Metal-containing ionic liquids and ionic liquid crystals based on imidazolium moiety

Ionic liquids and ionic liquid crystals of imidazolium salts composed of various transition and main group metals have been reviewed. Ionic metal complexes of imidazoles and N-heterocyclic carbenes possess the similar properties were also included. These types of ILs and ILCs have been realized as potential solvents, catalysts, catalyst precursors and reagents for many organic transformations and provide ecofriendly protocols. They have also been found to play key roles in material science. Many of these IL systems are air- and moisture stable and are considered as alternatives for air- and moisture sensitive chloroaluminate-based ILs.     

Solution-processed bilayer photovoltaic devices with nematic liquid crystals

The cross-linking of polymerisable liquid crystalline semiconductors is a promising approach to solution-processable, multilayer, organic photovoltaics. Here we demonstrate an organic bilayer photovoltaic with an insoluble electron-donating layer formed by cross-linking a nematic reactive mesogen. We investigate a range of perylene diimide (PDI) materials, some of which are liquid crystalline, as the overlying electron acceptor layer. We find that carrier mobility of the acceptor materials is enhanced by liquid crystallinity and that mobility limits the performance of photovoltaic devices.     

Optical storage effects in liquid crystals

The electro-optic behavior of two chemically identical liquid crystal mixtures was compared. It was determined that ethoxybenzylidene-p′-n-butylaniline (Eobuta) doped with 10% 1-methanol exhibited pronounced optical storage, whereas Eobuta doped with 10% racemic menthol only exhibited dynamic scattering. Since Eobuta doped with 1-menthol is cholesteric and Eobuta doped with racemic menthol is nematic, it was concluded that optical storage capability is an intrinsic property of the cholesteric state, and in agreement with the observable textures, should be viewed as a reversible plane ? focal-canic texture change.     

Digital optical memory devices based on polymer-dispersed liquid crystals films: appropriate polymer matrix morphology

ABSTRACT

Permanent memory effect (PME) in polymer dispersed liquid crystal (PDLC) allows a greater applicability range than traditional PDLCs. One of the most interesting application could be the possible storing of optical information, the so-called Digital Optical Memory (DOM) devices. To test this application it would be required a display structure having an array of pixels addresses. Each pixel was filled with PDLC film with PME and electric field can be independently applied to different PDLC elements to define on/off pixel states (transparent or scattering states).PDLC films were obtained from a mixture of E7 nematic liquid crystal and poly(ethylene glycol) dimethacrylate with 875 g mol^-1oligomer as precursor of the polymeric matrix. The effect of the curing temperature and the UV light intensity as well time exposure during the polymerisations on the electro-optical performance of PDLC films were investigated. In this way, a high transparency state (T_OFF=55%) for a long period of time at room temperature even after the applied voltage has been switched off were obtained, started from an opaque state (T₀=0%) and after reaching a transparent state (T_MAX=75%), which causes 73% PME. The application to an 8x8 passive matrix using PDLC with PME is also demonstrated as proof-of-principle.     

Liquid crystals for holographic optical data storage

A tutorial review is presented to inform and inspire the reader to develop and integrate strong scientific links between liquid crystals and holographic data storage, from a materials scientist's viewpoint. The principle of holographic data storage as a means of providing a solution to the information storage demands of the 21st century is detailed. Holography is a small subset of the much larger field of optical data storage and similarly, the diversity of materials used for optical data storage is enormous. The theory of polarisation holography which produces holograms of constant intensity, is discussed. Polymeric liquid crystals play an important role in the development of materials for holographic storage and photoresponsive materials based on azobenzene are targeted for discussion due to their ease of photo-reversion between trans- and cis-states. Although the final polymer may not be liquid crystalline, irradiation can induce ordered domains. The mesogens act in a co-operative manner, enhancing refractive indices and birefringences. Surface relief gratings are discussed as a consequence of holographic storage. Cholesteric polymers comprising azobenzene are briefly highlighted. Irradiation causing cis-trans-isomerisation can be used to control helix pitch. A brief mention of liquid crystals is also made since these materials may be of future interest since they are optically transparent and amenable to photo-induced anisotropy.     

Dynamic temperature measurement in microfluidic devices using thermochromic liquid crystals

Thermochromic liquid crystals (TLCs) are used to explore the temperature transients during thermal cycling for microchip-based polymerase chain reaction (PCR). By analyzing the reflected spectra of the TLCs over time, temperature vs. time trajectories were extracted and overshoots/undershoots were estimated. To our knowledge, this is the first report of TLC-based dynamic temperature measurements in a microfluidic device for all PCR temperature stages.     

