Enantiotropic SmC Research Articles

Symmetric mesogenic twins derived from salicylaldimines

The synthesis, characterization, and mesomorphic properties of a series of symmetric dimeric twins derived from mesogenic salicylaldimines 1a–e are reported. In order to understand the structure–property relations, the bridged spacer (m=5, 6, 7, 8, 9), and alkoxy chains (n=6, 8, 10, 12, 14) were systematically varied. Except for 1a, all derivatives 1b–e exhibited mesophases whose mesomorphic properties strongly depended on the spacer parity and the alkoxy chains. Dimers 1a exhibited crystal or SmX phases. Other dimers 1b–e were truly mesogenic, giving enantiotropic SmC, SmA or/and N phases. Odd–even effect was observed on phase temperatures, and nematic phases were observed for derivatives with odd spacer, and a value of d/l=0.92 (1a; m=5, n=8)–0.84 (1c; m=7, n=14) was obtained by powder X-ray diffraction data, therefore, a monolayer structure in mesophase was proposed.

An Azo-Bridged Ferroelectric Liquid Crystal with Highly Enhanced Second and Third Harmonic Generation

A laterally azo-bridged trimer ferroelectric liquid crystal (FLC) incorporating a strong chromophore along its polar axis was synthesized and characterized by polarized-light optical microscopy, differential scanning calorimetry, two-dimensional X-ray diffraction analysis, electro-optical measurements, and nonlinear optical (NLO) investigations. This mesogen exhibits a thermodynamically stable enantiotropic SmC* phase and a bistable ferroelectric switching in a surface stabilized cell with bookshelf geometry. It gives the resonance-enhanced d(22) coefficient of 28 pm V(-1) (λ = 1.369 μm) for second harmonic generation (SHG), the largest NLO susceptibility reported to date for all FLCs. At the same wavelength, a new type of helicoidal phase matching assisted by the helical SmC* structure was identified. When the second harmonic wavelength of 780 nm is far away from the resonance wavelength (λ(max) = 572 nm), the d(22) coefficient is reduced to 6.8 pm V(-1) (λ = 1.56 μm). In addition to a strong SHG activity, the trimer also shows a strong third harmonic generation (THG) with an estimated third-order nonlinear susceptibility of χ((3)) = ~3 × 10(-11) esu (λ = 1.56 μm), among the largest χ((3)) value reported from THG measurements for liquid crystals. This work enables viable applications of FLCs in nonlinear optics and offers an innovative approach to develop new FLCs with larger NLO strength.

Novel photoluminescent mesogenic Schiff-base ligands bearing [N4O4] donors and their bimetallic Zn(II) complexes

Novel photoluminescent salicylaldimine ligands condensed from 3/, 3/, 4/, 4/-tetraminobiphenyl and 4-substituted long alkoxy salicylaldehyde possessing two sets of tetradentate [N2O2] donor site and their binuclear zinc(II) complexes have been synthesized. The mesogenic and photophysical properties were investigated. The compounds were characterized by FT-IR, 1H and 13C NMR, UV–vis, elemental analyses, solution electrical conductivity measurements and FAB mass spectrometry. The mesomorphic behavior of these compounds was probed by differential scanning calorimetry and polarized optical microscopy. The ligand with six carbon chain length showed monotropic nematic mesomorphism at 128°C. However, the ligand with alkoxy tail of carbon length 12 showed enantiotropic SmC phase. The complexes are devoid of any mesomorphism. The low molar conductance values in CH2Cl2 indicate that the complexes are non-electrolytes. At 330nm excitation, the ligand emits green light at ~516nm (Φ=30%) and ~549nm (Φ=16%) in solution and solid state, respectively. At similar excitation wavelength, the complexes exhibit blue light in solution at ~452nm (Φ=20%) and green light in solid state ~555nm (Φ=11%). The DFT calculations were performed using DMol3 program at BLYP/DNP level to ascertain the stable electronic structure of the complex.

