Dendritic europium complex as a single dopant for white-light electroluminescent devices

Abstract

Three novel dendritic europium complexes containing grafted carbazole units as their functional peripheries were prepared and characterized. The Förster energy transfer from the peripheries of carbazole units to β-diketonates as well as from the ligands to the central Eu(III) ions in these complexes has been demonstrated. Under the indirect excitation of carbazole units, a characteristic red emission peak at 615 nm due to the Eu(III) ion transition of ⁵D₀ → ⁷F₂ was observed for the complexes in solution or solid films. As a result of the light-harvesting and site isolation of carbazole dendrons, compared with the reference compound Eu(BPPD)₃(Phen), PL efficiencies of Eu(MCPD)₃(Phen), Eu(BCPD)₃(Phen) and Eu(TCPD)₃(Phen) in the film state are 3.3, 7.9 and 4.5 fold increased, respectively. Notably, white light emission with the CIE coordinates of (0.333, 0.348) has been achieved with a single dopant of Eu(TCPD)₃(Phen) in an EL device of configuration ITO/NPB/CBP : Eu(TCPD)₃(Phen)/BCP/Mg : Ag. Its maximum external quantum efficiency is more than 1.1% and maximum brightness reached 229 cd m⁻². The origin of the blue and green emission is attributed to the dendron-grafted carbazole units forming an exciplex and an electroplex with CBP (4,4′-N,N′-dicarbazole biphenyl) and ligand β-diketonate, respectively. The results suggest that modifying ligands can not only tune the carrier-transporting properties of complexes, but also provide a useful clue to the use of electroplexes or exciplexes to realize broad or even white electroluminescence.