INSTITUTO DE QUIMICA, FISICA DE LOS MATERIALES, MEDIOAMBIENTE Y ENERGIA
Unidad Ejecutora - UE
Photoisomerization of azobenzenes and spirocompounds in nematic and in twisted nematic liquid crystals
M. BOSSI; D. H. MURGIDA; P. F. ARAMENDIA
JOURNAL OF PHYSICAL CHEMISTRY B
American Chemical Society
Año: 2006 vol. 110 p. 13804 - 13804
Samples of a nematic mixture of ZLI1132 and of a twisted nematic mixture composed of ZLI1132 and chiral inductor S811 including 1 - 10 % w/w 4-N,N-dimethylaminoazobenzene (DAB), (4´-nitro)-4-N,N-dimethylaminoazobenzene (NDAB), spiropyran (SP), or spirooxazine (SO) were irradiated to produce the photochromic transformation of the dopant. The changes in the system were monitored by time-resolved transmission spectroscopy, by time-resolved birefringence or by polarized Raman scattering. The medium sensitivity of the kinetics and spectroscopy of some of the probes was used to derive information on polarity of the medium. In the systems studied, apart from the changes in absorption spectrum, great changes in birefringence can be photoinduced and the order of the nematic phase can be changed in either direction depending on the dopant. The open form of SP can discriminate orientation polarity. While the polarity parallel to the mesogenic director is similar to acetone, the perpendicular orientation has a polarity similar to acetonitrile. In agreement with this observation, the kinetics of the Z ® E isomerization of NDAB, oriented parallel to the mesogenic director, also experiences a polarity close to acetone. The decay rate constant of the open form of SP displays a linear relation between its Arrhenius parameters, which is universal in a great variety of homogeneous solvents, solvent mixtures, and liquid crystals, therefore validating the hypothesis that the same type of transformation is observed in all these cases, namely the decay of the open form monomer. The dopants used showed to be adequate probes of bulk-like properties in locally heterogeneous systems as liquid crystals.