Yuji Kawanishi

Nematic Homogeneous Alignment Regulated by the Polarization Photochromism of Surface Azobenzenes

Abstract Alignment modulation of a nematic liquid crystal was performed by a polarization-selective photochromic reaction of surface azobenzenes. The alignment was changed from an initial perpendicular state to a homogeneous planar state by exposure to the linearly polarized UV light. The induced homogeneous axis was perpendicular to the electric polarization of the reaction light. Rotation of the homogeneous axis was demonstrated by changing the light polarization, and the corresponding relaxation time was measured by laser pulse experiments.

Liquid crystalline conjugated oligomers: synthesis and mesomorphic properties of laterally and terminally alkyl-substituted oligo(1,4-phenyleneethynylene)s

A new series of liquid crystalline conjugated oligomers, namely laterally and terminally alkyl-substituted derivatives of oligo(1,4-phenyleneethynylene)s, were synthesized, and the relationships between the molecular structures and the mesomorphic properties were investigated. The mesomorphic properties of the oligomer derivatives were examined by differential scanning calorimetry and polarizing microscopy. In the alkyl-substituted oligomers with varying core lengths, C8H17C6H4CC[C6H2(C2H5)2CC]nC6H4C8H17 where n = 2–5, the trimer (n = 2) exhibited a monotropic nematic phase, and the others showed enantiotropic nematic phases. In the alkyl-substituted tetramers, R1C6H4CC[C6H2(R2)2CC][C6H2(R3)2CC][C6H2(R2)2CC]C6H4R1 where R1, R2, and R3 = alkyl chains of varying lengths and H, most of the tetramer derivatives exhibited nematic phases, whereas the others did not show any mesophases. It was found that the formation of the mesophase and the crystalline–nematic and nematic–isotropic transition temperatures are strongly affected by the structural features of the oligomer molecules: (i) the length of the oligomer core, (ii) the length, number, and position of the lateral chains, and (iii) the length of the terminal chains.

Tuning reverse ring closure in the photochromic and thermochromic transformation of 1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline] analogues by ionic moieties

Abstract The photochromism and thermochromism of eight 1′,3′,3′-trimethyl-6-nitrospiro[2 H -1-benzopyran-2,2′-indoline] analogues bearing cationic, neutral and anionic moieties were examined in acetonitrile at 318 K. The rate constants of forward colouration ( k 1 ) and reverse ring closure ( k −1 ) were evaluated. The transition energy ( E T ) of the merocyanines and k −1 are strongly correlated with the ionic charge of the substituent moiety and with the solvent polarity. A linear correlation ( ln k −1 =αE T +β ) was observed for the series of merocyanines in acetonitrile (similar to that obtained on changing the solvent). The slope α obtained in the present case was more than threefold steeper than that observed for the conventional solvent effect; k 1 was almost constant regardless of the solvent or ionic charge. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the coloured form and the formation energy of the transition state in the reverse reaction were calculated. Both increased in the same order of ionic charge: −1

Reversible alignment change of liquid crystals induced by photochromic molecular films: Properties of azobenzene chromophores covalently attached to silica surfaces

Abstract In connection with the mechanism of reversible alignment change of nematic liquid crystals induced by photoisomerizable azobenzene molecules attached to a quartz surface, a static contact angle measurement was carried out for azo-modified plates before and after UV irradiation. The results were not in line with the so-called Friedel-Creagh-Kmetz rule which claims that the alignment is governed by a mutual relationship between a substrate surface energy and surface tension of a liquid crystal. On the other hand, electronic absorption spectra of the plate demonstrated that the ratio of an absorbance at around 340 nm to that at around 240 nm of the azo-chromophore on the surface is reduced by wetting the surface with a UV-transparent nematic liquid crystal. This reflects that the surface azobenzene units are induced to stand approximately perpendicular to the surface upon contact with the liquid crystal. Further discussion is made concerning the photo-induced homogeneous alignment triggered by a rub...

Reversible Alignment Change of Liquid Crystals Induced by Photochromic Molecular Films

The alignment relaxation of nematic liquid crystals regulated by the photochromic reaction of azobenzene (Az) monolayer on substrates was measured upon excitation with an excimer laser. The time of the relaxation between the homeotropic and the parallel alignments resided in the range of 50 and 300 ms, depending on the cell conditions, which values were identical with the electro-optical decay time reported.

