Efficient Layers of Emitting Ternary Lanthanide Complexes for Fabricating Red, Green, and Yellow OLEDs

A series of novel nona- and octacoordinate
highly volatile and luminescent complexes, [Eu­(hfaa)3­(indazole)3] and [Ln­(hfaa)3­(indazole)2] (Ln
= Tb, Dy, and Lu), were synthesized using a monoanionic bidentate
hexafluoroacetylacetone (hfaa–) and a neutral monodentate
indazole ligand. The X-ray diffraction analyses of their single-crystals
indicate that the complexes are mononuclear. The Eu complex is nonacoordinate
and has a distorted monocapped square antiprismatic structure whereas
the terbium and dysprosium complexes are octacoordinate and possess
a trigonal bicapped prism geometry. The indazole units are involved
in π–π stacking interaction and N–H···F
hydrogen bonding with the fluorine atoms of hfaa–. The photophysical studies of indazole and the complexes show that
the triplet states are at the appropriate positions and make ligand-to-metal
energy transfer process efficient. A strong protective shield is provided
by the coordination of three hfaa– moieties (which
have low frequency C–F vibrational oscillators), and two/three
ancillary indazole ligands around these metal ions ascribe higher
quantum yields and longer radiative life times (ΦEu = 69% ±
10, 989 ± 1 μs, ΦTb = 33% ± 10, 546 ± 1
μs, and ΦDy = 2.5% ± 10, 13.6 ± 1 μs)
to these novel compounds. The emission from europium, terbium, and
dysprosium are, respectively, red, green, and yellow. Finally, these
compounds were used, as emitting layers, to fabricate electroluminescent
devices of their respective colors. The best devices are found with
the following structure: ITO/CuPc (15 nm)/[Eu complex]:CBP or [Tb
complex]:CBP or [Dy complex]:CBP (80 nm)/BCP (25 nm)/AlQ (30 nm)/LiF
(1 nm)/Al (100 nm), which indicates an improved EL performance for
the Eu device over the Eu devices reported in the literature. The
ligand, indazole, is a good sensitizer for trivalent europium, terbium,
and dysprosium ions. It together with hfaa– plays
an important role in fabricating OLEDs, especially, processed at low
temperature.