Thermal and optical properties of newly synthesized dicholesteryl esters with a phenylene oxide link in the normal and solidified cholesteric phases

A series of liquid crystalline compounds with a central phenylene oxide group to which two cholesteryl groups are attached via two alkanoate spacers has been synthesized and investigated. All the compounds except the one with ethanoate spacers showed a cholesteric phase as a single mesophase. The phase transition temperatures, the corresponding enthalpy changes and the wavelengths of selective reflection associated with the cholesteric phases showed a strong odd-even effect as a function of the spacer length. By rapid cooling of the compounds from the cholesteric phase to 0°C, solid films maintaining a cholesteric molecular order were obtained. At room temperature, the solid film showed stable cholesteric colours controlled by changing the temperature at which the rapid cooling was begun. Heating the cholesteric solid film of the compound with hexanoate spacers gave two forms of crystals above 80°C, whose ratio changed depending on the colour of the starting solid films. This result suggests the existence of two conformational isomers in the liquid crystalline state. Since it is possible repeatedly to fix stable red, green and blue colours by thermal treatment of this compound, we may apply it to a rewritable full colour recording in the thermal mode.     

Optical and liquid crystalline properties of dicholesteryl derivatives having different configurations of the hydrocarbon linkages

The liquid crystalline and optical properties of four dicholesteryl derivatives having various linkage configurations, but with the same number of connecting carbons (18), have been investigated, using differential scanning calorimetry, polarizing optical microscopy, Fourier transform infrared (FTIR) spectroscopy and UV-Vis spectroscopy. Quite different phase behaviours and optical properties have been found for these compounds, while all of them exhibited the behaviour specific for a cholesteric phase. By rapid cooling from the cholesteric phase to 0°C, the iridescent colours of some of the dicholesteryl esters were fixed in the cholesteric glassy state. It was difficult but possible to fix various stable colours in the cholesteric glassy state of one compound with a polymethylene (CH₂)₁₈ linking chain. It was much easier to fix less stable colours for samples with the linking groups (CH₂)₆ and C=C double bonds between them. Very unstable colours could also be fixed for the compound with the (CH₂)₇ chains and a diyne group. The fourth compound with a branched linking chain gave a cholesteric phase which showed colours at room temperature. FTIR spectroscopy measurements provided interesting results concerning the average change in molecular configuration between the blue cholesteric glass and the crystal for the compound with the (CH₂)₇ polymethylene chains which crystallized within a few hours; also frequency changes associated with some bands were found and reported.     

λ-Shaped liquid crystal trimers with dual terminal cholesteryl moieties: synthesis and concomitant of N*, SmA and cholesteric glassy phases

Two homologous series of λ-shaped chiral liquid crystal trimers composed of a laterally substituted benzylidene-aniline as the central core armed by two cholestryl ester moieties via odd-even alkyl spacer are synthesised. All the compounds are mesogenic exhibiting both chiral nematic (N*) and SmA phases except for trimers bearing long spacers (n = 9–10). A pronounced odd-even effect is observed on the phase transition temperatures and clearing enthalpies when the spacer length is varied in which the even-parity members show higher value. The widening of N* phase upon elongation of the alkyl spacer can be interpreted as a result of the destabilisation of SmA phase. Full recrystallisation from the cholesteric phase upon cooling is not observed for all the compounds bearing long spacer, especially oxydecanoyl spacer. Instead, the anisotropic fluid vitrified to form the cholesteric glassy state characteristic of polymer at low temperature. This finding is also evident from the polarising optical microscope whereby non-crystalline texture which resembles the Grandjean texture with various reflection colours is observed upon cooling to low temperature. The effect of the lateral substituents on the liquid-crystalline behaviour is also discussed.     

Synthesis and liquid crystalline properties of a series of cholesterol-based dimesogenic compounds

The synthesis and mesomorphic properties of a series of novel dimesogenic compounds containing the cholesteryl ester unit and a phenyl benzoate group are reported. The two mesogenic units of these compounds are linked through dicarboxylic ester bonds, with alkylene spacer lengths of 2, 4, 6 and 8 methylene units. The chemical structures and liquid crystalline properties of this series of compounds were characterized by FTIR, ¹H NMR, hot stage-coupled polarizing microscopy and DSC. The results show that this series of compounds are cholesteric liquid crystals over a wide range, both during heating and cooling, and they exhibit iridescent colours in the liquid crystalline state.     

