An elastic metal-organic crystal with a densely catenated backbone.

What particular mechanical properties can be expected for materials composed of interlocked backbones has been a long-standing issue in materials science since the first reports on polycatenane and polyrotaxane in the 1970s1-3. Here we report a three-dimensional porous metal-organic crystal, which is exceptional in that its warps and wefts are connected only by catenation. This porous crystal is composed of a tetragonal lattice and dynamically changes its geometry upon guest molecule release, uptake and exchange, and also upon temperature variation even in a low temperature range. We indented4 the crystal along its a/b axes and obtained the Young's moduli of 1.77 ± 0.16 GPa in N,N-dimethylformamide and 1.63 ± 0.13 GPa in tetrahydrofuran, which are the lowest among those reported so far for porous metal-organic crystals5. To our surprise, hydrostatic compression showed that this elastic porous crystal was the most deformable along its c axis, where 5% contraction occurred without structural deterioration upon compression up to 0.88 GPa. The crystal structure obtained at 0.46 GPa showed that the catenated macrocycles move translationally upon contraction. We anticipate our mechanically interlocked molecule-based design to be a starting point for the development of porous materials with exotic mechanical properties. For example, squeezable porous crystals that may address an essential difficulty in realizing both high abilities of guest uptake and release are on the horizon.

Self-assembled soft alloy with Frank-Kasper phases beyond metals.

Soft building blocks, such as micelles, cells or soap bubbles, tend to adopt near-spherical geometry when densely packed together. As a result, their packing structures do not extend beyond those discovered in metallic glasses, quasicrystals and crystals. Here we report the emergence of two Frank-Kasper phases from the self-assembly of five-fold symmetric molecular pentagons. The µ phase, an important intermediate in superalloys, is indexed in soft matter, whereas the ϕ phase exhibits a structure distinct from known Frank-Kasper phases in metallic systems. We find a broad size and shape distribution of self-assembled mesoatoms formed by molecular pentagons while approaching equilibrium that contribute to the unique packing structures. This work provides insight into the manipulation of soft building blocks that deviate from the typical spherical geometry and opens avenues for the fabrication of 'soft alloy' structures that were previously unattainable in metal alloys.

Expanding quasiperiodicity in soft matter: Supramolecular decagonal quasicrystals by binary giant molecule blends.

The quasiperiodic structures in metal alloys have been known to depend on the existence of icosahedral order in the melt. Among different phases observed in intermetallics, decagonal quasicrystal (DQC) structures have been identified in many glass-forming alloys yet remain inaccessible in bulk-state condensed soft matters. Via annealing the mixture of two giant molecules, the binary system assemblies into an axial DQC superlattice, which is identified comprehensively with meso-atomic accuracy. Analysis indicates that the DQC superlattice is composed of mesoatoms with an unusually broad volume distribution. The interplays of submesoatomic (molecular) and mesoatomic (supramolecular) local packings are found to play a crucial role in not only the formation of the metastable DQC superlattice but also its transition to dodecagonal quasicrystal and Frank-Kasper σ superlattices.

Thiourea as a "Polar Hydrophobic" Hydrogen-Bonding Motif: Application to Highly Durable All-Underwater Adhesion.

Here, we report that, in contrast to urea, thiourea functions as a "polar hydrophobic" hydrogen-bonding motif. Although thiourea is more acidic than urea, thiourea exchanges its N-H protons with water at a rate that is 160 times slower than that for urea at 70 °C. This suggests that thiourea is much less hydrated than urea in an aqueous environment. What led us to this interesting principle was the serendipitous finding that self-healable poly(ether thiourea) adhered strongly to wet glass surfaces. This discovery enabled us to develop an exceptionally durable all-underwater adhesive that can maintain large adhesive strength for over a year even in seawater, simply by mechanically mixing three water-insoluble liquid components on target surfaces. Because thiourea is hydrophobic, its hydrogen-bonding networks within the adhesive structure and at the adhesive-target interface are presumed to be dehydrated. For comparison, a reference adhesive using urea as a representative "polar hydrophilic" hydrogen-bonding motif was durable for less than 4 days in water. Highly durable all-underwater adhesives are needed in various fields of marine engineering and biomedical sciences, but their development has been a major challenge because a hydration layer that spontaneously forms in water always inhibits adhesion.

