Masayoshi Tabata

Synthesis and solid state helix to helix rearrangement of poly(phenylacetylene) bearing n-octyl alkyl side chains

Highly stereoregular polymerisations of p-n-octyphenylacetylene (pOcPA) were performed using a [Rh(norbornadiene)Cl]2-triethylamine catalyst in ethanol at −20 and 25 °C to afford yellow and orange polymers, Poly(Y) and Poly(O), in yields of 64 and 99%, respectively. The XRD patterns of Poly(Y) showed a hexagonal columnar crystal with a contracted cis–cisoid helix, HexaPoly(Y)CC. The XRD pattern of Poly(O) matched that of Poly(Y) when heated to 80 °C. The heat treatment of HexaPoly(Y)CC at 100 °C generated two tetragonal crystals: Tetra1Poly(R)CC, containing contracted cis–cisoid helices, and Tetra2Poly(R)CT, containing stretched cis–transoid helices. The helical diameters of HexaPoly(Y)CC before and after heat treatment were estimated using XRD and were consistent with the results of MMFF 94 calculations, although the n-octyl alkyl chains of HexaPoly(Y)CC and Tetra2Poly(R)CT did not have linear alkyl chains; a bend in the chains was confirmed by 13C CP-MAS NMR. When HexaPoly(Y)CC was heated to 100 °C in the solid phase, the λmax in the diffuse reflective UV-vis spectra shifted from 448 nm to 565 nm. Furthermore, the endothermic transition for HexaPoly(Y)CC occurred at 100 °C in the DSC trace. Therefore, these data corroborated the assertion that HexaPoly(Y)CC thermally converted into Tetra1Poly(R)CC and Tetra2Poly(R)CT.

Helix oscillation of polyacetylene esters detected by dynamic 1H NMR, IR, and UV-vis methods in solution

An aliphatic substituted acetylene, n-heptyl propiolate, was stereoregularly polymerised using a catalyst, [Rh(nbd)Cl]2, at 40 °C in methanol to obtain the corresponding helical polymer, PnHepP. The changes in the line shapes and splitting patterns of the 1H NMR spectra were interpreted consistently as representing restricted rotation around the ester O–Ce bond in –O–CeHe2(R)– (R = n-hexyl alkyl chains), rather than helix inversion, with the aid of a 3-site jump model. A two-line peak corresponding to the ester methylene protons of –O–CH2– observed at 30 °C suggested the formation of three rotamers designated as A, B, and C based on the presence of contracted helix and stretched helix forms, each of which has an intrinsic helical pitch with a helical cis–cisoid structure. Furthermore, an accordion-like helix oscillation (HELIOS) along the main chain axis was proposed to explain the temperature dependence of spectral changes observed in the 1H NMR, UV-vis, and IR spectra. The temperature dependence of the UV-vis and 1H NMR spectra of PnHepP corroborated the presence of contracted and stretched one-handed helix sense polymers in solution.

Configuration and conformation of poly(3-carbazolylacetylene) including cis and trans radicals revealed by ESR spectroscopy

π-Conjugated cis and trans radicals of poly(N-isobutyl-3-carbazolylacetylene), P(iBCzA), were stereospecifically prepared using the [Rh(norbornadiene)Cl]2-triethylamine catalyst in toluene as the polymerization solvent at room temperature, and their geometric structures were determined using the electron spin resonance (ESR) method. The deuterated analogue, P(iBCzA-d1), with vinyl protons replaced with deuterium, D–C, was used to corroborate the structures together with the simulation of the observed ESR spectra and the calculation of the spin densities of the two radicals. The cis and trans radicals were generated at 1 : 1 ratio with spin densities delocalized on the main chain rather than the side-chain, unlike other substituted polyacetylenes, which have relatively small side-chains. This finding indicates that the large and planar carbazole moieties of the two radicals were perpendicular to the π-conjugated main-chain and that fairly stiff π-conjugated main chains were generated.

Synthetic Molecular Springs: Stretched and Contracted Helices with Their Interconversions of Monosubstituted Polyacetylenes Prepared with a Rhodium Complex Catalyst

Stereoregular monosubstituted polyacetylenes (SPAs) can be prepared by polymerizing the corresponding monosubstituted acetylene monomers using a bidentate rhodium–diene complex with either an amine or alcohol cocatalyst. SPAs have advantages over polyacetylene (PA) in terms of handling and application because SPAs are stable in air and soluble in organic solvents, whereas PA is unstable in air and insoluble. The main chains of SPAs form unique helical structures because of the steric hindrance between neighboring monomers. SPAs can be switched between stretched and contracted helices by controlling the polymerization solvents, substituents, and temperature, leading to color changes in the solid-state materials. An “accordion-like helix oscillation” (HELIOS) was observed in a solution of a SPA with aliphatic ester groups, where the restricted rotation of the ester O–C bond was dynamically synchronized with changes in the helical pitch of the SPA molecules.

Mutual Conversion between Stretched and Contracted Helices and its External Stimuli Induced Drastic Colors and Geometrical Structures Changes of Substituted Polyacetylenes Prepared with an Organo-Rhodium Catalyst

Unique conversions between stretched and contracted helices of mono-substituted polyacetylenes (SPA)s prepared with an organo rhodium complex-amine catalytic system were demonstrated. The conversions of aromatic SPAs with a drastic color change were caused by polymerization solvents used, substituents in the phenyl ring, and external stimuli like heat- and solvent-treatments which were induced in the solid state. The helices of aliphatic polyacetylenes was oscillating just like spring or coil which was synchronizing with the restricted rotation around the ester O-*C bond in the solution. The oscillation mode was named as an accordion-like helix oscillation “HELIOS”. All the conversions were attributed to the difference of the thermodynamical stability between stretched and contracted helices.