Orthogonal Self-Assembly of a Two-Step Fluorescence-Resonance Energy Transfer System with Improved Photosensitization Efficiency and Photooxidation Activity.

During the past few decades, fabrication of multistep fluorescence-resonance energy transfer (FRET) systems has become one of the most attractive topics within supramolecular chemistry, chemical biology, and materials science. However, it is challenging to efficiently prepare multistep FRET systems with precise control of the distances between locations and the numbers of fluorophores. Herein we present the successful fabrication of a two-step FRET system bearing specific numbers of anthracene, coumarin, and BODIPY moieties at precise distances and locations through an efficient and controllable orthogonal self-assembly approach based on metal-ligand coordination and host-guest interactions. Notably, the photosensitization efficiency and photooxidation activity of the two-step FRET system gradually increased with the number of energy transfer steps. For example, the two-step FRET system exhibited 1.5-fold higher 1O2 generation efficiency and 1.2-fold higher photooxidation activity than that of its corresponding one-step FRET system. This research not only provides the first successful example of the efficient preparation of multistep FRET systems through orthogonal self-assembly involving coordination and host-guest interactions but also pushes multistep FRET systems toward the application of photosensitized oxidation of a sulfur mustard simulant.

Visible-Light-Driven Rotation of Molecular Motors in Discrete Supramolecular Metallacycles.

The organization of molecular motors in supramolecular assemblies to allow the amplification and transmission of motion and collective action is an important step toward future responsive systems. Metal-coordination-driven directional self-assembly into supramolecular metallacycles provides a powerful strategy to position several motor units in larger structures with well-defined geometries. Herein, we present a pyridyl-modified molecular motor ligand (MPY) which upon coordination with geometrically distinct di-Pt(II) acceptors assembles into discrete metallacycles of different sizes and shapes. This coordination leads to a red-shift of the absorption bands of molecular motors, making these motorized metallacycles responsive to visible light. Photochemical and thermal isomerization experiments demonstrated that the light-driven rotation of the motors in the metallacycles is similar to that in free MPY in solution. CD studies show that the helicity inversions associated with each isomerization step in the rotary cycle are preserved. To explore collective motion, the trimeric motor-containing metallacycle was aggregated with heparin through multiple electrostatic interactions, to construct a multi-component hierarchical system. SEM, TEM, and DLS measurements revealed that the photo- and thermal-responsive molecular motor units enabled selective manipulation of the secondary supramolecular aggregation process without dissociating the primary metallacycle structures. These visible-light-responsive metallacycles, with intrinsic multiple rotary motors, offer prospects for cooperative operations, dynamic hierarchical self-assembled systems, and adaptive materials.

Acid-Activated Motion Switching of DB24C8 between Two Discrete Platinum(II) Metallacycles.

The precise operation of molecular motion for constructing complicated mechanically interlocked molecules has received considerable attention and is still an energetic field of supramolecular chemistry. Herein, we reported the construction of two tris[2]pseudorotaxanes metallacycles with acid-base controllable molecular motion through self-sorting strategy and host-guest interaction. Firstly, two hexagonal Pt(II) metallacycles M1 and M2 decorated with different host-guest recognition sites have been constructed via coordination-driven self-assembly strategy. The binding of metallacycles M1 and M2 with dibenzo-24-crown-8 (DB24C8) to form tris[2]pseudorotaxanes complexes TPRM1 and TPRM2 have been investigated. Furthermore, by taking advantage of the strong binding affinity between the protonated metallacycle M2 and DB24C8, the addition of trifluoroacetic acid (TFA) as a stimulus successfully induces an acid-activated motion switching of DB24C8 between the discrete metallacycles M1 and M2. This research not only affords a highly efficient way to construct stimuli-responsive smart supramolecular systems but also offers prospects for precisely control multicomponent cooperative motion.

Aggregation-Induced Emissive Carbon Dots Gels for Octopus-Inspired Shape/Color Synergistically Adjustable Actuators.

