Iridium phosphors with peripheral triphenylamine encapsulation: Highly efficient solution-processed red electrophosphorescence

Article abstract of DOI:10.1039/c2cc17515k

The peripheral triphenylamine-encapsulated red-emitting iridium(iii) complexes have been designed and synthesized. External quantum efficiency over 15% has been realized in single-layer polymer light-emitting diodes, which is the highest ever reported for solution-processed red phosphorescence. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17515k

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Iridium phosphors with peripheral triphenylamine encapsulation: highly
efficient solution-processed red electrophosphorescencew
Minrong Zhu,a Yanhu Li,b Sujun Hu,b Chen’ge Li,a Chuluo Yang,*a Hongbin Wu,*b
Jingui Qina and Yong Caob
Received 1st December 2011, Accepted 17th January 2012
DOI: 10.1039/c2cc17515k
The peripheral triphenylamine-encapsulated red-emitting iridium(III)
complexes have been designed and synthesized. External quantum
efficiency over 15% has been realized in single-layer polymer
light-emitting diodes, which is the highest ever reported for
solution-processed red phosphorescence.
phosphorescent lifetimes and their applications as efficient
saturated red emitters in single-layer polymer light-emitting
diodes (PLEDs). The advantages of choosing triphenylamine
units as chelating ligands are threefold: in addition to the high-lying
HOMO (highest occupied molecular orbital) level (ca. ꢀ5.2 eV)
and sufficient triplet energy (ca. 2.9 eV) to be used as good antenna
for the charge transfer and/or energy transfer to the emissive
center, the arylamine group aids directly to get rid of the close
molecular packing in the solid state.10
Phosphorescent organic light-emitting diodes (PhOLEDs) unfurl
a bright future for the next generation flat-panel displays and
solid-state illumination sources.1,2 For full-color applications,
it is of significance to search for three primary color (red, green,
and blue) emitters of relatively equal stability, efficiency, and color
purity.3 Green PhOLEDs have achieved very high efficiency
during the past decades compared with their blue and red
counterparts.4 The development of efficient, saturated red emission
suffers from intrinsic obstacles because the luminescence quantum
yields tend to decrease with the increasing emission peak
wavelength according to the energy gap law.5
Scheme 1 outlines the overall route for preparation of the
phosphors. The ligand L1 was facilely prepared through the
Suzuki-coupling reaction of pinacol boronic ester 1 with commer-
cially available 2,4-dibromopyridine. The stepwise preparation of
L2 started with the synthesis of 2, which was then subjected to a
modified Buchwald cross-coupling methodology to generate 3
in 85% yield.11 Subsequent borylation reaction afforded the
desired pinacol boronic ester 4 in 70% yield. The Suzuki–
Miyaura cross-coupling of 4 with 2,4-dibromopyridine gave
the L2 in 80% yield. The final step in the preparation of
targeted phosphors was the complexation of all-aromatic
ligands with iridium source. Regarding the thermal stability of
phosphors and better site-isolating effect on the emission center,
forming homoleptic complexes other than the heteroleptic counter-
parts is the preferred strategy, which involves treating Ir(acac)3
(acac = 2,4-pentanedionate) with 3.2 equiv. of free ligand in
glycerol at refluxing temperature.7 The complexation for R1
was accomplished in nearly quantitative yield with a modified
procedure (ESIw). In addition, 40% of yield was obtained for
R2 bearing 18 triphenylamine units. The two phosphors exhibit
higher thermal-decomposition temperatures (Td, corresponding
to 5% weight loss) than those for Ir(ppy)3 (413 1C) and Ir(piq)3
(384 1C).9 Together with their high glass transition temperatures
(Tg, Table 1), the good thermal properties of R1 and R2 are
essential for the morphological stability of thin films, which is
highly desirable for the applications in electroluminescent
(EL) devices.
