Effect of substitution of methyl groups on the luminescence performance of IrIII complexes: Preparation, structures, electrochemistry, photophysical properties and their applications in organic light-emitting diodes (OLEDs)

Article abstract of DOI:10.1002/ejic.200400114

A series of dimethyl-substituted tris(pyridylphenyl)iridium(III) derivatives [(n-MePy-n′-MePh)₃Ir] [n = 3, n′ = 4 (1); n = 4, n′ = 4 (2); n = 4, n′ = 5 (3); n = 5, n′ = 4 (4); n = 5, n′ = 5 (5)] have been synthesized and characterized to investigate the effect of the substitution of methyl groups on the solid-state structure and photo- and electroluminescence. The absorption, emission, cyclic voltammetry and electroluminescent performance of 1-5 have also been systematically evaluated. The structures of 2 and 4 have been determined by a single-crystal X-ray diffraction analysis. Under reflux (> 200 °C) in glycerol solution, fac-type complexes with a distorted octahedral geometry are predominantly formed as the major components in all cases. Electrochemical studies showed much smaller oxidation potentials relative to Ir(ppy)₃ (Hppy = 2-phenylpyridine). All complexes exhibit intense green photoluminescence (PL), which has been attributed to metal-to-ligand charge transfer (MLCT) triplet emission. The maximum emission wavelengths of thin films of 1, 3, 4 and 5 at room temperature are in the range 529-536 nm, while 2 displays a blue-shifted emission band (λ_max = 512 nm) with a higher PL quantum efficiency (Φ_PL = 0.52) than those of complexes 1 and 3-5; this is attributed to a decrease of the intermolecular interactions. Multilayered organic light-emitting diodes (OLEDs) were fabricated by using three (2, 3 and 4) of these Ir^III derivatives as dopant materials. The electroluminescence (EL) spectra of the devices, which have the maximum peaks at 509-522 nm, with shoulder peaks near 552 nm, are consistent with the PL spectra in solution at 298 K. The devices show operating voltages at 1 mA/cm ² of 4,9, 5.6, 5,1, and 4.6 V for Ir(ppy)₃, 2, 3, and 4, respectively. In particular, the device with 2 shows a higher external quantum efficiency (η_ext = 11% at 1 mA/cm²) and brightness (4543 cd/m² at 20 mA/cm²) than Ir(ppy)₃ (η_ext = 6.0% at 1 mA/cm²; 3156 cd/m² at 20 mA/cm²) and other Ir(dmppy)₃ derivatives, (dmppy = dimethyl-substituted ppy), under the same conditions. The methyl groups at the meta (Ph) and para (Py) positions to the Ir metal atom have a great influence on absorption, emission, redox potentials and electroluminescence. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400114

FULL PAPER
Effect of Substitution of Methyl Groups on the Luminescence Performance of
IrIII Complexes: Preparation, Structures, Electrochemistry, Photophysical
Properties and Their Applications in Organic Light-Emitting Diodes (OLEDs)
Sungouk Jung,[a] Youngjin Kang,*[b] Hyung-Sun Kim,[a] Yun-Hi Kim,[a] Chang-Lyoul Lee,[c]
Jang-Joo Kim,[c] Sung-Koo Lee,[d] and Soon-Ki Kwon*[a]
Keywords: Electrochemistry / Electrophosphorescence / Iridium / Luminescence / N ligands
A
series of dimethyl-substituted tris(pyridylphenyl)iridi-
higher PL quantum efficiency (ΦPL = 0.52) than those of com-
um(III) derivatives [(n-MePy−nЈ-MePh)3Ir] [n = 3, nЈ = 4 (1); plexes 1 and 3−5; this is attributed to a decrease of the inter-
n = 4, nЈ = 4 (2); n = 4, nЈ = 5 (3); n = 5, nЈ = 4 (4); n = 5, nЈ = molecular interactions. Multilayered organic light-emitting
5 (5)] have been synthesized and characterized to investigate
the effect of the substitution of methyl groups on the solid-
state structure and photo- and electroluminescence. The ab-
sorption, emission, cyclic voltammetry and electrolumines-
diodes (OLEDs) were fabricated by using three (2, 3 and 4)
of these IrIII derivatives as dopant materials. The electrolumi-
nescence (EL) spectra of the devices, which have the maxi-
mum peaks at 509−522 nm, with shoulder peaks near
cent performance of 1−5 have also been systematically eva- 552 nm, are consistent with the PL spectra in solution at
luated. The structures of 2 and 4 have been determined by a
single-crystal X-ray diffraction analysis. Under reflux (Ͼ 200
°C) in glycerol solution, fac-type complexes with a distorted
octahedral geometry are predominantly formed as the major
components in all cases. Electrochemical studies showed
much smaller oxidation potentials relative to Ir(ppy)3 (Hppy =
2-phenylpyridine). All complexes exhibit intense green pho-
toluminescence (PL), which has been attributed to metal-to-
298 K. The devices show operating voltages at 1 mA/cm2 of
4.9, 5.6, 5.1, and 4.6 V for Ir(ppy)3, 2, 3, and 4, respectively.