Synthesis and properties of novel ferroelectric liquid crystalline polyacetylenes containing terphenyl mesogens with chiral groups

Novel ferroelectric liquid crystalline (FLC) polyacetylenes, namely, “side-end-fixed” type of PAM 6 OTPhOR* and “side-on-fixed” type of PAM 3 OCO(TPh)OR* were designed and synthesized in high yields (89.3 and 62.0%), respectively, where the terphenyl was linked at the different positions. The thermal stability of PAM 3 OCO(TPh)OR* is better than PAM 6 OTPhOR* owing to the “jacket effect” from terphenyl pendant linked at the waist position well protecting the main chain from the perturbations. The PAM 3 OCO(TPh)OR* shows enantiotropic chiral smectic A phase (SmA*), but it is noteworthy that the PAM 6 OTPhOR* exhibits enantiotropic chiral smectic C phase (SmC*) responsible for ferroelectric liquid crystallinity. Compared to “side-end-fixed” type of polymer, the “side-on-fixed” type shows better light emitting property, ascribed to the mesogen linked at the waist position has stronger tendency to enhance the main-chain coplanarity. Furthermore, the circular dichroism (CD) spectra demonstrate that the asymmetric force field generated by the chiral center affects the secondary structure of PAM 3 OCO(TPh)OR*. Novel ferroelectric liquid crystalline polyacetylenes (PAM 6 OTPhOR* and PAM 3 OCO(TPh)OR*) containing terphenyl pendants with chiral end groups were synthesized. The “side-on-fixed” type of PAM 3 OCO(TPh)OR* showed enantiotropic SmA*, but it is noteworthy that the “side-end-fixed” type of PAM 6 OTPhOR* showed enantiotropic SmC* responsible for ferroelectric liquid crystallinity. Compared to “side-end-fixed” type of polymer, the “side-on-fixed” type shows better light emitting property ascribed to the mesogen linked at the waist position have stronger tendency to enhance the main-chain coplanarity.

Laterally Azo-Bridged H-Shaped Ferroelectric Dimesogens for Second-Order Nonlinear Optics: Ferroelectricity and Second Harmonic Generation

Two classes of laterally azo-bridged H-shaped ferroelectric liquid crystals (FLCs), incorporating azobenzene and disperse red 1 (DR-1) chromophores along the FLC polar axes, were synthesized and characterized by polarized light microscopy, differential scanning calorimetry, 2D X-ray diffraction analysis, and electro-optical investigations. They represent the first H-shaped FLC materials exhibiting the ground-state, thermodynamically stable enantiotropic SmC* phase, i.e., ground-state ferroelectricity. Second harmonic generation measurements of one compound incorporating a DR-1 chromophore at the incident wavelength of 1064 nm give a nonlinear coefficient of d(22) = 17 pm/V, the largest nonlinear optics coefficient reported to date for calamitic FLCs. This value enables viable applications of FLCs in nonlinear optics.

Novel bent-shaped liquid crystalline compounds: IV. Dimesogenic compounds containing 2-hydroxy-1,3-dioxypropylene and azobenzene mesogens

Three bent-shaped dimesogenic liquid crystalline compounds having odd-numbered terminal alkyloxy chains ( n = 5, 7, 9) with a 2-hydroxy-1,3-dioxypropylene as central linking unit and two identical azobenzene mesogens were synthesized. Their mesomorphic properties were then characterized by polarized light microscopy and differential scanning calorimeter. The terminal alkoxy chain length was the key factor to provide the liquid crystallinity in this series. The compound of n = 7 showed a monotropic SmC phase, and that of n = 9 displayed an enantiotropic SmC phase. The compound of n = 5, however, did not show liquid crystal phase. The thermal stability of the smectic phases also increased with the terminal alkoxy length.

