Highly efficient blue-green and white light-emitting electrochemical cells based on a cationic iridium complex with a bulky side group

Article abstract of DOI:10.1021/cm100993j

Blue-green-emitting cationic iridium complex with high luminescent efficiencies in both solutions and solid-states are essential for high-performance white light-emitting electrochemical cells (LECs). We report here an efficient blue-green-emitting cation

Full text of DOI:10.1021/cm100993j

Chem. Mater. 2010, 22, 3535–3542 3535
DOI:10.1021/cm100993j
Highly Efficient Blue-Green and White Light-Emitting Electrochemical
Cells Based on a Cationic Iridium Complex with a Bulky Side Group
Lei He, Lian Duan,* Juan Qiao, Guifang Dong, Liduo Wang, and Yong Qiu*
Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
Received March 11, 2010
Blue-green-emitting cationic iridium complex with high luminescent efficiencies in both solutions and
solid-states are essential for high-performance white light-emitting electrochemical cells (LECs). We
report here an efficient blue-green-emitting cationic iridium complex [Ir(dfppz)2(tp-pyim)]PF6, using
1-(2,4-difluorophenyl)-1H-pyrazole (dfppz) as the cyclometalated ligand and 2-(1-(4-tritylphenyl)-1H-
imidazol-2-yl)pyridine (tp-pyim) as the ancillary ligand. [Ir(dfppz)2(tp-pyim)]PF6 emits efficient blue-
green light with a luminescent quantum yield of 0.54 in CH3CN solution. Because of the sterically bulky
group 4-tritylphenyl that is attached to the ancillary ligand, the intermolecular interaction and excited-
state self-quenching of [Ir(dfppz)2(tp-pyim)]PF6 in solid states is significantly suppressed. Theoretical
calculations reveal that the emission from [Ir(dfppz)2(tp-pyim)]PF6 has both metal-to-ligand charge-
3
transfer and ligand-centered π-π* character. LECs based on [Ir(dfppz)2(tp-pyim)]PF6 show highly
efficient blue-green electroluminescence with peak current efficiency, external quantum efficiency, and
power efficiency of 18.3 cd A-1, 7.6%, and 18.0 lm W-1, respectively. White LECs based on [Ir(dfppz)2-
(tp-pyim)]PF6 give warm-white light, with Commission Internationale de L’Eclairage coordinates of
(0.37, 0.41), a color-rendering index up to 80, and a peak power efficiency of 11.2 lm W-1
.
Introduction
and tunable light emission color.2,3,8-22 Using carefully
designed cationic iridium complexes, highly efficient
green,10,12,14,18,21 yellow,8,15,21,22 orange,13,14,20,21 and red10,21
LECs have been achieved, with the external quantum effi-
ciency (EQE) and power efficiency for green LECs app-
roaching 15% and 40 lm W-1, respectively.18 However,
the EQE and power efficiency of blue-green LECs, with
In recent years, light-emitting electrochemical cells (LECs)
based on ionic transition metal complexes (iTMCs) have
spurred much interest because of their great potential in
solid-state lighting applications.1-3 In addition to the
notable features of conventional polymer-based LECs
(single layer, air-stable cathodes, solution process, etc.),4
iTMC-based LECs have high electroluminescent (EL)
efficiency because of the phosphorescent nature of iTMCs.
For iTMC-based LECs, high brightness and efficiency
can be achieved at relatively low operating voltages as a
result of efficient carrier injection and balanced carrier
recombination in LECs.4-7
(9) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal,
R. A.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712.
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Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 8723.
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€
De Angelis, F.; Fantacci, S.; Gratzel, M. Inorg. Chem. 2006, 45,
9245.
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€
Among all iTMCs, ionic iridium complexes are most
widely used because of their high luminescent efficiency
Chem. Mater. 2006, 18, 2778.
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261118.
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Wong, K.; Wu, C. Adv. Funct. Mater. 2007, 17, 1019.
(15) Bolink, H. J.; Cappelli, L.; Cheylan, S.; Coronado, E.; Costa,
*Corresponding author. Tel: (þ86)10-6277-1964 (Y.Q.); (þ86)10-6277-9988
(L.D.). E-mail: qiuy@mail.tsinghua.edu.cn (Y.Q); duanl@mail.tsinghua.edu.cn
(L.D).
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R. D.; Lardies, N.; Nazeeruddin, M. K.; Ortı, E. J. Mater. Chem.
´
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(1) Slinker, J. D.; Bernards, D.; Houston, P. L.; Abruna, H. D.;
2007, 17, 5032.
(16) Su, H.; Chen, H.; Fang, F.; Liu, C.; Wu, C.; Wong, K.; Liu, Y.;
Peng, S. J. Am. Chem. Soc. 2008, 130, 3414.
(17) He, L.; Duan, L.; Qiao, J.; Wang, R.; Wei, P.; Wang, L.; Qiu, Y.
Adv. Funct. Mater. 2008, 18, 2123.
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(18) Bolink, H. J.; Coronado, E.; Costa, R. D.; Lardies, N.; Ortı, E.
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Inorg. Chem. 2008, 47, 9149.
(19) Colman, E. Z.; Slinker, J. D.; Parker, J. B.; Malliaras, G. G.;
Bernhard, S. Chem. Mater. 2008, 20, 388.
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R.; Batsanov, A. S.; Bryce, M. R.; Monkman, A. P. Adv. Funct.
Mater. 2009, 19, 2038.
(21) He, L.; Qiao, J.; Duan, L.; Dong, G. F.; Zhang, D. Q.; Wang, L. D.;
Qiu, Y. Adv. Funct. Mater. 2009, 19, 2950.
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(22) Costa, R. D.; Ortı, E.; Bolink, H. J.; Graber, S.; Schaffner, S.;
Bernhard, S.; Malliaras, G. G. Chem. Commun. 2003, 2392.
(2) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker, J. B.; Bernhard,
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S.; Abruna, H. D.; Malliaras, G. G. J. Mater. Chem. 2007, 17, 2976.
