ARAMENDIA Pedro Francisco
congresos y reuniones científicas
Photochromism in nematic and in cholesteric liquid crystals
Congreso; 16th IAPS winter conference; 2005
p { margin-bottom: 0cm; direction: ltr; color: rgb(0, 0, 0); widows: 2; orphans: 2; }p.western { font-family: "Arial",sans-serif; font-size: 11pt; }p.cjk { font-family: "Times New Roman",serif; font-size: 11pt; }p.ctl { font-family: "Arial",sans-serif; font-size: 10pt; }a:link { color: rgb(0, 0, 255); } Photochromic compounds were dissolved in nematic (ZLI1132) and in chiral nematic phases (ZLI1132 with chiral inductor S811). When necessary, samples were oriented by surface induced alignment. Three photochromic families were studied: azobenzenes (azobenzene, AB, 4-(NN-dimethylamino)azobenzene, DAB, and 4?nitro-DAB, NDAB), spirocompounds (6-nitro-spirobenzopyrane, SP, and a phenanthrospirooxazine, SO), and fulgides (phenylfulgide, PF, and furylfulgide, FF). The photochromic compounds differ in lifetime of the photoisomer (from 10-3 to 103 s) as well as in the geometry change between isomers (big geometry change in E-Z isomerization in azobenzenes or in ring opening of spirocompounds and slight change in pericyclic reactions of fulgides). The photochromic transformation was studied by polarized spectroscopy (linear and circular dichroism), birefringence, and polarized microscopy as a function of temperature. Azobenzenes are the most perturbing probes. They decrease the order of the mesophase upon E-Z isomerization. Great changes in transmittance are obtained for aligned nematic phases placed between polarizers (S/N better than 10), even with 10% conversion of samples containing 2-3% w/w AB?s. On the contrary, the build up of the more rod like merocyanine form of SO and SP increases the order of the mesophase. The highly dipolar form of SP displays orientational solvatochromism. The polarity sensed by this compound is similar to the one obtained from the kinetic decay of NDAB. The MC  SP transformation has a high activation entropy, which correlates with the entropy change in common solvents. In chiral nematic phases only DAB and NDAB can change the position and shape of the reflection band near the Ch-I transition temperature. A photoinduced phase transition is detected at high photoconversions. Fulgides show a photoinduced circular dichroism in the visible, which is a consequence of the chiral order of the phase and not of chiral enrichment. From the magnitude of this effect, the linear anisotropy is calculated. In the systems studied changes in birefringence, linear dichroism, reflection band, nature of the stable phase, and circular dichroism can be photoinduced.