Absorption hexaphenyl films

From the optical absorption of two different hexaphenyl films, one with its chains predominantly standing upright on the substrate, the other with the chains randomly distributed in all orientations, similar structure property relations can be concluded [139]. By comparing the calculated absorption coefficient [139J perpendicular to the chains with the observed optical absorption spectra of both films we see that the optical absorption, plotted in Figure 9-9, in the visible and... 

FIGURE 8.6. Triplet absorption, singlet emission, and absorption (from left to right) of (from top to bottom) m-LPPP161 and hexaphenyl films 162 at 90 K, and para-quater-, and terphenyl at 77 K in tetrahydro-2-methylfuran.25... 

The optical properties of materials are determined by the so-called dielectric function. This dielectric function was determined for PPP as a result of first principles band structure calculations. In Fig. 30.3 we depict one of the main results, namely the dependence of the imaginary part of the dielectric function (which is proportional to the optical absorption coefficient) on the orientation parallel (cc) and perpendicular ( , e ) to the chain axis. From comparison with the experiment one can see that the optical absorption in the visible and ultraviolet range is mainly determined by the dielectric function parallel to the polymer chain. This is shown in Fig. 30.4, where the calculated absorption coefficient along the chain is compared with experimental data. The observed n-n transition is therefore the result of optical excitation with the dipole moment parallel to the PPP chains. Another experimental proof for this are the absorbance properties of hexaphenyl single crystals [43]. The absorbance spectra for thin films with the hexaphenyl chains oriented perpendicular and parallel to the substrate plane are shown in Fig. 30.5 together with the photoluminescence (PL) and electroluminescence (EL) emission spectra. The above-described anisotropy of the optical absorption is also valid for photoluminescence emission and can be used to obtain polarized EL spectra from Langmuir-Blodgett films made from soluble PPPs [44]... 

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