Synthesis, photophysical and electroluminescent properties of a novel bright light-emitting Eu³⁺ complex based on a fluorene-containing bidentate aryl phosphine oxide

A novel bidentate aryl phosphine oxide (APO) ligand with fluorene as the chromophore (9,9-diethyl-9H-fluorene-2,7-diyl)bis(diphenylphosphine)oxide (EFDPO), as well as the corresponding Eu³⁺ complex 1 were designed and synthesized. The strong absorption antennae effect of the functional APO ligands was proved. The effect of EFDPO in the intramolecular singlet and triplet energy transfer was also investigated. It is showed that the rigid structure and the appropriate energy levels of EFDPO not only reduce the energy loss induced by the structure relaxation, but also restrain the solvent quenching and facilitate the energy transfer from EFDPO to Eu³⁺. The thermal analysis was also performed to prove the improved thermal stability and phase stability of the complex. The complex exhibited good electroluminescent (EL) performance, such as maximum brightness more than 600 cd m^-2, E.Q.E. around 2% and stable monochromic red emission at 616 nm.