Host-guest Assembly Research Articles

The construction of an AIE-based controllable singlet oxygen generation system directed by a supramolecular strategy.

An aggregation-induced emission (AIE) based smart singlet oxygen (1O2) generation system has been successfully fabricated based on supramolecular host-guest assembly. The controllable 1O2 generation can be achieved by conveniently changing the molar ratio between the macrocyclic host (WP5) and the guest molecule (TPEPY). Moreover, reversible control of 1O2 generation and fluorescence emission of supramolecular nanoassemblies can be achieved via adding Fe3+ and EDTA, which allows qualitatively monitoring the singlet oxygen generation efficiency by the naked eye.

Atomic Decomposition Scheme of Noncovalent Interactions Applied to Host-Guest Assemblies.

The design of novel stimuli-responsive supramolecular systems based on host-guest chemistry implies a thorough understanding of the noncovalent interactions involved. In this regard, some computational tools enabling the extraction of the noncovalent signatures from local descriptors based on the electron density have been previously proposed. Although very useful to detect the existence of such interactions, these analyses provide only a semi-quantitative description, which represents a limitation. In this work, we present a novel computational tool based on the local atomic descriptor IGM-δginter/At, which is able to decompose the fragment interaction into atomic contributions. Then, the role played by each atom in the formation of the host-guest assembly is quantified by an integrated Δginter/At score. Herein, we apply the IGM-Δginter/At approach to some challenging systems, including multimetallic arrays, buckycatchers, and organic assemblies. These systems exhibit unique structural features that make it difficult to determine the host/guest atoms that contribute the most to the guest encapsulation. Here, the Δginter/At score proves to be an appealing tool to shed light on the guest accommodation on a per-atom basis and could be useful in the rational design of more selective target agents. We strongly believe that this novel approach will be useful for experimental teams devoted to the synthesis of supramolecular systems based on host-guest chemistry.

A corneal-penetrating eye drop formulation for enhanced therapeutic efficacy of soft corticosteroids against anterior uveitis

Eye drop administration suffers from low bioavailability due to inadequate preocular retention and tissue penetration, which leads to multiple administration to achieve ocular therapeutic concentration. We tested the hypothesis that a cluster bomb releasing corneal-penetrating bomblet of drug assembles would increases drug bioavailability and reduce the administration frequency of corticosteroids in uveitis treatment. The microparticulate drug delivery system was obtained from host-guest assembly of (2-hydroxypropyl)-γ-cyclodextrin with loteprednol etabonate, a hydrophilic polymer, hydroxypropyl methylcellulose, and a stabilizer, caffeine, and polysorbate 80. The pharmacokinetics and pharmacodynamics studies of the proposed microparticles were performed on normal and on experimental uveitis model of New Zealand White rabbits, respectively. In comparison to the commercial microparticulate formulation Lotemax®, our formulation showed 5.4 and 3.5-fold increase of maximum drug concentration in aqueous humor and iris/ciliary body (target tissue of anterior uveitis) in vivo, respectively, while the intraocular pressure remains normal. Along with the enhanced drug exposure in target tissue, the drug dosing frequency can be reduced into half with a comparable outcome to the typical steroid treatment against autoimmune-induced uveitis by using our eye drop formulation. The microparticulate drug delivery system was developed to provide a potential solution to increase drug bioavailability in ocular tissues and with which the administration frequency could be reduced. Our results suggest that these goals could be successfully achieved by using the proposed eye drop formulation.

Stimulus-Responsive Supramolecular Host-Guest Assembly of a Cationic Pyrene Derivative with Sulfated β-Cyclodextrin.

