Evidence of the molecular orientation of Pt- And Pd-containing organometallic polymers in thick layers: A NEXAFS study

The molecular orientation of rod-like organometallic polymers [-Me(Pn-Bu-3)(2)-C=C-C6H4-C6H4-C=C-](n), Me = Pt, Pd, in the form of thick films, has been determined from the study of their near-edge X-ray absorption spectra (NEXAFS) recorded in the photon-energy region 280-320 eV, The C K-edge resonances, recorded at grazing (theta = 25 degrees) and normal (theta = 90 degrees) X-ray beam incidence angles, show a strong polarization dependence of the majority of the peaks, Quantitative evaluation of the intensity ratios suggests that the organic moiety of the polymer, that is -C=C-C6H4-C6H4-C=C-, is oriented at about 40 degrees with respect to the surface normal, the two benzene rings are nearly coplanar, and the plane containing the metal and the phosphines (Pn-Bu-3) is probably aligned with the plane of the rings, since the butyl groups of the Pn-Bu-3 ligands also show a non-isotropic character. Although the repeat unit of the polymer is quite complex, the assignment of the pi* and sigma* resonances has been developed on the basis of (i) the chemical structure provided by traditional spectroscopic investigations, (ii) ab initio LCAO-MO-SCF calculations performed on a simple model molecule (phenylacetylene), and (iii) curve fitting of the spectra.