Highly improved electroluminescence from a series of novel Eu¹¹¹ complexes with functional single-coordinate phosphine oxide ligands: Tuning the intramolecular energy transfer, morphology, and carrier injection ability of the complexes

The functional single-coordinate phosphine oxide ligands (4-diphenylaminophenyl)diphenylphosphine oxide (TAPO), (4-naphthalen-1-yl- phenylaminophenyl)diphenylphosphine oxide (NaDAPO), and 9-[4- (diphenylphosphinoyl)phenyl]-9H-carbazole (CPPO). as the direct combinations of hole-transporting moieties, and electron-transporting triphenylphosphine oxide (TPPO) were designed and synthesized (amines or carbazole), together with their Eu^III complexes [Eu(tapo)₂(tta)₃] (1). [Eu(nadapo):(tta)₃] (2), and [Eu(cppo)₂(tta)₃] (3; TTA: 2-thenoyltrifluoroacetonate). The investigation indicated that by taking advantage of the modification inertia of the phosphine oxide ligands, the direct introduction of the hole-transport groups as chromophore made TAPO, NaDAPO. and CPPO obtain the most compact structure and mezzo Si and T ₁ energy levels, which improved the intramolecular energy transfer in their Eu^III complexes. The amorphous phase of 1-3 proved the weak intermolecular interaction, which resulted in extraordinarily low self-quenching of the complexes. The excellent double-carrier transport ability of the ligands was studied with Gaussian calculations, and the bipolar structure of TAPO and CPPO was proved. The great improvement of the double-carrier transport ability of 1-3 was shown by cyclic voltammetry. Their HOMO and LUMO energy levels of around 5.3 and 3.0 eV, respectively, are the best results for Eu^III complexes reported so far. A single-layer organic light-emitting diode of 2 had the impressive brightness of 59 cd m ^-2 which, to the best of our knowledge, is the highest reported so far. Both of the four-layer devices based on pure 1 and 2 had a maximum brightness of more than 1000 cdm^-2, turn-on voltages lower than 5 V, maximum external quantum yields of more than 3% and excellent spectral stability.