In particular, the device with 2 shows a higher external quan-
tum efficiency (ηext = 11% at 1 mA/cm2) and brightness (4543
cd/m2 at 20 mA/cm2) than Ir(ppy)3 (ηext = 6.0% at 1 mA/cm2;
3156 cd/m2 at 20 mA/cm2) and other Ir(dmppy)3 derivatives,
(dmppy = dimethyl-substituted ppy), under the same condi-
tions. The methyl groups at the meta (Ph) and para (Py) posi-
ligand charge transfer (MLCT) triplet emission. The maxi- tions to the Ir metal atom have a great influence on absorp-
mum emission wavelengths of thin films of 1, 3, 4 and 5 at
room temperature are in the range 529−536 nm, while 2 dis-
tion, emission, redox potentials and electroluminescence.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
plays a blue-shifted emission band (λmax = 512 nm) with a Germany, 2004)
Introduction
materials because of their ability to achieve maximum in-
ternal quantum efficiency, nearly 100%, as well as high ex-
ternal quantum efficiency in OLEDs.[2] Based on previous
reports, a promising phosphorescent metal complex in
OLEDs should have a short lifetime in the excited state and
high phosphorescent efficiency. Based on these consider-
ations, a number of researchers have focused recently on the
development of cyclometallated (C∧N)IrIII complexes.[2] In
particlar, luminescent Ir(ppy)3 (Hppy ϭ 2-phenylpyridine)
derivatives have been described where adjustment of the
band gap between the HOMO and LUMO energy levels
could be achieved by modification of the substituents on
the ppy rings.[3] Accordingly, to control the energy of the
Ir(ppy)3 system from blue light emission to red, a judicious
incorporation of electron donors/acceptors as substituents
on the ppy frameworks is an essential prerequisite.[4,5] For
example, introduction of electron-donating groups to the
pyridine ring and electron-withdrawing groups to the phe-
nyl ring can cause a decrease of the HOMO level and an
increase of the LUMO level, respectively, inducing a larger
Luminescent transition metal complexes, such as those of
groups 9 or 10, are currently of great interest due to their
various applications in photochemistry, organic light-emit-
ting diodes (OLEDs), and chemical sensors for small mol-
ecules.[1] Among these transition metal complexes, IrIII
complexes have been regarded as excellent phosphorescent
[a]
School of Applied Chemical Engineering and Engineering
Research Institute, Gyeongsang National University
Chinju 660-701, South Korea
Fax: (internat.) ϩ 82-55-753-6311
E-mail: skwon@nongae.gsnu.ac.kr
[b]
Division of Science Education, Kangwon National University
Chuncheon 200-701, South Korea
kangy@kangwon.ac.kr
[c]
School of Materials Science and Engineering, Seoul National
University
Seoul 151-744, South Korea
[d]
LG Electronic Institute of Technology
16 Woomyeon-Dong, Seocho-Gu, Seoul 137-724, South Korea
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2004, 3415Ϫ3423
DOI: 10.1002/ejic.200400114
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Y. Kang, S.-K. Kwon et al.,
FULL PAPER
band gap than that of Ir(ppy)3. Although extensive research
on the investigation of luminescent properties of IrIII de-
rivatives containing two ortho-chelating (C∧N) ligands and
a bidentate ancillary LX (e.g. monoanionic chelating li-
gands, acac ϭ acetylacetonate, pic ϭ picolinate, sal ϭ sal-
icyliminate), and their use and performance as triplet emit-
ters in EL devices has been well documented over several
years, the illustration of the effect of substituents, such as
simple functional groups, and the dependence on their posi-
tions on the phenyl or pyridine rings in the ppy backbone,
still remains virtually unexplored.[1e,5] During our ongoing
investigation of phosphorescent Ir(ppy)3 derivatives, we
have observed that attached methyl groups as substituents
on the ppy ligand have a remarkable influence on the elec-
trochemical and photophysical properties.[6] Our interest in
the development of highly efficient phosphors for appli-
cation in OLEDs has prompted us to synthesize Ir(ppy)3
derivatives and investigate their photophysical and electrol-
uminescent characteristics. The methyl moiety was chosen
for the following reasons: (i) The emissions of Ir(ppy)3 de-
rivatives come from phosphorescence with triplet character
of both ligand-centered (πϪπ*) and MLCT (metal-to-li-
gand charge transfer) transitions. This phosphorescence is
strongly affected by the triplet energy of the ortho-chelating
C∧N ligands. Therefore substituents, even methyl groups,
on the ppy rings could play a key role in the alteration of
the triplet energy. (ii) Although a methyl group is a weaker
electron-donating group than other functional groups (e.g.
OMe, NR2 etc.), incorporating the methyl group as a sub-
stituent on the ppy rings has an influence on the HOMO
Scheme 1
There is a possibility of the formation of two isomers (fac
and mer) during the reaction of Ir(acac)3 and the Hdmppy
ligands.[8a] However, we found that the fac isomer was
formed as the major component, although some mer isomer
was also formed. The formed mer isomers were not isolated
because of their extremely low yields. All complexes are
very stable up to 300 °C without degradation in air. In the
1H NMR spectra, complexes 1Ϫ5 exhibit six well-resolved
peaks due to the ppy rings, indicating that all complexes
3
and/or the LUMO level, as well as the MLCT state of the
complex. Herein we describe the results of our systematic
investigation on the preparation, structural characteriz-
ation, electrochemical behavior, photophysical properties
and the fabrication of multiplayer phosphorescent light-
emitting devices of a series of fac-[IrIII(dmppy)3] complexes
[Hdmppy ϭ n-methyl-2-(nЈ-methylphenyl)pyridine].