Fluoro-Crown Ether Phosphate as Efficient Cell-Permeable Drug Carrier by Disrupting Hydration Layer.

Fast and direct permeation of drug molecules is crucial for effective biotherapeutics. Inspired by a recent finding that fluorous compounds disrupt the hydrogen-bonded network of water, we developed fluoro-crown ether phosphate CyclicFP-X. This compound acts as a fast cell-permeating agent, enabling direct delivery of various bioactive cargos (X) into cancer cells without endocytic entrapment. In contrast, its nonfluorinated cyclic analog (CyclicP-X) failed to achieve cellular internalization. Although the acyclic fluorous analog AcyclicFP-X was internalized, this process occurred slowly owing to the involvement of an endocytic trapping pathway. Designed with a high fluorine density, CyclicFP-X exhibits compactness, polarity, and high-water solubility, facilitating lipid vesicle fusion by disrupting their hydration layers. Raman spectroscopy confirmed the generation of dangling -OH bonds upon addition of CyclicFP-OH to water. Furthermore, conjugating CyclicFP-X with fluorouracil (FU, an anticancer drug) via a reductively cleavable disulfide linker (CyclicFP-SS-FU) demonstrated the general utility of fluoro-crown ether phosphate as a potent carrier for biotherapeutics.

Clay Nanosheet-Based Nanocomposite Supramolecular Hydrogel Enabling Rapid, Reversible Phase Transition Only with Visible Light.

High mechanical properties and rapid sol/gel phase transition are mutually exclusive in the hydrogels reported to date, most likely because the 3D crosslinked networks of mechanically robust hydrogels comprise bundled thick fibers that are not rapidly dissociable or formable.  Herein, we report a visible light-responsive hydrogel that showed a rapid, reversible sol/gel phase transition despite its relatively high mechanical properties (storage modulus ~1000 Pa).  To construct its 3D crosslinked network, we used a design strategy analogous to that employed for our highly water-rich yet mechanically robust nanocomposite supramolecular hydrogel ("aqua material").  In this case, multiple poly(ethylene glycol) chains carrying ortho-tetramethoxyazobenzene termini (AzoPEG) were noncovalently crosslinked by clay nanosheets (CNSs) with surface-immobilized ß-cyclodextrin units using their seven guanidinium ion (Gu+) pendants (GuCD) via a multivalent salt-bridge.  When exposed to visible light at 625 and 450 nm, the azobenzene termini isomerized from trans-to-cis and cis-to-trans, respectively, and were detached from and attached to the surface-immobilized GuCD units.  The advantage of this CNS-based nanocomposite supramolecular system is its simple 3D network structure, which forms and breaks rapidly without slow chain entangling and disentangling processes.

"Photochemical Surgery" of 1D Metal-Organic Frameworks with a Site-Selective Solubilization/Crystallization Strategy.

One-dimensional (1D) hybrid MOFs are attractive if they consist of different MOF blocks with interconnected channels. However, the precision synthesis of such 1D multiblock MOFs with the desired block lengths and sequences remains a formidable challenge. Herein we propose the "photochemical surgery" method, which combines top-down and bottom-up approaches to enable the site-selective solubilization (removal)/crystallization (reconstruction) of 1D MOFs. We employed photoreactive MOFs, which were prepared by complexing either Cd2+ or Zn2+ with a mixture containing a photochromic bispyridyl ligand (PyDTEopen or PyDTZEopen) and an isophthalate (5-nitroisophthalate (nip2-) or 5-bromoisophthalate (bip2-)). These MOFs were obtained as high-aspect-ratio, needlelike, colorless crystals that bore 1D channels oriented parallel to the long needle axis. When photoreactive DTECdMOFNO2 ([Cd(nip)(PyDTEopen)(H2O)]n), for example, was immobilized at both ends with a metal alloy on a glass substrate and exposed to UV light through a photomask for 60 min in N,N-dimethylformamide/methanol (DMF/MeOH), the unmasked part was removed via solubilization to produce a 50 µm gap. The resulting specimen was immersed for 24 h at 25 °C in DMF/MeOH containing the necessary components for the construction of DTZECdMOFNO2 ([Cd(nip)(PyDTZEopen)(H2O)]n). Eventually, the gap was filled with DTZECdMOFNO2 to produce a triblock hybrid MOF (DTECdMOFNO2-DTZECdMOFNO2-DTECdMOFNO2). The result of a guest diffusion experiment confirmed that the newly formed DTZECdMOFNO2 block shared its 1D channels with the host DTECdMOFNO2 blocks. "Photochemical surgery" can be applied to synthesize 1D hybrid MOFs bearing unconventional sequences and morphologies, e.g., honeycomb- and inverted-honeycomb-patterned hybrids.

