Potential solution processible phosphorescent iridium complexes toward applications in doped light-emitting diodes: Rapid syntheses and optical and morphological characterizations

Article abstract of DOI:10.1021/jo060840h

A series of carbazol-9-yl end-capped red-emitting heteroleptic iridium complexes was readily synthesized using post Suzuki coupling. The appealing solubility, photoluminescent characteristics, and morphological stability enabled the current heteroleptic i

Full text of DOI:10.1021/jo060840h

Based on solution processing, phosphorescent iridium com-
plexes are usually doped into semiconducting polymeric hosts.
More recently, there emerged a class of electrophosphorescent
dendrimers. Due to their solution-processibility, they act as host
emitters or can be doped into a bipolar molecular host of CBP
[4,4′-bis(N-carbazolyl)biphenyl].3,4 Very high external device
efficiencies have been realized in these cases.3-5 A potential
problem associated with doped LEDs is molecular crystallizations/
aggregations and thus phase segregations in the doped emitting
layers. With the promise of easy materials processing and high
device efficiencies of doped light-emitting diodes, phosphores-
cent molecular/dendritic iridium complexes endowed with high
solubility and amorphous morphological stability resisting the
occurrence of crystallizations/aggregations would be extremely
desirable as dopant emitters for solution-processed doped LEDs
in terms of film homogeneity and long-term device stability.6
Generally, syntheses of molecular electroluminescent iridium
complexes involved a chloro-bridged dimeric precursor, fol-
lowed by substitution of a third ligand.7 In the preparation of a
simple green-emitting iridium complex [2-(4-tert-butylphenyl)-
pyridine]2Ir(acac), we found that it appears hard to separate this
heteroleptic complex from the residual dimeric presursor due
to their very close polarity as a result of introduction of the
solubilizing tert-butyl groups (acac ) acetylacetonate, a coligand
generally used in heterolepic iridium complexes). In the present
work, we present an efficient synthesis of a new series of
solution-processible red-emitting heteroleptic cyclometalated
iridium(III) complexes 4a and 4b (Scheme 1) using the post-
Suzuki coupling route.8 Very recently, this approach has been
used to prepare conjugated electrophosphorescent iridium
polymers.9,10 To effect rapid synthesis and easy separation of
the highly soluble coupling products, we introduced 2-(2-
pyridyl)benzimidazole as a polar coligand. The 9,9-dioctylfluo-
renyl groups are key to improving the solubility and thus
solution processibility,11 and the hole-transporting carbazoyl
groups are key to driving the emission into the red region and
increasing the morphological stability.12 In addition, functional
Potential Solution Processible Phosphorescent
Iridium Complexes toward Applications in Doped
Light-Emitting Diodes: Rapid Syntheses and
Optical and Morphological Characterizations
Yi-Heng Sun,† Xu-Hui Zhu,*,†,‡ Zhao Chen,†
Yong Zhang,† and Yong Cao†
Institute of Polymer Optoelectronic Materials and DeVices,
Key Laboratory of Special Functional Materials and AdVanced
Manufacturing Technology, South China UniVersity of
Technology (SCUT), Guangzhou 510640, China, and
Key Laboratory of Molecular Engineering of Polymers,
Fudan UniVersity 200433, Shanghai, China
xuhuizhu@scut.edu.cn
ReceiVed April 21, 2006
A series of carbazol-9-yl end-capped red-emitting hetero-
leptic iridium complexes was readily synthesized using post
Suzuki coupling. The appealing solubility, photoluminescent
characteristics, and morphological stability enabled the
current heteroleptic iridium complexes to be highly desirable
phosphorescent dopant emitters in doped light-emitting
diodes for the purpose of resisting molecular aggregations
in the doped emitting layers.
(3) Lo, S. C.; Anthopoulos, T. D.; Lo, S. C.; Burn, P. L.; Samuel, I. D.
W. Samuel, AdV. Mater. 2005, 17, 1945;
(4) (a) Cumpstey, N.; Bera, R. N.; Burn, P. L.; Samuel, I. D. W.