Ferroelectric and antiferroelectric liquid crystals of chiral swallow-tailed materials with an ester group linked to achiral alkyl chains

Two homologous series of chiral materials, 3-pentyl (S)-2-{6-[4-(4′-alkanoyloxyphenyl)benzoyloxy]-2-napthyl}propionate, PmPBNP(m=7-11), and 2-ethyl-1-butyl (S)-2-{6-[4-(4′-alkanoyloxyphenyl)benzoyloxy]2-napthyl}propionate, EBmPBNP(m=7-11), derived from a chiral moiety, (S)-2-(6-methoxynaphthyl-2)propionic acid, in conjugation with swallow-tailed groups attached to the external side of the chiral centre, have been synthesized and their mesomorphic phases studied. Members of the PmPBNP series, with the exception of P7PBNP and P8PBNP, showed an enantiotropic antiferroelectric SmC*A whereas, members of the EBmPBNP series, with the exception of EB7PBNP, showed an enantiotropic ferroelectric SmC*A phase. The maximum spontaneous polarization for these materials was in the range 20–60 nC cm−2. The electro-optic responses measured for both series of materials in ferroelectric and antiferroelectric phases displayed thresholdless V-shaped switching.

Effect of the variation of swallow-tailed groups on the mesomorphic and electro-optical properties of chiral compounds derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid

Three homologous series of chiral swallow-tailed compounds, alkyl (S)-2-{6-[4-(4′-alkoxyphenyl)benzoyloxy]-2-naphthyl}propionates, (S)HNP(p,n,q) derived from (S)-2-(6-hydroxy-2-naphthyl)propionic acid in conjugation with a variety of swallow-tailed groups, attached to the external side of the chiral centre, have been synthesized and their mesomorphic and electro-optical properties studied. Both (S)HNP(p,1,2) and (S)HNP(p,1,3) exhibited an enantiotropic antiferroelectric SmC*A phase. This implys that the swallow-tailed groups in the molecules favour zigzag pairing of the molecules in the smectic phase. The maximum P S values of compounds (S)HNP(p,1,2) in the antiferroelectric phase were measured in the range 21–30 nC cm-2; those of compounds (S)HNP(p,1,3) were in the range 15–23 nC cm-2, indicating that these chiral compounds possess low polarity. The electro-optical response of the compounds in the antiferroelectric SmC*A phase displayed thresholdless V-shaped switching.

A convenient synthesis and the mesomorphic properties of new chiral ferroelectric thiobenzoates†

A secondary chiral (R)-(−)-2-alcohol underwent the Mitsunobu reaction with triphenylphosphine, diethyl azodicarboxylate and ethyl 4-hydroxybiphenylcarboxylate, resulting in the desired (S)-(+)-product with high enantiomeric purity (>99% ee), with the chiral branched chain attached to the biphenyl. This method is operationally simple and provides the very important chiral precursor in good yields (62% in dry THF and 72% in dry Et2O). The condensation of the (S)-(+)-acid chloride from this material and a suitable 4-n-alkylthiophenol in toluene in the presence of pyridine or triethylamine furnishes the chiral (S)-(+)-thiobenzoate liquid crystals in good yields (80–83% in pyridine and 65–68% in Et3N). (S)-(+)-4-(1-Methylheptyloxy)biphenyl 4-alkylthiobenzoates are abbreviated (S)-MHOBSn , where n varies from 4 to 10 and denotes the number of carbon atoms in the alkyl chain. DSC, polarizing microscopy and X-ray diffraction showed that the (S)-MHOBSn series possesses a rich phase polymorphism: two highly ordered tilted phases CrG* and SmI*, as well as the ferroelectric smectic C (SmC*) and chiral nematic (N*) phase. In this series, the seldom observed transition between the chiral phases SmI*–SmC* is seen. All the compounds possess stable enantiotropic SmC* and N* phases. The existence of weak intermolecular hydrogen-bonding in (S)-MHOBSn was confirmed by FTIR spectroscopy.

Synthesis and mesomorphic properties of chiral liquid crystals derived from ( S )-lactic acid with 3-pentanol

The chiral swallow-tailed liquid crystals, 1-ethylpropyl (R)-2-[4-(4′-alkoxybiphenylcarbonyloxy)-phenoxy]propionates, EPmPBPP (m = 8−12), were prepared by using chiral (S)-lactic acid with 3-pentanol as starting materials. Mesophases and their corresponding transition temperatures were determined by polarizing microscopic textures and DSC. The results showed that all the chiral materials exhibited enantiotropic BP, N*, TGBA*, SmA*, and SmC* phases. Spontaneous polarization, dielectric constant and electro-optical response for the materials in the ferroelectric SmC* phase were investigated. It was noted that the electro-optical response of transmittance versus applied voltage obtained from the ferroelectric phase of material EPmPBPP (m = 10) displayed V-shaped switching, while that of other materials displayed the typical characteristics of ferroelectric hysteresis switching or U-shaped switching.

