High-efficiency solution processable electrophosphorescent iridium complexes bearing polyphenylphenyl dendron ligands

Article abstract of DOI:10.1016/j.jorganchem.2008.12.009

We report the synthesis and electrophosphorescent behavior of a series of novel iridium complex materials (Complexes A-F), which are composed of ligands bearing polyphenylphenyl dendron groups and acetylacetonate. Yellow to saturated red organic light-emitting diodes (OLEDs) based on these newly developed Ir complexes were fabricated through solution process by doping the complex materials into polyvinyl carbazole (PVK)/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) matrices. The emission wavelengths of the materials could be effectively tuned from 549 nm to 640 nm by changing the conjugation of the ligands either through incorporating additional aromatic segment (e.g. phenyl or fluorenyl group) onto the basic dendron ligand or fusing two of the phenyl rings on the polyphenylphenyl dendron group. High performance devices with the configuration of ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) (50 nm)/PVK:PBD (40%):Ir complex (6%) (70 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (12 nm)/Alq₃ (20 nm)/Mg:Ag (150 nm) have been demonstrated. For example, when Complex B was used as the emissive layer, maximum current efficiency of 34.0 cd/A and external quantum efficiency of 10.3% have been achieved. When 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) was used as the block layer, the efficiencies can be further improved to 46.3 cd/A and 13.9%, respectively. These solution processed OLED devices demonstrated quite stable EL efficiencies over a large range of current density, which indicated that triplet-triplet annihilation in electrophosphorescence could be effectively suppressed by incorporation of the polyphenylphenyl dendron structure into iridium complexes.

Full text of DOI:10.1016/j.jorganchem.2008.12.009

Journal of Organometallic Chemistry 694 (2009) 1317–1324
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
High-efficiency solution processable electrophosphorescent iridium complexes
bearing polyphenylphenyl dendron ligands
c
c
Chun Huang a,b, Chang-Gua Zhen a, Siew Ping Su a, Zhi-Kuan Chen a, , Xiao Liu , De-Chun Zou ,
*
Yan-Rong Shi c, Kian Ping Loh b,
*
a Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, Singapore
b Department of Chemistry, National University of Singapore, 3 Science Drive, Singapore 117543, Singapore
c Department of Chemistry, Peking University, Beijing, 100871, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis and electrophosphorescent behavior of a series of novel iridium complex mate-
rials (Complexes A–F), which are composed of ligands bearing polyphenylphenyl dendron groups and
acetylacetonate. Yellow to saturated red organic light-emitting diodes (OLEDs) based on these newly
developed Ir complexes were fabricated through solution process by doping the complex materials into
polyvinyl carbazole (PVK)/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) matrices. The
emission wavelengths of the materials could be effectively tuned from 549 nm to 640 nm by changing
the conjugation of the ligands either through incorporating additional aromatic segment (e.g. phenyl
or fluorenyl group) onto the basic dendron ligand or fusing two of the phenyl rings on the polyphenyl-
phenyl dendron group. High performance devices with the configuration of ITO/poly(3,4-ethylenedioxy-
thiophene):poly(styrenesulfonic acid) (PEDOT:PSS) (50 nm)/PVK:PBD (40%):Ir complex (6%) (70 nm)/2,
9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (12 nm)/Alq3 (20 nm)/Mg:Ag (150 nm) have been
demonstrated. For example, when Complex B was used as the emissive layer, maximum current effi-
ciency of 34.0 cd/A and external quantum efficiency of 10.3% have been achieved. When 1,3,5-tris(N-
phenylbenzimidazol-2-yl) benzene (TPBI) was used as the block layer, the efficiencies can be further
improved to 46.3 cd/A and 13.9%, respectively. These solution processed OLED devices demonstrated
quite stable EL efficiencies over a large range of current density, which indicated that triplet–triplet anni-
hilation in electrophosphorescence could be effectively suppressed by incorporation of the polyphenyl-
phenyl dendron structure into iridium complexes.
Received 30 September 2008
Received in revised form 3 December 2008
Accepted 4 December 2008
Available online 16 December 2008
Keywords:
Electrophosphorescent
Polyphenylphenyl dendron
Iridium complexes
OLEDs
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
such a drawback will limit the application of iridium complexes
for OLEDs. To address this issue, bulky ligands have been used to
Electroluminescent materials have attracted significant scien-
tific and industrial attention because of their potential for full-col-
or flat panel displays [1–3]. Recently, heavy metal ion based
electrophosphorescent materials, especially iridium-based com-
plex materials, have been intensively investigated as promising
candidates for highly efficient OLEDs because: (1) they can harvest
both triplet excitons and singlet excitons for light emission in de-
vices due to the strong spin–orbital coupling caused by the heavy
metal atoms; (2) their emission can cover the whole visible range
through tuning of the ligand structures of the complexes [4–18].
