Copper-Free Synthesis of Cationic Glycidyl Triazolyl Polymers.

Copper-free synthesis of cationic glycidyl triazolyl polymers (GTPs) is achieved through a thermal azide-alkyne cycloaddition reaction between glycidyl azide polymer and propiolic acid, followed by decarboxylation and quaternization of the triazole unit. For synthesizing nonfunctionalized GTP (GTP-H), a microwave-assisted method enhances the decarboxylation reaction of carboxy-functionalized GTP (GTP-COOH). Three variants of cationic GTPs with different N-substituents [N-ethyl, N-butyl, and N-tri(ethylene glycol) monomethyl ether (EG3)] are synthesized. The molecular weight of GTP-H is determined via size exclusion chromatography. Thermal properties of all GTPs are characterized using differential scanning calorimetry and thermogravimetric analysis. The ionic conductivities of these cationic GTPs are assessed by impedance measurements. The conducting ion concentration and mobility are calculated based on the electrode polarization model. Among three cationic GTPs, the GTP with the N-EG3 substituent exhibits the highest ionic conductivity, reaching 6.8 × 10-6 S cm-1 at 25 °C under dry conditions. When compared to previously reported reference polymers, the reduction of steric crowding around the triazolium unit is considered to be a key factor in enhancing ionic conductivity.

Gold nanowire mesh electrode for electromechanical device.

Ionic polymer-metal composite (IPMC) actuators were prepared with Nafion film as the ionic polymer and gold nanowire (Au-NW) mesh film as the metal electrodes by hot-pressing, which shortened preparation time within 1 h. As a reference, IPMC actuator consisting of Nafion film and gold foil (Au-foil) was also prepared. Au-NW mesh film can be an electrode with thinner (about 150 nm) and lower surface resistivity (about 0.5 Ω sq-1) than the conventional electrode prepared by electroless plating. Larger contact area of the Au-NW mesh electrode than the Au-foil electrode resulted in better actuation performance (60% larger peak-to-peak displacement in actuation). It was confirmed that the transformation behavior of Au-NWs differed depending on the external stimuli condition. Namely Au-NWs transformed to Au nanoparticles in the case of the heat stimulus only. Meanwhile, Au-NWs transformed to plates in the case of the heat and pressure stimuli. While higher temperature improved the adhesion of Au-NW mesh electrode to the Nafion surface, it induced the transformation of nanowire to plates. The IPMC actuator that the Au-NW mesh electrodes were hot-pressed at 90 ºC exhibited the highest capacitance and the largest peak-to-peak displacement in actuation. This research expanded the application field of gold nanowires to the electromechanical devices.

Facile Synthesis of Tetra-Branched Tetraimidazolium and Tetrapyrrolidinium Ionic Liquids.

A facile synthetic route for tetra-branched tetraimidazolium and tetrapyrrolidinium ionic liquids was developed. In contrast to the previous synthetic scheme, the new synthetic route requires only three reaction steps instead of seven. The total yield of tetracation was also improved from 17-21 to 39-41%. Using the new synthetic scheme, four kinds of tetracations were synthesized from the combination of two cationic units (imidazolium and pyrrolidinium) and two counteranions [bis(fluorosulfonyl)imide (FSI) and bis(trifluoromethanesulfonyl)imide (TFSI)]. Basic physical properties including glass transition temperature, thermal decomposition temperature, density, viscosity, and ionic conductivity were determined. The counterion exchange from TFSI to FSI resulted in lower glass transition temperature and higher ionic conductivity. Tetrapyrrolidinium exhibited higher viscosity and lower ionic conductivity than tetraimidazolium. The counterion exchange from TFSI to FSI resulted in lower viscosity in the case of tetraimidazolium, while the opposite result was obtained in the case of tetrapyrrolidinium. Tetracations composed of ethyl imidazolium units, diethylene glycol spacers, and FSI counterions exhibited the highest ionic conductivity of 3.5 × 10-4 S cm-1 at 25 °C under anhydrous conditions. This is the best ionic conductivity in the tetracations ever reported.

Poly(ionic liquid)s Based on Copolymers of Poly(ethylene oxide) and Cationic Glycidyl Triazolyl Polymers with Tribranched Side Chains.

