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(tripheny lphosphine oxide) europium (Eu(TTA)(3)(TPPO)(2)), as the light-emitting lay er, 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 la yer, the triple-layer electroluminescent (EL) device emits red light charac teristic of Eu3+ emission. However, as the mixture of Eu(TTA)(3)(TPPO)(2) a nd ALQ is co-evaporated as the light-emitting layer to form a bilayer EL de vice, a new wide-banded emission peaked at c. 640 nm was obtained. This emi ssion is neither from ALQ nor from the europium complex. The photoluminesce nce (PL) of the thin film on quartz substrate evaporated from one mixed sol id powder of Eu(TTA)(3)(TPPO)(2) and ALQ is composed of distinct PL emissio ns 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 emiss ion 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 t he highest occupied molecular orbital (HOMO) of ALQ. (C) 2000 Elsevier Scie nce B.V. All rights reserved.