However, it was found that the quantum efficiency of iridium com-
plex-based devices drops quickly due to the triplet–triplet annihi-
lation and concentration quenching of the materials. Obviously,
prevent interactions between Ir-complex molecules and the per-
formance of devices fabricated from these compounds has im-
proved [19,20]. For example, dendrimer type iridium complexes
have been demonstrated to prevent the interaction between mol-
ecules in solid state effectively and highly efficient OLED devices
have been achieved through solution process fabrication with
these materials [21–25]. Recently, it has been demonstrated that
organic light-emitting materials with fully conjugated polyphenyl-
phenyl dendron groups can also effectively prevent molecular
aggregation or
p–p stacking and excellent EL efficiencies for both
fluorescent small molecular OLEDs and polymeric light-emitting
diodes (PLEDs) have been realized [26–29]. The polyphenylphenyl
dendron groups could be easily incorporated into the material
structure through a simple synthetic procedure. Complex materials
with ligands bearing such a polyphenylphenyl dendron structure
should also be able to retard the interactions between the mole-
cules and prevent the quenching of electrophosphorescensce in
* Corresponding authors.
E-mail addresses: zk-chen@imre.a-star.edu.sg (Z.-K. Chen), chmlohkp@nus.
edu.sg (K.P. Loh).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.12.009

1318
C. Huang et al. / Journal of Organometallic Chemistry 694 (2009) 1317–1324
devices [30,31]. On the basis of this idea, we recently have devel-
oped a series of iridium complex materials (Complexes A–F, struc-
tures are shown in Fig. 1) bearing polyphenylphenyl dendron
groups in the ligands through simple synthesis and purification
procedure [32]. It was found that the emission wavelengths of
the materials could be effectively tuned by changing the conjuga-
tion of the ligands either through incorporating additional aro-
matic segment onto the basic dendron ligand or fusing two of
the phenyl rings on the polyphenylphenyl dendron group. Good
performance of OLED devices emitting yellow to saturated red light
has been achieved by using these newly developed complex mate-
rials as emissive layer through solution process. Although the syn-
thesis of Complex D has been reported by Lin et al. [30], no OLED
device has been demonstrated yet. Thus, Complex D is also in-
cluded in this report and its OLED performance will be compared
with other complexes.
(yield 90%) of 2-(trimethylsilyl)ethynyl-5-bromopyridine (1b).
MS: m/z 252.8 (47%), 254.8 (47%). 1H NMR (400 MHz, CDCl3) d
(ppm): 8.640 (s, 1H), 7.801 (d, J = 8.0 Hz, 1H), 7.368 (d, J = 8.0 Hz,
1H), 0.289 (s, 9H).
2.1.3. Synthesis of compound 2a (Ligand A)
To a solution of 2-(trimethylsilyl)ethynyl-pyridine (1a) (0.88 g,
5 mmol) in a mixture of THF (10 ml) and methanol (2 ml) was
added 1 ml of NaOH (5 N). After the reaction mixture was stirred
for 1 h at room temperature, 50 ml of ethyl acetate was added.
The mixture was washed with water and brine and dried with
anhydrous magnesium sulfate. After the solvent was removed in
vacuo, the residue was refluxed with tetraphenylcyclopentadie-
none (2.0 g, 5.2 mmol) in 50 ml o-xylene overnight. After being
cooled down to room temperature, the solvent was removed by
flash column and the residue was purified by recrystallization in
ethanol 2–3 times to offer 1.95 g of pure 2-(2’,3’,4’,5’-tetra-
phenyl)phenylpyridine (yield 85%). MS: m/z 458.1 (100%). 1H
NMR (400 MHz, CDCl3) d (ppm): 8.06 (d,J = 4.0 Hz, 1H), 7.82 (s,
1H), 7.384 (t, 1H), 7.18 (m, 5H), 7.09 (m, 1H), 6.98–6.89 (m,
14H), 6.825 (m, 2H). 13C NMR (100 MHz, CDCl3) d (ppm): 142.00,
142.03, 141.55, 140.82, 140.54, 140.32, 140.24, 140.12, 139.51,
135.37, 131.87, 131.84, 131.76, 131.57, 130.39, 127.86, 127.47,
127.29, 127.04, 126.60, 126.04, 125.80, 125.62.
2. Experimental
2.1. Materials synthesis
2.1.1. Synthesis of compound 1a
To a solution of 2-bromopyridine (4.74 g, 0.030 mol), CuI
(0.14 g, 0.74 mmol), and Pd(PPh3)2Cl2 (0.52 g, 0.74 mmol) in
100 ml of diisopropylamine was added (trimethylsilyl)acetylene
(3.0 g, 0.030 mol). The mixture was stirred at room temperature
overnight under nitrogen atmosphere. After removal of the solvent
in vacuo, the residue was distilled under reduced pressure to offer
5.0 g (yield 95%) of pure 2-(trimethylsilyl)ethynyl-pyridine (1a).
MS: m/z 175.2 (27%). 1H NMR (400 MHz, CDCl3) d (ppm): 8.52 (d,
J = 4.7 Hz, 1H), 7.57 (d, J = 7.7 Hz, 1H), 7.39 (m, 1H), 7.16 (m, 1H),
0.22 (s, 9H).