Macromolecules 2005, 38, 9564. (b) Lo, S. C.; Lo, S. C.; Burn, P. L.;
Samuel, I. D. W. Macromolecules 2003, 36, 9721. (c) Lo, S. C.; Male, N.
A. H.; Markham, J. P. J.; Magennis, S. W.; Burn, P. L.; Salata, O. V.;
Samuel, I. D. W. AdV. Mater. 2002, 14, 975.
(5) (a) Jiang, C. Y.; Yang, W.; Peng, J. B.; Xiao, S.; Cao, Y. AdV. Mater.
2004, 16, 537. (b) Yang, X. H.; Neher, D.; Hertel, D.; Daubler, T. K. AdV.
Mater. 2004, 16, 161. (c) Yang, X. H.; Neher, D. Appl. Phys. Lett. 2004,
84, 2476. (d) Yang, X. H.; Mu¨ller, D. C.; Neher, D.; Meerholz, K. AdV.
Mater. 2006, 18, 948.
(6) (a) Ostrowski, J. C.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C.
Chem. Commun. 2002, 784. (b) Gong, X.; Ostrowski, J. C.; Bazan, G. C.;
Moses, D.; Heeger, A. J.; Liu, M. S.; Jen, A. K. Y. AdV. Mater. 2003, 15,
45.
Electrophosphorescent molecular and polymeric materials
containing platinum group metal centers are attractive active
emitters in high efficiency light-emitting diodes (LEDs) due to
their propensity to harvest both singlet and triplet excited states
leading to a potential internal device efficiency of unity.1,2 As
triplet emitters, recent studies showed that cyclometalated
organometallic iridium complexes appear of particular interest,
among others.2 They exhibit appealing characteristics such as
highly emissive3 MLCT excited states at room temperature in
the doped/neat solid films and facilely tunable emission color
in the visible. They are made into electroluminescent devices
either via vacuum thermal deposition or solution processing
techniques. At present, it is generally believed that the latter
methods are more advantageous for the purposes of low cost
processing and large area display.
(7) (a) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767. (b) Lamansky,
S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.;
Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001,
123, 4304.
(8) Miyaura N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
(9) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby,
C. E.; Kohler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004,
126, 7041.
(10) (a) Zhen, H. Y.; Jiang, C. Y.; Yang, W.; Jiang, J. X.; Huang, F.;
Cao, Y. Chem.sEur. J. 2005, 11, 5007. (b) Zhen, H. Y.; Luo, C.; Yang,
W.; Song, W. Y.; Du, B.; Jiang, J. X.; Jiang, C. Y.; Zhang, Y.; Cao, Y.
Macromolecules 2006, 39, 1693.
† South China University of Technology.
‡ Fudan University.
(1) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley,
S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Ma, Y. G.;
Zhang, H. Y.; Shen, J. C.; Che, C. M. Synth. Metals 1998, 94, 245.
(2) Review article: Holder, E.; Langeveld, B. M. W.; Schubert, U. S.
AdV. Mater. 2005, 17, 1109 and references therein.
10.1021/jo060840h CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/11/2006
J. Org. Chem. 2006, 71, 6281-6284
6281

SCHEME 1. Synthetic Routes to Soluble Red-Emitting
Complexes 4a and 4b
NMR showed absence of the iridium dimer. Due to the marked
molecular polarity upon coordination of 2-(2-pyridyl)benzimi-
dazole, 2 could be easily separated from unreacted dimer 1 if
present.
