Substituent effects of iridium complexes for highly efficient red OLEDs

Article abstract of DOI:10.1039/b417058j

This study reports substituent effects of iridium complexes with 1-phenylisoquinoline ligands. The emission spectra and phosphorescence quantum yields of the complexes differ from that of tris(1-phenylisoquinolinato-C ², N) iridium(in) (Irpiq) depending on the substituents. The maximum emission peak, quantum yield and lifetime of those complexes ranged from 598-635 nm, 0.17-0.32 and 1.07-2.34 μs, respectively. This indicates the nature of the substituents has a significant influence on the kinetics of the excited-state decay. The substituents attached to phenyl ring have an influence on a stability of the HOMO. Furthermore, those substituents have effect on the contribution to a mixing between ³π-π and ³MLCT for the lowest excited states. Some of the complexes display the larger quantum yield than Irpiq, which has the quantum yield of 0.22. The organic light emitting diode (OLED) device based on tris [1-(4-fluoro-5-methylphenyl) isoquinolinato-C²,N]iridium(III) (Ir4F5Mpiq) yielded high external quantum efficiency of 15.5% and a power efficiency of 12.41m W^-1 at a luminance of 218 cd m^-2. An emission color of the device was close to an NTSC specification with CIE chromaticity characteristics of (0.66, 0.34). The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b417058j

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F U L L P A P E R
Substituent effects of iridium complexes for highly efficient red
OLEDs
Shinjiro Okada, Keiji Okinaka, Hironobu Iwawaki, Manabu Furugori, Masashi Hashimoto,
Taihei Mukaide, Jun Kamatani, Satoshi Igawa, Akira Tsuboyama, Takao Takiguchi and
Kazunori Ueno*
OD project Canon Inc., 5-1, Morinosato Wakamiya, Atsugishi, Kanagawa, Japan.
E-mail: ueno.kazunori@canon.co.jp, okada.shinjiro@canon.co.jp
Received 9th November 2004, Accepted 2nd March 2005
First published as an Advance Article on the web 24th March 2005
This study reports substituent effects of iridium complexes with 1-phenylisoquinoline ligands. The emission spectra
and phosphorescence quantum yields of the complexes differ from that of tris(1-phenylisoquinolinato-C2,N)
iridium(III) (Irpiq) depending on the substituents. The maximum emission peak, quantum yield and lifetime of those
complexes ranged from 598–635 nm, 0.17–0.32 and 1.07–2.34 ls, respectively. This indicates the nature of the
substituents has a significant influence on the kinetics of the excited-state decay. The substituents attached to phenyl
ring have an influence on a stability of the HOMO. Furthermore, those substituents have effect on the contribution to
a mixing between 3p–p* and 3MLCT for the lowest excited states. Some of the complexes display the larger quantum
yield than Irpiq, which has the quantum yield of 0.22. The organic light emitting diode (OLED) device based on tris
[1-(4-fluoro-5-methylphenyl)isoquinolinato-C2,N]iridium(III) (Ir4F5Mpiq) yielded high external quantum efficiency
of 15.5% and a power efficiency of 12.4 lm W−1 at a luminance of 218 cd m−2. An emission color of the device was
close to an NTSC specification with CIE chromaticity characteristics of (0.66, 0.34).
Introduction
Experimental
The development of the first thin film hetero-junction organic
light emitting diode in 1987 by Tang and VanSlyke1 had
stimulated the research in OLEDs for practical applications
using fluorescent materials, for the last 15 years. Especially,
a lot of efforts have been concentrated on developing new
materials with high quantum yields, for the realization of higher
efficiency. Recently, OLEDs using phosphorescent materials are
attracting great interest due to its potential in the application of
flat panel display. Iridium(III) complexes with 2-phenylpyridine
are well known to exhibit high triplet quantum yields due
to the mixing of the singlet excited state and the triplet
excited state via spin–orbit coupling, which leads to high
phosphorescence yields, enhancing the triplet state.2–4 The goal
of material research is to achieve high power efficiency and
low power consumption when OLED display is used in mobile
appliance.5–8
NMR spectra were recorded on a Bruker model DPX-400 (1H:
400 MHz, 13C: 100 MHz) or a Bruker model AVANCE-500
(13C: 125 MHz) NMR spectrometer. Mass spectra data were
measured by JEOL JMS-700 operating in electron impact (EI).
Elemental analysis was carried out with an elemental analyzer
Vario EL CHNOS from Elementar Co.
Crystals of iridium complex suitable for X-ray analysis
were obtained from chloroform solutions at room temperature.
Diffraction data were collected at 93.1 on a Rigaku RAXIS-
RAPID imaging plate diffractometer equipped with graphite-
˚
monochromated Mo Ka radiation (k = 0.71069 A). The crystal
structure was solved by the direct method using SIR92.20
The crystal structure was refined by the full-matrix least-
squares method on F2. All non-hydrogen atoms were refined
anisotropically, while hydrogen atoms were refined isotropically.