2.1.4. Synthesis of compound 2b
To a solution of 2-(trimethylsilyl)ethynyl-5-bromopyridine (1b)
(1.27 g, 5 mmol) in a mixture of THF (10 ml) and methanol (2 ml)
was added 1 ml of NaOH (5 N). The reaction mixture was stirred
for 1 h at room temperature. Then 50 ml of ethyl acetate was added
and the mixture was washed with water, brine and dried with
anhydrous magnesium sulfate. After the solvent was removed,
the residue was refluxed with tetraphenylcyclopentadienone
(2.0 g, 5.2 mmol) in 50 ml o-xylene overnight. After being cooled
down to room temperature, the solvent was removed by flash col-
umn and the residue was purified by recrystallization in ethanol 2–
3 times to offer 2.17 g of pure 2-(2’,3’,4’,5’-tetraphenyl)phenyl-5-
bromopyridine (2b) (yield 81%). MS: m/z 537.9 (100%), 535.8
(100%). 1H NMR (400 MHz, CDCl3) d (ppm): 8.659 (d, J = 2 Hz, 1H),
7.803 (s, 1H), 7.490 (d, 1H), 7.165 (s, 5H), 7.017-6.769 (m, 16H).
2.1.2. Synthesis of compound 1b
To a solution of 2,5-dibromopyridine (3.56 g, 0.015 mol), CuI
(0.07 g, 0.37 mmol), and Pd(PPh3)2Cl2 (0.26 g, 0.37 mmol) in
100 ml of diisopropylamine was added (trimethylsilyl)acetylene
(1.47 g, 0.015 mol). The mixture was stirred at room temperature
overnight under nitrogen atmosphere. After removal of the solvent
in vacuo, the residue was purified by flash column to offer 3.45 g
R
Cyclometallated dimer A ;
R
H
Cl
Cl
N
Ir
Ir
R
N
Cyclometallated dimer B :
Cyclometallated dimer C :
2
R
R
2
C6H13
C6H13
H
R
Cyclometallated dimer D ;
Cyclometallated dimer E :
R
Cl
Cl
N
Ir
Ir
R
N
2
R
R
Cyclometallated dimer F :
2
C6H13
C6H13
Complex D ;
R
H
Complex A ;
Complex B :
Complex C :
R
H
O
O
O
O
R
Complex E :
Complex F :
R
Ir
Ir
N
N
2
2
R
R
R
R
C6H13
C6H13
C6H13
C6H13
Fig. 1. Chemical structures of Complexes A–F.

C. Huang et al. / Journal of Organometallic Chemistry 694 (2009) 1317–1324
1319
2.1.5. Synthesis of Ligand B
refluxed with phencyclone (2.0 g, 5.2 mmol) in 50 ml o-xylene
overnight. After being cooled down to room temperature, the sol-
vent was removed by flash column and the residue was purified
by recrystallization in ethanol 2–3 times to offer 2.00 g (about
yield 75%) of 3b with small amount of impurity. Compound 3b
was used for next step synthesis without further purification.
In an argon flushed two-neck round-bottom flask, a mixture of
1.60 g (3.0 mmol) of compound 2b, 0.5 g (4 mmol) of phenyl boro-
nic acid, 36 mg (1 mol%) of tetrakis(triphenylphosphine)palla-
dium(0), 15 ml of 2 M sodium carbonate and 30 ml of toluene
was added and heated at reflux for 2 h. After being cooled down
to room temperature, the reaction mixture was extracted with
ethyl acetate and the organic phase was washed with water and
brine and dried over magnesium sulfate. The solvent was then re-
moved in vacuo and the residue was purified by flash column
eluted with hexane/CH2Cl2 (3:1) followed by recrystallization in
ethanol to provide 1.48 g of Ligand B (yield 92%). MS: m/z 534.2
(100%). 1H NMR (400 MHz, CDCl3) d (ppm): 8.68 (s, 1H), 7.91
(s, 1H), 7.60 (d, 3H), 7.50 (d, 2H), 7.42 (m, 1H), 7.20 (t, 5H),
7.00–6.83 (m, 16H). 13C NMR (100 MHz, CDCl3) d (ppm): 158.48,
147.89, 142.19, 142.03, 141.64, 140.93, 140.55, 140.33, 140.29,
139.68, 139.57, 138.02, 134.22, 133.74, 131.90, 131.84, 131.79,
131.69, 130.41, 129.93, 129.42, 128.35, 127.89, 127.62, 127.38,
127.32, 127.07, 126.64, 126.34, 126.07, 125.83, 125.51.
2.1.9. Synthesis of Ligand E
The procedure used is the same as that for preparation of Ligand
B. 1.60 g (3.0 mmol) of 3b, 0.5 g (4 mmol) of phenyl boronic acid,
36 mg (1 mol%) of tetrakis(triphenylphosphine)palladium(0),
15 ml of 2 M sodium carbonate and 30 ml of toluene were added
in a round-bottom flask and heated at reflux for 2 h. After normal
work-up, the crude product was purified by flash column eluted
with hexane/CH2Cl2 (3:1) followed by recrystallization in ethanol
to provide 1.43 g of Ligand E (yield 92%). MS: m/z 532.2 (100%).