Under Ullmann reaction conditions, 9-(4-bromophenyl)-9H-
carbazole and 9-(2-bromo-9,9-dioctylfluoren-7-yl)-9H-carbazole
were efficiently prepared by monoamination of 1,4-dibromoben-
zene and 2,7-dibromo-9,9-dioctylfluorene with 9H-carbazole,
respectively.15 Successive treatment of the above two bromides
with n-butyllithium and 2-isoproxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane gave the corresponding boronic esters 9-(4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carba-
zole 5 and 9-(9,9-dioctyl-7-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)fluoren-2-yl)-9H-carbazole 3a. Unexpectedly, when 5 was
reacted with 2,7-dibromo-9,9-dioctylfluorene in a 1:1 molar ratio
under conventional Suzuki reaction conditions, the totally
debrominated product 9-(4-(9,9-dioctylfluoren-7-yl)phenyl)-9H-
carbazole was separated as the major species, instead of the
monobromide product 9-(4-(2-bromo-9,9-dioctylfluoren-7-yl)-
phenyl)-9H-carbazole 6. Alternatively, with an excess of
2-bromo-7-iodo-9,9-dioctylfluorene16 under similar reaction
conditions, 6 was successfully isolated, emphasizing the im-
portance of iodides in highly selective palladium-catalyzed
carbon-carbon coupling reactions. Consequently, 3b was
obtained analogously to 3a.
The boronic esters 3a and 3b thus obtained were reacted with
the dibromide precursor 2 under conventional Suzuki coupling
conditions, respectively. Target iridium complexes 4a and 4b
were isolated by column chromatography using 1:40 (v/v) ethyl
acetate/dichloromethane as the eluent and are highly soluble in
common solvents such as dichloromethane, chloroform, chlo-
robenzene and o-dichlorobenzene at room temperature. The high
1
purity of the prepared samples was confirmed by H NMR,
elemental analysis, and mass spectroscopy.
1
The H NMR spectrum of the dimeric iridium intermediate
conjugated carbazole derivatives are excellent hosts for phos-
phorescent dopants.13 The optical, thermal, and morphological
studies showed that 4a and 4b are potentially highly valuable
solution-processed triplet emitters for doped light-emitting
diodes.
1 showed the two methyl groups on the naphthalenyls as one
single signal at 2.24 ppm. Upon substitution by 2-(2-pyridyl)-
benzimidazolate, the singlet was split into two peaks with an
equal intensity in bis[5-bromo-2-(1-methylnaphthalen-4-yl)-
pyridine]Ir[2-(2-pyridyl)benzimidazolate] 2 at 2.44 and 2.43
ppm, respectively, and thus in 4a (2.48 and 2.47 ppm) and 4b
(2.47 and 2.46 ppm). We reason that the presence of the two
signals could be attributed to the nonequivalent conjugated
cyclometalated ligands induced by the asymmetric co-ligand.17
This makes the spectra and detailed interpretation complicated.
The ESI MS of 4a showed a molecular weight of 1932 (100),
corresponding to MH+ (calcd for C126H127IrN7, 1930.98). The
molecular weight of 4b was determined by MALDI-TOF mass
spectrometry using dithranol as the matrix (dithranol MW )
226). It showed a series of major peaks at 1886.8 (100) [M -
2-(2-pyridyl)benzimidazolate]+ (calcd 1887.96); 2081.6 (100)
[MH+] (calcd 2083.04); 2305.4 (100) [MH+ + dithranol] (calcd
2309.1) and 2529.1 (100) [MH+ + 2-dithranol] (calcd 2535.16).
The fragment peak at m/z 1886.8 resulted from the thermal
process in the mass measurement.
The target iridium complexes 4a and 4b were readily
synthesized by the post-Suzuki coupling route, involving a key
intermediate bis[5-bromo-2-(1-methylnaphthalen-4-yl)pyridine]-
Ir[2-(2-pyridyl)benzimidazolate] 2. Due to the differential
reactivity of 2,5-dibromopyridine in the palladium-catalyzed
cross-coupling reactions,14 5-bromo-2-(1-methylnaphthalen-4-
yl)pyridine could be isolated as a white solid via Suzuki coupling
using Pd(PPh3)4 as the catalyst and aqueous Na2CO3 as the base.
