Daichi Kitagawa

Morphology, wettability and photomicropatterning of superhydrophobic surface with high adhesive force by crystal growth of a photochromic diarylethene

A photochromic diarylethene, 1,2-bis(2-methyl-6-nitro-1-benzothiophen-3-yl)perfluorocyclopentene (1a), can undergo crystallization from the amorphous film in poly(methyl methacrylate) (PMMA) at 130 °C. The contact angle with water on the film changed from 82 to 132° upon crystallization. Moreover, by heating at 180 °C, the contact angle increased to 154°, showing a superhydrophobic surface (water contact angle >150°). The change in the contact angle can be ascribed to the surface roughness change by the crystal growth. The superhydrophobic surface also showed high-water adhesive properties. Patterning of the superhydrophobic and hydrophobic surfaces was performed using a photomask by photoirradiation leading to a resolution of about 10 μm. The contact angle on the photoirradiated area was reduced to 138° along with lowering of the surface roughness because of poorer crystal growth of the photoirradiated crystals. Such a superhydrophobic surface with high adhesive force is useful for water capturing devices.

Alkyl substituent effects in photochemical and thermal reactions of photochromic thiophene-S,S-dioxidized diarylethenes

Thiophene-S,S-dioxidized diarylethenes introducing various alkyl groups at the reactive positions were newly synthesized. The diarylethenes showed reversible photochromism, whereas the photocycloreversion reaction was suppressed by thiophene-oxidation. The diarylethene closed-ring isomers with secondary alkyl groups at the reactive positions were found to undergo thermal bleaching reactions to produce byproducts. The thermal bleaching reactions were accelerated by introducing a more bulky substituent at the reactive positions. The relationship between the rate constant of the thermal bleaching reactions and the bulkiness of the substituent at the reactive positions can be correlated using a steric substituent constant. Moreover, it was noted that introduction of a methyl group at the 4-position of the thiophene in the oxidized diarylethenes accelerates the thermal byproduct formation. The rate of the thermal bleaching reactions was found to depend on the difference in the ground state energy between the closed-ring isomer and one of the byproducts, as can be estimated by theoretical calculation of the energy level. Such materials can be used in application such as light-starting irreversible thermosensors.

Control of Surface Wettability and Photomicropatterning with a Polymorphic Diarylethene Crystal upon Photoirradiation

A photochromic diarylethene crystal of 1,2-bis(2-methyl-6-nitro-1-benzothiophen-3-yl)perfluorocyclopentene (1 a) was found to undergo a thermodynamic phase transition at 180 °C to form a needle-like crystal, designated as 1 a-γ. The phase transition involves melting of the initial α-crystal and growth of the γ-crystal. The phase transition temperature decreased with the presence of the closed-ring isomer (1 b) in the crystal because of the decrease in the melting temperature. Upon irradiation with ultraviolet (UV) light, compound 1 a in the α-crystal was converted into 1 b to an extent of 20 %. Consequently, the α-crystal containing 20 % of 1 b underwent the phase transition accompanied by melting of the crystal and growth of the γ-crystal even at 170 °C. Photomicropatterning by the phase transition upon irradiation with UV light using a photomask, followed by heating at 170 °C, was successfully accomplished with a resolution in the microcrystalline pattern of about 20 μm. The contact angle with water on the γ-microcrystalline phase on a glass substrate was larger than that on the α-microcrystalline phase by 20°. This can be ascribed to a difference in the roughness of the surface. Furthermore, the γ-microcrystal was also found to be formed upon heating an amorphous film of 1 a in poly(methyl methacrylate) for 2 min at 130 °C. The crystallized area exhibited a higher water contact angle than the amorphous area. Upon irradiation of the amorphous film with UV light, such crystallization did not take place because of the impurity effect of 1 b in 1 a. Photomicropatterning by the crystallization in the polymer showed a pattern with a higher resolution of about 4 μm, which was much better than that of the neat crystal. This photopatterning process represents a useful tool for controlling the surface wettability in relevant applications.

Control of Photomechanical Crystal Twisting by Illumination Direction

Photomechanical molecular crystals have been investigated as mesoscopic photoactuators. Here, we report how the photomechanical twisting of 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) crystals depends on illumination direction. The ribbon-like crystal of 1a could be successfully prepared by a sublimation method. The ribbon crystal exhibited reversible photomechanical crystal twisting upon alternating irradiation with ultraviolet (UV) and visible light. Moreover, changing the UV illumination direction with respect to the crystal resulted in different twisting modes, ranging from helicoid to cylindrical. Control of photomechanical crystal deformation by illumination direction provides a convenient and useful way to generate a variety of photomechanical motions from a single crystal.