In general, aggregation-prone organic molecules are prevented from self-aggregation in the presence of macrocyclic hosts like β-cyclodextrin because of their preference for the formation of inclusion complex with guest molecules. On the contrary, sulfate-laced β-cyclodextrin has been recently reported to induce the aggregation of some of the non-aggregation-prone organic dyes, which have been subsequently utilized for biosensing applications. In the present contribution, we report the interaction of a cationic organic probe molecule, 1-pyrene methyl amine (PMA), which belongs to one of the most useful families of organic fluorescent probes, that is, pyrene, with a sulfated β-cyclodextrin derivative (SCD). Interaction of a cationic probe with a β-cyclodextrin derivative was studied using a variety of photophysical methods such as ground-state absorption, steady-state emission, and time-resolved emission techniques. Detailed photophysical investigations have revealed that SCD induces the ground-state association of PMA molecules. This SCD-induced aggregation of PMA molecules has been attributed to the charge neutralization of the cationic probe by negatively charged sulfate groups, which subsequently lead to their association because of the close proximity on the rims of cyclodextrin. This monomer-dimer equilibrium of the PMA-SCD system is found to be extremely responsive to external chemical stimuli like temperature, pH, ionic strength of the medium, and organic solvent (dimethyl sulfoxide), which projects them as potential platforms for various sensing applications including bioanalytes. The supramolecular assembly has been demonstrated to sense arginine.

Lighting Up the Gold Nanoclusters via Host-Guest Recognition for High-Efficiency Antibacterial Performance and Imaging.

Au nanoclusters (Au NCs) with a unique size effect on the antibacterial performance provide a promising nanoprobe for developing an efficient nanomedicine. However, little progress has been made owing to the low quantum yield and poor stability of Au NCs. In this work, protamine (Prot) functionalized Au NCs (Prot/MTU-Au NCs) with high stability were achieved through a simple mixing with 6-methyl-2-thiouracil-capped Au NCs (MTU-Au NCs) due to the hydrogen bonding between 5-methyl-2-thiouracil (MTU) and the guanidine groups from Prot. Interestingly, a distinctly enhanced photoluminescence from Prot/MTU-Au NCs (ca. 28-fold) was observed due to the formation of rigid host-guest assemblies. We inferred that the cross-linked structure and supramolecular hydrogen bonds both contributed to the fluorescence enhancement and stability. The extra small size of the NCs and the efficient antibacterial capability from the capping shell of Prot encouraged us to probe its antibacterial performance systemically. It was found that the Prot/MTU-Au NCs with highly stable loading of positively charged antibacterial reagents were likely to penetrate into the bacteria and thus enhance the ability to kill both Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (multiple-resistant Staphylococcus aureus). The synergetic effect between the unique size and the capping layers enabled the minimal inhibitory concentration of the as-derived Prot/MTU-Au NCs reduced by ∼100-fold compared to that with individual Au nanoparticle. The antibacterial mechanism further revealed that membrane injury occurred and reactive oxygen species were generated after the incubation of the bacteria with Prot/MTU-Au NCs. Moreover, the highly luminescent fluorescence and positive surface charge of Prot/MTU-Au NCs could image the bacteria easily, which held great potential for imaging-guided antibacterial platform.

Controlling the Photophysics of Aromatic Guests Using a Cyclic Porphyrin Dimer: Synthesis, Structure, and Encapsulation‐Mediated “ON‐OFF” Switch

A cyclic zinc(II) porphyrin dimer with flexible linkers has been employed as an artificial molecular container that can efficiently encapsulate aromatic guests (pyrene/anthracene) through π–π interactions. Such molecular entrapment inside the container completely alters the photophysical pathways of the encapsulated guests that are normally favored in solution. As a consequence, the exciplex emission (in case of pyrene), and photodimerization (in case of anthracene) properties are suppressed inside the container. The structural integrity and compactness of the host–guest assemblies are the reason that prevents the guest molecules to come out from the container, thus switching “OFF” their favored solution pathways. The effects of such supramolecular encapsulation have been extensively investigated using UV/Visible, 1H NMR, fluorescence, and X‐ray structure determination of the host–guest complexes. The high association constant for the complexes manifests the robust nature of the host–guest assemblies in solution. Moreover, the exciplex emission and photodimerization properties of the guests can be recovered by the addition of bidentate N‐donor ligand (1,4‐diazabicyclo[2.2.2]octane, DABCO), which immediately replaces the encapsulated guests by coordinating with the metal centers of the host. Such controllable and dynamic features of the bisporphyrinic container will further enhance the choice of incoming guest molecules and extend the range of various applications in supramolecular host–guest chemistry.