(3) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S. Adv.
Mater. 2009, 21, 4418.
(4) Pei, Q. B.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science. 1995,
269, 1086.
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H. D.; Marohn, J. A.; Malliaras, G. G. Nat. Mater. 2007, 6, 894.
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r
2010 American Chemical Society
Published on Web 05/10/2010
pubs.acs.org/cm

3536 Chem. Mater., Vol. 22, No. 11, 2010
Scheme 1. Synthesis of the Ligands and [Ir(dfppz)2(tp-pyim)]PF6
He et al.
records of 4.6% and 11 lm W-1, respectively, still lag behind
those of other colors.10 To achieve high-performance
white LECs, efficient blue-green-emitting ionic iridium
complexes and corresponding blue-green LECs are highly
desired.16,21 During the operation of iTMC-based LECs,
severe excited-state self-quenching always happens because
the active layers of LECs comprise neat complexes that are
closely packed.15,17 To resolve the excited-state-quenching
problem and improve the EL efficiency of LECs, we should
introduce enhanced steric hindrance or bulky side groups
into the complexes.14,20 This technique has been proven
very effective to obtain highly efficient green and orange
LECs14,20 but has never been attempted for blue-green
LECs.
In this paper, we report highly efficient blue-green and
white LECs based on a cationic iridium complex with a bulky
side group, namely [Ir(dfppz)2(tp-pyim)]PF6 (Scheme 1).
1-(2,4-Difluorophenyl)-1H-pyrazole (dfppz), instead of
2-(2,4-difluorophenyl)pyridine (dfppy), was used as the
cyclometalated ligand because dfppz can blue-shift the
emission of the complex via stabilizing the HOMO (highest
occupied molecular orbitals) of the complex.10,16 2-(1-(4-
Tritylphenyl)-1H-imidazol-2-yl)pyridine (tp-pyim) was used
as the ancillary ligand because this pyim-type ligand
(pyim is pyridinylimidazole) is more effective than the
commonly used bpy-type ligand (bpy is 2,20-bipyridine) in
terms of blue-shifting the emission of the complex.21
More importantly, the pyim-type ligand affords an active
N atom in the imidazole ring to which functional groups
can be conveniently anchored. The sterically bulky group
4-tritylphenyl (TP) is thus anchored to the imidazole ring
and shields the luminescent iridium core in an umbrella
shape, suppressing the excited-state self-quenching in
solid states significantly and rendering the complex an
excellent film-forming property. LECs based on [Ir(dfppz)2-
(tp-pyim)]PF6 showed highly efficient blue-green electro-
luminescence with Commission Internationale de L’Eclairage
(CIE) coordinates of (0.22, 0.41). At 3.2 V, the peak cur-
rent efficiency, EQE and power efficiency of the blue-
green LEC reached 18.3 cd A-1, 7.6%, and 18.0 lm W-1
,
respectively. By doping a small amount of red-emitting
ionic complex into the blue-green LEC, white LECs were
fabricated, with CIE coordinates of (0.37, 0.41), a color-
rendering index (CRI) up to 80 and a peak power effi-
ciency of 11.2 lm W-1. The efficiencies of the blue-green
and white LECs are the highest among blue-green9-11,17,21
and white16,21 iTMC-based LECs reported so far.
Experimental Details
General Experiments. All reactants and solvents, unless other-
wise stated, were purchased from commercial sources and used
as received. Absorption and photoluminescent (PL) spectra were
recorded with a UV-vis spectrophotometer (Agilent 8453) and
a fluorospectrophotometer (Jobin Yvon, FluoroMax-3), respec-
tively. The PL decay lifetimes were measured on a transient
spectrofluorimeter (Edinburgh Instruments, FLSP920) with time-
correlated single-photon counting technique. The photolumine-
scent quantum yields (PLQYs) (excited at 350 nm) in CH3CN

Article
Chem. Mater., Vol. 22, No. 11, 2010 3537
solutions were measured with quinine sulfate (Φp = 0.545 in 1 M
H2SO4) as the standard.23 The solutions were degassed by three
freeze-pump-thaw circles before measurements. The PLQYs
in thin films were measured with an integrating sphere in a fluoro-
spectrophotometer (Jobin Yvon, FluoroMax-3) according to a
reported procedure.24 The surface morphology of thin films was
characterized with Atomic Force Microscopy (AFM, SPA-400).
Cyclic voltammetry was performed on a Princeton Applied
Research potentiostat/galvanostat model 283 voltammetric ana-
lyzer in CH3CN solution (1ꢀ10-3 M) at a scan rate of 100 mV/s,
with a platinum plate as the working electrode, a silver wire as
the reference electrode and a platinum wire as the counter elec-
trode. The supporting electrolyte was tetrabutylammonium
hexafluorophosphate (0.1 M) and ferrocene (Fc) was used as
the internal standard. The solutions were degassed with argon
before measurements.
by column chromatography on silica gel (200-300 mesh) with
CH2Cl2/acetone (30:1) as the eluent. The product was then recrys-
tallized from dichloromethane/hexane, yielding a light-yellow pow-
der (0.54 g, 0.46 mmol). Yield: 68%. 1H NMR (dimethylsulfoxide-
d6, 600 MHz, δ[ppm]): 8.63(d, J=2.7 Hz, 2H), 8.03-7.92(m, 3H),
7.68(s, 2H), 7.54-7.47(m, 4H), 7.41-7.35(m, 7H), 7.31-7.23(m,
9H), 7.14-7.06(m, 2H), 6.91(d, J = 1.38 Hz, 1H), 6.83-6.79(m,
2H), 6.75(d, J=8.22 Hz, 1H), 5.67(td, J=8.25 and 2.04 Hz, 2H).
ESI-MS [m/z]: 1014.3 [M - PF6]þ. Anal. Found: C, 52.68; H, 3.38;
N, 8.35. Anal. Calcd. for C51H35F10N7PIr: C, 52.85; H, 3.04; N,
8.46.