Systematic study on the thermal cycloreversion reactivity of diarylethenes with alkoxy and alkyl groups at the reactive carbons

The relationship between the thermal cycloreversion reactivity of diarylethenes and the bulkiness of the substituents at the reactive carbons was systematically investigated. Two photochromic diarylethenes, 1,2-bis(2-isobutoxy-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-neopentoxy-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were newly synthesized and their optical properties and thermal cycloreversion reactivity were examined, because there is insufficient data for diarylethenes with alkoxy groups at the reactive carbons. The steric substituent constant was employed to correlate the relationship between the thermal cycloreversion reactivity of diarylethenes with alkyl and alkoxy groups at the reactive carbons and the bulkiness of the substituent. A good correlation was obtained for the substituent constant using CH2 instead of oxygen in the alkoxy groups. The results indicate that this is a very useful strategy for the design of novel diarylethenes with desired thermal cycloreversion reactivity.

Thermodynamic Phase Transition Through Crystal-to-Crystal Process of Photochromic 1,2-Bis(5-phenyl-2-propyl-3-thienyl)perfluorocyclopentene

A photochromic diarylethene, 1,2-bis(5-phenyl-2-propyl-3-thienyl)perfluorocyclopentene (1a), was found to have two polymorphic crystal forms, α- and β-crystals. From X-ray crystallographic analysis, the space groups of α- and β-crystals were determined to be P21 /c and C2/c, respectively. The difference between two crystal forms is ascribed to the orientation of two of four molecules in the unit cell. The thermodynamic phase transition from α- to β-forms occurred via a crystal-to-crystal process, as confirmed by differential scanning calorimetry measurements, optical microscopic observations in the reflection mode and under crossed Nicols, and powder X-ray diffraction measurements. The movement of the molecules in the crystal was evaluated by analyzing the change of face indices before and after the phase transition.

Optical properties and solvatofluorochromism of fluorene derivatives bearing S,S-dioxidized thiophene

We synthesized fluorene derivatives having phenylthiophene (FPT) or benzothiophene (FBT), and their S,S-dioxidized compounds (FPTO2, FPTO4, FBTO2 and FBTO4), which are prepared by oxidation of the thiophene rings in FPT and FBT with m-chloroperoxybenzoic acid. FPT and FBT exhibited similar optical properties for absorption maximum wavelength, fluorescence maximum wavelength and fluorescence quantum yield. However, the absorption and fluorescence spectra of FPTO2 were largely shifted toward a longer wavelength in comparison with those of FPT, and their fluorescence quantum yields dramatically decreased with oxidation. In contrast, the absorption and fluorescence spectra and the fluorescence quantum yields of FBTO2 and FBTO4 were similar to those of FBT. Moreover, FPTO2 and FBTO2 showed strong solvatofluorochromism. Such solvent dependent properties are ascribed to the charge transfer character of the molecule.

Photoinduced stepwise bending behavior of photochromic diarylethene crystals

Diarylethene crystals were found to exhibit photoinduced stepwise bending behavior. The bending rate changed significantly during continuous irradiation with ultraviolet (UV) light. This is a new type of photomechanical motion reported previously for various photoresponsive molecular crystals. Furthermore, the delayed bending behavior was observed even after UV light was turned off. The mechanism for the stepwise bending is discussed.

Strategy for Molecular Design of Photochromic Diarylethenes Having Thermal Functionality

Thermal reactivities of photochromic diarylethene closed-ring isomers can be controlled by the introduction of substituents at the reactive positions. Diarylethenes having bulky alkyl groups undergo thermal cycloreversion reactions. When bulky alkoxy groups are introduced, the diarylethenes have both thermal cycloreversion reactivities and low photocycloreversion quantum yields. Such photochromic compounds can be applied to thermally reusable photoresponsive-image recordings. The thermal cycloreversion reactivity of the closed-ring isomers can be evaluated using specific steric substituent constants and be correlated with the parameters. By introduction of trimethylsilyl or methoxymethyl groups at the reactive positions, the diarylethene closed-ring isomers undergo thermal irreversible reactions to produce by-products at high temperatures. These diarylethenes may be useful for secret-image recordings. Furthermore, thiophene-S,S-dioxidized diarylethenes having secondary alkyl groups at the reactive positions undergo thermal by-product formation reactions, in addition to the photostability of the colored closed-ring isomers. Such materials may be used for light-starting thermosensors. The thermal by-product formation reactivity can be evaluated by the specific substituent constants and theoretical calculations of quantum chemistry. These results supply the strategy for the molecular design of the photochromic diarylethenes having thermal functionality.

Photomechanically Induced Magnetic Field Response by Controlling Molecular Orientation in 9‐Methylanthracene Microcrystals

A surfactant-assisted seeded-growth method is used to form single-crystal platelets composed of 9-methylanthracene with two different internal molecular orientations. The more stable form exhibits a photoinduced twisting, as observed previously for 9-methylanthracene microribbons grown by the floating drop method. However, the newly discovered elongated hexagonal platelets undergo a photoinduced rolling-up and unrolling. The ability of the rolled-up cylindrical shape to trap superparamagnetic nanoparticles enables it to be carried along in a magnetic field gradient. The new photoinduced shape change, made possible by a novel surfactant-assisted crystal growth method, opens up the possibility of using light to modulate the crystal translational motion.