Photophysical properties of substituted homoleptic and heteroleptic phenylimidazolinato Ir(III) complexes as a blue phosphorescent material

Article abstract of DOI:10.1021/ic400950u

Iridium complexes are one of the most important materials for fabrication of organic light emitting diodes (OLEDs). There are difficulties in the preparation of blue phosphorescent complexes with respect to chromaticity, emission efficiency, and stability of the material, compared with green and red phosphorescent complexes. Control of the frontier orbital energy level (HOMO-LUMO) is the sole method to achieve better blue phosphorescent iridium complexes by appropriate ligand selection and the introduction of adequate substituents. Homoleptic and heteroleptic iridium(III) tris(phenylimidazolinate) complexes were synthesized, and the effect of the substituents on their nature in the excited state was examined. Density functional theory calculation showed that the imidazolinato complexes have the HOMO localized at the iridium d- and phenyl π-orbitals. The LUMO is also localized on the phenyl moiety with a much higher population than HOMO. This LUMO is quite different from other complexes, such as iridium(III) tris(phenylpyridinate) and tris(phenylpyrazolinate) complexes. Therefore, substitution with π-electron donating groups and electron withdrawing groups induces blue and red spectral shifts, respectively, which is the reverse shift exhibited by other complexes. The ancillary ligand (acetylacetone) acts as a path for nonradiative deactivation in the blue phosphorescent complexes.

Full text of DOI:10.1021/ic400950u

Article
pubs.acs.org/IC
Photophysical Properties of Substituted Homoleptic and
Heteroleptic Phenylimidazolinato Ir(III) Complexes as a Blue
Phosphorescent Material
Takashi Karatsu,* Masatomo Takahashi, Shiki Yagai, and Akihide Kitamura
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japan
S
* Supporting Information
ABSTRACT: Iridium complexes are one of the most important
materials for fabrication of organic light emitting diodes (OLEDs).
There are difficulties in the preparation of blue phosphorescent
complexes with respect to chromaticity, emission efficiency, and
stability of the material, compared with green and red phosphorescent
complexes. Control of the frontier orbital energy level (HOMO−
LUMO) is the sole method to achieve better blue phosphorescent
iridium complexes by appropriate ligand selection and the introduction
of adequate substituents. Homoleptic and heteroleptic iridium(III)
tris(phenylimidazolinate) complexes were synthesized, and the effect of
the substituents on their nature in the excited state was examined.
Density functional theory calculation showed that the imidazolinato
complexes have the HOMO localized at the iridium d- and phenyl π-
orbitals. The LUMO is also localized on the phenyl moiety with a much
higher population than HOMO. This LUMO is quite different from other complexes, such as iridium(III) tris(phenylpyridinate)
and tris(phenylpyrazolinate) complexes. Therefore, substitution with π-electron donating groups and electron withdrawing
groups induces blue and red spectral shifts, respectively, which is the reverse shift exhibited by other complexes. The ancillary
ligand (acetylacetone) acts as a path for nonradiative deactivation in the blue phosphorescent complexes.
bis(4,6-difluorophenylpyridinato)picolinate (FIrpic),7 and
1. INTRODUCTION
other iridium complexes,8,9 the sum of the x, y coordinates is
Iridium complexes have a wide range of applications: in
particular, the organic light emitting diode (OLED) is one of
more than 0.3, and the color is called sky blue. The second
difficulty is insufficient emission efficiency. A blue phosphor-
important industrial applications due to its high phosphor-
escent complex has a higher energy emission state (3MLCT,
escent efficiency at ambient temperature.1,2 Emitting material of
OLEDs was changed from fluorescent complexes,3,4 to
metal-to-ligand charge-transfer) than other color complexes,
due to the large transition energy. This enables thermal
phosphorescent materials such as fac-tris(2-phenylpyridyl)-
iridium(III) (fac-Ir(ppy)3).5 The external efficiencies of the
OLED device were increased from 1% to 7.5% during this
change. These reports boosted the research activity in this field.
The efficiency was jumped up to 29%, also using fac-Ir(ppy)3,
as reported by Kido and co-workers,6 which meant an internal
emission quantum yield that reached 100%. Here, not only
triplet excitons (75%) but also singlet excitons (25%) were
used after intersystem crossing.
Red and green phosphorescent iridium complexes have been
used in commercial devices.1 However, there are three main
difficulties associated with the development of blue phosphor-
escent OLEDs, which are the barrier to achieve full color total
phosphorescent OLED display and some types of white color
lighting. The first is that they do not have sufficient color purity.
The National Television Standards Committee (NTSC)
determined that the Commission Internationale de l’Eclairage
(CIE) coordinates for blue are (x, y = 0.14, 0.08). However, in
typical blue phosphorescent complexes, such as iridium(III)
activation to the metal-centered excited state (d*), which
promotes nonradiative deactivation. For example, fac-tris(1-
phenylpyrazolyl)iridium(III) (fac-Ir(ppz)3) has strong blue
phosphorescence (ΦPL = 1.0) at 77 K, but has almost no
emission at 298 K (ΦPL = 0.001).10,11 The last difficulty is the
development of host and carrier transport materials. For blue
phosphorescent materials, because of the high triplet excitation
energy, confinement of the excitation energy is becoming
difficult.7
Computational investigation of fac-Ir(ppy)3 revealed that the
HOMO is mainly localized at the iridium d-orbital and phenyl
moiety, and the LUMO is localized at the pyridyl moiety.
Therefore, control of the HOMO−LUMO energy gap has been
attempted by the modification of green phosphorescent fac-
Ir(ppy)3 using the following three strategies. (1) One strategy is
Received: April 17, 2013
© XXXX American Chemical Society
A
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Figure 1. Complexes synthesized with phenylimidazole that have been studied.
stabilization of the HOMO by the introduction of electron
withdrawing groups (EWGs) on a phenyl ring. Iridium(III)
bis(4,6-difluorophenylpyridinato)acetylacetonate (FIr(acac))
and FIrpic exhibited very high efficiency emission;7 λmax for
FIr(acac) is blue-shifted 40 nm compared with that for
iridium(III) bis(2-phenylpyridinato)acetylacetonate (Ir-
(ppy)2(acac)). These results indicate that the introduction of
fluoro groups on the phenyl stabilizes the HOMO, because the
HOMO is partly localized at the phenyl moiety. λmax for fac-
iridium(III) tris(4,6-difluorophenylpyridinate) ( fac-Ir-
(dfppy)3)10 was the same as FIrpic (λmax = 468 nm). De
Cola and co-workers reported 3,4,6-trifluorophenyl derivative
(fac-Ir(F3ppy)3) (459 nm), and 3,4,5,6-tetrafluorophenyl
derivative (fac-Ir(F4ppy)3) (468 nm).12 Interestingly, the
increased number of F substitution, from three to four, resulted
in red-shift of λmax. These attempts demonstrate the
effectiveness and limitations of design for blue phosphorescent
Ir complexes by using EWGs. In addition, a device composed of
FIrpic produced a large amount of defluorinated product,13
bringing a shift to fluorine-free materials.14(2) A second
strategy is the use of strong EWG ancillary ligands: At the
first stage, use of homoleptic complex was major. A typical
Ir(dfppy)3 (λmax = 450 nm). However, these complexes in
solution were poorly phosphorescent at room temperature
(ΦPL < 0.001). Thompson and co-workers proposed that this
was due to the bond weakness between iridium and the ligand
nitrogen atoms, and they therefore synthesized a complex with
a carbene type ligand, tris(1-phenyl-3-methylbenzimidazolin-2-
ylidene)iridium(III) (Ir(pmb)3).11 However, its device external
quantum efficiency was only 2.6%.21 We examined the
substituent effect of the Ir(pmb)3. fac-Ir(CF3pmb)3 and fac-
Ir(CH3Opmb)3 exhibited deep blue phosphorescence with λmax
at 396 and 403 nm (ΦPL = 0.84 and 0.76), respectively.22
However, there are no appropriate host and charge carrier
materials for such large band gap complexes; therefore, the
external quantum efficiency of the OLED device was only
2.6%.21
Samuel and co-workers reported a series of phenyl triazole
type complexes.23 The fac-iridium(III) tris(1-methyl-5-phenyl-
3-propyl-[1,2,4]triazolate) (Ir(pptz)3, x, y = 0.16, 0.20) and 4-
fluorophenyl and 4,6-difluorophenyl derivatives (Ir(fpptz)3 and
Ir(dfpptz)3, respectively) had λmax at 449 nm (ΦPL = 0.66), 428
(ΦPL = 0.27), and 425 nm (ΦPL = 0.03), respectively. ΦPL
decreased by the increase of the number of substituted fluorine
atoms and increase of band gap energies.
preparation is conducted through a μ-Cl dimer complex;
therefore, use of a diketonate complex through a dimer
complex became more popular lately.15 They are easily
synthesized even they have equivalent or slightly lower
emission efficiency than the homoleptic complexes.16 Many
types of ancillary ligands have been added to the complexes
after FIrpic exhibited 20 nm (860 cm−1) shorter λmax than
bis(4,6-difluorophenylpyridinato)acetylacetonate (FIracac).
