A novel solution-processible heterodinuclear AIIII/Ir III complex for host-dopant assembly OLEDs

Article abstract of DOI:10.1021/ic8002806

A discrete heterodinuclear AI^III/Ir^III complex shows bright-orange light emission when used as an active layer in host-dopant assembly organic light-emitting diodes based on a solution process.

Full text of DOI:10.1021/ic8002806

Inorg. Chem. 2008, 47, 6566-6568
A Novel Solution-Processible Heterodinuclear AlIII/IrIII Complex for
Host-Dopant Assembly OLEDs
Jung Oh Huh,† Min Hyung Lee,† Hyosook Jang,† Kyu Young Hwang,† Jong Soon Lee,‡ Se Hun Kim,‡
and Youngkyu Do*,†
School of Molecular Science BK-21 and Center for Molecular Design and Synthesis, Department
of Chemistry, KAIST, Daejeon 305-701, Korea, and R&D Center, Dongwoo FineChem Company,
Ltd., 1177 Pyeongtaek-Si, Gyeonggi-Do 451-764, Korea
Received February 13, 2008
A discrete heterodinuclear AlIII/IrIII complex shows bright-orange light
emission when used as an active layer in host-dopant assembly
organic light-emitting diodes based on a solution process.
complexes7 and host polymers can be overcome. In the present
work, such a challenge has been met by coupling two lumi-
nophores, one as a host and the other as a phosphorescent
emitter. As a result, an unprecedented heterodinuclear AlIII/IrIII
complex, [(3,5-tBu2)salenAl(µ-hpbpy)Ir(ppy)2]+ [PF6]- (1), that
can serve as a multifunctional light-emitting material for
host-dopant assembly OLEDs based on a solution process was
discovered as described below.
Luminescent metal complexes have been the focus of
numerous studies owing to their potential applications in various
areas, such as organic light-emitting diodes (OLEDs),1 chemical
sensors,2 and photovoltaic devices.3 In OLED applications in
particular, the development of new solution-processible phos-
phorescent small molecules having the multifunctionality of host
and emitter properties is extremely attractive. They can be used
as light-emitting materials for host-dopant assembly OLEDs4
based on a solution process, and the efficiency of small
molecules can be maximized. Furthermore, easy and cheap
fabrication of OLEDs can be achieved, and the drawbacks of
phase separation5 and excimer formation6 associated with the
spin-coating approach of blends of phosphorescent metal
* To whom correspondence should be addressed. E-mail: ykdo@
kaist.ac.kr.
†
KAIST.
‡
Dongwoo FineChem Company, Ltd.
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Recently, an efficient hole-blocking layer material 2 was
developed in the form of a salen-based pentacoordinated AlIII
complex for a phosphorescent OLED.8 The HOMO-LUMO
energy gap was observed to be 2.9 eV for 2 independent of
the type of aryloxy ancillary ligand. Because this energy band
gap appears to be suitable for a host, 4′-(4-hydroxyphenyl)-
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(11) For 5, the observation of a distinct absorption band at 420 nm and a
slight change in the position of an emission maximum are somewhat
different from those reported previously.9 This may be involved with
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6566 Inorganic Chemistry, Vol. 47, No. 15, 2008
10.1021/ic8002806 CCC: $40.75  2008 American Chemical Society
Published on Web 06/28/2008

