Yuki Arakawa

Synthesis of diphenyl-diacetylene-based nematic liquid crystals and their high birefringence properties

We synthesized two series of diphenyl-diacetylene (DPDA)-based materials with alkoxy and alkyl tails of length m (DPDA–OCm and DPDA–Cm, respectively), and measured their nematic-phase birefringence (Δn) as a function of wavelength and temperature. We found that Δn decreases with an increase in m, possibly by a dilution effect of the low-Δn alkyl tail. Further, of the two series, Δn was found to be relatively higher in the DPDA–OCm materials, with the highest value of 0.4 obtained for DPDA–OC1 at 550 nm at 10 °C below the isotropic-to-nematic transition temperature. Further, we observed the temperature dependence for Δn, which is proportional to the order parameter (s). From extrapolation to s = 1 (the perfect orientation state), it is speculated that the DPDA–O moiety has the potential to afford a very large Δn of 0.9.

Stereocomplex Crystallization and Homocrystallization of Star-Shaped Four-Armed Stereo Diblock Poly(lactide)s with Different l-Lactyl Unit Contents: Isothermal Crystallization from the Melt

The effects of l-lactyl unit content on star-shaped four-armed stereo diblock poly(lactide) (4-LD) polymers and star-shaped four-armed poly(l-lactide) (4-L) on the isothermal crystallization from the melt were investigated. Solely stereocomplex (SC) crystallites were formed in equimolar 4-LD polymer with l-lactyl unit content of about 50%, irrespective of crystallization temperature (Tc) values. 4-L and 4-LD polymers with l-lactyl unit contents of 100 and 93% formed only homocrystallites, regardless of Tc, whereas only SC crystallites with traceable amounts were formed in 4-LD polymers with l-lactyl unit contents of 72 and 31% at a limited narrow Tc range of 110-120 °C, when crystallization was continued for as long as 24 h. About 20% deviation of l-lactyl unit content from 50% dramatically decreased the spherulite growth rate (G) values of SC crystallites, whereas a 7% decrease of l-lactyl unit content from 100% significantly decreased the G values of homocrystallites and largely decreased the overall homocrystallization rates. Branching architecture rather than diblock architecture hindered the simultaneous formation of SC and homocrystallites of non-equimolar 4-LD polymers.

Synthesis and evaluation of dinaphthylacetylene nematic liquid crystals for high-birefringence materials

We synthesised 1,2-bis(6-alkoxy-2-naphthyl)acetylene as a new dinaphthylacetylene liquid crystal (LC) compound with alkoxy tails of length m (DNA–OCm) and characterised their phase behaviour by polarising optical microscopy and differential scanning calorimetry, where all compounds exhibited a nematic phase. Also, we measured the actual, and not extrapolated, Δn values of this target compound, and our results show the birefringence potential of the pure LC compound. They have high birefringence values, with the highest value of approximately 0.5 obtained for DNA–OC2 at 550 nm at 10°C above the nematic−crystal transition temperature.

Assembly of thioether-containing rod-like liquid crystalline materials assisted by hydrogen-bonding terminal carboxyl groups

We designed a hydrogen-bonding tolane-based molecule with terminal carboxyl and alkylsulfanyl groups in an effort to realize thioether-containing rod-like liquid crystalline materials. The molecule successfully exhibits a stable enantiotropic mesophase, in contrast to non-hydrogen bonding derivatives, including tolane and diphenyldiacetylene. In addition, it shows two remarkable characteristics compared to analogues with alkyl or alkoxy groups. First, the mesophase of the alkylsulfanyl derivative shows strong long-range correlation. Second, the birefringence of the alkylsulfanyl derivative is highly temperature-dependent, achieving a maximum value of 0.36. These remarkable characteristics are believed to be due to the high polarisability of sulfur atoms and S–S contacts. These findings will be helpful for the design of novel sulfur-containing rod-like liquid crystalline materials.

Synthesis and evaluation of high-birefringence polymethacrylate having a diphenyl-diacetylene LC moiety in the side chain

We synthesized polymethacrylate with a diphenyl-diacetylene moiety in the side chain. It formed a nematic LC, which appears in the wide temperature region and vitrified at room temperature. The polymer exhibited a high-birefringence of ∼0.3 at 550 nm at room temperature.

Design of an extremely high birefringence nematic liquid crystal based on a dinaphthyl-diacetylene mesogen

We designed dinaphthyl-diacetylene-based nematic liquid crystals with alkoxy tails (DNDA–OCm) and evaluated their birefringence (Δn). Actual measurements of pure target compounds showed that DNDA–OC2 had the highest value of 0.62 at 550 nm at Tc + 10 °C.

Development of novel bistolane-based liquid crystalline molecules with an alkylsulfanyl group for highly birefringent materials

In order to generate high-birefringence liquid crystal (LC) materials, the introduction of highly polarisable groups into the terminal positions of the mesogen represents one of the most important design strategies. Even though the alkylsulfanyl group is a potentially interesting option for this purpose, it has so far been very difficult to obtain enantiotropic mesophases for LCs containing this group. Herein, we report novel high-birefringence LC molecules based on 1,4-bis(2-phenylethynyl)benzene (bistolane) with alkylsulfanyl groups in the terminal positions. The incorporation of a fluorine atom into the central benzene ring of an alkylsulfanyl-substituted bistolane led to the formation of a well-defined enantiotropic nematic phase. In contrast, the nonfluorinated analogue with alkylsulfanyl groups did not exhibit a mesophase. In comparison with an alkoxy-substituted derivative, the birefringence of the alkylsulfanyl-substituted analog was substantially higher (Δn = 0.42 at 550 nm). Furthermore, the birefringence properties of the alkylsulfanyl-substituted derivative were observed to be proportional to the order parameter and largely temperature-dependent over the entire temperature range. These results thus provide not only fundamental insights into structure–reactivity relations, but also furnish practical design principles for the synthesis of new sulfur-containing, rod-shaped LC materials with optical applications.

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.

Large extraordinary refractive index in highly birefringent nematic liquid crystals of dinaphthyldiacetylene-based materials

The extraordinary and ordinary refractive indices in highly birefringent dinaphthyldiacetylene-based nematic liquid crystals with different alkoxy tails were accurately measured by the multiple-beam interference method. The high birefringence in the nematic liquid crystals was attributed to the high value of the extraordinary refractive index, ne. The value of ne increased from 1.8 to 2.1 as the number of carbon atoms in the alkoxy tail decreased from 12 to 3, whereas the ordinary index, no, remained at approximately 1.6.