Angewandte
Communications
Chemie
ure S2d), which showed an oxidation potential of 0.90 V for
of FlAnt (about 15.7%) as a result of a stronger intermo-
FlAnt and 0.95 V for 2A. The calculated deep HOMO levels
of À5.50 and À5.55 eV favor the high environmental stability
of FlAnt and 2A, respectively. The thermal, optical, and
electrochemical parameters of FlAnt and 2A are listed in
Table S1 of the Supporting Information.
lecular force at the excitation wavelength of 370 nm. It was
found that the fluorescence lifetime for FlAnt at 450 nm was
1.93 ns (see Figure S4 for time-resolved fluorescence spectra
of the thin films). The emissions from 2A at 450, 510, and
550 nm had unequal fluorescence lifetimes of 0.55, 5.33, and
7.34 ns, respectively, which further prove a difference in
emission at 450 and 550 nm.
Figure 1b shows the photoluminescence spectra of FlAnt
and 2A in the solid state. To our surprise, FlAnt showed
a deep-blue emission with CIE coordinates of (0.16, 0.11),
whereas 2A demonstrated a broad yellow emission centered
at 550 nm in addition to blue emission at 450 nm, which
constitute a greenish-yellow emission with CIE coordinates of
(0.30, 0.55). Figure 1c shows the similar excitation spectra for
emissions at 450, 510, and 550 nm in the thin film of 2A, which
demonstrates the same source of these three emissions and
thus excludes the formation of new fluorophore aggregates.[5a]
The electroluminescence (EL) spectrum of 2A shown in
Figure 1d is similar to the PL spectrum in the thin film, which
excludes the possibility of the formation of electromers and
exciplexes in solid-state films.[5c] Thus, the yellow emission
from 2A can be attributed to the excimers. Photolumines-
cence images of the powders are shown in Figure S5 of the
Supporting Information.
We fabricated OLEDs with the structure indium tin oxide
(ITO)/MoO3 (1 nm)/NPB (60 nm)/FlAnt (20 nm)/TPBi
(40 nm)/LiF (1 nm)/Al (OLED-1; see Figure S6a) to inves-
tigate the electroluminescence properties of FlAnt. N,N’-
bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (NPB) and
1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi)
were chosen as the hole- and electron-transfer material,
respectively. The electroluminescence spectra are shown in
Figure 1d. The OLEDs based on FlAnt showed deep-blue
emission with CIE coordinates of (0.16, 0.11) and performed
well with a low turn-on voltage of 2.7 V, a high brightness of
2778 cdmÀ2 at 10 V, and an external quantum efficiency
(EQE) of 0.91% (see Figure S7a,d). Similarly, OLEDs based
on 2A adopted the structure ITO/HAT-CN (30 nm)/HTL1
(50 nm)/HTL2 (10 nm)/2A (20 nm)/ETL1:Liq (30 nm)/Liq
(1 nm)/Al (OLED-2; see Figure S6b). Dipyrazino[2,3-f:2’,3’-
h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN)was
chosen as hole-injection material, N,N,11-tri([1,1’-biphenyl]-
4-yl)-11H-benzo[a]carbazol-5-amine (HTL1) and 11-(4-
([1,1’-biphenyl]-4-yl)quinazolin-2-yl)-8-(9-phenyl-9H-carba-
zol-3-yl)-11H-benzo[a]carbazole (HTL2) were chosen as the
hole-transfer material, and 2-(4-(9,10-di(naphthalen-2-yl)an-
thracen-2-yl)phenyl)-1-phenyl-1H-benzo[d]imidazole:lithium
8-hydroxyquinolinolate (ETL1:Liq) was chosen as the elec-
tron-transfer material. The OLEDs based on 2A emitted
greenish-yellow light with the CIE coordinates (0.29, 0.49)
and operated with a low turn-on voltage of 2 V, a high
brightness of 5655 cdmÀ2 at 10 V, and an EQE of 0.46% (see
Figure S7b,e).
To correlate the optoelectronic properties with the
molecular-packing structure, we grew single crystals of
FlAnt and 2A by sublimation for X-ray crystal-structure
analysis.[8] The FlAnt molecule keeps a distance of 5.037–
6.049 ꢀ from the surrounding molecules, whereas the 2A
molecule keeps
a shorter distance of 4.676–5.958 ꢀ
(Figure 2). Thus, we assume that the two molecules with the
shortest distance of 4.676 ꢀ tend to form excimers: When one
of the molecules becomes an exciton, it combines with the
adjacent molecule to form an excimer for the sake of lowering
the energy, which reduces the blue emission and gives narrow-
band-gap light emission. The fluorescence quantum yield in
the thin films of 2A is about 13.9%, which is lower than that
The PL of the thermally deposited thin films of Mixture1/
9 showed much broader emission and CIE coordinates of
(0.28, 0.36). The OLED devices based on this simple mixture
with the OLED-1 device structure showed a low turn-on
voltage of 3 V, a high brightness of 2560 cdmÀ2 at 10 V, EQE
of 0.16% and exhibited white emission with a combination of
blue and broad greenish-yellow emissions (see Figure S7c,f).
When the driving voltage increased from 4 to 8.5 V, the green
part of the emission (515 nm) decreased considerably, and the
red part of the emission (610 nm) increased (Figure 1e), so
that the CIEx coordinate increased from 0.25 to 0.26, and the
CIEy coordinate decreased sharply from 0.35 to 0.31, towards
white emission. The CIE coordinates moved from (0.1, 0.29),
corresponding to bluish-green emission, to (0.33, 0.34),
corresponding to pure-white emission, when the driving
voltage was increased from 2.6 to 10 V (Figure 1 f). The
simple mixing of FlAnt and 2A and evaporation in a source
stove provides a new and straightforward method for the
production of white-emitting OLEDs.
The X-ray crystallographic results (Figure 2) showed that
the FlAnt crystal belongs to the P2(1)/c space group with
a monoclinic crystal structure and crystal parameters of a =
Figure 2. a) Molecular packing of a) the FlAnt single crystal and b) the
2A single crystal.
Angew. Chem. Int. Ed. 2016, 55, 1 – 7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!
Chen, Mengyun