Some living organisms such as the octopus have fantastic abilities to simultaneously swim away and alter body color/morphology for disguise and self-protection, especially when there is a threat perception. However, it is still quite challenging to construct artificial soft actuators with octopus-like synergistic shape/color change and directional locomotion behaviors, but such systems could enhance the functions of soft robotics dramatically. Herein, we proposed to utilize unique hydrophobic carbon dots (CDs) with rotatable surficial groups to construct the aggregation-induced emission (AIE) active glycol CDs polymer gel, which could be further employed to be interfacially bonded to an elastomer to produce anisotropic bilayer soft actuator. When putting the actuator on a water surface, glycol spontaneously diffused out from the gel layer to allow water intake, resulting in a color change from a blue dispersion fluorescence to red AIE and a shape deformation, as well as a large surface tension gradient that can promote its autonomous locomotion. Based on these findings, artificial soft swimming robots with octopus-like synergistic shape/color change and directional swimming motion were demonstrated. This study provides an elegant strategy to develop advanced multi-functional bio-inspired intelligent soft robotics.

Metallo-Helicoid with Double Rims: Polymerization Followed by Folding by Intramolecular Coordination.

In this study, we established a feasible strategy to construct a new type of metallo-polymer with helicoidal structure through the combination of covalent polymerization and intramolecular coordination-driven self-assembly. In the design, a tetratopic monomer (M) was prepared with two terminal alkynes in the outer rim for polymerization, and two terpyridines (TPYs) in the inner rim for subsequent folding by selective intramolecular coordination. Then, the linear covalent polymer (P) was synthesized by polymerization of M via Glaser-Hay homocoupling reaction. Finally, intramolecular coordination interactions between TPYs and Zn(II) folded the backbone of P into a right- or left-handed metallo-helicoid (H) with double rims. Owing to multiple positive charges on the inner rim of helicoid, double-stranded DNA molecules (dsDNA) could interact with H through electrostatic interactions. Remarkably, dsDNA allowed exclusive formation of H with right handedness by means of chiral induction.

Rotaxane-Branched Dendrimers with Enhanced Photosensitization.

During the past few decades, fabrication of functional rotaxane-branched dendrimers has become one of the most attractive yet challenging topics within supramolecular chemistry and materials science. Herein, we present the successful fabrication of a family of new rotaxane-branched dendrimers containing up to 21 platinum atoms and 42 photosensitizer moieties through an efficient and controllable divergent approach. Notably, the photosensitization efficiencies of these rotaxane-branched dendrimers gradually increased with the increase of dendrimer generation. For example, third-generation rotaxane-branched dendrimer PG3 revealed 13.3-fold higher 1O2 generation efficiency than its corresponding monomer AN. The enhanced 1O2 generation efficiency was attributed to the enhancement of intersystem crossing (ISC) through the simple and efficient incorporation of multiple heavy atoms and photosensitizer moieties on the axles and wheels of the rotaxane units, respectively, which has been validated by UV-visible and fluorescence techniques, time-dependent density functional theory calculations, photolysis model reactions, and apparent activation energy calculations. Therefore, we develop a new promising platform of rotaxane-branched dendrimers for the preparation of effective photosensitizers.

Giant Concentric Metallosupramolecule with Aggregation-Induced Phosphorescent Emission.

Fluorescent metallosupramolecules have received considerable attention due to their precisely controlled dimensions as well as the tunable photophysical and photochemical properties. However, phosphorescent analogues are still rare and limited to small structures with low-temperature phosphorescence. Herein, we report the self-assembly and photophysical studies of a giant, discrete metallosupramolecular concentric hexagon functionalized with six alkynylplatinum(II) bzimpy moieties. With a size larger than 10 nm and molecular weight higher than 26 000 Da, the assembled terpyridine-based supramolecule displayed phosphorescent emission at room temperature. Moreover, the supramolecule exhibited enhanced aggregation-induced phosphorescent emission compared to the ligand by tuning the aggregation states through intermolecular interactions and significant enhancement of emission to CO2 gas.

Daisy Chain Dendrimers: Integrated Mechanically Interlocked Molecules with Stimuli-Induced Dimension Modulation Feature.