Pathway Complexity in Nanotubular Supramolecular Polymerization: Metal-Organic Nanotubes with a Planar-Chiral Monomer.

Here, we report an anomalous pathway complexity in the supramolecular polymerization of a chiral monomer, which displays an unusual chiroptical feature that does not follow any of the known stereochemical rules such as "chiral self-sorting" and "majority rule". We newly developed a planar-chiral ferrocene-cored tetratopic pyridyl monomer FcL, which underwent AgBF4-mediated supramolecular polymerization to give nanotubes FcNTs composed of metal-organic nanorings FcNRs. Although FcNRs must be homochiral because of a strong geometrical constraint, FcNRs were formed even efficiently from racemic FcL and AgBF4. Detailed studies revealed the presence of two competing pathways for producing homochiral FcNRs as the constituents of FcNTs: (i) spontaneous cyclization of initially formed acyclic polymers -[FcL-Ag+]n- and (ii) template (FcNR)-assisted cyclization via a Ag+···Ag+ metallophilic interaction. The dominance of the two pathways changes depending on the %ee of chiral FcL. Namely, when the %ee of FcL is high, -[FcL-Ag+]n- must contain sufficiently long homochiral sequences that can be readily cyclized into FcNRs. Meanwhile, when the %ee of FcL is low, the homochiral sequences in -[FcL-Ag+]n- must be short and therefore are hardly eligible for spontaneous cyclization. Why were FcNRs formed? Even though the probability is very low, homochiral -[FcL-Ag+]n- can be statistically generated and undergo spontaneous cyclization to give FcNRs minutely. We found that FcNRs can be amplified by heterochirally templating their own synthesis using metallophilic interaction. Because of this stereochemical preference, the growth of FcNRs into FcNTs via the template-assisted mechanism occurs only when both (R,R)FcL and (S,S)FcL are present in the polymerization system.

Simulating Assembly Landscapes for Comprehensive Understanding of Supramolecular Polymer-Solvent Systems.

Complexity in supramolecular polymer systems arises from interactions between different components, including solvent molecules. By varying their concentration or temperature in such multicomponent systems, complex phenomena can occur such as thermally bisignate and dilution-induced assembly of supramolecular polymers. Herein, we demonstrate that both these phenomena emerge from the same underlying interaction mechanism between the components. As a model system, amide-decorated supramolecular polymers of porphyrins were investigated in combination with aliphatic alcohols as hydrogen-bond scavengers, and thermodynamic mass-balance models were applied to map the three-dimensional assembly landscapes. These studies unveiled that the interaction between hydrogen-bond scavengers and monomers is temperature-dependent and becomes dominant at high monomer concentrations. With these insights, we could exploit competitive monomer-alcohol interactions to prompt the dilution-induced assembly of various common monomers as well as bisignate assembly events. Moreover, kinetic insights were obtained by navigating through the assembly landscape. Similar to phase diagrams of covalent polymers, these assembly landscapes provide a comprehensive picture of supramolecular polymerizations, which helps to precisely regulate the system properties. The generality of this approach using assembly landscapes makes it relevant for any supramolecular system, and this enhanced control will open the door to build complex and functional supramolecular polymer systems.