Host–guest assembly of H33258 dye in cucurbit[7]uril and specific recognition for HPO42− anion

In the present work, a well-known host–guest inclusion complex comprising a cucurbit[7]uril (Q[7]) and an important biological dye, Hoechst-33258 (H33258) was selected as a fluorescence probe. In contrast to the pH conditions in which H33258 is exhibited in the literature, these being, mono- or di-protonated states, in the present work, we selected an extreme case, whereby H33258 was displayed in an anionic state (pH = 11). The investigation of the host–guest interaction demonstrated similar inclusion modes in a wide pH range, that is the dumbbell-like inclusion complex in which one Q[7] includes piperazine moiety and another Q[7] includes the phenol moiety of H33258. Moreover, investigation on the binding capacity of the 15 selected anions under three representative pH levels (pH = 2, pH = 4.5 and pH = 11, respectively) indicated that the H33258@Q[7] probe displayed unique selectivity on HPO42− anion in medium at pH = 11. More importantly, the probe could open a new platform in Q[n]-chemistry, that is the Q[n]-based host–guest interaction systems can apply in anion recognition or response.

Bioinspired γ-Cyclodextrin Pseudorotaxane Assembly Nanochannel for Selective Amino Acid Transport.

Amino acid metabolic abnormalities will cause serious diseases, which arise from the different biological effect of enantiomers. Therefore, it is necessary to design and construct an artificial chiral amino acid gated transport system, which can effectively recognize and selectively transport chiral amino acids. Herein, based on the host-guest interaction, a switchable chiral discrimination system for amino acid enantiomers is reported, and the biomimetic nanochannel was functionalized with N-(1-naphthyl) ethylenediamine (NEDA), which acts as the guest and γ-CD was used as the host. The artificial nanochannel performs highly chiral selective gating for l-phenylalanine and well regulates the process of amino acid transportation. Moreover, the channel can easily actualize the reversible switching under the influence of a pH change; thus, our biomimetic nanochannel with a host-guest assembly can be widely used in chiral detection and drug delivery.

An injectable thermosensitive photothermal-network hydrogel for near-infrared-triggered drug delivery and synergistic photothermal-chemotherapy

Near-infrared (NIR)-responsive hydrogels have exhibited remarkable advantages in biomedical applications especially for in situ therapeutic delivery, because of their deep-tissue penetration capacity, minimal invasiveness, and high spatiotemporal selectivity. Nevertheless, conventional NIR-responsive nanocomposite hydrogels suffer from the disadvantages of limited photothermal effect and potential leakage of the physically mixed photothermal nanoagents. To overcome these limitations, we herein designed an injectable thermosensitive photothermal-network hydrogel (PNT-gel) through the host-guest self-assembly of a photothermal conjugated polymers and ɑ-cyclodextrin. The conjugated-polymer backbones can directly convert incident light into heat, endowing the PNT-gel with high photothermal conversion efficiency (η = 52.6%) and enhanced photothermal stability. Meanwhile, the mild host-guest assembly enable the shear-thinning injectability, photothermally-driven and reversible gel-sol conversion of the hydrogel. Consequently, the remotely controlled on-demand release of doxorubicin (DOX) was achieved via photothermal-induced gel-sol transition. Because the backbone of the hydrogel absorbs NIR light and mediates the photothermal conversion itself, the PNT-gel demonstrated the advantage of a prolonged retention time and thus permitting repeatable NIR treatment after a one-time intratumoral injection of this hydrogel. Under repeated NIR laser irradiation (0.15 W cm−2), the synergistic photothermal-chemotherapy mediated by the PNT-gel almost completely eradicated 4T1 breast cancer. This work not only presents a multifunctional therapeutic platform integrated with inherent photothermal characteristic and reversible stimuli responsiveness for on-demand delivery and combinatorial photothermal-chemotherapy, but also provides a new strategy for the development of the next-generation of light-modulated intelligent hydrogels. Statement of SignificanceThe conventional NIR-responsive nanocomposite hydrogels suffer from the disadvantages of limited photothermal effect and possible leakage of the physically mixed photothermal nano-components. To overcome these limitations, we hereby fabricated a NIR-responsive themosensitive photothermal-network hydrogel through the supramolecular assembly of conjugated polymer. The conjugated polymeric backbones of the hydrogel directly convert NIR light to heat, endowing the hydrogel with good photothermal effect and long-term photothermal stability. Meanwhile, the dynamic crosslinkages via supramolecular assembly enabled the shear-thinning injectability and reversible gel-sol transition of the hydrogel, facilitating the photothermal-induced drug release. Our strategy demonstrated the efficacy of using conjugated polymer as the backbone of hydrogel for the construction of a new injectable NIR-responsive hydrogel system with enhanced photothermal capabilities and improved therapy outcomes.