Copolymers comprising poly(ethylene oxide) and cationic glycidyl triazolyl polymer with tribranched side chains (PEO-co-GTP·3X) were synthesized from glycidyl azide copolymer (PEO-co-GAP) and the tricationic alkyne. A synthetic route for the tricationic alkyne was also improved. Bis(trifluoromethanesulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI) were used as counteranions. Copolymers PEO-co-GTP·3TFSI and PEO-co-GTP·3FSI were characterized by NMR, IR, size exclusion chromatography, DSC, TGA, rheological, and impedance measurements. The NMR results suggested that the main chain of the copolymer was more flexible than that of the homopolymer. However, no major changes were detected in the glass transition temperature and ionic conductivity of the homopolymer and copolymer with TFSI counteranions. The counterion exchange from TFSI to FSI resulted in an increase in the storage modulus and complex viscosity because of the ionic association. Despite its unfavorable viscoelastic properties, PEO-co-GTP·3FSI exhibited higher ionic conductivity than PEO-co-GTP·3TFSI (3.9 × 10-5 S cm-1 at 25 °C under anhydrous conditions).

Redox-Responsive and Thermoresponsive Supramolecular Nanosheet Gels with High Young's Moduli.

Supramolecular gels made from 2D building blocks are emerging as one of the novel multifunctional soft materials for various applications. This study reports on a class of supramolecular nanosheet gels formed through a reversible self-assembly process involving both intramolecular folding and intermolecular self-assembly of poly[oligo(ethylene glycol)-co-(phenyl-capped bithiophenes)]. Such hierarchical self-assembled structure allows the gels to switch between sol and gel states under either redox or thermostimulus. Moreover, the gels illustrate high Young's moduli, compared to their controls that are made from the same oligo(ethylene glycol) and phenyl-capped bithiophenes blocks but have highly covalent-crosslinked structures. The example might open a window for emerging supramolecular 2D materials to develop mechanically robust and stimuli-responsive soft materials without compromising their intrinsic functions.

Glycidyl Triazolyl Polymers: Poly(ethylene glycol) Derivatives Functionalized by Azide-Alkyne Cycloaddition Reaction.

Glycidyl triazolyl polymer (GTP), which is the product of the Huisgen dipolar cycloaddition reaction between glycidyl azide polymer and alkyne derivatives, is featured here. GTP is the multifunctionalized poly(ethylene glycol) (PEG). The drawback of PEG is that linear PEG has the functional group only at both ends. The low loading capability of the functional groups limits the possibilities of PEG applications. GTP facilitates the synthesis of multifunctionalized PEG derivatives. In this article, 74 examples of GTP homopolymers and copolymers are introduced. The synthetic protocols and work-up processes of GTP are summarized. In addition, application studies are reviewed: for example, stimuli-responsive and self-healing materials, materials for electrical memory devices, ion-conductive materials, and biomedical materials. Finally, some issues on GTP synthesis and future directions for GTP-based polymer materials are proposed.

Stacked-Layer Heterostructure Films of 2D Thiophene Nanosheets and Graphene for High-Rate All-Solid-State Pseudocapacitors with Enhanced Volumetric Capacitance.

Stacked-layer heterostructure films of 2D thiophene nanosheets and electrochemically exfoliated graphene are constructed for ultrahigh-rate all-solid-state flexible pseudocapacitors and micro-supercapacitors with superior volumetric capacitance due to the synergetic effect of the ultrathin pseudocapacitive thiophene nanosheets and the capacitive electrochemically exfoliated graphene.

Control of optical and electrical properties of nanosheets by the chemical structure of the turning point in a foldable polymer.

Oligomers of tetra(ethylene glycol)-disubstituted phenyl-capped bithiophene (Ph2TPh) linked by catechol and resorcinol were prepared. Catechol and resorcinol link the monomers via the ortho- and meta-positions of the benzene ring, respectively, and function as turning points in the folding process of the polymer. It was confirmed that the ortho-linked 8mer (o-8mer) and meta-linked 8mer (m-8mer) could form nanosheets through the self-assembly of folded polymers in o-dichlorobenzene. We confirmed that the arrangement of thiophene units inside the nanosheets was controllable by changing the chemical structure of the turning point. The different arrangements of the Ph2TPh units led to changes in other physical properties such as UV-Vis absorption, nanosheet thickness and charge carrier transport. The absorption spectrum of the o-8mer nanosheets suggested that the Ph2TPh units are arranged vertical to the lateral direction of the nanosheets. On the other hand, the Ph2TPh units in the m-8mer nanosheets were considered to have a tilted orientation. The change in the Ph2TPh tilt angle inside the nanosheets was supported by the different thicknesses of the o-8mer and m-8mer nanosheets. The relationship between the absorption spectrum and Ph2TPh unit arrangement was discussed based on the DFT calculation. Intrinsic charge carrier transport properties were evaluated by a noncontact microwave-based method. The o-8mer nanosheets showed higher conductivity than the m-8mer and triazole-linked-8mer nanosheets. The lifetime of charge carriers in the nanosheet was longer than that in the lamellar structure of the drop-cast film.

