Rational design of chelating phosphine functionalized os(II) emitters and fabrication of orange polymer light-emitting diodes using solution process

Abstract

A new series of charge neutral Os(II) pyridyl azolate complexes with either bis(diphenylphosphino)methane (dppm) or cis-1,2- bis(diphenylphosphino)ethene (dppee) chelates were synthesized, and their structural, electrochemical, photophysical properties and thermodynamic relationship were established. For the dppm derivatives 3a and 4a, the pyridyl azolate chromophores adopt an eclipse orientation with both azolate segments aligned trans to each other, and with the pyridyl groups resided the sites that are opposite to the phosphorus atoms. In sharp contrast, the reactions with dppee ligand gave rise to the formation of two structural isomers for all three kind of azole chromophores, with both azolate or neutral heterocycles (i.e., pyridyl or isoquinolinyl fragments) located at the mutual trans-disposition around the Os metal (denoted as series of a and b complexes). These chelating phosphines Os(II) complexes show remarkably high thermal stability, among which and several exhibit nearly unitary phosphorescence yield in deaerated solution at RT. A polymer light-emitting device (PLED) prepared using 0.4 mol % of 5a as dopant in a blend of poly(vinylcarbazole) (PVK) and 30 wt % of 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PBD) exhibits yellow emission with brightness of 7208 cd m-2, an external quantum efficiency of 10.4% and luminous efficiency of 36.1 cd A-1 at current density of 20 mA cm-2. Upon changing to 1.6 mol % of 6a, the result showed even better brightness of 9212 cd m-2, external quantum efficiency of 12.5 % and luminous efficiency of 46.1 cd A-1 at 20 mA cm-2, while the max. external quantum efficiency of both devices reaches as high as 11.7% and 13.3%, respectively. The high PL quantum efficiency, non-ionic nature, and short radiative lifetime are believed to be the determining factors for this unprecedented achievement. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.