Effect of guest structure on amylose-guest inclusion complexation

Amylose-guest inclusion complexes are a type of supramolecular host-guest assembly that can provide protection for and controlled release of guest molecules. The successful and efficient complexation between amylose and guest molecules is governed by factors including: guest structure and chemistry, and process method and parameters. Here we investigated the formation, crystalline structure, and thermal stability of amylose inclusion complexes with a total of ten guest molecules differing in alkyl chain length (C10 and C16), molecular shape (linear vs. branched), and functional groups (alcohol, aldehyde, carboxylic acid, and ester). Their ability to complex with amylose was evaluated using two complexation methods (partitioning from water after heating and partitioning from a DMSO/water solution), and two annealing temperatures (60 and 90 °C). The extent of complexation differed for the two methods, likely due to guest solubility and partitioning behavior in the respective solvent systems. Annealing temperature created inclusion complexes of different structure and dissociation temperature using the water approach. Here we suggest that the so-called “Form I” and “Form II” V-type amylose inclusion complexes differ in their crystal size, crystallinity and arrangement of guest molecules in the helical cavity, rather than being amorphous or crystalline as previously reported. Chain length, molecular shape, and functional groups affected the thermal stability of the inclusion complexes. Shorter chain length, unsaturation, and short branched chains formed inclusion complexes with lower dissociation temperatures. We propose the Form II as a tail-to-tail arrangement of molecules in the helices that leaves the functional groups at the helical openings. Guest compounds that either failed to form complexes from water or formed poor complexes were able to form inclusion complexes with amylose using the DMSO approach, suggesting solubility of the guest, flexibility of the amylose chain, or the partitioning of the guest between the solvent and the helix core affected complexation.

Neutral Heteroleptic Lanthanide Complexes for Unravelling Host-Guest Assemblies in Organic Solvents: The Law of Mass Action Revisited.

The binding of lanthanide containers [Ln(β-diketonate)3dig] [dig = 1-methoxy-2-(2-methoxyethoxy)ethane] to aromatic tridentate N-donor ligands (L) in dichloromethane produces neutral nine-coordinate heteroleptic [LLn(β-diketonate)3] complexes, the equilibrium reaction quotients of which vary with the total concentrations of the reacting partners. This problematic drift prevents the determination of both reliable thermodynamic stability constants and intrinsic host-guest affinities. The classical solution theory assigns this behavior to changes in the activity coefficients of the various partners in nonideal solutions, and a phenomenological approach attempts to quantitatively attribute this effect to some partition of the solvent molecules between bulk-innocent and contact-noninnocent contributors to the chemical potential. This assumption eventually predicts an empirical linear dependence of the equilibrium reaction quotient on the concentration of the formed [LLn(β-diketonate)3] complexes, a trend experimentally supported in this contribution for various ligands L differing in lipophilicity and nuclearity and for lanthanide containers grafted with diverse β-diketonate coligands. Even if the origin of the latter linear dependence is still the subject of debate, this work demonstrates that this approach can be exploited by experimentalists for extracting reliable thermodynamic constants suitable for analyzing and comparing host-guest affinities in organic solvents.