Changing the ancillary ligand to the EWG picolinate stabilized
the HOMO. Thompson and co-workers reported iridium(III)
bis(4,6-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate
(FIr6) with borate as an ancillary ligand, and this complex had
λmax at 457 nm.8 De Cola and co-workers reported that λmax and
ΦPL of iridium(III) bis(4,6-difluorophenylpyridinato)-
pyridyltriazole (FIrpyptz) were 461 nm and 0.27, respectively.17
At this stage, pyridyl-azole type ligands became popular.9,18 Chi
and co-workers reported a phosphine type ligand;19 iridium-
(III) bis(4,6-difluorophenylpyridinato)(2,4-difluorobenzyl)-
diphenylphosphinate (FIrdfphdpp) had λmax at 457 nm (ΦPL
= 0.19).20 (3) The third strategy is breakaway from phenyl
pyridinato complexes, using phenylheterocycles: Complexes
with many types of ancillary ligands have been synthesized;
however, the shift of λmax was only 10 nm (392 cm−1), and
color clarity of the blue was less than satisfactory. Further blue-
shift is very difficult using difluorophenylpyridine as a ligand;
therefore, an approach to tune the resonance stabilization
energy by changing the pyridyl group to other heterocycles was
attempted. Complexes with phenyl pyrazole type ligand were
reported.10 fac-Ir(ppz)3 and the 4,6-difluorophenyl derivatives
fac-Ir(46dfppz)3 exhibited phosphorescence at 77 K with λmax at
414 and 390 nm, respectively. This was a significant blue-shift
compared to phenyl pyridine based complexes such as fac-
There have been only a few reports of phenylimidazole
derivatives. Gratzel and co-workers reported a diketonate
̈
complex, iridium(III) bis(1-methyl-2-phenylimidazolato)-
acetylacetonate (N966),24 which gives a broad emission
between 440 and 800 nm that is applicable to a single
molecular white lighting OLED. They also reported sub-
stituents effects for N966,25 to improve efficiency up to ΦPL
=
0.95. Perumal and co-workers also reported substituent and
solvent effects on the emission of diketonate complexes.26 The
aim of that research was to achieve white phosphorescence, and
they did not deal with the blue phosphor. In addition, there is
no report of homoleptic complexes.
In this study, we initiated blue phosphorescence-oriented
research of phenylimidazolinato complexes. In particular, we
have synthesized complexes with various substituents intro-
duced on the phenyl ring (Figure 1), and examined their effects
on the photophysical properties and frontier orbitals (HOMO
and LUMO).
2. EXPERIMENTAL SECTION
2.1. General Information and Materials. All chemicals used for
synthesis were purchased from Aldrich, Kanto Chemical, TCI, Wako
Pure Chemical Industries, and Furuya Metal and used without further
purification. Electroluminescence (EL) grade FIrpic was purchased
from Luminescence Technology Corp. 1H nuclear magnetic resonance
(NMR) and 13C NMR spectra were recorded on Jeol JNM-LA 400
and Bruker DPX 300 spectrometers. Fast atom bombardment and
electrospray ionization (ESI) mass spectra were recorded on a Jeol
JMS-AX500 double focusing mass spectrometer and Thermo Scientific
Exactive mass spectrometer, respectively. Elemental analysis was
conducted using an Exeter Analytical Inc. CE-440F analyzer.
B
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2.2. Synthesis of Ligands. Methyl-2-phenyl-1H-imidazole
(MHpiH) was synthesized using the method reported.27 The reaction
yield was 38%, and details were described in the Supporting
Information.
1-Butyl-2-(4-fluorophenyl)-1H-imidazole (FpiH) was synthesized
using the method reported.28 The reaction yield was 11%. Other
ligands, 1-butyl-2-(4-(trifluoromethyl)phenyl)-1H-imidazole
(CF3piH), 1-butyl-2-(4-(trifluoromethoxy)phenyl)-1H-imidazole
(OCF3piH), 1-butyl-2-(4-methoxyphenyl)-1H-imidazole (OCH3piH),
and 1-butyl-2-phenyl-1H-imidazole (HpiH), were also synthesized
using the same method, and their yields were 35%, 18%, 30%, and
36%, respectively, and details were described in the Supporting
Information.
110 °C. After cooling to ambient temperature, water (100 mL) and
brine (30 mL) were added. The organic layer was extracted three
times with CH2Cl2. The solvent was removed in vacuo, and crude μ-Cl
dimer complex was obtained. A mixture of acetylacetone (250 mg, 2.50
mmol) and potassium carbonate (354 mg, 2.56 mmol) in 25 mL of 2-
ethoxyethanol was added, and this reaction mixture was stirred for 6 h
at 110 °C. After cooling to ambient temperature, water (100 mL) and
brine (30 mL) were added. The organic layer was extracted three
times with CH2Cl2. Brown powder was obtained after removal of the
solvent in vacuo. The product was purified by precipitation (CH2Cl2
and hexane) after the column chromatography (alumina, CH2Cl2), and
the complex was obtained as a pale yellow powder (52 mg, 13%).
1H NMR (CDCl3, 300 MHz, 300 K): δ (ppm) 7.28−7.26 (m, 2H),
6.99 (d, J = 1.4 Hz, 2H), 6.92 (d, J = 1.4 Hz, 2H), 6.74−6.69 (m, 2H),
6.62−6.57 (m, 2H), 6.37−6.35 (m, 2H), 5.15 (s, 1H), 4.46−4.27 (m,
4H), 2.00−1.90 (m, 4H), 1.77 (s, 6H), 1.53−1.41 (m, 4H), 1.00 (t, J =
7.4 Hz, 6H).
2.3. Synthesis of Homoleptic Complexes. The fac-tris[2-(4-
trifluoromethylphenyl)-3-butyl-[1,3]-imidazolynato-C2,N1]iridium(III)
complex (fac-CF3) is given as an example.29
Iridium(III) trisacetylacetonate (763 mg, 1.56 mmol), CF3piH
(2.10 g, 7.83 mmol), and 9 mL glycerol that was deoxygenated by
argon bubbling at 70 °C for 10 min were placed in a 50 mL three-
necked flask. The mixture was stirred and heated up to 230 °C under
nitrogen for 3.5 h. CH2Cl2 (60 mL) was added after cooling down to
ambient temperature. An insoluble yellow precipitate was observed in
the organic layer. AcOEt (40 mL) was added, and the precipitates
were separated by filtration. The organic layer was washed twice with a
mixture of water (100 mL) and sat. aq NaCl (40 mL), and then once
with water, dried with anhydrous sodium sulfate, followed by solvent
evaporation. The yellow powder was reprecipitated twice using
CH2Cl2 and methanol to yield 398 mg. High-performance liquid
chromatography (HPLC) analysis revealed that the reaction mixture
contained equal amounts of the meridional isomer. Therefore, the
powder was well stirred in 100 mL of CH2Cl2, and the residual facial
isomer was obtained as ocher powder by filtration and dried under
vacuum (156 mg, 0.16 mol, yield 10%). An HPLC chromatograph
gave a single peak assigned to the facial isomer.