Quantum Chemical Calculations. Density functional theory
(DFT) and time-dependent DFT (TD-DFT) at the B3LYP level
were adopted for calculations on the ground and excited elec-
tronic states of the complex.28,29 “Double-ξ” quality basis sets
were employed for the C, H, N and F (6-31G*) and the Ir
(LANL2DZ). An effective core potential replaces the inner core
electrons of Ir leaving the outer core (5s)2(5p)6 electrons and the
(5d)6 valence electrons of Ir(III). The geometry of the singlet
ground state (S0) was fully optimized with a C1 symmetry con-
straint. All calculations were carried on with Gaussian 03 soft-
ware package using a spin-restricted formalism.30
Material Synthesis. 1-(2,4-Difluorophenyl)-1H-pyrazole (dfppz).
Dfppz was synthesized by a slight modification of a reported proce-
dure.25 1,1,3,3-Tetramethoxy propane (5.5 g, 33 mmol) was dissol-
ved in 95% ethanol (50 mL) and concentrated aqueous HCl solution
(5 mL) was then added. Under stirring, (2,4-difluorophenyl) hydra-
zine hydrochloride (5.4 g, 30 mmol) dissolved in 95% ethanol
(100 mL) was slowly added to the solution. The solution was
refluxed for 24 h. After neutralization with aqueous Na2CO3 solu-
tions, the solution was concentrated under vacuum and the resi-
due was purified by column chromatography on silica gel (200-
300 mesh) with petroleum ether/ethyl acetate (20:1) as the eluent,
yielding a clear oil liquid (4.3 g, 24 mmol). Yield: 80%. 1H NMR
(chloroform-d6, 600 MHz, δ[ppm]): 7.92(t, J=2.8 Hz, 1H), 7.88-
7.82(m, 1H), 7.73(s, 1H), 7.02-6.95(m, 2H), 6.47(t, J = 2.1 Hz,
1H). ESI-MS [m/z]: 181.1 [M þ H]þ.
Fabrication of LECs. Indium-tin-oxide (ITO) substrates
with sheet resistance of 15 Ω/0 were sufficiently cleaned and
treated with UV-zone before use. The poly(3,4-ethylenedioxy-
thiophene: poly(styrene sulfonate) (PEDOT: PSS) layer was
spin-coated onto the ITO substrate in air and baked at 200 °C
for 10 min. The light-emitting layer was spin coated onto the
PEDOT-coated substrate in a nitrogen-filled glovebox from
CH3CN solutions and baked at 70 °C for 30 min. The substrate
was then transferred into a metal-evaporating chamber, where
ꢀ
the aluminum cathode was evaporated at 8-10 A/s. The resul-
2-(1-(4-Tritylphenyl)-1H-imidazol-2-yl)pyridine (tp-pyim).
2-(1H-imidazol-2-yl)pyridine and 4-trityl-iodobenzene were pre-
pared by reported procedures.26,27 2-(1H-Imidazol-2-yl)pyridine
(0.29 g, 2 mmol) and 4-trityl-iodobenzene (1.11 g, 2.5 mmol),
1,10-phenanthroline (0.24 g, 1.2 mmol), copper(I) iodide (0.11 g,
0.6 mmol), and cesium carbonate (1.63 g, 5 mmol) were dissol-
ved in dimethylformamide (40 mL). The mixture was refluxed in
the dark for 20 h under an argon atmosphere and then cooled
to room temperature. The solvent was removed under vacuum.
The residue was extracted with dichloromethane (150 mL) and
purified by column chromatography on silica gel (200-300 mesh)
with dichloromethane/methanol (50:1) as the eluent, yielding a
white solid (0.35 g, 0.76 mmol). Yield: 38%. 1H NMR (acetone-
d6, 600 MHz, δ[ppm]): 8.25(d, J = 4.08 Hz, 1H), 8.05(d, J =
7.56 Hz, 1H), 7.81(td, J=7.56 and 2.04 Hz, 1H), 7.38-7.31(m,
7H), 7.27-7.17(m, 15H). ESI-MS [m/z]: 464.5 [MþH]þ.
[Ir(dfppz)2(tp-pyim)]PF6. The dichloro-bridged diiridium com-
plex [Ir(dfppz)2Cl]2 (0.40 g, 0.34 mmol) and tp-pyim (0.33 g, 0.7
mmol) were dissolved in 1,2-ethanediol (30 mL). The mixture was
refluxed at 150 °C for 15 h under an argon atmosphere and then
cooled to room temperature. To the reaction mixture was added
50 mL methanol. NH4PF6 (1.8 g, 11 mmol) in deionized water
(50 mL) was then slowly added into the solution under stirring,
resulting in a light-yellow suspension. The suspension was filtrated
and the resultant precipitate was washed with deionized water and
dried under vacuum at 70 °C for 12 h. The crude product was purified
tant device was transferred back to the glovebox and carefully
encapsulated. The device was characterized with a Keithley 4200
semiconductor characterization system in ambient conditions.
The EL spectrum was collected with a Photo Research PR705
spectrophotometer.
Results and Discussion
Synthesis and Characterization. Scheme 1 depicts the
synthetic route of the ligands and [Ir(dfppz)2(tp-pyim)]PF6.
Dfppz was synthesized by the condensation of (2,4-difluoro-
phenyl)hydrazineand1,1,3,3-tetramethoxypropane. Tp-pyim
was synthesized by copper(I)-catalyzed C-N coupling
reactions between 2-(1H-imidazol-2-yl)pyridine (Hpyim)
and 4-trityl-iodobenzene. Because of the active N-H
bond in the imidazole, the bulky TP group can be easily
attached to the imidazole ring. [Ir(dfppz)2(tp-pyim)]PF6
was synthesized, with a good yield of ca. 70%, by reacting
dimeric iridium(III) intermediates [Ir(dfppz)2Cl]2 with
tp-pyim, followed by a counterion exchange reaction from
Cl- to PF6-. All the ligands and the complex were fully
characterized by ESI (electron spray ionization) mass spec-
1
troscopy, H NMR, and elemental analysis (see Experi-
mental Section).
