Shin'ya Morino

The reversion of photoalignment direction of a liquid crystal induced by a polymethacrylate with coumarin side chains

The linearly polarized light induced photoalignment of liquid crystals is critically influenced by the structure of polymethacrylates with coumarin side chains. While the photoalignment direction is in parallel with the electric vector of linearly polarized 313 nm light for polymethacrylates having coumarin moieties with ethylene and hexamethylene spacers, the reversion of photoalignment from parallel into perpendicular orientation is observed for a polymethacrylate having coumarin side chains without spacer.

Third-order nonlinear optical properties of aromatic polyisoimides

Abstract Several aromatic polyisoimides were prepared, and their third-order nonlinear susceptibilities, χ (3) (−3 ω , ω , ω , ω ), were measured by optical third-harmonic generation. Obtained χ (3) values are in the order of 10 −12 e.s.u. at a fundamental wavelength of 1907 nm. The largest value, χ (3) =3.4×10 −12 e.s.u., was obtained for the polyisoimide prepared from pyromellitic dianhydride (PMDA) and p -phenylenediamine(PDA). Thermal stability of the polyisoimide prepared from PMDA and PDA was tested. The contents of isoimide structure decreases due to thermal imidization above 200°C.

Photo-probe study of siloxane polymers. I. Local free volume of an MQ-type silicone resin containing crosslinked nanoparticles probed by photoisomerization of azobenzene

Abstract We have investigated the local free volume of an MQ-type silicone resin consisting only of M (R 3 SiO 1/2 ) and Q (SiO 4/2 ) units by using photoisomerization of azobenzene molecularly dispersed in the matrices. Photoisomerization profiles in the uncured and cured MQ resin over a wide temperature range (4–300 K) are discussed based on three parameters: first-order plots, final cis fraction and quantum yields for trans-to-cis photoisomerization of azobenzene. For uncured MQ resins or prepolymers, for which the structure was proposed to be an assembly of crosslinked nanoparticles, a large fluctuation of local free volume was observed between room temperature and 20 K. At 4 K, a drastic decrease in the final cis fraction was observed in both the uncured prepolymer mixture and the cured MQ resin, implying the complete suppression of the free-volume fluctuation at 4 K. The formation of heterogeneous sites after curing is shown by the deviation from first-order kinetics for the cured MQ resins. Data for the cured MQ resin are analyzed quantitatively with the three-component approximation and the validity of a free-volume fluctuation model is verified. Comparison of the results for linear polysiloxanes with those for poly(methyl methacrylate) shows that siloxane chains are very flexible at low temperatures down to 20 K.

Photochemical Behavior and the Ability to Control Liquid Crystal Alignment of Polymethacrylates with Styrylpyridine Side Chains

Photoproduct distribution of styrylpyridine (SP) side chains tethered to polymethacrylate backbones upon irradiation with 313 nm light was analyzed UV-spectroscopically in solutions and films of a homopolymer and copolymers of 2-[2-(4-methacryloxyphenyl)ethenyl]pyridine. Whereas E-to-Z photoisomerization of SP moieties proceeds in solution, followed by the slow oxidative photocyclization to give an azaphenanthrene, the predominant photochemistry in films consists of photodimerization and E-to-Z photoisomerization. Irradiation of films of the SP-polymers with linearly polarized 313 nm light generated dichroism of the chromophores due to the axis-selective consumption of the E-isomer. Exposing films of SP-polymers to relatively lower doses of linearly polarized light induced homogeneous alignment of a nematic liquid crystal with a perpendicular orientation with respect to the polarization plane of the light. The photoalignment was thermally stabilized by efficient photodimerization leading to crosslinked structures. However, prolonged irradiation with the polarized light deteriorated the photoalignment. On the basis of these results, it is concluded that the homogeneous photoalignment is determined by remaining E-isomers whereas the photodimerization yielding crosslinks enhances the thermal stability of the alignment.

Effect of Methylene Spacers in Poly(methacrylate)s Bearing Styrylpyridine Side Chains on the Ability to Control Liquid Crystal Alignment

The photochemistry, photoinsolubilization and ability to control surface-assisted liquid crystal (LC) alignment of poly(methacrylates) tethering styrylpyridine (SP) side chains through methylene spacers (m = 2,6 and 12) are described. The extent of (2 + 2) photodimerization and E-to-Z isomerization of SP side chains was markedly affected by the lenght of methylene spacers. In the early stage of photoirradiation, the photodimerization took place preferentially for SP-polymers with a shorter methylene spacer, whereas the photodimerization was accompanied by the E-to-Z photoisomerization for the polymers with longer spacers even in the early stage of photoirradiation. Irradiation of thin films with linearly polarized light resulted in the generation of dichroism of SP moieties owing to the angular-selective consumption of E-isomer, leading to surface-assisted homogeneous LC alignment. The direction of LC photoalignment was reversed from perpendicular to parallel orientation when SP moieties were tethered to polymer backbones through spacers. The working mechanism of the alignment reversion was discussed, based on the angular-selective E-to-Z photoisomerization and photodimerization.

