Synthesis, characterization, and electroluminescent properties of iridium(III) 2-phenylpyridine-type complexes containing trifluoromethyl substituents and various main-group moieties

Article abstract of DOI:10.1002/ijch.201400076

New heteroleptic cyclometalated iridium(III) 2-phenylpyridine-type complexes with trifluoromethyl substituents and various main-group moieties were synthesized and their photophysical, electrochemical, and electroluminescent (EL) properties studied. The emission color can be tuned by a facile derivatization of the phenyl moiety of 2-phenylpyridine with various main-group moieties, and we have prepared new yellowish-green to orange triplet emitters with enhanced charge injection/charge transporting features, which can furnish attractive EL performance in phosphorescent organic light-emitting devices (OLEDs). Attempts were also made to fabricate two-color white-light OLEDs based on a combination of fluorescent blue and phosphorescent orange emitters.

Full text of DOI:10.1002/ijch.201400076

Full Paper
DOI: 10.1002/ijch.201400076
Synthesis, Characterization, and Electroluminescent
Properties of Iridium(III) 2-Phenylpyridine-type
Complexes Containing Trifluoromethyl Substituents and
Various Main-Group Moieties
Cheuk-Lam Ho,*[a, b] Ching-Shan Lam,[a] Ning Sun,[c] Dongge Ma,*[c] Li Liu,*[d] Zhen-Qiang Yu,*[e]
Liqin Xue,[f] Zhenyang Lin,[f] Hua Li,[g] Yih Hsing Lo,*[h] and Wai-Yeung Wong*[a, b, i]
Abstract: New heteroleptic cyclometalated iridium(III) 2-
phenylpyridine-type complexes with trifluoromethyl substitu-
ents and various main-group moieties were synthesized and
their photophysical, electrochemical, and electroluminescent
(EL) properties studied. The emission color can be tuned by
a facile derivatization of the phenyl moiety of 2-phenylpyri-
dine with various main-group moieties, and we have pre-
pared new yellowishꢀgreen to orange triplet emitters with
enhanced charge injection/charge transporting features,
which can furnish attractive EL performance in phosphores-
cent organic light-emitting devices (OLEDs). Attempts were
also made to fabricate two-color white-light OLEDs based
on a combination of fluorescent blue and phosphorescent
orange emitters.
Keywords: iridium · main group elements · organic light-emitting diodes · phosphorescence · synthesis design
1 Introduction
angles, and lower power attributes that are capable of
In view of the numerous advantages of organic light-emit-
ting diodes (OLEDs) over liquid-crystal displays (LCDs)
as flat-panel displays, such as thinner films, higher con-
trast, lighter weight, faster response, wider viewing
meeting the most stringent demands of future display ap-
plications, many researchers have continued to devote
their efforts to this frontier in both academia and indus-
[a] C.-L. Ho, C.-S. Lam, W.-Y. Wong
[e] Z.-Q. Yu
Institute of Molecular Functional Materials
Partner State Key Laboratory of Environmental and Biological
Analysis
College of Chemistry and Chemical Engineering
Shenzhen University, Shenzhen 518060 (P. R. China)
e-mail: zqyu@szu.edu.cn
Department of Chemistry and Institute of Advanced Materials
Hong Kong Baptist University, Waterloo Road
Kowloon Tong, Hong Kong (S. A. R. China)
e-mail: rwywong@hkbu.edu.hk
[f] L. Xue, Z. Lin
Department of Chemistry
The Hong Kong University of Science and Technology
Clearwater Bay, Hong Kong (S. A.R. China)
[b] C.-L. Ho, W.-Y. Wong
[g] H. Li
HKBU Institute of Research and Continuing Education
Shenzhen Virtual University Park, Shenzhen, 518057 (P. R.
China)
College of Life and Environmental Sciences & Beijing Engineer-
ing Research Center of Food Environment and Public Health
Minzu University of China, Beijing 100081 (P. R. China)
[c] N. Sun, D. Ma
[h] Y. H. Lo
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun 130022 (P. R. China)
e-mail: mdg1014@ciac.jl.cn
Department of Applied Physics and Chemistry
University of Taipei, Taipei 100
e-mail: yhlo@utaipei.edu.tw
[i] W.-Y. Wong
[d] L. Liu
Institute of Polymer Optoelectronic Materials and Devices
State Key Laboratory of Luminescent Materials and Devices
South China University of Technology, Guangzhou 510640 (P. R.
China)
Hubei Collaborative Innovation Center for Advanced Organic
Chemical Materials
Ministry of Education Key Laboratory for the Synthesis and
Application of Organic Functional Molecules
School of Chemistry and Chemical Engineering
Hubei University, Wuhan 430062 (P. R. China)
e-mail: liulihubei@gmail.com
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/ijch.201400076.
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try.[1] While the electroluminescence (EL) efficiencies for
the devices based on the singlet-state fluorescent materi-
als are limited, according to spin statistics, organic phos-
phorescent complexes of heavy transition metals, espe-
cially cyclometalated iridium(III) congeners, provide
a major breakthrough in the advancement of high-effi-
ciency phosphorescent OLEDs because they can harness
both singlet and triplet excitons in the emission process.
The research work generally covered the search for new
emissive and host materials within the redꢀgreenꢀblue
(RGB) color spectrum, optimization of device structures
for EL efficiency enhancement, and best operational sta-
bilities.[2] It was not until the discovery of metalated phos-
phors that researchers began intensive investigations on
white OLEDs (WOLEDs) as a practical avenue towards
thin-film solid-state lighting sources. For example, the
power efficiency should be about 10–15 lmWꢀ1 for a con-
ventional white-light incandescent lamp and a power effi-
ciency of 70 lmWꢀ1 is required for a fluorescent lamp.[3]
As a result, the development of highly efficient organic
phosphors of various colors becomes crucial for the even-
tual success of WOLEDs. White-light emission can be ob-
tained by mixing three primary colors (RGB) in a multi-
or a single-layer structure from small molecules and/or
polymers.[1a,4] More recently, two-color systems involving
complementary blue- and orange- or red-light emitters
can be employed to mimic the white-light spectrum.[5]
Many reported WOLEDs with dual emission layers typi-
cally display a clear valley in the green-light region of
their EL spectra,[6] indicating their poor color rendering
index (CRI), especially for green-colored objects viewed
in white light. Although such a drawback can, in some in-
stances, be partially tackled using the emission tail from
the blue-emitting component, a proper green or yellow-
ishꢀgreen light emission would still be highly beneficial
for many practical applications.
The success of cyclometalated iridium(III) complexes
with 2-phenylpyridine (Hppy)-type ligands in the field of
OLEDs has triggered substantial research attention in the
design, synthesis, and EL characterization of this class of
luminescent metal complexes.[7] Generally, the emission
wavelength of an iridium(III)ꢀppy complex is redshifted
when an electron-donating group is introduced into the
phenyl moiety of ppy, the p-conjugation length of the
ligand is extended or an electron-withdrawing group is
added to the pyridyl moiety of Hppy.[7] It is clear that the
highest occupied molecular orbital (HOMO) of such iridi-
um complexes consists principally of a mixture of phenyl
p and iridium d orbitals, whereas the lowest unoccupied
molecular orbital (LUMO) is localized largely on ppy,
with the predominant contribution derived from the pyr-
idyl p orbitals. Many iridium(III) complexes with yellow,
orange, and red emission are known; these usually con-
tain electron-donating groups on the phenyl moiety and
electron-withdrawing groups on the pyridyl moiety or
a more extended ligand chromophore.[8] Blue-emitting
congeners are designed either by substituting the pyridyl
moiety with heteroaromatic systems that are less easily
reduced than the former or by replacing H with electron-
withdrawing F groups on the phenyl moiety of the li-
gands.[9] Furthermore, it is anticipated that non-metallic
functional main-group elements, such as N and S, show
distinct electronic features according to their intrinsic
nature.[10] On the other hand, the use of CF3 substituents
on the pyridyl ring can generally lead to improved quan-
tum efficiency and redshifted emission and may also play
a role in enhancing the device performance with its in-
trinsic chemical nature.[11]
With the strong incentive to obtain efficient WOLEDs,
we describe herein the synthesis and electroluminescent
properties of some new heteroleptic iridium(III) cyclome-
talates with various main-group moieties on the phenyl
ring and electron-withdrawing CF3 substituents on the
pyridyl ring.