Photophysical Properties. Figure 1 shows the absorp-
tion and PL spectra of [Ir(dfppz)2(tp-pyim)]PF6 in CH3CN
(23) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229.
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3538 Chem. Mater., Vol. 22, No. 11, 2010
He et al.
the emissive excited states contain much LC 3π-π*
character. The calculated nonradiative decay rate (Knr)
(1.16 ꢀ 105 s-1) of [Ir(dfppz)2(tp-pyim)]PF6 in CH3CN
solution is much lower than those of other blue-green-
emitting cationic iridium complexes (in the range of 5-7ꢀ
105 s-1).17,21 This is presumably due to the shielding effect
of the nonpolar bulky TP group, which protects the
luminescent core from being interacted with polar solvent
molecules, thus suppressing the nonradiative deactiva-
tion of the excited states in solutions.14,20
Table 2 summarizes the PL characteristics of [Ir(dfppz)2-
(tp-pyim)]PF6 in thin films and, as comparisons, those of
[Ir(dfppy)2(ph-pyim)]PF6 are also shown. [Ir(dfppz)2(tp-
pyim)]PF6 exhibits vibronic emission spectra in both the
5% (by weight) doped poly(methyl methacrylate) (PMMA)
film and neat film (see Figure S2 in the Supporting Infor-
mation), which indicates that the emissive excited states
of [Ir(dfppz)2(tp-pyim)]PF6, with much LC 3π-π* char-
acter, remain the same in the 5% doped PMMA film and
neat film. In contrast, [Ir(dfppy)2(ph-pyim)]PF6 exhibits
a vibronic emission spectrum in the 5% doped PMMA
film but a featureless and largely red-shifted emission
spectrum in neat film (see Figure S3 in the Supporting
Information), which indicates that the emissive exci-
ted states of [Ir(dfppy)2(ph-pyim)]PF6 change from LC
3π-π* to 3MLCT on going from the 5% doped PMMA
film to neat film. The change of the emissive excited states
should be due to the change of the local environment of
the complex, as previously observed for a similar catio-
nic iridium complex.15 For Ir(dfppz)2(tp-pyim)]PF6, the
emissive excited states are unaltered and the emission
spectra are less red-shifted on going from the 5% doped
PMMA film to neat film (see Figure S2 in the Supporting
Information), which indicates that the bulky TP group
helps to separate the molecules from each other and
maintain the color purity of the blue-green emission in
concentrated films.
The PL decays of [Ir(dfppz)2(tp-pyim)]PF6 in doped or
neat films all exhibit monoexponential characteristics.
In the 5% doped PMMA film, the PL decay lifetime of
[Ir(dfppz)2(tp-pyim)]PF6 is 4.42 μs, whereas in the neat
film, the PL decay lifetime is 3.04 μs, which does not de-
crease significantly as compared to that in the 5% doped
PMMA film. In the 5% doped PMMA film, the PLQY of
[Ir(dfppz)2(tp-pyim)]PF6 is 0.79, whereas in the neat film,
the PLQY remains as high as 0.54. In contrast, the PL
decay of [Ir(dfppy)2(ph-pyim)]PF6 in neat film exhibits a
biexponential characteristic [1.06 μs (49%), 0.40 μs (51%)]
due to the strong intermolecular interactions. Moreover,
the PLQY of [Ir(dfppy)2(ph-pyim)]PF6 in neat film re-
mains only 0.14, decreasing sharply as compared to that
(0.72) in the 5% doped PMMA film. The largely retained
PL decay lifetime and PLQY of [Ir(dfppz)2(tp-pyim)]PF6
in neat film indicates that the sterically bulky TP group
works effectively in suppressing the intermolecular inter-
actions and excited-state self-quenching of the complex in
solid states.14,20 In the film of [Ir(dfppz)2(tp-pyim)]PF6
mixed with an ionic liquid 1-butyl-3-methylimidazolium
hexafluorophosphate (BMIMPF6), the PL decay lifetime
Figure 1. Absorption and PL spectra of [Ir(dfppz)2(tp-pyim)]PF6 in CH3-
CN solution (1 ꢀ 10-5 M) and neat film.
solution and as neat film. Those in the CH2Cl2 solution
are shown in the Supporting Information (Figure S1).
Detailed photophysical characteristics of [Ir(dfppz)2-
(tp-pyim)]PF6 in CH3CN solution are summarized in
Table 1. The intense absorption in the ultraviolet region
below 350 nm belongs to 1π-π* transitions of the ligands
while the relatively weak absorption bands extending
1
to the visible region correspond to MLCT (metal-to-
ligand charge-transfer), 1LLCT (ligand-to-ligand charge-
transfer), 3MLCT, 3LLCT, and ligand-centered (LC)
3π-π* transitions. The spin-forbidden 3MLCT, 3LLCT,
and LC 3π-π* transitions gain much intensity by mixing
1
with the higher-lying spin-allowed MLCT transitions
because of the strong spin-orbit coupling induced by
the heavy iridium atom.31
In CH3CN solution, [Ir(dfppz)2(tp-pyim)]PF6 emits
efficient blue-green light with a peak emission and a
shoulder emission at 494 and 472 nm, respectively. The
vibronic structure in the emission spectra indicates that
the emissive excited states contain considerable LC
3π-π* character.32-34 Upon cooling the solution to 77 K,
the emission spectrum becomes much more structured
and exhibits a considerable blue-shift of 15 nm, which indi-
cates that the emissive excited states contain considerable
3
3
3LLCT or MLCT character as well as the LC π-π*
character.10,34 The photophysical properties of [Ir(dfppz)2-
(tp-pyim)]PF6 contrast with those of the blue-green-emitting
complex [Ir(dfppy)2(ph-pyim)]PF6 (complex 2 in refer-
ence 21), which uses dfppy as the cyclometalated ligands
and 2-(1-phenyl-1H-imidazol-2-yl)pyridine (ph-pyim) as
the ancillary ligand and contains no bulky side groups.21
In CH3CN solution, [Ir(dfppy)2(ph-pyim)]PF6 exhibits a
featureless emission spectrum with a sole peak at 489 nm
and the emission has been assigned to 3MLCT or 3LLCT
states, with little LC 3π-π* contribution.21
In diluted CH3CN solution, [Ir(dfppz)2(tp-pyim)]PF6
has a high PLQY of 0.54 and a PL decay lifetime of 3.99 μs.