A convenient preparation of photoactive monolayers for liquid crystal photoalignment by the surface adsorption of an aminoalkylated azobenzene on poly(acrylic acid) films

Spin-cast poly(acrylic acid) (pAA) films on a substrate plate are immersed in a hexane solution of 4-butyl-4′-(10-aminodecyloxy)azobenzene, resulting in surface-selective adsorption and leading to the formation of photoactive monolayers. The surface adsorption behaviour is critically affected by the concentration of the azobenzene adsorbate solutions, and a 2.5×10–5 mol dm–3 solution gives an azobenzene monolayer with a surface density of 2 molecules nm–2 without any modification of the surface morphology of the pAA film. The surface density of the azobenzene is controlled by a binary system consisting of the azobenzene and n-octadecylamine. Photoirradiation of the surface-modified film with linearly polarised UV light for E-to-Z photoisomerisation results in the orientational transformation of the nematic liquid crystal layer from homeotropic to homogeneous alignment. It has been found that the minimum exposure energy of polarised UV light required for the liquid crystal alignment photocontrol is determined by the surface density of the azobenzene. There is a critical density; no photoresponse is observed for average densities of the azobenzene smaller than ca. 0.7 molecules nm–2 . With densities larger than this value, the smaller the density of the chromophore on pAA surface is, the faster the photoreorientation of the liquid crystal effectively takes place.

Photochemistry Determined by Light Propagation. Part 1 Three-dimensional Photomanipulation of Self-organized Azobenzenes in Liquid-Crystalline Polymers

Clear-cut observation of the reorientation and self-organization of azobenzene chromophores to the propagation direction of actinic light is described, using spin-cast thin films of a liquid crystalline azobenzene polymer.

Surface-Assisted Orientational Control of Discotic Liquid Crystals by Light

Abstract This paper reports photoalignment control of discotic liquid crystal (DLC), a pentakis-(phenylethynyl)benzene derivative, using a thin film of an azobenzene-containing polymer. The polymer films was irradiated with non-polarized light obliquely, subsequently annealed at 240°C and coated with a DLC solution to give a mono-domain texture of DLC films with excellent optical quality. Photopatterning of DLC films was achieved by imagewise irradiation of the polymer film at an oblique incident.

Chiroptical Properties of Cholesteric Liquid Crystals Induced by Chiral Photochromic 3,3′-Dialkoxyazobenzenes

Abstract The spectroscopic features and photoisomerization behaviors of chiral photochromic 3,3′-dialkoxyazobenzenes were evaluated in an isotropic solution and a nematic solvent. The effect of photoisomerization on the helical twisting power and handedness of these chiral dopants in induced cholesteric liquid crystals were studied. Depending on the distance of the chiral center from the azo-core, elongation of the helical pitch and reversion of the handedness upon E-to-Z photoisomerization were observed. These characteristics are interesting for optical devices based on light-controlled phase switching.

Photogeneration of pretilt angles of nematic liquid crystals by azobenzene-containing monolayers on poly(acrylic acid) films

A convenient procedure to fabricate azobenzene-containing monolayers capable of performing the photoalignment of nematic liquid crystals (LCs) with pretilt angles has been elaborated. The photoactive monolayers were readily prepared by the surface-selective adsorption of p-butylazobenzene substituted with an ω-aminoalkyl residue to the outermost surface of a poly(acrylic acid) thin film. After the fabrication of an LC cell, the monolayers generated homeotropic LC orientation, which was transformed into homogeneous alignment with tilting angles by oblique irradiation with non-polarized UV light. The azimuthal direction of the photoinduced LC alignment was consistent with the propagation direction of the UV light, suggesting that the LC photoalignment results from direction-selective E to Z photoisomerization leading to re-orientation of the azobenzene chromophores. The photogenerated pretilt angles of LC alignment were significantly dependent on the dielectric anisotropy of LC and the surface density of the azobenzene in the monolayers.