Solvent-free autocatalytic supramolecular polymerization.

Solvent-free chemical manufacturing is one of the awaited technologies for addressing an emergent issue of environmental pollution. Here, we report solvent-free autocatalytic supramolecular polymerization (SF-ASP), which provides an inhibition-free template-assisted catalytic organic transformation that takes great advantage of the fact that the product (template) undergoes a termination-free nucleation-elongation assembly (living supramolecular polymerization) under solvent-free conditions. SF-ASP allows for reductive cyclotetramerization of hydrogen-bonding phthalonitriles into the corresponding phthalocyanines in exceptionally high yields (>80%). SF-ASP requires the growing polymer to form hexagonally packed crystalline fibres, which possibly preorganize the phthalonitriles at their cross-sectional edges for their efficient transformation. With metal oleates, SF-ASP produces single-crystalline fibres of metallophthalocyanines again in exceptionally high yields, which grow in both directions without terminal coupling until the phthalonitrile precursors are completely consumed. By taking advantage of this living nature of polymerization, multistep SF-ASP without/with metal oleates allows for the precision synthesis of multi-block supramolecular copolymers.

Effects of the Magnetic Orientation of M13 Bacteriophage on Phage Display Selection.

Although phage display selection using a library of M13 bacteriophage has become a powerful tool for finding peptides that bind to target materials on demand, a remaining concern of this method is the interference by the M13 main body, which is a huge filament >103  times larger than the displayed peptide, and therefore would nonspecifically adhere to the target or sterically inhibit the binding of the displayed peptide. Meanwhile, filamentous phages are known to be orientable by an external magnetic field. If M13 filaments are magnetically oriented during the library selection, their angular arrangement relative to the target surface would be changed, being expected to control the interference by the M13 main body. This study reports that the magnetic orientation of M13 filaments vertical to the target surface significantly affects the selection. When the target surface was affinitive to the M13 main body, this orientation notably suppressed the nonspecific adhesion. Furthermore, when the target surface was less affinitive to the M13 main body and intrinsically free from the nonspecific adhesion, this orientation drastically changed the population of M13 clones obtained through library selection. The method of using no chemicals but only a physical stimulus is simple, clean, and expected to expand the scope of phage display selection.

Blending to Make Nonhealable Polymers Healable: Nanophase Separation Observed by CP/MAS 13 C NMR Analysis.

Can commodity polymers are made to be healable just by blending with self-healable polymers? Here we report the first study on the fundamental aspect of this practically challenging issue. Poly(ether thiourea) (PTUEG3 ; Tg =27 °C) reported in 2018 is extraordinary in that it is mechanically robust but can self-heal even at 12 °C. In contrast, poly(octamethylene thiourea) (PTUC8 ; Tg =50 °C), an analogue of PTUEG3 , cannot heal below 92 °C. We found that their polymer blend self-healed in a temperature range above 32 °C even when its PTUEG3 content was only 20 mol %. Unlike PTUEG3 alone, this polymer blend, upon exposure to high humidity, barely plasticized, keeping its excellent mechanical properties due to the non-hygroscopic nature of the PTUC8 component. CP/MAS 13 C NMR analysis revealed that the polymer blend was nanophase-separated, which possibly accounts for why such a small amount of PTUEG3 provided the polymer blend with humidity-tolerant self-healable properties.

ATP-Responsive Nanoparticles Covered with Biomolecular Machine "Chaperonin GroEL".

Herein, we report an ATP-responsive nanoparticle (GroEL NP) whose surface is fully covered with the biomolecular machine "chaperonin protein GroEL". GroEL NP was synthesized by DNA hybridization between a gold NP with DNA strands on its surface and GroEL carrying complementary DNA strands at its apical domains. The unique structure of GroEL NP was visualized by transmission electron microscopy including under cryogenic conditions. The immobilized GroEL units retain their machine-like function and enable GroEL NP to capture denatured green fluorescent protein and release it in response to ATP. Interestingly, the ATPase activity of GroEL NP per GroEL was 4.8 and 4.0 times greater than those of precursor cys GroEL and its DNA-functionalized analogue, respectively. Finally, we confirmed that GroEL NP could be iteratively extended to double-layered ( GroEL ) 2 ${{^{({\rm GroEL}){_{2}}}}}$ NP.