The relatively long PL decay lifetime further confirms that
(31) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem.
Soc. 1984, 106, 6647.
€
(32) Colombo, M. G.; Gudel, H. U. Inorg. Chem. 1993, 32, 3081.
€
(33) Colombo, M. G.; Hauser, A.; Gudel, H. U. Inorg. Chem. 1993, 32,
3088.
(34) Lowry, M. S.; Hudson, W. R.; Pascal, R. A., Jr.; Bernhard, S.
J. Am. Chem. Soc. 2004, 126, 14129.

Article
Chem. Mater., Vol. 22, No. 11, 2010 3539
Table 1. Photophysical and Electrochemical Characteristics of [Ir(dfppz)2(tp-pyim)]PF6 in CH3CN Solutions
room-temperature emission
electrochemical datac
Eox1/2 (V) Ered1/2 (V)
absorptiona λ (nm)
emission at
77 K λ (nm)
(ε (ꢀ 104 M-1 cm-1))
λ (nm)
Φp (τ (μs)) b
Kr (ꢀ 105 s-1
)
Knr (ꢀ 105 s-1
)
243 (4.18), 304 (1.98)
325 (1.11)
472
494
0.54 (3.99)
1.35
1.16
459, 491
522
1.21
-1.94
a In CH3CN solutions (1 ꢀ 10-5 M). ε denotes the molar extinction coefficients. b In degassed CH3CN (1 ꢀ 10-5 M) solutions. The Φp was measured
versus quinine sulfate (Φp = 0.545 in 1 M H2SO4). c Collected in CH3CN solutions (10-3 M). The data were versus Fcþ/Fc.
Table 2. PL Characteristics of [Ir(dfppz)2(tp-pyim)]PF6 and [Ir(dfppy)2(ph-pyim)]PF6 in Thin Films.a
[Ir(dfppz)2(tp-pyim)]PF6
[Ir(dfppy)2(ph-pyim)]PF6
PL λ (nm)
τ (μs)
PLQY
0.54
PL λ (nm)
τ (μs)
PLQY
0.14
neat film
476, 499
3.04
501
1.06 (49%)
0.40 (51%)
b
film with BMIMPF6
5 wt % doped PMMA film
473, 494
466, 490
3.77
4.42
0.67
0.79
469, 488
2.94
0.72
a Thin films (ca. 100 nm thick) were deposited on quartz substrates and excited at 350 nm. b The molar ratio between [Ir(dfppz)2(tp-pyim)]PF6 and
BMIMPF6 is 1:1.
Theoretical Calculations. Figure 3 shows the optimized
geometry of [Ir(dfppz)2(tp-pyim)]PF6, together with the
selected molecular surfaces. More molecular surfaces
can be found in the Supporting Information (Table S1).
As shown in Figure 3, the bulky TP group shields the
luminescent iridium core in an umbrella shape. In con-
trast to other cationic iridium complexes whose HOMOs
reside on the iridium ion and the phenyl groups of the
cyclometalated ligands,21 the HOMO of [Ir(dfppz)2(tp-
pyim)]PF6 resides on the TP group of tp-pyim ligand. The
HOMO-1 resides on the iridium ion and the phenyl
groups of dfppz ligands. The HOMO and HOMO-1, with
the calculated energy levels of -8.03 eV and -8.06 eV, res-
Figure 2. Cyclic voltammogram of [Ir(dfppz)2(tp-pyim)]PF6 in CH3CN
pectively, are almost equivalent in energy. The HOMO-3,
solution (1 ꢀ 10-3 M) (the small wave around -1.38 V arises from some
HOMO-4, HOMO-6, HOMO-8 and HOMO-9 all reside
on the TP group of tp-pyim ligand (see Table S1 in the
Supporting Information). As revealed by the following
TDDFT calculations, these occupied orbitals residing on
the TP group are not involved in the low-lying emissive
excited-states. The HOMO-2, HOMO-5, HOMO-10, and
HOMO-11 all reside on the iridium ion and the dfppz
ligands. The HOMO-7 resides mainly on the iridium ion
and dfppz ligands, with a small distribution on the pyim
of tp-pyim ligand. The HOMO-12 resides on the pyim of
tp-pyim ligand (see Table S1 in the Supporting Infor-
mation). The LUMO and LUMOþ1 of [Ir(dfppz)2(tp-
pyim)]PF6 reside on the pyim of tp-pyim ligand, with
nearly no distribution on the TP group (Figure 3c and 3d).
To gain deep insight into the emission nature of [Ir(dfppz)2-
(tp-pyim)]PF6, we calculated the first three triplet states
of [Ir(dfppz)2(tp-pyim)]PF6 through the TD-DFT app-
roach. Table 3 summarizes the vertical excitation energies
and molecular orbitals involved in the excitations. The
T1 state originates from the excitation of HOMO-1 f
LUMO, with character of mixed 3MLCT (Ir f pyim of tp-
pyim) and 3LLCT (dfppz f pyim of tp-pyim). The T2 state
originates mainly from the excitations of HOMO-7 f
LUMO and HOMO-12 f LUMO, with character of mixed
3MLCT (Ir f pyim of tp-pyim), 3LLCT (dfppz f pyim of
tp-pyim), and LC 3π-π* (pyim-based). The T3 state
unknown impurities in the conducting electrolyte). Potentials were recor-
ded versus Fcþ/Fc.