2 Results and Discussion
2.1 Synthesis and Characterization
By using ligands with different main-group moieties, the
photophysical and electrochemical characteristics of
iridium(III) complexes can be finely adjusted.[12]
Scheme 1 shows the synthetic route to ligands L1–L4.
The bromo precursor of each ligand was converted into
its corresponding boronic acid, which then reacted with 2-
chloro-5-trifluoromethylpyridine using palladium-cata-
lyzed Suzuki cross-coupling to afford L1, L2, L3a, and
L4. Further oxidation of L3a by using excess m-CPBA in
chloroform at 08C gave L3.
The synthesis of heteroleptic iridium(III) complexes in-
volves two steps.[11] In the first step, IrCl3·nH2O was treat-
ed with an excess amount of each cyclometalating ligand
to give a chloride-bridged iridium(III) dimer. This dimer
could be readily cleaved to give the monomeric complex
by replacing the bridging chlorides with acetylacetonate
anions (acacꢀ) derived from the deprotonation of acetyla-
cetone (Hacac) using sodium carbonate. Purification of
the mixture by preparative TLC gave 1–4 as air-stable
powders in high purity with colors ranging from orange
to red, depending on the nature of the chelating ligands.
The synthetic strategies to the new iridium(III) complexes
1–4 are depicted in Scheme 2.
All of the synthesized compounds are air stable and
can be stored without any special precautions. The identi-
ties of the products were confirmed through NMR spec-
troscopic (except for those complexes that did not have
enough solubility for 13C NMR spectroscopic measure-
1
ments) and mass spectrometric methods. In all cases, H
NMR resonances arising from the protons of the organic
moieties were observed with accurate integrals and were
in agreement with their assigned structures. For example,
the ortho proton on the pyridyl ring shows a characteristic
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Scheme 1. Synthetic strategies of L1–L4. m-CPBA=m-chloroperbenzoic acid.
Scheme 2. Synthesis of new heteroleptic iridium(III) complexes 1–4.
downfield signal, while the CH resonance and two methyl
groups of the acacꢀ ligand are located typically at dꢁ5.3
and 1.8 ppm, respectively. This is in agreement with the
literature values reported.[13,14] In their 13C NMR spectra,
the most downfield carbon signal was observed for the
C=O group of acacꢀ at dꢁ185 ppm, while the CH3 group
of acacꢀ was located at dꢁ27 ppm. The formulas of the
iridium(III) complexes were also successfully established
by the appearance of intense molecular ion peaks in their
mass spectra.
chelate disposition. Due to the bulky cyclometalating li-
gands, the IrꢀC and IrꢀN bond lengths are not identical;
the IrꢀC bond is shorter than that of the IrꢀN bond. The
shortest coordination distances are associated with the
IrꢀC bonds, while the longest ones correspond to the Irꢀ
O bonds. The NꢀIrꢀN bond angles at the metal centers
are 174.34 and 176.58 ꢀ for 1 and 3, respectively.
2.3 Photophysical Properties
To gain more insight into the effect of cyclometalating li-
gands on the photophysical properties of Ir(III) com-
plexes, the absorption spectra of ligands L1–L4 were first
recorded in CH2Cl2 (Table 3). Generally, their absorption
spectra are dominated by p to p* transitions in the near-
UV region. With the same pyridyl functionality, the iden-
tity of the main group attached to the phenyl ring of the
CN ligands did not have a significant influence on the ab-
sorption spectra generally, except for the NPh2-containing
one. L4 shows a marked redshift in its absorption profile,
compared with the others, probably due to the strong
electron-donating nature of the triphenylamine moiety.
2.2 X-ray Crystallography
The molecular structures of L4, 1, and 3 were further con-
firmed by X-ray single-crystal diffraction studies. The per-
spective drawings of 1 and 3 are depicted in Figures 1 and
2 and their selected bond lengths and angles are listed in
Tables 1 and 2. For 1 and 3, each of the molecular struc-
tures reveals that the central iridium center is coordinated
by two anionic NC ligands and one chelating acacꢀ
anion. Coordination around the iridium center is distort-
ed octahedral, with the cis-O,O, cis-C,C, and trans-N,N
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Figure 1. Molecular structure of complex 1, with the thermal ellip-
soids shown at the 25% probability level. The labels on carbon
atoms (except for those bonded to Ir(1)) and all hydrogen atoms
are omitted for clarity.
All of the cyclometalating ligands emit in the near-UV
and blue region in solution under UV irradiation at room
temperature. Their emissions are attributed to fluores-
cence because of the measured lifetimes being in the
nanosecond range.
Figure 2. Molecular structure of complex 3, with the thermal ellip-
soids shown at the 25% probability level. The labels on carbon
atoms (except for those bonded to Ir(1)) and all hydrogen atoms
are omitted for clarity.
The absorption spectra of all Ir(III) cyclometalated
complexes were then measured in CH2Cl2 at room tem-
Table 1. Selected bonding parameters of complex 1.
Selected Bond
Bond Length [ꢀ]
Selected Angle
Bond Angle [8]
Selected Angle
Bond Angle [8]
Ir(1)ꢀC(1)
Ir(1)ꢀC(13)
Ir(1)ꢀN(1)
Ir(1)ꢀN(2)
Ir(1)ꢀO(1)
Ir(1)ꢀO(2)
1.999(4)
1.990(4)
2.029(3)
2.028(3)
2.155(3)
2.156(3)
C(1)ꢀIr(1)ꢀC(13)
N(1)ꢀIr(1)ꢀN(2)
O(1)ꢀIr(1)ꢀO(2)
C(1)ꢀIr(1)ꢀN(1)
C(1)ꢀIr(1)ꢀN(2)
C(13)ꢀIr(1)ꢀN(1)
C(13)ꢀIr(1)ꢀN(2)
89.17(17)
174.34(14)
87.70(12)
80.42(16)
94.63(16)
96.29(16)
80.81(16)
C(1)ꢀIr(1)ꢀO(1)
C(13)ꢀIr(1)ꢀO(1)
C(1)ꢀIr(1)ꢀO(2)
C(13)ꢀIr(1)ꢀO(2)
N(1)ꢀIr(1)ꢀO(1)
N(1)ꢀIr(1)ꢀO(2)
N(2)ꢀIr(1)ꢀO(1)
N(2)ꢀIr(1)ꢀO(2)
93.90(14)
172.83(15)
173.93(15)
89.91(15)
90.61(13)
93.72(13)
92.47(13)
91.15(13)
Table 2. Selected bonding parameters of complex 3.
