Seamless, Self-Transformation of Thermoplastic Polyesters into Vitrimers Through Bond Exchange-Triggered Cross-Linking.

In this study, a seamless, self-transformation system of linear thermoplastic polyesters into the sustainable cross-linked polymers, vitrimers, is demonstrated. The key is the use of polyesters bearing abundant hydroxyl side groups, which are synthesized via the reaction using dithiol molecules bearing ester units and diepoxy molecules. The polymerization reaction progresses efficiently at relatively low temperature due to the click nature of the thiol-epoxy reaction, which provides the hydroxyl side groups along the polyester chain. The tin catalyst (stannous octoate) is added in the initial polymerization, and the catalyst also works to cross-link the polyesters via intermolecular transesterification bond exchange simply by heating at high temperatures. By adjusting the degrees of cross-linking, the mechanical properties as well as the thermal properties are well tuned. The bond exchange can still be activated in the final cross-linked sample; and thus, the material behaves as vitrimers, exhibiting mechanical recyclability. The application of a new type of hot melt adhesive, where the post-coating tuning/enhancement of adhesion strength is realized, is also demonstrated. On the whole, the present system is very simple but proposes a new application window of bond exchange concept into self-transformation polymers.

Trapping bond exchange phenomenon revealed for off-stoichiometry cross-linking of phase-separated vitrimer-like materials.

Vitrimer materials combined with nano-phase separated structures have attracted attention, expanding the tuning range of physical properties, such as flow and creep properties. We recently demonstrated a preparation of vitrimer-like materials with phase-separated nanodomains in which dissociative bond exchange via trans-N-alkylation of quaternized pyridine was operated. In this study, we demonstrate a new finding about the bond exchange mechanism: that is, the trapping bond exchange phenomenon. The component polymer is a poly(acrylate) containing pyridine side groups randomly along the chain, which is cross-linked by diiodo molecules via pyridine-iodo quaternization, where the quaternized pyridines are aggregated to form nano-size domains. When the cross-linking reaction is performed at an off-stoichiometric pyridine : iodo ratio (i.e., an excess of pyridine groups), free pyridine groups are located in the matrix phase. Since the bond exchange in the present system progresses in an inter-domain manner, the dissociated unit bearing pendant iodo is trapped by the free pyridine groups in the matrix, which generates other small aggregates. This trapping phenomenon greatly affects the relaxation and creep properties, which are very different from those found in conventional knowledge about vitrimer physics.

Noncovalent tailoring of coordination complexes by resorcin[4]arene-based supramolecular hosts.

Molecular recognition of guest molecules in a confined cavity is one of the important phenomena in biological and artificial molecular systems. When the guest is trapped within an artificial nano-space, its conformation is fixed in an unusual fashion by noncovalent interactions with host frameworks, and also the guest is kept away from the bulk solvent by the steric effect of the host. Therefore, host-guest formations lead to the effective modulation of the chemical and physical properties of guests via noncovalent interactions. In contrast to the many examples of organic guests, the examples of host-guest formation using coordination complex guests have been less explored. This is simply due to the size and shape complementarity problem between small hosts and large coordination complex guests. Resorcin[4]arene-based supramolecular hosts have been shown to provide internal cavities that are large enough to fully accommodate coordination complexes within the internal spaces via effective molecular interactions. In this article, we focus on supramolecular strategies to control the chemical and physical properties of the coordination complex guests within resorcin[4]arene-based supramolecular hosts. By the careful selection of the host and guest complexes, these combinations can produce a new supramolecular system, showing unusual structures, redox, catalytic, and photophysical properties derived from the entrapped coordination complexes.

Unusual Stress Upturn in Elastomers Prepared Using Macro Cross-Linkers with Multiple Vinyl Side Groups.