All analyses were performed by the crystallographic software
package CrystalStructure 3.6.0.21
Our previous reports on electroluminescence from Ir com-
plex with 1-phenylisoquinoline as the ligands, (Irpiq: Tris(1-
[C45H27N3F3Ir][CHCl3], M 978.32 g mol−1, T 93.1 K, mon-
oclinic, P21/n, ◦Z 4, a 13.3262(4), b 19.4561(4), c 14.3648(3)
phenylisoquinolinato-C2,N)iridium(III)) doped into
a
4,4ꢀ-
3
−1
˚
˚
bis(carbazole-9-yl)biphenyl (CBP) emphasized its dramatically
increased power efficiency of 6.8 lm W−1 (8.2 cd A−1) at
100 cd m−2 and excellent color CIE characteristics (0.68, 0.32)
for red.9,10
A, b 96.785(1) , V 3698.4(2) A , l(MoKa)/mm , graphite
monochromator, 10795 reflections used, R 0.032, Rw 0.006.
CCDC reference number 240125.
See http://www.rsc.org/suppdata/dt/b4/b417058j/ for cry-
stallographic data in CIF or other electronic format.
Photoluminescence spectra were recorded on a Hitachi F4500
fluorescence spectrometer using 464 nm excitation pulse. Spec-
tral data were corrected with the use of emission spectra of
tris(2-phenylpyridinato-C2,N)iridium(III) (fac-Irppy) and fac-
Irpiq measured by Spectroradiometer SR1 from TOPCON Ltd.
Phosphorescence quantum yields were measured at room tem-
perature using an N2-saturated toluene solution of 10−5 M. The
quantum yields were determined using fac-Irppy as a reference
(φ = 0.4).22
The substitution effect on 2-phenylpyridine ligand of
iridium,11–13 cyclometalating ligands of platinum complexes14
and iridium complexes,15,16 diimine ligands of platinum
complexes17 and the effect of phenanthroline ligand sub-
stituents on radiative and nonradiative process concerning
ruthenium(II) and iridium(III) complexes had been also reported
previously.18Y. J. S u et al. had reported the study on fluorine
substituted Irpiq.19 In the present work we report the effect of
a series of substituent on the phenyl ring of Irpiq. Also the
luminescence, quantum yield, lifetime, analysis of luminescing
state and the performance of organic EL device using these
complexes are discussed in detail. These results enable us to
expand our knowledge of fine tuning capabilities of ligand
substituents and establish a guideline for designing novel red
phosphorescent molecules.
Emission lifetime measurements were carried out using a
Hamamatsu Photonics C4334 streakscope with excitation light
(k = 420 nm) from a N2 laser (LN120C from Laser Photonics).
UV-visible spectra were recorded on a Shimadzu UV3100S
spectrophotometer.
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 1 5 8 3 – 1 5 9 0
1 5 8 3
©

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The cyclic voltammograms of the complexes were measured in
CH2Cl2 (Eox1/2), THF (Ered1/2) solutions containing n-Bu4NClO4
(0.1 M) at a 20 mV s−1 scan rate using a Pt rod (Eox1/2) working
electrode, a glassy carbon (Ered1/2) working electrode, a Pt rod
counter electrode, and an Ag/Ag+ reference electrode under an
N2 atmosphere. All potentials are reported against an Ag/Ag+
reference.
MS (70 eV): m/z 805 (M+), 601 ([M − ligand]+), 402 (M2+), 204
([ligand]+).
Ir4Fpiq (2). Tris[1-(4-fluorophenyl)isoquinolinato-C2,N]-
1
iridium(III). H NMR (CDCl3, 400 MHz) d (ppm): 8.89 (m,
3H), 8.17 (m, 3H), 7.74 (m, 3H), 7.66 (m, 6H), 7.29 (d, 3H, J =
6.1 Hz), 7.12 (d, 3H, J = 6.1 Hz), 6.66–6.70 (m, 6H), 6.60 (m,
3H). 13C NMR (CDCl3, 100 MHz) d (ppm): 167.2 (d, JC–F
5.8 Hz), 166.5, 163.9 (d, JC–F = 253.1 Hz), 141.4 (d, JC–F
=
=
Synthesis, general procedure
1.9 Hz), 139.6, 136.7, 131.8 (d, JC–F = 9.7 Hz), 130.4, 127.8,
127.5, 127.1, 126.3, 122.5 (d, JC–F = 16.4 Hz), 120.2, 107.3 (d,
JC–F = 22.2 Hz). Anal. Calc. for C45H27F3IrN3: C, 62.93; H,
3.17; N, 4.89. Found: C, 62.97; H, 3.41; N, 4.78%. MS (70 eV):
m/z 859 (M+), 637 ([M − ligand]+), 429 (M2+), 222 ([ligand]+).
All ligands were prepared through Suzuki coupling. Ligands
with isoquinoline core were synthesized by the reaction of
corresponding halogenated isoquinoline and aryl boronic acids.
Other ligands with substituted 1-phenylisoquinolines were syn-
thesized by analogous reactions. The ligand compounds were
purified by silica gel column chromatography with CHCl3 as
an eluent. Phenyl isoquinoline ligands were prepared from
isoquinoline N-oxide as a precursor. The synthesis scheme of
1-phenylisoquinoline is given below (Scheme 1).
