METAL CONJUGATED POLYMER INTERFACES - A LOCAL-DENSITY FUNCTIONAL-STUDY OF ALUMINUM POLYENE INTERACTIONS

The interactions between aluminum atoms and model molecules representi ng trans-polyacetylene are studied quantum chemically by a local densi ty functional method. We focus on the chemical and electronic structur e of the organoaluminum complexes. Special emphasis is put on a compar ison between results at the local spin density approximation and ab in itio Hartree-Fock levels. In unmetallized polyenes, the density functi onal method provides a very good description of the carbon-carbon bond lengths of conjugated systems; in the case of hexatriene, it reproduc es the bond dimerization in very good agreement with experimental meas urements. Upon metallization, a strong covalent interaction between al uminum and carbon is found. The Al-C bond formation induces an interru ption of the bond alternation pattern and reduces the pi-conjugation i n the oligomer, in qualitative agreement with photoelectron spectrosco py data and previous theoretical results at the Hartree-Fock level. No tably, the pi-electron levels in the organoaluminum complexes maintain delocalization. In contrast to Hartree-Fock results where an aluminum atom binds to a single carbon, the interactions calculated with the l ocal spin density approximation lead to (i) formation of multicenter a luminum-carbon bonding; (ii) near planarity of the polyene molecule; a nd (iii) a lower degree of charge transfer from the metal atom to the polymer.