Photocurable Urushiol Analogues Bearing Methacryloxy-Containing Side chains.

Photocurable urushiol analogues were synthesized using eugenol (an ingredient of clove oil) as the starting material. Photo-induced radical polymerization with 2,2-dimethoxy-2-phenylacetophenone as a photo-initiator took place in the film prepared from the urushiol analogue-bearing methacryloxy groups at the ends of their side chains. Successful polymerization was confirmed by infrared spectroscopy measurements of the film before and after photo-irradiation. Strain-induced elastic buckling instability for mechanical measurement tests revealed that the Young's moduli of the photo-irradiated samples were 4-5 times higher than the films without photo-irradiation. This was attributed to the formation of a highly cross-linked structure through polymerization of the methacrylic moieties and oxidative polymerization of the catechol moieties. Photo-induced surface texturing was also performed for the films prepared on a substrate using a photomask. Negative-tone patterns were successfully obtained after development by soaking in cyclohexanone over several minutes. The preparation of such patterned surfaces was of particular relevance as the obtained surface can serve as a scaffold for cell adhesion, protein immobilization, and the immobilization of other chemicals with spatial disposition.

Light-Induced Reworkable Adhesives Based on ABA-type Triblock Copolymers with Azopolymer Termini.

Photocurable adhesives based on polymers and resins are an integral part of different production processes because of their fast curing and local area bonding ability. Recently, dismantlable adhesives have attracted a lot of attention for recycling adherends or replacement of adhesion defects. However, adhesives that allow repeatable bonding and debonding solely by light irradiation, i.e., without heat activation, are lacking. Here, ABA-type triblock copolymers consisting of poly(meth)acrylates bearing an azobenzene moiety (A block) and 2-ethylhexyl (B block) side chains were synthesized and utilized as photocurable adhesives. In contrast to the azo homopolymers, the block copolymer structure and incorporation of the soft middle block actualized a low concentration of the azobenzene moiety and consequently, higher flexibility of the resultant copolymers. This enabled film formation of the azobenzene-based adhesives and light-induced bonding for the first time. On the basis of the photoisomerization of the azobenzene moiety, changes in their viscoelastic property, i.e., softening and hardening, were induced by UV irradiation at 365 nm (50-100 mW cm-2) and green light irradiation at 520 nm (40 mW cm-2), respectively. In fact, two glass substrates were bonded with the self-standing polymer film, which was sequentially softened and hardened upon UV and green light irradiations. They exhibited shear strengths of 1.5-2.0 MPa, and UV irradiation lowered the adhesion strength to 0.5-0.1 MPa. Interestingly, the repeatable bonding and debonding abilities of the polymers were accomplished without loss of the adhesion strength.

Unusual photoresponses in the upper critical solution temperature of polymer solutions mediated by changes in intermolecular interactions in an azo-doped liquid crystalline solvent.

Photoinduced changes in the upper critical solution temperature (UCST) were investigated for polymer solutions in an azobenzene-doped liquid crystal solvent. The UCST of poly(methyl methacrylate) (PMMA) and polystyrene (PS) solutions dropped upon irradiation with UV light, which induces trans-cis photoisomerization of the doped azo dye. In the case of PMMA solutions, the photoinduced drop in UCST was significantly larger than that expected from previous studies using azo-based polymers and common solvents. Moreover, the UCST of PS solutions also decreased under photoirradiation, in a direction opposite to that expected from the contribution of polarity. X-ray diffraction data of the solvent suggest that the decreased intermolecular interaction in the solvent (i.e. larger distance between the solvent molecules) is responsible for the photoresponsive behavior of the UCST. The proposed mechanism is consistent with the Flory-Huggins theory. Using such photoresponses in the UCST, the isothermal transition between 2-phase and 1-phase states by photoirradiation was demonstrated.

Biobased Coatings Based on Eugenol Derivatives.

