Search

We studied the photo-induced dynamics in a LC polymer film including azobenzene by using a time-resolved and a microscopic technique. The film was confined in a liquid crystal cell, while it is a photomobile film under a free standing condition, which is triggered by the photoisomerization of azobenezene. From the result of the time-resolved measurements, the change inside the film induced by UV irradiation was highly anisotropic polarization change. Microscopic observation revealed that the film was consisted of ordered and disordered region with a patched structure, and the UV induced change was travelled in the ordered region on the order of seconds.

We studied the effect of the ratio between the monomer and cross-linker molecules in the azobenene included liquid crystal polymer films by using the heterodyne transient grating (HD-TG) technique, which is one of the time-resolved measurement techniques. Depending on the ratio, the magnitude of the refractive index change, its anisotropy, and the lifetime of the cis isomer of azobenzene, generated by a UV pulse irradiation. By increasing the cross-linker ratio, the refractive index change and its anisotropy was reduced, indicating less ability for the motion, while slower lifetime was observed by increasing the monomer ratio, indicating that the film is difficult to return the original shape by a visiblelight irradiation. The obtained dynamics was consistent with the functionality of the films.

Shortcoming of a planar self-emitting device is only a small fraction of light generated in the device available due to a total internal reflection at the air/substrate interface. To increase a luminance efficiency of the device, microlens arrays are most useful: they extract the total internal reflection of light at the air/substrate interface. In this study, we investigated photoinduced reorientation behavior of dye-doped polymerizable liquid crystals to fabricate a novel planar microlens array film as an optical component for a self-emitting device.

Amorphous copolymers were synthesized with azobenzene methacrylate, cyanobiphenyl methacrylate, and alkyl methacrylate, in which the azobenzene acts as a photoresponsive part to undergo photoisomerization, the cyanobiphenyl is involved in a change in refractive index triggered by the photoisomerization of the azobenzene, and the alkyl methacrylate is to prevent light scattering, respectively. The obtained polymers showed no LC phase in spite of the incorporation of the common mesogenic group. When two writing beams were interfered in the thick polymer films, the diffraction of the probe beam was observed. The first-order diffraction efficiency reached more than 70 % within 500 ms. On the other hand, the intensity of the zeroth-order beam decreased in proportion to the increase in that of the first-order diffracted beam. It is clear that the intensity of the zeroth-order beam is transferred to that of the first-order diffraction with no optical loss due to the formation of the Bragg gratings. When the writing beams were turned off, the diffraction efficiency remained unchanged. When the recorded grating was kept in the dark at room temperature, sufficient diffraction efficiency was generally retained after a year. However, the grating was easily erased when heated up above Tg for several seconds, and rewritability was confirmed more than 100 cycles. No shrinkage was observed through those processes.

Various kinds of crosslinked liquid-crystalline polymers were synthesized with acrylate monomers and diacrylate crosslinkers containing azobenzene moieties. The effect of spacer length on photoinduced bending behavior of the polymer films was investigated. The films with long spacers exhibited low glass transition temperatures and underwent bending and unbending behavior at room temperature. Mechanical force generated during the process of UV light irradiation was also measured. It was found that the maximum force increased with the increment of the cross-linking density and the intensity of UV light.

Azobenzene-containing polymer, in which a change in birefringence can be induced by photoisomerization, is one of promising materials for holographic applications. To develop high-performance holographic materials using azobenzene polymers, it is necessary to prepare optically transparent thick films with capability of a large photoinduced change in refractive index to obtain high diffraction efficiency and fast response in the Bragg regime. A number of materials have been studied so far, but no attention has been paid to the combination of thick amorphous copolymers films containing photoresponsive and mesogenic groups in the side chain and polarization gratings formed in the Bragg regime. In this paper, we prepared optically transparent thick polymer films containing an azobenzene moiety with photosensitivity and a tolane moiety with high birefringence, and formed intensity and polarization gratings with linearly polarized beams and orthogonal circularly polarized beams, respectively. It was found that the nearly theoretically maximum diffraction efficiency was successfully achieved, and the recorded hologram could be erased thermally or photochemically. In addition, a large change in refractive index was induced in the polarization grating, and high diffraction efficiency (~90%) and quick response (920 ms) was realized simultaneously.

Holography is one of the most promising techniques that enables ultra-high density optical data storage with a simple and small optical setup. The key component in holographic devices is a holographic material that meets various requirements such as high diffraction efficiency, fast response, and stability. Furthermore, additional problems, rewritability and reversibility, must be solved in rewritable holographic materials which are more convenient. In this paper, we successfully achieved rewritable holograms with 55 multiplicities by a simple formulation of optically transparent copolymer films containing azobenzene and mesogenic moieties. The recorded hologram was stable for more than six months and was rewritable over 300 times. In addition, holograms with angle and polarization multiplicity were recorded by controlling the incident angle and the polarization direction of writing beams independently.