1H NMR (400 MHz, CDCl3)
d (ppm): 8.93 (s, 1H), 8.48, (d,
J = 8.0 Hz, 2H), 7.98 (s, 1H), 7.82 (d, J = 8.0 Hz, 1H), 7.63 (d, 3H),
7.58–7.38 (m, 11H) 7.28–7.24 (m, 5H), 7.16 (t, 1H), 7.06 (t, 1H),
6.72 (d, J = 8.0 Hz, 1H). 13C NMR (100 MHz, CDCl3) d (ppm):
158.48, 147.89, 142.19, 142.03, 141.64, 140.93, 140.55, 140.33,
140.29, 139.68, 139.57, 138.02, 134.22, 133.74, 131.90, 131.84,
131.79, 131.69, 130.41, 129.93, 129.42, 128.35, 127.89, 127.62,
127.38, 127.32, 127.07, 126.64, 126.34, 126.07, 125.83, 125.51.
2.1.6. Synthesis of Ligand C
The procedure used is the same as that for preparation of Ligand
B. 1.60 g (3.0 mmol) of 2b, 1.51 g (4 mmol) of 2-(9,9-dihexyl)-flu-
orenyl boronic acid, 36 mg (1 mol%) of tetrakis(triphenylphos-
phine)palladium(0), 15 ml of 2 M sodium carbonate and 30 ml of
toluene were added in a round-bottom flask and heated at reflux
for 2 h. After normal work-up, the crude product was purified by
flash column eluted with hexane/CH2Cl2 (4:1) followed by recrys-
tallization in ethanol to provide 1.99 g of Ligand C (yield 84%). MS:
m/z 791.4 (100%). 1H NMR (400 MHz, CDCl3) d (ppm): 8.93 (s, 1H),
7.98 (s, 1H), 7.78 (m, 2H), 7.66 (d, 1H), 7.57 (m, 2H), 7.38 (m, 3H),
7.20 (t, 5H), 7.03–6.96 (m, 9H), 6.90 (m, 5H), 6.83 (m, 2H), 2.03 (t,
4H), 1.13–1.06 (m, 12H), 0.79–0.67 (m, 10H). 13C NMR (100 MHz,
CDCl3) d (ppm): 158.50, 152.08, 151.36, 148.02, 142.24, 142.03,
141.64, 141.52, 140.90, 140.56, 140.37, 140.34, 139.70, 139.50,
136.38, 134.73, 133.67, 131.89, 131.84, 130.41, 127.89, 127.71,
127.64, 127.31, 127.24, 127.06, 126.62, 126.33, 126.21, 126.06,
125.81, 125.50, 123.32, 121.56, 120.55, 120.26, 55.62, 40.78,
31.86, 30.07, 24.14, 22.96, 14.38.
2.1.10. Synthesis of Ligand F
The procedure used is the same as that for preparation of Ligand
B. 1.60 g (3.0 mmol) of 3b, 1.51 g (4 mmol) of 2-(9,9-dihexyl)-flu-
orenyl boronic acid, 36 mg (1 mol%) of tetrakis(triphenylphos-
phine)palladium(0), 15 ml of 2 M sodium carbonate and 30 ml of
toluene were added in a round-bottom flask and heated at reflux
for 2 h. After normal work-up, the crude product was purified by
flash column eluted with hexane/CH2Cl2 (4:1) followed by recrys-
tallization in ethanol to provide 2.0 g Ligand F (yield 85%). MS: m/z
789.1 (100%). 1H NMR (400 MHz, CDCl3) d (ppm): 8.99 (s, 1H), 8.48
(d, J = 8.0 Hz, 2H), 8.00 (s, 1H), 7.81 (d, J = 8.0 Hz, 2H), 7.76 (d,
J = 8.0 Hz, 1H), 7.60 (d, J = 8.0 Hz, 1H), 7.56–7.36 (m, 13H), 7.30
(m, 5H), 7.14 (t, 1H), 7.07 (t, 1H), 6.74 (d,, J = 8.0 Hz, 1H), 2.04 (t,
4H), 1.14 (12H), 0.77 (10H). 13C NMR (100 MHz, CDCl3) d (ppm):
158.50, 152.13, 151.38, 148.21, 144.75, 142.42, 141.57, 140.85,
138.99, 136.98, 136.69, 134.70, 133.52, 132.52, 132.37, 132.15,
132.11, 131.88, 131.42, 131.09, 130.56, 130.25, 130.10, 129.38,
129.18, 127.73, 127.56, 127.42, 127.26, 126.86, 126.24, 126.02,
125.79, 125.69, 123.65, 123.60, 123.34, 121.60, 123.34, 121.60,
120.58, 120.28, 55.63, 40.78, 31.87, 30.09, 24.16, 22.97, 14.40.
