Photochromic liquid crystalline compounds

The relation between the type of photochromic compound and its effectiveness to induce a phase transition is a point of interest. When unsubstituted azobenzene is added to 5CB, the phase transition is not induced even after prolonged irradiation. BMAB is by itself liquid crystalline whereas azobenzene is not. It seems to be essential for a triggering photochromic compound to have effective interactions with the host liquid crystal. 

The properties of photochromic molecules such as DHI 7 and related compounds are dependent not only on structure but to a large extent also on their environment. Clear differences are to be expected if solution or solid phases of photochromic molecules are compared, i.e. (1) containing supramolecular anchor groups, (2) in liquid crystalline phases, or (3) polymer-containing photochromic systems. It is necessary to use the appropriate spectral methods for the detection of different properties. In addition to normal methods, photoacoustic spectroscopy (PAS), polarized light, and others may be applied. 

Studies on photochromic compounds in connection with chirality have been continuously done from the several viewpoints such as photochemical control of chirality-related liquid crystalline (LC) properties, photochemical control of biological activities by attaching a photochromic compound to biological macromolecules, and use for the optical memory media, as well as the purely scientific interests. 

Liquid-crystalline phases with two-dimensional order (Table 1) influence photochromic processes because their molecular orientation is reversibly altered by photochromic reactions owing to the marked structural changes of the photoactive molecules.3 Intensive interest has been focused on the photoinduced alteration of mesophases admixed with photochromic compounds, since photochromism can be transformed to the reversible modification of the optical anisotropy of liquid-crystalline states. This phenomenon is applicable to optical storage of information as nondestructive readout can be performed with light of wavelengths far from those causing the photochromic reactions. 

It is not necessary to carry out synthesis, if the triggering photochromic compound has good affinity to the polymer matrix. A mixture of the polyacrylate with BMAB which exhibits an excellent function as trigger is equally photoresponsive. While the monomer model compound (i.e. the acrylate before polymerization) does not provide a liquid crystalline phase, the polymer shows a clear nematic - isotropic transition at ca. 61 °C and the glass transition temperature at 24 °C as shown in Fig. 4. Tj j depends very much on the length of the alkyl spacer. In comparison with the... 

H. Hattori, T. Uryu, Photochromic chiral liquid crystalline systems containing spiro-oxazine with a chiral substituent I. Synthesis and characterization of compounds. Liq. Cryst. 28, 25-34 (2001)... 

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