Reflux of a mixture of the above-obtained bromide and IrCl3‚
xH2O in 3:1 (v/v) 2-methoxyethanol and H2O yielded the chloro-
bridged dimeric iridium complex 1.7 Treatment of 1 with 2-(2-
pyridyl)benzimidazole using K2CO3 as the base in DMF
afforded 2 in 72% yield without chromatography. TLC and 1H
(11) (a) Woo, E. P.; Inbasekaran, M.; Shiang, W.; Roof, G. R. WO97/
05184, 1997. (b) Lee, J. I.; Klaerner, G.; Miller, R. D. Chem. Mater. 1999,
11, 1083.
The solution and solid UV-vis and PL spectra of 4a and 4b
are shown in Figure 1. The absorption and emission maxima
(12) Shirota, Y. J. Mater. Chem. 2005, 15, 75.
(13) (a) van Dijken, A.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.;
Langeveld, B. M. W.; Rothe, C.; Monkman, A.; Bach, I.; Stossel, P.;
Brunner, K. J. Am. Chem. Soc. 2004, 126, 7718. (b) Brunner, K.; van Dijken,
A.; Borner, H.; Bastiaansen, J. J. A. M.; Kiggen, N. M. M.; Langeveld, B.
M. W. J. Am. Chem. Soc. 2004, 126, 6035.
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443.
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Jing, X. B.; Wang, F. S. J. Mater. Chem. 2004, 14, 895.
(16) Lee, S. H.; Nakamura, T.; Tsutsui, T. Org. Lett. 2001, 3, 2005.
(17) Yeh, S. J.; Wu, M. F.; Chen, C. T.; Song, Y. H.; Chi, Y.; Ho, M.
H.; Hsu, S. F.; Chen, C. H. AdV. Mater. 2005, 17, 285.
6282 J. Org. Chem., Vol. 71, No. 16, 2006

FIGURE 2. DSC curves of 4a and 4b.
the UV-vis and PL spectra negligibly, whether in solution or
in the solid state. This allows us to continue to modify this series
of carbazolyl-end capped molecular structures to tune the
physical properties, for instance, solubility, morphological
stability, and emission quantum efficiency while not affecting
substantially the emission color. The absolute photoluminescent
quantum efficiencies of 4a and 4b in the neat films were
measured in the integrating sphere. They exhibited close
phosphorescent efficiencies of ca. 2.7%.
Cyclic voltammetry showed that the spin-coated films of 4a
and 4b on the electrodes exhibited similar electrochemical
behavior.10a The oxidation onset voltages were approximately
0.95 V vs saturated calomel electrode (SCE). The HOMO
FIGURE 1. UV-vis absorption and photoluminescence spectra of 4a
(a) and 4b (b) in CH2Cl2 solutions and in the films on quartz.
TABLE 1. UV-vis and PL Spectral Data
UV-vis (λabsmax, nm)
PL (λemmax, nm)
energy levels of 4a and 4b were then calculated to be -5.35
solution
solid
solution
solid
onset
eV using the empirical formula HOMO ) -e(Eox
+ 4.4)
4a
4b
294, 347, 471
294, 346,466
296, 351, 473
296, 353, 470
621 (657a)
622 (650a)
622 (654a)
623 (652a)
eV.20 Based on their optical band gaps of ca. 2.36 eV in the
solid films, the LUMO energy levels were calculated as -2.99
eV.
a Vibronic shoulder.
Compounds 4a and 4b showed remarkable thermal stability.
The onset decomposition temperatures are well over 350 °C.
They held a high decomposition temperature of ca. 437 °C,
defined by a starting 5% weight loss. DSC measurements on
the as-prepared samples of 4a and 4b revealed an amorphous
state with Tg of 123 and 127 °C, respectively (Figure 2). The
slightly higher glass transition temperature of 4b was coincident
with its higher molecular size and weight. Remarkably, on
heating to 300 °C, no crystallization and melting were observed.