Selected Bond
Bond Length [ꢀ]
Selected Angle
Bond Angle [8]
Selected Angle
Bond Angle [8]
Ir(1)ꢀC(12)
Ir(1)ꢀC(30)
Ir(1)ꢀN(1)
Ir(1)ꢀN(2)
Ir(1)ꢀO(5)
Ir(1)ꢀO(6)
1.999(4)
1.992(4)
2.032(4)
2.043(4)
2.131(3)
2.123(3)
C(12)ꢀIr(1)ꢀC(30)
N(1)ꢀIr(1)ꢀN(2)
O(5)ꢀIr(1)ꢀO(6)
C(12)ꢀIr(1)ꢀN(1)
C(12)ꢀIr(1)ꢀN(2)
C(30)ꢀIr(1)ꢀN(1)
C(30)ꢀIr(1)ꢀN(2)
94.14(18)
176.58(15)
89.29(14)
80.37(18)
98.24(18)
95.87(18)
81.09(18)
C(12)ꢀIr(1)ꢀO(5)
C(30)ꢀIr(1)ꢀO(5)
C(12)ꢀIr(1)ꢀO(6)
C(30)ꢀIr(1)ꢀO(6)
N(1)ꢀIr(1)ꢀO(5)
N(1)ꢀIr(1)ꢀO(6)
N(2)ꢀIr(1)ꢀO(5)
N(2)ꢀIr(1)ꢀO(6)
87.21(16)
176.50(16)
174.09(16)
89.62(17)
87.54(14)
94.74(16)
95.53(14)
86.83(15)
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Table 3. Electronic absorption and emission data of L1–L4.
Compound
lmax [nm][a]
lem [nm]
Fluorescence quantum yields[c]
(298 K) [%]
[b]
CH2Cl2
CH2Cl2
(298 K)
L1
L2
L3
L4
253 (1.09), 280 (1.08)
229 (0.88), 294 (1.54)
228 (0.84), 262 (1.56), 284 (1.89)
229 (1.35), 300 (1.24), 364 (2.74)
356 (1.49)
356 (1.10)
349, 359 (1.24)
471 (4.68)
0.8
17.3
0.5
74.0
[a] Extinction coefficients (104 Mꢀ1 cmꢀ1) are shown in parentheses. [b] Fluorescence lifetimes tP [ns] at 298 K are shown in parentheses. [c]
Fluorescence quantum yields (FF) are measured in CH2Cl2 relative to anthracene.
Table 4. Photophysical properties of Ir(III) complexes.
lmax [nm][a]
lem [nm]
Frozen CH2Cl2 (77 K)[d]
540, 575 sh (8.61)
[b]
[c]
CH2Cl2
CH2Cl2
(298 K)
FP
(298 K) [%]
(kr)P
(knr)P
(106 sꢀ1)
(106 sꢀ1)
269 (2.12), 357 sh (0.35), 482 sh (0.12)
554
(0.13)
533
(0.12)
571
(0.42)
560
27.4
2.1
2.2
1.7
1.9
5.6
6.2
0.7
5.7
231 (2.34), 285 (2.80), 317 (2.68), 467 (0.17)
26.0
72.3
24.9
520, 556 sh
(11.7)
564, 606 sh
(8.8)
553, 596 sh
(7.2)
232 (2.25), 267 (3.40), 283 (3.62), 396 sh (0.32), 498 sh (0.22)
228 (1.88), 257 (2.84), 279 (3.03), 421 (2.69), 475 sh (1.85)
(0.13)
[a] Extinction coefficients (104 Mꢀ1 cmꢀ1) are shown in parentheses. [b] Phosphorescence lifetimes, tP (ms), at 298 K are shown in parenthe-
ses. [c] Phosphorescence quantum yields, FP, are measured in CH2Cl2 relative to fac-[Ir(ppy)3]. [d] Phosphorescence lifetimes, tP (ms), at 77 K
are shown in parentheses.
perature and the data are summarized in Table 4. As
shown in Figure 3, all of the complexes exhibit two major
bands in their absorption spectra, in which the intense
UV bands are assigned to the spin-allowed S0!S1 transi-
tions of the organic chromophores (i.e., pꢀp* bands),
whereas the weaker, low-energy bands are caused by
attachment of CF3 would generally result in a bathochro-
mic shift of the emission spectrum.[11] For example, with
reference to the parent complex [Ir(ppy)2(acac)] (lem
ꢁ512 nm), complex 1 shows a significant redshift of
about 32 nm in the emission maximum. The OPh moiety,
which has a minor participation in both HOMO and
LUMO levels (see below), cannot offer a good electron
1
both S0! MLCT (MLCT=metal to ligand charge trans-
3
fer) and S0! MLCT transitions.[15] The oscillator strength
3
of S0! MLCT transition is close to that of the lower-
1
energy S0! MLCT one (less than a factor of two in their
extinction coefficients for the two MLCT bands),[16] indi-
3
cating that the S0! MLCT transition is strongly allowed
by significant singletꢀtriplet state mixing owing to the
spinꢀorbit coupling induced by the heavy iridium metal.
Upon light irradiation at 400 nm, complexes 1–4 show
intense yellowish-green to orange photoluminescence
(PL) bands with emission maxima ranging from about
533 to 571 nm. The spectra of 1–4 and the parent complex
[Ir(ppy)2(acac)][17] have virtually identical shapes, indicat-
ing that the same excited and/or ground states are in-
volved in the phosphorescent transitions. The structure-
less pattern of the PL spectra is suggestive of their pre-
dominantly MLCT character for the lowest excited triplet
states at 298 K (Figure 3).
A general trend of redshifted emission spectra was ob-
served by comparing the complexes with CF3 substituents
to those without. This is in agreement with the idea that
Figure 3. Absorption and emission spectra of Ir(III) complexes 1–4
in CH2Cl2.
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sink for the stabilization of MLCT states of OPh-contain-
ing trifluoromethylpyridine-based iridium complex 2;
thus raising the LUMO level through its inherent good p-
donating ability with the phenyl ring attached, and caus-
ing a blueshift in the emission relative to 1. Clearly, for
NPh2-containing complex 4, it exhibits a redshifted emis-
sion wavelength at 560 nm relative to 1 because the NPh2
group can push up the HOMO energy by electron dona-
tion compared with the parent compound 1. According to
the traditional color-tuning strategy reported in the litera-
ture,[18] the emission maximum of an iridium(III) cyclo-
metalated complex typically shows a hypsochromic shift
when an electron-withdrawing group (such as F) is intro-
duced into the phenyl ring of ppy. In contrast, our new
SO2Ph-substituted complex 3, which consists of an elec-
tron-withdrawing group on the phenylpyridine moiety,
demonstrated the reverse trend and emitted at a longer
wavelength of approximately 550 nm than 1. The induc-
tively electron-withdrawing SO2Ph moieties can somehow
act as the destination for electrons in the MLCT process
and stabilize the MLCT states accordingly and contribute
to the LUMO stabilization to a large extent.[19]
All of the Ir(III) complexes emit strong phosphores-
cence in glass matrices at 77 K.[19] Typically, these spectra
display more structured spectral profiles and experience
rigidochromic blueshifts upon cooling the solutions to
77 K. The hypsochromic shifts observed at 77 K are
mainly due to solvent reorganization in a fluid solution at
low temperature that can stabilize the charge-transfer
states prior to emission.[20] This event is largely impeded
in a rigid matrix at 77 K, resulting in phosphorescence at
a higher energy. The relatively short triplet lifetime along
with the structureless emission profile at room tempera-
ture are suggestive of the predominantly MLCT character
for these iridium(III) complexes at room temperature. At
77 K, the vibronic fine structures become more apparent
in the emission bands, which suggests the involvement of
some ligand-centered character.