In this study, the unique tensile properties of acrylate elastomers prepared using macro cross-linker polymers with multiple vinyl side groups are analyzed. For the preparation of the macro cross-linker, poly(ethyl acrylate) copolymers bearing hydroxy functional groups are synthesized, followed by the hydroxy-isocyanate reaction with 2-isocyanatoethyl acrylate. Subsequently, the elastomers samples are prepared by UV polymerization of ethyl acrylate in the presence of the macro cross-linkers. The tensile properties of the elastomers in the small elongation region are similar to those of typical elastomers prepared using divinyl cross-linkers, whereas the stress upturn in the large elongation region is considerably different. The stress upturn varies based on the fraction of vinyl side groups in the macro cross-linkers, whereas stress in the small elongation region remains unchanged. These properties are analyzed using various theoretical models. The results reveal that there is artificial inhomogeneity in the cross-link density for samples prepared by the macro cross-linkers, where the short poly(ethyl acrylate) strands inside the macro cross-linker limit the overall chain stretchability. On the whole, this study demonstrates a new method for tuning elastomer properties, especially at large deformation.

Self-healable fiber-reinforced vitrimer composites: overview and future prospects.

To achieve sustainable development goals, approaches towards the preparation of recyclable and healable polymeric materials is highly attractive. Self-healing polymers and thermosets based on bond-exchangeable dynamic covalent bonds, so called "vitrimers" could be a great effort in this direction. In order to match the industrial importance, enhancement of mechanical strength without sacrificing the bond exchange capability is a challenging issue, however, such concerns can be overcome through the developments of fiber-reinforced vitrimer composites. This article covers the outstanding features of fiber-reinforced vitrimer composites, including their reprocessing, recycling and self-healing properties, together with practical applications and future perspectives of this unique class of materials.

Strong Proton-Electron Coupling in π-Planar Metal Complex with Redox-Active Ligands.

Proton-coupled electron transfer (PCET) of metal complexes has been widely studied, especially in biochemistry and catalytic chemistry. Although metal complexes bearing redox-active ligands play a part in these research areas, those with π-planar structure remain entirely unexplored, which are vital for future development of iono-electronics. Here, proton-electron coupling of a π-planar nickel complex bearing redox-active N,S-ligands, Ni(itsq)2 , was investigated by combining experimental and theoretical approaches. Strong proton-electron coupling was manifested in a large potential shift, which is twice greater than that of a typical PCET-type π-planar metal complex with redox-inactive ligands, [Ni(dcpdt)2 ]2- . Theoretical calculations affirmed that the stabilization of frontier orbitals by protonation is greater in Ni(itsq)2 than that in [Ni(dcpdt)2 ]2- . These results indicate that π-planar metal complexes with redox-active ligands are promising for developing novel PCET-type materials.

Various Stacking Patterns of Two-Dimensional Molecular Assemblies in Hydrogen-Bonded Cocrystals: Insight into Competitive Intermolecular Interactions and Control of Stacking Patterns.

Control over the stacking patterns in 2D molecular assemblies is demonstrated using chemical modification. A target system is a hydrogen-bonded cocrystal (2:1) composed of 2-pyrrolidone (Py) and chloranilic acid (CA) (PyCA). X-ray crystallography showed that weak intersheet interactions give rise to a variety of metastable overlapping patterns comprised of the 2D assemblies mainly formed via hydrogen bonds, affording reversible and irreversible structural phase transitions. We prepared cocrystals of Py and anilic acids bearing different halogens, in which 2D assemblies isostructural with those observed in PyCA exhibit various overlapping patterns. The order of stability for each overlapping pattern estimated using calculations of the intermolecular interactions did not completely coincide with those indicated by our experimental results, which can be explained by considering the entropic effect: the molecular motion of Py as detected using nuclear quadrupole resonance spectroscopy.

Implantation of Recyclability and Healability into Cross-Linked Commercial Polymers by Applying the Vitrimer Concept.