Results and Discussion
˚
Table 1. Selected bond lengths [A] and angles [°] for 2 and 4
Syntheses and Structures
Compound 2
The methyl-substituted ppy ligands were synthesized by
the Suzuki coupling[7] reaction of 2-bromo-n-methylpyri-
dine with the corresponding nЈ-methylphenylboronic acid in
the presence of K2CO3 and [Pd(PPh3)4] catalyst (Scheme 1).
These ligands were obtained in good yields (70Ϫ90%). C(25)ϪIr(1)ϪN(2)
Complexes 1Ϫ5 were synthesized by a slight modification
Ir(1)ϪC(12)
Ir(1)ϪC(25)
Ir(1)ϪN(2)
C(12)ϪIr(1)ϪN(1)
2.015(8) Ir(1)ϪC(38)
2.034(8) Ir(1)ϪN(1)
2.114(7) Ir(1)ϪN(3)
2.016(9)
2.131(6)
2.125(7)
79.3(3)
95.3(3)
97.7(3)
95.5(3)
80.1(3)
78.5(3)
86.4(3)
174.0(3)
C(38)ϪIr(1)ϪN(3)
C(12)ϪIr(1)ϪC(38)
C(38)ϪIr(1)ϪC(25)
C(12)ϪIr(1)ϪC(25)
C(38)ϪIr(1)ϪN(1)
C(38)ϪIr(1)ϪN(2)
of the previous synthetic methodology reported by Watt et
al.[8b] The reaction of [Ir(acac)3] with the corresponding
Compound 4[a]
methyl-substituted ppy ligands at high temperature (Ͼ 200
°C), gave 1Ϫ5 in moderate yields. Even though the ligands
Ir(1)ϪC(12)
2.002(5) Ir(1)ϪN(1)
79.3(2)
2.111(5)
C(12)ϪIr(1)ϪN(1)
C(12A)iϪIr(1)ϪC(12) 95.2(3)
C(12)ϪIr(1)ϪN(1B)ii 91.6(2)
methylated at the 6- or 3Ј-position were synthesized success-
C(12A)iϪIr(1)ϪN(1B)ii 171.6(2)
fully in moderate yields, the complexation of these ligands
with the IrIII starting material hardly occurs under the same
reaction conditions. We attribute this mainly to steric fac-
tors.
N(1B)iiϪIr(1)ϪN(1)
94.48(19)
[a]
Symmetry transformations used to generate equivalent atoms: i:
z, x, y; ii: y, z, x.
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Eur. J. Inorg. Chem. 2004, 3415Ϫ3423

Effect of Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes
FULL PAPER
Figure 1. Top: Molecular structure of 2 with atom labeling schemes and 50% thermal ellipsoids; all hydrogen atoms have been omitted
for clarity; bottom: crystal packing diagram between two adjacent molecules of 2 showing the lack of a πϪπ stacking interaction in the
solid state
have a facial geometry around the Ir atom. Complexes 1Ϫ5 in the dmppy ligand arrange in a trans disposition to the
were fully characterized by mass spectrometry and elemen- dative N atoms of adjacent ppy ligands. The IrϪN bonds
˚
tal analyses, and crystal structures of 2 and 4 were obtained. [2.111(5)Ϫ2.131 (6) A] observed in 2 and 4 are slightly
˚
Selected bond lengths and angles are presented in Table 1; longer than those of [Ir(acac)(tpy)2] [2.040(5) A] and [Ir-
[5b]
˚
refinement and structure-solution data can be found in the (acac)(ppy)2] [2.010(9) A]. These observed elongations im-
Exp. Sect. Complexes 2 and 4 exhibit only the fac configu- ply a stronger trans influence of the phenyl ring in compari-
ration, with a distorted octahedral geometry around the Ir son to the pyridyl groups having a trans-N,NЈ configuration.
atom, as shown in Figures 1 and 2, respectively. The di- In the crystal packing, 4 displays an effective overlap of the
hedral angle between the phenyl and pyridine rings in 2 pyridine rings between two adjacent molecules (Figure 2).
˚
(8.81°) is approximately three times larger than that of 4 The shortest separation distance is about 3.22 A, which is
(3.4°), indicating a decreased conjugation of the 4,4Ј-ppy sufficiently short to suggest πϪπ stacking interactions.
ligands. The IrϪC bond lengths, ranging from 2.002(5) to Complex 2 does not exhibit an effective overlap in the crys-
˚
2.034 A, are within the range reported for other mononu- tal packing, as shown in Figure 1. Recently, many reports
clear complexes with the Ir(ppy)2 moiety [1.97(2)Ϫ2.13(6) have described that intermolecular πϪπ interactions have a
[9]
A], and are also close to values reported for [(tpy)2Ir(µ- direct influence on luminescent properties.[2c,5] The influ-
˚
Cl)2Ir(tpy)2] [Htpy ϭ 2-(p-tolyl)pyridine] and other substi- ence of this intermolecular interaction on the luminescence
tuted fac-[Ir(ppy)3] derivatives.[8a,10] The σ-donor C atoms of the IrIII complexes in this work is discussed below.
