Yasuo Norikane

Light-induced crawling of crystals on a glass surface

Motion is an essential process for many living organisms and for artificial robots and machines. To date, creating self-propelled motion in nano-to-macroscopic-sized objects has been a challenging issue for scientists. Herein, we report the directional and continuous motion of crystals on a glass surface when irradiated simultaneously with two different wavelengths, using simple azobenzenes as a photoresponsive organic compound. The direction of the motion can be controlled by the position of the light sources, and the crystals can even climb vertical surfaces. The motion is driven by crystallization and melting at the front and rear edges of the crystal, respectively, via photochemical conversion between the crystal and liquid phases induced by the trans–cis isomerization of azobenzenes. This finding could lead to remote-controlled micrometre-sized vehicles and valves on solid substrates.

Crystal Melting by Light: X-ray Crystal Structure Analysis of an Azo Crystal Showing Photoinduced Crystal-Melt Transition

Trans-cis photoisomerization in an azo compound containing azobenzene chromophores and long alkyl chains leads to a photoinduced crystal-melt transition (PCMT). X-ray structure analysis of this crystal clarifies the characteristic coexistence of the structurally ordered chromophores through their π···π interactions and disordered alkyl chains around room temperature. These structural features reveal that the PCMT starts near the surface of the crystal and propagates into the depth, sacrificing the π···π interactions. A temporal change of the powder X-ray diffraction pattern under light irradiation and a two-component phase diagram allow qualitative analysis of the PCMT and the following reconstructive crystallization of the cis isomer as a function of product accumulation. This is the first structural characterization of a compound showing the PCMT, overcoming the low periodicity that makes X-ray crystal structure analysis difficult.

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.

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.

Realization of highly photoresponsive azobenzene-functionalized monolayers

We report the successful fabrication of azobenzene-functionalized self-assembled monolayers (SAMs) exhibiting high and reversible photoswitching between trans and cis states on a flat gold surface. Azobenzene thiols (MeSH and EtSH) containing meta and/or ortho substituents were chosen based on the occupied area per molecule as well as intermolecular interactions between the azobenzene aromatic rings (formation of H-aggregates). Theoretical predictions of the geometrical structures were performed to clarify the correlation between the molecular structure and photoisomerization characteristics in monolayer systems. The striking difference in absorption spectra of a trans-EtSH SAM and a cis-EtSH SAM by alternating UV and visible light irradiation was in good agreement with that in their contact angles for water, strongly indicating that the structural changes were controlled by light wavelength. By contrast, despite there being sufficient free space for each MeSH molecule, the strong tendency of the planar azobenzene units to generate H-aggregates even during cis-MeSH SAM formation lessened the trans-to-cisphotoisomerization yield in a monolayer.

Light-driven modulation of fluorescence color from azobenzene derivatives containing electron-donating and electron-withdrawing groups

We report a simple preparation and color-tunable fluorescence of a series of azobenzene derivatives. The introduction of an electron-withdrawing or electron-donating group at the X position of azobenzenes (1–8) containing a biphenyl unit makes it possible to modulate the fluorescence color of the UV-exposed azobenzene solutions from blue to yellow, which correlates with the electron-donating abilities of the respective substituents. Theoretical calculations suggest that changes in both the dihedral angles between two phenyl rings of the biphenyl unit and the dipole moments between the trans and cis forms depending on the substituents seem to be important factors in determining the photochemical properties of chromophores.

Photochemical Liquid–Solid Transitions in Multi-dye Compounds

We report the synthesis of liquid crystalline materials, which repeatedly liquefied and solidified when irradiated with ultraviolet (UV) and visible light at room temperature. The materials are the derivatives of the sugar alcohols, D-mannitol, D-sorbitol, and allitol, in which all hydroxyl groups were substituted with multi-mesogenic azobenzenes. The thermal phase transition behavior and stability of the liquid crystal phase were changed according to the sugar alcohols, but photochemical phase transition behaviors were not different. In addition, when applied as an adhesive layer to glass substrates, the materials exhibited changes in the adhesion force when irradiated with light.

Photochromism of a spiroperimidine compound in polymer matrices

The photochromic behavior of a spiroperimidine compound, 2,3-dihydro-2-spiro-7′-(8′-imino-7′,8′-dihydronaphthalen-1′-amine)perimidine (PNI), doped in various polymers has been studied. Upon irradiation with 405 nm light, PNI exhibits photochromism, and a color change (light yellow to brown) is observed. The initial (closed) and colored (open) forms exhibit absorption maxima (λmax) around 400 and 500 nm, respectively. The open form possesses a wide absorption band, extending to ∼750 nm. Little effect of the polymer matrix on the λmax of either the closed or open forms was observed, and these values were also similar to those in solution. The lifetime of the open form depends mainly on the glass transition temperature (Tg) of the polymer, and ranges from, for example, 11 min to 5.4 d. This indicates that the lifetime of the open form can be readily tuned by simply choosing a suitable polymer matrix. Photochromism in different polymer substrates are demonstrated herein.

Azobenzene crystals swim on water surface triggered by light

Crystals of 4-methoxyazobenzene move on water surface and the motion is triggered by irradiation with ultraviolet light. The propulsion is produced by dissolution of the photo-generated cis isomer, which is produced by the photoinduced crystal–liquid phase transition. A photoresponsive boat was also prepared by a filter paper adsorbed with azobenzene.