The phosphorescent quantum yields (FP) for our new
triplet emitters were measured in CH2Cl2 against the
[Ir(ppy)3] standard. Their FP values are in the range of
24.9–72.3%. The exceptionally high FP values for 3 might
be attributed to the presence of electron-withdrawing
SO2Ph groups that can facilitate the formation of MLCT
states. This clearly demonstrates the merit of introducing
electron-withdrawing moieties to the [Ir(ppy)2(acac)]
emissive core to afford highly efficient phosphorescent
iridium(III) complexes. The observed phosphorescence
lifetimes, tP, have magnitudes of about 0.13–0.42 ms, which
are shorter than that of the reference compound
[Ir(ppy)3] (tP ꢁ0.5 ms). It is widely accepted that a longer
lifetime of a phosphorescent emitter would cause severe
tripletꢀtriplet annihilation, especially at high current den-
sity.[21] Thus, this detrimental issue in many phosphores-
cent OLEDs is likely to be relieved with the inclusion of
these main-group moieties. Accordingly, the radiative life-
times (tr) of the triplet excited state deduced from tr =tP/
FP are as short as 0.16–1.25 ms.[22] The triplet radiative
and nonradiative rate constants, (kr)P and (knr)P, can be
derived from FP and tP using the relationships FP =
FISC{(kr)P/[(kr)P +(knr)P]}
and
tP =[(kr)P +(knr)P)]ꢀ1 [23]
.
Here, FISC is the intersystem-crossing yield, which can
safely be assumed to be 1.0 for iridium complexes be-
cause of the strong spinꢀorbit coupling interaction caused
by the heavy-atom effect of iridium. The (kr)P value, in
the order of 106 sꢀ1 for our complexes, are higher than
that found for [Ir(ppy)3] (8.0ꢁ105 sꢀ1). This would be ad-
vantageous to the molecular design of highly efficient de-
vices based on light-energy harvesting from the triplet ex-
citons.
2.4 Computational Studies
Figure 4 shows the DFT contour plots of complexes 1 and
3 based on their X-ray data, which reveal that the
HOMOs are derived from the out-of-phase combinations
of the Ir ꢂt2gꢃ set of d orbitals and the p orbitals of the iri-
dium-bonded phenyl rings of the C^N bidentate ligands.
In each case, the Ir d orbitals make about 46.0–47.2%
contribution and the N(amine) pp orbitals also contribute
to the HOMOs (Table 5). The LUMOs contain the p* or-
bitals of the pyridine rings of the ligands.
2.5 Electrochemical Properties
The electrochemical properties of our materials were in-
vestigated by cyclic voltammetry (CV) of solutions of the
complexes in CH3CN containing 0.1 M tetrabutylammoni-
um hexafluorophosphate as the supporting electrolyte at
a scan rate of 100 mVsꢀ1. Ferrocene was used as the inter-
nal standard. The results are listed in Table 6. It is widely
accepted that the oxidation process involves the iridium
moiety of the complexes, and the HOMO levels are
mostly due to the d orbitals of iridium with some p-orbi-
tal contributions from the ligand.[15] Generally, with at-
tachment of CF3 substituents, the LUMO level of the
complexes is significantly stabilized to a greater extent
than that of the HOMO level. For instance, when com-
paring [Ir(ppy)2(acac)] to 1, the decrease in the LUMO
level is larger than that of the HOMO, which results in
a narrower band gap and a redshifted emission spectrum
(i.e., a stabilized MLCT transition). Due to the low par-
ticipation of the OPh group, the HOMO and LUMO
levels would not be affected significantly in 2 relative to
its parent complex without the OPh group. Owing to the
addition of electron-rich diphenylamino groups, complex
4 shows an elevated HOMO level (ꢀ5.38 eV) relative to
1 (ꢀ5.40 eV), which will facilitate hole injection (HI) to
the complex upon electrical excitation. The electron-rich
ligand also makes these complexes much more difficult to
reduce, leading to
a slightly higher LUMO in 4
(ꢀ2.45 eV) with respect to 1 (ꢀ2.53 eV). With the induc-
Isr. J. Chem. 2014, 54, 999 – 1014
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Figure 4. Contour plots of HOMOs and LUMOs for 1 (top) and 3 (bottom). (1 au=27.2114 eV)
Table 5. Percentage contribution of the metal dp orbitals to the HOMO and LUMO together with the time-dependent density functional
theory (TDDFT) calculation results.
Compound Contribution of d orbitals
to the HOMO
Contribution of d orbitals Largest coefficient in the CI expan-
to the LUMO
Oscillator strength (f) of the
S0!S1 transition
sion of the S1 state[a]
1
3
47.2%
46.0%
4.4%
3.4%
H!L: 0.65
H!L: 0.67
0.0286
0.0329
[a] H!L represents the HOMO to LUMO transition. CI stands for configuration interaction.
Table 6. Electrochemical properties of 1–4.
Eox [V][a]
HOMO [eV]
Ered [V][a]
LUMO [eV]
Tdec [8C]
1
2
3
4
0.60
0.66
0.89
0.58
ꢀ5.40
ꢀ5.46
ꢀ5.69
ꢀ5.38
ꢀ2.27
ꢀ2.31
ꢀ1.80
ꢀ2.35
ꢀ2.53
ꢀ2.49
ꢀ3.00
ꢀ2.45
362
379
378
398
[a] 0.1 M [Bu4N]PF6 in CH3CN, scan rate 100 mVsꢀ1, versus Fc/Fc+ couple.
tively electron-withdrawing SO2Ph moieties, SO2Ph-con-
taining complex is readily reduced and gives the first Ered
of ꢀ1.79 V for 3, which should be ascribed to the reduc-
tion of SO2Ph. Compared with that of 1 (ꢀ2.27 V), the
higher reduction potential for SO2Ph-containing complex
3 indicates its lower LUMO energy (ꢀ3.00 eV), highly fa-
voring the electron injection/electron transporting (EI/
ET) character. Therefore, these complexes provide good
examples of orange-emitting iridium(III) phosphors with
promising EI/ET features. Likewise, the SO2Ph-substitut-
ed complex is more difficult to be oxidized than its non-
substituted parent species. Complex 3 has a higher oxida-
tion potential (0.89 V), which would not promote hole in-
jection/hole transporting (HI/HT) properties. In general,
the HI/HT feature is common in iridium complexes be-
cause many of them possess p-conjugated organic ligands,
which would make the EI/ET unlikely. However, these
CF3-substituted complexes with main-group moieties
show good EI/ET properties, which may offer a more bal-
anced charge-carrier injection during the OLED opera-
tion, and would be an added advantage to the overall per-
formance of phosphorescent OLEDs prepared with these
triplet emitters. Most of the complexes show quasi-rever-
sible or reversible waves, which imply a good reversibility
in the electrochemical behavior and may be advantageous
to the overall stability of the OLEDs.
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2.6 Thermal Properties
layer configuration indium tin oxide (ITO)/N,N’-di(1-
naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine
(NPB; 40 nm)/x% Ir:4,4’-N,N’-dicarbazolebiphenyl (CBP;
20 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP; 10 nm)/tris(8-hydroxyquinolinato)aluminum (Alq3;
30 nm)/LiF (1 nm)/Al. Thanks to its ambipolar charge-
transporting property and suitable triplet level, CBP was
used as the carrier combination host for our phosphors.
