Investigation of chromophore-chromophore interaction by electro-optic measurements, linear dichroism, x-ray scattering, and density-functional calculations

Free-beam interferometry and angle-resolved absorption spectra are used to investigate the linear electro-optic coefficients and the linear dichroism in photoaddressable bis-azo copolymer thin films. From the first- and second order parameters deduced, the chromophore orientation distribution is calculated and displayed for several poling temperatures and chromophore concentrations. The influence of dipole-dipole interaction on the overall polymer dynamics is discussed. The first order parameter, and therefore the Pockels effect, peaks for a poling temperature of around 10 degrees C above the glass transition. The decrease of the Pockels effect above this temperature region is triggered by a head-to-tail chromophore orientation, i.e., a transition to a microcrystalline phase, increasing the second order parameter. Comparison of the experimentally observed absorption spectra and those obtained by density-functional calculations support the picture of differently aligned bis-azo dye molecules in a trans,trans configuration. Complementary wide-angle x-ray scattering is recorded to confirm the various kinds of ordering in samples poled at different temperatures.