ESIMS: m/z = 749.2236, theoretical mass = 749.2250, δ = −1.85
ppm.
Anal. Calcd for C31H35F2IrN4O2: C, 51.30; H, 4.86; N, 7.72. Found:
C, 51.42; H, 4.78; N, 7.48.
The iridium(III) bis[2-(4-trifluoromethylphenyl)-3-butyl-[1,3]-imi-
dazolynato-C2,N1]acetylacetonate (CF3acac) was obtained as yellow
powder (yield 63%) using the same method.
1H NMR (CDCl3, 300 MHz, 300 K): δ (ppm) 7.32 (d, J = 7.5 Hz,
2H), 7.04 (d, J = 1.5 Hz, 2H), 7.02 (d, J = 1.5 Hz, 2H), 6.97 (d, J = 7.5
Hz, 2H), 6.46 (s, 2H), 5.19 (s, 1H), 4.58−4.22 (m, 4H), 1.95−1.87
(m, 4H), 1.80 (s, 6H), 1.51−1.39 (m, 4H), 0.98 (t, J = 7.3 Hz, 6H).
ESIMS: m/z = 827.2347, theoretical mass = 827.2366, δ = −2.35
ppm.
Anal. Calcd for C33H35F6IrN4O2: C, 47.99; H, 4.27; N, 6.78. Found:
C, 48.18; H, 4.15; N, 6.91.
The iridium(III) bis[2-(4-trifluoromethoxyphenyl)-3-butyl-[1,3]-
imidazolynato-C2,N1]acetylacetonate (OCF3acac) was obtained as
yellow powder (yield 41%) using the same method.
1H NMR (CD2Cl2, 300 MHz, 300 K): δ (ppm) 7.33 (d, J = 8.5 Hz,
2H), 7.06 (d, J = 1.5 Hz, 2H), 6.97 (d, J = 1.5 Hz, 2H), 6.68 (dd, J =
8.5 and 1.3 Hz, 2H), 6.07 (d, J = 1.3 Hz, 2H), 5.22 (s, 1H), 4.49−4.26
(m, 4H), 1.98−1.88 (m, 4H), 1.79 (s, 6H), 1.51−1.41 (m, 4H), 0.99
(t, J = 7.4 Hz, 6H).
ESIMS: m/z = 859.2247, theoretical mass = 859.2265, δ = −2.07
ppm.
Anal. Calcd for C33H35F6IrN4O4: C, 46.20; H, 4.11; N, 6.53. Found:
C, 46.17; H, 4.11; N, 6.52.
1H NMR (CD2Cl2, 300 MHz, 300 K): δ (ppm) 7.47 (d, J = 8.2 Hz,
3H), 7.10 (d, J = 8.2 Hz, 3H), 7.06 (s, 3H), 6.90 (d, J = 1.4 Hz, 3H),
6.37 (d, J = 1.4 Hz, 3H), 4.44−4.23 (m, 6H), 1.93−1.83 (m, 6H),
1.48−1.36 (m, 6H), 0.96 (t, J = 7.4 Hz, 9H).
ESI-MS: m/z = 995.3013, theoretical mass = 995.3029, δ = −1.60
ppm.
Anal. Calcd for C42H42F9IrN6: C, 50.75; H, 4.26; N, 8.45. Found: C,
50.63; H, 4.26; N, 8.29.
The fac-tris[2-(4-trifluoromethoxyphenyl)-3-butyl-[1,3]-
imidazolynato-C2,N1]iridium(III) complex (fac-OCF3) was obtained
as an ocher powder (yield 23%) using the same method.
1H NMR (CD2Cl2, 300 MHz, 300 K): δ (ppm) 7.40 (d, J = 8.5 Hz,
3H), 6.86 (d, J = 1.4 Hz, 3H), 6.72 (d, J = 8.5 Hz, 3H), 6.67 (s, 3H),
6.35 (d, J = 1.4 Hz, 3H), 4.37−4.19 (m, 6H), 1.93−1.80 (m, 6H),
1.49−1.37 (m, 6H), 0.97 (t, J = 7.3 Hz, 9H).
The iridium(III) bis[2-(4-methoxyphenyl)-3-butyl-[1,3]-imidazoly-
nato-C2,N1]acetylacetonate (OCH3acac) was obtained as pale yellow
powder (yield 20%) using the same method.
1H NMR (CDCl3, 300 MHz, 300 K): δ (ppm) 7.20 (d, J = 8.5 Hz,
2H), 6.94 (d, J = 1.4 Hz, 2H), 6.84 (d, J = 1.4 Hz, 2H), 6.30 (dd, J =
8.5 and 2.6 Hz, 2H), 5.90 (d, J = 2.6 Hz, 2H), 5.14 (s, 1H), 4.40−4.18
(m, 4H), 3.55 (s, 6H), 1.96−1.86 (m, 4H), 1.77 (s, 6H), 1.52−1.39
(m, 4H), 0.98 (t, J = 7.4 Hz, 6H).
ESI-MS: m/z = 1043.2870, theoretical mass = 1043.2877, δ = −0.65
ppm.
Anal. Calcd for C42H42F9IrN6O3: C, 48.41; H, 4.06; N, 8.07. Found:
C, 48.23; H, 3.96; N, 7.82.
The fac-tris[2-(4-fluorophenyl)-3-butyl-[1,3]imidazolynato-C2,N1]-
iridium(III) complex (fac-F) was obtained as an ocher powder
(yield 1%) using the same method.
ESIMS: m/z = 750.2732, theoretical mass = 750.2752, δ = −2.54
ppm.
Anal. Calcd for C33H41IrN4O4: C, 52.85; H, 5.51; N, 7.47. Found:
C, 52.51; H, 5.45; N, 7.34.
1H NMR (CD2Cl2, 300 MHz, 300 K): δ (ppm) 7.36 (dd, J = 8.2
Hz, JFH = 5.6 Hz, 3H), 6.82 (d, J = 1.3 Hz, 3H), 6.60−6.51 (m, 6H),
6.30 (d, J = 1.3 Hz, 3H), 4.37−4.17 (m, 6H), 1.93−1.82 (m, 6H),
1.49−1.37 (m, 6H), 0.97 (t, J = 7.3 Hz, 9H).
The iridium(III) bis[2-phenyl-3-butyl-[1,3]-imidazolynato-C2,N1]-
acetylacetonate (Hacac) was obtained as yellow powder (yield 18%)
using the same method.
ESI-MS: m/z = 845.3115, theoretical mass = 845.3125, δ = −1.17
ppm.
1H NMR (CDCl3, 300 MHz, 300 K): δ (ppm) 7.27 (d, J = 7.5 Hz,
2H), 6.99 (d, J = 1.4 Hz, 1H), 6.92 (d, J = 1.4 Hz, 1H), 6.72 (dt, J =
7.5 and 1.2 Hz, 2H), 6.60 (dt, J = 7.5 and 1.3 Hz, 2H), 6.36 (dd, J =
7.5 and 1.2 Hz, 2H), 5.15 (s, 1H), 4.46−4.27 (m, 4H), 2.00−1.90 (m,
4H), 1.77 (s, 6H), 1.53−1.41 (m, 4H), 1.00 (t, J = 7.4 Hz, 6H).
ESIMS: m/z = 690.2511, theoretical mass = 690.2540, δ = −4.24
ppm.
Anal. Calcd for C39H42F3IrN6: C, 55.50; H, 5.02; N, 9.96. Found: C,
55.21; H, 4.95; N, 9.71.
2.4. Syntheses15,30 of Diketonate Complexes. Iridium(III)
bis[2-(4-fluorophenyl)-3-butyl-[1,3]-imidazolynato-C2,N1]-
acetylacetonate (Facac) is given as an example.
