Highly anisotropic molecular thin films prepared by oriented growth on poly(tetrafluoroethylene) surfaces

Highly anisotropic molecular thin films have been successfully prepared by depositing bisazo dyes under vacuum through evaporation on an oriented poly(tetrafluoroethylene) (PTFE) surface aligned by rubbing or a friction-transfer technique. Polarized absorption spectra (UV-visible and IR), electron diffraction, optical microscopy, and Raman scattering were employed to investigate the molecular orientation in the films. The results indicate that the dye molecules are oriented with their long axes parallel to the PTFE chains and that their phenyl rings are fairly parallel to the substrate surface. Slight modifications of the dye molecules lead to significant changes in the degree of the orientation. In particular, the films from a bisazo dye containing a pyrrolidinyl group exhibit a remarkable orientation, showing dichroic ratios up to 130 on the polarized UV-visible spectra.