Koichi Sakajiri

Highly birefringent polymer films from the photo-crosslinking polymerisation of bistolane-based methacrylate monomers

The photo-polymerisation of mixtures of mono- and di-methacrylates, containing a bistolane moiety with a central fluorine-substituted benzene ring, resulted in the formation of highly birefringent polymer films (Δn = 0.40), which were obtained in a nematic liquid crystal (NLC) phase. While the dimethacrylate forms enantiotropic NLCs at T = 110–138°C, smectic phases at T = 50–138°C and crystallises at T = 50°C, whereas the monomethacrylate forms NLCs at a wider temperature range (T = 98–185°C) and crystallises at a lower temperature (T = 98°C). These methacrylates were infinitely miscible and a 20/80 (w/w) mixture of the dimethacrylate/monoacrylate was able to form an NLC phase over a broad temperature range (T = 73–179°C). The mixed NLC phase exhibited a Δn value of 0.36, even though the Δn values of the mono- and di-methacrylates were determined as 0.35 and 0.25, respectively, suggesting that the Δn of the mixture follows an additivity rule. Furthermore, the Δn of the NLC phase could be increased to 0.40 by photo-polymerisation.

Synthesis of new wide nematic diaryl-diacetylenes containing thiophene-based heteromonocyclic and heterobicyclic structures, and their birefringence properties

New monocyclic and bicyclic thiophene-based diaryl-diacetylene liquid crystal (LC) materials were synthesised and demonstrated to exhibit wide enantiotropic nematic phases. One thiophenyl-benzene derivative, in particular, displayed a stable nematic phase across a temperature range of over 100°C. The birefringence properties of the compounds were measured using a multiple-beam interference (MBI) method, with the highest value found to be for the same thiophenyl-benzene molecule (Δn = 0.57, 550 nm) at 10°C above the crystal-to-nematic transition temperature. Furthermore, the effects of the cyclic structure on the mesomorphic behaviours, transition temperatures, and birefringence properties were investigated in detail from both experimental and theoretical viewpoints.

Stable supramolecular helical structure of C6-symmetric hydrogen-bonded hexakis(phenylethynyl)benzene derivatives with amino acid pendant groups and their unique fluorescence properties

A highly stable supramolecular helical structure was formed by the self-assembly of novel C6-symmetric hydrogen-bonded discotic molecules, hexakis(phenylethynyl)benzene derivatives with chiral alanine parts, and exhibited orange excimer emission with a large Stokes shift.

Reversible helix–helix transition of poly(β-phenylbutyl l-aspartate) and poly(β-phenylpentyl l-aspartate), involving a screw-sense inversion in the solid state

Abstract We newly synthesized poly( β -phenylbutyl l -aspartate) with the side chain R=–(CH 2 ) 4 Ph and poly( β -phenylpentyl l -aspartate) with R=–(CH 2 ) 5 Ph to extend the study on a helix–helix transition in the solid state. In these two polymers, the helix–helix transition takes place reversibly during a heating and cooling cycle similarly as in poly( β -phenylpropyl l -aspartate) with R=–(CH 2 ) 3 Ph. From the combined observations by circular dichroism, X-ray, 1 3 C-NMR and IR measurements, we postulated that the reversible transition is due to an interconversion of the helical screw sense between a right-handed α -helix and a left-handed ω -helix. The effect of the side-chain length on the transition behavior was examined.

Diphenyltriacetylenes: novel nematic liquid crystal materials and analysis of their nematic phase-transition and birefringence behaviours

Improvements in birefringence properties of LC materials are actively being pursued because of the increase in their applications in optical devices. 1,6-Diphenyl-1,3,5-hexatriyne, i.e., diphenyltriacetylenes (DPTAs; number of acetylene units, n = 3) are expected to have wide nematic mesophases and exhibit extremely high birefringence properties because of the high aspect ratio of their conjugated structures. Therefore, we synthesised DPTA derivatives as novel, highly birefringent nematic liquid crystal materials. The derivatives exhibit stable enantiotropic nematic phases with longer alkoxy carbon chain lengths (i.e., with dodecyloxy groups, m = 12). High birefringence values of Δn = 0.46 at 550 nm and 140 °C and a high extraordinary refractive index (ne) of over 2.0 were exhibited for triyne DPTA–OC6. Furthermore, we investigated the effect of the acetylene bond on the nematic transition behaviours and refractive index parameters of the conjugated oligoyne or polyyne compounds with two terminal aromatic rings. An increasing number of acetylene units (n = 1–3), i.e., extended conjugation, led to wider nematic phases and higher birefringence; the increment in Δn per acetylene unit was estimated to be approximately 0.14.

