Oriented liquid-crystalline network films with various cross-linking densities were prepared
by polymerization of mixtures of mono- and diacrylates, both of which contain azobenzene
chromophores. The optical anisotropy in the films was evaluated by polarizing optical
microscopy and polarized IR spectroscopy. The dichroic ratios and the order parameters of
the films were estimated from polarized UV absorbance. The free-standing films were found
to show photoinduced bending and unbending behavior upon alternate exposure to
unpolarized UV light at 366 nm and unpolarized visible light at >540 nm. It was observed
that the films with different cross-linking densities exhibited different bending extents and
speeds.
All-optical switching of selectively reflected colors based on isomerization of an azobenzene (Azo) and subsequent change in transmittance was investigated in cholesteric liquid crystals (LCs) which reflect light in the visible region. Photoirradiation at 366 nm, which causes an efficient trans-cis isomerization of Azo, led to change in color of cholesteric LCs toward shorter wavelengths with a concomitant lowering of phase transition temperature from a cholesteric to an isotropic phase (T Ch-I ). Reversible change in color was induced all-optically by irradiating alternatively at effective wavelengths for reversible isomerization of Azo. A remarkable change in transmittance was also observed when the photoinduced change in colors was measured by a probe light with the same handedness as the helical sense of the cholesteric LCs. In the wavelength regions of reflected colors before photoirradiation, the spectral position played an important role in producing a normal-mode and a reverse-mode switching in photoinduced modulation of transmittance of the cholesteric samples with respect to the probe light.
Polymer/liquid crystal (LC) composite films possessing an azobenzene LC as a photoresponsive molecule (AzoCFs) were prepared by the thermal polymerization-induced phase separation method. The composite films (AzoCFs) showed a strong light-scattering state after polymerization, and their optical properties were strongly affected by the compositional ratio of the liquid crystals in the composite film. Change in the transmittance between a light-scattering and a transparent state could be induced isothermally by photoirradiation. Upon irradiation at 366 nm, AzoCFs changed from the light-scattering state to the transparent state. This is ascribed to nematic (N)-isotropic (I) phase transition due to transcis isomerization of the azobenzene molecules in the LC domains within the polymer matrix. Furthermore, restoration of the initial state could be achieved by visible-light irradiation (>420 nm), resulting from the I-N phase transition induced by cis-trans back-isomerization of the azobenzene guest molecules.
Dynamic holographic gratings were investigated with thin films of polymer azobenzene liquid crystals by periodic induction of photochemical nematic-to-isotropic phase transition. On irradiation of writing beams (488 nm), multiple diffraction beams of a reading beam (633 nm) were immediately observed. The firstorder diffraction efficiency reached to a maximum value within several tens of milliseconds: the rise time was approximately 50 ms for PM6AB2 films and 30 ms for PA6ABCN films. Quite large modulation in refractive index (∼0.08) was obtained in both polymer films. Studies on effects of temperature and light intensity on the grating formation suggested that photochemical reactions of azobenzene moieties followed by photochemical phase transition in bright fringes of the interference pattern would be responsible for the formation of holographic gratings. By turning on and off the writing beams, the diffraction beams could be switched dynamically without significant fatigue.
To characterize active defect sites in the near-surface and bulk phase of MgO in Li-doped MgO, structural analyses were carried out by means of XRD, XPS, Mg K-edge XANES, and SEM techniques. For Li-MgO calcined at 873 K, Li doping at a low content (2.5 wt % as Li) brings about the formation of defect species only in the near-surface, which is due to the localization of doped Li ions in the surface. In this case, the catalytic species containing a Li + -Ocenter exists in the surface region. At higher contents of more than 7.5 wt %, Li ions penetrate the MgO bulk, and its crystallinity decreases. The defect species possibly exist in the bulk phase. After oxidative coupling of methane reaction, the defect species are formed in the near-surface over MgO and Li-MgO. However, the C 2 selectivity is much higher on Li-MgO than on MgO. It is concluded that the defect species containing Li + -Oin both surface and bulk can act as the active species for producing C 2 compounds with high selectivity.
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