Light-induced orientation of dyes in azobenzene containing LC polymers

Abstract

Liquid crystalline polymethacrylates with benzanilide and photochromic azobenzene side groups and a related terpolymer containing an additional side group with a phenylethynyl substituted anthracene chromophore were oriented by the irradiation with linearly polarized light. The orientation of both polymers were compared irradiating with polarized visible or, alternatively, UV light. Caused by the co-operativity of the photoorientation process, the light-induced orientation of the azobenzene groups is connected to the alignment of the non-photochromic side groups below the glass transition temperature. The light-induced order generated in the glassy state was significantly amplified by the subsequent annealing of the irradiated films at temperatures in the mesophases. Factors of amplification of about 30 were found in the case of both polymers. The photo-induction process and its amplification by thermotropic self-organization were investigated in dependence on the polym er composition, the irradiation dose and the wavelength of the incident light with respect to the absorption of the dye and its limited photo-stability. The required dose or the irradiation time, respectively, were significantly reduced by the optimization of the light-induced and thermal processing. In this way, dichroic films of co- and terpolymers were created. However, the green fluorescence of the anthracene chromophore is effectively quenched by the azobenzene side group within the film

Photo-induced anisotropic films based on liquid crystalline copolymers containing stilbene units

“…Secondly, a small dichroism D of about 0.03e0.09 for different irradiation conditions was induced in the films of Am/Stl and CNT/Stl (Table 3). These values are significantly small compared to polymers with azobenzene moieties, but they correspond to values that can be induced in cinnamate-containing polymers [25]. These small photo-induced anisotropies at room temperature were amplified by annealing above T g , in particular at 93 C, in the mesophase range of the polymers.…”

“…In the presence of oxygen, the dihydrophenanthrene is irreversibly converted to phenanthrene. However, for donoreacceptor disubstituted stilbenes, this last photoreaction is less efficient compared to the non-substituted or the donoredonor disubstituted derivatives [24,25]. With respect to the required free volume, the E/Z photoisomerisation of stilbenes is more restricted in polymeric films as that of azobenzene derivatives; moreover, the Z isomer of stilbenes is more stable as that of azobenzenes excluding the thermal back reaction to the E isomer [26,27].…”

Photoinduced Optical Anisotropy in New Polymers containing Azobenzene Side Groups

“…Rotating the polarizer, the spectra were measured at 5 3 each to find the maximum and minimum of the orientational distribution. The dichroism was calculated using equation (1). D 8 1abs max 4 abs min 231abs max 7 abs min 2…”

“…Functional polymers containing photochromic groups have generated much interest in the field of optical layer technologies. The use of linearly polarized light offers a new approach to the induction of anisotropic properties in polymer films [1][2][3][4]. In azobenzenecontaining polymers a photoorientation process can be induced by angular-dependent excitation, which causes a number of E/Z photoisomerization cycles and subsequently a reorientation of azobenzene units [5][6][7].…”

Light-Induced Properties in Polyimides Containing Side Azobenzene Groups

“…The use of linearly polarized light offers a new approach to induce anisotropic properties in polymer films [1,2]. In azobenzene-containing polymers the conventional photoorientation process occurs via angular-dependent excitation, a number of E/Z photoisomerization cycles and rotational diffusion within the steady state of the photo-reaction [3,4].…”