Helical polyacetylene

Helix is the most common higher-order structure of synthetic polymers such as peptides, polymethacrylates, polychloral, polyisocyanides, polyisocyanates, and polysilanes. Polyacetylenes bearing appropriate substituents also form a helix. Substituted helical polyacetylenes are promising candidates for enantioselective permeable materials, polarization-sensitive electro-optical materials, asymmetric electrodes, and hence their synthesis is currently under intensive research. This section overviews the synthesis and properties of helical polyacetylenes recently reported. 

A water-soluble salt of the above synthetic helical polyacetylene derivative, (I), in the current application having no asymmetric carbon atom was prepared by Sakajiri et al. (1) and used in biological research associated with biomimesis. 

Crown ether binaphthyl derivatives 128-131 (Scheme 71) were synthesized and investigated by Akagi [139], Compounds 128-131 were used to induce chiral nematic phases (N ) in liquid crystals. It was found that the helical twisting power increased with decreasing ring size. Helical polyacetylenes were synthesized in the N phases. It was found that the interdistance between the fibril bundles of the helical polyacetylene was equal to the half-helical pitch of the N liquid... 

A cast film of 28 also responded to the chirality of liquid and solid chiral amines, and exhibited an ICD in the UV-visible region. The observed Cotton effect patterns were similar to those of 28 induced by the chiral amines in solution [147]. These methods are more convenient and practically feasible to sense the chirality of chiral amines than the solution method, and may be applicable to other dynamic helical polyacetylenes. 

As another example of a helical polyacetylene, the single-handed helical polyacetylene fibril, whose structure was studied by SEM, was prepared by the polymerization of acetylene within a chiral nematic liquid crystalline phase.192... 

K. Akagi, G. Piao, S. Kaneko, K. Sakamaki, H. Shirakawa, M. Kyotani, Helical polyacetylene synthesized with a chiral nematic reaction field. Science 282, 1683-1686 (1998)... 

K. Akagi, Helical polyacetylene asymmetric polymerization in a chiral liquid-crystal field. Chem. Rev. 109, 5354-5401 (2009)... 

Helical Polyacetylene Synthesized in Chiral Nematic Liquid Crystal... 

Chiral Dopants and Chiral Nematic Liquid Crystals Acetylene Polymerization in Chiral Nematic Liquid Crystal Characterization of Helical Polyacetylene Film... 

Closer observation of SEM images indicates that helical polyacetylenes synthesized in the (R)- and (S)-chiral nematic LCs form screwed bundles of fibrils and screwed fibrils with counterclockwise and clockwise directions, respectively (Figure 3.5). This result implies that the screw direction of helical polyacetylene is controllable by choosing the helicity, i.e., optical configuration of the chiral dopant, so far as the chiral nematic LC induced by the chiral dopant is employed as an asymmetric polymerization solvent. In addition, it is clear that the screw directions of bundle and fibrils are opposite to those of the (R-l)- and (S-1)-chiral nematic LCs used as solvents. This is an unexpected and even surprising result, requiring a sound interpretation that is to be discussed later. 

The bundles of fibrils for helical polyacetylenes synthesized in the (fi-2)- and (S-2)-chiral nematic LCs are screwed counterclockwise and clockwise, respectively The screw directions of helical polyacetylene are opposite to those of the corresponding (fi-2)- and (S-2)-chiral nematic LCs whose directions are confirmed to be clockwise and counterclockwise, respectively, through the miscibility test with choles-teryl oleyl carbonate. This is the same situation as that of the (fi-1)- and (S-l)-chiral nematic LCs including (fi)- and (S)-PCH506-Binol. 

FIGURE 3.4 Hierarchical spiral morphology of helical polyacetylene film, (a) and (b) show scanning electron microscope (SEM) photographs of multidomain-type spiral morphology and helical bundles of fibrils in a domain, respectively. 

FIGURE 3.5 SEM photographs of helical polyacetylene films synthesized in the chiral nematic LCs including (R)- and (S)-PCH506-Binol. 

From the above mentioned results, it can be remarked that counterclockwise and clockwise helical polyacetylene chains are formed in (R)- and (S)-chiral nematic LCs, respectively, and that these hehcal chains are bundled through van der Waals interactions to form hehcal fibrils with the opposite screw directions to those of the chiral nematic LCs. The bundles of fibrils further form the spiral morphology with various sizes of domains (Figure 3.9). 

FIGURE 3.7 Schematic representation of plausible mechanism for acetylene polymerization in the chiral nematic LC. The helical polyacetylene with counterclockwise (left-handed) screw direction blue arrow) grows up starting from catalytic species in the clockwise (right-handed) chiral nematic LC. 

FIGURE 3.8 Circular dichroism (CD) spectra of helical polyacetylene films. The polyacetylene films synthesized in the (R-2)- and (S-2)-chiral nematic LCs including (R)- and (S)-6,6 -PCH506-2,2 -Et-Binol are designated as PA by R-2 and PA by S-2, respectively. 

FIGURE 3.9 Hierarchical helical structures from primary to higher order in helical polyacetylene. 

Helical polyacetylene was synthesized in asymmetric reaction field consisting of chiral nematic LC. The chiral nematic LC was prepared by adding a chiroptical binaphthyl derivative as a chiral dopant to a mixture of two nematic LCs. Acetylene polymerizations were carried out using the catalyst Ti(0-n-Bu)4— EtsAl dissolved in the chiral nematic LC solvent. The polyacetylene film consisted of clockwise or counterclockwise helical structure of fibrils in SEM. Cotton effect was observed in the region of ir — ir transition of the polyacetylene chain in CD spectrum. The high-electrical conductivities of 1.5 1.8 x... 

S/cm after iodine doping and the chiral helicity of the present films should be available for novel electromagnetic and optical properties. Macroscopic alignment of helical polyacetylene has also been successfully carried out to prepare the samples that are feasible for examination of novel electromagnetic properties, which will be presented in the near feature. 

FIGURE 16.2 SEM images o (a) polyacetylene nanofiber ropes. (From Park, J.H., Electronic and scanning tunneling spectroscopic studies of conducting polymer nanostructures Polyacetylene nanofibers, PPV nanotubes and MEH-PPV nanowires, Ph.D. thesis, Seoul National University, Seoul, 2004.) (b) R-helical polyacetylene nanofiber ropes. (From Akagi, K., Unpublished data, 2004.)... 

Akagi, K., et al. 1998. Helical polyacetylene synthesized with a chiral nematic reaction field. Science 282 1683. 

Piao, G., et al. 2001. Synthesis of well-controlled helical polyacetylene films using chiral nematic liquid crystals. Curr Appl Phys 1 121. 

Lee, H.J., et aL 2004. Dispersion and current—voltage characteristics of helical polyacetylene single libers. J Am Chem Soc 126 16722. 

Suh, D.-S., et al. 2001. Helical polyacetylene heavily doped with iodine Magnetotransport. / Chem Phys 114 7222. 

Helical polyacetylene (PA), 3-3-3-12, 16-3, 16-6, 16-11 Helical single PA fibers, 16-7 Helical solid, 9-23 Helix Formation, 3-12... 

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