A HEXAGONAL STRUCTURE FOR ALKALI-METAL DOPED POLY (P-PHENYLENE)

An hexagonal structure (space group p6BAR2m, a = 8.6 angstrom) is proposed for sodium-doped poly(p-phenylene), PPP. The diffraction pattern calculated using only one freely adjustable parameter (the distance between the alkalimetal column and the polymer backbone) is in good agreement with the observed electron diffraction patterns. A similar structure (a = 9.2 angstrom) is also suggested by diffraction data for potassium-doped PPP. This hexagonal structure is analogous to that reported for sodium-doped poly(p-phenylene vinylene), lithium-doped polyacetylene, and sodium-doped polyacetylene. The three chain per column arrangement provides a fundamental structural motif which maximizes the coordination of the negatively charged carbon atoms with both the alkali metal ions and the hydrogens, and maximizes interchain, intercolumn, and hydrogen to alkali-metal separations. The size of the dopant-ion relative to the cross-sectional dimensions of the host polymer determines whether the dopant-ion columns are formed in triangular (three chain per column) or tetragonal (four chain per column) channels.