Photoinduced bending and unbending behavior of cross-linked polymer films, prepared by in-situ polymerization of liquid-crystalline (LC) monomers with azobenzene moieties, is discussed. Various modes of bending have been achieved with various alignments of the photoactive mesogens in the elastomers. Exposure of a monodomain film to UV light to cause trans-cis isomerization of the azobenzene moieties leads to the bending of the film toward irradiation direction of the actinic light. Irradiation of a polydomain film with linearly polarized light results in the bending along the polarization direction of the light, which enables bending of the film along any direction. The LC alignment in the films significantly affected the photoinduced bending behavior.

To create polarized organic light emitting diodes (OLEDs), liquid-crystalline polymers (LCPs) containing oxadiazole and amine moieties in the side chain were design and synthesized. These LCPs emitted intense blue fluorescence with high quantum yield over 0.6 in a solution. Furthermore, polarized emissions of fluorescence were observed in a LC phase, and order parameters estimated by the polarized emission were about 0.2. They have proven to function as polarized blue-emitting materials for OLEDs.

Photochemically induced nematic (N)-isotropic (I) phase transition behavior and domain formation of the azobenzene/liquid crystal (LC) mixtures in the presence of non-mesogenic dopants were evaluated. When a bent-shaped non-mesogenic molecule was added to the azobenzene LCs, formation of I domains was formed effectively on photoirradiation to cause the trans-cis isomerization of the azobenzene. The domain formation results from the aggregation of cis isomers, which could be enhanced to more extent with the aid of the bent-shaped dopant with a common structural feature to the cis-azobenzene. When the formation of the I domain proceeds quickly upon photoirradiation, the photochemical phase transition is induced more effectively.

Photoinduced two-dimensional (2-D) and three-dimensional (3-D) alignment behavior of polymer liquid crystals (LCs) with azobenzene moieties was investigated. Two-dimensional alignment of the polymer LCs was brought about on irradiation with linearly polarized light. The effects of various factors on the phenomena were discussed in detail, and experimental conditions and structural parameters of the polymer LCs were optimized. It was found that the response could be enhanced by two methods: chemically tailoring the structure of the polymer LCs and physically modifying the alignment procedure. Three- dimensional alignment of the azobenzene moieties was achieved on irradiation with unpolarized light. It was revealed that the azobenzene moieties were aligned along the propagation direction of the incident light. Photochemically inert mesogens underwent reorientation together with azobenzene moieties in the 2-D as well as 3-D alignment process.

A simple casting film of Poly(6-[4-(4- cyanophenyl)diazenylphenyl oxy] hexyl methacrylate) (PM6ABCN) was made on an untreated surface of a glass substrate and adopted as a typical sample to study photoinduced orientation of azobenzene liquid crystalline side chain homopolymers. Below Tg and after irradiation of light at 366 nm, 400 nm, and 436 nm, respectively, the same;l shows different UV spectra which can give an expedition on the steady state of the photoinduced orientation. Under various conditions photoinduced orientation of PM6ABCN was examined, and results show that the orientation process is not only dependent on the light strength, but also on the temperature and the wavelength of polarized light used. The maximum extent of orientation below Tg can be obtained within 100 s under irradiation of polarized light of 20 mW/cm2 at 400 nm, and irradiation at 366 nm causes a lower extent of orientation than that by irradiation at 400 nm and 436 nm because relative more cis isomer was produced in the steady state.

Manipulation of light by light as a stimulus by the use of liquid-crystalline materials was explored, which was aimed at optical switching, optical image storage and optical display. Two types of the liquid-crystalline materials are used: nematic liquid crystals and ferroelectric liquid (FLCs). Several kinds of optical switching and image storage systems were demonstrated by means of photochromic molecules and photosensitive LC alignment layer to induce the change in alignment of the LCs: 1) Fast nematic to isotropic phase transition of azobenzene LCs was induced by the laser pulse in two modes of analysis, transmission mode and reflection mode. 2) Optical switching in FLCs doped with the azobenzene was investigated with regard to photochemical flip of polarization of the FLCs. 3) Optical control of polymer- dispersed LC was explored by means of the azobenzene LC. 4) Optical switching and fabrication of nonrubbed alignment layer based on the novel photophysical principle was achieved in terms of photosensitive polyimide as an LC alignment layer. 5) Optical control of conductivity was performed by the use of LC thiophene derivatives.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Image storage into polymer liquid crystals (PLCs) on the basis of an isothermal phase transition is presented. Photochromic guest molecules are incorporated into PLCs just physically or covalently at concentrations below several mole and irradiated so as to bring about a photoisomerization. In the case where azobenzenes are used as guests, a trans-cis photoisomerization caused by photoirradiation acts as a trigger to induce nematic (N) to isotropic (I) phase transition at the irradiated sites owing to lowering of the N-I phase transition temperature (TNI) by accumulation of the sphere-like cis-form of the azobenzenes. This photochemical phase transition is reversible and the cis to trans isomerization restores the system to the initial state. The photochemical phase transition depends on such factors as temperature, structures of guests, and hosts. Guest molecules with high affinity to hosts induce the photochemical phase transition effectively and hosts with low order parameters are also effective for the phase transition. Unlike the response to electric field, PLCs show a similar photoresponsiveness to low-molecular-weight LCs.