A mixture of iridium(III) chloride n-hydrate (201 mg, 0.54 mmol),
FpiH (293 mg, 1.34 mmol), 2-ethoxyethanol (30 mL), and H2O (10
mL) were set in the flask, purged with nitrogen, and stirred for 6 h at
Anal. Calcd for C31H37IrN4O2: C, 53.97; H, 5.41; N, 8.12. Found:
C, 53.72; H, 5.37; N, 7.85.
C
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Iridium(III) bis[2-phenyl-3-methyl-[1,3]-imidazolynato-C2,N1]-
acetylacetonate (MHacac) was obtained as yellow powder (yield
47%) using the same method.
(BCP) (20 nm)/CsF (2 nm)/Al (100 nm). PEDOT was spin-coated
directly onto the ITO glass and dried at 200 °C for 10 min. 3,3″,5,5″-
Tetra(9H-carbazol-9-yl)-1,1′:3′,1″-terphenyl (mB-4Cz)42 was synthe-
sized and used as a wet processable host material. The light emitting
layer was spin-coated on the PEDOT layer using 1,2-dichloroethane as
a solvent, and dried at 60 °C for 3 h under vacuum. The BCP layer,
which was grown by thermal sublimation in a vacuum of 2 × 10−6
Torr, was used as an electron transport layer that blocked holes and
confined excitons. The cathode CsF/Al alloy was subsequently
deposited onto the BCP layer using an Ulvac VPC 1100 vacuum
deposition system.
1H NMR (CDCl3, 300 MHz, 300 K): δ (ppm) 7.36 (d, J = 7.5 Hz,
2H), 6.99 (d, J = 1.4 Hz, 2H), 6.89 (d, J = 1.4 Hz, 2H), 6.73 (t, J = 7.5
Hz, 2H), 6.63 (t, J = 7.5 Hz, 2H), 6.40 (d, J = 7.5 Hz, 2H), 5.15 (s,
1H), 4.10 (s, 6H), 1.78 (s, 6H).
ESIMS: m/z = 629.1475, theoretical mass = 629.1499, δ = −3.87
ppm.
Anal. Calcd for C25H25IrN4O2: C, 49.57; H, 4.16; N, 9.25. Found:
C, 49.31; H, 4.05; N, 8.99.
2.5. X-ray Crystallography. Single crystals of fac-CF3 and fac-
OCF3 were grown in CH2Cl2−hexane. Diffraction data were collected
on Bruker SMART APEX II CCD diffractometer at 173 K with
graphite-monochromated Mo Kα radiation. Initial atomic positions
were determined using direct methods. The structures of the
compound were refined using least-squares methods with the XShell
program. ORTEP31 and packing diagrams were constructed using
Mercury 3.0.32 Optical micrographs of the single crystals were
obtained with a microscope (Olympus DP71).
2.6. Electrochemistry. Cyclic voltammetry (CV) was performed
using a standard compartment cell equipped with a Pt working
electrode (BAS), a platinum wire counter electrode, a Ag/Ag+ (Ag/
AgNO3) reference electrode, and a ALS/CH Instruments model 440A
analyzer. Argon-purged anhydrous tetrahydrofuran (THF) was used as
a solvent, and tetrabutylammonium hexafluorophosphate (0.1 M) was
used as a supporting electrolyte. The scan rate was 100 mV/s. The
HOMO energy level was estimated using the value of ferricenium/
ferrocene couple (4.8 eV33) and a potential increase of 100 mV by
using eq 1.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Molecular Structure. We have
synthesized nine homo- and heteroleptic iridium complexes,
as detailed in the Experimental Section. 1-Methyl-2-phenyl-1H-
imidazole (MHpiH) was obtained by methylation of 2-
phenylimidazole using dimethyl carbonate.27 However, other
ligands have n-butyl chains on the imidazolyl ring and
substituents on the phenyl ring; therefore, a one-pot synthesis
method has been applied to easily obtain reactants and save
time.28 Introduction of the n-butyl group helps to increase the
stability43 and solubility of the complexes. All ligand were
obtained as liquids and yields were 38% (MHpiH) and between
11% and 36% (one-pot syntheses).
Diketonate iridium(III) complexes were synthesized through
the μ-Cl dimer complexes obtained from iridium(III)
trischloride n-hydrate and corresponding ligand molecules.15,30
Purification by silica gel chromatography gave a brownish
eluent solution, which indicates some decomposition; however,
the product was successfully purified using alumina chromatog-
raphy. The yields were 63% for CF3acac, around 40% for
OCF3acac and MHacac, and 10−20% for others.
ferrocene
HOMO
ferrocene
onset
EHOMO = (E
−E
) + Eonset = 4.8 − 0.1 + Eonset
(1)
2.7. Photophysical Properties. UV−vis absorption spectra were
measured using a Jasco V570 spectrophotometer. Photoluminescence
(PL) spectra were measured using a Jasco F6010 fluorescence
spectrophotometer. PL lifetimes were measured using a Horiba
NAES-550 single photon counting spectrophotometer. Stabilizer-free
anhydrous 2-MeTHF (refluxed and distilled over sodium metal) was
used as a solvent for PL quantum yield and PL lifetime measurements.
A rectangular quartz cuvette equipped Pyrex tube was placed attaching
to a metal block in the spectrometer and maintained at a temperature
constant. The Pyrex tubes were submerged into a liquid-nitrogen-filled
Dewar flask equipped with a rectangular quartz optical window (77 K).
Sample solutions in 2-MeTHF were carefully deaerated using three
freeze−pump−thaw cycles. Emission quantum yields were determined
at 298 K using quinine sulfate dehydrate as a standard (in 0.5 M
sulfuric acid, excitation at 365 nm, ΦPL = 0.54634,35). fac-Ir(ppz)3 was
used as a standard (ΦPL = 1.0 in 2-MeTHF) at 77 K.36 The refractive
index of 2-MeTHF at 298 K was 1.405 08.37 Chromaticity coordinates
were measured using a Konica Minolta CS-100A luminance meter.
The radiative (kr) and nonradiative (knr) rate constants were calculated
from ΦPL and τ using the following equations (eqs 2 and 3):
At first, it was considered that the homoleptic complexes
could be synthesized through the diketonate complexes;15
however, the cyclometalated ligand exchange reaction of
iridium(III) trisacetylacetonate was employed to save time.28
We have successfully synthesized CF3piH (10%), OCF3piH
(23%), and FpiH (1%) with relatively low yields (yields in
parentheses).
Single crystals of homoleptic complexes of fac-CF3 and fac-
OCF3 were grown using a method with n-hexane vapor diffused
into CH2Cl2. The resultant crystals were brown and had a
distorted hexagonal columnar shape (Figure S1, Supporting
Information). X-ray crystallographic data including essential
bond lengths are listed in Table 1 and Table S1 in the
Supporting Information.
ORTEP diagrams31 of fac-CF3 and fac-OCF3 are presented in
Figure 2. Three cyclometalated ligands were hexagonally
coordinated around the iridium metal center. There was little
variation in the lengths of each of the three Ir−N and Ir−C
ΦPL = kr/(kr + knr)
(2)
τ = 1/(kr + knr)
(3)
Table 1. Bond Lengths for fac-CF3, fac-OCF3, and fac-
2.8. DFT and Time Dependent (TD)-DFT Calculations. DFT
and TD-DFT calculations were performed using the Gaussian 03
package38 at the RB3LYP39/ LANL2DZ40 level. The structures were
fully optimized, and TD-DFT calculations were performed with the
ground-state geometry to obtain the vertical excitation energies of the
low-lying singlet and triplet excited state of the complexes. The
contributions to the HOMO and LUMO were calculated using Gauss
Sum (ver 2.2).41
2.9. Fabrication of OLED Devices. The EL devices were
fabricated with the following configuration: Indium tin oxide (ITO)/
poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PE-
DOT:PSS, 40 nm)/light emitting layer (50 nm)/bathocuproine
Ir(tfmppz)3
bond distance [Å]
10
bond type
fac-CF3
fac-OCF3
fac-Ir(tfmppz)3
Ir1−C1
Ir1−C2
Ir1−C3
Ir1−N1
Ir1−N2
Ir1−N3
2.018(3)
2.017(4)
2.018(3)
2.109(3)
2.108(4)
2.108(3)
2.019(2)
2.019(2)
2.018(3)
2.105(2)
2.105(2)
2.106(3)
2.016(8)
2.016(8)
2.013(8)
2.114(7)
2.113(7)
2.116(8)
D
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Figure 2. ORTEP diagram of (a) fac-CF3 and (b) fac-OCF3 (hydrogen atoms are neglected for clarity).