Diazo liquid crystals for potential infrared applications

Nine series of liquid crystals containing an electron-donating group at one end of a conjugated system and an electron-withdrawing group at the other end have been synthesized. The electron-donating group is 4-n-alkylpiperazine and the electron-withdrawing group is the nitro group; and the conjugated system is diphenyldiazene with zero, one or two substituents on the phenyl ring not containing the nitro group. The substituents are-F,-Cl, and-CH₃. Most of the compounds synthesized are nematogenic and exhibit rather broad liquid crystalline ranges. The effects of the lateral substituents on the optical absorption and phase transition temperatures are correlated with their nature and position of substitution. Dielectric anisotropy and birefringence measurements were made using mixtures containing 10 wt % of synthesized liquid crystals dissolved in commercial mixture E7.     

Diazo liquid crystals for potential infrared applications

Nine series of liquid crystals containing an electron-donating group at one end of a conjugated system and an electron-withdrawing group at the other end have been synthesized. The electron-donating group is 4-n-alkylpiperazine and the electron-withdrawing group is the nitro group; and the conjugated system is diphenyldiazene with zero, one or two substituents on the phenyl ring not containing the nitro group. The substituents are-F,-Cl, and-CH₃. Most of the compounds synthesized are nematogenic and exhibit rather broad liquid crystalline ranges. The effects of the lateral substituents on the optical absorption and phase transition temperatures are correlated with their nature and position of substitution. Dielectric anisotropy and birefringence measurements were made using mixtures containing 10 wt % of synthesized liquid crystals dissolved in commercial mixture E7.     

Third order optical non-linearities of nematic liquid crystals

Liquid crystals can exhibit large third order optical non-linearities. Using the Z-scan technique, we have measured the intensity dependence of the non-linear absorption and non-linear refractive indices on time-scales ranging from milliseconds to picoseconds for nematic liquid crystals. This method allows the determination of the non-linear absorption coefficients as well as the signs and magnitudes of the non-linear refractive indices for different polarizations. A two pulse technique further enables estimation of the response time of the dominant mechanism. Using CW argon and Q-switched and mode-locked pumped dye lasers, we have carried out Z-scan measurements on aligned liquid crystal samples as a function of temperature, as well as wavelength. In the geometries studied, director reorientation is not expected to take place. On the nanosecond time-scale, all materials studied were self-focusing for polarization perpendicular to the director, and self-defocusing for parallel polarization. On the picosecond timescale, the samples were self-focusing for all polarizations, but strong non-linear birefringence was typically observed. An attempt is made to relate the bulk response to the molecular structures.     

Non-linear optical properties of metallorganic ferroelectric liquid crystals

Second harmonic generation (SHG) efficiencies d eff were determined for Pt-containing ferroelectric liquid crystals (FLCs). A relatively simple method is presented which allows for the simultaneous determination of the second order susceptibility tensor coefficients d ij together with the dispersion and birefringence at the fundamental frequency. The relation between the d ij coefficients is qualitatively interpreted in terms of the molecular shape and of a scheme proposed for the molecular orientation in the ferroelectric phase. Contrary to the usual situation with calamitics, d 23 is much higher than the rest of the coefficients. This peculiarity, which reflects the 3D character of the molecular response, is interesting since d 23 is directly involved in d eff for phase matched SHG.     

Engineering liquid crystals for optimal uses in optical communication systems

Liquid crystal technology has become, nowadays, one of the core technologies for implementing optical functions for WDM networks. Their advantage, with respect to other materials, is their unequalled engineering capability which is nowadays decisive in providing more flexible and cost effective systems. First, we briefly review the main driving physical features determining their optical and dynamic behaviour, illustrated through various pure or composite liquid crystal materials. Second, we discuss their suitability for use in some telecommunication requirements. Finally we give examples illustrating the huge variety of their possible effects, in particular when engineered with polymer chains.     

Side-chain liquid crystalline polyesters for optical information storage

Azobenzene side-chain liquid crystalline polyester structures suitable for permanent optical storage are described. The synthesis and characterization of the polyesters together with differential scanning calorimetry and X-ray investigations are discussed. Optical anisotropic investigations and holographic storage in one particular polyester are described in detail and polarized Fourier transform infrared spectroscopic data complementing the optical data are presented. Optical and atomic force microscope investigations point to a laser-induced aggregation as responsible for permanent optical storage.     

Optical storage of hidden images in ultraviolet-cured polymer dispersed liquid crystals

Among the different methods used to prepare polymer dispersed liquid crystals (PDLCs), polymerization induced phase separation can be successfully exploited to obtain optical recording of high resolution holographic gratings and binary images in these materials. In this paper we report a new method that allows hidden images to be obtained in PDLCs that are not detectable by light in the visible range. The possibility of storing invisible images during the curing process will be described and discussed. The binary images obtained can be detected by illuminating them with low power UV radiation, thus opening the way to interesting applications in the field of optical storage of reserved information.     