2.1.7. Synthesis of compound 3a (Ligand D)
The procedure used is the same as that for preparation of com-
pound 2a. 0.88 g (5 mmol) of 2-(trimethylsilyl)ethynyl-pyridine
(1a) was converted to 2-ethynyl-pyridine in a mixture of THF
(10 ml) and methanol (2 ml) containing 1 ml of NaOH (5 N). The
obtained crude product of 2-ethynyl-pyridine was refluxed with
phencyclone (2.0 g, 5.2 mmol) in 50 ml o-xylene overnight. After
being cooled down to room temperature, the solvent was removed
by flash column and the residue was purified by recrystallization in
ethanol 2–3 times to offer 1.95 g (yield 85%) of Ligand D. MS: m/z
2.2. General procedure for synthesis of Cyclometallated dimmer A–F
In a mixture solvent of 2-ethoxyethanol and water (3:1)
(30 ml), 0.2 g (0.57 mmol) of IrCl3Á3H2O and 1.45 mmol of ligand
compound were added. The reaction mixture was refluxed over-
night and precipitate was formed. The precipitate was filtered
when the reaction was cooled down to room temperature and
washed with water and ethanol successively. The cyclometallated
dimer products were obtained after drying in vacuo. Cyclometal-
lated dimer A, yield 78%; Cyclometallated dimer B, yield 68%;
Cyclometallated dimer C, yield 71%; Cyclometallated dimer D,
yield 68%; Cyclometallated dimer E, yield 71%; Cyclometallated
dimer F, yield 75%.
456.1 (100%). 1H NMR (400 MHz, CDCl3)
d (ppm): 8.68 (d,
J = 4.0 Hz, 1H), 8.48 (d, J = 8.0 Hz, 2H), 7.91 (s, 1H), 7.81 (d,
J = 8.0 Hz, 1H), 7.56–7.21 (m, 16H), 6.95 (m, 1H), 6.71 (m, 1H).
13C NMR (100 MHz, CDCl3) d (ppm): 159.90, 149.70, 144.76,
142.37, 139.41, 138.97, 136.95, 135.24, 132.34, 132.26, 132.08,
132.04, 131.86, 131.38, 131.08, 130.56, 130.27, 130.09, 129.37,
129.02, 127.54, 127.30, 127.22, 126.85, 126.00, 125.96, 125.69,
123.63, 123.60, 121.48.
2.1.8. Synthesis of compound 3b
To a solution of 2-(trimethylsilyl)ethynyl-5-bromopyridine (1b)
(1.27 g, 5 mmol) in a mixture of THF (10 ml) and methanol (2 ml)
was added 1 ml of NaOH (5 N). The reaction mixture was stirred
for 1 h at room temperature. 50 ml of ethyl acetate was added
and washed with water, brine and then dried with anhydrous mag-
nesium sulfate. After the solvent was removed, the residue was
2.3. General procedure for synthesis of Complexes A–F
In an argon flushed two-neck 50 ml round-bottom flask, a mix-
ture of 0.1 mmol of cyclometallated dimer product, 0.1 g (1 mmol)
of 2,4-pentanedione in 1 ml ethanol, 0.5 ml tetrame-
thylammoniumhydroxide (25% in methanol), 5 ml ethanol, and

1320
C. Huang et al. / Journal of Organometallic Chemistry 694 (2009) 1317–1324
30 ml of CH2Cl2 was added and heated at reflux for 5 h. After
cooled down to room temperature, the reaction mixture was
washed with brine and dried over magnesium sulfate. The solvent
was then removed in vacuo and the residue was refluxed in hep-
tane later filtered when it is still hot to provide the complex mate-
rial with a yield of 70–80%.
Calc. for C87H59IrN2O2 1356.421; found 1356.526. Element Anal.:
Calc. for C87H59IrN2O2: C, 77.02; H, 4.38; N, 2.06; found: C, 77.28;
H, 4.72; N, 2.19%.
2.3.6. Complex F
Yield: 70%. 1H NMR (400 MHz, CDCl3) d (ppm): 8.586–8.567 (d,
J = 7.6 Hz, 2H), 8.364–8.318 (t, J = 9.2 Hz, 6H), 7.952–7.949 (d,
J = 1.2 Hz, 2H), 7.758–7.728 (m, 4H), 7.593–7.574 (d, J = 7.6 Hz,
2H), 7.473–7.458 (m, 2H), 7.432–7.335 (m, 8H), 7.319–7.226 (m,
6H), 7.215–7.088 (m, 6H), 7.046–7.025 (d, J = 8.4 Hz, 2H), 6.878–
6.860 (d, J = 7.2 Hz, 2H), 6.842–6.803 (t, J = 7.8 Hz, 2H), 6.758–
6.721 (t, J = 7.4 Hz, 2H), 6.694–6.655 (t, J = 7.8 Hz, 2H), 6.616–
6.581 (t, J = 7.0 Hz, 2H), 6.527–6.446 (m, 4H), 5.864–5.842 (d,
J = 8.8 Hz, 2H), 5.097 (s, 1H), 2.182 (s, 6H), 2.135–1.983 (m, 8H),
1.226–1.021 (m, 24H), 0.792–0.759 (t, J = 6.6 Hz, 12H), 0.744–
0.610 (m, 8H). MS (MALDI-TOF): m/z Calc. for C125H115IrN2O2
1868.859; found 1868.965. Element Anal.: Calc. for C125H115
IrN2O2: C, 80.31; H, 6.20; N, 1.50; found: C, 80.14; H, 6.04; N,
1.52%.
