Facile Catch and Release of Fullerenes Using a Photoresponsive Molecular Tube

Hydrogels Self-Assembled from an Azobenzene Building Block: Stability toward UV Irradiation in the Gel and Thin-Film States

Azobenzene and its derivatives are widely used as photoresponsive units for the fabrication of photoresponsive smart materials. In this work, two azobenzene derivatives were designed and investigated as hydrogelators, namely N-4-azodiphenyl-maleimic acid (ADPMA) and N-4-azodiphenyl-succinic acid (ADPSA) bearing azobenzene and carboxylic acid segments. In the process of deprotonation/protonation of the carboxylic acids by pH variation, the self-assembly of these two gelators was triggered. ADPMA could transform from solution to hydrogel while the solution of ADPSA formed a precipitate under the same conditions. The solution-gel/precipitate transformation can be repeated by changing the pH. In contrast to the conventional responsiveness to UV-light irradiation for azobenzene-based gels, the ADPMA hydrogel shows typical trans-cis isomerization of the azobenzene unit in solution, yet the hydrogel demonstrates remarkable stability to UV light irradiation in both the bulk gel and thin film states.

Exploiting the labile site in dinuclear [Pd2L2]n+ metallo-cycles: multi-step control over binding affinity without alteration of core host structure

Synthetic metallosupramolecular systems have generally been binary (on/off) when they have control over molecular recognition. This report details a dipalladium(ii) system with four-step graduated control over recognition for a guest.

Reversible reduction drives anion ejection and C60 binding within an FeII 4L6 cage

Fe^II₄L₆ tetrahedral cage 1 was prepared from a redox-active dicationic naphthalenediimide (NDI) ligand. The +20 charge of the cage makes it a good host for anionic guests, with no binding observed for neutral aromatic molecules. Following reduction by Cp₂Co, the cage released anionic guests; subsequent oxidation by AgNTf₂ led to re-uptake of anions. In its reduced form, however, 1 was observed to bind neutral C₆₀. The fullerene guest was subsequently ejected following cage re-oxidation. The guest release process was found to be facilitated by anion-mediated transport from organic to aqueous solution. Cage 1 thus employs electron transfer as a stimulus to control the uptake and release of both neutral and charged guests, through distinct pathways.

Fe^II₄L₆ cage 1 binds anionic guests but not neutral guests. In its reduced form, the cage can bind neutral C₆₀. Reduction and oxidation of the cage could thus be used as a stimulus to control the uptake and release of both neutral and charged guests.

Self-Healing Heterometallic Supramolecular Polymers Constructed by Hierarchical Assembly of Triply Orthogonal Interactions with Tunable Photophysical Properties

Here, we present a method for the building of new bicyclic heterometallic cross-linked supramolecular polymers by hierarchical unification of three types of orthogonal noncovalent interactions, including platinum(II)-pyridine coordination-driven self-assembly, zinc-terpyridine complex, and host-guest interactions. The platinum-pyridine coordination provides the primary driving force to form discrete rhomboidal metallacycles. The assembly does not interfere with the zinc-terpyridine complexes, which link the discrete metallacycles into linear supramolecular polymers, and the conjugation length is extended upon the formation of the zinc-terpyridine complexes, which red-shifts the absorption and emission spectra. Finally, host-guest interactions via bis-ammonium salt binding to the benzo-21-crown-7 (B21C7) groups on the platinum acceptors afford the cross-linked supramolecular polymers. By continuous increase of the concentration of the supramolecular polymer to a relatively high level, supramolecular polymer gel is obtained, which exhibits self-healing properties and reversible gel-sol transitions stimulated by various external stimuli, including temperature, K⁺, and cyclen. Moreover, the photophysical properties of the supramolecular polymers could be effectively tuned by varying the substituents of the precursor ligands.

Temperature Controls Guest Uptake and Release from Zn4L4 Tetrahedra

We report the preparation of triazatruxene-faced tetrahedral cage 1, which exhibits two diastereomeric configurations (T1 and T2) that differ in the handedness of the ligand faces relative to that of the octahedrally coordinated metal centers. At lower temperatures, T1 is favored, whereas T2 predominates at higher temperatures. Host–guest studies show that T1 binds small aliphatic guests, whereas T2 binds larger aromatic molecules, with these changes in binding preference resulting from differences in cavity size and degree of enclosure. Thus, by a change in temperature the cage system can be triggered to eject one bound guest and take up another.

