SUBMILLIMETER WAVE ABSORPTION IN A HOMOLOGOUS SERIES OF LIQUID-CRYSTALS

The far-infrared absorption spectra of the first seven homologs (C 1-C7) of the p,p-di-n-alkoxyazoxy-benzene series of liquid crystals have been studied in the range 30-200 cm-1. Comparison of the different spectra in the isotropic phase shows that the intensity of absorption is maximum in C1. In C2 and C3, the intensity as also the peak position of the central band decrease as compared to C1. A broad and shallow absorption profile without any central band is observed in C4-C7. The various factors that can possibly contribute to the far-infrared absorption in the fluid phases of these mesogens are considered. It is found that the Poley absorption may be the dominant factor in C4-C7, but it can not account for the distinct changes associated with the central band in C1-C 3. The observed decrease in the intensity and the shift in the peak position of this band on going from C1 to C2 strongly suggest that this band originates from the torsion of the end alkoxy groups around the phenyl-oxygen bonds. This assignment is also consistent with (i) the virtual disappearance of the central band in C4-C7 (ii) the dichroic behavior of the band in C1 in the homogeneously aligned nematic phase and (iii) the considerably reduced intensity of absorption in the liquid phase of the related, but nonmesomorphic compound p,p′- azoxytoluene. We conclude that in the composite absorption profiles due to a superposition of the Poley and torsional bands, the latter contribution becomes increasingly important for the lower homologs of this series. © 1979 American Institute of Physics.