Solvent-induced chirality inversion involving supramolecular helix transformation and color-tunable fluorescence of a C6-symmetric hexakis(phenylethynyl)benzene derivative

A C(6)-symmetric disk-like molecule, a hexakis(phenylethynyl)benzene derivative bearing chiral alanine parts, L-1, exhibited a solvent-induced supramolecular helix-sense inversion involving conformational changes followed by destruction of the supramolecular helical column. This phenomenon has been found by investigating the supramolecular assembly state of L-1 in mixed solvents of various chloroform (CHCl(3))/n-hexane (Hx) ratios. L-1 forms a stable helical columnar assembly via multiple noncovalent bonding interactions in nonpolar Hx, while the molecules in relatively polar CHCl(3) are in a molecularly dispersed state. Although one would expect disruption of the helical column with the addition of nonhelicogenic CHCl(3), an opposite-handed helical columnar structure was formed at 8-15 vol% of CHCl(3), and subsequently the inverted helical column was disassembled by a further increase of CHCl(3). In addition, this morphological transformation was accompanied by a significant change in fluorescent color, which varies over a wide visible range from orange in an original helical columnar state to light blue in a molecularly dispersed state through yellow in an inverted helical columnar state. These unprecedented behaviors are shown by the spectroscopic results, and the molecular conformations of L-1 and the driving force for the helical sense inversion are discussed.

Odd–even effect on viscoelastic properties of twin-dimer nematic liquid crystals

By using a combination of dynamic light scattering and capacitance measurements, we investigate the viscoelastic properties of nematic liquid crystalline α,ω-bis(4-cyanobiphenyl-4′-yloxy)alkane (BCBO-m) dimers and the corresponding 6OCB monomer. The results show that the K11 and K33 elastic constants are larger for the even-m dimers than for the odd-m dimers. K11 and K33 are twice as large for the odd-m dimers than for the 6OCB monomer, and the ratios K33/K11 ~ 2 are similar to that of 6OCB over the entire range of temperature investigated. The even-m dimers are characterised by large K11 and K33, which are respectively 3 and 10 times larger than the corresponding values for 6OCB at T/TNI = 0.980. Furthermore, K33/K11 = 6 upon decreasing T/TNI to 0.943. In contrast, the viscosities ηsplay and ηbend are not characteristic of dimers. The viscosity ηsplay increases with increasing molecular mass and ηbend is comparable to the corresponding value for 6OCB.

Cholesteric films exhibiting expanded or split reflection bands prepared by atmospheric photopolymerisation of diacrylic nematic monomer doped with a photoresponsive chiral dopant

Cholesteric liquid crystal (CLC) films with gradient pitches are prepared by atmospheric UV irradiation of mixtures of a diacrylic nematic monomer (M), a macrocyclised chiral cinnamate dimer (CD), and a photoinitiator (PI). UV light induces the intramolecular photodimerisation of the two cinnamate moieties in the CD to decrease the helical twisting power. The CLC of 90/10 (w/w) mixture of M/CD exhibits an optical pitch of 482 nm, which increases homogeneously to 815 nm upon atmospheric UV irradiation of the film. When the CLC doped with 1.5 wt% PI is formed into a planar-aligned film and irradiated with UV light in the atmosphere, the CLC film exhibits pitches that increases toward the air-interface side so as to result in a wide reflection band ranging from 420 to 890 nm; in addition, the film assumes a silver appearance. When the PI concentration and UV intensity are increased, the spectra of the CLC films exhibit two reflection bands and films assume a magenta colour. Such pitch distributions result from the confliction between untwisting and polymerisation of the CLC, both of which are induced by the UV light, although the polymerisation rate is decreased by the diffusion of atmospheric oxygen into the film.

High-density poly(hexyl methacrylate) brushes offering a surface for near-zero azimuthal anchoring of liquid crystals at room temperature

High-density poly(hexyl methacrylate) (PHMA) brushes offered a near-zero azimuthal anchoring surface for nematic liquid crystals (NLCs) at room temperature. NLCs in a cell with PHMA-grafted substrates aligned along comb-shaped electrodes after a light push. The LC cell was driven at a threshold voltage five times lower than that of a conventional cell.

Enlarged helical twisting power in chiral cyclic dimers based on conventional chiral alkyl diols

We prepared a homologous series of chiral cyclic liquid crystal (LC) dimers containing two biphenyl mesogens linked by two alkyl spacers, one of which was based on a R-enantiomeric chiral diol. In solution, the circular dichroism spectra of all the R-enantiomeric dimers showed the first negative and second positive Cotton effects, which indicated that the two biphenyl mesogens were located in a skewed overlapping alignment with counter-clockwise helicity. The helical twisting power (HTP) of these cyclic dimers was confirmed by the cholesteric LCs in mixtures with the nematic host. The induced cholesteric LCs in all mixtures had the same counter-clockwise helical sense as the skewed conformation of the two biphenyl mesogens. Furthermore, HTP values and Cotton effect amplitudes were linearly correlated, indicating direct induction of cholesteric twisting by the skewed biphenyl moieties. The induced HTP values were quite large, irrespective of the use of conventional chiral diols; the largest HTP value exceeded 100 μm−1 and was obtained in the cyclic dimer based on the R,R-1,2-diphenylethane diol.