Ir4F5Mpiq (3). Tris[1-(4-fluoro-5-methylphenyl)isoquino-
linato-C2,N]iridium(III). 1H NMR (CDCl3, 400 MHz) d (ppm):
8.99 (m, 3H), 8.02 (d, 3H, J = 8.0 Hz), 7.70 (m, 3H), 7.64 (m,
6H), 7.27 (d, 3H, J = 6.2 Hz), 7.07 (d, 3H, J = 6.2 Hz), 6.55 (d,
3H, J = 11.0 Hz), 2.21 (s, 9H). 13C NMR (CDCl3, 100 MHz) d
(ppm): 166.6, 163.6 (d, JC–F = 6.8 Hz), 162.8 (d, JC–F = 253.1 Hz),
141.3 (d, JC–F = 1.9 Hz), 139.7, 136.7, 133.2 (d, JC–F = 5.8 Hz),
130.2, 127.6, 127.5, 127.0, 126.4, 122.0 (d, JC–F = 16.4 Hz), 120.0,
115.8 (d, JC–F = 19.3 Hz), 15.0 (d, JC–F = 3.9 Hz). Anal. Calc. for
C48H33F3IrN3: C, 63.99; H, 3.69; N, 4.66. Found: C, 64.10; H,
4.10; N, 4.60%. MS (70 eV): m/z 901 (M+), 665 ([M − ligand]+),
451 (M2+), 236 ([ligand]+).
Scheme 1
Complexes were synthesized according to a previous paper.13
Ir(acac)3 [iridium(III) acetylacetonate] (0.5 g, 1.0 mmol) and
cyclometalating ligand (ca. 5 mmol) were dissolved in 50 ml
glycerol. The solution was refluxed under nitrogen stream for 6 h.
After completion of the reaction, addition of 1 M HCl resulted
in precipitation of the product, which was filtered, washed with
water, and dried at 100 ◦C in vacuum. The product was purified
by silica gel column chromatography with CHCl3 as an eluent.
Yields of the products were in the range from 11 to 45% based
on Ir(acac)3. Molecular structures of the complexes in this paper
and their abbreviations are listed in Table 1.
Ir4MOpiq (4). Tris[1-(4-methoxyphenyl)isoquinolinato-C2,
N]iridium(III). H NMR (CDCl3, 400 MHz) d (ppm): 8.88 (m,
1
3H), 8.11 (d, 3H, J = 8.5 Hz), 7.70 (m, 3H), 7.60 (m, 6H), 7.62
(m, 6H), 7.26 (d, 3H, J = 6.2 Hz), 7.03 (d, 3H, J = 6.2 Hz), 6.58
(m, 3H), 6.54 (m, 3H), 3.56 (s, 9H). 13C NMR (CDCl3, 100 MHz)
d (ppm): 168.2, 167.2, 160.6, 139.8, 138.4, 136.7, 131.3, 129.9,
127.6, 127.1, 126.9, 126.1, 120.3, 119.0, 106.8, 54.6. Anal. Calc.
for C48H36IrN3O3: C, 64.41; H, 4.05; N, 4.69. Found: C, 64.50; H,
4.30; N, 4.60%. MS (70 eV): m/z 895 (M+), 661 ([M − ligand]+),
448 (M2+), 234 ([ligand]+).
IrC4piq (5). Tris[1-(4-butylphenyl)isoquinolinato-C2,N]-
The 1H NMR spectra and elemental analysis data of the
complexes 1 to 8 are as follows.
1
iridium(III). H NMR (CDCl3, 400 MHz) d (ppm): 8.94 (m,
3H), 8.07 (d, 3H, J = 8.2 Hz), 7.69 (m, 3H), 7.60 (m, 6H), 7.23
(d, 3H, J = 6.2 Hz), 7.03 (d, 3H, J = 6.2 Hz), 6.82 (d, 3H, J =
2.0 Hz)6.78 (m, 3H), 2.37 (m, 6H), 1.46 (m, 6H), 1.22 (6H, m),
0.82 (t, 9H, J = 7.2 Hz). 13C NMR (CDCl3, 100 MHz) d (ppm):
167.9, 165.3, 144.4, 142.8, 139.9, 137.6, 136.6, 129.94, 129.87,
127.8, 127.1, 126.9, 126.4, 119.9, 119.7, 35.6, 33.5, 22.4, 14.0.
Anal. Calc. for C57H54IrN3: C, 70.34; H, 5.59; N, 4.32. Found:
C, 70.10; H, 5.60; N, 4.10%. MS (70 eV): m/z 973 (M+), 713
([M − ligand]+), 487 (M2+), 260 ([ligand]+).
Irpiq (1). Tris(1-phenylisoquinolinato-C2,N)iridium(III). 1H
NMR (CDCl3, 400 MHz) d (ppm): 8.96 (m, 3H), 8.18 (d, 3H,
J = 7.9 Hz), 7.71 (m, 3H), 7.62 (m, 6H), 7.33 (d, 3H, J =
6.1 Hz), 7.09 (d, 3H, J = 6.1 Hz), 6.94–6.99 (m, 6H), 6.84 (t, 3H,
J = 7.4 Hz). 13C NMR (DMF-d6, 125 MHz) d (ppm): 168.8,
166.4, 147.4, 141.7, 138.8, 138.6, 132.2, 131.7, 130.9, 129.8,
129.2, 128.7, 128.2, 122.4, 121.2. Anal. Calc. for C45H30IrN3:
C, 67.14; H, 3.76; N, 5.22. Found: C, 67.05; H, 3.80; N, 5.25%.
