Mixtures of cholesteric liquid crystals doped with high clearing temperature azobenzene nematic liquid crystals are shown to possess large, fast, and reversible dynamic photosensitive features. Selective wavelength shifts approaching 400 nm are reported, and depending on the host cholesteric liquid crystal, both red‐shifted and blue‐shifted wavelength changes can be induced. The photoinduced states of these material systems are shown to be stable for long periods of time upon removal of the radiation source, completely reversible, and dynamically fast. These phototunable features are demonstrated using both continuous wave (CW) and nanosecond laser beams. The latter is used to change the selective reflection wavelength from blue to green with a single nanosecond pulse and the ability to write information into these films using these processes are demonstrated.
This study compares optical switching capabilities of liquid crystal (LC) materials based on different classes of azobenzene dyes. LCs based on molecules containing benzene rings with nearly symmetrical pi-pi conjugation respond more efficiently to a cw beam than to a nanosecond laser pulse and maintain the changes induced by the beam for tens of hours. Using azo dye molecules containing two benzene rings with push-pull pi-pi conjugation we demonstrate high photosensitivity to both a cw beam as well as nanosecond laser pulse with only 1 s relaxation of light-induced changes in material properties. Even faster, 1 ms restoration time is obtained for azo dye molecules containing hetaryl (benzothiazole) ring with enhanced push-pull pi-pi conjugation. These materials respond most efficiently to pulsed excitation while discriminating cw radiation.
Photosensitive chiral liquid crystals (CLCs) are an important class of optical materials due to the opportunities of controlling their Bragg reflection properties with the aid of light beams. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] We recently enhanced the capabilities of suchCLCs by using room temperature azobenzene nematic LCs (azo NLCs) [17,18] as photosensitizing dopants. The resultant materials revealed extraordinarily large photoinduced shifts of the Bragg reflection band in the visible-near IR range of wavelengths. [19] This behavior was enabled by the high clearing temperature of the novel azo-liquid crystal compounds and the high dopant concentrations achievable due to their liquid crystallinity. These compounds, derivatives of 4-n-alkyl-4′-n-alkoxyazobenzenes, [18] possess different effective twisting powers in their cis-and trans-isomer configurations. In the present paper we study optically induced processes originating from the photoinduced isotropic (PHI) state of these azo NLC doped CLCs. This PHI state, a cis-isomer rich, isotropic phase induced by UV irradiation of the CLC phase containing a large concentration of azo-LC dopant, is stable for tens of hours due to the long lifetime of the cis isomers.[20] UV illumination well beyond the time required for transformation of a mesophase into an isotropic phase creates this metastable state which we show here has interesting thermodynamic and optical properties. We demonstrate the feasibility of a photoinduced phase transformation between this metastable isotropic phase and the reflective cholesteric phase when interrogated with low power visible laser beams. Complex two dimensional patterns of reflective and isotropic regions can easily be written and erased in thin films of this material by varying the wavelength of radiation. These reconfigurable images are stable over long periods of time. Restoration of the CLC reflection from the PHI state over a wide spectral band covering the visible spectrum is shown utilizing a pitch gradient across the area of a cell. One may envision 'patterned' latent colorimetric information that can be induced by spatially rastering laser irradiation. In addition, we demonstrate autonomous nonlinear feedback of an optical signal using this photo-induced bandgap formation in these unique materials. This process of bandgap restoration (isotropic to reflective) is accompanied by nonlinear transmission properties for the bandgap-sensitive polarization component of radiation.A series of uniformly aligned cholesteric liquid crystal cells exhibiting selective reflection across the NIR-visible spectra were fabricated. Upon irradiation with UV light, the peak Bragg reflection wavelength is blue shifted as shown in Figure 1a (details in ref. [19]). The change in the Bragg wavelength for CLC54 and CLC57 is as large as 250 nm before they are transformed into the isotropic phase. Upon extended irradiation, the films undergo a classic isothermal phase transition to a photoinduced isotropic (PHI) state wherein the film...
We discuss materials that reveal fundamental intercoupling of light and chirality in creation of complex structures. These materials are based on cholesteric liquid crystals (CLCs) photosensitized by azobenzene nematics. Transformation of the one-dimensional periodic structure of such CLCs into complex spatial patterns takes place on macroscopic scales, over the whole area of the CLC layer, under the influence of low power radiation including LED, ambient illumination, and sunlight. The obtained structures, with their origin in the strain of the CLC layers caused by trans-cis photoisomerization precede a shift in the bandgap position of the CLCs. The effect is observed both in red-shifting as well as blue-shifting CLCs.
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