White electroluminescence of lanthanide complexes resulting from exciplex formation

Bimetallic complexes of the visible-emitting Eu³⁺, Tb ³⁺ and Sm³⁺ ions with 2,2′-bipyrimidine (bpm) and acetylacetonate (acac) ligands have been synthesized and crystallographically characterized. They have been found to be isomorphous with the general formula [{Ln(acac)₃}₂(μ-bpm)] (Ln = Eu, 1; Ln = Tb, 2; Ln = Sm, 3). Their X-ray crystal structure shows that the aromatic polyamine ligand bridges the two metal ions. These volatile complexes have been thermally evaporated to be introduced in multilayer OLEDs as dopants. In addition to the electroluminescence of the lanthanide ion, a broad emission in the yellow-green region was obtained. This resulted in white electroluminescence of the devices doped with 1 or [{Eu(tta)₃}₂(μ-bpm)] (4) complexes (tta = 2-thenoyltrifluoromethanesulfonato). This broad emission is shown to be caused by the formation of an exciplex between the dopant and the host matrix within the emitting layer of the devices.