Such soluble materials are particularly useful in solution-
processed light-emitting diodes where materials are more prone
to aggregate and crystallize during solvent evaporation processes
or in the operating devices with time. Moreover, it is worth
noting that materials with high Tgs are advantageous in certain
specific display devices that have to endure a higher ambient
temperature.
are given in Table 1. The current UV-vis spectral characteristics
led to a facile assignment of the spectra. The solution spectrum
of 4a tailed into the visible region with an emerging broad and
featureless absorption band centered around 471 nm, which we
attribute to the coexistent spin-allowed metal-to-ligand charge
transfer 1MLCT and spin-forbidden 3MLCT/3π-π* transitions
as a result of iridium-induced spin-orbital coupling enhancement
(heavy atom effect). Under excitation, 4a gave a structured red
emission at 621 nm with a vibronic shoulder around 657 nm,
revealing that the emissive 3MLCT excited states are mixed with
the ligand-centered 3π-π*.18 As compared to [(NaPy)2Ir
(acac)],19 4a showed a nearly 20 nm bathochromic emission
shift, due largely to end-capping by electron-donating and hole-
transporting carbazoyl groups (NaPy ) 2-(naphthalen-1-yl)-
pyridine). Generally, the solid emission spectra are more
sensitive to the formation of aggregates/excimers of photolu-
minescent materials in the solid states. The “perfect” overlap
of both the solution and solid emission spectra of 4a implies
nonexistence of any appreciable aggregates in the solid state, a
key merit of electroluminescent materials.
Usually, solubilities have to be traded to the glass transition
temperatures. The high amorphous morphological stabilities of
4a and 4b are attributed, to a large extent, to the carbazolyl
end groups. The X-ray crystal structural analysis of m-CP, a
host material for blue-emitting iridium complexes, revealed that
the asymmetric bulky carbazolyl groups involved large twist
With respect to 4a, the introduction of an additional single
1,4-phenylene group into the conjugated ligands in 4b affected
(19) Zhu, W.-G.; Ke, Y.; Wang, F.; Yuan, M.; Cao, Y. Synth. Met. 2003,
137, 1079.
(20) Agrawal, A. K.; Jenekhe, S. A. Chem. Mater. 1996, 8, 579.
(18) Colombo, M. G.; Brunold, T. C.; Riedener, T.; Gu¨del, H. U.; Fo¨rtsch,
M.; Bu¨rgi, H.-B. Inorg. Chem. 1994, 33, 545.
J. Org. Chem, Vol. 71, No. 16, 2006 6283

7.06-7.16 (m, 5H), 7.28-7.36 (m, 5H), 7.42-7.48 (m, 10H),
7.50-7.63 (m, 7H), 7.74-7.91 (m, 6H), 7.96-8.08 (m, 5H), 8.16-
8.19 (m, 4H), 8.35 (s, 1H), 8.55-8.68 (m, 5H). Anal. Calcd for
C126H126IrN7: C, 78.39; H, 6.58; N, 5.08. Found: C, 78.12; H, 6.65;
N, 5.00. ESI MS: m/z 1932 (100), MH+ (calcd 980).
angles of the central phenylene core of 41.8 and 112.9°,
respectively [m-CP ) 1,3-bis(carbazol- 9-yl)benzene]. This
might explain why m-CP could exist in an amorphous state with
a Tg of 55 °C for the simplest molecular structure17 and why
many other structurally closely related functional carbazoles
showed more morphologically stable amorphous states.12
In summary, a post-Suzuki-coupling route was utilized to
synthesize a series of soluble carbazolyl end-capped saturated
red-emitting iridium complexes using 2-(2-pyridyl)benzimida-
zolate as the coligand. The appealing solubility, optical char-
acteristics, and amorphous morphological stability promise the
current hole-transporting enhanced iridium complexes great
potential in solution-processible high-efficiency doped light-
emitting devices to address fatal phase segregations in the doped
emitting layer. The current synthetic approach can be useful in
the efficient synthesis of new judiciously designed heteroleptic
iridium complex for the purpose of solution-processible light-
emitting devices. Intensive efforts in these directions are
underway, including dedicated device engineering, and the
results will be reported in due course.
