A series of novel poly(2,5-dialkoxy-p-phenyleneethynylene) (PPEs) has been prepared by the palladium-catalyzed cross-coupling polycondensaton of aryl acetylenes and aryl iodides. Different alkoxy side chains including n-hexadecyloxy, n-octyloxy, (2-ethylhexyl)oxy, (2-methylpropyl)oxy, (3-(dimethylamino)propyl)oxy, and (7-carboxyheptyl)oxy groups were attached to the rigid-rod polymer main chain. With this structural concept, polymers having an identical conjugated backbone but different supramolecular structures in the solid state could be achieved. X-ray diffraction measurements on thin films show that the polymers which have sterically hindered side chains are essentially disordered, while those with only linear side chains can form lamellar structures with significant degrees of long-range order. High photoluminescence (PL) quantum yields, up to 0.86 in solution and 0.36 in the solid state, have been measured for the new materials. While the solution quantum yields are independent of the functionalization, solid-state quantum efficiencies were found to be related to the degree of long-range order in the samples, decreasing with increasing order. The coplanar orientation of the conjugated polymer backbones is assumed to lead to the formation of excimer complexes which provide nonemissive decay channels for the excited states. These nonemissive orientations are more significant in the materials having a greater degree of long-range order.