2.3.1. Complex A
Yield: 79%. 1H NMR (400 MHz, CDCl3) d (ppm): 8.562– 8.551 (d,
J = 4.4 Hz, 2H), 7.380–7.359 (d, J = 8.4 Hz, 2H), 7.320–7.303 (d,
J = 6.8 Hz, 2H), 7.236–7.120 (m, 8H), 7.101–7.013 (m, 6H), 6.979–
6.966 (m, 2H), 6.938–6.870 (m, 4H), 6.870–6.787 (m, 4H), 6.777–
6.758 (d, J = 7.6 Hz, 2H), 6.738–6.708 (m, 2H), 6.688–6.669 (d,
J = 7.6 Hz, 2H), 6.619–6.602 (d, J = 6.8 Hz, 2H), 6.531–6.516 (d, J =
6.0 Hz, 2H), 6.467–6.446 (d, J = 8.4 Hz, 2H), 6.205–6.168 (t,
J = 7.4 Hz, 2H), 6.062–6.043 (d, J = 7.6 Hz, 2H), 5.981–5.964 (d,
J = 6.8 Hz, 2H), 5.208 (s, 1H), 2.048 (s, 6). MS (MALDI-TOF): m/zCalc.
for C75H55IrN2O2 1208.389; found 1208.365. Element Anal.: Calc.
for C75H55IrN2O2: C, 74.54; H, 4.59; N, 2.32; found: C, 74.13; H,
4.38; N, 1.99%.
2.2. Spectroscopy and electrochemical measurement
2.3.2. Complex B
Yield: 71%. 1H NMR (400 MHz, CDCl3) d (ppm): 8.198–8.193 (d,
J = 2.0 Hz, 2H), 7.384–7.353 (t, J = 6.4 Hz, 8H), 7.350–7.297 (m, 4H),
7.26–7.213 (m, 2H), 7.146–7.112 (m, 4H), 7.030–7.018 (m, 2H),
7.005–6.977 (dd, J = 9.0 Hz, 2.4 Hz, 2H), 6.909–6.890 (d, J = 7.6 Hz,
2H), 6.86–6.81 (m, 4H), 6.81–6.74 (m, 8H), 6.724–6.704 (m, 2H),
6.66–6.61 (m, 4H), 6.605–6.522 (tt, J = 7.2 Hz, 4H), 6.484–6.465
(d, J = 7.6 Hz, 2H), 6.379–6.342 (t, 7.4 Hz, 2H), 6.164–6.128 (t,
7.6 Hz, 2H), 5.900–5.877 (d, J = 9.2 Hz, 2H), 5.056 (s, 1H), 1.791
(s, 6H). MS (MALDI-TOF): m/z Calc. for C87H63IrN2O2 1360.452;
found 1360.482. Element Anal.: Calc. for C87H63IrN2O2: C, 76.80;
H, 4.67; N, 2.06; found: C, 76.99; H, 4.63; N, 2.02%.
Nuclear magnetic resonance (NMR) spectra were collected on a
Bruker ACF 400 spectrometer using chloroform-d as a solvent and
tetramethylsilane (TMS) as an internal standard. Ultraviolet–Visi-
ble (UV–Vis) spectra of the materials in CHCl3 at room temperature
with concentration of ꢀ1 Â 10À5 M were obtained using a Shima-
dzu UV 3101PC UV–Vis-NIR spectrophotometer with a xenon lamp
in the scan rate of 300 nm/min. Photoluminescence (PL) spectra of
the materials in CHCl3 at room temperature with concentration of
ꢀ3 Â 10À6 M were obtained with a Perkin Elmer LS 50B lumines-
cence spectrometer with the excitation at kmax of the UV–Vis
absorption spectra. Cyclic voltammetry (CV) experiments were
conducted on an AUTOLAB (model PGSTAT30) workstation under
argon atmosphere. All potentials were measured in a three-elec-
trode cell with 0.10 M tetrabutylammonium perfluorate phosphate
(Bu4NPF6) in anhydrous acetonitrile as the electrolyte, using a Ag/
AgCl (3 M KCl) electrode as the reference electrode (À0.040 V ver-
sus SCE), a platinum wire as the counter electrode and a platinum
disc as the working electrode. All experimental values were cor-
rected with respect to SCE. The HOMOs of the compounds are esti-
2.3.3. Complex C
Yield: 73%. 1H NMR (400 MHz, CDCl3) d (ppm): 8.087–8.084 (d,
J = 1.2 Hz, 2H), 7.707–7.688 (d, J = 7.6 Hz, 4H), 7.37–7.30 (m, 6H),
7.291–7.253 (t, J = 7.2 Hz, 4H), 7.25–7.17 (m, 4H), 7.17–7.09 (m,
8H), 7.030–7.010 (d, J = 8.0 Hz, 2H), 6.89–6.81 (m, 6H), 6.800–
6.782 (m, 4H), 6.759–6.739 (d, J = 8.0 Hz, 2H), 6.73–6.69 (m, 2H),
6.69–6.61 (m, 6H), 6.492–6.474 (d, J = 7.2 Hz, 2H), 6.46–6.39 (m,
2H), 6.345–6.326 (m, 4H), 6.049–6.027 (d, J = 8.8 Hz, 2H), 5.325
(s, 1H), 1.998–1.945 (m, 8H), 1.922 (s, 6H), 1.124–1.050 (m, 24H),
0.755–0.724 (t, J = 6.2 Hz, 12H), 0.686–0.547 (m, 8H). MS (MAL-
DI-TOF): m/z Calc. for C125H119IrN2O2 1872.890; found 1872.943.
