EVIDENCE OF THE MOLECULAR-ORIENTATION OF PT-CONTAINING 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 fil ms, has been determined from the study of their near-edge X-ray absorp tion 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 stron g polarization dependence of the majority of the peaks, Quantitative e valuation 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 degr ees with respect to the surface normal, the two benzene rings are near ly coplanar, and the plane containing the metal and the phosphines (Pn -Bu-3) is probably aligned with the plane of the rings, since the buty l groups of the Pn-Bu-3 ligands also show a non-isotropic character. A lthough the repeat unit of the polymer is quite complex, the assignmen t of the pi and sigma* resonances has been developed on the basis of (i) the chemical structure provided by traditional spectroscopic inves tigations, (ii) ab initio LCAO-MO-SCF calculations performed on a simp le model molecule (phenylacetylene), and (iii) curve fitting of the sp ectra. The organometallic polymers containing two different transition metals, Pt and Pd, investigated here, show the same molecular structu re and orientation. (C) 1997 Elsevier Science B.V.