Synthesis and liquid crystalline properties of a series of cholesterol‐based dimesogenic compounds

The synthesis and mesomorphic properties of a series of novel dimesogenic compounds containing the cholesteryl ester unit and a phenyl benzoate group are reported. The two mesogenic units of these compounds are linked through dicarboxylic ester bonds, with alkylene spacer lengths of 2, 4, 6 and 8 methylene units. The chemical structures and liquid crystalline properties of this series of compounds were characterized by FTIR, ¹H NMR, hot stage‐coupled polarizing microscopy and DSC. The results show that this series of compounds are cholesteric liquid crystals over a wide range, both during heating and cooling, and they exhibit iridescent colours in the liquid crystalline state.     

Polymeric infrared reflective thin films with ultra-broad bandwidth

A polymeric liquid crystalline film was fabricated by photo-induced diffusion and mesogenic phase structural transition of photopolymerisable acrylate monomer mixtures that presented phase transition between cholesteric (Ch) and smectic (SmA) phases. By controlling the curing temperature to be close to SmA–Ch phase transition, a novel architecture that combined Ch and SmA-like short-range ordering (SSO) nanostructures was obtained, which reflected light with the wavelength from 0.5 to 11.5 µm. Experimental results showed that the existence of UV-absorbing dye and curing temperature were critical for the creation of asymmetrical super wide pitch gradient distribution. Moreover, the reflection wavelength and bandwidth of the cholesteric liquid crystal (CLC) films were tuneable by adjusting proportion of the compounds, and the bandwidth could be broadened up to 13 μm. It was expected to have great potential applications in fields like architectural energy conservation or infrared-stealth.     

Vitrification and water induced crystallization of zinc(II) dodecylbenzene sulphonate

Crystallization of zinc(II) dodecyl benzene sulphonate has been suppressed by either the removal of water of hydration or the rapid cooling of the compound from the liquid crystalline state. The anisotropic structure is preserved in the solid state when the hydrated compound in the LC phase is rapidly quenched, while removal of the water of hydration at temperatures in or above the LC phase and the subsequent slow cooling produces a stable isotropic structure.     

Dimesogenic compounds consisting of cholesterol and azobenzene-based moieties: dependence of liquid crystal properties on spacer length and fluorination of the terminal chain

The liquid crystalline (LC) properties of two series of non-symmetric dimesogenic compounds consisting of cholesterol and azobenzene-based moieties interconnected by ω-oxyalkanoyl spacers of varying length are compared: one series (AOC-n) has an octyloxy chain attached to the azobenzene mesogen unit while the other (AOCF-n) has a perfluoroheptylmethyloxy chain. In general, compounds bearing the fluorinated alkoxy chain exhibited LC properties over a much broader temperature range than those with the alkoxy chain. In addition, the AOC-n series exhibited the chiral smectic C (SmC*), smectic A (SmA) and cholesteric (N*) phases depending on the length of the central spacer, whereas the AOCF-n series favoured the formation of only the SmA phase with the N* phase completely suppressed. Both series showed an odd-even dependence of the isotropization temperature on spacer length.     

Synthesis and mesomorphic properties of a series of dimers derived from thioether-terminated and cholesteryl

ABSTRACT

The synthesis and material properties of a series of new liquid crystalline compounds containing thioether and cholesteryl, these homologues with different alkyl chain lengths of 2–8, are reported. Thermal analysis shows that all oligomers have wide mesophase temperature ranges with high thermal stability. The oligomers were determined by differential scanning calorimetry (DSC) and polarising optical microscopy (POM). The molecule not only successfully exhibits strong optical properties and rainbow colours, but also the cholesteric helical pitch decreased with increasing temperature. The mesogenic incidence and tendency were found to be strongly dependent on the numbers of carbon in the flexible alkyl chain. Even members formed widely mesophase compared to odd members that showed narrower ones. The reflection wavelengths of 6S8Ch are almost across the entire visible region when they are heated, which offer tremendous potential for various optical applications. Also, it not only shows a lower transition temperature but also has a narrower cholesteric phase compared to analogues with alkoxy groups. These results not only provide practical design principles for the synthesis of new sulphur-containing LC materials with optical applications, also make a significant contribution to use as thermally sensitive liquid crystal devices requiring fast response.     