Vitrimers are a new class of cross-linked materials that are capable of network topology alternation through the associative dynamic bond-exchange mechanism, which has recently been invented to solve the problem of conventional cross-linked materials, such as poor recyclability and healability. Thus far, the concept of vitrimers has been applied to various commercial polymers, e.g., polyesters, polylactides, polycarbonates, polydimethylsiloxanes, polydienes, polyurethanes, polyolefins, poly(meth)acrylates, and polystyrenes, by utilizing different compatible bond-exchange reactions. In this review article, the concept of vitrimers is described by clarifying the difference from thermoplastics and supramolecular systems; in addition, the term "associative bond-exchange" in vitrimers is explained by comparison with the "dissociative" term. Several useful functions attained by the vitrimer concept (including recyclability and healability) are demonstrated, and recent molecular designs of vitrimers are classified into groups depending on the types of molecular frameworks. This review specifically focuses on the vitrimer molecular designs with commercial polymer-based frameworks, which provide useful hints for the practical application of the vitrimer concept.

Role of d-Elements in a Proton-Electron Coupling of d-π Hybridized Electron Systems.

We first demonstrate the influence of d-elements on the electronic-state alternation of molecules coupled with proton transfer in d-π hybridized electron systems. Compact and planar metal complexes with protonated 2,3-pyrazinedithiolates (L), M(HL)2 (M = Ni, Pd, and Pt), were synthesized and subsequently determined to be assembled by hydrogen bond (H-bond) interactions between pyrazine moieties. Structural and theoretical investigations revealed that these complexes are regarded as d-π hybridized electron systems based on a M(S2C2)2 core, especially, significant d-π hybridization in the Pt(S2C2)2 core was indicated. The pH-dependent optical and electrochemical measurements revealed that the Ni complex has a higher proton-accepting character and a stronger pH dependence for redox potential compared with the Pt complex. This indicates that the Ni complex has a larger amount of π-electron density on ligands than the Pt complex because the significant d-π hybridization in the Pt complex could reduce the amount of π-electron reconstructed by attaching/detaching proton. Cyclic voltammetry of Ni and Pt complexes that form an H-bonded multimer showed a potential splitting at the first redox wave (ΔE1/2 = 0.28 V for M = Ni and 0.17 V for M = Pt) corresponding to a mixed-valence state coupled with proton transfer. The ΔE1/2 values indicate that the change in electronic states by proton transfer is remarkable in the Ni complex, but moderate in the Pt complex. These experimental results lead that the d-element substitution plays a role in controlling the degree of proton-electron coupling in d-π hybridized electron systems.

Rational Design of Proton-Electron-Transfer System Based on Nickel Dithiolene Complexes with Pyrazine Skeletons.

To understand the effect of chemical modification on the stability and proton-electron coupling in neutral radical molecules with a proton-electron-transfer (PET) state, we investigate a nickel dithiolene complex with cyano-substituted pyrazine skeletons using experimental and theoretical methods. A Pourbaix diagram constructed from absorption spectroscopic and cyclic voltammetric measurements strongly suggests that the PET state of the complex is significantly more stable compared with that of the nonsubstituted complex. Theoretical calculations predicted that the introduction of electron-withdrawing groups leads to stabilization of the PET state mainly because of a greater delocalized electron distribution in the molecule. Crystallographic studies, with the support of theoretical calculations, revealed that the degree of coupling between protons and electrons varies depending on the Hammett σ value of the substituents; the electronic state of the nonsubstituted complex appears to be most sensitive to the protonated state mainly owing to the spatially confined π-electron system.

Probing charge transfer characteristics in a donor-acceptor metal-organic framework by Raman spectroelectrochemistry and pressure-dependence studies.