Supramolecular Polymerization of a Photo-Fluttering Chiral Monomer: A Temporarily Suspendable Chain Growth by Light.

Using a photochemically fluttering thiophene-fused cyclooctatetraene derivative (COT) as a nonplanar chiral monomer, we have succeeded in remotely suspending the supramolecular polymerization in a temporal manner by a completely new strategy. The COT monomer with an 8π electron core adopts a saddle shape in the ground state and flutters 5.8 × 103 times faster upon photoirradiation than in the dark as a result of the stabilized planar conformation by the excited-state aromaticity (Baird aromaticity). Detailed investigation revealed that without photoirradiation the rate constant of the fluttering motion is 1/560 times smaller than that of the chain elongation, indicating that the fluttering of COT does not affect the chain elongation in the dark. In contrast, under photoirradiation (365 nm), the fluttering of COT is at least 11 times more rapid than the chain elongation, thereby suppressing the elongation event. The rapid fluttering of COT to suspend the chain elongation is not accompanied by a decrease in active monomer concentration, leading to depolymerization.

Mechanically Robust, Self-Healable Polymers Usable under High Humidity: Humidity-Tolerant Noncovalent Cross-Linking Strategy.

Although mechanically robust polymer materials had not been thought to self-heal, we recently found that poly(ether thiourea) PTUEG3, which is a glassy polymer with high mechanical strength, self-heals even at ambient temperatures. This finding updated the above preconception. Nevertheless, it should also be noted that PTUEG3, under high humidity, absorbs water and is plasticized to lose its mechanical strength. Humidity-induced plasticization is a general problem for polymers with polar groups. Herein, we report that PTUEG3, if designed by copolymerization to contain only 10 mol % of a dicyclohexylmethane (Cy2M) thiourea unit (TUCy2M), serves as a humidity-tolerant, mechanically robust polymer material that can self-heal at ambient temperatures. This copolymer contained, in its ether thiourea (TUEG3)-rich domain, a humidity-tolerant, noncovalently cross-linked 3D network with mechanical robustness formed by stacking of the Cy2M group. The present work provides a promising design strategy for mechanically robust, self-healable polymers usable under high humidity.

Photoreactive Molecular Glue for Enhancing the Efficacy of DNA Aptamers by Temporary-to-Permanent Conjugation with Target Proteins.

We developed a photoreactive molecular glue, BPGlue-N3, which can provide a universal strategy to enhance the efficacy of DNA aptamers by temporary-to-permanent stepwise stabilization of their conjugates with target proteins. As a proof-of-concept study, we applied BPGlue-N3 to the SL1 (DNA aptamer)/c-Met (target protein) conjugate system. BPGlue-N3 can adhere to and temporarily stabilize this aptamer/protein conjugate multivalently using its guanidinium ion (Gu+) pendants that form a salt bridge with oxyanionic moieties (e.g., carboxylate and phosphate) and benzophenone (BP) group that is highly affinitive to DNA duplexes. BPGlue-N3 is designed to carry a dual-mode photoreactivity; upon exposure to UV light, the temporarily stabilized aptamer/protein conjugate reacts with the photoexcited BP unit of adhering BPGlue-N3 and also a nitrene species, possibly generated by the BP-to-N3 energy transfer in BPGlue-N3. We confirmed that SL1, covalently conjugated with c-Met, hampered the binding of hepatocyte growth factor (HGF) onto c-Met, even when the SL1/c-Met conjugate was rinsed prior to the treatment with HGF, and suppressed cell migration caused by HGF-induced c-Met phosphorylation.

Accumulated Lattice Strain as an Internal Trigger for Spontaneous Pathway Selection.

