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. The organometallic polymers containing two different transition metals, Pt and Pd, investigated here, show the same molecular structure and orientation.