Different Modes of Anion Response Cause Circulatory Phase Transfer of a Coordination Cage with Controlled Directionality

Controlled directional transport of molecules is essential to complex natural systems, from cellular transport up to organismal circulatory systems. In contrast to these natural systems, synthetic systems that enable transport of molecules between several spatial locations on the macroscopic scale, when external stimuli are applied, remain to be explored. Now, the transfer of a supramolecular cage is reported with controlled directionality between three phases, based on a cage that responds reversibly in two distinct ways to different anions. Notably, circulatory phase transfer of the cage was demonstrated based on a system where the three layers of solvent are arranged within a circular track. The direction of circulation between solvent phases depended upon the order of addition of anions.

Bent Anthracene Dimers as Versatile Building Blocks for Supramolecular Capsules

This Account provides a comprehensive summary of our 1-decade-long investigations into bent anthracene dimers as versatile building blocks for supramolecular capsules. The investigations initiated in 2008 with the design of an anthracene dimer with a meta-phenylene spacer bearing two substituents on the convex side. Using the bent polyaromatic building block, we began to develop novel supramolecular capsules from two different synthetic approaches. One is a coordination approach, which was pursued by converting the building block into a bent ligand with two pyridine units at the terminal positions. The ligands quantitatively assemble into an M₂L₄-type capsule through coordination bonding with metal ions. The other is a π-stacking approach, which was followed by utilizing the block as a bent amphiphilic molecule with two trimethylammonium groups at the spacer. In water, the amphiphiles spontaneously assemble into a micelle-type capsule through the hydrophobic effect and π-stacking interactions. Simple modification of the building block allowed us to prepare a wide variety of coordination capsules as well as π-stacking capsules, bearing different hydrophilic side-chains, terminal substitutions, connecting units, polyaromatic panels, or spacer units. The coordination capsule possesses a rigid cavity, with a diameter of ∼1 nm, surrounded by multiple anthracene panels. The spherical polyaromatic cavity binds various synthetic molecules (e.g., paracyclophanes, corannulene, BODIPY, and fullerene C₆₀) in aqueous solutions. With the aid of the polyaromatic shell, photochemically and thermally reactive radical initiators and oligosulfurs are greatly stabilized in the cavity. Biomolecules such as hydrophilic sucrose and oligo(lactic acid)s as well as hydrophobic androgenic hormones are bound by the capsule with high selectivity. In addition, long amphiphilic poly(ethylene oxide)s are threaded into the closed shell of the capsule(s) to generate unusual pseudorotaxane-shaped host-guest complexes in water. In contrast, the π-stacking capsule furnishes a flexible cavity, adaptable to the size and shape of guest molecules, encircled by multiple anthracene panels. In water, the capsule binds hydrophobic fluorescent dyes (e.g., Nile red and DCM) in the cavity. Simple grinding of the bent amphiphile with highly hydrophobic nanocarbons such as fullerenes, nanographenes, and carbon nanotubes (followed by sonication) as well as metal-complexes such as Cu(II)-phthalocyanines and Mn(III)-tetraphenylporphyrins leads to the efficient formation of water-soluble host-guest complexes upon encapsulation. Red emission from otherwise water-deactivated Eu(III)-complexes is largely enhanced in water through encapsulation. Moreover, the incorporation of pH- and photoswitches into the amphiphile affords stimuli-responsive π-stacking capsules, capable of releasing bound guests by the addition of acid and light irradiation, respectively, in water. The host functions of the coordination and π-stacking capsules are complementary to each other, which enables selection of the capsule-type depending on the envisioned target. We are convinced that continued investigation of the present supramolecular capsules featuring the bent anthracene dimer and its derivatives will further increase their value as advanced molecular tools for synthetic, analytical, material, biological, and/or medical applications.

