Host Guest Recognition Motif Research Articles

Constructing a solid-state supramolecular polymer based on host-guest recognition between perethylated pillar[5]arene and tetrathiafulvalene.

Herein we present a novel linear supramolecular polymeric structure formed in both the solution and solid state, utilizing the host-guest recognition motif between perethylated pillar[5]arene and tetrathiafulvalene.

Highly Strong and Tough Supramolecular Polymer Networks Enabled by Cryptand-Based Host-Guest Recognition.

Supramolecular polymer networks (SPNs) demonstrate great potential in the development of smart materials owing to their attractive dynamic properties. However, as they suffer from the inherent weak bonding of most noncovalent cross-links, it remains a significant challenge to construct SPNs with outstanding mechanical performance. Herein, we exploit the cryptand/paraquat host-guest recognition motifs as cross-links to prepare a class of highly strong and tough SPNs. Unlike those supramolecular cross-links with relatively weak binding abilities, the cryptand-based host-guest interactions have a high association constant and steady complexing structure, which effectively stabilizes the network and resists mechanical deformation under external force. Such favorable structural stability endows our SPNs with greatly enhanced mechanical performance, compared with the control-1 cross-linked by the weakly complexed crown ether/secondary ammonium salt motif (tensile strength: 21.1±0.5 vs 2.8±0.1 MPa; Young's modulus: 102.6±4.8 vs 2.1±0.3 MPa; toughness: 90.4±2.0 vs 10.8±0.6 MJ m-3 ). Moreover, our SPNs also retain abundant dynamic properties including good abilities in energy dissipation, reprocessability, and stimuli-responsiveness. These findings provide novel insights into the preparation of SPNs with enhanced mechanical properties, and promote the development of high-performance intelligent supramolecular materials.

A General Picture of Cucurbit[8]uril Host–Guest Binding: Recalibrating Bonded Interactions

Atomic-level understanding of the dynamic feature of host-guest interactions remains a central challenge in supramolecular chemistry. The remarkable guest binding behavior of the Cucurbiturils family of supramolecular containers makes them promising drug carriers. Among Cucurbit[n]urils, Cucurbit[8]uril (CB8) has an intermediate portal size and cavity volume. It can exploit almost all host-guest recognition motifs formed by this host family. In our previous work, an extensive computational investigation of the binding of seven commonly abused and structurally diverse drugs to the CB8 host was performed, and a general dynamic binding picture of CB8-guest interactions was obtained. Further, two widely used fixed-charge models for drug-like molecules were investigated and compared in great detail, aiming at providing guidelines in choosing an appropriate charge scheme in host-guest modelling. Iterative refitting of atomic charges leads to improved binding thermodynamics and the best root-mean-squared deviation from the experimental reference is 2.6 kcal/mol. In this work, we focus on a thorough evaluation of the remaining parts of classical force fields, i.e., the bonded interactions. The widely used general Amber force fields are assessed and refitted with generalized force-matching to improve the intra-molecular conformational preference, and thus the description of inter-molecular host-guest interactions. The interaction pattern and binding thermodynamics show a significant dependence on the modelling parameters. The refitted system-specific parameter set improves the consistency of the modelling results and the experimental reference significantly. Finally, combining the previous charge-scheme comparison and the current force-field refitting, we provide general guidelines for the theoretical modelling of host-guest binding.

Multicolor Iridium(III) Complexes with Host-Guest Recognition Motifs for Enhanced Electrochemiluminescence and Modular Labeling.

Cyclometalated Ir(III) complexes with high electrochemiluminescence (ECL) efficiency and appropriate bioconjugation sites are urgently needed in ECL immunoassays (ECLIA). Herein, we report the synthesis, photophysics, electrochemistry, and ECL of six new Ir(III) complexes bearing naphthyl (nap) or adamantane phenyl (adap) substitutions, four of which emit cyan, green, or red light and display 1.7- to 7.5-fold increases in ECL intensity. In combination with DFT/TDDFT calculations, this enhancement is rationalized to the augmented radiative rate that arises from both the strengthened spin-orbit coupling (SOC) and the increased transition dipole moment. In addition, the adap-based Ir(III) complex shows high binding affinity with β-cyclodextrin (β-CD) due to the strong hydrophobic interaction, which enables us to develop a modular strategy for the labeling of Ir(III) complexes with biomolecules and to use hydrophobic luminophores in the aqueous-phase detection. As demonstrated, a novel ECLIA is built up and exhibits a wide linear range from 1 ng/mL to 10 μg/mL and a detection limit of 72 pg/mL for the determination of C-reactive protein (CRP). These findings provide new insights into the design, synthesis, and bio-labeling of highly emissive Ir(III) complexes and pave the way for the development of novel ECLIA based on host-guest recognition motifs.