Eur. J. Inorg. Chem. 2004, 3415Ϫ3423
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Y. Kang, S.-K. Kwon et al.,
FULL PAPER
Figure 2. Top: Molecular structure of 4 with atom labeling schemes and 50% thermal ellipsoids; all hydrogen atoms have been omitted
for clarity; bottom: crystal packing diagram between two adjacent molecules of 4 showing the intermolecular interaction in the solid state
Photoluminescent and Electrochemical Properties
patterns of 1Ϫ5 and [Ir(ppy)3] are similar in the
250Ϫ550 nm region.[11] However, the MLCT and MLCT
peaks in 1, 3, 4 and 5 appear at lower energies than in
[Ir(ppy)3], while those of 2 occur at much higher energy
with larger extinction coefficients. These observed blue- or
red-shifted MLCT bands show that the methyl groups on
the dmppy rings have a significant effect on the electronic
1
3
Table 2 summarizes the photophysical data of 1Ϫ5. The
UV/Vis spectra of all compounds exhibit intense absorption
bands between 250 and 310 nm (ε Ͼ 104 molϪ1·dm3·cmϪ1),
indicating that the electronic transitions are mostly ligand-
centered (LC) πϪπ * transitions (Figure 3). The absorption
Table 2. Photophysical and electrochemical data of 1Ϫ5[a] and [Ir(ppy)3]
UV
Em (λmax
Solution
)
ΦPL
Redox
E1/2 [V]
(ε [103 L·molϪ1·cmϪ1])
Film
[ox]
[Ir(ppy)3]
283(4.9), 380(4.1), 405(4.0), 455(3.5), 490(3.1)
284(4.9), 384(4.1), 408(4.0), 455(3.6), 493(3.2)
278(4.9), 374(4.2), 407(4.0), 452(3.6), 484(3.3)
284(4.9), 391(4.1), 414(4.0), 456(3.6), 493(3.2)
284(4.9), 383(4.1), 412(4.0), 459(3.5), 493(3.2)
284(4.9), 384(4.0), 414(3.9), 458(3.4), 497(3.0)
514
522
509
524
524
524
516
529
512
534
536
532
0.40[b]
0.41
0.52
0.32
0.34
0.29
0.71
0.51
0.54
0.49
0.52
0.47
1
2
3
4
5
[a]
[b]
All the data were obtained in CH2Cl2 at ambient temperature with the same concentration. Ref.[8a]
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Eur. J. Inorg. Chem. 2004, 3415Ϫ3423

Effect of Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes
FULL PAPER
transition energies. The emission spectra of all complexes
in solution and as thin films at room temperature are domi-
nated by phosphorescence in the region 509Ϫ534 nm. Inter-
estingly, both the absorption and emission spectra of 2 are
blue-shifted relative to the other complexes and [Ir(ppy)3].
This result can be explained by correlating the electronic
and steric effects due to the methyl substituents. As re-
ported previously,[5] the emission energy of [Ir(ppy)3] can
be controlled readily by adding substituents to the rings. To
investigate the electronic effects caused by the addition of
methyl groups, cyclic voltammetry experiments were carried
out for all complexes, including [Ir(ppy)3]. Due to the lim-
ited range available in CH2Cl2 and the inability of our in-
strument to measure the reduction potentials in the range
Ϫ2.7 to Ϫ3.5 V, we obtained only the oxidation potentials
for complexes 1Ϫ5 (Table 2). For the electrochemical be-
Figure 3. Absorption and emission spectra of 2 (triangles) and 4
(squares) in CH2Cl2 at room temperature; inset: MLCT bands of
2 and 4 in the UV/Vis region
havior of Ir(ppy)3 analogues it has been proposed that the
oxidative and reductive processes involve the IrϪphenyl and
Ϫpyridine moiety of the C∧N ligand, respectively. DFT cal-
culations on a similar methyl-substituted Ir(ppy)3 deriva-
tive, tris[2-(3-methylpyrid-2-yl)phenyl]iridium(), provide
2
evidence that the HOMO levels are mostly due to the Ir dz
orbital with some π-orbital contributions from the phenyl
rings, while the LUMO levels in this complex are mostly
due to the pyridine π* orbital.[8c] The same description
complex has also been reported theoretically for the parent
[Ir(ppy)3] complex by Hay.[12] Reversible oxidation waves
are observed for all complexes between 0.47 and 0.54 V. It is
noteworthy that all of the complexes show lower oxidative
potentials than [Ir(ppy)3], indicating the relative ease of oxi-
dation (see Figure 4). This can be ascribed to the great
electron-donating ability of the methyl group. Complexes 3
and 5, in particular, exhibit lower oxidation potentials than
those of the other methyl-substituted complexes 1, 2, and
4. Based on our DFT calculations, there are nodes at the 4Ј-
and 6Ј-positions in the HOMO. In the HOMO, the electron
density is mostly dispersed at the 3Ј- and 5Ј-positions, and
also involves the 1Ј- and 2Ј-positions in the phenyl ring.
