Haruhisa Akiyama

Photo‐optical liquid crystal cells driven by molecular rotors

Photochemical regulation of in‐plane alignment of a nematic liquid crystal is presented. A quartz plate surface was modified with 4‐hexyl‐4’‐hexyloxyazobenzene substituted with triethoxysilyl group through a spacer at the 2’ position to introduce the side‐on type azobenzene unit of the surface. A liquid crystal hybrid cell fabricated with the azo‐modified plate was irradiated with linearly polarized visible light for the n‐π* transition. This resulted in the in‐plane reorientation of the liquid crystal molecules to afford homogeneous alignment. Discussion is made on the mechanism of the in‐plane alignment regulation, and the concept of a molecular rotor is proposed.

Photochemically Reversible Liquefaction and Solidification of Single Compounds Based on a Sugar Alcohol Scaffold with Multi Azo‐Arms

Sugar alcohol derivatives with multi azobenzene arms are photochemically and isothermally liquefied from a powdered solid upon irradiation with ultraviolet light at room temperature, and then solidified on irradiation with visible light, where the transition between solid and liquid are reversible. These compounds possess similar chemical structures to comb-like liquid crystalline oligomers.

THREE-DIMENSIONAL ORIENTATIONAL CONTROL OF MOLECULES BY SLANTWISE PHOTOIRRADIATION

When a film of poly[2-(4-phenylazophenyloxy)ethyl methacrylate] is subjected to irradiation with nonpolarized 436 nm light for n-π* excitation at an oblique incident angle with respect to a film surface, dichroism is induced as a result of the reorientation of azobenzene chromophores. The orientational direction is determined reversibly by an incident plane of the light while azobenzene residues reorient in a direction parallel with the light propagation. This procedure is applicable to the generation of tilt angles of nematic liquid crystals assisted by obliquely photoirradiated films as a function of exposure doses, maintaining homogeneous alignment during photoirradiation.

A novel method for creation of free volume in a one-component self-assembled monolayer. Dramatic size effect of para-carborane

We fabricated the self-assembled monolayers (SAMs) composed of 4-(12-(10-mercaptodecyl)-1,12-dicarba-closo-dodecaborane)-4′-octyloxy azobenzene and 4-(10-mercaptodecyloxy)-4′-octyloxyazobenzene on a gold surface, and performed photoswitching experiments. The SAM including p-carborane exhibited a much higher and more reversible photochemical response in comparison with the SAM in the absence of the p-carborane. This result suggests that the free volume of the azobenzene moiety to change its conformation is increased due to the size effect of the p-carborane.

Photochemically Reversible Liquefaction and Solidification of Multiazobenzene Sugar-Alcohol Derivatives and Application to Reworkable Adhesives

Multiazobenzene compounds, hexakis-O-[4-(phenylazo)phenoxyalkylcarboxyl]-D-mannitols and hexakis-O-[4-(4-hexylphenylazo)phenoxyalkylcarboxyl]-D-mannitols, exhibit photochemically reversible liquefaction and solidification at room temperature. Their photochemical and thermal phase transitions were investigated in detail through thermal analysis, absorption spectroscopy, and dynamic viscoelasticity measurements, and were compared with those of other sugar-alcohol derivatives. Tensile shear strength tests were performed to determine the adhesions of the compounds sandwiched between two glass slides to determine whether the compounds were suitable for application as adhesives. The adhesions were varied by alternately irradiating the compounds with ultraviolet and visible light to photoinduce phase transitions. The azobenzene hexyl tails, lengths of the methylene spacers, and differences in the sugar-alcohol structures affected the photoresponsive properties of the compounds.

Reversible work function changes induced by photoisomerization of asymmetric azobenzene dithiol self-assembled monolayers on gold

We measured reversible changes in the work function (ΔϕAu) of gold substrates modified by asymmetric azobenzene dithiol self-assembled monolayers (SAMs) following photoisomerization and thermal recovery of the azo unit. The azobenzene derivative SAMs were photoisomerized to cis form by UV irradiation. ΔϕAu was monitored in real time during thermal recovery to trans form by ultraviolet photoelectron spectroscopy using a synchrotron light source. Changing the substituted functional group in the p′ position of the azobenzene from electron donating to electron withdrawing resulted in opposite responses of ΔϕAu against photoisomerization. Hence, a direct correlation between ΔϕAu and changes in molecular dipole moments was obtained.

Role of E/Z Photoisomerization of Cinnamate Side Chains Attached to Polymer Backbones in the Alignment Photoregulation of Nematic Liquid Crystals*1

The mechanism of azimuthal reorientation of nematic liquid crystals induced by linearly polarized light irradiation of thin films of polymers with cinnamate side chains has been discussed. It has been concluded that the photoalignment is induced not by the polarized photodimerization, but by polarized photoisomerization, based on the following results. First, spectral analysis of olefinic chromophores upon UV irradiation has shown that a predominant photochemistry in films is the E/Z photoisomerization which results in the dichroism of the chromophores. Second, the alignment direction can be altered by subsequent irradiation with polarized light with a different electric vector. Photodimerization plays a role in enhancing the thermal stability of photoalignment.

Control of the Orientation and Photoinduced Phase Transitions of Macrocyclic Azobenzene

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups (1) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal-isotropic phase-crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be -0.84. Heating enhances the thermal cis-to-trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.

Photoinduced crystal-to-liquid phase transitions of azobenzene derivatives and their application in photolithography processes through a solid-liquid patterning.

The direct and reversible transformation of matter between the solid and liquid phases by light at constant temperature is of great interest because of its potential applications in various manufacturing settings. We report a simple molecular design strategy for the phase transitions: azobenzenes having para-dialkoxy groups with a methyl group at the meta-position. The photolithography processes were demonstrated using the azobenzene as a photoresist in a single process combining development and etching of a copper substrate.

Command surfaces 12 [1]. Factors affecting in-plane photoregulation of liquid crystal alignment by surface azobenzenes on a silica substrate

Abstract In-plane alignment of nematic liquid crystals was regulated by polarized-light-irradiation of a cell assembled with a silica plate, the surface of which was modified by attaching 4-hexyl-4′-hexyloxyazobenzene at its o-position through surface silylation. The photoisomerizability of the chemisorbed azo-chromophore was affected by their surface density and the nature of photoinactive co-modifiers. The efficiency of the photoregulation of liquid crystal alignment was optimized by two-dimensional dilution of the chromophore with ethyltriethoxysilane (ETS) or 3-aminopropyltriethoxysilane (ATS). As a result, favourable procedure was to modify a silica surface with a crude azo-silylating reagent contaminated by ATS. The rate of the photoinduced reorientation of liquid crystals was followed by monitoring the alteration of the alignment direction of a dichroic dye dissolving in a mesophasic layer upon exposure to linearly polarized light. Exposure energy for the in-plane reorientation of a liquid crystal ...