Element Anal.: Calc. for C125H119IrN2O2: C, 80.13; H, 6.40; N,
1.50; found: C, 80.33; H, 6.21; N, 1.65%.
mated from onset potentials for oxidation of the p-doping
ox
processes using the equation EHOMO = À[4.4 + E
] eV [33,34]
onset
and the LUMOs are thus estimated incorporating the HOMOs from
electrochemical measurements and the HOMO–LUMO gap
estimated from the UV spectra edge from equation of
Egap = À[EHOMO – ELUMO]. The onset potentials were determined
from the intersection of the two tangents drawn at the rising cur-
rent and baseline charging current of the CV curves.
2.3.4. Complex D
Yield: 66%. 1H NMR (400 MHz, CDCl3) d (ppm): 8.580–8.560 (d,
J = 8.0 Hz, 2H), 8.362–8.298 (m, 6H), 8.087–8.069 (d, J = 7.2 Hz, 2H),
7.750–7.735 (m, 4H), 7.465–7.398 (m, 4H), 7.382–7.261 (m, 8H),
7.013–6.973 (t, J = 8.0 Hz, 4H), 6.865–6.794 (m, 4H), 6.538–6.509
(m, 4H), 6.449–6.399 (m, 4H), 5.822–5.801 (d, J = 8.4 Hz, 2H),
5.001 (s, 1H), 2.104 (s, 6H). MS (MALDI-TOF): m/z Calc. for C75H51Ir-
N2O2 1204.358; found 1204.440. Element Anal.: Calc. for C75H51Ir-
N2O2: C, 74.79; H, 4.27; N, 2.33; found: C, 74.91; H, 4.25; N, 2.24%.
2.3. LED device fabrication
A first layer of poly(3,4-ethylenedioxythiophene) doped with
poly(styrenesulfonic acid) (PEDOT:PSS) was spin-coated on a pre-
treated glass substrate with patterned ITO to form a hole injection
layer with a thickness of about 50 nm. After being dried in oven at
120 °C for 15 min, a solution containing 4 ml chlorobenzene, 27 mg
PVK, 20 mg PBD, and 3.0 mg iridium complex material was spin-
coated onto the first layer to form the emitting layer with a thick-
ness of about 70 nm. On the emissive layer, 12 nm of BCP, 20 nm of
Alq3, 150 nm of Mg:Ag, and 10 nm of Ag were thermally deposited
sequentially under vacuum of 3 Â 10À4 Pa. The organic electrolu-
minescent devices obtained were examined in a glove box. EL spec-
tra were recorded with an Ocean Optics USB2000 Miniature Fiber
Optic Spectrometer.
2.3.5. Complex E
1H NMR (400 MHz, CDCl3) d (ppm): 8.357–8.336 (d, J = 8.4 Hz,
2H), 8.305–8.284 (d, J = 8.4 Hz, 2H), 8.136–8.117 (d, J = 7.6 Hz,
2H), 7.839–7.770 (m, 4H), 7.468–7.394 (m, 10H), 7.338–7.191 (m,
16H), 7.030–6.992 (t, J = 7.6 Hz, 2H), 6.966–6.939 (dd, J = 8.8 Hz,
J = 1.0 Hz, 2H), 6.856–6.817 (t, J = 7.8 Hz, 2H), 6.525–6.489 (t,
J = 7.2 Hz, 2H), 6.395–6.310 (m, 6H), 5.873–5.851 (d, J = 8.8 Hz,
2H), 5.025 (s, 1H), 1.847 (s, 6H). Yield: 81%. MS (MALDI-TOF): m/z

C. Huang et al. / Journal of Organometallic Chemistry 694 (2009) 1317–1324
1321
65% to 80%. Column purification is also not necessary in the final
step. Pure Complex A–F can be achieved by treating the crude
products in refluxed heptane and filtering out the solid product.