Therefore, the donating effect from the methyl groups at
either the 4Ј- or the 6Ј-position does not have a significant
influence on the HOMO energy levels, while methyl groups
Figure 4. Cyclic voltammograms of 1 and [Ir(ppy)3] in 0.1 
nBu4NPF6 at a scan rate of 50 mV/s
at the 3Ј- or 5Ј-positions effectively donate electron density dered spacers, such as pinene.[2c] Although the effect on lu-
to the dmppy moiety, leading to a decrease in the oxidation minescence, such as excimer formation and from intermo-
potentials. The reduction process of the Ir(ppy)3 derivatives lecular πϪπ interactions, is less demonstrated in octahedral
was also shown to be due to the C∧N ligand and is also complexes than in square-planar PtII complexes,[13] these
affected by the various substituents.[8a] However, one of the blue-shifted absorption and emission maxima for 2 could
key factors that affect the reduction potential in these sys- be attributed to a reduction of π-orbital conjugation be-
tems is the degree of conjugation between the phenyl and tween the phenyl and pyridine rings as well as the lack of
pyridine rings. More effective conjugation stabilizes the intermolecular interactions in the solid state.[16] Therefore,
LUMO, leading to a decreased reduction potential. Al- we believe that the observed blue-shifted absorption and
though we could not observe the reductive process, the re- emission spectra of complex 2 are probably due to steric
duction potential of 2 should be higher than that of com- factors with a contribution from the strong electron-donat-
plex 4, as supported by the structural data.
ing effect of the methyl group.
Another fact affecting the nature of the luminescence in
The PL quantum yields were obtained relative to
[Ir(ppy)3] is the degree of steric hindrance. Lee et al. have [Ir(ppy)3] (ΦPL
demonstrated that higher photoluminescence quantum ef- [Ir(4,4Јdmppy)3]
ficiencies (ΦPL) and shorter lifetimes in [Ir(ppy)3] deriva- [Ir(5,4Јdmppy)3]
ϭ 0.4) and increase in the order
(2)
(4)
Ͼ
Ͼ
[Ir(3,4Јdmppy)3]
[Ir(4,5Јdmppy)3]
(1)
(3)
Ͼ
Ͼ
tives can be achieved by the introduction of sterically hin- [Ir(5,5Јdmppy)3] (5). The observed high ΦPL value for 2 can
Eur. J. Inorg. Chem. 2004, 3415Ϫ3423
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Y. Kang, S.-K. Kwon et al.,
FULL PAPER
be attributed partially to the reduction of intermolecular dopants gives rise to increasing driving voltages in
interactions. The lifetimes of the excited state in all com- OLEDs.[17] If the dopant material functions as a hole trap,
plexes at room temperature are longer (Ͼ 1 µs) than that of the HOMO level of the dopant could be above that of the
[Ir(ppy)3], indicating that the substituents generate a num- host material. In other words, materials having a lower oxi-
ber of vibronic modes in the excited state of [Ir(dmppy)3] dation potential can function as effective hole traps in
(see Supporting Information).[15]
OLEDs. Therefore, the observed higher operating voltages
Based on our observation with respect to 2, the blue- for 2 and 4, relative to [Ir(ppy)3], are predominantly caused
shifted emission energy, high PL efficiency and absence of by effective hole trapping.[18] These results are consistent
intermolecular interactions are due to the incorporation of with the electrochemical behaviors of 1Ϫ5 and [Ir(ppy)3].
methyl groups at the para position of the pyridine and the The brightness of the devices at 20 mA/cm2 are 4543 cd/m2,
meta position of the phenyl rings relative to the Ir atom. 4470 cd/m2, 3551 cd/m2 and 3235 cd/m2, for 2, 3, 4 and
This phenomenon shows that the position of the substi- [Ir(ppy)3], respectively. With increasing current density, the
tution on the pyridine or phenyl rings as well as the elec- external quantum efficiency of devices containing 2, 3, 4
tronic effect of the substituent have an influence on the and [Ir(ppy)3] gradually decreased. Similar results have been
phosphorescence of [Ir(ppy)3] and related systems. When reported previously, and are attributed to the increase of
compared to the other positions, the 4- and 4Ј-positions are triplet-triplet annihilation of the exciton at high voltage
quite sensitive to both the photophysical and structural as- levels. As expected, the device fabricated using complex 2
pects.
exhibits higher brightness and external quantum efficiency,
as well as stronger EL intensity, than those of the other
complexes or [Ir(ppy)3] (see Figures 8 and 9). In contrast,
Electroluminescent Properties
The electroluminescent properties of compounds 2, 3 and the device efficiencies of 1 and 4 are lower than that of
4 were investigated. Devices with the Ir phosphor doped [Ir(ppy)3] under the same conditions. The positions of the
into the emissive layer were fabricated. EL-device perform- substituents as well as the nature of the substituents on the
ance was conducted using complexes 2, 3 or 4 as dopant ppy rings could be important factors in achieving high elec-
and 4,4Ј-bis(9-carbazolyl)-1,1Ј-biphenyl (CBP) as host in all troluminescent efficiency in substituted Ir(ppy)3 systems.
cases. In addition, to compare with previous works, EL de-
vices with a similar structure were also fabricated with
[Ir(ppy)3], except for the cathode, as reported by Thompson
et al.,[1b] by vacuum-deposition techniques. The device
structure used in this work is a typical multi-layer, em-
ploying indium tin oxide (ITO) as the anode, copper-
phthalocyanine (CuPc: 10 nm) as the hole injection
material, 4,4Ј-bis[(1-naphthyl)(phenyl)amino]-1,1Ј-biphenyl
(NPD: 40 nm) as the hole-transport material, 2, 3, or 4 as
the dopant material, CBP as the host material (20 nm), 2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP: 10 nm) as
the hole-blocking material, aluminium tris(8-quinolinolate)
(Alq3: 40 nm) as the electron-transport material, LiF as the
electron-injection layer, (1 nm) and Al (100 nm) as the cath-
ode, as shown in Figure 5. All complexes display green em-
ission in the range of 508Ϫ520 nm in the EL spectra, which
Figure 5. Device structures and molecular structure used in this
study
are well matched to those of the PL spectra in solution.
