EFFICIENT SOLID-STATE PHOTOLUMINESCENCE IN NEW POLY(2,5-DIALKOXY-P-PHENYLENEETHYNYLENE)S

A series of novel poly(2,5-dialkoxy-p-phenyleneethynylene) (PPEs) has been prepared by the palladium-catalyzed cross-coupling polycondensato n of aryl acetylenes and aryl iodides. Different alkoxy side chains in cluding n-hexadecyloxy, n-octyloxy, (2-ethylhexyl)oxy, (2-methylpropyl )oxy, (3-(dimethylamino)propyl)oxy, and (7-carboxyheptyl)oxy groups we re attached to the rigid-rod polymer main chain. With this structural concept, polymers having an identical conjugated backbone but differen t supramolecular structures in the solid state could be achieved. X-ra y 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) q uantum yields, up to 0.86 in solution and 0.36 in the solid state, hav e been measured for the new materials. While the solution quantum yiel ds are independent of the functionalization, solid-state quantum effic iencies were found to be related to the degree of long-range order in the samples, decreasing with increasing order. The coplanar orientatio n of the conjugated polymer backbones is assumed to lead to the format ion of excimer complexes which provide nonemissive decay channels for the excited states. These nonemissive orientations are more significan t in the materials having a greater degree of long-range order.