Synthesis of 4b. This compound was synthesized by a similar
method for 4a, using 2 and 3b. Yield: 50%. TLC Rf (40:1
dichloromethane/ethyl acetate v/v): 0.62. 1H NMR (300 MHz,
CDCl3) δ (ppm): 0.62 (m, 8H), 0.80 (t, 12H, J ) 6.6 Hz), 1.07
(m, 40H), 1.78-2.08 (m, 8H), 2.4643 (s, 3H), 2.4732 (s, 3H), 6.21
(d, 1H, J ) 8.2 Hz), 6.50 (s, 1H), 6.53 (s, 1H), 6.70 (t, 1H, J ) 7.5
Hz), 7.03-7.15 (m, 5H), 7.30 (s, 1H), 7.33 (d, 4H, J ) 7.2 Hz),
7.42-7.53 (m, 10H), 7.56-7.72 (m, 12H), 7.75 (d, 1H, J ) 3.0
Hz), 7.74-7.85 (m, 4H), 7.87-7.91 (m, 4H), 7.96-8.09 (m, 5H),
8.18 (d, 4H, J ) 7.8 Hz), 8.35 (s, 1H), 8.55-8.68 (m, 5H). Anal.
Calcd for C138H134IrN7: C, 79.58; H, 6.48; N, 4.71. Found: C,
79.20; H, 6.70; N, 4.53. MALDI-TOF: m/z 1886.8 (100) [M -
2-(2-pyridyl)benzimidazolate]+ (calcd 1887.96); 2081.6 (100)
[MH+] (calcd 2083.04); 2305.4 (100) [MH+ + dithranol] (calcd
2309.1) and 2529.1 (100) [MH+ + 2-dithranol] (calcd 2535.16).
Acknowledgment. We gratefully acknowledge the financial
support of MOST of China (Grant No.2002CB613403). X.-H.Z.
thanks SCUT for a career award and SRF of State Education
Ministry for a program supporting the returned overseas Chinese
scholars. Ms. Chao Zheng and Chan Sun are acknowledged with
gratitude for the assistance in MALDI-TOF mass measurement
at Fudan University. X.-H.Z. wishes to thank Dr. Nicolas
Mercier at CIMMA, University of Angers, France, for discus-
sions on the X-ray structure of m-CP and for the long time
support.
Experimental Section
Synthesis of 4a. Pd(PPh3)4 (0.03 g, 0.026 mmol) was added to
a mixture of 2 (0.20 g, 0.20 mmol) and 3a (0.29 g, 0.43 mmol) in
toluene (50 mL), aqueous Na2CO3 (2 M, 10 mL), and ethanol (5
mL) under an inert atmosphere of nitrogen. The reaction was heated
at 90 °C for 48 h. After being cooled to room temperature, distilled
water was added. The organic layer was separated, dried over
MgSO4, filtered, and concentrated under reduced pressure. The
residual was purified on a silica gel column by gradient elution
with a mixture of dichloromethane and ethyl acetate to afford a
yellow solid. Yield: 0.25 g (63.6%). TLC Rf (40:1 dichloromethane/
Supporting Information Available: Full experimental details
and characterization of precursors to 4a and 4b; 1H NMR and mass
spectra of 4a and 4b; CIF file of m-CP. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
ethyl acetate v/v): 0.64. H NMR (300 MHz, CDCl3) δ (ppm):
0.62 (m, 8 H), 0.80 (t, 12H, J ) 6.6 Hz), 1.08-1.26 (m, 40H),
1.72-1.94 (m, 8H), 2.4663 (s, 3H), 2.4776 (s, 3H), 6.21 (d, 1H, J
) 8.2 Hz), 6.49 (s, 1H), 6.53 (s, 1H), 6.69 (t, 1H, J ) 7.5 Hz),
JO060840H
6284 J. Org. Chem., Vol. 71, No. 16, 2006