is 3.77 μs and the PLQY remains 0.67, both of which are
higher than those of the neat film because of the diluting
effect of the ionic liquid.14,16,18,22
Electrochemical Properties. Figure 2 shows the cyclic
voltammogram of [Ir(dfppz)2(tp-pyim)]PF6 in CH3CN
solution and the redox potentials are listed in Table 1. As
shown in Figure 2, both the oxidation and reduction pro-
cesses are reversible, which are beneficial for the applica-
tion of [Ir(dfppz)2(tp-pyim)]PF6 in electroluminescent
devices. The oxidation potential (1.21 V) of [Ir(dfppz)2-
(tp-pyim)]PF6 is shifted anodically (by 70 mV) with
respect to that (1.14 eV) of [Ir(dfppy)2(ph-pyim)]PF6,21
which is consistent with the previous report that the dfppz
ligand stabilizes the HOMO of the complex as compared
to the dfppy ligand.10,16 The reduction potential (-1.94 V)
of [Ir(dfppz)2(tp-pyim)]PF6 is the same as that (-1.94 V)
of [Ir(dfppy)2(ph-pyim)]PF6.21 Because the reduction of
the complexes occurs on the ancillary ligands where the
lowest unoccupied molecular orbitals (LUMOs) of the
complexes are localized,21 the same reduction potentials
of [Ir(dfppz)2(tp-pyim)]PF6 and [Ir(dfppy)2(ph-pyim)]PF6
suggest that the bulky TP group exerts little influence on
the LUMO level of the complex.

3540 Chem. Mater., Vol. 22, No. 11, 2010
He et al.
Figure 3. Molecular surfaces of [Ir(dfppz)2(tp-pyim)]PF6. (a) HOMO; (b) HOMO-1; (c) LUMO; (d) LUMOþ1. All the MO surfaces correspond to an
isocontour value of |Ψ| = 0.025.
Table 3. Calculated Triplet States of [Ir(dfppz)2(tp-pyim)]PF6 through TDDFT Approach
states
E [eV]a
excitationsb
nature
T1
T2
2.73
2.90
H-1 f L (100%)
H-7 f L (44%)
dπ(Ir)-π(dfppz) f π*(pyim of tp-pyim)
dπ(Ir)-π(dfppz) f π*(pyim of tp-pyim)
π(pyim of tp-pyim) f π*(pyim of tp-pyim)
π(pyim of tp-pyim) f π*(pyim of tp-pyim)
π(pyim of tp-pyim) f π*(pyim of tp-pyim)
π(tp of tp-pyim) f π*(pyim of tp-pyim)
H-12 f L (51%)
H-12 f Lþ1 (5%)
H f L (100%)
T3
3.03
a Calculated excitation energies for the triplet states. b H and L denote the HOMO and LUMO, respectively; data in parentheses are the contributions
of the excitations.
originates mainly from the excitation of HOMO f LUMO,
with intraligand-charge-transfer character (TP of tp-
pyim f pyim of tp-pyim). According to the photophysi-
cal characterizations and the fact that T1 and T2 states are
close-lying in energy (within 0.17 eV), it is believed that
the emission of [Ir(dfppz)2(tp-pyim)]PF6 originates mainly
from the T1 and T2 states, that is, the observed light emis-
sion originates not only from the 3MLCT (Ir f pyim of
3
tp-pyim) and LLCT (dfppz f pyim of tp-pyim) states
but also from the 3π-π* states (pyim-based).
Previously, we performed TDDFT calculations on [Ir-
(dfppy)2(ph-pyim)]PF6, of which the T1 state has 3MLCT,
Figure 4. AFM topographic images of thin films (ca. 100 nm thick) on top
of PEDOT:PSS: (a) neat [Ir(dfppz)2(tp-pyim)]PF6, (b) [Ir(dfppz)2(tp-
pyim)]PF6 mixed with BMIMPF6 (molar ratio 1:1).
3LLCT character, and the T2 and T3 states have 3MLCT,
LC 3π-π* character.21 In CH3CN solution or neat film,
the emission of [Ir(dfppy)2(ph-pyim)]PF6 with featureless
spectra has been assigned to 3MLCT or 3LLCT states (the
T1 state).21 However, in the 5% doped PMMA film, the
emission with structured spectra can be assigned to LC
3π-π* states (T2 or T3 states). On going from the 5% doped
PMMA film to neat film, the emissive excited states of
[Ir(dfppy)2(ph-pyim)]PF6 change from the T2 or T3 states
to the T1 state, because of the change of the local environ-
ment of the complex.15 This change in the emissive excited
states does not happen for [Ir(dfppz)2(tp-pyim)]PF6 be-
cause of the bulky TP group attached to the complex.
Film-Forming Properties. Because of the film-forming
properties of organic electronic materials are crucial for
the performances of the resultant devices, the surface morpho-
logies of thin films of [Ir(dfppz)2(tp-pyim)]PF6 were exa-
mined. As shown in Figure 4, smooth and amorphous
thin films can be formed by spin-coating [Ir(dfppz)2(tp-
pyim)]PF6 from CH3CN solutions. The root-mean-square
(rms) roughness of neat [Ir(dfppz)2(tp-pyim)]PF6 film is
as small as 0.34 nm, indicating that [Ir(dfppz)2(tp-pyim)]PF6
has an excellent film-forming property. For the thin film
of [Ir(dfppz)2(tp-pyim)]PF6 mixed with BMIMPF6 (molar
ratio 1:1), the rms value increases slightly to 0.38 nm.