Porphyrin-Based Multicomponent Metallacage: Host-Guest Complexation toward Photooxidation-Triggered Reversible Encapsulation and Release.

The development of supramolecular hosts with effective host–guest properties is crucial for their applications. Herein, we report the preparation of a porphyrin-based metallacage, which serves as a host for a series of polycyclic aromatic hydrocarbons (PAHs). The association constant between the metallacage and coronene reaches 2.37 × 107 M–1 in acetonitrile/chloroform (ν/ν = 9/1), which is among the highest values in metallacage-based host–guest complexes. Moreover, the metallacage exhibits good singlet oxygen generation capacity, which can be further used to oxidize encapsulated anthracene derivatives into anthracene endoperoxides, leading to the release of guests. By employing 10-phenyl-9-(2-phenylethynyl)anthracene whose endoperoxide can be converted back by heating as the guest, a reversible controlled release system is constructed. This study not only gives a type of porphyrin-based metallacage that shows desired host–guest interactions with PAHs but also offers a photooxidation-responsive host–guest recognition motif, which will guide future design and applications of metallacages for stimuli-responsive materials.

Crown Ether-Based Supramolecular Polymers: From Synthesis to Self-Assembly.

Supramolecular polymers not only possess many advantages of traditional polymers, but also have many unique characteristics. Supramolecular polymers can be constructed by self-assembly of various noncovalent interactions. Host-guest interaction, as one important type of noncovalent interactions, has been widely applied to construct supramolecular polymers. From the perspective of classification of the recognition system motifs, host-guest recognition motifs mainly include crown ether, cyclodextrin, calixarene, cucurbituril, and pillararene-based host-guest recognition pairs. Crown ethers, as the first-generation macrocyclic hosts, have played a very important part in the development of supramolecular chemistry. Due to the easy modification of crown ethers, various crown ether derivatives have been prepared by attaching some functional groups to the edges of crown ethers, which endowed them with some interesting properties and made them ideal candidates for the fabrication of supramolecular polymers. This review gives a review of the preparation of crown ether-based supramolecular polymers (CSPs) and summarizes crown ether-based recognition pairs, organization methods, topological structures, stimuli-responsiveness, and functional characteristics.

Molecular Entrapment of Polymers by Pyrogallol[4]arenes.

Macromolecular recognition systems are difficult to construct because extremely high recognition ability is required to form a stable host-guest complex toward macromolecules. Herein, we report a novel host-guest recognition motif based on C-propylpyrogallol[4]arene (PgC3) and a commercially available polymer, polyethylene glycol (PEG). The results show that PgC3 can selectively entrap PEG with higher molecular weights to form bilayered host-guest complex structures. Interestingly, this host-guest recognition is strong enough that PgC3 is able to adsorb PEG from an aqueous solution efficiently.

Cover Feature: Counter Cation Dependent and Stimuli Responsive Supramolecular Polymers Constructed by Calix[4]pyrrole Based Host–Guest Interactions (Eur. J. Org. Chem. 4/2019)

The Cover Feature shows the counter cation dependent and pH-responsive supramolecular polymers based on the host–guest recognition motif of a well-known macrocycle calix[4]pyrrole and its tethered carboxylate unit. Unlike its congener tetrabutylammonium cation, when cetyltrimethylammonium (CTA) was used as the counter cation, they were found to be a direct part of supramolecular polymer via cation-π interactions between cone-shaped calix[4]pyrroles and CTA. This allowed supramolecular polymers to establish further hydrophobic interactions and to form gels at elevated concentrations. More information can be found in the Full Paper by A. Aydogan et al.

Photolysis of a calixpyridinium-based supramolecular amphiphilic assembly and its selective turn-on fluorescence recognition of lysine in water.

A new calixpyridinium-based light-responsive host-guest recognition motif was found in this work. This host-guest recognition motif was further discovered to be applied as a selective turn-on fluorescent sensor for lysine over other natural amino acids.

