Novel electron-withdrawing π-conjugated pyrene-containing poly(phenylquinoxaline)s

Abstract

65 In recent time, conjugated polymers have attracted considerable attention of researchers not only from scientific but also from a practical viewpoint due to their fundamental optoelectronic properties and application as electroactive materials for organic light emitting diodes [1], field effect transistors [2], and photovoltaic devices [3]. Conjugated polymers are prepared by Stille, Suzuki, and Yamamoto reactions using nickel (Yamamoto) and palladium (Suzuki and Stille) catalysts [4, 5]. However, the high cost of the processes and difficulties of purification from metal catalyst residues, which are undesirable impurities for optoelectronic devices, are considerable drawbacks of these reactions. New noncatalytic methods for the synthesis of high purity conjugated polymers should be developed to solve these problems. It has also been found that intense electroluminescence and high charge mobility in a polymer require a high electron– hole conduction of a medium due to charge carriers mobility and a balance of injection of carriers of both signs from opposite electrodes into the emission poly mer layer. However, the majority of conjugated poly mers provide hole transport that causes imbalance of charge injection and a poor quantum yield. The search for new efficient electron transport polymers is necessary to improve the characteristics of electrooptical devices, which is provided, in particu lar, by the introduction of electron withdrawing frag ments—triazole [6], oxodiazole [7], quinoline [8], and quinoxaline [9]—into macromolecules. The lat ter are the least studied and show higher electron withdrawing properties due to the presence of four electron withdrawing heteroatoms. π Conjugated thiophene containing poly(phenylquinoxaline)s (PPQs) have been obtained recently. These PPQs behave as excellent electron transport materials for organic light emitting diodes [10]. We prepared new conjugated pyrene containing PPQs as high purity polymers by noncatalytic polycondensation reaction to provide the balance of the hole and electron con duction. The introduction of flat structures, such as pyrene, into a conjugated polymer is supposed to enhance intermolecular electronic interaction in the solid state and result in formation of π–π stacking structures. In continuation of our studies [11, 12], we prepared and studied new bipolar pyrene containing PPQs as efficient electron hole and transport materi als and thereby synthesized, previously unknown bis(α diketone)s IV and VIII.