Figure 3. MOs of fac-OCF3 (left side) and OCF3acac (right side).
bonds, which indicates high symmetry. Disorder was observed
for the trifluoromethoxy group of fac-OCF3. A similar complex
with trifluoromethyl substituents, fac-tris(1-(4-
(trifluoromethyl)phenyl)pyrazolato) iridium(III) (fac-Ir-
(tfmppz)3), was reported by Thompson, which had similar
bond lengths.10 The packing diagram indicates that the
rhombohedral lattice is composed of a pair of optical isomers;
i.e., these are racemic crystals (Figure S2 in Supporting
Information). A combination of three rhombohedral lattices
results in hexagonal columnar crystals.
therefore, other diketonate complexes may also have similar
structures.
3.2. DFT Calculation. Complex structures were optimized
using the DFT method with the B3LYP/LANL2DZ basis set
level.23,44 Optimized structures and the HOMO−LUMO of
fac-OCF3 and OCF3acac are plotted in Figure 3. For the
molecular orbitals (MOs), the atomic orbital coefficients of
each component, such as iridium metal (Ir), phenyl (Ph),
imidazolyl (Im), pyridyl (Py), and acetylacetone (acac)
moieties, are obtained using the Gauss Sum program, as
shown in Table 2.
In contrast, the CF3acac and OCF3acac diketonate complexes
gave fine needle-like crystals. Therefore, we have not succeeded
in measuring single crystal X-ray diffraction. MHacac (N966 in
ref 25) has been reported to have a meridional configuration;
The Ir−C (mean value 2.042 Å) and Ir−N (mean value
2.127 Å) bond lengths in fac-OCF3 were obtained by structural
optimization and are similar to the Ir−C (mean value 2.019 Å)
E
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is less effective to stabilize the HOMO than in the case of fac-
Ir(ppy)3.
Table 2. Calculated Energy Levels of the HOMO, LUMO,
LUMO + 1, and LUMO + 2 and Contribution of Iridium
Metal (Ir), Phenyl (Ph), Imidazolyl (Im), Pyridyl (Py), and
Acetylacetonate (acac) Moieties
The LUMO localized on the ligand (nearly 100%) is the
23
same result as that for fac-Ir(ppy)3 and fac-Ir(ppz)3.44 These
results strongly indicate a HOMO−LUMO transition with
MLCT character. Details of the LUMO in the phenyl (Ph)
moieties (53.6−64.1%) of homoleptic fac-CF3, fac-OCF3, and
fac-F have a higher contribution than those of the imidazolyl
(Im) parts (35.5−45.8%); therefore, these populations are
Ir
Ph
Im or Py acac E [eV]
fac-CF3
fac-OCF3
fac-F
LUMO
0.4
64.1
30.7
57.0
31.8
53.6
32.8
1.2
35.5
13.3
42.5
15.0
45.8
14.8
1.0
1.49
5.11
1.19
5.11
0.77
4.73
1.05
1.41
1.54
5.16
1.03
1.15
1.25
5.16
0.74
0.87
0.88
4.86
0.31
0.43
0.54
4.43
0.50
0.64
0.64
4.48
0.59
0.71
0.73
4.56
1.47
4.95
HOMO
56.0
0.5
LUMO
23
HOMO
53.2
0.6
quite different from the LUMO of fac-Ir(ppy)3 with 73.5%
LUMO
pyridyl (Py) and 25.9% phenyl moieties. The LUMO of
diketonate complexes also has a higher coefficient at the phenyl
moiety (51.2−62.3%) than at the imidazole moiety (35.7−
46.3%), similar to the homoleptic complexes. However, there
are two exceptions, OCH3acac and Hacac, where the LUMO is
localized at acetylacetone (93.6% and 83.8%, respectively). In
addition to these two extreme cases, other diketonate
complexes have a LUMO + 1 (MHacac and Facac) or
LUMO + 2 (CF3acac and OCF3acac) localized at acetylacetone
(e.g., Figure 3, right side). Complexes with trifluoromethyl
substituents, fac-CF3 and CF3acac, have LUMO highly localized
at the phenyl moiety with 64.1% and 62.3%, respectively.
The calculated results indicate that the HOMOs of iridium
phenylimidazolinate complexes have similar or slightly smaller
contributions at the phenyl moiety, as with fac-Ir(ppy)3;
however, much larger localization of the LUMO at the phenyl
moiety. Therefore, substitution of the phenyl group by EWGs
affects not only the HOMO, but also the LUMO. This type of
HOMO−LUMO relation has also been reported for iridium
phenyltriazolinate complexes.23
HOMO
52.4
2.2
CF3acac
LUMO + 2
LUMO + 1
LUMO
95.6
2.6
2.8
60.3
62.3
36.8
10.5
43.0
55.4
37.0
42.3
7.5
34.3
35.7
8.6
1.6
0.4
HOMO
48.8
2.8
5.8
OCF3acac
Facac
LUMO + 2
LUMO + 1
LUMO
8.8
77.9
20.3
0.4
2.2
34.5
42.3
9.9
1.9
HOMO
47.6
3.6
5.5
LUMO + 2
LUMO + 1
LUMO
39.7
7.4
14.4
83.7
0.4
1.4
2.1
51.9
38.4
46.4
50.9
2.2
45.6
10.2
46.2
46.8
2.6
HOMO
46.4
3.0
5.0
OCH3acac
Hacac
LUMO + 2
LUMO + 1
LUMO
4.4
2.0
0.3
1.6
93.6
4.6
HOMO
46.4
3.6
38.9
42.2
51.7
7.4
10.0
39.8
45.8
7.4
LUMO + 2
LUMO + 1
LUMO
14.4
0.4
2.1
1.4
83.8
4.8
3.3. Electrochemical Properties. Oxidation potentials
were determined by cyclic voltammetry in anhydrous THF to
estimate the effect of substituents on the HOMO energy level.
Voltammograms are shown in Figure S3 in the Supporting
Information, and the results are summarized in Table 3. All of
HOMO
47.0
3.7
39.8
37.6
11.4
51.2
39.8
25.9
38.9
8.4
MHacac
fac-Ir(ppy)3
LUMO + 2
LUMO + 1
LUMO
36.4
11.3
46.3
8.2
22.3
75.9
0.4
1.4
2.1
HOMO
47.1
0.2
4.9
9
LUMO
73.5
8.2
Table 3. HOMO Energy Levels Corresponding to the Rise-
Up Potentials of Oxidative Cyclic Voltammograms
HOMO
52.8
Eonset [mV]
EHOMO [eV]
fac-CF3
fac-OCF3
fac-F
237
210
161
222
377
307
92
5.04
5.00
4.96
5.02
5.18
5.11
4.89
4.96
4.92
4.833
and Ir−N (mean value 2.105 Å) bond lengths obtained by X-
ray crystallography. The Ir−C (mean value 2.017 Å) and Ir−N
(mean value 2.042 Å) bond lengths obtained by DFT for
OCF3acac are shorter than those of fac-OCF3. The results
correspond to the X-ray crystallographic results for fac-tris(2-
(p-tolyl)pyridinato) iridium(III) (fac-Ir(tpy)3) and iridium(III)
bis(2-(p-tolyl)pyridinato)- acetylacetonate (Ir(tpy)2(acac)) re-
ported by Thompson.11
CF3acac
OCF3acac
Facac
OCH3acac
Hacac
159
118
100
MHacac
ferrocene
MO analysis indicated that the HOMO is mainly localized in
the d-orbital (46.4−56.0%) and phenyl moiety (30.7−39.8%).