Effect of polymerization on hierarchical self-assembly into nanosheets.

The oligomers consisting of phenyl-capped bithiophene and tetra(ethylene glycol)s linked by azide-alkyne Huisgen cycloaddition were synthesized. The relationship between the degree of polymerization and self-assembling ability was investigated in o-dichlorobenzene and dimethyl sulfoxide. From the absorption spectrum, it was confirmed that the critical degree of polymerization (CDP) for thiophene unit aggregation was 4. The morphology of the aggregated product was observed by atomic force microscopy. The oligomers 4mer and 5mer could not self-assemble into well-defined structures due to the weak driving force for the self-assembly. In the cases of 6mer and 7mer, aggregates with nonwell-defined and nanosheet structures coexisted. In the cases of 8mer and 9mer, the nanosheet was the main product. The critical point between 7mer and 8mer could be confirmed by different aggregation behaviors in the cooling process of the solution (nonsigmoidal and sigmoidal). In the cases of 8mer and 9mer, polymer folding prior to intermolecular self-assembly, which was supported by sigmoidal aggregation behavior, leads to the nanosheet formation. On the contrary, shorter oligomers than 8mer experience intermolecular aggregation prior to intramolecular polymer folding, which was supported by the nonsigmoidal aggregation behavior. This is the first report to prove the existence of CDP for folded polymer nanosheet formation which requires hierarchical self-assembly, i.e., polymer folding followed by intermolecular self-assembly.

Electrochemically durable thiophene alkanethiol self-assembled monolayers.

Thiophene-based redox-active self-assembled monolayers (SAMs) were prepared on gold substrates. The alkanethiol derivatives of 1TPh-OC12SH and ETPh-OC12SH contain thiophene (1T) and 3,4-ethylenedioxythiophene (ET) units, respectively, with unprotected (nonsubstituted) thiophene α-carbons. PhETPh-OC12SH contains the ET unit, and all thiophene carbons are protected. Using these thiophene alkanethiol derivatives, we characterized the effect of thiophene carbon protection on the redox behavior of the thiophene SAMs by cyclic voltammetry. The formation of SAMs was confirmed by X-ray photoelectron spectroscopy and reflective IR. The IR peaks in the fingerprint region were assigned with the help of DFT calculations. Although 1TPh-OC12SH and ETPh-OC12SH SAMs lost their electrochemical activity during the first anodic scan, PhETPh-OC12SH SAMs are stable and maintain their electrochemical activity for at least 1200 redox cycles.

Electrochromic properties of polythiophene polyrotaxane film.

The electrochromic properties of a polythiophene polyrotaxane film consisting of a polythiophene backbone wrapped by the tetra-cationic cyclophane, cyclobis(paraquat-p-phenylene), were characterized. A naked reference polythiophene film, i.e., polythiophene without tetra-cationic cyclophane, was also characterized. The surface morphology and thickness of the film (L) were observed by atomic force microscopy. The surface of the naked reference polythiophene film has micrometer-scale polythiophene aggregates, which causes the darker color of the film and smaller color contrast in the electrochromic process. The polythiophene polyrotaxane gives a more homogeneous and brighter colored film owing to the suppression of molecular interactions between the polythiophene chains by the tetra-cationic cyclophanes. Potential-step chronoamperometric measurement provided the area density of the oxidizable sites (Γ) and the apparent diffusion coefficient of the charge transport in the film. From linear relationship between L and Γ, the concentrations of the oxidizable sites in the polythiophene polyrotaxane and naked reference polythiophene films were calculated to be 1.3 and 2.4 mmol cm(-3), respectively. Interestingly, the polythiophene polyrotaxane film afforded a significantly larger apparent diffusion coefficient than the naked reference polythiophene film. This result suggests that the rate-determining step of the charge transport is not the electron hopping between the polythiophene chains but the transport of charge-compensating counterions from the solvent into the polythiophene. We believe that the counteranions of the tetra-cationic cyclophane provide a pathway allowing the charge-compensating counteranions to migrate from the solvent to polythiophene. The polythiophene polyrotaxane film showed faster color change than the naked reference polythiophene film in the electrochromic reaction. These results indicate that our polythiophene polyrotaxane is a better electrochromic material than the naked reference polythiophene.