The stimuli responsive behaviour of charge transfer donor-acceptor metal-organic frameworks (MOFs) remains an understudied phenomenon which may have applications in tuneable electronic materials. We now report the modification of donor-acceptor charge transfer characteristics in a semiconducting tetrathiafulvalene-naphthalene diimide-based MOF under applied electrochemical bias and pressure. We employ a facile solid state in situ Raman spectroelectrochemical technique, applied for the first time in the characterisation of electroactive MOFs, to monitor the formation of a new complex TTFTC˙+-DPNI from a largely neutral system, upon electrochemical oxidation of the framework. In situ pressure-dependent Raman spectroscopy and powder X-ray diffraction experiments performed in a diamond anvil cell revealed blue shifts in the donor and acceptor vibrational modes in addition to contractions in the unit cell which are indicative of bond shortening. This study demonstrates the utility of in situ Raman spectroscopic techniques in the characterisation of redox-active MOFs and the elucidation of their electronic behaviours.

Drastic rearrangement of self-assembled hydrogen-bonded tapes in a molecular crystal.

A 2 : 1 hydrogen-bonded crystal of 2-pyrrolidone and chloranilic acid shows structural phase transitions accompanied by the drastic rearrangement of hydrogen-bonded tapes. Such a phenomenon is attributed to the selective and directional character of hydrogen bonds.

Effectiveness of personal genomic testing for disease-prevention behavior when combined with careful consultation with a physician: a preliminary study.

OBJECTIVES: There are many direct-to-consumer (DTC)-type personal genomic testing (PGT) services commercially available to the public, providing the specific disease susceptibilities of individuals. While these services do not appear to stimulate disease-prevention behavior, few studies have addressed the methods to do so. We investigated the effectiveness of combining a consultation with a physician with the delivery of test results from a DTC-type PGT, as a preliminary study to identify the effective genomic testing for disease-prevention. A prepared physician disclosed the PGT results of twenty healthy subjects and provided a specific consultation on the high-risk diseases for each subject. The effects on the sense of health, understanding of possible future diseases, and preventive behaviors for each subject were examined pre-PGT, post-PGT, and 3, 6, and 12 months post-PGT. RESULTS: Significant increases between the pre- and post-PGT scores were observed for the awareness of lifestyle effects on developing those diseases (P < 0.05) and the awareness of the ability to influence disease onset (P < 0.01). The follow-up questionnaire results showed that over 60% of the subjects changed their lifestyles in favor of disease prevention. These results suggest that combining the DTC-PGT with a careful physician consultation may be effective at motivating people toward preventive behavior.

N-Heterocyclic Carbene Initiated Anionic Polymerization of (E,E)-Methyl Sorbate and Subsequent Ring-Closing to Cyclic Poly(alkyl sorbate).

A diene-based cyclic polymer has been synthesized by the anionic polymerization of methyl sorbate (MS) by an N-heterocyclic carbene (NHC) in the presence of a bulky aluminum Lewis acid. We first polymerized methyl sorbate (MS) initiated by NHC in N,N-dimethylformamide (DMF) at 25 °C, poly(MS) with a number-average molecular weight (Mn) of 3.5 × 103 (Mw/Mn = 2.1) was obtained with a conversion of 93%. The structure was confirmed by 1H and 13C NMR and IR spectra, which revealed that the propagation proceeded via 1,2-addition as well as 1,4-addition. Although the polymerization did not occur in toluene in the absence of any additive, quantitative monomer consumption was observed in the presence of methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) to afford the poly(MS) with a 1,4-trans structure, 86% of threo diastereoselectivity, and a Mn of 23.0 × 103 with narrow molecular weight distribution (Mw/Mn = 1.17). From the matrix assisted laser desorption/ionization (MALDI-TOF) mass spectra of poly(MS) and the hydrogenated analogue, ring-closing occurred by nucleophilic attack of the anionic propagating center into the adjacent carbon of the α-terminal imidazolimium group to afford cyclic poly(MS). The cyclic formation in the present synthesis system was confirmed by DSC and viscosity measurements.

Discovery of Hexagonal Structured Pd-B Nanocrystals.

