Electrophosphorescent light-emitting diodes

for which both singlet and triplet excitons contribute to the light emission

have attracted great scientific and commercial attention because of their high quantum efficiency.Iridium-based complexes are the popular selection as the efficient dopant due to a relatively short phosphorescent lifetime.Aseries of electrophosphorescent conjugated polymers based on substituted para-polyphenylenes are synthesized by Suzuki polycondensation.Apart of N-alkyl benzene units were replaced by di-ketone group ended to provide a ligand for the formation of Ir complexes in side chain in the presence of 2-phenylpyridine.Poly (para-phenylene) with broad band gap makes the energy level better matching between the host and the guest.Therefore

energy transfer from the host to Ir-complex becomes more efficient.The incorporation of long alkoxy group into the side-chain can substantially raise the flexibility of polymer that leads to the improvement of solubility of polymer

which is helpful for the fabrication of devices.On the other hand

the activity of β-diketone group to iridium core is raised due to more rotating freedom of β-diketone-alkyl around the oxygen atom.ELemission from the backbone of the polymers is completely quenched even though the feed content ratio of Ir-complex is as low as 1%.They all show only the emission from iridium complex

while the host emission disappeared.It indicates that efficient energy transfer from the main chain to the Ir-complex occurs.The maximum external quantum efficiency of 2.6% was achieved for PPPIrPPy2 with the feed content ratio of Ir-complex of 2% at the current density of 0.35 mA/cm

2

.