There are no characteristic emission peaks from CBP or
Alq3, indicating that the emission originates mostly from
the dopant (Figure 6). Furthermore, these results support
the proposal that effective energy transfer from the host
(CBP) to the dopant (Ir complex) occurs in the emis-
sive layer. More detailed EL characteristics are shown in
Table 3. The turn-on voltages (defined as 1 mA/cm2) of
[Ir(ppy)3], 2, 3 and 4 are 3.4, 5.6, 5.1 and 4.6 V, respectively
(Figure 7). The intermolecular interactions (e.g. πϪπ stacks
in molecular crystals) in the solid state and the ability to
trap charges in the emissive layer are considered the most
important factors in determining the operating voltage. The
strong electronic coupling and intermolecular interactions
in the molecular crystals contribute to effective injection
Figure 6. EL spectra of [Ir(ppy)3], (X), 2 (triangles), 3 (squares)
and transport of charges, leading to lower operating volt-
and 4 (diamonds) device structure: ITO/NPD/CBP-Ir(dmppy)3/
ages.[16] On the other hand, effective charge-trapping by the BCP/Alq3/LiF-Al)
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Eur. J. Inorg. Chem. 2004, 3415Ϫ3423

Effect of Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes
FULL PAPER
Conclusion
A series of dimethyl-substituted [Ir(dmppy)3] derivatives
have been synthesized and characterized, including their
photo-electroluminescent and electrochemical properties.
The nature of their solid-state structures depends on the
position of the methyl groups on the phenyl and pyridine
rings. These methyl groups give rise to several distinct trip-
let states, affecting the electrochemical behavior and electro-
luminescence of the IrIII complexes. As expected, they also
considerably perturb the absorption, redox potentials, and
photo- and electroluminescent efficiency. Lower oxidation
potentials than that of [Ir(ppy)3] were observed in all cases,
indicating that the methyl group induces a relatively easy
oxidation and leads to effective hole trapping. Complex 2,
which has methyl groups at the 4Ј- and 4-positions exhibits
a brighter green emission and higher ΦPL than [Ir(ppy)3].
In addition, a significant improvement of the external
quantum efficiency (ηext ϭ 11%), which is nearly 200%
higher than that of [Ir(ppy)3], is also observed in OLEDs.
Further investigation on the effects on luminescent proper-
ties depending on other bulky functional groups into vari-
ous positions of the ppy ligand is currently in progress.
Figure 7. Voltage versus current-density characteristics of
[Ir(ppy)3]- (X), 2- (triangles), 3- (squares), and 4-doped (dia-
monds) devices
Experimental Section
General Considerations. All experiments were performed under dry
N2 using standard Schlenk techniques. All solvents were freshly
distilled from appropriate drying reagents prior to use. All starting
materials were purchased from either Aldrich or Strem and used
without further purification. Methyl-substituted phenyl or pyridine
compounds were synthesized by slight modification of previous re-
ports.[8]
Figure 8. Luminance versus current-density characteristics of
[Ir(ppy)3]- (X), 2- (triangles), 3- (squares), and 4-doped (dia-
monds) devices
Measurements: 1H NMR and mass spectra were recorded with a
Bruker DRX 500 MHz spectrometer and JEOL-JMS 700 instru-
ment, respectively. UV/Vis and photoluminescent spectra for all
samples with concentrations in the range of 10Ϫ50 µ were ob-
tained with
a Lambda 900 UV/Vis spectrometer and a
PerkinϪElmer Luminescence spectrometer LS 50B. All solutions
for photophysical experiments were degassed with more than three
repeated freeze-pump-thaw cycles in a vacuum line. Melting points
were determined with an Electrothermal 9100 apparatus. Emission
lifetimes were measured with the fourth harmonic of a Q-switched
Nd-YAG laser using the upper excited state energy transfer with a
pulse duration of 6 ns and a repetition rate of 10 Hz. The laser
beam had a diameter of 5 mm and an optical power of about 50
mJ. The emission from the film was recorded by a gated intensified
diode array detector through a monochromator. The system allows
for an integration time (gate width) of detection from 100 ns to 10
Figure 9. External quantum efficiency versus current-density
characteristics of of [Ir(ppy)3]- (X), 2- (triangles), 3- (squares), and
4-doped (diamonds) devices
Table 3. Electroluminescent data of [Ir(ppy)3] and 2-, 3- and 4-doped OLEDs
Luminance [Cd/m2]
at 20 mA/cm2
Operating voltage
[V][a]
External quantum efficiency (%)
at 10 mA/cm2
at 1 mA/cm2
at 50 mA/cm2
Ir(ppy)3
3156
4543
4470
3551
4.9
5.6
5.1
4.6
6.0
11
10.9
8.6
5.4
8.2
7.6
6.5
4.7
6.8
5.8
5.0
2
3
4
[a]
The operating voltages in all cases were defined as 1 mA/cm2.