Ir4Mpiq (6). Tris[1-(4-methylphenyl)isoquinolinato-C2,N-
Table 1 Tris-cyclometalated Ir complexes and abbreviations
1
]iridium(III). H NMR (CDCl3, 400 MHz) d (ppm): 8.94 (m,
3H), 8.06 (d, 3H, J = 8.2 Hz), 7.70 (m, 3H), 7.61 (m, 6H), 7.28
(d, 3H, J = 6.2 Hz), 7.04 (d, 3H, J = 6.2 Hz), 6.81 (s, 3H), 6.77
(d, 3H, J = 8.2 Hz), 2.15 (s, 9H). Anal. Calc. for C48H36IrN3: C,
68.06; H, 4.28; N, 4.96. Found: C, 68.05; H, 4.64; N, 4.75. 13C
NMR (CDCl3, 100 MHz) d (ppm): 167.7, 165.2, 142.7, 139.8,
139.6, 138.0, 136.6, 129.91, 129.86, 127.8, 127.1, 126.9, 126.4,
120.6, 119.6, 21.7 Anal. Calc. for C54H30IrN3: C, 68.06; H, 4.28;
N, 4.96. Found: C, 68.00; H, 4.60; N, 4.70%. MS (70 eV): m/z
847 (M+), 629 ([M − ligand]+), 423 (M2+), 218 ([ligand]+).
Attached position
Ir5iPrpiq (7). Tris[1-(5-isopropylphenyl)isoquinolinato-C2,
No.
Complex
X1
X2
1
N]iridium(III). H NMR (CDCl3, 400 MHz) d (ppm): 8.95 (m,
3H), 8.02 (s, 3H), 7.70 (m, 3H), 7.61 (m, 6H), 7.24 (d, 3H,
J = 6.2 Hz), 7.06 (d, 3H, J = 6.2 Hz), 6.81 (m, 6H), 2.87 (m,
3H), 1.26 (t, 18H, J = 7.2 Hz). 13C NMR (CDCl3, 100 MHz)
d (ppm): 168.2, 161.3, 145.1, 139.9, 139.3, 136.9, 136.6, 129.9,
128.7, 128.3, 127.8, 127.2, 126.9, 126.6, 120.0, 33.9, 24.3, 24.2.
Anal. Calc. for C54H48IrN3: C, 69.65; H, 5.20; N, 4.51. Found: C,
69.60; H, 5.20; N, 4.50%. MS (70 eV): m/z 931 (M+), 685 ([M −
ligand]+), 465 (M2+), 246 ([ligand]+).
1
2
3
4
5
6
7
8
Irpiq
Ir4Fpiq
F
F
Ir4F5Mpiq
Ir4MOpiq
IrC4piq
Ir4Mpiq
Ir5iPrpiq
Ir5Mpiq
Methyl
Methoxy
Butyl
Methyl
Isopropyl
Methyl
1 5 8 4
D a l t o n T r a n s . , 2 0 0 5 , 1 5 8 3 – 1 5 9 0

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Ir5Mpiq (8). Tris[1-(5-methylphenyl)isoquinolinato-C2,N]-
iridium(III). H NMR (CDCl3, 400 MHz) d (ppm): 8.96 (m,
A molecular plot of Ir4Fpiq (2) is shown in Fig. 2. Some
selected bond lengths (A) and angles ( ) for Ir4Fpiq (2) and Irpiq
◦
1
˚
3H), 8.01 (s, 3H), 7.70 (m, 3H), 7.62 (m, 6H), 7.28 (d, 3H, J =
6.2 Hz), 7.06 (d, 3H, J = 6.2 Hz), 6.47 (d, 3H, J = 7.5 Hz), 6.72
(m, 3H) 2.30 (s, 9H). Anal. Calc. for C48H36IrN3: C, 68.06; H,
4.28; N, 4.96. Found: C, 69.60; H, 5.2; N, 4.5. 13C NMR (CDCl3,
100 MHz) d (ppm): 167.7, 165.6, 145.4, 139.9, 137.2, 136.6,
131.3, 130.7, 129.9, 128.0, 127.8, 127.3, 126.9, 126.6, 120.2, 21.5.
Anal. Calc. for C48H36IrN3: C, 68.06; H, 4.28; N, 4.96. Found:
C, 68.0; H, 4.5; N, 4.9%. MS (70 eV): m/z 847 (M+), 629 ([M −
ligand]+), 424 (M2+), 218 ([ligand]+).
(1) are presented in Table 2. The detailed characteristic data of
Irpiq (1) had been reported.10 All bonds and angles of those
two complexes are very similar. The two complexes have facial
1
octahedral structure. The H NMR spectra of all complexes 1
to 8 are consistent with the facial structure, which indicates that
the number of coupled spins is equal to that of protons on one
ligand because the three ligands are magnetically equivalent due
to the 3-fold symmetry.
OLED fabrication and evaluation
The device geometry of the OLED is shown in Fig. 1.
Fig. 2 ORTEP drawing of Ir4Fpiq (2). The thermal ellipsoids repre-
senting a 50% probability level. Hydrogen atoms are omitted for clarity.
The absorption and photoluminescence (PL) properties of the
newly synthesized iridium complexes are summarized in Table 3.