A Dynamic Hydrogen-Bonded Azo-Macrocycle for Precisely Photo-Controlled Molecular Encapsulation and Release

A light-responsive system constructed from hydrogen-bonded azo-macrocycles demonstrates precisely controlled propensity in molecular encapsulation and release process. A significant decrease in the size of the cavity is observed in the course of the E→Z photoisomerization based on the results from DFT calculations and traveling wave ion mobility mass spectrometry. These macrocyclic hosts exhibit a rare 2:1 host-guest stoichiometry and guest-dependent slow or fast exchange on the NMR timescale. With the slow host-guest exchange and switchable shape change of the cavity, quantitative release and capture of bipyridinium guests is achieved with the maximum release of 68 %. This work underscores the importance of slow host-guest exchange on realizing accurate release of organic cations in a stepwise manner under light irradiation. The light-responsive system established here could advance further design of novel photoresponsive molecular switches and mechanically interlocked molecules.

Light triggered encapsulation and release of C60 with a photoswitchable TPE-based supramolecular tweezers

Stimuli responsive hosts for C₆₀ can control its binding and release on demand. A photoswitchable TPE based supramolecular host can encapsulate C₆₀ in the Z-form with a markedly different visual change in the colour. In addition, the Z-1 bound C₆₀ has been characterized by various spectroscopic methods and mass spectrometry. Upon exposure to visible light (>490 nm), the host switches to the E-form where the structural complementarity with the guest is destroyed as a result of which the C₆₀ is disassembled from the host. The results described herein reveals an actionable roadmap to pursue further advances in component self-assembly particularly light-induced association and dissociation of a guest molecule.

Suite of Organoplatinum(II) Triangular Metallaprism: Aggregation-Induced Emission and Coordination Sequence Induced Emission Tuning

A series of triangular metallaprisms with a kinetically inert Pt-N bond have been synthesized from the stepwise assembly of a Pt-corner, linear linker 4,4'-bipy (4,4'-bipy = 4,4'-bipyridine) and triangular ligand [tpb or tpt, tpb = tris(4-pyridyl)benzene, tpt = tris(4-pyridyl)triazine]. The use of an unsymmetrical [Pt(HL)]-corner (H₂L = 2,6-diphenylpyridine) leads to novel isostructural products. Phenyl rotation at the metal-corners endows these complexes with good aggregation-induced emission (AIE) function, with varied activities across the isostructural complexes. The coordination sequence of electron-deficient ligand tpt also imparts significant influence on the complex emission. These organoplatinum triangular metallaprisms thus provide a good model to study the influence of building blocks and coordination sequence on the luminescence of supramolecules.

Hierarchical Hybrid Nanostructures Constructed by Fullerene and Molecular Tweezer

For the construction of well-defined hierarchical superstructures of pristine [60]fullerene (C₆₀) arrays, pyrene-based molecular tweezers (PT) were used as host molecules for catching and arranging C₆₀ guest molecules. The formation of host-guest complexes was systematically studied in solution as well as in the solid state. Two-dimensional proton nuclear magnetic resonance spectroscopic studies revealed that PT-host and C₆₀-guest complexes were closely related to the molecular self-assembly of PT. Ultraviolet and fluorescence spectroscopic titrations indicated the formation of stable 1:1 and 2:1 (PT/C₆₀) complexes. From the nonlinear curve-fitting analysis, equilibrium constants for the 1:1 (log K₁) and 2:1 (log K₂) complexes were estimated to be 4.96 and 5.01, respectively. X-ray diffraction results combined with transmission electron microscopy observations clearly exhibited the construction of well-defined layered superstructures of the PT-host and C₆₀-guest complexes. From electron mobility measurements, it was demonstrated that the well-defined hierarchical hybrid nanostructure incorporating a C₆₀ array exhibited a high electron mobility of 1.7 × 10^-2 cm² V^-1 s^-1. This study can provide a guideline for the hierarchical hybrid nanostructures of host-guest complex and its applications.