We report on hexagonal close-packed (hcp) palladium (Pd)-boron (B) nanocrystals (NCs) by heavy B doping into face-centered cubic (fcc) Pd NCs. Scanning transmission electron microscopy-electron energy loss spectroscopy and synchrotron powder X-ray diffraction measurements demonstrated that the B atoms are homogeneously distributed inside the hcp Pd lattice. The large paramagnetic susceptibility of Pd is significantly suppressed in Pd-B NCs in good agreement with the reduction of density of states at Fermi energy suggested by X-ray absorption near-edge structure and theoretical calculations.

An Electrically Conductive Single-Component Donor-Acceptor-Donor Aggregate with Hydrogen-Bonding Lattice.

An electrically conductive D-A-D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex 1H2). The crystal structure of complex 1H2 has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex 1H2 is semiconducting (Ea = 0.29 eV, ρrt = 9.1 × 104 Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (Ea = 0.13 eV, ρrt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π-π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D-A-D aggregate.

Highly Extensible Supramolecular Elastomers with Large Stress Generation Capability Originating from Multiple Hydrogen Bonds on the Long Soft Network Strands.

Highly extensible supramolecular elastomers are prepared from ABA triblock-type copolymers bearing glassy end blocks and a long soft middle block with multiple hydrogen bonds. The copolymer used is polystyrene-b-[poly(butyl acrylate)-co-polyacrylamide]-b-polystyrene (S-Ba-S), which is synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Tensile tests reveal that the breaking elongation (εb ) increases with an increase in the middle block molecular weight (Mmiddle ). Especially, the largest S-Ba-S with Mmiddle of 3140k, which is synthesized via high-pressure RAFT polymerization, achieves εb of over 2000% with a maximum tensile stress of 3.6 MPa, while the control sample without any middle block hydrogen bonds, polystyrene-b-poly(butyl acrylate)-b-polystyrene with Mmiddle of 2780k, is merely a viscous material due to the large volume fraction of soft block. Thus, incorporation of hydrogen bonds into the large molecular weight soft middle block is found to be beneficial to prepare supramolecular elastomers attaining high extensibility and sufficiently large stress generation ability simultaneously. This outcome is probably due to concerted combination of entropic changes and internal potential energy changes originating from the dissociation of multiple hydrogen bonds by elongation.

A compact planar low-energy-gap molecule with a donor-acceptor-donor nature based on a bimetal dithiolene complex.

We present the first report of a compact, planar and low-energy-gap molecule based on a π-conjugated bimetal system comprising a tetrathiooxalate (tto) skeleton. The observed low HOMO-LUMO energy gap (1.19 eV) is attributed to its donor-acceptor-donor (D-A-D) nature because the skeleton acts as an electron acceptor as well as a tiny and noninnocent bridging moiety.

Hybrid materials of Ni NP@MOF prepared by a simple synthetic method.

We demonstrate a novel, simple synthetic method for metal (Ni) NPs in a MOF using the partial thermal decomposition of nickel(II) 2,5-dihydroxyterephthalate (Ni-MOF-74). The Ni NPs inside the Ni-MOF-74 are several nanometers in size, and the size can be precisely controlled by the heating conditions.

Coordination programming of photofunctional molecules.

Our recent achievements relating to photofunctional molecules are addressed. Section 1 discloses a new concept of photoisomerization. Pyridylpyrimidine-copper complexes undergo a ring inversion that can be modulated by the redox state of the copper center. In combination with an intermolecular photoelectron transfer (PET) initiated by the metal-to-ligand charge transfer (MLCT) transition of the Cu(I) state, we realize photonic regulation of the ring inversion. Section 2 reports on the first examples of heteroleptic bis(dipyrrinato)zinc(II) complexes. Conventional homoleptic bis(dipyrrinato)zinc(II) complexes suffered from low fluorescence quantum yields, whereas the heteroleptic ones feature bright fluorescence even in polar solvents. Section 3 describes our new findings on Pechmann dye, which was first synthesized in 1882. New synthetic procedures for Pechmann dye using dimethyl bis(arylethynyl)fumarate as a starting material gives rise to its new structural isomer. We also demonstrate potentiality of a donor-acceptor-donor type of Pechmann dye in organic electronics.