Novel bent-shaped liquid crystalline compounds: 1. Synthesis and structure analysis of dimesogenic compounds with azo units

Abstract A new series of bent-shaped liquid crystalline compounds (2An), which were composed of two identical mesogens (azobenzene-type), terminal alkyloxy chains (n=6, 8, 10, 12) and 2-hydroxy-1,3-dioxypropylene as central linking unit, have been prepared. Their mesomorphic properties were characterized by polarized light microscopy, differential thermal analysis, and X-ray diffraction (XRD). For 2An series, n=8, 10 and 12 compounds showed enantiotropic SmC phase, but n=6 compound did not show liquid crystal phase, as was confirmed by the XRD patterns.

Synthesis and Liquid Crystalline Properties of (S)-[4-n-Alkyloxy-benzoyloxyphenyl]-4′-[(2-n-alkyloxy)propionyloxy]benzoate

New series of chiral liquid crystalline compounds, (S)-[4-n-alkyloxy-benzoyloxyphenyl]-4′-[(2-n-alkyloxy)propionyloxy]benzoate (QMn/m), were synthesised. Polymorphism of the new compounds was investigated by polarising microscopy and differential scanning calorimetry. All members of the homologous series showed enantiotropic SmC* and N* mesophases except one compound having the longest alkyl chain in both terminal groups. The spontaneous polarisation of these ferroelectric liquid crystalline compounds is about 110–130 nC/cm2. The temperature dependence of the dielectric permittivity was measured for all the compounds studied. Several ferroelectric binary mixtures were prepared in order to get a wide enantiotropic chiral SmC* phase at ambient temperature.

Novel antiferroelectric liquid crystals with a phenylpiperazine moiety in the mesogenic core structure

A novel series of antiferroelectric liquid crystalline compounds having a phenylpiperazine unit and a biphenyl or a phenyl moiety in the core structure was synthesized using the optically active alcohols 2-methyl octanol and 1,1,1-trifluoromethyl octanol. Most of the compounds exhibited enantiotropic SmC*A, SmC* and SmA mesophases. The phase transition temperatures, response times, sponta A neous polarizations and the field-transmittance hysteresis loops were monitored. These data were compared with those of the MHPOBC compounds, which represent the most conventional core structure of the biphenyl/phenyl combination. The replacement of the biphenyl moiety, with the phenylpiperazine unit, reduced the isotropization temperature as well as the SmC*A-SmC transition point. It was found that some of the new compounds, which consisted ofAan octyl or a nonyl terminal chain with a 2-methyloctyl 2-fluorophenylcaboxylate group in the core structure, could have a potential application in display devices. Some work on blends of these compounds is also described.

Liquid Crystalline Phase Transition in Hexakis(4-(N-(41-alkoxyphenyl)iminomethyl)phenoxy)cyclotriphosphazene

A new series of cyclotriphosphazenes bearing Schiff bases with different carbon number alkoxy end groups(HAICP) in the side chains were synthesized and their phase transitions and liquid crystallinity were studied using DSC measurements and polarizing microscope observations. We found a monotropic Sml phase for n = 3 and an enantiotropic Sml phase for n = 4 −5, enantiotropic Sml, SmC and SmA phases for n = 6 – 7, enantiotropic SmC and SmA phases for n = 8 – 10, and enantiotropic Sm2, SmC and SmA phases for n=11 and 12 in the HAICP series. The Sml phase is a low-order uniaxial smectic phase and Sm2 phase is a low-order biaxial smectic phase. The melting point decreased with an increase in the end alkoxy carbon number, n, of the cyclotriphosphazenes, but the clearing point decreased with an increase in n. As a result, the mesomophic region expands with an increase in n. The melting and clearing points do not show clear odd-even effects, which is caused by the peculiar shape of the cyclotriphosphazenes.