The complexes have quantum yield ranging from 0.17 to 0.32,
emission maximum peak (kmax)), ranging from 598 to 635 nm
and lifetimes ranging from 1.07 to 2.34 ls, indicating that the
nature of the substituents and therefore ligands have a significant
influence on the kinetics of the excited-state decay. A maxmum
peak of the phosphorescent spectrum and phosphorescent life
time of Ir4Fpiq (2) were consistent with the observation of
Y. J. S u et al.19
Hartree–Fock calculations were carried out on Irpiq (1),
Ir4Fpiq (2) and Ir4F5Mpiq (3) using the Gaussian98 software
package (Gaussian, Inc.)23 with a LanL2DZ basis set.14,24–27 The
Hartree–Fock calculations suggest that the highest occupied
molecular orbital (HOMO) of the three complexes are mainly
localized on the phenyl ring and iridium center. On the contrary,
the lowest unoccupied molecular orbital (LUMO) is mainly
localized on the isoquinoline ring portion of the cyclometalating
ligand (Fig. 3). The orbital picture predicts that variation in the
electronic properties of the ligands should have an effect on the
energy of the excited state. The calculated HOMOs of Irpiq (1),
Fig. 1 Device structure and the molecular structures of the compounds
used in the device.
The organic materials were deposited by thermal evaporation
at pressures of less than 10−4 Pa. The devices were fabricated on
indium tin oxide (ITO) film (150 kX m−2, thickness 120 nm, from
Nippon Sheet Glass Co.) with a 3.14 mm2 round-patterned area.
A four layer device employed an ITO as an anode, a 40 nm thick
a-NPD (4,4ꢀ-bis(N-1-naphthyl-N-phenylamino)biphenyl) as the
hole transport material, Ir4F5Mpiq (3) as the luminescent ma-
terial, CBP (4,4ꢀ-bis(carbazole-9-yl)biphenyl) as a host material
of a 20 nm thick light emitting layer, a 50 nm thick Bphen
(4,7-diphenyl-1,10-phenanthroline) as the electron transport
material, a 1 nm thick potassium fluoride as an electron injection
layer and 150 nm thick Al as the cathode. The device structure
and the structural materials are shown in Fig. 1. The light-
emitting layer was prepared by coevaporating a host (CBP) and a
phosphorescent material as a dopant, with both deposition rates
being controlled with two independent quartz crystal oscillators.
The doping concentration of the dopant was a 10 wt%.
After the device fabrication, they were encapsulated with
calcium oxide as desiccant. Current–voltage characteristics of
the devices were measured by an automated homemade set
up consisting a waveform generator Yokogawa AG1200, and
a digital oscilloscope TEKTRONIKS TDS420. A luminance
colorimeter BM-7 from TOPCON Ltd was used for measuring
the luminance of the OLED.
◦
˚
Table 2 Selected bond distances (A) and angles ( ) for Irpiq (1) and
Ir4Fpiq (2)
Irpiq (1)
Ir4Fpiq (2)
˚
Bond lengths/A
Ir(1)–N(1)
Ir(1)–C(15)
N(1)–C(1)
N(1)–C(9)
2.135(5)
2.009(6)
1.374(8)
1.339(8)
2.134(3)
2.006(3)
1.364(4)
1.348(4)
Results and discussion
Bond angles/◦
N(1)–Ir(1)–C(15)
Ir(1)–N(1)–C(1)
Ir(1)–N(1)–C(9)
Ir(1)–C(15)–C(10)
C(1)–N(1)–C(9)
78.5(2)
124.8(4)
115.1(4)
116.3(4)
119.7(5)
78.4(1)
123.5(2)
116.1(2)
115.8(2)
120.3(3)
(1) Characterization of the complexes
The iridium center of Ir4Fpiq (2) is octahedrally coordinated
by three bidentate ligands with a facial structure and a 3-fold
rotational axis passing through the iridium atom.
D a l t o n T r a n s . , 2 0 0 5 , 1 5 8 3 – 1 5 9 0
1 5 8 5

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Table 3 absorption and photoluminescence properties of complexes 1–8
c
d
Quantum yield
Lifetime
s/ls
krb
×
knr
×
Absorptiong k/nm (emol
10−3/M−1 cm−1
×
Hammett
constantf
No.
Complex
Ua
103/s−1
103/s−1
)
kmaxe,g/nm
1
2
3
4
5
6
7
8
Irpiq
Ir4Fpiq
0.22
0.32
0.26
0.23
0.25
0.26
0.17
0.17
1.12
2.34
1.65
1.81
1.36
1.29
1.07
0.97
194.2
138.2
154.7
125.0
187.2
203.2
161.4
177.7
698.2
288.6
451.4
426.3
550.8
569.6
777.6
853.20
430(15), 477(11), 544(4), 595(2)
404(19), 456(12), 568(1)
621
598
607
610
620
621
635
635
0.00
0.34
0.17
Ir4F5Mpiq
Ir4MOpiq
IrC4piq
Ir4Mpiq
Ir5iPrpiq
Ir5Mpiq
419(17), 471(10), 518(2)h
418(29), 469(15), 516(4)h
0.12
428(17), 476(11), 550(4), 598(1)
428(18), 475(13), 534(5), 589(2)
446(16), 456(13), 534(5), 589(2)
444(15), 486(12), 558(4), 606(2)
−0.08
−0.07
−0.15
−0.17
a Quantum yield at room temperature in toluene below 10−6 M measured with 464 nm excitation Relative value on the basis of Irppy as the quantum
yield is 0.4. b Radiative rate constant calculated from kr = U/s. c Nonradiative rate constant calculated from knr = 1/ s − kr. d Molar absorbance
coefficient. e Corrected maximum emission peak by PMA11 (Hamamatsu Photonics). f Hammett constant (rp, rm).24,25,26 g Absorption and emission
peak, kmax, were measured in toluene solution. h Shoulder peak.
These results are in agreement with the calculation (Fig. 3).