Moreover, most of the phosphorescent emitters have a long
lifetime, which aggravates the occurrence of triplet–triplet annihila-
tion at high currents.6 Among the phosphorescent dyes based on
third row transition metals, orthometalated iridium complexes
exhibit relatively short excited-state lifetimes.7 For instance, the
first red phosphor 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine
platinum(II) (PtOEP) shows an observed lifetime of 91 ms in a
polystyrene matrix, which can achieve a maximum external
quantum efficiency of 1.3%;8 while the saturated red-light
emitter fac-tris(1-phenylisoquinolinato-C2,N)iridium [Ir(piq)3]
with a radiative emission lifetime of 0.74 ms exhibits an
external quantum efficiency of 10.3%.9 Thus, a reasonably
short phosphorescent lifetime of the phosphors is critical for
the performances of the devices.
Herein, we report the design and synthesis of the peripheral
triphenylamine-encapsulated iridium complexes with short
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan 430072,
P. R. China. E-mail: clyang@whu.edu.cn
The absorption spectra of the red phosphors in toluene and
photoluminescence (PL) spectra both in toluene and films are
presented in Fig. 1. The intense bands below 350 nm can be
assigned to spin-allowed intraligand p–p* transitions and
triphenylamine-centered p–p* absorption, which are enhanced
in intensity with the increasing number of outer triphenyl-
amine units.10a The weak absorption shoulders in the range of
b Institute of Polymer Optoelectronic Materials and Devices, State
Key Laboratory of Luminescent Materials and Devices, South China
University of Technology, Guangzhou 510640, P. R. China.
E-mail: hbwu@scut.edu.cn
w Electronic supplementary information (ESI) available: Synthetic
procedures, cyclic voltammograms, TGA and DSC thermograms,
OLEDs fabrication and characterization. See DOI: 10.1039/
c2cc17515k
c
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Chem. Commun., 2012, 48, 2695–2697 2695

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Scheme 1 Synthetic routes of the complexes.
Table 1 The photophysical, thermal and electrochemical data of the complexes
Phosphors
labsa/nm (e ꢁ 10ꢀ5
307 (0.95)
424 (0.10)
304 (1.98)
420 (1.37)
)
lema/nm
lemb/nm
Fc (%)
tc/ms
Tg/Tmd/Td (1C)
HOMOe/LUMOe (eV)
4.75/2.45
R1
608
614
42 (28)
0.18 (0.25)
186/295/434
R2
608
613
43 (34)
0.35 (0.38)
226/—/550
5.08/2.58
a
b
Measured in toluene with a concentration of 10ꢀ5 M. Extinction coefficients at lmax of the absorption spectra are given in parentheses. Neat
film data measured with the excitation wavelength of 420 nm for the PL spectra. Solid state quantum yields using an integration sphere and the
c
lifetimes of the complexes under an excitation wavelength of 420 nm were measured at 298 K in PMMA at 10 wt%, values in neat films are in
d
e
parentheses. All PLQYs are ꢂ5%. Melting point (Tm) is not detected for R2. The HOMO energies were estimated from the onset of oxidation
potentials and the LUMO energies were deduced from the energy gap (Eg) and HOMO.
implies that the troublesome intermolecular interactions
between the emissive cores in the ground state could be
shielded effectively by the outer triphenylamine groups.
Solid state photoluminescence quantum yields (PLQYs) of the
complexes are measured using an integrating sphere apparatus
on the quartz plate and the data are listed in Table 1. When
doped into a poly(methyl methacrylate) (PMMA) matrix at 10%
by weight, the complexes are highly emissive in the pure red
spectral region. The PLQYs in doped films are around 42%,
while the values in neat films are still around 30% within the
experimental error. For comparison, we also measured the values
of Ir(piq)3 under the same conditions. In the polymer matrix, the
PLQY turns out to be around 46%, which is close to the value
reported by reference.12 In neat films, the PLQY is negligible due
Fig. 1 Electronic absorption spectra of the phosphors in toluene and
to the concentration quenching. This further indicates that the
triphenylamine-functionalized iridium complexes provide good
their emission spectra recorded both in toluene (open) and films
(solid). The emission scale is arbitrary and the spectra have been
encapsulation for emissive cores and effectively suppress the
normalized and offset for comparison.
