Mesomorphism of (E)-4-[(4-propyloxyphenyl)diazenyl]phenyl alkanoates (3OABOOCm)

Several compounds from the group of (E)-4-[(4-propyloxyphenyl)diazenyl]phenyl alkanoates (3OABOOCm, for m = 1 - 19) were synthesized, with the derivatives for m >= 8 being reported for the first time. The phase situation was investigated by polarizing optical microscopy, thermo-optical analysis, and differential scanning calorimetry methods. The enantiotropic nematic mesophase was noted for compounds where m = 2 - 13, whereas the monotropic nematic phase was observed for m = 1, 14, and 15. Multiple crystalline phase were noticed in selected derivatives (for m = 1 - 3, 9, 13, and 14). The X-ray powder diffraction confirmed the polymorphism of the crystalline phase for the 3OABOOC2 compound. The trans-cis photoisomerization process is attributed to the presence of the diazenyl (azo) group. Absorption spectra, collected as a function of UV light exposure time, indicated a first-order reaction comprising two components. The determined values of the kinetic constant k were of the order of 10-2 s - 1.