The precise construction of the high-order mechanically interlocked molecules (MIMs) with well-defined topological arrangements of multiple mechanically interlocked units has been a great challenge. Herein, we present the first successful preparation of a new family of daisy chain dendrimers, in which the individual [c2]daisy chain rotaxane units serve as the branches of dendrimer skeleton. In particular, the third-generation daisy chain dendrimer with 21 [c2]daisy chain rotaxane moieties was realized, which might be among the most complicated discrete high-order MIMs comprised of multiple [c2]daisy chain rotaxane units. Interestingly, such unique topological arrangements of multiple stimuli-responsive [c2]daisy chain rotaxanes endowed the resultant daisy chain dendrimers controllable and reversible nanoscale dimension modulation through the collective and amplified extension/contraction of each [c2]daisy chain rotaxane branch upon the addition of acetate anions or DMSO molecules as external stimulus. Furthermore, on the basis of such an intriguing size switching feature of daisy chain dendrimers, dynamic composite polymer films were constructed through the incorporation of daisy chain dendrimers into polymer films, which could undergo fast, reversible, and controllable shape transformations when DMSO molecules were employed as stimulus. The successful merging of [c2]daisy chain rotaxanes and dendrimers described herein provides not only a brand-new type of high-order mechanically interlocked systems with well-defined topological arrangements of [c2]daisy chain rotaxanes, but also a successful and practical approach toward the construction of supramolecular dynamic materials.

Construction of Supramolecular Liquid-Crystalline Metallacycles for Holographic Storage of Colored Images.

Design and construction of new functionalized supramolecular coordination complexes (SCCs) via coordination-driven self-assembly strategy is highly important in supramolecular chemistry and materials science. Herein, we present a family of well-defined metallacycles decorated with mesogenic forklike dendrons through the strategy of coordination-driven self-assembly. Due to the existence of mesogenic forklike dendrons, the obtained metallacycles displayed the smectic A liquid crystal phase at room temperature while their precursors exhibited the rectangular columnar liquid crystal phase. Interestingly, by taking advantage of the electrostatic interactions between the positively charged metallacycle and the negatively charged heparin, the doping of heparin induced a significant change of the liquid-crystalline behaviors of metallacycles. More importantly, the prepared liquid-crystalline metallacycles could be further applied for holographic storage of colored images. Notably, the rhomboidal metallacycle and hexagonal metallacycle gave rise to different holographic performances although they featured a similar liquid crystal phase behavior. Therefore, this research not only provides the first successful example of supramolecular liquid-crystalline metallacycles for holographic storage of colored images but also opens a new door for supramolecular liquid-crystalline metallacycles toward advanced optical applications.

Light-Controlled Generation of Singlet Oxygen within a Discrete Dual-Stage Metallacycle for Cancer Therapy.

Noninvasive control over the reversible generation of singlet oxygen (1O2) has found the practical significance in benefiting photodynamic therapy. In this study, we developed a new dual-stage metallacycle (M) by using a photosensitizer and photochromic switch as the functional building blocks, which enables the noninvasive "off-on" switching of 1O2 generation through the efficient intramolecular energy transfer. Due to the proximal placement of the functional entities within the well-defined metallacyclic scaffold, 1O2 generation in the ring-closed form state of the photochromic switch (C-M) is quenched by photoinduced energy transfer, whereas the generation of 1O2 in the ring-open form state (O-M) is activated upon light irradiation. More interestingly, the metallacycle-loaded nanoparticles with relatively high stability and water solubility were prepared, which allow for the delivery of metallacycles to cancer cells via endocytosis. Their theranostic potential has been systematically investigated both in vitro and in vivo. Under the light irradiation, the designed ring-open form nanoparticles (O-NPs) show remarkable higher cytotoxicity against cancer cells compared to the ring-closed form nanoparticles (C-NPs). In vivo experiments also revealed that tumors can be very efficiently eliminated by the designed nanoparticles under light irradiation with the ability to regulate in vivo generation of singlet oxygen. All these results demonstrated that the supramolecular coordination complexes with a dual-stage state provide a highly efficient nanoplatform for noninvasive control over the reversible generation of 1O2, thus allowing for their promising applications in tumor treatment and beyond.

Helical Sulfono-γ-AApeptides with Aggregation-Induced Emission and Circularly Polarized Luminescence.