COMMUNICATION
Scheme 1. Synthesis of 1 and 4a
a
Reagents and conditions: (i) 5, MeCN, 90 °C, 57%; (ii) 3, toluene,
120 °C, 83%.
Figure 2. UV-vis absorption of 1 (solid line), 4 (dashed line), and 5 (dash-
dotted line) and emission spectrum of 1 (open circles) in acetonitrile (1.0
× 10-5 M; λex ) 360 nm) at room temperature. Inset: emission spectrum
of 1 in the solid state.
The room temperature PL spectrum of 1 irradiated at 360
nm (Figure 2) displays emission maxima at 475 and 596 nm,
which correspond exactly to those of 4 and 5, respectively
[Figure S2a in the Supporting Information (SI)].11 Measurement
of the lifetime of the emissions at 475 and 596 nm for 1 as
<10 and 196 ns, respectively, further reveals that the former is
assignable to the Al-centered fluorescence and the latter to the
Ir-centered phosphorescence. The PL spectra of 1, 4, and 5 with
420-nm excitation (Figures S2b and S3 in the SI) also show
invariant appearance of the emission maxima position, indicat-
ing the presence of little electronic effect of the Al- and Ir-
centered luminophores on the energy band gap of each center,
which results in independent emission in the solution. This
feature might be a consequence of the unique feature of the
nonplanar molecular structure of 1. It is also consistent with
the observation8 that the energy band gap of mononuclear salen-
based aryloxyaluminum complexes is independent of the type
of aryloxy ancillary ligand.
A comparison of the emission intensities for 1, 4, and 5 at
the same concentration reveals decreased intensity of the 475-
nm band of 1 compared to 4 and increased intensity of the 596-
nm band of 1 compared to 5 (Figure S4 in the SI). This change
in intensity suggests the involvement of energy transfer from
the Al-centered luminophore to the Ir-centered luminophore in
1. In order to clarify the energy-transfer pathway, low-
temperature PL studies were carried out on 1 and an equimolar
mixture of 4 and 5 at 77 K with excitation at 360 nm. The
spectra reveal the blue-shifted emission maxima13 and the
relative intensity decrease at 441 nm and increase at 539 nm
of the emission peaks from the Al- and Ir-centered luminophores
of 1, respectively, compared to the emission intensities of the
mixture (Figure S5 in the SI). This indicates the occurrence of
intramolecular energy transfer14 from the Al-centered lumino-
phore to the Ir-centered luminophore in a rigid matrix. On the
other hand, the solid-state PL spectrum of 1 only shows a strong
Ir-centered phosphorescent emission (τ ) 313 ns) at 583 nm
with the quenching of the excited-state lifetime of the Al-
centered emission (inset of Figure 2), thus implying that energy
transfer takes place completely in the solid state probably via
both intra- and intermolecular pathways.4b
-
Figure 1. ORTEP diagram of 1 (50% ellipsoids). The counterion (PF6
)
and H atoms are omitted for clarity.
6′-phenyl-2,2′-bipyridine (Hhpbpy, 3)9 was chosen as an
aryloxy ancillary ligand to develop a potential host center
(3,5-tBu2)salenAl(hpbpy) (4). There are two additional merits
9
of 3: its IrIII complex [Ir(ppy)2(hpbpy)]+[PF6 ] (5),
a
-
potential phosphorescent emitter, is known, and its rigid
framework may facilitate efficient energy transfer from the
Al host center to the Ir emitter.
Thus, new complexes 1 and 4 were synthesized according
to the procedures outlined in Scheme 1.8
While H and 13C NMR, IR spectroscopy, and elemental
1
analysis results confirm the identity of 1, a single-crystal X-ray
diffraction study clearly establishes the dinuclear nature of 1,
in which the Al and Ir centers are linked via hpbpy, a
deprotonated form of 3, to form a [(3,5-tBu2)salenAl(µ-
hpbpy)Ir(ppy)2]+ cation (Figure 1). Further structural analysis
reveals that the Al-salen moiety is bent away from the distorted
octahedral Ir center with an Al-O3-C42 angle of 131.7(4)°
and retention of the rigid arrangement of both moieties.
The UV-vis absorption and the photoluminescence (PL)
spectra of the heterodinuclear complex 1 and its closely related
mononuclear complexes 4 and 5 were recorded in a degassed
solution of acetonitrile at room temperature (Figure 2). While
4 shows an absorption band centered at 360 nm due to the Al-
salen moiety,10 the absorption spectrum of 5 exhibits lower
energy bands at 420 nm assignable to metal-to-ligand charge
transfer (MLCT)11 as well as strong bands from ligand-centered
(LC) π-π* transitions in a region ranging from 280 to 320
nm.9,12 On the other hand, the high-energy absorption bands
of 1 were slightly broadened by possible overlapping of LC
transitions originating from 4 and 5. In contrast to the distinct
transition in 5, the MLCT transition of 1 appears as a broad
tail toward the visible region (ca. 500 nm).
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Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2001, 40, 1704. (b) Tamayo, A. B.; Alleyne, B. D.;
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Inorganic Chemistry, Vol. 47, No. 15, 2008 6567