Eur. J. Inorg. Chem. 2004, 3415Ϫ3423
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3421

Y. Kang, S.-K. Kwon et al.,
Table 4. Crystallographic data for 2 and 4
FULL PAPER
ms and a variable delay after excitation. Cyclic voltammetry was
performed with an Autolab potentiostat from Echochemie under
nitrogen in a one-compartment electrolysis cell consisting of a plati-
num wire working electrode, a platinum wire counter electrode, and
a quasi Ag/AgCl reference electrode. Cyclic voltammograms were
monitored at scan rates of either 100 mV·sϪ1 or 50 mV·sϪ1 and
recorded in distilled dichloromethane. The concentration of the
complex was maintained at 0.5 m or less and each solution con-
tained 0.1  of tetrabutylammonium hexafluorophosphate (TBAP)
as the electrolyte. The ferrocenium/ferrocene couple (0.40V) was
used as the internal standard.
2
4
Empirical formula
Formula mass
Space group
C39H36IrN3·0.5C6H6
777.96
P21/n
15.5695(9)
14.6261(8)
16.1088(9)
111.8840(10)
3403.0(3)
4
1.518
39.56
56.62
21580
C39H36IrN3
738.91
I43d
23.4008(10)
23.4008(10)
23.4008(10)
90
12814.2(9)
16
1.532
41.99
56.64
39294
¯
˚
a [A]
˚
b [A]
˚
c [A]
β [°]
3
˚
V [A ]
Z
X-ray Crystallographic Analysis: Suitable crystals of 2 and 4 were
obtained from slow vapor diffusion of toluene/benzene/hexane
(2:1:1) into a solution of 2 or 4 in CH2Cl2. The crystal of 2 or 4
was attached to a glass fiber and mounted on a Bruker SMART
diffractometer equipped with a graphite-monochromated Mo-Kα
Dcalcd, [Mg·cmϪ3
]
µ [cmϪ1
2θmax [°]
]
No. of refections measured
No. of refections used
Rint
7996
0.1391
415
2650
0.1027
130
˚
(λ ϭ 0.71073 A) radiation, operating at 50 kV and 30 mA and a
CCD detector; 45 frames of two-dimensional diffraction images
were collected and processed to obtain the cell parameters and
orientation matrix. All data collections were performed at 173 K.
No. of parameters
Final R [I Ͼ 2σ(I)]
R1[a]
0.0525
0.1182
0.0341
0.0755
wR2[b]
The data collection 2θ ranges were 3.1Ϫ56.62° for
2 and
R (all data)
4.26Ϫ56.56° for 4. No significant decay was observed during the
data collection. The raw data were processed to give structure fac-
tors using the SAINT program. Each structure was solved by direc-
tion methods and refined by full-matrix least squares against F2 for
all data using the SHELXTL software (version 5.10).[19] All non-
hydrogen atoms in compounds 2 and 4 were anisotropically refined.
All other hydrogen atoms were included in the calculated positions
and their contributions in structural factor calculations were in-
cluded. Compound 2 cocrystallizes with benzene in the monoclinic
crystal system with space group P21/n. The disordered benzene
molecule was not modeled successfully. However, hydrogen atoms
in benzene were included in the calculated positions and their con-
tributions included in the structure factor. Compound 4 belongs to
R1[a]
0.0950
0.1582
1.006
0.0429
0.0800
1.253
wR2[b]
Goodness of fit on F2
NPD (40 nm)/CBP-Ir(dmppy)3 (20 nm)/BCP (10 nm)/Alq3 (40 nm)/
LiF (1 nm)/Al (100 nm).
General Preparation of [Ir(n,nЈ-dmppy)3]: The corresponding com-
plexes were synthesized by a slight modification of Watt’s pro-
cedures.[8b] A solution of the corresponding methyl-substituted
phenylpyridine ligand (6 equiv., 2.64 mmol) in glycerol was added
at ambient temperature under N2 to a stirred anhydrous degassed
glycerol solution (20 mL) of [Ir(acac)3] (0.217 g, 0.44 mmol). The
reaction mixture was refluxed for 24Ϫ48 h. The reaction mixture
was then cooled to room temperature, 1  HCl was added, and the
mixture filtered to give a crude product. The pure complexes were
obtained by chromatographic workup on silica gel using ethyl acet-
ate/hexane as eluent.
¯
the cubic crystal system (I43d space group). Crystal data for 2 and
4 are summarized in Table 4. Selected bond lengths and angles for
2 and 4 are given in Table 1. In the case of 2, since the large positive
Ϫ3
(3.236 e/A ) and negative (Ϫ3.306 e/AϪ3) difference Fourier peaks
˚
˚
˚
˚
are located at short distances from Ir (0.84 A and 0.90 A, respec-
tively), these peaks can be attributed to ghosts of the heavy Ir
atom.[20] CCDC-220262 (2) and -220263 (4) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
[or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223-336
033; E-mail: deposit@ccdc.cam.ac.uk].