Even at this high doping concentration of the ionic liquid
in [Ir(dfppz)2(tp-pyim)]PF6 (molar ratio 1:1), no particular
aggregation or phase separation between the two species
were observed, indicating the good compatibility of [Ir-
(dfppz)2(tp-pyim)]PF6 with BMIMPF6.14
LECs. LECs were fabricated with a structure of ITO/
PEDOT:PSS (40 nm)/ [Ir(dfppz)2(tp-pyim)]PF6:BMIMPF6
(molar ratio 1:1) (100 nm) /Al (120 nm), where the PEDOT:
PSS layer was used to smooth the ITO surface and BMIMPF6

Article
Chem. Mater., Vol. 22, No. 11, 2010 3541
Table 4. Electrical Characteristics of LECs Biased at 3.2 V
d
LECs
1:x: ya
ton (min) b
tmax (min) c
Bmax (cd m-2
)
η
c, max (cd A-1
)
EL λ (nm)
CRI
CIE (x, y)
blue-green
white
1:1:0
1:1: 0.008
4.5
3.5
29.0
22.5
14.5
7.9
18.3 (7.6%)
11.4 (5.6%)
474, 494
495, 600
(0.22, 0.41)
(0.37, 0.41)
80
a The device structure is ITO/PEDOT: PSS (40 nm)/[Ir(dfppz)2(tp-pyim)]PF6:BMIMPF6:[Ir(ppy)2(qlbi)]PF6 (1:x:y, molar ratio) (100 nm)/Al (100
nm), where 1:x:y denotes the molar ratio between [Ir(dfppz)2(tp-pyim)]PF6, BMIMPF6, and [Ir(ppy)2(qlbi)]PF6. b The time required to reach 1 cd m-2
.
c The time required to reach the maximum brightness. d The number in the parentheses denotes the corresponding EQE values.
Figure 5. EL spectrum of the blue-green LEC (biased at 3.2 V) and PL
spectrum of the light-emitting layer.
Figure 6. Time-dependent current-density and brightness curves of the
blue-green and white LECs biased at 3.2 V.
was added to shorten the response time of LECs.35 With
this device structure, LECs can be routinely fabricated
with good reproducibility in electroluminescent perfor-
mances. Detailed electrical characteristics of LECs are
summarized in Table 4. Figure 5 shows the EL spectrum
of the blue-green LEC, with a peak emission and a shoul-
der emission at 494 and 474 nm, respectively. The CIE co-
ordinates of the blue-green electroluminescence are (0.22,
0.41), which are better than those [(0.25, 0.46)] of the LEC
based on [Ir(dfppy)2(ph-pyim)]PF6,21 because of the main-
tained color purity of the blue-green emission of [Ir(dfppz)2-
(tp-pyim)]PF6 in concentrated films. The EL spectrum
resembles the PL spectrum of the light-emitting layer,
except that a long tail develops in the long-wavelength
region of the EL spectrum (Figure 5).
Figure 6 shows the time-dependent current density and
brightness curves of LECs. The biasing voltage is chosen
at 3.2 V, which is close to the band gap (3.15 eV) of
[Ir(dfppz)2(tp-pyim)]PF6. As shown in Figure 6, both the
current density and brightness increase gradually with
time, which is a typical characteristic of LECs.4,8 The
peak current efficiency, EQE, and power efficiency rea-
ched 18.3 cd A-1, 7.6%, and 18.0 lm W-1, respectively. At
3.2 V, it took 29 min for the device to reach a maximum
brightness of 14.5 cd m-2. When biased at a higher
voltage of 4.0 V, the device showed a faster response
and it took 3 min for the device to reach a maximum
brightness of 94 cd m-2. At 4.0 V, the peak current
efficiency remained 16.6 cd A-1. The high EL efficiency
is attributed to the high luminescent efficiency of [Ir-
(dfppz)2(tp-pyim)]PF6, the suppressed excited-state self-
quenching in solid states and the efficient carrier injection
and recombination in LECs.
At the same biasing voltage of 4.0 V, the current density
of the LEC based on [Ir(dfppz)2(tp-pyim)]PF6 is lower
than that of the LEC based on [Ir(dfppy)2(ph-pyim)]-
PF6,21 because the bulky TP group increases the intersite
distance and thereby hinders charge-hopping between
molecules in the active layer.20,36 However, at 4.0 V, the
maximum brightness and efficiency (94 cd m-2, 18.3 cd
A-1, respectively) of the LEC based on [Ir(dfppz)2(tp-
pyim)]PF6 are higher than those (39 cd m-2, 8.4 cd A-1
,
respectively) of the LEC based on [Ir(dfppy)2(ph-pyim)]-
PF6, because of the enhanced PLQY of [Ir(dfppz)2(tp-
pyim)]PF6 in concentrated films. As previously reported,
bulky side groups attached to the complex would hinder
charge-hopping between molecules and so decrease the
brightness of LECs.20,36 This negative effect, as shown
here, can be largely counteracted by the simultaneously
enhanced PLQY of the complex in concentrated films.
On the basis of the efficient blue-green LEC, white
LECs were fabricated. The device structure is ITO/PEDOT:
PSS(40nm)/[Ir(dfppz)2(tp-pyim)]PF6:BMIMPF6:[Ir(ppy)2-
(qlbi)]PF6 (molar ratio 1:1:0.008) (100 nm)/Al (120 nm),
where [Ir(ppy)2(qlbi)]PF6 is bis(2-phenylpyridinato-N,
0
C2 )iridium(III)[3-(1-phenyl-1H-benzo[d]imidazol-2-yl)-
isoquinoline] hexafluorophosphate, a red-emitting catio-
nic iridium complex developed in our lab.21 The biasing
voltage is still chosen at 3.2 V. Figure 7 shows the EL
spectrum of the white LEC, along with the PL spectrum
of the light-emitting layer. The relative intensity of the red
component to the blue one in the EL spectrum is much
larger than that in the PL spectrum, which indicates that,
as well as the energy transfer from [Ir(dfppz)2(tp-pyim)]PF6
to [Ir(ppy)2(qlbi)]PF6, charge-trapping on [Ir(ppy)2(qlbi)]-
PF6 plays an important role for the operation of the white
(35) Parker, S. T.; Slinker, J. D.; Lowry, M. S.; Cox, M. P.; Bernhard, S.;
Malliaras, G. G. Chem. Mater. 2005, 17, 3187.
(36) Rudmann, H.; Shimada, S.; Rubner, M. F. J. Am. Chem. Soc. 2002,
124, 4918.

