Liquid crystals photochromic reactions

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

Y. Kawanishi, T. Tamaki, T. Seki, M. Sakuragi, and K. Ichimura, Alignment of nematic liquid crystals controlled by the photochromic reaction of aggregated surface azobenzenes, J. Phot-polym. Sci. Technol. 4, 271-278 (1991). 

The photochromic reaction in liquid ctystals (LCs) is one of the effective stimuli inducing modificaticm of the LC ( tical prqralies. For example, photochromic reactions in cholestoic liquid crystals (CLCs) can induce dianges in the cholesteric iridescent reflections (reflected colors) due to the helical molecular alignment.(i, 2) Since the molecular aligrunoit of the CLC... 

The incorporated SP exhibited photochromism in both of the immobilized bilayer complexes with montmorillonite and PSS. Kinetic measurements of the thermal isomerization (decoloration) were carried out for the annealed film. The decoloration reaction rate is dependent on the mobility of the surroundings and, in polymer matrices, is influenced by the glass transition. It was found that the reaction rates abruptly increased near the gel-to-liquid-crystal phase-transition temperature (54°C) of the immobilized bilayer due to increased matrix mobility in this system. The film prepared with montmorillonite gives more homogeneous reaction environments for the chromophore than those with the linear polymer (PSS). This leads to drastic changes in the reaction rate at the crystal-to-liquid-crystal phase transition of the bilayer, showing the effect of the phase transition of immobilized bilayers to be more pronounced than that of the glass transition of amorphous polymer matrices. 

---