Engineering Blue Fluorescent Bulk Emitters for OLEDs: Triplet Harvesting by Green Phosphors

Triplet harvesting in organic light-emitting diodes (OLEDs) from a blue fluorescent to a green phosphorescent emitter is important for the development of highly efficient white OLEDs for lighting applications. Here, we report new blue fluorescent bulk emitters with high triplet energies for triplet harvesting in OLEDs. Based on the chemical structure of the highly efficient blue emitter N,N'-di-1-naphthalenyl-N,N'-diphenyl-[1,1':4',1 '':4 '',1'''-quaterphenyl]-4,4''' diamine (4P-NPD), two new quarterphenylene compounds are designed and synthesized. Their experimentally obtained ionization potentials, singlet, and triplet levels are compared to quantum chemical calculations. We show that the incorporation of repulsive methyl groups leads to a twist of the outer phenyl rings of the molecule and increases the triplet level. Finally, triplet harvesting OLEDs comprising these two new compounds as bulk emitters are electrically and optically characterized. We demonstrate that both materials allow triplet harvesting by the green phosphorescent emitter iridium(III)bis(2-phenylpyridinato-N,C2')acetylacetonate (Ir(ppy)(2)(acac)).