NPB serves as the hole-transport layer, BCP is the hole
blocker, Alq3 is the electron transporter, and LiF is the
electron-injection layer. To optimize the device efficiency,
a doping concentration ranging from 4 to 10 wt% were
employed. Devices A–D gave intense yellowish-orange to
orange EL spectra. The close resemblance of the EL
spectra (Figure 6) with the corresponding PL spectra in
each case implies that EL originates from the triplet
states of the phosphor. Their EL spectra show negligible
voltage dependence, resulting in an improved color stabil-
ity with increasing voltage. No clear emission from CBP
or Alq3 was detected from each device, even at high cur-
rent densities, suggesting efficient energy transfer from
the host exciton to the phosphor molecule upon electrical
excitation and the effective hole blocking effect of BCP.
Table 7 summarizes the EL data for devices A–D at
the optimum Ir doping levels. Device D3 prepared from 4
(8 wt%) showed promising results with an external quan-
tum efficiency, hext, of 11.5%, a luminance efficiency, hL,
of 38.9 cdAꢀ1, power efficiency, hP, of 27.0 lmWꢀ1, a lumi-
Since good thermal stabilities of the phosphors are impor-
tant for OLED applications, the onset decomposition
temperatures (Tdec) of 1–4 were determined by thermog-
ravimetric analysis (TGA) under a stream of nitrogen
(Table 6). From the onset of the TGA curves, all com-
plexes show good thermal stabilities with their Tdec values
ranging from 362 to 3988C. The first weight loss of some
of the compounds is mostly due to dissociation of the
acacꢀ ancillary ligand. For example, we observe a sharp
weight loss of 12.4% for 4, which is ascribed to the re-
moval of acacꢀ groups from the complex. Their results in-
dicated that the incorporation of main-group moieties
into [Ir(ppy)2(acac)] could provide higher thermal and
chemical stability.[24] These data render the materials suit-
able for the fabrication of OLEDs by using the vacuum
thermal deposition method.
2.7 Electroluminescent Properties
2.7.1 Orange OLEDs (A–D) Based on Ir(III) Complexes 1–4
With high stability under thermal sublimation, complexes
1–4 were used to fabricate vacuum-evaporated OLEDs.
Figure 5 illustrates the architecture of electrophosphores-
cent devices and the molecular structures of the com-
pounds employed. All of the devices consist of the multi-
Figure 5. The general structure for OLED devices and the molecular structures of the relevant compounds used.
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Figure 6. EL spectra of devices A1 (a), B1 (b), C1 (c), and D3 (d).
Table 7. Performance of the best Ir-doped OLEDs made from 1–4.
[d]
Device
Dopant
Conc.
[wt%]
Vturn-on
[V]
L
hext
[%]
hL
[cdAꢀ1]
hp
[lmWꢀ1]
lmax
[nm]
[cdmꢀ2]
A1
1
4
4
4
8
3.8
3.9
3.8
3.3
11240[a]
15.15
11.50
16.70 (7.7)
11.06
8.34
12.47 (9.9)
6.28
5.05
6.36 (9.7)
9.25
6.46
56.16
42.79
61.88 (7.7)
42.20
31.85
47.67 (9.9)
18.42
14.83
18.66 (9.7)
30.87
21.59
18.88
11.24
29.16 (6.1)
10.75
6.85
17.05 (7.9)
5.64
3.79
6.52 (8.7)
11.28
6.43
552 (0.42, 0.57)
42726[b]
48371 (12.5)[c]
8510[a]
B1
C1
D3
2
3
4
532 (0.35, 0.62)
564 (0.50, 0.49)
560 (0.48, 0.51)
31809[b]
48388 (15.7)[c]
3750[a]
14858[b]
48260 (14.7)[c]
6169[a]
21591[b]
48413 (12.5)[c]
11.51 (5.5)
38.89 (5.1)
26.97 (4.3)
[a] Values collected at 20 mAcmꢀ2. [b] Values collected at 100 mAcmꢀ2. [c] Maximum values of the device. Values in parentheses are the vol-
tages at which the maximum values were obtained. [d] CIE coordinates [x, y] in parentheses.
nance. Lmax, of 48413 cdmꢀ2 at 12.5 V, and a turn-on volt-
age, Vturn-on, of 3.3 V. Phosphor 4, with a NPh2 moiety, is
expected to afford highly efficient OLEDs due to its in-
trinsic hole-trapping ability; however, to our surprise, we
observed that devices A1–A4 doped with 1 showed
a much better EL performance than devices D1–D4.
Phosphor A1 gave the best EL performance at 4 wt%
doping concentration with hext of 16.7%, hL of
61.9 cdAꢀ1, hP of 29.1 lmWꢀ1, Lmax of 48371 cdmꢀ2 at
12.5 V, and Vturn-on of 3.8 V. Provided that phosphors
1 and 4 are associated with similar HOMO levels (1:
ꢀ5.40 eV, 4: ꢀ5.38 eV), and hence, a similar HI/HT abili-
ty; this may be attributed to a closer match of the LUMO
level of 1 (LUMO: ꢀ2.53 eV) to the corresponding host
CBP (ꢀ3.2 eV) than that of the 4 (ꢀ2.45 eV), leading to
a more effective EI/ET, and thus, enhanced efficiencies.
Another set of eye-catching yellowish-orange-emitting
OLEDs refers to those made from B1–B4 with a maxi-
mum hext of 12.5%, hL of 47.7 cdAꢀ1, hP of 17.1 lmWꢀ1,
Lmax of 48388 cdmꢀ2 at 15.7 V, and Vturn-on of 3.9 V at
4 wt% doping concentration. Device C1, which was fabri-
cated from 3, turned on at 3.8 V, but it only exhibited effi-
ciencies of 6.4%, hL of 18.7 cdAꢀ1, hP of 6.5 lmWꢀ1, and
Lmax of 48260 cdmꢀ2 at 3.8 V. The lower performance of
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devices C1–C4 may be attributed to the much higher oxi-
dation potential of 3 (0.89 V) than other heteroleptic
complexes, resulting in reduced HI/HT features during
device operation. Representative current densityꢀvolt-
ageꢀluminance (JꢀVꢀL) characteristics and the efficiency
versus the current density relationship of devices A1, B1,
C1, and D3 are shown in Figure 7. For A1, B1, C1, and
D3, they only showed a moderate drop in hext of about
13.3–24.6%. This desirable character of the OLEDs
might be attributed to the short lifetime of the phosphors
and their sterically congested CF3 groups, which effective-
ly blocks the non-emissive pathways provided by various
intermolecular excited-state interactions to some extent.
Notably, other factors, such as carrier recombination and
emission width, would also control the roll-off character-
istics.[25]
With favorable performance data for our orange triplet
emitters, complexes 1, 3, and 4 were selected in the fabri-
cation of highly efficient fluorescence/phosphorescence
(F/P) two-element WOLEDs (W-1 to W-3) by color
mixing with a complementary blue fluorophore, 4,4’-bis[4-
(diphenylamino)styryl]biphenyl (DSA-Ph), in the device
configuration of ITO/MoO3 (5 nm)/NPB (100 nm)/CBP:1
(4%, 20 nm)/NPB (2 nm)/2-methyl-9,10-di(2-napthyl)an-
thracene (MADN):DSA-Ph (0.75%, 40 nm)/bis(2-methyl-
8-quinolinolato)(4-phenylphenolato)aluminum(III)
(BAlq; 10 nm)/LiF (1 nm)/Al (see Figure 8). Transparent
MoO3 acts as the charge generation/injection layer, which
has a very small HI barrier with NPB. BAlq acts as the
ET and hole-blocking material for better confinement of
excitons in the emission layer.[19] MADN was used as the
host for the blue dopant DSA-Ph.[26] The similarity of the
HOMO levels between MADN and BAlq enhances HT
in the device. A thin layer of NPB was added between
the F and P regions as a spacer to facilitate HT into the
blue layer and block electrons at the blue layer/spacer in-
terface. Hence, the main recombination zone should be
located near the blue layer, enhancing the DSA-Ph emis-
sion, which is usually weaker than that of the electrophos-
phor.
