Solid-state structures of phenyleneethynylenes: Comparison of monomers and polymers

The solid-state structures of a series of 2,5-dialkyl-substituted poly(p-phenyleneethynylene)s (PPEs 4, R = hexyl, dodecyl, isopentyl, ethylbutyl, ethylhexyl, H) and of 2,5-didodecyl(1,4-phenylenebutadiynylene) were investigated by X-ray powder and electron diffraction. In addition, X-ray single-crystal structures of the monomeric (1,4-dipropynyl-2,5-dialkyl)benzenes were obtained and compared to the structures of the corresponding polymers. In the case of the ethylhexyl-substituted PPE, the packing of the monomer resembles that of the polymer in the solid state. All of the examined PPEs show lamellar morphologies, in which the extended geometry of the side chains determines the value of the lamellar spacing obtained by powder diffraction. A second d value, which could be obtained for all samples represents the pi-pi-stacking distance of the main chains. This distance is around 3.8 Angstrom, a typical value for stacking of aromatic systems. It implies strong electronic interaction of the chromophoric backbones, leading to the observed sharp aggregate band in CHCl3/MeOH solutions and in thin films. From the scattering data, staggered packing of the benzene rings and the phenyleneethynylene main chains in PPEs 4 was inferred. This packing avoids repulsive electrostatic interactions between adjacent conjugated phenyleneethynylene chains and at the same time leads to optimal packing of the side chains.