G. Piao

Helical polyacetylene synthesized under chiral nematic liquid crystals

Abstract We synthesized helical polyacetylene films under asymmetric and anisotropic reaction field consisting of chiral nematic (N * ) liquid crystals. A series of axially chiral binaphthols and chiral phenylcyclohexyl derivatives were used as chiral dopants to induce N * liquid crystals. Acetylene polymerizations were carried out with Ti(O- n -Bu) 4 -Et 3 Al under N * reaction field. Measurements of SEM and CD spectra showed that the polyacetylene films are composed of clockwise or counter-clockwise helical structures not only in fibril but also in each polyene chain. By virtue of its high electrical conductivity of 1.51.8 x 103 S/cm after iodine doping, the helical polyacetylene might serve as a molecular solenoid based on conducting polymer. The helicity of the present film also suggests a feasibility of second-order nonlinear optical material.

Helical polyacetylene heavily doped with iodine: Magnetotransport

The magnetoconductivity of two kinds of iodine doped helical polyacetylene, R- and S-polyacetylene, were investigated to understand the effect of the intrafibril and the interfibrillar interaction in the polyacetylene system. The zero-field resistivity ratio, ρr=ρ(1.2 K)/ρ(300 K), is comparable to that of stretch-oriented high-density polyacetylene film, which indicates the partial alignment of chains inside a polymer fiber. At low magnetic fields, the small negative component of magnetoconductivity (positive magnetoresistance) was observed and its magnitude increases as the ρr value increases. In the high field region, the magnetoconductivity is positive and it clearly shows the linear dependence on the magnetic field up to H=30 T. The linear field dependence of magnetoconductivity is different from what is expected in the three-dimensional localization-interaction picture. For the same ρr value samples, the magnitude of negative magnetoconductivity of S-polyacetylene is much bigger than that of R-polyac...

Synthesis of well-controlled helical polyacetylene films using chiral nematic liquid crystals

Abstract Helical polyacetylene recently developed is anticipated to show novel electromagnetic and optical properties. We have synthesized for the first time helical polyacetylene films with left- and right-handed screw structures under chiral nematic reaction field. We discuss the influence of the chiral nematic reaction field on film properties, morphology, crystal structure, electrical conductivity, and optical activity. The films are characterized with scanning electron microscopy (SEM), X-ray diffraction analysis, the four-probe method and circular dichroism (CD) spectroscopy.

Chiroptical titanium complexes as catalytically active chiral dopants available for asymmetric acetylene polymerization

Abstract Chiroptical titanium complexes are expected not only to serve as a chiral dopant to nematic liquid crystals(LCs) to give an induced chiral nematic (N * ) phase, but also to play a role of catalyst itself. Here, we synthesized some kinds of chiral titanium complexes mat might be available for synthesis of helical (CH) x . Axially chiral binaphthol derivatives, ( R )and ( S )-6,6′- di -4-( tranas -4- n pentylcyclohexyl) phenoxy-1-hexyl)-2,2′-dihydroxy-1,1′-binaphtyl (6, 6′-PCH506-Binol) and chiral center-containing ( R )and ( S )-2-octanol (HO-2-octyl) were adopted as coordination ligands of titanium complexes to yield ( R )and ( S )-Ti(O-6,6′-PCH506-Binaphtyl) 2 (1), and Ti(O-2-octyl) 4 (2). We found that these complexes induce the N * phase when dissolved in nematic LCs and they are catalytically active for acetylene polymerization. Positive or negative Cotton effect was observed in the region of π → gp * transition of polyene chain in circular dichroism(CD) spectra of the (CH) x thin films. It is evident that the main chain is helically screwed, taking account of no existence of Chiroptical substituent in side chains

Synthesis and properties of helical polyacetylene

Abstract We have synthesized for the first time helical polyacetylene [(CH) x ] with leftand right-handed screwstructures under chiral nematic reaction field. We discuss the influence of the chiral nematic reaction field on film properties, morphology, crystal structure, electrical conductivity, and optical activity. These properties are characterized by scanning electron microscopy (SEM), X-ray diffraction, the four-probe method and circular dichroism (CD) spectroscopy.

Synthesis of Vertically Aligned Helical Polyacetylene Films in Homeotropic Chiral Nematic Liquid Crystals

Abstract Vertically aligned helical polyacetylene thin films were synthesized by adding a chiral dopant into homeotropic nematic LC solvent to induce chiral nematic phase with homeotropic orientation. A scanning electron microscope (SEM) measurements showed that twisted polyacetylene fibrils are grown along the direction of film thickness, giving vertically aligned helical fibrillar morphology.