As seen from Table 4, electron donating substituent on phenyl
ring such as alkyl group is effective to raise the energy of the
1/2
HOMO (the Eox is lower) and consequently reducing the
energy gap, while the phenyl rings with electron-withdrawing
substituents such as fluorine lowers the energy of the HOMO
to make the energy gap wider. The emission energies increase
along the series, 4-fluoro(4F) > 4-fluoro-5-methyl (4F5M) >
4-methoxy (4MO) > 4-butyl (C4) > H > 4-methyl (4M) > 5-
isopropyl (5iPr) > 5-methyl (5M) as shown in Table 3. This
tendency arises from the substituent effect of the d orbital (t2g)
stabilization of iridium metal through the carbon atom–iridium
bonding. And this tendency identifies with the inductive trend of
the substituents. Therefore the HOMO stability and the emission
energy gap are controlled by the nature of substituents and its
inductive influence on the aromatic ring.
(3) A relationship between HOMO and Hammett constant
Fig. 3 Hartree–Fock results of HOMO (left) and LUMO (right) for
Skarda et al. had explained the substitution effect of 4,4ꢀ-
Irpiq (1). The HOMO is mainly localized on the phenyl ring and iridium
and 5,5ꢀ-disubstituted 2,2ꢀ-bipyridyls by plotting the reduction
and the LUMO is localized on the isoqunoline ring mainly.
potential, Ered1/2, against the Hammett constant r.28 We tried to
Ir4Fpiq (2) and Ir4F5Mpiq (3) were −7.37, −7.63 and −7.83 eV,
quantify the substitution effect for HOMO level by plotting
respectively. And the LUMOs of the above mentioned complexes
were 1.57, 1.27 and 1.34 eV, respectively (shown in Fig 3, inset).
1/2
oxidation potential Eox
against the Hammett constant r
(Fig. 4). In the case of double substitution at meta- and para-
positions on a phenyl ring, the sum of both rm and rp was
applied.28–31 It is clear from Fig. 4 that there exists a good
correlation between oxidation potentials of iridium complexes
and the rs of substituents on their ligands were obtained. In this
work we proposed the HOMO level of the complexes could be
rationalized to Hammett constant. The emission energy from
a series of the Ir complexes, 1–8 versus Hammett constants is
plotted in Fig. 5, which shows a linear relation between the
emission energy and Hammett constant (Line A). The ligands
that have the substituent with the larger Hammett constant
(electron withdrawing substituents) lead to large emission energy
by the lowering of the HOMO. On the contrary, the smaller
Hammett constant (electron donating substituents) leads to a
smaller emission energy by raising the HOMO, as shown in
(2) Substituent effect of phenyl ring on HOMO
Table 4 shows electrochemical data of the complexes 1–6 which
have substituents on phenyl ring. It is obvious that the HOMO
level of the compounds 1–6 is strongly affected by a kind
of substituent on the phenyl ring. Especially, the oxidation
potential of the electron-withdrawing group ‘fluorine’ [4Fpiq
(2), 4F5Mpiq (3)] is higher in comparison with the electron
donating alkyl groups [C4piq (5), 4Mpiq (6)]. On the other
hand, substituent effect on the LUMO is very small. As given
in Table 4, the difference of the reduction potential between the
highest 4Fpiq (2) and the lower 4Mpiq (6) was only 0.17 V. The
oxidation and reduction potentials increase along the series Irpiq
(1) < Ir4F5Mpiq (3) < Ir4Fpiq (2).
Table 4 Electrochemical data for the complexes
No.
Complex
Hammett constant
Eox1/2/V (DE/mV)a
Ered1/2/V (DE/mV)a
1
2
3
4
5
6
7
8
Irpiq
Ir4Fpiq
0
0.34
0.17
0.12
−0.08
−0.07
−0.15
−0.17
0.45 (60)
0.65 (70)
0.54 (60)
0.43 (70)
0.36 (60)
0.44 (120)
0.36 (310)
0.32 (80)
−2.18 (70)
−2.08 (90)
−2.17 (110)
−2.28 (100)
−2.18 (90)
−2.25 (120)
N.d. (—)
Ir4F5Mpiq
Ir4MOpiq
IrC4piq
Ir4Mpiq
Ir5iPrpiq
Ir5Mpiq
N.d. (—)
1/2
1/2
a Eox and Ered are the oxidation and reduction potentials between peaks, respectively. DE is the peak potential difference.
1 5 8 6
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3
complexes are predominantly MLCT as same as that of Irpiq
(1).10,35,36 For more detail, we should consider degree of mixing
to understand the photochemical differences of those complexes
according to eqn. (3-1).
UT = aUT(pp*) + bUT(MLCT)
(3-1)
where a and b are the normalized coefficients and UT(pp*)
3
3
and UT(MLCT) are the wave functions of p–p* and MLCT
excited states, respectively. For the iridium complex, the wave
function of the triplet state (UT) responsible for phosphorescence
is principally expressed as eqn. (3-1). Eqn. (3-1) implies that the
excited triplet states of the iridium complexes are mixture of
UT(pp*) and UT(MLCT)10,37,38
Four typical characteristics regarding the emission spectrum
and the absorption spectrum of the 3p–p* state were proposed.
The first is the vibrational sideband pattern of the luminescence
from the 3p–p* state. On the contrary, the broad shape of
luminescence spectrum is a typical characteristic of the emission
Fig. 4 Oxidation potential versus Hammett constant for substituted
2-phenylisoqunoline Ir complexes shown in Table 3.