Aggregation-induced emission (AIE) was intensively studied because of packing of small molecules and polymers; however, mid-molecular-weight (1000-3000) molecular scaffold containing a precise number of AIE luminogens is rare. Herein, we report the investigation of three tetraphenylethylene (TPE)-modified sulfono-γ-AApeptides in which multiple TPE moieties are conjugated to the chiral right-handed helical peptidomimetic backbone as functional side chains. The crystal structure of the TPE-α/sulfono-γ-AA peptide 1 demonstrates that because of the rigid helical scaffold of the TPE-α/sulfono-γ-AA peptides, the intramolecular rotations of the TPE with short linker are restricted, therefore leading to the boosted fluorescent emission in solution. Peptides 2 and 3 exhibit aggregation-induced emission enhancement (AIEE), possibly because of the combination of both AIE and rotation restriction. Moreover, because of their preoriented assembly induced by the right-handed helical scaffold, these emissive chiral luminogens show effective circularly polarized luminescence signals with high dissymmetry factor glum. Finally, the amphiphilic nature of TPE-α/sulfono-γ-AA peptides could enable them to penetrate the bacterial membranes and exhibit strong fluorescence. Their antimicrobial activity and labeling-free character could further augment their potential applications in both materials and biomedical sciences.

Radical-Induced Hierarchical Self-Assembly Involving Supramolecular Coordination Complexes in Both Solution and Solid States.

To explore a new supramolecular interaction as the main driving force to induce hierarchical self-assembly (HSA) is of great importance in supramolecular chemistry. Herein, we present a radical-induced HSA process through the construction of well-defined rhomboidal metallacycles containing triphenylamine (TPA) moieties. The light-induced radical generation of the TPA-based metallacycle has been demonstrated, which was found to subsequently drive hierarchical self-assembly of metallacycles in both solution and solid states. The morphologies of nanovesicle structures and nanospheres resulting from hierarchical self-assembly have been well-illustrated by using TEM and high-angle annular dark-field STEM (HAADF-STEM) micrographs. The mechanism of HSA is supposed to be associated with the TPA radical interaction and metallacycle stacking interaction, which has been supported by the coarse-grained molecular dynamics simulations. This study provides important information to understand the fundamental TPA radical interaction, which thus injects new energy into the hierarchical self-assembly of supramolecular coordination complexes (SCCs). More interestingly, the stability of TPA radical cations was significantly increased in these metallacycles during the hierarchical self-assembly process, thereby opening a new way to develop stable organic radical cations in the future.

Supramolecular Transformation of Metallacycle-linked Star Polymers Driven by Simple Phosphine Ligand-Exchange Reaction.

As a common phenomenon in biological systems, supramolecular transformations of biomacromolecules lead to specific biological functions as outputs, which thus inspire people to construct biomimetic dynamic systems through supramolecular transformation strategy. It should be noted that well-modulating the artificial macromolecules to fine-tune their properties is of great significance yet still remains a big challenge in polymer chemistry. In this study, through the combination of coordination-driven self-assembly and postassembly ring-opening polymerization, a six-armed star polymer linked by well-defined hexagonal metallacycle as core was successfully prepared. At the same time, the trans-platinum acetylide moieties as transformation sites were anchored onto the discrete metallacycle scaffold. Subsequently, the simple phosphine ligand-exchange reaction induced the conversions of platinum acetylide building blocks with the varied binding angles, which thus resulted in the successive hexagon-rhomboid-hexagon transformations of metallacyclic scaffold, therefore allowing for the corresponding supramolecular transformation of metallacycle-linked star polymers. More importantly, accompanied by such transformation process, property modulation of the resultant polymers has been successfully realized. In a word, by taking advantage of dynamic nature of metal-ligand coordination bonds and simple phosphine ligand-exchange reactions, facile architecture transformation of a star polymer to a linear polymer and back to a star polymer was successfully realized, which may provide a promising approach toward the construction of new dynamic polymeric materials.

Self-assembly of emissive metallocycles with tetraphenylethylene, BODIPY and terpyridine in one system.

Tetraphenylethylene (TPE) related (supra)molecules have been intensively investigated due to their aggregation-induced emission (AIE) effect based on the restriction of intramolecular rotation (RIR). Meanwhile, boron-dipyrromethene (BODIPY) tends to emit intense fluorescence with high quantum yields. Herein, we combined TPE, BODIPY and terpyridine (TPY) into one system to study the emissive behaviour of organic building block as well as a self-assembled metallo-supramolecule. The TPY and BODIPY substituents with bulky sizes provide strong hindrance to restrict the rotation of the phenyl groups on TPE, leading to enhancement of emissive properties in both solution and aggregation states. Furthermore, the BODIPY-TPE-TPY ligand (L) was assembled with Zn (II) through coordination-driven self-assembly to form a cyclic dimer (D) with typical AIE characteristics.