Optical storage of hidden images in ultraviolet-cured polymer dispersed liquid crystals

Among the different methods used to prepare polymer dispersed liquid crystals (PDLCs), polymerization induced phase separation can be successfully exploited to obtain optical recording of high resolution holographic gratings and binary images in these materials. In this paper we report a new method that allows hidden images to be obtained in PDLCs that are not detectable by light in the visible range. The possibility of storing invisible images during the curing process will be described and discussed. The binary images obtained can be detected by illuminating them with low power UV radiation, thus opening the way to interesting applications in the field of optical storage of reserved information.     

Dielectric versus optical response of chevron ferroelectric liquid crystals

Dielectric and optical methods to investigate the response of surface‐stabilized ferroelectric liquid crystals (SSFLCs) of the chevron structure are examined and compared in the case of the azimuthal mode of collective relaxation processes. It is found that the variation of an effective (averaged over the chevron cell volume) dielectric permittivity tensor under the influence of a weak alternating external electric field is approximately equivalent to the transformation of this tensor as a consequence of the rotation of the laboratory frame around the axis perpendicular to the smectic plane about a small angle. Then, using an analytic solution of the equation of motion describing the azimuthal rotation of molecules, it is shown that both of the analysed approaches to calculate and measure the response of SSFLCs yield consistent results for these rotational dynamic processes. This allows the calculation of the spontaneous polarization of the unit volume of chevron slabs, provided that the pretilted azimuthal angle (in the absence of an applied electric field) within the smectic plane is known.     

Strongly non-linear optical ferroelectric liquid crystals for frequency doubling

Abstract

From our research for novel non-linear optical (NLO) materials for frequency doublers and optical modulators we report on new ferroelectric liquid crystals, which for the first time, exhibit second order NLO coefficients (for example d ₂₂ = 5 pm V^?1, which are comparable to those of state of the art inorganic NLO materials. The novel compounds contain 5-amino-2-nitrophenyl groups attached close to the chiral centres. The switching behaviour of the new compounds, their spontaneous polarization, as well as their frequency doubling of Nd:YAG laser pulses in the S*_c and in the glass state, are reported. Moreover their waveguiding properties are presented.     

The optical properties of polymethylmethacrylate polymer dispersed liquid crystals

The effect of the addition of polymer liquid crystals as dispersed molecules to polymethylmethacrylate (PMMA) on the optical properties in the UV-visible and near infrared regions is investigated. From transmission, absorption and reflection spectra the absorption coefficient (ω) and refractive index (n) at angular frequency of radiation (ω) have been calculated at room temperature. The values of the optical band gap (E_opt) have been obtained from the direct allowed transitions in k-space. The width of the tails of localized states in the band gap (ΔE) was evaluated from Urbach edges. Both the parameters (E_opt) and (ΔE) vary with the mixing ratio of dispersed liquid crystals.     

Dielectric versus optical response of chevron ferroelectric liquid crystals

Dielectric and optical methods to investigate the response of surface-stabilized ferroelectric liquid crystals (SSFLCs) of the chevron structure are examined and compared in the case of the azimuthal mode of collective relaxation processes. It is found that the variation of an effective (averaged over the chevron cell volume) dielectric permittivity tensor under the influence of a weak alternating external electric field is approximately equivalent to the transformation of this tensor as a consequence of the rotation of the laboratory frame around the axis perpendicular to the smectic plane about a small angle. Then, using an analytic solution of the equation of motion describing the azimuthal rotation of molecules, it is shown that both of the analysed approaches to calculate and measure the response of SSFLCs yield consistent results for these rotational dynamic processes. This allows the calculation of the spontaneous polarization of the unit volume of chevron slabs, provided that the pretilted azimuthal angle (in the absence of an applied electric field) within the smectic plane is known.     

Strongly non-linear optical ferroelectric liquid crystals for frequency doubling

From our research for novel non-linear optical (NLO) materials for frequency doublers and optical modulators we report on new ferroelectric liquid crystals, which for the first time, exhibit second order NLO coefficients (for example d₂₂ = 5 pm V^-1, which are comparable to those of state of the art inorganic NLO materials. The novel compounds contain 5-amino-2-nitrophenyl groups attached close to the chiral centres. The switching behaviour of the new compounds, their spontaneous polarization, as well as their frequency doubling of Nd:YAG laser pulses in the S*_c and in the glass state, are reported. Moreover their waveguiding properties are presented.