Facile Construction of a Supramolecular Organic Framework Using Naphthyl Viologen Guests and CB[8] Host via Charge-Transfer Complexation.

Herein, we report the synthesis of guest–host systems comprising naphthyl–viologen–naphthyl (Np–Vio–Np) and viologen–naphthyl–viologen (Vio–Np–Vio) guest molecules and their subsequent supramolecular polymerization in the presence of a CB[8] host in water. In addition, the guest complexation of ethyl-terminated trimeric viologen (ETV) with Np–Vio–Np and CB[8] was investigated. As a result of supramolecular interactions, 2D supramolecular organic frameworks with high internal periodicity were constructed. 1H NMR studies clearly demonstrated the formation of a host-stabilized charge-transfer complex via folding back (Np–Vio–Np and Vio–Np–Vio) in the presence of CB[8]. In the case of ETV + Np–Vio–Np + CB[8], a large polymeric network was formed as indicated by the NMR titrations. UV–vis and fluorescence studies clearly confirm the formation of an inter/intra molecular CT complex upon complexation with cucurbit[8]uril. The size obtained using the dynamic light scattering (DLS) method pinpoints the formation of larger supramolecular aggregates in the order of μm through host–guest assembly, which is further complemented by FESEM and TEM. SAXS measurements indicate the formation of a 2D supramolecular polymer/polymer aggregate with long-range order.

Efficient and Monochromatic Electrochemiluminescence of Aqueous-Soluble Au Nanoclusters via Host-Guest Recognition.

Inspired by the enhanced photoluminescence of Au nanoclusters (AuNCs) with a rigid shell, the formation of rigid host-guest assemblies on AuNC surfaces was employed to screen novel electrochemiluminophores with 6-aza-2-thiothymine(ATT)-protected AuNCs (ATT-AuNCs) and l-arginine (ARG) as models for the first time. The rigid host-guest assemblies formed between ARG and ATT on the ATT-AuNC surface enabled aqueous-soluble ARG/ATT-AuNCs with a dramatically enhanced electrochemiluminescence (ECL) compared to ATT-AuNCs. This includes one cathodic ECL process (-1.30 V) and three anodic ECL processes (+0.78, 0.90, and 1.05 V) in a so-called half-scan experiment without a co-reactant, as well as a 70-fold enhanced oxidative-reduction ECL at +0.78 V with tri-n-propylamine as a co-reactant. Importantly, the ECL of the ARG/ATT-AuNCs is highly monochromatic with an emission maximum around 532 nm and a full width at half-maximum of 36 nm, which is of great interest for color-selective ECL assays.

Efficient and Monochromatic Electrochemiluminescence of Aqueous‐Soluble Au Nanoclusters via Host–Guest Recognition

AbstractInspired by the enhanced photoluminescence of Au nanoclusters (AuNCs) with a rigid shell, the formation of rigid host–guest assemblies on AuNC surfaces was employed to screen novel electrochemiluminophores with 6‐aza‐2‐thiothymine(ATT)‐protected AuNCs (ATT‐AuNCs) and l‐arginine (ARG) as models for the first time. The rigid host–guest assemblies formed between ARG and ATT on the ATT‐AuNC surface enabled aqueous‐soluble ARG/ATT‐AuNCs with a dramatically enhanced electrochemiluminescence (ECL) compared to ATT‐AuNCs. This includes one cathodic ECL process (−1.30 V) and three anodic ECL processes (+0.78, 0.90, and 1.05 V) in a so‐called half‐scan experiment without a co‐reactant, as well as a 70‐fold enhanced oxidative‐reduction ECL at +0.78 V with tri‐n‐propylamine as a co‐reactant. Importantly, the ECL of the ARG/ATT‐AuNCs is highly monochromatic with an emission maximum around 532 nm and a full width at half‐maximum of 36 nm, which is of great interest for color‐selective ECL assays.