Construction of Porphyrin-Containing Metallacycle with Improved Stability and Activity within Mesoporous Carbon.

The successful construction of porphyrin functionalized metallacycle in the confined cavity of mesoporous carbon FDU-16 (3⊂C) is presented in this study. Because of high dispersity of metallacycles within the mesoporous cavities, the stability and activity of porphyrin-containing metallacycles were obviously improved. For example, 1O2 generation efficiency of 3⊂C is ca. 6-fold faster than that of free metallaycles in solution. Thus, the resultant hybrid material has been successfully employed as a heterogeneous catalyst for photooxidation of sulfides.

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.

Hierarchical Self-Assembly of an Alkynylplatinum(ll) Bzimpy-Functionalized Metallacage via Pt···Pt and π-π Interactions.

A new discrete supramolecular metallacage functionalized with an alkynylplatinum(ll) 2,6-bis(benzimidazole-2'-yl)-pyridine (bzimpy) moiety has been successfully constructed via coordination-driven self-assembly. A study on the hierarchical self-assembly behavior of the obtained metallacage revealed that it displayed a solvent-induced emission switch accompanied by enhancement of the emission intensity as a result of the change in intermolecular Pt···Pt and π-π interactions. More interestingly, the metallacage has been found to spontaneously self-assemble into a transparent metallogel at room temperature without a heating-cooling process.

Photoresponsive Chirality-Tunable Supramolecular Metallacycles.

Chirality-tunable supramolecular metallacycles containing two light-responsive dithienylethene units and two chiral 1,1'-bi-2-naphthol (BINOL) units have been successfully constructed via coordination-driven self-assembly. These new metallacycles are well-characterized with 1D multinuclear NMR (1 H and 31 P NMR), 2D 1 H-1 H COSY and DOSY, ESI-TOF-MS, and PM6 semiempirical molecular orbital methods. Interestingly, upon irradiation with ultraviolet and visible light, the conformation of these metallacycles can undergo reversible transformation between ring-open and ring-closed forms accompanied with the obvious change of CD signals. Further investigation reveals that the photoisomerization of the dithienylethene moieties induces the change in the dihedral angle of the binaphthyl rings, thus leading to the chiral modulation of supramolecular metallacycles. Thus, this study provides very few examples of the light-induced chirality-tunable metallosupramolecular assemblies, which may find potential application in mimicking the function of natural systems in the future.

Construction of π-Surface-Metalated Pillar[5]arenes which Bind Anions via Anion-π Interactions.

By simple ligand exchange of the cationic transition-metal complexes [(Cp*)M(acetone)3 ](OTf)2 (Cp*=pentamethylcyclopentadienyl and M=Ir or Rh) with pillar[5]arene, mono- and polynuclear pillar[5]arenes, a new class of metalated host molecules, is prepared. Single-crystal X-ray analysis shows that the charged transition-metal cations are directly bound to the outer π-surface of aromatic rings of pillar[5]arene. One of the triflate anions is deeply embedded within the cavity of the trinuclear pillar[5]arenes, which is different to the host-guest behavior of most pillar[5]arenes. DFT calculation of the electrostatic potential revealed that the metalated pillar[5]arenes featured an electron-deficient cavity due to the presence of the electron-withdrawing transition metals, thus allowing encapsulation of electron-rich guests mainly driven by anion-π interactions.

Direct Self-Assembly of a 2D and 3D Star of David.

Two- and three-dimensional metallosupramolecules shaped like a Star of David were synthesized by the self-assembly of a tetratopic pyridyl ligand with a 180° diplatinum(II) motif and PdII ions, respectively. In contrast to other strategies, such as template-directed synthesis and stepwise self-assembly, this design enables the formation of 2D and 3D structures in one step and high yield. The structures were characterized by both one-dimensional (1 H, 13 C, 31 P) and two-dimensional (COSY, NOESY, DOSY) NMR spectroscopy, ESI-MS, ion-mobility mass spectrometry (IM-MS), AFM, and TEM. The stabilities of the 2D and 3D structures were measured and compared by gradient tandem mass spectrometry (gMS2 ). The high stability of the 3D Star of David was correlated to its high density of coordination sites (DOCS).