Temperature photochromic liquid crystalline

S. Ivanov, I. Yakovlev, S. Kostromin, V Shivaev, L. Lasker, J. Stumpe, and D. Kreysig, Laser-induced birefringence in homeotropic films of photochromic comb-shaped liquid-crystalline copolymers with azobenzene moieties at different temperatures, Makromol. Chem. Rapid... 

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... 

In reality, the liquid crystalline-to-isotropic phase transition temperature Tc) of the copolymer or LCP mixture with the cw-form Tec) is lower than that with the trans-isomer (Tct). The presence of the cw-azobenzene moieties depresses the transition temperature of the host liquid crystalline phase based on the concentration. If the temperature of the mixture T) is set between Tct and Tec and the sample is irradiated to cause trans-to-cis photoisomerization of azobenzene molecules, Tc decreases with an accumulation of the number of cw-isomer. When Tc becomes lower than the irradiation temperature, the liquid crystalline-to-isotropic phase transition is induced. The photoinduced phase transitions are interpreted in terms of a change in the phase transition temperature of PLCP systems on accumulation of one isomer of the photochromic molecule. As expected from Figure 7.5, AT (= Tct - Tec) is one of the most important parameters in the phase transition [1], When the temperature of systems is set below Tec, no phase transition is induced. When the temperature is set close to Tct, the amount of cw-azobenzene needed to lower Tc is relatively small, enabling one to effectively induce phase transition upon UV irradiation [10]. 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

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