Hydrogen bond directed molecular recognition in water in a strapped-porphyrin-cyclodextrin assembly

A water soluble, phenanthroline-strapped zinc porphyrin bearing four arylsulfonate groups formed a stable host–guest complex with two per-[Formula: see text]-methylated [Formula: see text]-cyclodextrin cavities. In the host–guest assembly, the zinc porphyrin was capable of binding imidazole within the cavity between the zinc(II) ion and the phenanthroline strap in an aqueous medium. The formation of a hydrogen bond between the imidazole NH and the nitrogen atoms of the phenanthroline was an essential element of the binding event, as shown by comparative binding studies with a non-strapped tetrasulfonated zinc porphyrin and with [Formula: see text]-methylimidazole. This hydrogen bonding in an aqueous medium was possible due to the protected hydrophobic environment created by the cyclodextrins around the phenanthroline strap. This type of binding event may provide a biomimetic approach to study water soluble heme protein models.

Terminal Substituent Induced Differential Anion Coordination and Self-Assembly: Case Study of Flexible Linear Bis-Urea Receptors

For a comprehensive analysis of host–guest binding propensity in their neutral form, three linear flexible bis-urea receptors (L1–L3) with different terminal substituents have been synthesized. It has been established that, with the existence of electron-withdrawing or π-acidic phenyl substituents, they act as a possible system that can proficiently coordinate with anions of diverse dimensions constantly initiated by the size of the countercations. The 3,5-bis(trifluoromethyl)phenyl-derived isomer (L1) can readily form cooperative neutral self-assemblies irrespective of the size the monovalent halides (viz. chloride, bromide, and iodide anions) and noncooperative neutral self-assemblies with planar divalent carbonate anion. The meta isomer L2 captures spherical halides, i.e. chloride and bromide, in an isostructural way, forming a 1:2 host–guest assembly, whereas the isomeric para receptor L3 shows cooperative binding with chloride anions, having coordination number 3. However, due to the greater flexibility and lesser hydrophobicity of receptor L2 and L3 in comparison to receptor L1, successful crystallization of any oxyanion complexes through the meta and para isomers was not successful.

Encapsulation driven conformational changes in n-alkanes inside a hydrogen-bonded supramolecular cavitand assembly

Molecular chaperones are known to assist proteins in achieving their native state conformations by providing them a confined environment through encapsulation and altering their free energy landscapes. Instances of structural rearrangements in molecules confined to narrow nano-dimensional spaces are often encountered in vivo where the intermolecular interactions play a decisive role in determining the molecular conformations. As a simple analogue, we model a system of n-alkanes encapsulated into a supramolecular host cavitand forming a capsular host-guest assembly of volume less than half a cubic nanometer. Here, we provide a detailed insight of process of host-induced conformational changes in guests by exploring the conformational dynamics. Interestingly, the conformational analysis reveals that the host apart from being able to fold some of the guests into coiled conformations also unfolds a few of the guests while rearranging its hydrogen-bonded network.

A New Approach for Improving the Antibacterial and Tumor Cytotoxic Activities of Pipemidic Acid by Including It in Trimethyl-β-cyclodextrin.

Pipemidic acid (HPPA) is a quinolone antibacterial agent used mostly to treat gram-negative infections of the urinary tract, but its therapeutic use is limited because of its low solubility. Thus, to improve drug solubility, natural cyclodextrins (CDs) are used for their ability of including guest molecules within their cavities. The aim of this work was to evaluate the antibacterial activity and the preliminary anticancer activity of HPPA included into Heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) as a possible approach for a new innovative formulation. The inclusion complex of HPPA with TRIMEB was prepared in solid state by the kneading method and confirmed by FT-IR and powered X-ray diffraction. The association in aqueous solutions of pipemidic acid with TRIMEB was investigated by UV-Vis spectroscopy. Job’s plots have been drawn by UV-visible spectroscopy to confirm the 1:1 stoichiometry of the host–guest assembly. The antibacterial activity of HPPA, TRIMEB and of their complex was tested on Escherichia coli, Pseudomonas aeruginosa, and Staphilococcus aureus. The complex was able to increase 47.36% of the median antibacterial activity of the free HPPA against E. coli (IC50 = 249 µM vs. 473 µM). Furthermore, these samples were tested on HepG-2 and MCF-7. After 72 h, the median tumoral cytotoxicity exerted by the complex was increased by 78.08% and 94.27% for HepG-2 and MCF-7 respectively, showing a stronger bioactivity of the complex than the single HAPPA.

