Masako Sakuragi

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.

REINVESTIGATION OF SYNTHETIC METHODS FOR ZINC MESO-TETRAPHENYLTETRABENZOPORPHYRIN

Abstract The reaction of 3-benzylidenephthalimidine (3-BPI) with zinc acetate for the preparation of the meso- tetraphenyltetrabenzoporphyrin zinc complex (ZnP4TBP) is reinvestigated. From the structural analyses of the reaction product with ultraviolet-visible absorption spectra, field desorption mass spectra (FDMS) and 1H NMR spectra, it is shown that this reaction procedure gives a complicated mixture of ZnTBP derivatives with meso-phenyl substituent(s) the number of which is from one to four. The product obtained by the reaction of isoindole with benzaldehyde in the presence of zinc acetate is also discussed.

Laser photolysis studies of nitrospiropyrans intramolecularly linked with a triplet quenching or sensitizing side group

Abstract Nanosecond laser photolysis studies have been done for the family of nitrospiropyrans which have a triplet quenching or sensitizing side groups. The transient absorption spectra for all compounds examined are identical, indicating that the triplet state makes a minor contribution to the generation of photomerocyanine.

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...

Formation of tetrabenzoporphine skeleton by the reactions of phthalimide with zinc carbonates

Abstract The high temperature reactions of phthalimide potassium salt (PIK) with various zinc carbonates were carried out. The reaction of PIK with zinc acetate yielded tetrabenzoporphinato zinc (ZnTBP). Tetratert-butyltetrabenzoporphinato zinc (ZntBu 4 TBP), whose four tert-butyl substituents were positioned on the TBP periphery, was also prepared in a similar way using the 4-tert-butylphthalimide potassium salt (4-tBuPIK). These results were explained on the basis of the function of zinc acetate as the meso -methine carbon source of the TBP structure. The use of zinc phenylacetate instead of zinc acetate in the reaction of PIK led to the production of meso -phenyl substituted ZnTBPs, although they were different in the number of meso -phenyl substituent(s). The attempt to introduce alkyl residues at the meso -position(s) by the reaction of PIK in the presence of zinc alkylacetate having a long alkyl chain was not successful, except for the substitution of the methyl group using zinc propionate.

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.

The photochromism of nitrospiropyran included in γ-cyclodextrin

SummaryThe inclusion compounds of 1′, 3′, 3′-trimethyl-6-nitrospiro-[2H, 1-benzo-pyran-2, 2′-indoline] (NSP) with γ-cyclodextrin (γ-CD), 2,3,6,0-permethylated γ-CD (γ-MCD), and naphthyl modified γ-CD (γ-NCD) showed normal photochromism in the solid state with high light sensitivity comparable with that of NSP dispersed in polymer matrices. This is in sharp contrast with NSP in the crystalline state, which is much less photosensitive. These results suggest that the host cavity offers a sufficient free volume for phototransformation of the guest molecule. The colored form of the inclusion compounds were found to be more stable both chemically upon prolonged UV irradiation and thermally in the dark as compared with that in a PMMA film.

Singlet oxygen generation by tetrabenzoporphyrins as photosensitizer

Abstract The efficiency of the photosensitized generation of singlet oxygen ( 1 O 2 ) with meso -phenyl substituted tetrabenzoporphyrin metallo complexes as sensitizer was determined from the limiting quantum yield of the (+)-limonene photooxygenation and compared with that of hematoporphyrin (HP), which has been known to serve as an effective sensitizer, using a HeNe laser light (633 mm). Among the diamagnetic (Mg, Zn, and Cd) complexes treated here, the Zn complexes were most efficient for the (+)-limonene photooxygenation with the limiting quantum yields of Φ(−A)=0.4. This result was comparable to that of HP. The direct observation of the intensity of 1 O 2 emission by the laser flash photolysis technique supported these results.

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.