Conformer-dependent self-assembled metallacycles with photo-reversible response

Discrete, well-defined metallacycles and metallacages with stimuli-responsive behaviors have been largely predominated by the organic donor/metal acceptor paradigm with spontaneous formation of coordination bonds. However, light-driven self-assembly systems usually show relatively low utilization yield of photons and low fatigue resistance. Given that almost no example illustrates the different self-assembly behaviors of antiparallel and parallel conformers in the traditional photochromic diarylethene (DAE) system, here we have for the first time constructed a unique series of photoactive conformer-dependent metallacycles, focusing on the characterization and comparison of self-assembly behavior in different ligand conformers with different di-platinum(ii) acceptors. Their photoswitchable scaffold sizes and shapes are precisely controlled by photochromically separable parallel or anti-parallel conformers via coordination-driven self-assembly. The ap-conformer and closed form provide larger bending angles upon coordination with di-Pt(ii) acceptors into hexagon [6 + 6] or [3 + 3] while the p-conformer only can form smaller polygon cycles. Notably, in contrast with the non-photoactive parallel conformer, the reversible interconversion of anti-parallel ring-open and ring-closed conformer metallacycles can be achieved by alternate irradiation with UV and visible light, respectively, along with a relatively high conversion ratio and good fatigue resistance. This work provides a potential way to construct smart materials for use in sensing, catalysis and drug delivery systems.

Insights into a novel class of azobenzenes incorporating 4,6-O-protected sugars as photo-responsive organogelators

A novel class of 4,6-O-butylidene/ethylidene/benzylidene β-d-glucopyranose gelator functionalized with photo-responsive azobenzene moieties were designed and synthesized and also characterized using different spectral techniques. These azobenzene-based organogelators can gel even at lower concentrations (critical gelation concentration – 0.5% and 1%). A morphological study of the gels shows one-dimensional aggregated bundles and helical fibres. The main driving force for the self-assembly is through cooperative interactions exhibited by the different groups viz., sugar hydroxyl (hydrogen bonding interaction), azobenzene (aromatic π–π interaction) and alkyl chain of the protecting group (van der Waals interaction).

A novel class of 4,6-O-butylidene/ethylidene/benzylidene β-d-glucopyranose gelator functionalized with photo-responsive azobenzene moieties were designed and synthesized and also characterized using different spectral techniques.

Supramolecular Polymer Cross-Linked by Discrete Tris-[2]pseudorotaxane Metallacycles and Its Redox-Responsive Behavior

A new discrete hexagonal metallacycle M containing tris-[2]pseudorotaxane moiety has been successfully designed and synthesized via coordination-driven self-assembly. The newly designed tris-[2]pseudorotaxane metallacycle was well characterized with nuclear magnetic resonance and mass spectra analysis. Such tris-[2]pseudorotaxane metallacycle M and pillar[5]arene dimer (PD) could further form a new family of cross-linked redox-responsive supramolecular polymer M⊃(PD)₃ through a host-guest interaction. Interestingly, the polymer M⊃(PD)₃ displayed redox-responsive behavior and showed tuned weight-average diffusion coefficients D upon redox stimuli, which is attributed to the changed coordination geometries of [Cu(phen)₂]⁺ and [Cu(phen)₂]²⁺ in such system.

Orthogonal Stimuli Trigger Self-Assembly and Phase Transfer of FeII4L4 Cages and Cargoes

Two differently protected aldehydes, A and B, were demonstrated to deprotect selectively through the application of light and heat, respectively. In the presence of iron(II) and a triamine, two distinct Fe^II₄L₄ cages, 1 and 2, were thus observed to form from the deprotected A and B, respectively. The alkyl tails of B and 2 render them preferentially soluble in cyclopentane, whereas A and 1 remain in acetonitrile. The stimulus applied (either light or heat) thus determines the outcome of self-assembly and dictates whether the cage and its ferrocene cargo remain in acetonitrile, or transport into cyclopentane. Cage self-assembly and cargo transport between phases can in this fashion be programmed using orthogonal stimuli.

Hierarchical Self-Assembly of a Water-Soluble Organoplatinum(II) Metallacycle into Well-Defined Nanostructures

A water-soluble metallosupramolecular hexagon containing pendant methyl viologen (MV) and trimethylammonium units at the vertices has been synthesized via an organoplatinum(II) ← pyridyl coordination-driven self-assembly reaction. The MV units of the metallacycle were further utilized in the formation of a heteroternary complex with cucurbit[8]uril and a galactose-functionalized naphthalene derivative, yielding a metallacycle-cored carbohydrate cluster that was subsequently ordered into nanospheres and tapes, depending upon the concentration.