23
Similar behavior has been observed in fac-Ir(ppy)3 and fac-
Ir(ppz)3.44 More detailed analysis of the MOs of fac-CF3, fac-
OCF3, and fac-F revealed the HOMO contribution of the
phenyl moiety was 30.7−32.8%, which is smaller than that of
fac-Ir(ppy)3 (38.9%). Accordingly, the contribution of the
imidazole ring part (13.3−15.0%) is higher than that of the
pyridine part (8.2%) in fac-Ir(ppy)3. On the other hand, the
contributions of phenyl ring and heterocyclic ring parts of
diketonate complexes were similar to those of fac-Ir(ppy)3;
however, the contribution of the iridium d-orbital had smaller
values 46−48%, and these decreases appeared with an increase
of acetylacetone parts (4.6−5.8%). Therefore, in fac-CF3, fac-
OCF3, and fac-F, the substitution of EWGs on the phenyl ring
the complexes showed poor reversibility for oxidation
voltammograms. Complexes having F-atom, such as OCF3acac,
Facac, fac-F, and complex Hacac, showed reversibilities that
were not good. All the complexes examined gave irreversible
reduction potentials. Samuel, Ma, and their co-workers
reported difficulty in observing reversible reduction potentials
for blue phosphorescent complexes.23,44 These instabilities may
reflect on the OLED device instability. Gratzel and co-workers
̈
reported the oxidation potential of MHacac (N966)25 and
estimated the HOMO energy level to be 4.97 eV. In the present
experiment, the value is estimated to be 4.91 eV by the same
method as that given in the literature. The results of DFT
F
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calculations indicate that the HOMO of these complexes is
localized at the d-orbital of iridium and the π-orbital of the
phenyl moiety. The oxidation potentials shift toward the more
oxidative direction by substitution with EWGs. This is
explained by the stabilization of the HOMO due to
delocalization of the iridium d-orbital electron toward the
phenyl π-orbitals by substitution with EWGs. On the other
hand, the substitution of electron donating groups (EDGs)
induces destabilization of the HOMO, which is observed as a
more negative shift of the oxidation peak potential.
The Swain−Lupton constant,45 as a modification of the
Hammett rule, is used as an electron accepting and donating
parameter, as shown in Table 4. A larger F value is more σ-
Table 4. Substituents and their Swain−Lupton Constants45
F value
R value
CF3
0.38
0.39
0.45
0.29
0.03
0.01
−0.01
0.16
OCF3
F
−0.04
−0.39
−0.56
0.00
OCH3
H
CH3
−0.18
−0.15
(CH2)3CH3
electron inductive (inductive effect), and a smaller R is more π-
electron donative (resonance effect). While the F value of the F
substituent has the highest value at 0.45, fac-F and Facac have
oxidation potentials that are 50−70 mV smaller than those of
fac-OCF3 and OCF3acac. This is explained by the small R value
of −0.39 for the F atom due to the π-electron donating ability
of the nonbonding lone-pair electrons, which results in
destabilization of the HOMO.
The fac-CF3 complex has an oxidation potential 15 mV
smaller than that of CF3acac, which is explained by the stronger
electron donating ability of the phenylimidazole ligand
compared with that of acetylacetone, and this causes
destabilization of the iridium d-orbital.
For further investigation, the HOMO energy levels (EHOMO
)
Figure 4. Absorption spectra in 2-MeTHF at 298 K.
derived from the oxidation potentials are listed in Table S2 in
the Supporting Information. Complexes substituted with
trifluoromethyl groups, fac-Ir(tfmppz)3, fac-Ir(tfmpmb)3, and
fac-Ir(tfmptz)3, have EHOMO values in the range 5.4−5.5
eV,10,22,23 and the fac-CF3 imidazolinato complex has EHOMO
= 5.04 eV. This value is smaller than that for fac-Ir(ppy)3:
EHOMO = 5.11 eV.10 This provides an important insight: that
phenylimidazolinato complexes examined in this paper have
higher HOMO energy levels, by approximately 0.4 eV, than
those of complexes with similar skeleton structures, such as
phenylpyridinato, phenylpyrazolato, phenylimidazolinato car-
bene type, and phenyltriazolato complexes. Thus, this nature is
due to the imidazole moiety.46
3.4. Absorption Spectra. UV−vis absorption spectra of
complexes measured in 2-MeTHF are shown in Figure 4. The
absorption maxima (λmax) and molar extinction coefficients (ε)
are summarized in Table 5. Triplet excitation energies (Eg)
were assumed using the following equation (eq 4), where λonset
is the rise-up wavelengths of weak spin-forbidden 3MLCT
absorptions, h is the Planck constant, c is light speed, and e is
electron charge.
Table 5. Characteristics of Absorption Spectra in 2-MeTHF
Measured at 298 K
a
λonset
[nm]
Eg
[eV]
λabs (log ε) [nm]
fac-CF3
251 (4.60), 273 (4.53, sh), 288 (4.36),
355 (4.01)
460
2.70
fac-OCF3
249 (4.58), 266 (4.53, sh), 302 (4.11),
341 (4.02)
428
2.90
fac-F
248 (4.33), 302 (3.89), 331 (3.75)
265 (4.61), 336 (4.01), 366 (3.88, sh)
258 (4.52), 304 (4.15, sh)
413
466
436
423
423
449
3.00
2.66
2.84
2.93
2.93
2.76
CF3acac
OCF3acac
Facac
253 (4.50), 304 (4.17, sh)
OCH3acac 271 (4.57), 370 (3.73, sh)
Hacac
257 (4.52), 303 (4.08, sh), 351 (3.80, sh),
379 (3.65, sh)
MHacac
256 (4.46), 303 (4.00, sh), 350 (3.73, sh)
445
2.79
a
sh in parenthese indicates a peak observed as a shoulder.
The absorption band of iridium complexes can be generally
separated into two regions. The absorption band below 290 nm
Eg = hc/eλonset[eV]
is assigned to the spin-allowed 1LC (ligand-centered) transition
(4)
G
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Figure 5. Phosphorescence spectra of diketonate complexes measured in 2-MeTHF (at 298 K, left side) and in a glassy matrix (at 77 K, right side).
Photographs of phosphorescence of diketonate complexes measured in 2-MeTHF (at 298 K) and in a glassy matrix (at 77 K).
of the phenylimidazole moiety. The band around 350 nm is
Table 6. Phosphorescence Characteristics
1
1
assigned to spin-allowed MLCT. Bands longer than MLCT
λmax [nm]
CIE coordinates (x, y)
298 K 77 K
3
3
are assigned to a mixture of spin-forbidden LC and MLCT.
1
298 K
77 K
For the homoleptic complexes, the MLCT absorption band
appeared as a moderate peak; however, the diketonate
complexes showed a gradual decrease of absorption, as similarly
reported for fac-Ir(ppz)3 and Ir(ppz)2(acac).44 The change of
Eg by the introduction of substituents will be discussed in
section 3.5.
