Photoactive Cholesteric Polymeric Material With Dual Photochromism
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Photoactive Cholesteric Polymeric Material With Dual Photochromism Alexey Bobrovsky, Natalia Boiko and Valery Shibaev Department of Chemistry, Moscow State University, Lenin Hills, 119899 Moscow, Russia ABSTRACT New cholesteric polymeric material containing a liquid crystalline (LC) copolymer with side phenylbenzoate nematogenic and photochromic spiropyran groups and 5 mol % chiral photochromic dopant based on cinnamic acid and isosorbide was prepared. It was shown that UV irradiation and subsequent annealing of planarly-oriented mixture films lead to untwisting of the cholesteric helix and cause an irreversible shift of selective light reflection to the long-wavelength region. This process is explained by the E-Z isomerization of chiral photochromic groups of the dopant relative to a C=C bond accompanied by a decrease in their twisting ability. Upon UV irradiation of films at room temperature, a thermally reversible transformation of spiropyran groups into a merocyanine form takes place, which is followed by the appearance of an intense absorption maximum in the visible spectral region. It was shown, that the use of the material prepared in this work provides an opportunity to record two images at a time: one image is due to a change in the helical pitch and another image (above) is due to the photochromism of spiropyran groups. It is pertinent to note that using the approach developed in this work allows one to widely vary the range of materials exhibiting dual photochromism. It was demonstrated that the mixture under examination shows promise as new photosensitive material for optics, optoelectronics, and data recording. INTRODUCTION The rapid growth of research in the field of information technologies in the last few years creates prerequisites for more extensive search of new materials for data recording, storage, and processing of information. From this point of view, photochromic polymeric materials are of indubitable interest [1–3]. Recently, a new approach to designing cholesteric polymeric materials with a helical pitch controlled under the action of light was suggested [4–10]. This approach is based on the introduction of chiral photochromic groups capable of E-Z photoisomerization into a nematogenic or cholesteric matrix. During this process, the configuration of the above groups changes and this change is accompanied by a decrease in the anisometry and the helical twisting power of these fragments. The vast majority of the polymers synthesized can form a chiral nematic phase and are capable of reversible or irreversible change of the cholesteric helical pitch and the maximum selective light reflection wavelength. Benzylidene-pmenthane-3-one [4–8] and azobenzene [9–10] derivatives were used as the functional chiral photochromic groups. Another approach to the preparation of new materials for data recording and storage involves the synthesis of cholesteric copolymers containing chiral, photochromic and nematogenic moieties based on spiropyran [11, 12] or diarylethylene [13]. In this case, an intense absor
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