Orthogonal self-assembly of an organoplatinum(II) metallacycle and cucurbit[8]uril that delivers curcumin to cancer cells

Curcumin (Cur) is a naturally occurring anticancer drug isolated from the Curcuma longa plant. It is known to exhibit anticancer properties via inhibiting the STAT3 phosphorylation process. However, its poor water solubility and low bioavailability impede its clinical application. Herein, we used organoplatinum(II) ← pyridyl coordination-driven self-assembly and a cucurbit[8]uril (CB[8])-mediated heteroternary host-guest complex formation in concert to produce an effective delivery system that transports Cur into the cancer cells. Specifically, a hexagon 1, containing hydrophilic methyl viologen (MV) units and 3,4,5-Tris[2-[2-(2-methoxyethoxy)ethoxy]ethoxy]benzoyl groups alternatively at the vertices, has been synthesized and characterized by several spectroscopic techniques. The MV units of 1 underwent noncovalent complexation with CB[8] to yield a host-guest complex 4. Cur can be encapsulated in 4, via a 1:1:1 heteroternary complex formation, resulting in a water-soluble host-guest complex 5. The host-guest complex 5 exhibited ca 100-fold improved IC₅₀ values relative to free Cur against human melanoma (C32), melanoma of rodents (B16F10), and hormone-responsive (MCF-7) and triple-negative (MDA-MB231) breast cancer cells. Moreover, strong synergisms of Cur with 1 and 4 with combinatorial indexes of <1 across all of the cell lines were observed. An induced apoptosis with fragmented DNA pattern and inhibited expression of phosphor-STAT3 supported the improved therapeutic potential of Cur in heteroternary complex 5.

Self-Assembly of a [1 + 1] Ionic Hexagonal Macrocycle and Its Antiproliferative Activity

A unique irregular hexagon was self-assembled using an organic donor clip (bearing terminal pyridyl units) and a complementary organometallic acceptor clip. The resulting metallamacrocycle was characterized by multinuclear NMR, mass spectrometry, and elemental analyses. Molecular modeling confirmed hexagonal shaped cavity for this metallamacrocycle which is a unique example of a discrete hexagonal framework self-assembled from only two building blocks. Cytotoxicity of the Pt-based acceptor tecton and the self-assembled Pt^II-based macrocycle was evaluated using three cancer cell lines and results were compared with cisplatin. Results confirmed a positive effect of the metallamacrocycle formation on cell growth inhibition.

Polyaromatic molecular tubes: from strategic synthesis to host functions

Ring- and tube-shaped molecules like crown ethers and cyclodextrins play a fundamental role in supramolecular chemistry since their initial discovery. To date, numerous intriguing properties and reactivities have been reported based on their unique inner microenvironments. While inner spaces encircled by aliphatic and/or small aromatic frameworks have been heavily investigated, tubular structures that feature polyaromatic frameworks remained largely unexplored until 2010, despite their undisputable potential. Polyaromatic rings provide appealing photophysical and electrochemical properties and thus allow for the construction of new functional cylindrical nanospaces. This feature article describes the recent progress in the synthesis and application of short tubular molecules bearing multiple (≥3) polyaromatic rings (e.g., anthracene, pyrene, chrysene, and HBC). The polyaromatic tubes reported herein display characteristic properties such as strong fluorescent emission, a selective molecular binding ability, efficient host-guest energy transfer and open-closed structural transformations.

A 'photorelease, catch and photorelease' strategy for bioconjugation utilizing a p-hydroxyphenacyl group

A bioorthogonal 'catch and photorelease' strategy, which combines alkyne-azide cycloaddition between p-hydroxyphenacyl azide and alkyne derivatives to form a 1,2,3-triazole adduct and subsequent photochemical release of the triazole moiety via a photo-Favorskii rearrangement, is introduced. The first step can also involve photorelease of a strained alkyne and its Cu-free click reaction with azide.