Counter Cation Dependent and Stimuli Responsive Supramolecular Polymers Constructed by Calix[4]pyrrole Based Host–Guest Interactions

Carboxylate tethered calix[4]pyrroles in the form of their tetrabutylammonium (TBA) and cetyltrimethylammonium (CTA) salts were synthesized via an acid base reaction between the corresponding carboxylic acid functionalized calix[4]pyrrole and tetrabutyl‐ or cetyltrimethylammonium hydroxide. The host–guest recognition motif based on these calix[4]pyrrole macrocycles and tethered carboxylate units was employed to construct novel A–B type supramolecular polymers from low molecular weight monomers in chloroform. These supramolecular polymers were found to be dependent on the counter cation in terms of the properties of supramolecular polymers. 1H‐, DOSY‐, NOESY‐NMR spectroscopic analyses, viscosity measurements, and SEM results were used to characterize supramolecular polymers and revealed that the CTA cation becomes a direct part of supramolecular polymer via cation–π interactions between the ammonium part of CTA and carboxylate‐bound calix[4]pyrrole, effecting the overall properties of supramolecular polymers such as viscosity, gel formation, and drawing fibers. Additionally, reversible pH‐ and thermo‐responsiveness of the supramolecular polymers were demonstrated for the first time for an anion recognition induced calix[4]pyrrole based supramolecular polymer.

Clip[5]arenes: A new family of molecular clips

Here we report the design and syntheses of two new triptycene-based rigid acyclic C-shaped hosts, clip[5]arenes C[5]OH and C[5]ME, and the strong host–guest complexation between C[5]OH and an electron-poor bipyridinium salt, paraquat G. The Ka value for the host–guest complex C[5]OH ⊃ G was calculated to be (1.09 ± 0.36) × 105 M−1 in acetone by using a non-linear curve-fitting method based on the UV–vis absorption titration experiments. Furthermore, based on this new host–guest recognition motif, a novel pseudopolyrotaxane-like supramolecular structure was constructed with C[5]OH threaded on polyviologen polymer VP-10.

Acid/Base-Controllable FRET and Self-Assembling Systems Fabricated by Rhodamine B Functionalized Pillar[5]arene-Based Host-Guest Recognition Motifs.

A novel supramolecular Föster resonance energy transfer (FRET) system was fabricated by utilizing rhodamine B (RB) functionalized pillar[5]arene (EtP5-RB) and cyano-modified boron dipyrromethene (BDP-CN) based on their host-guest recognition at 5.0 × 10-5 M, which could be turned "on" and "off" by adding trifluoroacetic acid (TFA) and triethylamine (TEA), respectively. At a higher concentration (1.0 × 10-4 M) in acetone, EtP5-RB self-assembled into vesicles while EtP5-RBH self-assembled into nanoribbons. After the addition of BDP-CN, both EtP5-RB⊃BDP-CN and EtP5-RBH⊃BDP-CN self-assembled into nanoparticles, which caused the fluorescence of the host-guest complexes to be quenched.

Barium cation-responsive supra-amphiphile constructed by a new twisted cucurbit[15]uril/paraquat recognition motif in water

A new, strong, and barium cation-responsive host–guest recognition motif based on twisted cucurbit[15]uril and paraquat was established in water.

A triply-responsive supramolecular amphiphile fabricated by a thermal-responsive pillar[5]arene-based host–guest recognition motif

A novel molecular recognition motif was built between a neutral water soluble pillar[5]arene and decyltrimethylammonium bromide in water. Its thermal-controlled complexation with G1 in water was investigated. Furthermore, based on this new thermal responsive host–guest recognition motif, we further constructed a supramolecular amphiphile between this pillar[5]arene and a trimethylammonium bromide derivative containing an azobenzene group at the other end. This supramolecular amphiphile showed triply-responsiveness, that is, thermal responsiveness of the host–guest complex, photo-responsiveness of the azobenzene group and chemical-responsiveness by adding β-CD.

Tetranitro-oxacalix[4]crown-Based Host-Guest Recognition Motif and a Related [2]Rotaxane-Based Molecular Switch.

