solution.

Dimeric Pillar[5]arene as a Novel Fluorescent Host for Controllable Fabrication of Supramolecular Assemblies and Their Photocatalytic Applications.

A dimeric fluorescent macrocycle m-TPE Di-EtP5 (meso-tetraphenylethylene dimeric ethoxypillar[5]arene) is synthesized based on the meso-functionalized ethoxy pillar[5]arene. Through the connectivity of two pillar[5]arenes by CC double bond, the central tetraphenylethylene (TPE) moiety is simultaneously formed. The resultant bicyclic molecule not only retains the host-guest properties of pillararenes but also introduces the interesting aggregation-induced emission properties inherent in the embedded TPE structure. Three dinitrile derivatives with various linkers are designed as guests (G1, G2, and G3) to form host-guest assemblies with m-TPE Di-EtP5. The morphological control and fluorescence properties of the assemblies are successfully realized. G1 with a shorter alkyl chain as the linker completely threads into the cavities of the host. G2, due to its longer chain length, forms a linear supramolecular polymer upon binding to m-TPE Di-EtP5. G3 differs from G2 by possessing a bulky phenyl group in the middle of the chain, which can be further assembled with m-TPE Di-EtP5 to form supramolecular layered polymer and precipitated out in solution, and can be efficiently applied to photocatalytic reactions.

Orthogonal Design of Supramolecular Prodrug Vesicles via Water-Soluble Pillar[5]arene and Betulinic Acid Derivative for Dual Chemotherapy.

Supramolecular prodrug vesicles with efficient property for dual chemotherapy have been successfully constructed based on the orthogonal self-assembly between a water-soluble pillar[5]arene host (WP5) and a betulinic acid guest (BA-D) as well as doxorubicin (DOX). Under the acidic microenvironment of cancer cells, both the encapsulated anticancer drug DOX and prodrug BA-D can be effectively released from DOX-loaded WP5⊃BA-D prodrug vesicles for combinational chemotherapy. Furthermore, bioexperiments indicate that DOX-loaded prodrug vesicles can obviously enhance the anticancer efficiency based on the cooperative effect of DOX and BA-D, while remarkably reducing the systematic toxicity in tumor-mice, displaying great potential applications in combinational chemotherapy for cancer treatments.

Tetraphenylethylene-embedded [15]paracyclophanes: AIEgen and macrocycle merged novel supramolecular hosts used for sensing Ni2+ ions.

Transformation of [15]paracyclophanes ([15]PCP) into fluorophores has been achieved by embedding tetraphenylethene (TPE) units into their skeletons at the meso-positions. The obtained two hosts demonstrated distinct aggregation-induced emission (AIE) properties and their fluorescence could be selectively quenched by Ni2+ ions.

Artificial light-harvesting systems based on macrocycle-assisted supramolecular assembly in aqueous media.

Light-harvesting, which involves the conversion of sunlight into chemical energy by natural systems such as plants, bacteria, is one of the most universal routine activities in nature. Thus far, various artificial light-harvesting systems (LHSs) have been fabricated toward solar energy utilization through mimicking natural photosynthesis in simplified and altered ways. Macrocycles are supramolecular hosts with unique cavities, in which specific guest molecules can be recognized based on non-covalent interactions. They have been widely employed in constructing LHSs due to their ability to form supramolecular assembly and dynamic molecular activity. In this review, we mainly focus on some representative examples reported by our group and other groups. Specifically, the fabrication of LHSs and their related discussions, such as a high donor/acceptor ratio, driving force for the formation of supramolecular assemblies and energy transfer mechanisms using different water-soluble macrocycles such as cyclodextrins (CD), pillararenes (PA), calixarenes (CA), cucurbiturils (CB), and other macrocycles will be included. In addition, how the resulting supramolecular self-assembled LHSs could be potentially utilized for photocatalysis, sensing, and imaging is also explained in detail. Challenges and developing trends for photochemical solar energy conversion will also be presented.

Influence of water-soluble pillararene hosts on Kemp elimination.

Since pillar[5]arene was first discovered in 2008, it has developed into a multifunctional supramolecular host. Its application covers many fields from drug delivery and chemical sensing to the construction of molecular machines, and so on. Supramolecular catalysis based on pillar[n]arenes is one of the hot research topics that has emerged in recent years. In this paper, we have synthesized two water-soluble pillar[5]arenes with peripheral rims bearing opposite charges and investigated their influence on Kemp elimination reaction of 1,2-phenylisoxazole derivatives. It is found that both hosts have a moderate rate acceleration effect on the reaction, and the positively charged host H1 has a greater impact on the reaction rate than the negatively charged host H2.

Orthogonal Design of a Water-Soluble meso-Tetraphenylethene-Functionalized Pillar[5]arene with Aggregation-Induced Emission Property and Its Therapeutic Application.

An orthogonal strategy was utilized for synthesizing a novel water-soluble pillar[5]arene (m-TPEWP5) with tetraphenylethene-functionalized on the bridged methylene group (meso-position) of the pillararene skeleton. The obtained macrocycle exhibit both the aggregation-induced emission (AIE) effect and interesting host-guest property. Moreover, it can be made to bind with a tailor-made camptothecin-based prodrug guest (DNS-G) to form AIE-nanoparticles based on host-guest interaction and the fluorescence resonance energy transfer process for fabricating a drug delivery system. This novel type of water-soluble AIE-active macrocycle can serve as a potential fluorescent material for cancer diagnosis and therapy. In addition, the present orthogonal strategy for designing meso-functionalized aromatic macrocycles may pave a new avenue for creating novel supramolecular structures and functional materials.

Molecular protection of fatty acid methyl esters within a supramolecular capsule.

We describe the use of a supramolecular nano-capsule for selective protection of cis- and trans-C18 mono-unsaturated fatty-acid esters. In contrast to earlier studies revealing that protection of smaller esters is dictated by affinity, protection of these larger esters was found to be dependent on the packing motif of the guest.