Molecular motion of surface-immobilized double-decker phthalocyanine complexes.

The molecular motion of surface-immobilized double-decker phthalocyanine complexes was examined using STM. (C(8)OPc)(2)Ce (1), (C(12)OPc)(2)Ce (2), and (C(8)OPc)Ce(Pc) (3) double-decker complexes, of which two ligands contained Pc nuclei, formed well-ordered self-organized structures on their own. Square-shaped top Pc ligands were clearly observed for complexes 1, 2, and 3 even though free space presented around the top ligands caused by mixing the complexes with template molecules. However, the details of the shapes of the top ligands of complexes 1, 2, and 3 were changed by the surrounding environment. The surrounding environment was considered to have influenced the mobility of the top ligands. Another complex, (C(8)OPc)Ce(TPP) (4), had difficulty forming a self-organized structure by itself. Complex 4 could have been immobilized by coadsorbing on the substrate with the C(8)OPc template, but the intramolecular structure of the top ligands of complex 4 was difficult to observe. The results strongly suggested that combinations of molecules composed of double-decker complexes as well as the free space presented around a top ligand are important factors that control the molecular motion of immobilized double-decker complexes on solid surfaces.

Thermodynamic forecasting of mechanically interlocked switches.

Mechanically interlocked molecular (MIM) switches in the form of bistable [2]rotaxanes and [2]catenanes have proven to be--when incorporated in molecular electronic devices (MEDs) and in nanoelectromechanical systems (NEMS)--a realistic and viable alternative to the silicon chip density challenge. Structural modifications and chemical environment can have a large impact on the relaxation thermodynamics of the molecular motions, such as translation and circumrotation in bistable rotaxanes and catenanes responsible for the operation of devices based on MIMs. The effects of structural modifications on the difference in free energy (DeltaG(o)) for the equilibrium processes in switchable MIMs can be predicted by considering, firstly, the interactions present in their precursor pseudorotaxanes. By employing isothermal titration microcalorimetry (ITC) to investigate the thermodynamic parameters governing pseudorotaxane formation for a series of monosubstituted, acceptor host cyclophanes with various donor guests, in conjunction with X-ray crystallographic data, an obvious link between the noncovalent bonding interactions in pseudorotaxanes and MIMs that survive following the formation of the mechanical bond can be identified. It follows that the changes (DeltaDeltaG(o) values) in the difference of free energy during the formation of different pseudorotaxanes can subsequently be extrapolated to predict DeltaG(o) values for the thermodynamics associated with switching in analogous MIM switches, employing the same donor-acceptor recognition components. In this manner, a systematic and predictive thermodynamic approach to designing and tuning switchable MIMs and MIM-based materials has been established. Additionally, these thermodynamic relationships are reminiscent of the long forgotten concept of the 'parachor' as a molecular descriptor with respect to the additivity of physical properties in chemical systems dealing specifically with quantitative structure property-activity relationships (QSPR/QSAR).

From thiophene [2]rotaxane to polythiophene polyrotaxane.

The polythiophene polyrotaxane was synthesized through electrochemical polymerization of the [2]rotaxane consisting of the electron-rich dumbbell-shaped sexithiophene and the electron-deficient cyclophane of cyclobis(paraquat-p-phenylene). The optical and electrochemical property of the polythiophene polyrotaxane film was characterized. The material reported herein is attractive not only as a component for constructing the macromolecular machine but also as a new type of insulated molecular wire having donor-acceptor interaction between the macrocycle and the conductive polymer.

Beta-substituted terthiophene [2]rotaxanes.