Different from so far reported oxacalix[4]crown-based host-guest motifs in which oxacalix[4]crowns act only as hydrogen bond acceptors, a [2]pseudorotaxane-type tetranitro-oxacalix[4]crown/urea host-guest recognition motif was developed in which tetranitro-oxacalix[4]crown played a role as both a hydrogen bond donor and an acceptor to stabilize the resulting supramolecular complex. Furthermore, on the basis of a [2]pseudorotaxane complex formed from a tetranitro-oxacalix[4]crown and an axle containing a secondary ammonium ion and a urea group, a [2]rotaxane-based molecular switch was created, in which the oxacalix[4]crown wheel was able to reversibly translocate between the secondary ammonium binding site and the urea binding site of the axle under acid-base stimulation.

A water-soluble hybrid[4]arene: synthesis, host–guest complexation and application in the construction of a supra-amphiphile

A water-soluble hybrid[4]arene was synthesized for the first time and its pH-responsive host–guest complexation with paraquat in water was investigated. This new host–guest recognition motif was further applied in the construction of a supra-amphiphile.

Pillar[10]arene-based host–guest complexation promoted self-assembly: from nanoparticles to uniform giant vesicles

A novel host-guest recognition motif between a water-soluble pillar[10]arene and pyrene derivative was established and further applied in the fabrication of a pH-responsive supra-amphiphile.

Construction of pillar[6]arene-based CO2 and UV dual-responsive supra-amphiphile and application in controlled self-assembly.

A gas and photo dual-responsive host-guest recognition motif was established based on a water-soluble pillar[6]arene and an azobenzene derivative in an aqueous medium. This smart inclusion complex was further applied for the construction of a supra-amphiphile, which could reversibly self-assemble into vesicles upon UV/vis irradiation or bubbling with N2/CO2.

An Ag2O-responsive [2]pseudorotaxane based on the pillar[5]arene/bis(imidazolium) dication molecular recognition motif

A pillar[5]arene-based [2]pseudorotaxane was successfully synthesized. The damage and re-formation of it could be controlled by adding Ag2O and CF3COOH, respectively. The special metalloresponsiveness makes this novel host–guest recognition motif have broad potential applications in a number of areas.

Engineering responsive polymer building blocks with host-guest molecular recognition for functional applications.

CONSPECTUS: All living organisms and soft matter are intrinsically responsive and adaptive to external stimuli. Inspired by this fact, tremendous effort aiming to emulate subtle responsive features exhibited by nature has spurred the invention of a diverse range of responsive polymeric materials. Conventional stimuli-responsive polymers are constructed via covalent bonds and can undergo reversible or irreversible changes in chemical structures, physicochemical properties, or both in response to a variety of external stimuli. They have been imparted with a variety of emerging applications including drug and gene delivery, optical sensing and imaging, diagnostics and therapies, smart coatings and textiles, and tissue engineering. On the other hand, in comparison with molecular chemistry held by covalent bonds, supramolecular chemistry built on weak and reversible noncovalent interactions has emerged as a powerful and versatile strategy for materials fabrication due to its facile accessibility, extraordinary reversibility and adaptivity, and potent applications in diverse fields. Typically involving more than one type of noncovalent interactions (e.g., hydrogen bonding, metal coordination, hydrophobic association, electrostatic interactions, van der Waals forces, and π-π stacking), host-guest recognition refers to the formation of supramolecular inclusion complexes between two or more entities connected together in a highly controlled and cooperative manner. The inherently reversible and adaptive nature of host-guest molecular recognition chemistry, stemming from multiple noncovalent interactions, has opened up a new platform to construct novel types of stimuli-responsive materials. The introduction of host-guest chemistry not only enriches the realm of responsive materials but also confers them with promising new applications. Most intriguingly, the integration of responsive polymer building blocks with host-guest recognition motifs will endow the former with further broadened responsiveness to external stimuli and accordingly more sophisticated functions. In this Account, we summarize recent progress in the field of responsive polymeric materials containing host-guest recognition motifs with selected examples and highlight their versatile functional applications, whereas small molecule-oriented host-guest supramolecular systems are excluded. We demonstrate how the introduction of host-guest chemistry into conventional polymer systems can modulate their responsive modes to external stimuli. Moreover, the responsive specificity and selectivity of polymeric systems can also be inherited from the host-guest recognition motifs, and these features provide extra advantages in terms of function integration. The following discussions are categorized in terms of design and functions, namely, host-guest chemistry toward the fabrication of responsive polymers and assemblies, optical sensing and imaging, drug and gene delivery, and self-healing materials. A concluding remark on future developments is also presented. We wish this prosperous field would incur more original and evolutionary ideas and benefit fundamental research and our daily life in a more convenient way.