3
from MLCT excited state.33,39 Second is the smaller radiative
3
3
decay rate constant of p–p* state compared to MLCT state.
Therefore, for the third, the molar absorption constant of the
singlet–triplet absorption band of the 3p–p* state is smaller than
3
that of the MLCT excited state.24,39 The fourth is the short
3
phosphorescence lifetime for the MLCT. The phosphorescent
3
lifetime of the MLCT dominant complex is shorter than that
of the 3p–p* dominant complex.9,10,40,41
We examined the photophysical properties of complexes 1–8
in view of the above four criteria.
(i) The vibrational sideband patterns of the photolumines-
cence for the complexes 2, 3 and 4 are more prominent than
those of the complexes 1, 5, 6, 7 and 8 as shown in Fig. 6(a) and
(b).
(ii) The radiative decay rate constants of complexes 2, 3 and
4 are smaller than those of complexes 1, 5, 6, 7 and 8 as shown
in Table 3.
(iii) Furthermore, the absorption peak of the complexes 2, 3
and 4 were lower than that of the complexes 1, 5, 6, 7 and 8.
The molar absorption coefficients at the peaks around 590 nm
of the former complexes (those are considered as singlet–triplet
Fig. 5 (a) Emission energy versus the Hammett constant for substituted
2-phenylisoqunoline Ir complexes shown in Table 3; (b) effect of the
Hammett constant on the emission energy.
3
absorption peaks from ground singlet state to MLCT33,36,38
)
were smaller than that of the latter complexes. The molar
absorption coefficients of the complexes 1, 5, 6, 7 and 8 ranges
between 1000 and 2000 M−1 cm−1. On the contrary, the molar
absorption coefficients of the complexes 2, 3 and 4 are invisible
as shown in Table 3.
Fig. 5(b). Thus, by using Fig. 5(a), the maximum emission peak
wavelength from the substituted Irpiq, which has a variety of
substituent on the phenyl ring can be foreseen.
(iv) Phosphorescent lifetimes of the complexes 1, 5, 6, 7 and
8 were obviously shorter than those of the complexes 2, 3 and 4
as seen in Table 3.
From the above results (the differences of vibrational struc-
ture, kr, molar absorption coefficient of singlet–triplet absorp-
tion peaks and phosphorescent lifetime), it can be concluded that
the complexes 2, 3 and 4 (which have large Hammett constants
(4) The mixing of excited states
In the case of Irpiq (1), the lowest excited state was assigned to
3MLCT.10 An analogous argument is used to assign the excited
states for the complexes 2–8. Short lifetime, broad luminescence
spectra32 and similar absorption peaks33,34 of those complexes
show clearly that the lowest triplet excited states of those
Fig. 6 Emission spectra of Ir complexes 1–8. (a) Complexes 1, 5, 6, 7 and 8 with electron withdrawing substituents; (b) complexes 2, 3 and 4 with
electron donating substituents. Vibronic structure of (b) is clearly observed in comparison with (a).
D a l t o n T r a n s . , 2 0 0 5 , 1 5 8 3 – 1 5 9 0
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substituents) have excited states with large contributions of 3p–
p* in comparison with the complexes 1, 5, 6, 7 and 8 (which have
small Hammett constant substituents).
It is probable that increased electron population of phenyl
ring where the HOMO is located enhances the contribution of
3p–p* in the lowest triplet excited state in the iridium complexes.
And then, the behaviour of the charge transfer from iridium to
ligand decreases.
These results agree with the B3LYP density functional theory
(DFT) calculations14 through Mulliken charge difference on
iridium atom between the ground state and the lowest triplet
excited state. (The charge difference is defined here as a value
which charge of the ground state minus charge of the lowest
triplet excited state.) The calculations were done for Irppy, Irpiq
(1), Ir4F5Mpiq (3) and Ir4Fpiq (2) using Gaussian98 software
package (Gaussian, Inc.)24 with a LanL2DZ basis set. Irppy
was calculated for reference of well-known material that had the
3MLCT dominant lowest excited state. The calculated Mulliken
charge difference between the ground state and the lowest triplet
excited state was 0.45 for Irppy, 0.41 for Irpiq (1), 0.33 for
Ir4Fpiq (2) and 0.34 for Ir4F5Mpiq (3), respectively. The values
of former two complexes show the large differences and the
latter two complexes show relatively the small differences. The
large difference of the values means large charge transfer from
iridium to ligands. The calculation results strongly support the
Fig. 7 Quantum yield and decay rate constants of the complexes. (a)
Quantum yield versus emission peak wavelength; (b) radiative decay
rate constant versus emission peak wavelength; (c) nonradiative decay
rate constant versus emission peakwavelength; (d) plot of ln (knr) versus
emission energy gap.
3
differences between complexes. It is suggested that the p–p*
mixing of Ir4Fpiq (2) and Ir4F5Mpiq (3), is larger than Irppy
and Irpiq (1), owing to the small reduction of Mulliken charge
on iridium in the excited state.
found for Os(II), Ru(II)42,43 and Pt(II)17 diimine complexes which
have the MLCT excited states for the lowest excited triplet
(5) Quantum yield and photochemical properties
3
states. This correlation suggests that the change of nonradiative
decay rate constant is owing to the energy gap of the complexes.
Furthermore, this correlation suggests that the charge transfer
to the substituted 1-phenylisoquinoline excited states of the all
complexes 1, 3–9 have very similar vibrational and electronic
components.