Generally, comparison of the absorption spectra of
phosphors with the fluorescence spectrum of DSA-Ph
(PL maximum at 463 nm with a shoulder of 491 nm) indi-
cates good overlap of both spectra, which can justify effi-
cient energy absorption of blue fluorescence by the
orange triplet emitter. Both singlet and triplet excitons
are thus confined efficiently in two different recombina-
tion zones. Moreover, this interlayer, with a high triplet
energy between the two emitters, avoids phosphorescence
quenching, even if the triplet level of the blue emitter is
low and suppresses Dexter transfer of the phosphorescent
excitons to the nonradiative triplet state of DSA-Ph. It
also prevents direct singlet energy transfer from the blue
fluorophore to the lower-energy orange phosphor and
minimizes the exchange energy losses to increase the
WOLED efficiency.
Figure 7. Current densityꢀvoltageꢀluminance (JꢀVꢀL) characteristics and luminance (&), power (*), and external quantum (~) efficien-
cies as a function of the current density of devices A1 and B1.
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Figure 8. The general configuration for W-1 to W-3 and the molecular structures of the relevant compounds used in these devices.
Figure 9 depicts the EL device performance for W-1 to
W-3. The peak efficiencies and CIE coordinates (at 10 V)
were recorded as hext =10.3%, hL =28.4 cdAꢀ1, hP =
8.5 lmWꢀ1, and CIE at (0.33, 0.35) for W-3 (Table 8). It
was observed that the white EL from W-3 remained
almost constant with increasing applied voltage. In these
devices, the blue fluorescent dopant harnesses the majori-
ty of singlet excitons, with the remainder of lower energy
triplets diffusing through the conductive host to directly
19.0 lmWꢀ1, and CIE at (0.384, 0.438) for W-1; hext
=
7.1%, hL =16.7 cdAꢀ1, hP =11.2 lmWꢀ1, and CIE at (0.35,
0.34) for W-2 ; hext =4.1%, hL =10.6 cdAꢀ1, hP =
Figure 9. EL spectra of devices W-1 (a), W-2 (b), and W-3 (c).
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Table 8. Performance of Ir-doped WOLEDs W-1 to W-3.
[d]
Device
Dopant
Vturn-on
[V]
Lmax
hext
[%]
hL
[cdAꢀ1]
hp
[lmWꢀ1]
lmax
[nm]
[cdmꢀ2]
W-1
1
4.1
3.8
3.2
2977[a]
5.40
14.88
2.72
(0.453, 0.499) (6)
(0.427, 0.476) (8)
(0.384, 0.438) (10)
(0.354, 0.409) (12)
(0.333, 0.389) (14)
(0.322, 0.379) (16)
13155[b]
47908 (21.9)[c]
2052[a]
4.78
10.33 (4.7)
4.34
13.17
28.44 (4.7)
10.34
2.02
19.01 (4.7)
2.09
W-2
W-3
3
4
(0.407, 0.373) (7)
(0.388, 0.360) (8)
(0.371, 0.348) (9)
(0.354, 0.336) (10)
(0.339, 0.325) (11)
(0.326, 0.316) (12)
8581[b]
32798 (19.9)[c]
1836[a]
3.62
7.05 (4.49)
3.58
8.56
16.71 (4.49)
9.08
1.46
11.17 (4.49)
2.44
(0.328, 0.348) (8)
(0.330, 0.349) (9)
(0.334, 0.352) (10)
(0.336, 0.356) (11)
(0.337, 0.357) (12)
(0.336, 0.355) (13)
(0.332, 0.352) (14)
7243[b]
2.83
4.13 (6.5)
7.23
10.60 (6.50)
1.51
8.50 (3.29)
24904 (17.1)[c]
[a] Values collected at 20 mAcmꢀ2. [b] Values collected at 100 mAcmꢀ2. [c] Maximum values of the device. Values in parentheses are the vol-
tages at which the maximum values were obtained. [d] CIE coordinates [x, y], with the voltage shown in parentheses.
excite the orange phosphor. Diffusion of singlet excitons
to the phosphor dopant is negligible due to their intrinsi-
cally short diffusion lengths.[27] The trade-off between effi-
ciency and color stability is usually noted for two-element
WOLEDs because the main recombination zone should
be located near the blue fluorophore to get a more in-
tense blue emission relative to that from an orange phos-
phor. This, in turn, would strongly hinder the efficiency of
an Ir-doped device. At 10 V, devices W-1, W-2, and W-3
have a correlated color temperature (CCT) of 4267, 4572,
and 5455 K, respectively, which is suitable for illumina-
tion purposes. Among W-1 to W-3, W-3 exhibits a better
CRI of 77 at 8 V, whereas W-1 and W-2 remain at 56 and
65, respectively, for CRI at the same applied voltage.
switching of the charge-transfer characteristics of the
transition induced by the main-group moieties. While it is
common that the substituent effects are greatly depen-
dent on the substitution positions, future work should be
focused on the influence of substituent position on the
emission color. Given the ease of synthesis and color tun-
ability, as well as performance advantages, this work has
significant implications for future advances in this field.
4 Experimental Section
4.1 Materials and Reagents
All of the manipulations were performed under a dry ni-
trogen atmosphere by using Schlenk techniques. Solvents
were dried by standard methods and distilled prior to use,
except for those of spectroscopic grade for the photo-
physical and electrochemical measurements. All reagents
and chemicals, unless otherwise stated, were purchased
from commercial sources and used without further purifi-
cation. Separation and purification of products were ach-
ieved by column chromatography or preparative thin-
layer chromatography on glass plates (20 cmꢁ20 cm),
pre-coated with Merck Kieselgel 60 GF254 prepared in
our laboratory (0.7 mm). The bromo precursors of the cy-
3 Conclusions
We have demonstrated a versatile avenue for emission
color tuning of iridium(III) complexes through the facile
derivatization of the phenyl moiety of ppy with various
main-group moieties, and we have prepared new yellow-
ish-green to orange triplet emitters. The PL properties of
these phosphors at room temperature show that they are
promising candidates for the fabrication of monochromat-
ic OLEDs. These exciting results can be attributed to
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clometalating ligands were prepared according to
gram.[32] Molecular orbitals obtained from the B3LYP cal-
culations were plotted using the Molden 3.7 program
written by Schaftenaar.[33]
a method reported in the literature.[19]
4.2 Physical Measurements
4.4 Fabrication and Characterization of OLEDs
NMR spectra were measured in deuterated solvents as
the lock and reference on a Bruker AV 400 instrument
Commercial ITO-coated glass with a sheet resistance of
20–30 W/square was used as the starting substrate. Before
device fabrication, the ITO glass substrates were cleaned
by ultrasonic baths in organic solvents followed by ozone
treatment for 20 min. NPB was first deposited on the
ITO glass, the organic layers were evaporated and lami-
nated under 4ꢁ10–4 Pa without breaking the vacuum be-
tween each vacuum deposition process. The emissive
layer was then formed by co-deposition of the dopant
and the host. The evaporation rates were 1–2, 0.3, and 4–
6 ꢀsꢀ1 for organic materials. BCP, Alq3, LiF, or Al were
successively evaporated at a base pressure less than
10ꢀ6 torr, depending on the device structure used. The
layer thickness was monitored in situ using a quartz crys-
tal oscillator. The active area of the device was 5 mm2, as
defined by the shadow mask. The EL spectra and CIE co-
ordinates were measured with a PR650 spectra colorime-
ter. The JꢀVꢀL curves of the devices were recorded by
a Keithley 2400/2000 source meter and the luminance was
measured using a PR650 SpectraScan spectrometer. All
experiments and measurements were carried out at room
temperature under ambient conditions.