We have fabricated plant-based coating materials using urushiol analogues that were synthesized via a simple three-step route from eugenol (4-allyl-2-methoxyphenol), an allyl-substituted guaiacol. To mimic the chemical structure of urushiol, the allyl chain of eugenol was substituted with alkyl thiols by a thiol-ene reaction. The guaiacol backbone was modified to a catechol backbone through a silylation/desilylation reaction. Uniform thin films were obtained on various substrates by spin-coating a solution of the urushiol analogues and iron(II) acetate. The physical and chemical properties of these films were comparable to those of urushiol thin films, and the adhesion, mechanical strength, and antioxidant properties were superior. The hydrophobicity and Young's modulus of the film increased with the increase in the alkyl chain length. Because various functional units can be introduced to the catechol backbone, our method could be used to fabricate environmentally sustainable, multifunctional, high-performance coatings from eugenol.

Static Electricity-Responsive Supramolecular Assembly.

Stimuli-responsive materials can convert between molecular scale and macroscopic scale phenomena. Two macroscopic static electricity-responsive phenomena based on nanoscale supramolecular assemblies of a zinc porphyrin derivative are presented. One example involves the movement of supramolecular assemblies in response to static electricity. The assembly of a pyridine (Py) complex of the above-mentioned derivative in cyclohexane is drawn to a positively charged material, whereas the assembly of a 3,5-dimethylpyridine complex is drawn to a negatively charged material. The second phenomenon involves the movement of a non-polar solvent in response to static electrical stimulation. A cyclohexane solution containing a small quantity of the Py-complexed assembly exhibited a strong movement response towards negatively charged materials. Based on spectroscopic measurements and electron microscope observations, it was revealed that the assembled formation generates the observed response to static electricity.

Formation of Highly Pure and Patterned Carbon Nanotube Films on a Variety of Substrates by a Wet Process Based on Light-Induced Dispersibility Switching.

A simple fabrication method for patterned carbon nanotube (CNT) films is presented, using the concept of light-induced dispersibility switching with a photoresponsive dispersant. A comparison with other dispersants highlights the important role played by an azobenzene-derived cationic molecule as a photoisomerizable dispersant in the successful manufacture of patterned CNT films. Upon UV irradiation for a short time (∼0.5 min), a dispersion composed of CNTs and photoresponsive dispersant exhibited a dispersibility change due to the photoisomerization of the photoresponsive dispersant, and then the dispersant detached-CNT deposited onto the substrate. Our method enables patterned CNT films to be obtained directly from CNT dispersions onto various substrates such as glass, polyethylene terephthalate, and silicone rubber, expanding the possible applications of CNT films. Furthermore, the process minimizes the amount of the residual dispersant in the fabricated CNT film, reducing the amount of impurities, and improving the quality of the patterned CNT film.

Three Gel States of Colloidal Composites Consisting of Polymer-Brush-Afforded Silica Particles and a Nematic Liquid Crystal with Distinct Viscoelastic and Optical Properties.

Colloidal composites consisting of polymer-brush-afforded silica particles (P-SiPs) and a nematic liquid crystal (LC) exhibited three gel states with distinct viscoelastic and/or optical properties depending on temperature: (1) opaque hard gel, (2) translucent hard gel, and (3) translucent soft gel. We demonstrated that the transitions of the optical property and the hardness of the gels were due to the phase transition of the LC matrix and the glass transition of the grafted polymers of P-SiPs, respectively. We then revealed that the gelation (the formation of the translucent soft gel) was caused by the phase separation of P-SiPs and LC matrix in an isotropic phase based on spinodal decomposition. In addition, the particle concentration and molecular weight of the grafted polymer of P-SiPs were observed to significantly affect the elastic moduli and thermal stability of the composite gels. By the addition of an azobenzene derivative into an LC matrix, we achieved photochemical switching of the transparency of the composites based on the photoinduced phase transition of LCs, while keeping self-supporting ability of the composite gel.

Photopatterned Single-Walled Carbon Nanotube Films Utilizing the Adsorption/Desorption Processes of Photofunctional Dispersants.

We describe the application of photodetachable and recyclable dispersants for single-walled carbon nanotubes (SWNTs) in the fabrication of photopatterned SWNT thin films. Because adsorption and desorption of the dispersants on the SWNT surfaces affect not only their dispersibility in water but also their solubility, SWNT photopatterns were obtained on glass substrates in only three steps, i.e., casting the SWNT/dispersant solution, UV-light exposure of the casted SWNT/dispersant films through a photomask, and subsequent rinsing with neutral water. This patterning procedure is simple and scalable and will enable us to prepare microfabricated SWNT thin films.