Lastly, the radiative decay rate constants of complexes 2–4
are lower than those of complexes 1 and 5–8 due to the changes
of the mixing states along the lines of clause (4) [as shown in
Fig. 7(b)]. Consequently, complexes 2–4 have larger quantum
yield rather than complexes 1 and 5–8.
Table 3 and Fig. 7(a) show the quantum yields of the complexes
1–8. All the complexes have larger quantum yield than Irpiq
(1) except Ir5iPrpiq (7) and Ir5Mpiq (8). The quantum yield
has a tendency to shift to higher with shortening the maximum
emission peak wavelength.
The radiative and nonradiative decay rate constants, knr and
kr, are calculated from the phosphorescence yield, φ, and the
phosphorescence lifetime by following eqn. (3-2) and (3-3).
kr
φ =
(3-2)
kr + knr
1
(6) OLED properties using substituted Irpiq
s =
(3-3)
kr + knr
We have selected Ir4F5Mpiq (3) as a representative material
which would show high efficiency and proper red purity for
OLED. The OLED was fabricated with Ir4F5Mpiq (3) by the
thermal evaporation technique.
The evaluation of the device performance was done from the
plots of applied voltage versus luminance and power efficiency
[Fig. 8(a)] and the plots of applied voltage versus external
quantum efficiency [Fig. 8(b)].
The Commission Internationale de L’Eclairage (CIE) sys-
tem is the standard for evaluating color quality for visual
applications.44 The CIE characteristics of the OLEDs with three
complexes in the emission layer are shown in Table 5. The
device that used Ir4F5Mpiq (3) shows good CIE characteristics,
close to the National Television Standards Committee (NTSC)
recommended red for a cathode ray tube (CRT). The CIE
characteristics of the Ir4F5Mpiq (3) doped OLED are (0.66,
0.34).
It is clear that the EL emission originated from the triplet-
excited state of Ir4F5Mpiq (3), because the spectrum of the
device was consistent with PL spectrum of the same complex in
a dilute toluene solution.
From the viewpoint of the relationship between maximum
emission peak wavelength of photoluminescent spectrum and
decay rate constants, two trends are evident for the substituted
iridium 1-phenylisoquinoline derivatives. The nonradiative de-
cay rate constant (knr) increases as the maximum emission peak
shift to deep red. Additionally, the radiative decay rate constant
(kr) does not show monotonous change by the maximum
emission peak shift.
The maximum emission peak wavelength of the complexes
with substituent of large Hammett constant is shorter than that
of small Hammett constant as shown in Table 3. The energy gaps
for complexes 1–8 have been approximated using the maximum
emission peak of PL. The plot of ln (knr) versus the energy gap for
complexes 1–8 given in Fig. 7(d) shows linear relationship. There
is a good qualitative agreement with the “Energy Gap Law”.
The Energy Gap Law predicts that the rates of nonradiative
decay increase when the energy gap separating the ground and
excited state decreases. The relation is based on the vibrational
overlap between the ground state and the excited state. And
knr is a function of a Franck–Condon overlap integral. In a
case of complexes having similar excited states and vibrational
coupling, a simplified form of the Energy Gap Law is obtained
that predicts a linear relation between ln (knr) and the energy
gap.14 The slope of −8.1 eV−1 is similar in magnitude to those
The OLED device showed red electroluminescence with
excellent high external quantum efficiency and power efficiency.
A maximum external quantum efficiency of 15.5% and a power
efficiency of 12.4 lm W−1 were obtained at a luminance 218 cd
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Irpiq (1)
Ir4Fpiq (2)
Ir4F5Mpiq (3)
0.68
0.61
0.66
0.32
0.38
0.34
Deep red
Orange
Red
10
11
m−2 and a current density 1.23 mA cm−2. In spite of the gradual
decrease of the efficiencies due to triplet–triplet annihilation,45–48
an external quantum efficiency of 7.9% and a power efficiency
of 3.7 lm W−1 were obtained at a luminance of 10724 cd m−2 and
a current density 119.3 mA cm−2.
Summary
We have synthesized a series of the iridium complexes with
1-phenylisoquinoline as the ligand using various types of
substituents. The maximum emission peak wavelength of the
Ir complex is tuned from 598 to 635 nm depending on the
substituents.
The substituents with higher Hammett constants attached to
the phenyl ring of 1-phenylisoquinoline caused greater stabiliza-
tion of the HOMO, which was localized on that phenyl ring.
The substituents with the larger Hammett constants stabilized
the HOMO, and therefore the t2g orbitals of the iridium, more.
3
All the complexes discussed here showed MLCT predom-
inant mixing states for their lowest excited triplet states. But
3
3
the degree of the mixing between MLCT and p–p* states of
the lowest excited states varied. The large Hammett constant
substituents attached to the phenyl ring enhanced the mixing
contribution of 3p–p* excited state due to the reduction of
electron density on the iridium. Therefore, we can control the
nature of the lowest excited states of substituted Irpiq complexes.
And then, we can design the complex and OLED with the
objective of desirable emission color and high efficiency.
The OLED device using the Ir complex, Ir4F5Mpiq (3),
showed an excellent power efficiency of 12.4 lm W−1, external
quantum efficiency of 15.5% and sufficient CIE characteristics
(0.66, 0.34) for practical applications.
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D a l t o n T r a n s . , 2 0 0 5 , 1 5 8 3 – 1 5 9 0