1
with H and 13C NMR chemical shifts quoted relative to
tetramethylsilane (TMS) standard. The matrix-assisted
laser desorption ionization time-of-flight (MALDI-TOF)
mass spectra were recorded on an Autoflex Bruker
MALDI-TOF system. UV/Vis absorption spectra were
obtained on an HP-8453 diode array spectrophotometer.
The PL spectra were measured in dichloromethane with
a PTI Fluorescence Master Series QM1 spectrophotome-
ter. The phosphorescence quantum yields were deter-
mined in CH2Cl2 at 298 K against fac-[Ir(ppy)3] as a stan-
dard (FP =0.40).[28] Cyclic voltammograms were mea-
sured on a CHI model 600D electrochemistry station in
0.1 molLꢀ1 [Bu4N]PF6 in acetonitrile with a glassy carbon
working electrode and Pt wire for counter and reference
electrodes under a N2 atmosphere at a scan rate of
100 mVsꢀ1. Ferrocene was added as a calibrant after each
set of measurements, and all potentials reported were
quoted with reference to the ferroceneꢀferrocenium (Fc/
Fc+) couple. Crystals of our compounds suitable for X-
ray diffraction studies were grown by slow evaporation of
a solution in dichloromethane/hexane at room tempera-
ture and were mounted on a glass fiber. Geometric and
intensity data were collected by using graphite-monochro-
mated MoKa radiation (l=0.71073 ꢀ) on a Bruker Axs
SMART 1000 CCD diffractometer. The collected frames
were processed with the software SAINT[29] and an ab-
sorption correction (SADABS)[30] was applied to the col-
lected reflections. The structures of all compounds were
4.5 Synthesis of Cyclometalating Ligands
4.5.1 Synthesis of L1
In a flask, Pd(PPh3)4 (40 mg) was added to a mixture of
phenylboronic acid (1.56 g, 12.7 mmol) and 2-chloro-5-
(trifluoromethyl)pyridine (1.54 g, 8.48 mmol) in toluene
(40 mL) and 2 M aqueous Na2CO3 (4 mL) under an inert
atmosphere of nitrogen. The reaction was heated to
1008C for 48 h. After the mixture was cooled to room
temperature, water was added and the solution mixture
was extracted with ethyl acetate. The combined organic
layer was dried with MgSO4, filtered, and concentrated
under reduced pressure. The residue was washed with
hexane to afford a white solid (56%). MS (MALDI-
solved by direct methods (SHELXTLTM [31] in conjunction
)
with standard difference Fourier techniques and subse-
quently refined by full-matrix least-squares analyses on
F2. All non-hydrogen atoms were assigned with anisotrop-
ic displacement parameters. Hydrogen atoms were gener-
ated in their idealized positions and allowed to ride on
their respective parent carbon atoms. Crystal data are
given in Table S1 in the Supporting Information. CCDC
996471–996473 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
TOF): m/z: 223.1 [M+]; H NMR (CDCl3): d=8.95 (s,
1H; Ar), 8.05–7.98 (m, 3H; Ar), 7.87–7.85 (d, J=8 Hz,
1H; Ar), 7.54–7.47 ppm (m, 3H; Ar); 13C NMR (CDCl3):
d=160.67, 146.63, 146.59, 137.95, 133.93, 133.90, 130.05,
128.97, 127.26, 122.41, 119.95 ppm (Ar).
4.3 DFT and TD-DFT Computational Methods
DFT and TDDFT calculations at the B3LYP level were
performed based on experimental geometries from the X-
ray data. The basis set used for C, N, O, F, and H atoms
was 6-31G, while effective core potentials with
a LanL2DZ basis set were employed for Ir and S. All cal-
culations were carried out using the Gaussian 03 pro-
4.5.2 Synthesis of L2
A similar method was used to prepare this compound as
that for L1, but 4-phenoxyphenylboronic acid (1.0 g,
4.67 mmol) was used instead. The product was separated
by column chromatography using a mixture of CH2Cl2/
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4.6 Synthesis of Heteroleptic Iridium(III) Complexes
hexane as the eluent to afford a white solid (56%). MS
1
(MALDI-TOF): m/z: 315.1 [M+]; H NMR (CDCl3): d=
8.91 (s, 1H; Ar), 8.04–7.95 (m, 3H; Ar), 7.81–7.79 (d, J=
8 Hz, 1H; Ar), 7.40–7.36 (m, 2H; Ar), 7.19–7.06 ppm (m,
5H; Ar); 13C NMR (CDCl3): d=160.00, 159.39, 156.38,
146.59, 146.55, 133.93, 133.89, 132.67, 129.92, 128.86,
124.25, 123.96, 119.54, 119.39, 118.64 ppm (Ar).
In the synthesis of heteroleptic Ir(III) complexes, iridium-
(III) dimers were first prepared by reacting IrCl3·nH2O
with 2.7 equivalents of the corresponding cyclometalating
ligand in a mixture of 2-ethoxyethanol and water (3 :1, v/
v). The mixture was heated at reflux for 24 h and then
cooled to room temperature. Precipitates were gradually
formed during the reaction. The precipitate was collected
by filtration and washed with hexane followed by ethanol.
The solvent was then pumped dry completely to give the
crude Ir(III) dimer for subsequent reaction without fur-
ther purification. The crude dimer was mixed with 10
equivalents of Na2CO3 in 2-ethoxyethanol. Three equiva-
lents of acetylacetone were added and the reaction mix-
ture was then heated at reflux for 16 h. After the mixture
was cooled to room temperature, water was added fol-
lowed by extraction with CH2Cl2. The organic layer was
collected and solvent was subsequently removed. The res-
idue was then purified on preparative TLC plates using
a mixture of CH2Cl2/hexane as the eluent.
4.5.3 Synthesis of L3a
Similar to the synthesis of L1, L3a was prepared from
Suzuki coupling between 4-phenylsulfanylphenylboroinc
acid (1.13 g, 4.91 mmol) and 2-chloro-5-(trifluoromethyl)-
pyridine (0.59 g, 3.07 mmol). MS (MALDI-TOF): m/z:
1
331.0 [M+]; H NMR (CDCl3): d=8.92 (s, 1H; Ar), 7.98–
7.94 (m, 3H; Ar), 7.81–7.79 (d, J=8 Hz, 1H; Ar), 7.46–
7.45 (m, 2H; Ar), 7.39–7.32 ppm (m, 5H; Ar); 13C NMR
(CDCl3): d=149.87, 146.67, 146.63, 139.68, 135.99, 134.05,
133.98, 133.95, 132.44, 129.87, 129.45, 127.92, 127.83,
122.37, 119.62 ppm (Ar).