Multicomponent crystallization is universally important in various research fields including materials science as well as biology and geology, and presents new opportunities in crystal engineering. This process includes multiple kinetic and thermodynamic events that compete with each other, wherein "external triggers" often help the system select appropriate pathways for constructing desired structures. Here we report an unprecedented finding that a lattice strain accumulated with the growth of a crystal serves as an "internal trigger" for pathway selection in multicomponent crystallization. We discovered a "spontaneous" crystal transition, where the kinetically preferred layered crystal, initially formed by excluding the pillar component, carries a single dislocation at its geometrical center. This crystal "spontaneously" liberates a core region to relieve the accumulated lattice strain around the dislocation. Consequently, the liberated part becomes dynamic and enables the pillar ligand to invade the crystalline lattice, thereby transforming into a thermodynamically preferred pillared-layer crystal.

Alternating Heterochiral Supramolecular Copolymerization.

In stacking-based supramolecular polymerization, chiral hydrogen bonding (H-bonding) monomers often prefer to adapt a homochiral monomer sequence. Herein, we investigated the polymerization of a chiral thiophene-fused cyclooctatetraene (COT) as a novel nonplanar-core monomer and found the first example of the formation of an alternating heterochiral supramolecular copolymer. Although single enantiomer (-) or (+)-COT alone did not polymerize, when (-) and (+)-COT were mixed together, supramolecular polymerization took place to give a stereochemically alternating copolymer. By means of the microcrystal electron crystallography of a shorter side-chained COT analogue, we found that the resulting heterochiral supramolecular copolymer adapted an alternating arrangement of H-bonded and polar π-stacked parts. A computational study using density-functional theory (DFT) suggested that such an alternating heterochiral preference occurs because it allows two thiophene amide moieties facing each other to effectively cancel their in-plane dipole moments.

Ultralong Organic Phosphorescent Foams with High Mechanical Strength.

Ultralong organic phosphorescence (UOP) has aroused enormous interest in recent years. UOP materials are mainly limited to crystals or rigid host-guest systems. Their poor processability and mechanical properties critically hamper practical applications. Here, we reported a series of ultralong phosphorescent foams with high mechanical strength. Phosphorescence lifetime of the foam can reach up to 485.8 ms at room temperature. Impressively, lightweight gelatin foam can bear a compressive pressure of 4.44 MPa. Moreover, phosphorescence emission of polymer foam can be tuned from blue to orange through varying the excitation wavelength. Experimental data and theoretical calculations revealed that ultralong phosphorescence was ascribed to the fixation of multiple hydrogen bonds to the clusters of carbonyl groups. These results will allow for expanding the scope of luminescent foams, providing an ideal platform for developing ultralong phosphorescent materials with high mechanical strength.

An anisotropic hydrogel with electrostatic repulsion between cofacially aligned nanosheets.

Machine technology frequently puts magnetic or electrostatic repulsive forces to practical use, as in maglev trains, vehicle suspensions or non-contact bearings. In contrast, materials design overwhelmingly focuses on attractive interactions, such as in the many advanced polymer-based composites, where inorganic fillers interact with a polymer matrix to improve mechanical properties. However, articular cartilage strikingly illustrates how electrostatic repulsion can be harnessed to achieve unparalleled functional efficiency: it permits virtually frictionless mechanical motion within joints, even under high compression. Here we describe a composite hydrogel with anisotropic mechanical properties dominated by electrostatic repulsion between negatively charged unilamellar titanate nanosheets embedded within it. Crucial to the behaviour of this hydrogel is the serendipitous discovery of cofacial nanosheet alignment in aqueous colloidal dispersions subjected to a strong magnetic field, which maximizes electrostatic repulsion and thereby induces a quasi-crystalline structural ordering over macroscopic length scales and with uniformly large face-to-face nanosheet separation. We fix this transiently induced structural order by transforming the dispersion into a hydrogel using light-triggered in situ vinyl polymerization. The resultant hydrogel, containing charged inorganic structures that align cofacially in a magnetic flux, deforms easily under shear forces applied parallel to the embedded nanosheets yet resists compressive forces applied orthogonally. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, inspired by articular cartilage, will open up new possibilities for developing soft materials with unusual functions.