1
[Ir(3,4Јdmppy)3] (1): H NMR (CD2Cl2, 25 °C): δ ϭ 7.86 (d, J ϭ
8.4 Hz, 3 H), 7.46 (dd, J ϭ 7.5, 0.9 Hz, 3 H), 7.36 (dd, J ϭ 5.4,
1.5 Hz, 3 H), 6.67Ϫ6.78 (m, overlap, 9 H), 2.82 (s, 9 H, CH3), 2.12
(s, 9 H, CH3) ppm. MS (EI): m/z ϭ 739 [Mϩ]. C39H36IrN3 (739.25):
calcd. C 63.39, H 4.91, N 5.69; found C 63.31, H 4.89, N 5.67.
1
[Ir(4,4Јdmppy)3] (2): H NMR (CD2Cl2, 25 °C): δ ϭ 7.71 (s, 3 H),
Fabrication of Electroluminescent Devices: A glass substrate pre-
coated with indium tin oxide (ITO) was cleaned in an ultrasonic
bath of acetone, followed by 2-propanol. Surface treatment was
carried out by exposing ITO to a UV ozone plasma. OLEDs withg
CBP as host material were fabricated as follows. The hole-injecting
layer, a 10 nm thick film of CuPc was deposited on the ITO
surface by high-vacuum thermal evaporation and a 40 nm thick-
ness α-NPD as hole-transporting layer was deposited onto the
CuPc. An [Ir(dmppy)3]-doped (dmppy ϭ dimethyl-substituted ppy)
host CBP layer was thermally co-evaporated onto the α-NPD layer.
The [Ir(dmppy)3] doping concentration was 8% in CBP. A 10 nm
7.56 (d, J ϭ 8.1 Hz,, 3 H), 7.36 (d, J ϭ 5.47 Hz, 3 H), 6.71Ϫ6.74
(m, overlap, 6 H), 6.61 (dd, J ϭ 1.2, 0.3 Hz, 3 H), 2.44 (s, 9 H,
CH3), 2.12 (s, 9 H, CH3) ppm. MS (EI): m/z ϭ 739 [Mϩ].
C39H36IrN3 (739.25): calcd. C 63.39, H 4.91, N 5.69; found C
63.38, H 4.95, N 5.71.
1
[Ir(4,5Јdmppy)3] (3): H NMR (CD2Cl2, 25 °C): δ ϭ 7.74 (s, 3 H),
7.50 (s, 3 H), 7.42 (d, J ϭ 5.7 Hz, 3 H), 6.75 (dd, J ϭ 1.5, 0.9 Hz,
3 H), 6.64 (d, J ϭ 0.6 Hz, 6 H), 2.44 (s, 9 H, CH3), 2.27 (s, 9 H,
CH3) ppm. MS (EI): m/z ϭ 739 [Mϩ]. C39H36IrN3 (739.25): calcd.
C 63.39, H 4.91, N 5.69; found C 63.40, H 4.86, N 5.61.
1
thick BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) layer [Ir(5,4Јdmppy)3] (4): H NMR (CD2Cl2, 25 °C): δ ϭ 7.80 (d, J ϭ
was then deposited as the exciton-blocking layer, followed by a
40 nm thick Alq3 layer as an electron-transporting layer. Finally,
LiF (1 nm) and Al (100 nm) were deposited on top of the organic
layers by thermal evaporation. The fabricated multilayer organic
light-emitting devices have the structure of ITO/CuPc (10 nm)/α-
8.4 Hz, 3 H), 7.55 (d, J ϭ 7.8 Hz, 3 H), 7.47 (ddd, J ϭ 7.8, 2.1,
0.6 Hz, 3 H), 7.31 (dd, J ϭ 1.2, 0.9 Hz, 3 H), 6.72 (dd, J ϭ 7.8,
1.2 Hz, 6 H), 6.58 (d, J ϭ 0.9 Hz, 3 H), 2.15 (s, 9 H, CH3), 2.12 (s,
9 H, CH3) ppm. MS (EI): m/z ϭ 739 [Mϩ]. C39H36IrN3 (739.25):
calcd. C 63.39, H 4.91, N 5.69; found C 63.30, H 4.84, N 5.62.
3422
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 3415Ϫ3423

Effect of Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes
FULL PAPER
V. A. Petrov, Y. Wang, Chem. Commun. 2001, 1494. [5b] S. Lam-
ansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong,
I. Tsyba, M. Bortz, B. Mui, R. Bau, M. E. Thompson, Inorg.
Chem. 2001, 40, 1704.
1
[Ir(5,5Јdmppy)3] (5): H NMR (CD2Cl2, 25 °C): δ ϭ 7.83 (d, J ϭ
8.4 Hz, 3 H), 7.50 (m, overlap, 6 H), 7.37 (dd, J ϭ 1.2, 0.6 Hz, 3
H), 6.59Ϫ6.67 (m, overlap, 6 H), 2.27 (s, 9 H, CH3), 2.17 (s, 9 H,
CH3) ppm. MS (EI): m/z ϭ 739 [Mϩ]. C39H36IrN3 (739.25): calcd.
C 63.39, H 4.91, N 5.69; found C 63.43, H 4.85, N 5.67.
Supporting Information: Spectroscopic data (1H NMR, mass, and
absorption/emission spectra), lifetime determination, electrochemi-
cal data and crystal-packing diagrams.
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Eur. J. Inorg. Chem. 2004, 3415Ϫ3423
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3423