Advanced conjugated polymer— helical polyacetylene

Helical polyacetylene [(CH)x] thin films were synthesized in chiral nematic (N*) liquid crystal (LC) solvents. The N*LC was prepared by adding a chiral titanium complex as a chiral dopant to a mixture of two nematic LCs. The titanium complexes induced the N* phase when dissolved in nematic LCs and they were catalytically active for acetylene polymerization. Positive or negative Cotton effect was observed in the region of π π* transition of polyene chain in circular dichroism (CD) spectra for the (CH)x thin films synthesized under the N* LC. Spiral fibrillar morphology was observed through SEM measurement of the helical (CH)x film. Copyright

Novel liquid-crystalline titanocene complexes with catalytic activity for polymerizations of acetylene and phenylacetylene

Abstract We have synthesized a series of novel titanocene complexes with liquid-crystalline (LC) groups as coordination ligands, with the ultimate aim of developing LC catalytic species available for anisotropic polymerizations of acetylenes. The titanocene complexes were prepared by introducing LC groups into cyclopentadienyl ligands as substituents, (PCH506Cp)2TiCl2 (1), or coordinating them to a titanium atom as symmetric and asymmetric ligands, Cp2Ti(PCH5060)2 (2) and Cp2Ti(PCH5060)Cl (3) [where Cp = cyclopentadienyl, PCH506 = p-(trans-4-n-pentylcyclohexyl)phenoxyhexyl and PCH5060 = p-(trans-4-n-pentylcyclohexyl)phenoxyhexyloxy]. Polarizing optical microscope observations and differential scanning calorimetry measurements indicated that the titanocene complexes 2 and 3 are the first titanium complexes exhibiting LC behaviour. It is found that the titanocene complexes 1 and 2 can polymerize phenylacetylenes to give poly(phenylacetylene)s with the aid of triethylaluminium (Et3Al) as cocatalyst. The molecular weights and degrees of polymerization of the poly(phenylacetylene)s prepared were comparable or superior to those prepared with titanocene dichloride and tetra-n-butoxytitanium [Ti(O-n-Bu)4] catalysts. The LC titanocene complexes 2 and 3 were also confirmed to have catalytic activities for the polymerization of acetylene. Electron spin resonance spectra of the catalyst systems gave signals of trivalent titanium (Ti3+) with no hyperfine structure, suggesting that the catalytically active species is a mononuclear complex consisting of Ti3+ rather than a binuclear or trinuclear one.

Liquid crystalline titanocene with catalytic activity for polymerizations of acetylene derivatives

Abstract Two kinds of liquid crystalline (LC) titanocenes have been synthesized with an aim to construct a new type of anisotropic polymerization field. The titanocene derivatives were prepared by introducing LC groups into cyclopentadienyl ligands as substituents, (PCH506Cp) 2 TiCl 2 ( 1 ) or coordinating them to titanium atom as ligands, Cp 2 Ti(PCH5060) 2 ( 2 ). Observations of polarizing optical microscope and measurements of DSC indicated that 2 is the first titanium complex exhibiting a thermotropic LC phase. We found that both the titanocene derivatives can polymerize phenylacetylenes to give poly(phenylacetylene)s with an aid of triethylaluminium used as a cocatalyst. Molecular weights and polymerization degrees of the polymers were comparable or superior to those by catalysts such as a titanocene dichloride [Cp 2 TiCl 2 ] and a tetra- n -butoxytitanium [Ti(O- n -Bu) 4 ]. The LC titanocene derivative ( 2 ) was also confirmed to have a high catalytic activity for acetylene polymerization. These results should shed light on development of novel anisotropic reaction field available for syntheses of directly aligned conducting polymers.

Nanostructural properties of pyrolytic carbon from polyacetylene thin film

Carbonization of polyacetylene thin films was carried out by means of pyrolysis at a temperature of 800°C in a vacuum. During the pyrolysis, inadiation of an electron beam was carried out for a specimen. Nanostructural properties of the pyrolytic carbon were investigated using mainly electron microscopy. Ribbon-shaped graphite crystals in nanometer size were obtained. Effects of the electron beam irradiation on the formation of carbon crystals will be discussed.