A luminescent [Pd4Ru8]24+ supramolecular cage

A phosphorescent cage of the form [Pd₄Ru₈]²⁴⁺ is reported.

Platinum(II)-Based Convex Trigonal-Prismatic Cages via Coordination-Driven Self-Assembly and C60 Encapsulation

The development of three-dimensional (3D) supramolecular coordination complexes is of great interest from both fundamental and application points of view because these materials are useful in molecular catalysis, separation and purification, sensing, etc. Herein, we describe the synthesis of two Klärner's molecular-clip-based tetrapyridyl donors, which possess a C-shaped structure as shown by X-ray analysis, and subsequently use them to prepare four convex trigonal-prismatic cages via coordination-driven self-assembly with two 180° diplatinum(II) acceptors. The cages are fully characterized by multinuclear NMR (³¹P and ¹H) analysis, diffusion-ordered spectroscopy, electrospray ionization time-of-flight mass spectrometry, and UV/vis absorption spectroscopy. Moreover, the incorporation of molecular-clip-based ligands provides these cages with free cavities to encapsulate fullerene C₆₀ via aromatic interactions, which may be useful for fullerene separation and purification. The studies described herein enlarge the scope of the platinum(II)-based directional bonding approach in the preparation of curved 3D metallacages and their host-guest chemistry.

Polyaromatic molecular peanuts

Mimicking biological structures such as fruits and seeds using molecules and molecular assemblies is a great synthetic challenge. Here we report peanut-shaped nanostructures comprising two fullerene molecules fully surrounded by a dumbbell-like polyaromatic shell. The shell derives from a molecular double capsule composed of four W-shaped polyaromatic ligands and three metal ions. Mixing the double capsule with various fullerenes (that is, C60, C70 and Sc3N@C80) gives rise to the artificial peanuts with lengths of ∼3 nm in quantitative yields through the release of the single metal ion. The rational use of both metal-ligand coordination bonds and aromatic-aromatic π-stacking interactions as orthogonal chemical glue is essential for the facile preparation of the multicomponent, biomimetic nanoarchitectures.

Modeling Coordination-Directed Self-Assembly of M2L4 Nanocapsule Featuring Competitive Guest Encapsulation

Exploring the mechanism of self-assembly and guest encapsulation of nanocapsules is highly imperative for the design of sophisticated molecular containers and multistimuli-responsive functional materials. Here we present a molecular dynamics simulation protocol with implicit solvent and simulated annealing techniques to investigate the self-assembly and competitive guest (C₆₀ and C₇₀ fullerenes) encapsulation of a M₂L₄ nanocapsule that is self-assembled by the coordination of mercury cations and bent bidentate ligands. Stepwise formation of the nanocapsule and competitive fullerene encapsulation during dynamic structural changes in the self-assembly were detected successfully. Such processes were driven by coordination bonding and π-π stacking and obey the minimum total potential energy principle. Potential of mean force calculations for guest binding to the M₂L₄ nanocapsule explained the mechanism underlying the competitive encapsulations of C₆₀ and C₇₀. This work helps design new functional nanomaterials capable of guest encapsulation and release.

Hybrid[4]arenes with anthracene units and tuneable cavities

Acid catalyzed condensation between tetramethoxyanthracenes and formaldehyde in the presence of additional benzene-based building blocks leads to hybrid macrocyclic products that are further modified by the Diels–Alder reaction with benzyne to obtain macrocycles with expanded cavities.

Incarceration of Higher-Order Fullerenes within Cyclotriveratrylene-Based Hemicarcerands Allows Selective Isolation of C76 , C78 , and C84 from a Commercial Fullerene Mixture

Size-complementary cyclotriveratrylene (CTV)-based hosts can incarcerate C₇₆ , C₇₈ , and C₈₄ , thus allowing the selective isolation of these higher-order fullerenes from a commercially available mixture of fullerenes. The hemicarceplexes, formed after the encapsulation of the size-complementary fullerenes within the hosts, are isolated by column chromatography and released at elevated temperature, thereby leading to the isolation of C₇₆ /C₇₈ and C₈₄ in good purities (up to 95 and 88 %, respectively).