Rotational reorganization of doped cholesteric liquid crystalline films

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.     

New liquid crystalline di- and tetra-acrylates for network formation

New liquid crystalline diacrylates and tetra-acrylates containing four to six aromatic rings were synthesized and characterized, and their mesophase behaviour was investigated. They are designed to be used in combination with chiral molecules to form cholesteric mesophases which can be crosslinked by photopolymerization. The acrylates presented exhibit broad mesophase ranges since mesogenic moieties longer than three are employed. Most diacrylates show no isotropization, due to premature thermal polymerization above 180°C. Additionally, liquid crystalline dipropionates were synthesized as reference compounds which cannot be crosslinked, and selected examples of these exhibit isotropization temperatures as high as 238°C prior to thermal degradation. Substituents at the mesogenic moiety have a great influence on the mesophase characteristics. Bulky substituents such as the tert-butyl group, induce a nematic mesophase, whereas compounds with small substituents (e.g. OCH3) or unsubstituted molecules also exhibit smectic phases. Tetra-acrylates with unsubstituted and substituted mesogenic units feature nematic mesophases only as a result of the additional spacers attached. Here isotropization was observed without polymerization at temperatures around 120-160°C.     

Photoisomerizable chiral compounds derived from isosorbide and cinnamic acid

The syntheses of derivatives of isosorbide and cinnamic acid are described. These chiral compounds are photoisomerizable. The Z-isomers could also be obtained after irradiation of these E-isomeric cinnamic derivatives. The Z-isomers were found to have a much lower helical twisting power than the E-isomers. These compounds are very suitable for use in cholesteric colour filters for liquid crystal displays. This was investigated by functionalizing the E-isomeric derivatives with two acrylate groups. The reflection wavelength of cholesteric layers made with these diacrylates can be tuned by means of UV irradiation because the pitch of the cholesteric layer increases on isomerization to the Z-isomer. Subsequent photopolymerization results in cholesteric films with excellent thermal stability.     

Supramolecular self-assembly of a novel hydrogen-bonded cholesteric liquid crystal exhibiting macromolecular behaviour

The mesomorphic, thermoptic and glass-forming properties of 4-[6-((cholesteryloxy) carbonyl)oxy hexyloxy] benzoic acid (Ch-BA) have been investigated as a novel supramolecular hydrogen-bonded cholesteric mesogen. Fourier transform infrared and ¹H nuclear magnetic resonance studies have confirmed the chemical structure and the hydrogen-bond formation between the mesogens. According to polarising optical microscope observations, the compound exhibited smectic and chiral nematic phases. Differential scanning calorimetry indicated an unexpected glass transition (T _g) around 32°C and a liquid crystalline region between 32 and 122°C, in which the cholesteric phase appeared at 80°C. As a result of the glass formation, samples of Ch-BA which were rapidly cooled below the T _g were found to preserve the long-range ordering of the liquid crystalline state and retained the iridescent colours of the cholesteric phase. These results led to the conclusion that the formation of identical dimers by intermolecular hydrogen-bonding of the terminal carboxylic acids accompanying the lateral packing of the rigid cores, built a trimeric arrangement and this was responsible for the macromolecular behaviour of Ch-BA, despite its relatively simple structure and low molecular weight.     

Photocontrolled manipulation of a microscale object: a rotational or translational mechanism

In this paper the photocontrolled manipulation of solid materials on the surface of a liquid crystalline thin film is described. Three different types of films namely cholesteric liquid crystal (ChLC), compensated nematic liquid crystal (NLC) and nematic LC were used. The rotational and translational manipulation of the microscale solid object was induced by irradiation of light and mode of manipulation (either translational or rotational) was changed by changing the isomer of the azobenzene compound used to make the film. Rotational motion of the object was observed on the ChLC and compensated NLC films containing chirally pure azobenzene compound. The direction of rotational motion was controlled either by changing the optical isomer of the chiral azobenzene or by changing the irradiating light (from ultraviolet to visible). When racemic mixture of the chiral azobenzene compound was used, a translational motion of the object was observed. Even though the direction of the translational motion can be controlled by controlling irradiation position, more facile and precise manipulation of the objects was possible by spatially controlled irradiation of Ar(+) laser and diode UV laser.     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.     