Two kinds of beta-substituted terthiophene [2]rotaxanes were synthesized using the host-guest pairs of the electron-deficient cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) and the electron-rich terthiophenes with diethyleneglycol chains at the beta-position. One is made from the alpha-position non-substituted terthiophene (3 T-beta-Rx) and the other is made from the alpha-dibromo-substituted terthiophene (3 TBr-beta-Rx). The binding constants of the beta-substituted terthiophene threads were confirmed to be smaller than that of the alpha-substituted terthiophene analogue. By UV/Vis absorption measurements, we confirmed the charge-transfer (CT) band in the visible region with an extinction coefficient of approximately 10(2) (M(-1) cm(-1)). Strong, but not quantitative, quenching of the terthiophene fluorescence was confirmed for the [2]rotaxanes. Although the beta-substituted terthiophene thread was electrochemically polymerizable, the [2]rotaxane 3 T-beta-Rx was not polymerizable. This result indicates that the interlocked CBPQT(4+) macrocycle effectively suppresses the electrochemical polymerization of the terthiophene unit because electrostatic repulsive and steric effects of CBPQT(4+) hinder the dimerization of the terthiophene radical cations. In the electrochemical measurement, we confirmed the shift of the first reduction peak towards less negative potential compared to free CBPQT(4+) and the splitting of the second reduction peak. These electrochemical behaviors are similar to those observed for the highly-constrained [2]rotaxanes. The beta-substituted terthiophene [2]rotaxanes reported herein are important key compounds to prepare polythiophene polyrotaxanes.

A bistable poly[2]catenane forms nanosuperstructures.

Side-chain poly[2]catenanes at the click of a switch! A bistable side-chain poly[2]catenane has been synthesized and found to form hierarchical self-assembled hollow superstructures of nanoscale dimensions in solution. Molecular electromechanical switching (see picture) of the material is demonstrated, and the ground-state equilibrium thermodynamics and switching kinetics are examined as the initial steps towards processible molecular-based electronic devices and nanoelectromechanical systems.

Scanning tunneling microscopy observation of self-assembled monolayers of strapped porphyrins.

In this paper, we reveal that the free-base and zinc strapped porphyrins possessing long alkyl chains, C 24OPP-HQ and Zn(C 24OPP-HQ), respectively, can be arranged on surfaces. We used scanning tunneling microscopy (STM) to observe alkyl-chain-assisted self-assembled monolayers (SAMs) of these strapped porphyrins at the solid-liquid interface. STM images revealed that the strapped benzene moiety was detectable on the porphyrin core: that is, the strapped porphyrins could be differentiated from nonstrapped analogues. We compared the population of the nonstrapped porphyrin (C 24OPP) and either of the strapped porphyrins C 24OPP-HQ or Zn(C 24OPP-HQ) in the mixed SAMs. We then confirmed that Zn(C 24OPP-HQ) is more favorably incorporated in the mixed SAMs than C 24OPP-HQ. From (1)H NMR spectroscopic and X-ray crystallographic analyses, we concluded that the factors increasing the population of Zn(C 24OPP-HQ) in the mixed SAMs are the enhanced rigidity of the porphyrin core by the zinc coordination and the flat structure of the porphyrin moiety in the saddle conformation. This study demonstrates that strapped porphyrins possessing long alkyl chains are available to arrange the functional modules on the surface via chemical modification on the strapped moiety.

Thiophene donor-acceptor [2]rotaxanes.

A series of the thiophene donor-acceptor [2]rotaxanes have been synthesized based on the inclusion complexes of cyclobis(paraquat- p-phenylene) (CBPQT4+) with thiophene, bithiophene, and terthiophene. The maximum wavelength of the charge-transfer band strongly depends on the number of thiophene units, while the association constant does not. These donor-acceptor pairs will be fascinating constituents for optoelectronic and electromechanical materials.

Alkyl chain length dependence of the self-organized structure of alkyl-substituted phthalocyanines.

The alkyl chain length on alkyl-substituted phthalocyanines (C(n)OPc) dependence of their self-organized structures was examined in this study. STM results indicated that the symmetry of ordered structures decreased as the alkyl chain became longer, with the exception of C(6)OPc, which preferentially formed a quasi-3-fold symmetrical structure. This could be explained by the fact that the C(n)OPc molecules are most likely to form densely packed structures. With C(n)OPc, when n = 4 to 10, the self-organized structures were dependent on the competition between how densely the molecules were arranged and how loose the intermolecular interaction energy was, caused by the formation of the densely packed structure. However, with C(n)OPc, when n = 10-18, the molecules tended to form densely packed structures by reducing the symmetry, even though the C(n)OPc molecules were distorted. When C(12)OPc and cobalt phthalocyanine were coadsorbed, the mixed system exhibited a four-fold symmetrical structure, which is rarely observed in C(12)OPc.