1:
A bright orange powder (yield: 53%); MS
(MALDI-TOF): m/z: 736.1 [M+]; H NMR (CDCl3): d=
8.75 (s, 2H; Ar), 7.96–7.95 (m, 4H; Ar), 7.63–7.61 (d, J=
8 Hz, 4H; Ar), 6.89–6.86 (t, J=6 Hz, 2H; Ar), 6.79–6.75
(d, J=8 Hz, 2H; Ar), 6.26–6.24 (d, J=8 Hz, 3H; Ar), 5.27
(s, 1H; acac), 1.82 ppm (s, 6H; Ar); 13C NMR (CDCl3):
d=185.64 (acac), 172.33, 149.50, 14.46, 143.16, 134.47,
134.43, 133.56, 130.83, 125.64, 121.57, 118.37 (Ar), 101.11
(acac), 28.83 ppm (acac); elemental analysis calcd (%) for
C29H21N2F6O2Ir: C 47.34, H 2.88, N 3.81; found: C 47.21,
H 2.98, N 4.02.
1
4.5.4 Synthesis of L3
L3a (0.56 g, 1.69 mmol) was dissolved in chloroform at
08C and underwent oxidation to give L3 by adding m-
CPBA (0.64 g, 3.72 mmol, 2.2 equiv). The mixture was
stirred for 3 h. Concentration of the reaction mixture af-
forded the crude product, which was purified by column
chromatography using CH2Cl2/ethyl acetate as the eluent.
The final product was isolated as a white solid (83%).
1
MS (MALDI-TOF): m/z: 363.0 [M+]; H NMR (CDCl3):
2: An orange powder (yield: 31%). MS (MALDI-
1
d=8.97 (s, 1H; Ar), 8.18–8.16 (d, J=8 Hz, 2H; Ar), 8.09–
8.07 (d, J=8 Hz, 1H; Ar), 7.46–7.45 (m, 2H; Ar), 7.39–
7.32 (m, 5H; Ar). 8.04–8.02 (m, 1H; Ar), 8.00–7.97 (m,
2H; Ar), 7.88–7.86 (d, J=8 Hz, 1H; Ar), 7.59–7.51 ppm
(m, 3H; Ar); 13C NMR (CDCl3): d=158.51, 146.94,
146.90, 142.71, 142.35, 141.32, 134.33, 134.30, 133.41,
129.40, 128.32, 128.15, 127.74, 120.56 ppm (Ar).
TOF): m/z: 920.3 [M+]; H NMR (CDCl3): d=8.58 (s,
2H; Ar), 7.78–7.71 (q, J=9.3 Hz, 4H; Ar), 7.52–7.50 (d,
J=8 Hz, 2H; Ar), 7.23–7.22 (m, 2H; Ar), 7.09–7.05 (t, J=
6 Hz, 2H; Ar), 6.91–6.89 (d, J=8 Hz, 4H; Ar), 6.50–6.47
(m, 2H; Ar), 5.68–5.67 (d, J=4 Hz, 4H; Ar), 5.28 (s, 1H;
acac), 1.83 ppm (s, 6H; acac); 13C NMR (CDCl3): d=
184.38 (acac), 170.16, 158.41, 154.45, 150.43, 143.92,
136.42, 133.01, 128.44, 125.80, 122.81, 119.90, 119.42,
116.54, 109.67 (Ar), 99.97 (acac), 27.56 ppm (acac); ele-
mental analysis calcd (%) for C41H29N2F6O4Ir: C 53.53, H
3.18, N 3.05; found: C 53.44, H 3.30, N 3.17.
4.5.5 Synthesis of L4
This compound was prepared similarly as that for L1 in
65% yield by using Suzuki coupling, but diphenylamine-
4-phenylboronic acid (1.1 g, 3.8 mmol) was used instead
as the starting material. MS (MALDI-TOF): m/z: 390.2
3: An orangeꢀred powder (yield: 44%); MS (MALDI-
1
TOF): m/z: 1016.2 [M+]; H NMR (CDCl3): d=8.67 (2,
2H; Ar), 8.10–8.03 (q, J=9.3 Hz, 4H; Ar), 7.69–7.63 (m,
6H; Ar), 7.52–7.48 (m, 2H; Ar), 7.41–7.36 (m, 6H; Ar),
6.57 (s, 2H; Ar), 5.29 ppm (s, 1H; Ar); 13C NMR
(CDCl3): d=186.08 (acac), 170.26, 149.11, 147.92, 145.63,
141.68, 131.41, 135.48, 133.23, 131.11, 129.23, 127.79,
126.13, 125.76, 120.99, 120.03 (Ar), 101.36 (acac),
28.63 ppm (acac); elemental analysis calcd (%) for
C41H29N2F6O6S2Ir: C 48.47, H 2.88, N 2.76; found: C
48.60, H 3.04, N 2.88.
1
[M+]; H NMR (CDCl3): d=8.88 (s, 1H; Ar), 7.93–7.89
(m, 3H; Ar), 7.76–7.75 (d, J=8 Hz, 1H; Ar), 7.31–7.28
(m, 4H; Ar), 7.17–7.10 (m, 6H; Ar), 7.09–7.07 ppm (t, J=
4 Hz, 2H; Ar); 13C NMR (CDCl3): d=160.40, 149.95,
147.35, 146.78, 146.75, 146.70, 146.67, 134.00, 133.97,
132.64, 131.17, 129.82, 129.66, 129.37, 128.48, 128.34,
126.38, 125.47, 125.38, 124.29, 123.95, 123.03, 122.74,
122.60, 119.26 ppm (Ar).
Isr. J. Chem. 2014, 54, 999 – 1014
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4: An orange solid (yield: 30%); MS (MALDI-TOF):
1
m/z: 1070.2 [M+]; H NMR (CDCl3): d=8.41 (s, 2H; Ar),
7.53–7.45 (q, J=8 Hz, 4H; Ar), 7.33–7.31 (d, J=8 Hz,
2H; Ar), 7.17–7.14 (m, 8H; Ar), 7.01–6.95 (m, 14H; Ar),
6.54–6.52 (d, J=8 Hz, 2H; Ar), 5.63 (s, 2H; Ar), 5.26 (s,
1H; acac), 1.83 ppm (s, 1H; acac); 13C NMR (CDCl3): d=
185.16 (acac), 170.98, 149.88, 148.97, 146.76, 144.64,
135.75, 135.46, 133.18, 128.96, 128.25, 126.08, 125.89,
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30.32 ppm (acac); elemental analysis calcd (%) for
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H 3.89, N 5.12.
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We thank the National Basic Research Program of China
(973 Program) (2013CB834702), Hong Kong Baptist Uni-
versity (FRG2/12-13/083 and FRG1/12-13/046), Hong
Kong Research Grants Council (HKBU203011 and
HKUST2/CRF/10) and Areas of Excellence Scheme, Uni-
versity Grants Committee of HKSAR (project no. AoE/
P-03/08), Natural Science Foundation of Hubei Province
of China (2013CFA087), Hubei Province High-End
Talent Training Program, the National Natural Science
Foundation of China (project number 51373145) and the
Science, Technology, and Innovation Committee of
Shenzhen Municipality (JCYJ20120829154440583). The
work was also supported by Partner State Key Laborato-
ry of Environmental and Biological Analysis and Strate-
gic Development Fund of HKBU. H. Li thanks the 111
Project and Beijing Engineering Research Center of Food
Environment and Public Health from Minzu University
of China (No. B08044 and No. 10301-01404026) for finan-
cial support. We gratefully acknowledge the financial sup-
port from the National Science Council of Taiwan (NSC
102-2113-M-845-001) and the project of the specific re-
search fields in University of Taipei, Taiwan.
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Received: April 20, 2014
Accepted: May 20, 2014
Isr. J. Chem. 2014, 54, 999 – 1014
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.ijc.wiley-vch.de
1014