Luminescent columnar liquid crystals based on tristriazolotriazine

Five discotic molecules comprising a tris[1,2,4]triazolo[1,3,5] triazine core were designed and synthesized to obtain luminescent and charge-transporting columnar liquid crystalline materials. With the exception of one compound containing terminal hydroxyl groups all compounds presented a wide thermal range and stable columnar liquid crystalline phase, characterized by differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction (XRD) techniques. The phase formation appeared to be associated to some extent with interdigitation of the alkoxy and benzylalkoxy portion, as suggested by the XRD results. All compounds have a strong blue luminescence in solution and solid phase. At the temperature at which the compounds enter in the mesophase the luminescence decreases significantly. This result suggests that entrance into the Col(h) phase is accompanied by a better π-stacking of the peripheral phenyl rings compared to the solid phase, consistent with the intramolecular distances (3.5 ?) observed in the XRD analysis. These compounds based on tristriazolotriazine are quite robust with good optical and thermal properties for application as solid state emitters, and we anticipate that they may provide an interesting alternative to other discotic molecules based on N-heterocycles, which generally present a high-temperature Col(h) phase.     

Synthesis and properties of photochromic liquid‐crystalline copolymers containing both spironaphthoxazine and cholesteryl groups

Photochromic liquid‐crystalline copolymers consisting of a photochromic monomeric unit containing both a spironaphthoxazine group and an undecamethylene spacer, and a liquid‐crystalline monomeric unit containing both a cholesteryl group and a decamethylene spacer were prepared to investigate the effect of the thermal properties of the photochromic monomeric unit on the mesomorphic order of the side chain of the related copolymers. The photochromic liquid‐crystalline copolymers containing a photochromic liquid‐crystalline monomeric unit showed only a smectic phase. On the other hand, the photochromic liquid‐crystalline copolymers containing a photochromic non‐liquid‐crystalline monomeric unit showed a chiral nematic phase (cholesteric phase). The photochromic chiral nematic liquid‐crystalline copolymer containing 14 mol % photochromic monomeric unit reflected visible light around 104 °C. To lower the temperature range of reflection of visible light, cholesteryl oleyl carbonate was used as a chiral nematic plasticizer for the photochromic chiral liquid‐crystalline polymer systems. Photo‐induced pitch change of the mixture by means of UV irradiation was investigated and it was concluded that the pitch change observed under UV irradiation was mainly induced by thermal effect in the case of our system. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 887–894, 2000     

Photochemical tuning of the helical structure of cholesteric liquid crystals by photoisomerization of chiral azobenzenes, and their structural effects

Several chiral azobenzene compounds having different chiral substituents were synthesized. A cholesteric phase was induced by mixing each chiral azobenzene compound with a host non-chiral nematic liquid crystal (E44). The helical twisting power (HTP) as well as the change in HTP by trans-cis photoisomerization of the chiral azobenzene compound was dependent on the structure of the chiral substituents. A compensated nematic phase was induced by combination of E44, a chiral azobenzene compound and a non-photochromic chiral compound. Reversible switching between the compensated nematic phase and cholesteric phase was brought about by trans-cis photoisomerization of the chiral azobenzene compound in the liquid crystalline systems. An azobenzene compound substituted with a menthyl group showed the highest efficiency as the trigger for the switching; this efficiency was related to the compactness of the chiral group substituted within the azobenzene core moiety.     

Influence of spacer lengths on the properties of chiral triplet liquid crystals based on estradiol

A series of 3-[ω-(4-cyanobiphenyl-4'-yloxy)alkyl] ethers of estradiol 17-[ω(4-cyanobiphenyl-4'-yloxy)alkanoates] with variable spacers has been prepared as chiral triplet liquid crystals. The compounds show very broad range cholesteric phases and are transformed into cholesteric glasses at room temperature. They exhibit odd-even effects for their cholesteric-isotropic transition temperatures and the associated entropy changes as a function of spacer length. The odd-even effects are observed upon changing the parity of both the ester and of the ether spacer. The best ordering is observed for compounds with an even number of methylene groups both in the ester and the ether spacer. In these cases the three mesogen units are oriented more or less parallel when the alkyl spacers are in the all-trans-conformations. Odd-even effects are also observed for the selective reflection wavelength of the planar cholesteric phase, depending on the parity of both spacers. For several compounds with short spacers the selective reflection wavelength increases strongly with temperature, whereas for other compounds this is almost temperature independent.