COMMUNICATION
Figure 4. J-V-L characteristics of device OLED 1 (open squares, J-V;
closed circles, J-L).
OLED 1 relative to those of OLED 2 + 5, OLED PVK:1
(over 500%), and OLED PVK:5 (over 300%). This might
be an indication that device OLED 1 does not suffer from
the typical problems of phase separation and excimer
formation derived from the host-guest system,15 while
OLED 2 + 5, OLED PVK:1, and OLED PVK:5 are
influenced by the intrinsic properties of individual compo-
nents such as the charged metal complex.16
Another remarkable feature of OLED 1 is its low turn-on
voltage (VT) of 3.4 V. This is likely the consequence of the
good charge-transport property of OLED 1 arising from the
ionic nature of 1: the counterions PF6- of 1 are redistributed
under electrical excitation, leading to the formation of a high
electric field at the electrodes, and thereby the electronic charge
injection from the electrodes to the metal complex could be
facilitated.17
In summary, molecular coupling of a salen-based aluminum
complex as a host and a phenylpyridine-based ionic Ir complex
as a phosphorescent emitter into a discrete heterodinuclear
molecular complex 1 led to unprecedented multifunctional
OLED materials for host-dopant assembly OLED based on a
solution process. Bright-orange light emission along with stable
color purity, low turn-on voltage, and good device performance
were thereby obtained. The extension of a family of 1 and the
search for new solution-processible molecular multifunctional
OLED materials are in progress.
Figure 3. EL spectra of device OLED 1: 10 mA cm-2 (blue line), 20 mA
cm-2 (red line), 50 mA cm-2 (black line), 100 mA cm-2 (gray line). Insets:
device configuration (left) and a photograph of the working device (right).
Prior to the fabrication of an OLED based on 1 via the
solution process, the quality of a spin-coated thin film (40
nm) of 1 on a poly(styrenesulfonate)-doped poly(3,4-ethyl-
enedioxythiophene (PEDOT) layer was explored by atomic
force microscopy (AFM), revealing a film surface that is free
of pinholes or aggregates (Figure S6 in the SI). The root-
mean-square roughness of 0.542 nm further indicates the
formation of a uniform film surface. The good film-forming
ability of 1 can be ascribed to its high solubility in common
organic solvents as well as its poor crystallinity. On the basis
of this finding, four electroluminescence (EL) devices using
1, 2 + 5, PVK:1, and PVK:5 [PVK:1 or PVK:5 ) 1 or 5
blended with poly(N-vinylcarbazole)] as concomitant host-
emitting layer materials were constructed. The OLED
configuration is as follows: ITO/PEDOT (40 nm)/1 or 2 +
5 or PVK:1 or PVK:5 (40 nm)/BCP (20 nm)/Alq3 (20 nm)/
LiF (1 nm)/Al (100 nm) (BCP ) bathocuproine). For OLEDs
1 and 2 + 5, the emitting layer was directly spin-cast from
a 1,2-dichloroethane solution of 1 or an acetonitrile solution
of a blend of 2 and 5 onto the spin-coated PEDOT layer.
The EL spectra of 1 with the common maximum at 600
nm, shown in Figure 3, are consistent with its solid-state PL
spectrum, which has characteristics of phosphorescence
contributed solely from the Ir-based luminophore. This is a
clear indication that the heterodinuclear system 1 allows
complete energy transfer from the Al-based luminophore to
the Ir-based luminophore under electrical excitation. In
addition, the observed Commission Internationale de
L’Eclairage (CIE) coordinates of (0.54, 0.46) for OLED 1
remain nearly unchanged upon a change of the current
density.Figure4showsthecurrentdensity-voltage-luminance
(J-V-L) characteristics of the device. Device OLED 1 has
a maximum brightness (Lmax) of 1002 cd m-2 at 10.8 V and
a luminance efficiency (ηL) of 1.8 cd A-1, external quantum
efficiency (EQE) of 0.94%, and a luminous efficiency (LPW)
of 0.69 lm W-1 at 8.2 V. These results were compared with
the devices OLED 2 + 5, OLED PVK:1, and OLED PVK:
5, revealing an overwhelmingly superior performance of
Acknowledgment. We acknowledge the Korea Research
Foundation (Grant KRF-2006-312-C00578), the BK21 project,
and the PAL (Grant 2007-1041-15). We also thank Prof. Sang
Kyu Kim and Dr. Sun Jong Baek for the lifetime measurements.
Supporting Information Available: Experimental details, UV-vis
and PL spectra, photophysical data, AFM images, cyclic voltam-
mograms, and J-V-L, thermogravimetric analysis, and differential
scanning calorimetry curves. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC8002806
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6568 Inorganic Chemistry, Vol. 47, No. 15, 2008