detrimental intermolecular interaction. Phosphorescence life-
400–500 nm can be attributed to a spin-allowed singlet metal-
to-ligand charge transfer (1MLCT) band and spin-forbidden
3MLCT transitions. The two complexes exhibit almost identical
emission spectra both in solution and films, respectively. In
solution, R1 and R2 both display a structureless emission
centered at 608 nm. In films, the PL spectra show slight
bathochromic shifts (6 nm for R1 and 5 nm for R2), which
times of the complexes in the same polymer dispersions are
determined to be 0.18 and 0.35 ms for R1 and R2, respectively,
which are shorter than those reported for other well-known red
iridium-based phosphors such as Ir(piq)3 (0.74 ms) and
[Ir(btp)2(acac)] (btp = 2-(20-benzothienyl)pyridinato) (5.8 ms).9,13
Cyclic voltammetry (CV) was used to investigate the electro-
chemical behaviors of the compounds. Interestingly, the two
c
2696 Chem. Commun., 2012, 48, 2695–2697
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In conclusion, we have successfully synthesized two red-emitting
homoleptic Ir(III) complexes. By the peripheral triphenylamine
encapsulation of the emissive core, the phosphors formed thereby
are endowed with advantageous photophysical and thermal
properties compared with other known red phosphorescent
emitters. The facile preparation as well as excellent device
performance renders the triphenylamine-encapsulated iridium
phosphor very promising for use in full-color displays.
C. Yang thanks the National Science Fund for Distinguished
Young Scholars of China (No. 51125013), the National Basic
Research Program of China (973 Program 2009CB623602), the
National Natural Science Foundation of China (No. 90922020),
the Fundamental Research Funds for the Central Universities of
China; H. Wu thanks the National Natural Science Foundation
of China (61177022) for financial support.
Fig. 2 External quantum efficiency versus current density curves of
devices I–IV. Inset: EL spectra of the devices operated at 12 V.
complexes display reversible oxidation waves with slight differences
(Fig. S1, ESIw). The first oxidation at the lower potential for
R1 indicates that the oxidation occurs essentially on the iridium
complex at the core; whereas the oxidations at higher potentials
are related to the peripheral triphenylamine groups.14 However,
the emissive center of R2 becomes less electroactive since the
first oxidation process moves to a more positive potential. As
the outer dendritic shell enlarges, it seems that the inner
emissive core becomes less able to capture charges due to the
more congested cavum filled with dense dendrons.
Notes and references
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Due to its relatively short excited-state lifetime, we chose R1
as the dopant in the polymer matrix to evaluate its electro-
luminescent performance with the different doping concentra-
tions of 10 wt% (device I), 15 wt% (device II), 20 wt% (device III)
and 30 wt% (device IV). The devices were fabricated with
the simple configuration as follows: ITO/poly(ethylenedioxy-
thiophene) : poly(styrene sulfonic acid) (PEDOT : PSS, 50 nm)/
emitting layer (EML, 80 nm)/Ba (4 nm)/Al (150 nm). Poly
(N-vinylcarbazole) (PVK) was chosen as the host material
for its excellent film-forming property and hole-transport
characteristics, and 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole
(PBD) was included into the matrix to facilitate electron transport.
The EL spectra and external quantum efficiency versus current
density curves are depicted in Fig. 2, device performances are
tabulated in Table S1 (ESIw). All devices exhibit bright red
emission independent of the concentration of R1, indicating
that all EL emissions come from the triplet excited states of
R1. The peak external quantum efficiency (EQE) of device I
reaches 13.8%. Owing to the short phosphorescent lifetime of
the tailor-made complex and hence the reduced triplet–triplet
annihilation, the values are maintained as 11.8% and 10.9% at
the luminance of 100 and 1000 cd mꢀ2. In particular, the EL
efficiencies remain high as the doping ratio increases. Further-
more, device IV turns out to be brighter and more efficient
with a peak EQE of 15.3% with CIE coordinates of (0.61,
0.39), which is among the highest ever reported for solution-
processed red PhOLEDs.15 Such good performances at high
doping levels with simple device architecture can be attributed
to the structurally efficient protection of the emissive core.
¨
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2695–2697 2697