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 °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.

Figure 1

Schematic representation of the experimental setup for the measurement of linear dichroism.

Figure 2

Chemical structures of chromophore monomers used for computation in a trans,trans configuration (a) schematically of their relative positions with the distance $d$ between parallel molecular planes in parallel (b) and antiparallel (c) orientation, of monomers in a cis,trans configuration (d) and in a trans,cis configuration (e), and of the polymers used for experiment [statistical compositions of the units (f) and (g)].

Figure 3

Sample geometry.

Figure 4

The absorption coefficient $\alpha$ of the polymer (dotted line) and the refractive index $n$ (solid line) as a function of the wavelength $\lambda$. The circles indicate the values measured using the prism coupler.

Figure 5

Grazing incidence wide-angle x-ray scattering setup.

Figure 6

Spectra of absorption $A$ as a function of the wavelength $\lambda$ and as a function of the photon energy $E_a$, all with same scaling; graph a: trans,trans chromophore [solid curve; see Fig. 2a], of a trans,cis chromophore [dotted curve; see Fig. 2e], and of a cis, trans chromophore [dashed curve; see Fig. 2d]; graph b: an isolated single trans,trans chromophore (solid, same as in graph a), of a dimer in parallel (head-to-head) orientation [dashed, see Fig. 2b], and of a dimer in antiparallel (head-to-tail) orientation [dotted, see Fig. 2c]; graph c: material with 100% chromophore concentration poled at 135 °C measured with parallel (solid) and perpendicularly (dash-dotted) polarized light and of the one poled at 110 °C measured with parallel (dash-dot-dotted) and perpendicularly (dashed) polarized light. The dotted line represents a curve fit with the density-functional results of the antiparallel dimer.

Figure 7

The first (open circles) and second (filled squares) order parameter $a_1$ and $a_2$ as functions of the chromophore concentration $c_{\mathit{chr}}$ (top graph) and as functions of the previous poling temperature $T$ (graph below). $T_g$ marks the glass transition temperature.

Figure 8

Chromophore orientation distributions as functions of the polar angle $\theta$; graph a: polymers with different chromophore concentrations, previously corona poled at each one’s individual optimal poling temperature $\left[g_c\left(\theta \right)\right]$; graph b: a polymer with 100% chromophore concentration previously corona poled at different poling temperatures $\left[g_T\left(\theta \right)\right]$.

Figure 9

X-ray scattering images (left) of polymer films with 100% chromophore content and a glass transition temperature of 94 °C poled at 84 °C (graph a), 102 °C (graph b), and 124 °C (graph c). The right column schematically represents according diffracting structures with respect to the horizontal film pane: very small domains with very few diffracting planes and a large angular distribution with in-plane order (a), small domains with few diffracting planes and still a large angular distribution with out-of-plane order (b), and larger domains with more diffracting planes and a small angular distribution, again, with out-of-plane order (c).