Highly Sensitive Formation of Stable Surface Relief Structures in Bisanthracene Films with Spatially Patterned Photopolymerization.

A facile method for the fabrication of a highly sensitive surface relief is demonstrated, which operates on the principle of spatially patterned photopolymerization-induced mass transport in the amorphous films of a series of bisanthracene compounds. The stability of the resultant colorless transparent relief structure is dramatically improved owing to the polymerization of the bisanthracene.

Dual self-healing abilities of composite gels consisting of polymer-brush-afforded particles and an azobenzene-doped liquid crystal.

We prepared the composite gels from polymer-brush-afforded silica particles (P-SiPs) and an azobenzene-doped liquid crystal, and investigated their inner structure, dynamic viscoelastic properties, thermo- and photoresponsive properties, and self-healing behaviors. It was found that the composite gels had a sponge-like inner structure formed with P-SiPs and exhibited good elastic property and shape recoverability. The surface dents made on the composite gel could be repaired spontaneously at room temperature. Moreover, the composite gel exhibited a gel-sol transition induced by the trans-cis photoisomerization of the azo dye, and the transition could be used as a mending mechanism for surface cracks. Consequently, we successfully developed a material exhibiting two types of self-healing abilities simultaneously: (1) spontaneous repair of surface dents by means of the excellent elasticity of the composite gel and (2) light-assisted mending of surface cracks by photoinduced gel-sol transition.

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.

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 bulk-phase change of anthroyl compounds for photopatterning based on photodimerization in the molten state and thermal back reaction.

As new organic materials for rewritable photopatterning, 2-anthroyl and 9-anthroyl ester compounds were synthesized. Their bulk-phase changes (we use "bulk-phase change" as complete phase change in a mass of a material neither in a surface nor in a small quantity in this study) triggered by photodimerization under melting conditions (melt-photodimerization) and subsequent thermal back reactions were investigated. All the anthroyl compounds exhibited melting points lower than ca. 160 °C, and they were nearly quantitatively converted to the corresponding photodimers by UV irradiation at temperatures of ∼5 °C higher than their respective melting points. We found that there were two kinds of bulk-phase change behaviors through the photoreaction. Two of the anthroyl compounds remained isotropic and lost fluidity during the melt-photodimerization. The obtained photodimers exhibited robust solid-state amorphous phases at room temperature. In contrast, the other three anthroyl compounds showed crystallization during the melt-photodimerization. The resulting photodimers changed from isotropic to crystalline phases, even at high temperature. Various experiments revealed that the bulk phase of the photodimers was affected not by the existence of regioisomers but by their fluidity at the photoirradiation temperature. The latter three photodimers retained enough fluidity, reflecting their high molecular mobilities at the photoirradiation temperature at which the isothermal crystallization occurred. The other two products were not able to crystallize due to low fluidity, resulting in amorphous phases. We also found that all the photodimers reverted to the corresponding monomers by thermal back reaction and recovered their initial photochemical and thermal properties. Using these reversible bulk-phase changes of the anthroyl compounds, we successfully demonstrated rewritable photopatterning in not only negative images but also positive ones, based on the optical contrast between the ordered and disordered phases.

Two-dimensional structures of anthracene derivatives: photodimerization and host-guest chemistry.

By using a simple anthracene derivative with four alkoxy tails, a two-dimensional patterned surface was fabricated. The two-dimensional structures were directly visualized by scanning tunneling microscopy (STM) at the solid/liquid interface. The anthracene derivative formed highly ordered structures displaying cavities into which solvent molecules of 1-phenyloctane were coadsorbed. The functionality of the patterned surface was demonstrated by activating host-guest chemistry as the solvent molecules could be replaced by coronene, whose size is almost identical to the cavities formed by the anthracene derivative. Furthermore, [4 + 4] photodimerization of the anthracene derivative was performed at the solid/liquid interface and revealed that the physical height and electron density of the states were changed, resulting in the increase of an apparent height in the STM images. We demonstrate thus that the porous network of the two-dimensional pattern created by the anthracene derivative can be applied for selectively incorporating guest molecules and for photoprocessing.