Electroplex emission from a layer of a mixture of a europium complex and tris(8-quinolinolato) aluminum

With a europium complex, tris(alpha-thenoyltrifluoroacetonato) bis(triphenylphosphine oxide) europium (Eu(TTA)(3)(TPPO)(2)), as the light-emitting layer, N, N'-diphenyl-N, N'-di(m-tolyl)-benzidine (TPD) as the hole transport layer and tris(8-quinolinolato) aluminum (ALQ) as the electron transport layer, the triple-layer electroluminescent (EL) device emits red light characteristic of Eu3+ emission. However, as the mixture of Eu(TTA)(3)(TPPO)(2) and ALQ is co-evaporated as the light-emitting layer to form a bilayer EL device, a new wide-banded emission peaked at c. 640 nm was obtained. This emission is neither from ALQ nor from the europium complex. The photoluminescence (PL) of the thin film on quartz substrate evaporated from one mixed solid powder of Eu(TTA)(3)(TPPO)(2) and ALQ is composed of distinct PL emissions of Eu(TTA)(3)(TPPO)(2) and ALQ, denying an exciplex formation mechanism. It is impossible to form a host-guest system. We propose that the EL emission peaked at c. 640 nm is from an electroplex route: a transition between the lowest unoccupied molecular orbital (LUMO) of Eu(TTA)(3)(TPPO)(2) and the highest occupied molecular orbital (HOMO) of ALQ. (C) 2000 Elsevier Science B.V. All rights reserved.