Syntheses and emission properties of novel violet-blue emissive aromatic bis(diazaborole)sElectronic supplementary information (ESI) available: pictures of 1a???1e in DMF using 366 nm as excitation wavelength. See http://www.rsc.org/suppdata/jm/b2/b203485a/

Violet-blue emission from novel aromatic bis(diazaborole)s, which were synthesized by the reaction of 2,5-bis(hexyloxy)-1,4-phenylenediboronic acid and 1,2-phenylenediamine derivatives or diaminonaphthalene derivatives, is described. White-blue emission from a 4-methoxy-1,2-phenylenediamine-derived molecule was observed in DMF using 340 nm as the excitation wavelength.

Modulated photoregulation of liquid crystal alignment by azobenzene Langmuir-Blodgett layers: reversible alignment changes of liquid crystals induced by photochromic molecular films, Part 11

Abstract In-plane homogeneous alignment controls of a liquid crystal on photochromic azobenzene (Az) Langmuir-Blodgett layers are investigated. A nematic liquid crystal takes the parallel (planar) alignment with the cis-Az surface. In this mode LC molecules are oriented parallel to the dipping direction when the deposition number is more than 3. Under particular conditions a linearly polarized UV light induces subsequent in-plane reorientation of the LC molecules, the reorientation direction being perpendicular to the polarization plane.

Kinetic and Mechanistic Studies of Carbon-to-Metal Hydrogen Atom Transfer Involving Os-Centered Radicals: Evidence for Tunneling

We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C-H bond is accomplished by a single metal-centered radical. Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)2Os(•) and (η(5)-(i)Pr4C5H)(CO)2Os(•) radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons are in the range 1.5 × 10(5) M(-1) s(-1) to 1.7 × 10(7) M(-1) s(-1) at 25 °C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary deuterium kinetic isotope effects of 13.4 ± 1.0 and 16.8 ± 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)2OsH and (η(5)-(i)Pr4C5H)(CO)2OsH, respectively, at 25 °C. Temperature-dependent measurements of the kinetic isotope effects over a 60 °C temperature range were carried out to obtain the difference in activation energies (E(D) - E(H)) and the pre-exponential factor ratio (A(H)/A(D)). For hydrogen atom transfer from xanthene to (η(5)-(i)Pr4C5H)(CO)2Os(•), the (E(D) - E(H)) = 3.3 ± 0.2 kcal mol(-1) and A(H)/A(D) = 0.06 ± 0.02 values suggest a quantum mechanical tunneling mechanism.

Laser-induced orientational change of nematic liquid crystalline molecules mediated by photochromic reactions of surface azobenzenes

Abstract The alignment of nematic liquid crystalline (LC) molecules with respect to the LC cell surface was regulated between the homeotropic (H) state and the parallel (P) state by the photochromic trans—cis isomerization of surface-attached azobenzene (Az) moieties. By bringing the angular-selective photoreaction into the system, the orientational order in the P state was significantly improved to attain a universal orientational axis over the entire irradiated area, which was rotatable and erasable photochemically. Laser pulse experiments revealed that the allignment relaxation time τ was reduced from 1000 to 30 ms by increasing the pulse intensity from 10 to 20 mJ cm−2. Strong coupling between the LC and Az molecules was indicated.

Liquid crystalline alkyl-substituted oligo(p-phenyleneethynylene)s: synthesis and structure–property relationships

A new series of liquid crystalline oligo(p-phenyleneethynylene)s, namely laterally and terminally alkyl-substituted derivatives of phenyl-terminated phenyleneethynylene hexamer (H(C6H4CC)6C6H5), were synthesised, and the relationships between molecular structures and mesomorphic properties were investigated. The mesomorphic properties of the hexamer derivatives were examined by differential scanning calorimetry and polarising microscopy. In the alkyl-substituted hexamer derivatives, R1C6H4CC[C6H2(R2)2CC][C6H2(R3)2CC][C6H2(R4)2CC][C6H2(R3)2CC][C6H2(R2)2CC]C6H4R1, where R1, R2, R3 and R4 = alkyl chains of various lengths or H, most of the hexamer derivatives exhibited nematic phases, whereas the others did not show any mesophases. It was found that the formation of the mesophase and the crystalline–nematic and nematic–isotropic transition temperatures were strongly affected by the length, number and position of the lateral chains, and by the length of the terminal chains.