3542 Chem. Mater., Vol. 22, No. 11, 2010
He et al.
closer to the electrode interfaces, increasing the possibility
of exciton-quenching and thereby decreasing the bright-
ness and efficiency. Besides, the low luminescence effi-
ciency of [Ir(ppy)2(qlbi)]PF6 (PLQY = 0.03 in CH3CN
solution) should also be responsible for the lowered
brightness and efficiency of the white LEC.21 It is believed
that much higher performance will be achieved for the
white LECs by using red-emitting ionic complexes with
high luminescent efficiency and appropriate energy levels.
It is also noted that all LECs reported here showed
limited device stability, which has remained a big chal-
lenge for LECs. Currently, detailed mechanisms for
the degradation of iTMCs-based LECs are unclear. The
addition of large amounts of ionic liquid in LECs, though
shortening the response time, generally accelerates the
degradation of LECs.22,35 Several approaches have been
proposed to enhance the stability of LECs, such as freez-
ing the p-i-n junction once the junction is established,37
introducing supramolecular interactions within the mole-
cules38 or driving LECs under voltage pulses.39 For prac-
tical applications, further study to improve the stability of
LECs is greatly needed.
Figure 7. EL spectrum of the white LEC (biased at 3.2 V) and PL spec-
trum of the light-emitting layer. The inset on the top right depicts the
energy level diagram of the white LEC (the dashed lines represent the
energy levels of [Ir(ppy)2(qlbi))]PF6).
LEC.16,21 The inset in Figure 7 illustrates the energy level
diagram of the white LEC, from which it can be con-
cluded that both holes and electrons can be trapped by
small-gap [Ir(ppy)2(qlbi)]PF6. The CIE coordinates of the
white electroluminescence are (0.37, 0.41) and the CRI
value is up to 80, which corresponds to a warm-white light
that is appropriate for solid-state lighting applications.
The time-dependent current density and brightness curves
of the white LEC were depicted in Figure 6. Detailed elec-
trical characteristics were summarized in Table 4. The
peak current efficiency, EQE, and power efficiency of the
Conclusions
A highly efficient blue-green-emitting cationic iridium
complex [Ir(dfppz)2(tp-pyim)]PF6, which contains a bulky
side group in the ancillary ligand, has been synthesized
and fully characterized. LECs based on [Ir(dfppz)2(tp-
pyim)]PF6 showed highly efficient blue-green electrolu-
minescence, with peak current efficiency, EQE and power
efficiency of 18.3 cd A-1, 7.6% and 18.0 lm W-1, respec-
tively. White LECs based on [Ir(dfppz)2(tp-pyim)]PF6
showed warm white light, with CIE coordinates of
(0.37, 0.41), a CRI value up to 80 and a power efficiency
of 11.2 lm W-1. Further work will be focused on improv-
ing the performance of white LECs, with the aim to
white LEC reached 11.4 cd A-1, 5.6%, and 11.2 lm W-1
,
respectively, higher than those of the white LEC based on
[Ir(dfppy)2(ph-pyim)]PF6, BMIMPF6, and [Ir(ppy)2(qlbi)]-
PF6 (molar ratio 1:0.35:0.002).21
For the white LEC based on [Ir(dfppz)2(tp-pyim)]PF6,
the current density, brightness and efficiency are lower
than those of the blue-green LEC (Figure 6). This is quite
different from the situation in the white LEC based on
[Ir(dfppy)2(ph-pyim)]PF6, which showed higher current
density, brightness, and efficiency than the blue-green
LEC, attributed to the lowered carrier-injection barrier
upon doping the small-gap [Ir(ppy)2(qlbi)]PF6.21 In LECs
based on [Ir(dfppz)2(tp-pyim)]PF6 and BMIMPF6 (molar
ratio 1:1), the doping concentration of BMIMPF6 is large
enough so that adequate ions can accumulate at the
electrode interfaces, eliminating the carrier-injection bar-
riers. This can be confirmed by the evidence that the blue-
green LEC based on [Ir(dfppz)2(tp-pyim)]PF6 turned on
quickly at a low biasing voltage that is almost equal to the
band gap of the emitting complex. Therefore, in LECs
based on [Ir(dfppz)2(tp-pyim)]PF6 and BMIMPF6 (molar
ratio 1:1), lowering the carrier-injection barrier upon
doping the small-gap [Ir(ppy)2(qlbi)]PF6 is not likely to
change the device performance much as carrier-injection
barrier is no longer the key problem. On the contrary,
charge-trapping on [Ir(ppy)2(qlbi)]PF6 plays a key role on
the device performance. Because of the trap of electrons
and holes on [Ir(ppy)2(qlbi)]PF6, the white LEC showed
lower current density than the blue-green LEC; more-
over, the recombination zone in the white LEC moves
enhance the power efficiency over those (∼15 lm W-1
)
of electrical bulbs, and prolonging the operating lifetime
of LECs to the practical application level.
Acknowledgment. This work is supported by the National
Key Basic Research and Development Program of China
(Grant 2006CB806203 and 2009CB623604).
Supporting Information Available: Absorption and PL spec-
tra of [Ir(dfppz)2(tp-pyim)]PF6 in degassed CH2Cl2 solutions
(Figure S1); PL spectra of [Ir(dfppz)2(tp-pyim)]PF6 in the 5%
doped PMMA film and neat film (Figure S2); PL spectra of
[Ir(dfppy)2(ph-pyim)]PF6 in the 5% doped PMMA film and
neat film (Figure S3); the calculated molecular surfaces and
energy levels of [Ir(dfppz)2(tp-pyim)]PF6 (Table S1); the com-
plete author list of ref 30 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(37) Shao, Y.; Bazan, G. C.; Heeger, A. J. Adv. Mater. 2007, 19, 365.
(38) Graber, S.; Doyle, K.; Neuburger, M.; Housecroft, C. E.;
´
Constable, E. C.; Costa, R. D.; Ortı, E.; Repetto, D; Bolink,
H. J. J. Am. Chem. Soc. 2008, 130, 14944.
(39) Rudmann, H.; Rubner, M. F. J. Appl. Phys. 2001, 90, 4338.