Molecular Transmission: Visible and Rate-Controllable Photoreactivity and Synergy of Aggregation-Induced Emission and Host–Guest Assembly

The ability to modulate reaction rates like controlling speeds by the transmission of vehicles is highly desirable and challenging. Herein, a regio- and stereoselective photodimerization with four tunable and visible rates were realized at the molecular level via the synergy of host–guest chemistry and AIE technology: (i) normal rate without adding macrocycles (ν1), (ii) greatly accelerated rate by the formation of 2:2 complex between γ-cyclodextrin (γ-CD) and an AIE luminogen (AIEgen) (ν2), (iii) deactivated reaction by 1:1 binding of β-cyclodextrin (β-CD) with the AIEgen (ν3), and (iv) completely inhibited photoreaction in the β-CD-based hydrogel (ν4). The restriction of intramolecular motions of AIEgen activates the fluorescence of the transmission with the rate of ν2 > ν1 > ν3 > ν4 ≈ 0 and enhances the fluorescent contrast before and after photoirradiation, which not only provides in situ trackability to the microscopic process, but also endows intuitive mechanistic insights. The synergy of AIE and h...

Precise Modulation of Molecular Building Blocks from Tweezers to Rectangles for Recognition and Stimuli-Responsive Processes.

Alkynylplatinum(II) terpyridine complexes have been increasingly explored since the previous decades, mainly arising from their intriguing photophysical properties and aggregation affinities associated with their extensive Pt(II)···Pt(II) and π-π stacking interactions. Through molecular engineering, one can modulate their fundamental properties and assembly behavior by introduction of various functional groups and structural features. They can therefore serve as ideal candidates to construct metal complex-based molecular architectures to provide an alternative to organic compounds. The metal-based framework can be simultaneously built from predetermined building blocks, giving rise to their well-defined, unique, and discrete natures for molecular recognition. The individual constituents can contribute to molecular architectures with their integrated properties, allowing the manipulation of the various noncovalent intermolecular forces and interactions for selective guest capture. In this Account, our recent progress in the development of these metallomolecular frameworks based on the alkynylplatinum(II) terpyridine system and their recognition properties toward different guest molecules will be presented. Phosphorescent molecular tweezers have been constructed from the alkynylplatinum(II) terpyridine moiety to demonstrate host-guest interactions with cationic, charge-neutral and anionic platinum(II), palladium(II), gold(I), and gold(III) complexes and their binding affinities were found to be perturbed by different metal···metal, π-π and electrostatic interactions. The host-guest assembly process has also resulted in dramatic color changes, together with the turning on of near-IR (NIR) emissions as a result of extensive Pt(II)···Pt(II) interactions. Further work has also been performed to demonstrate that the tweezers can selectively recognize π-surfaces of different planar π-conjugated organic guests. The framework of molecular tweezers has been extended to a double-decker tweezers structure, or a triple-decker structure, which can bind two equivalents of square-planar platinum(II) guests cooperatively to induce a significant color change in solution, representing rare examples of discrete Magnus' green-like salts. By the approaches of structural modifications, we have further modulated the host architecture from molecular tweezers to molecular rectangles. The rectangles have been found to show selective encapsulation of different transition metal complex guests based on the size and steric environment of the host cavity. The molecular rectangles also exhibit reversible host-guest association, in which guest capture and ejection processes can be manipulated by the pH environment, illustrating a potential approach for precise and smart delivery of therapeutic reagents to the slightly more acidic cancer cells.