fac-CF3
fac-OCF3
fac-F
486, 518
483, 520, 560 (0.246,
0.467)
(0.225,
0.435)
461, 492
453, 482
498, 531
473, 503
539
454, 487, 525 (0.197,
0.315)
(0.173,
0.240)
446, 477, 509 (0.193,
0.289)
(0.172,
0.214)
3.5. Phosphorescence Spectra, Quantum Yields, and
Lifetimes. Phosphorescence spectra of the complexes
measured in deaerated 2-MeTHF are shown in Figure 5. The
characteristic phosphorescence data, such as emission λmax and
chromaticity coordinates, are summarized in Table 6. In
addition, the quantum yield (ΦPL) at 298 and 77 K, and
lifetimes (τ) at 298 K measured using the single-photon
counting method, are listed in Table 7. Emission decay profiles
for the homoleptic and CF3acac complexes showed single
exponential characteristics. Diketonate complexes other than
CF3acac showed double-exponential decay. One component
has a decay time of a microsecond, and another has less than a
couple of nanoseconds. The lifetime of MHacac in CH2Cl2 was
reported to be 24 ns.25
CF3acac
OCF3acac
Facac
492, 530, 572 (0.282,
0.531)
(0.256,
0.505)
466, 501, 539 (0.286,
0.434)
(0.197,
0.330)
455, 488, 523 (0.356,
0.435)
(0.215,
0.316)
OCH3acac 540
458, 491, 527 (0.426,
0.471)
(0.281,
0.404)
Hacac
485, 516, 542 472, 506, 545 (0.508,
0.491)
(0.211,
0.363)
MHacac
FIrpic
474, 512, 539 470, 504, 542 (0.371,
0.457)
(0.221,
0.353)
472
(0.174,
0.308)
In the case of the diketonate complexes at 298 K, only the
within 450−700 nm with poor ΦPL < 0.01. ΦPL for MHacac
was 0.0026, which is smaller than that reported for N966 (ΦPL
= 0.01525). This may be caused by solvent polarity; the
CF3acac complex showed strong phosphorescence (ΦPL
=
0.28), whereas the other complexes showed broad emission
H
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Table 7. Phosphorescence Quantum Yields and Lifetimes Measured in 2-MeTHF
ΦPL (298 K]
ΦPL (77 K)
τ [ns]
kr [105 s−1
]
knr [105 s−1
]
fac-CF3
fac-OCF3
fac-F
0.40
0.41
2.81 × 103
3.32 × 103
3.36 × 103
2.46 × 103
1.4
2.1
0.53
0.56
1.6
1.4
0.60
not measured
0.34
1.7
1.2
CF3acac
OCF3acac
Facac
0.28
1.1
2.9
0.0079
0.0037
0.0024
0.0022
0.0026
0.47
76.9 (96.6%), 0.960 (3.4%)
33.5 (82.5%), 257 (17.5%)
28.8 (48.8%), 0.465 (51.2%)
25.6 (89.7%), 1.01 (10.3%)
24.1 (48.9%), 0.463 (51.1%)
0.95
1.1
1.3 × 102
3.0 × 102
3.5 × 102
3.9 × 102
4.1 × 102
0.40
OCH3acac
Hacac
0.11
0.87
0.63
1.2
0.32
MHacac
0.28
emissions of imidazolyl diketonate complexes have been
reported to show strong solvent dependence.26
298 K (Figure 6). When the LUMO was localized on
acetylacetone (OCH3acac and Hacac), ΔE is treated as zero.
However, a clear vibrational structure appeared at 77 K, and
all complexes showed efficient blue to green phosphorescence
(ΦPL > 0.11). Observation of the emission 0,0 band in a glassy
matrix at 77 K assists understanding of the substituent effect on
the emission λmax. For example, λmax for CF3acac was red-shifted
by 20 nm (472−492 nm, 860 cm−1) relative to that of Hacac.
However, fac-Ir(ppy)3, which has small LUMO localization on
the phenyl moiety, had λmax = 494 nm with no spectral shift by
trifluoromethyl substitution at 77 K.29 DFT calculations
showed that the imidazolinate complexes studied here have
similar MO character with fac-Ir(ptz)3 reported by Samuel,
which exhibited a 13 nm (449−462 nm, 620 cm−1) red-shift by
trifluoromethyl substitution at 298 K.23 The red-shift of the
imidazolinate complexes and fac-Ir(ptz)3 by trifluoromethyl
substitution, where the LUMO is localized on the phenyl
moiety more than the HOMO, reveals the stabilization of the
LUMO. Comparison of the Swain−Lupton constants in Table
4 and λmax shows that the trifluoromethyl group has an F value
0.35 higher than that of H, which indicates the stronger
electron withdrawing effect. Complexes of OCF3acac, Facac,
and OCH3acac (except CF3acac) had λmax between 455 and
466 nm, which were blue-shifted compared with that for Hacac
at λmax = 472 nm. In particular, OCF3acac had λmax = 466 nm
that was 26 nm (1130 cm−1) shorter than that of CF3acac (492
nm). The F value of the trifluoromethoxy group is almost the
same as that of the trifluoromethyl group; however, the former
has an R value that is 0.20 smaller than that of latter. This
resonance effect destabilizes LUMO, and R values are even
smaller for the fluoro and methoxy groups; therefore,
destabilization of the LUMO gives a larger HOMO−LUMO
energy gap.
Figure 6. Plot of ΔE versus log ΦPL.
This plot indicates that a larger ΔE gives a larger ΦPL;
therefore, simple MO energy levels calculated for optimized
structure are useful to understand nonradiative deactivation
processes through the ancillary ligand. The weak and broad
emission of CF3acac and OCF3acac at 298 K is caused by the
quenching of excitation energy by the acetylacetone part. The
inefficiencies of other diketonated complexes at 298 K are due
to thermal activation to the upper excited state responsible for
nonradiative deactivation. Therefore, these thermal activations
are prohibited at 77 K. The smallest ΦPL of OCH3acac among
the diketonate complexes at 77 K is explained by the LUMO
being localized on acetylacetone.
In contrast, the homoleptic complexes showed efficient
emission, not only at 77 K, but also at 298 K (ΦPL = 0.40−
0.60). Phosphorescence spectra of homoleptic complexes are
shown in Figure 7. No significant difference of kr between the
homoleptic and diketonate complexes was evident, whereas knr
become smaller for the homoleptic complexes than for the
diketonate complexes. For example, in the case of fac-F and
Facac, kr is almost the same (1.1 and 1.7 × 105 s−1); however,
knr decreased by almost 1/300.
Substitution on the phenyl moiety with electron-donating
substituents is effective to obtain blue-shifted emissions. Hacac
and MHacac gave identical λmax; however, Hacac gave an x CIE
chromaticity coordinate that was 0.01 smaller than that of
MHacac. N-Butyl substitution caused a slight blue-shift than
that for N-methyl substitution (Figure 5 and Table 6).
As discussed in the DFT section, section 3.2, substitution has
a significant effect on the emission quantum efficiency. This can
be explained from the energy difference (ΔE) between the
LUMO and MO localized on the acetylacetone moiety. Table 2
shows the percentage MO (LUMO to LUMO + 2)
contribution of iridium metal (Ir), phenyl (Ph), imidazolyl
(Im), and acetylacetonate (acac) moieties, and the MO energy
levels (E) calculated by the DFT method. For example, in the
case of CF3acac, ΔE is 0.49 eV (ELUMO − ELUMO+2 = 1.54−
A blue-shift of λmax was observed for the homoleptic
complexes compared with those of the corresponding
diketonate complexes. For example, λmax values of CF3acac
and fac-CF3 were 492 and 483 nm at 77 K, respectively. Similar
blue-shifts of 8 nm (395 cm−1) were also observed for fac-
OCF3 and fac-F. In particular, that for fac-F (λmax = 453 nm) is
shorter than that for FIr6 (λmax = 458 nm47), which has one of
the shortest λmax of the phenylpyridinate complexes. The CIE
chromaticity coordinates (0.193, 0.289) were also equivalent to
those of FIr6 (0.16, 0.27).48 fac-OCF3 also had good
coordinates (0.197, 0.315) that are equivalent to those of
1.05), and for OCF3acac, ΔE is 0.22 eV (ELUMO − ELUMO+2
=
1.25−1.03). ΔE is plotted as function of the logarithm of ΦPL at
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Figure 7. Phosphorescence spectra for homoleptic complexes measured in 2-MeTHF (at 298 K) and in a glassy matrix (at 77 K). Photographs of
phosphorescence of homoleptic complexes measured in 2-MeTHF (at 298 K) and in a glassy matrix (at 77 K).
FIrpic (0.17, 0.34).49 fac-CF3 has low solubility; however, fac-
OCF3 has much better solubility in organic solvents. All of the
homoleptic complexes had emission lifetimes (2.81−3.36 μs)
suitable for device fabrication.
3.6. Temperature Dependence of Emission Lifetime.
Figure 8 and Table S3 in the Supporting Information show the
Figure 9. HOMO and LUMO energy levels calculated by the DFT
method (RB3LYP/LANL2DZ).
lifetime temperature dependence, a higher LUMO position
effectively decreases the emission lifetime. This is explained by
a thermal equilibrium between the state responsible for the
emission and the d* state responsible for nonradiative
deactivation. This phenomenon of phenylimidazolynato
Figure 8. Temperature dependence of emission lifetime measured in
complexes is similar to that observed for phenylpyridinato
2-MeTHF.
and phenylpyrazolynato complexes as reported by Thompson
and co-workers.50
effect of temperature on the emission intensity and lifetime for
fac-CF3, fac-OCF3, fac-F, CF3acac, and FIrpic between 283 and
353 K. In case of CF3acac, the lifetime at 298 K (2.46 μs) was
decreased 71% by increasing the temperature to 353 K (0.714
μs). This is caused by thermal activation of the nonradiative
deactivation process through the acetylacetone moiety.
However, almost no change was observed for fac-CF3 (2.81
μs at 298 K and 2.77 μs at 353 K). In the case of fac-OCF3,
lifetime decreased from 3.32 to 2.50 μs (25%) by increase of
the temperature from 298 to 353 K. The slope of the plot may
be larger for temperatures higher than 333 K. In the case of fac-
F, the lifetime was decreased 50%, from 3.36 μs (298 K) to 1.68
μs (353 K), by increase in the temperature. The frontier orbital
energy calculated by the DFT method is shown in Figure 9,
which indicates that the LUMO energy level is in the order of
fac-CF3 < fac-OCF3 < fac-F. In a comparison of this with the
3.7. OLED Device. OLED devices were fabricated for fac-
OCF3, which showed efficient luminescence in 2-MeTHF, and
FIrpic was used as a reference. Figure 10 shows the device
configuration. The wet-processable m-terphenyl derivative,
3,3″,5,5″-tetra(9H-carbazol-9-yl)-1,1′:3′,1″-terphenyl (mB-
4Cz), was synthesized as a host material.51 The HOMO−
LUMO energy levels are also indicated in the figure. The
reported HOMO energy level for FIrpic was obtained by CV,
and the HOMO−LUMO energy gap was determined by the
0−0 band (468 nm) in CH2Cl2 at 298 K.52 In this study, a
similar method was applied to determine the HOMO−LUMO
energy levels of fac-OCF3.
The performances of the fabricated devices were determined
from plots of current density (J, mA/cm2), luminance (L, cd/
m2), and current efficiency (η, cd/A) versus applied voltage (V,
V), as shown in Figure 11 and summarized in Table 8.
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Figure 10. Device structure and energy diagram51,52 (left, units in eV) and chemical structures of the materials (right).
Figure 11. (a) J−V, (b) L−V, and (c) η−V characteristics. (d) EL spectra and photographs of the fabricated devices.
smaller 0−0 bands than the 0−1 band, compared with the
spectra measured in 2-MeTHF (Figure 7). Therefore, the CIE
coordinates (x and y) were slightly increased. The coordinate of
FIrpic is more blue than that of fac-OCF3; however, there was
no significant difference observed by visual check.
The maximum luminance for fac-OCF3 and FIrpic was 889
and 3490 cd/m2, respectively. The lower luminance of fac-
OCF3 is explained by inefficient carrier injection into the
emitting layer. This is supported by both the inefficient current
density and larger driving voltage. The HOMO−LUMO energy
levels of fac-OCF3 were shifted by approximately 0.7 eV to
higher energy than those for FIrpic (Figure 10).
Table 8. Emission Characteristics for the OLED Devices
Measured in 2-MeTHF
λmax
Lmax
ηmax [cd/A], (J)
[nm] CIE (x, y) [cd/m2]
[mA/cm2]
fac-
OCF3
device
464,
494
(0.232,
0.354)
889
(15 V)
2.88 (13.2)
solution 461,
491
(0.197,
0.315)
FIrpic
device
474
(0.200,
0.345)
3490
(13 V)
3.86 (47.2)
solution 472
(0.174,
0.308)
From the energy diagram in Figure 10, the HOMO−LUMO
level of fac-OCF3 is moved by 0.7 V almost parallel to those of
FIrpic in the anodic direction. Therefore, charge injection from
EL spectra of both devices were red-shifted by 2−3 nm (140
and 89 cm−1 for fac-OCF3 and FIrpic, respectively) and had
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the charge conducting layer to the emitting layer became
difficult in the case of fac-OCF3. The HOMO level of fac-OCF3
is higher than that of PEDOT:PSS. The HOMO−LUMO
energy levels of imidazole have been reported to take high
values among some nitrogen-containing cyclic compounds,
according to ab initio calculations.46
The FIrpic device has the same configuration with the mB-
4Cz host materials. A hole only device (device fabricated
without a PEDOT:PSS layer) and an electron only device
(device fabricated with no BCP layer) were fabricated, and the
J−V characteristics were measured. The hole and electron only
devices showed current densities of 237 and 64 mA/cm2,
respectively, at an applied voltage of 10 V.42 Therefore, the high
hole and low electron transferability of mB-4Cz is partly
responsible for the moderate performance of these devices.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: karatsu@faculty.chiba-u.jp (T.K.). Fax: +81-43-290-
3401.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Dr. Koichi Ishibashi and Ms. Kyoko Endo
(Mitsubishi Chemical Corporation-Group Science and Tech-
nology Research Center) for stimulating discussions; Professor
Hyuma Masu, Professor Masami Sakamoto, and Dr. Fumitoshi
Yagishita (Analytical Center, Chiba University) for support
with single crystal X-ray crystallography; Professor Norihisa
Kobayashi and Mr. Naoki Ura (Department of Image Sciences,
Chiba University) for measurement of CV; and Dr. Lin Xu
(Department of Applied Chemistry and Biotechnology, Chiba
University) for fabrication of the OLEDs. The authors thank
the Analytical Center of Chiba University for help with the
materials analyses. This work was supported by a Grant-in-Aid
for Scientific Research (20550056) and the G-COE program
(Advanced School for Organic Electronics) from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT) of Japan.
4. CONCLUSIONS
Homoleptic and heteroleptic acetylacetonato phenylimidazoli-
nato Ir(III) complexes with various substituents were
synthesized. X-ray crystallographic analyses of single crystals
of fac-CF3 and fac-OCF3 showed their octahedral structures.
DFT calculations of all complexes revealed the HOMO
delocalized on the d-orbital of iridium and the phenyl moiety,
similar to that of fac-Ir(ppy)3. The LUMO of fac-Ir(ppy)3 is
localized at the pyridyl moiety; however, the LUMO of
imidazolinato complexes is localized at the phenyl moiety to a
greater extent than the HOMO.
These complexes had reversible oxidation voltammograms,
and the HOMO was destabilized by approximately 0.2−0.5 eV,
relative to those of phenylpyridinato, phenylpyrazolinato,
phenylimidazolinato-carbene type, and phenyltriazolinato
iridium(III) complexes.
Substitution on phenyl ring with trifluoromethyl groups
induced a red-shift of phosphorescence, and a blue-shift was
observed for other substituents, such as fluoro, methoxy, and
trifluoromethoxy groups. Therefore, π-electron donating
substituents (resonance effect) can induce a blue-shift of the
emission spectra. This is explained by the DFT results that
indicate localization of the LUMO on the phenyl moiety is
more significant than that for the HOMO. Diketonate
complexes with green phosphorescence gave efficient emission;
however, the blue phosphorescence complexes showed poor
efficiency. This is due to the shift of the emissive LUMO energy
level toward higher energy, which resulted in a decrease in the
energy difference between the LUMO and MO localized on the
diketonate ligand.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Crystallographic data in CIF format. H NMR, ESI-MS, CV,
time-correlated single photon counting decay profiles, and EL
spectra of complexes. These materials are available free of
charge via the Internet at http://pubs.acs.org.
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