The UV–Vis and photoluminescence (PL) spectra of the com-
plexes are shown in Fig. 2. The UV–Vis absorption over 400 nm
is assigned to metal-to-ligand charge transfer (MLCT) transition
[6,19]. The higher energy absorptions, which are more intensive,
3. Results and discussion
The ligand compounds and the resulting complex materials
Complex A–F were achieved with high yields through simple syn-
thesis and purification procedure following the synthetic routes
outlined in Scheme 1. Only recrystallization or flash column chro-
matography is needed to obtain the target compounds with satis-
factory purity. Ethynyl groups were firstly incorporated through
palladium-catalyzed Sonogashira coupling reaction onto pyridyl
ring with high selectivity and yield because of the high activity
of the 2-bromo position on pyridyl ring. Polyphenylphenyl
dendron groups were then introduced through the Diels-Alder
cycloaddition reaction of 2-ethynylpyridine or 2-ethynyl-5-bro-
mo-pyridine. Another aromatic segment was incorporated to form
Ligand B, C, E, and F through the Suzuki coupling reaction with
high yields. Precipitates of the cyclometallated iridium dimers
were formed after the ligand compounds were refluxed with irid-
ium trichloride in 2-ethoxyethanol/H2O overnight. The dimeric
compounds were then refluxed in methylene chloride in the
presence of 2,4-pentanedione, N(Me)4OH and a small amount of
ethanol to achieve the final complexes with yields ranging from
are assigned to the
p–
p* ligand absorption. Obviously, the energy
band gaps can be tuned by changing the conjugation of the li-
gands. Increasing the conjugation length by incorporating addi-
tional aromatic segments onto the pyridyl rings of the ligands,
or fusing two of the phenyl rings in the dendron groups, cause
significantly bathochromic shift of both the ligand absorption
band and metal-to-ligand charge transfer (MLCT) transition band.
For example, the absorption of the MLCT band of Complex A is
peaked at 405 nm; whilst the maxima MLCT transition of Com-
plexes B, C, D, E, F are red shifted to 443 nm, 455 nm, 476 nm,
495 nm, and 508 nm, respectively. The PL spectra of the com-
plexes follow the same trend. It was found that kmax of the PL
spectra of the complex materials in chloroform could be tuned
from 528 nm to 643 nm.
i) THF/MeOH, 5N NaOH
trimethylsilylethylene
R1
Br
R1
SiMe3
R1
ii) o-xylene
Ph
iPr2NH, CuI, Pd(PPh3)2Cl2
N
N
N
Ph
Ph
Ph
1a (R1 = H, 95%)
1b (R1 = Br, 90%)
2a (R1 = H, 85%)
2b (R1 = Br, 85%)
O
Ligand A: R2 = H
Ligand B: R2
=
arylboronic acid
toluene
2N Na2CO3, Pd(PPh3)4
(92%)
R2
N
Ligand C: R2
(84%)
=
H13C6
C6H13
i) THF/MeOH, 5N NaOH
ii) o-xylene
R1
R1
SiMe3
N
N
Ph
O
1a or 1b
3a (R1 = H, 81%)
3b (R1 = Br, 75%)
Ph
Ligand D: R2 = H
Ligand E: R2
=
=
arylboronic acid
(92%)
R2
toluene
2N Na2CO3, Pd(PPh3)4
N
Ligand F: R2
(85%)
H13C6
C6H13
Cyclometallated dimer A (78%)
Complex A (79%)
Complex B (71%)
Complex C (73%)
Complex D (66%)
Complex E (81%)
Complex F (70%)
Cyclometallated dimer B (68%)
Cyclometallated dimer C (71%)
Cyclometallated dimer D (68%)
Cyclometallated dimer E (71%)
Cyclometallated dimer F (75%)
IrCl3 3H2O
2-ethoxyethanol, H2O
Hacac, N(Me)4OH
CH2Cl2, ethanol
Ligand A-F
Scheme 1. The synthetic procedure for Complexes A–F.

1322
C. Huang et al. / Journal of Organometallic Chemistry 694 (2009) 1317–1324
Table 2
Devices performance data.
3.00
2.50
2.00
1.50
1.00
0.50
0.00
a
Complex A
Materials Turn-on
Lmax (V)
gcur
(max)
gext
EL
CIE (x, y)
Complex B
Complex C
Complex D
Complex E
Complex F
voltage
(V)
(cd/m2)
kmax
(max)
(cd/A)
(%)
(nm)
Complex
A
Complex
B
Complex
C
Complex
D
Complex
E
Complex
F
8.7
5.2
4.5
8.5
6.2
6.5
5701 (20 V)
4.31
1.23
10.3
10.9
2.12
5.12
5.60
549
559
582
632
638
640
(0.417, 0.575)
(0.474, 0.520)
(0.530, 0.467)
(0.635, 0.365)
(0.652, 0.343)
(0.668, 0.330)
50867 (20 V) 34.0
30543 (18 V) 30.9
3176 (20 V)
6138 (19 V)
5697 (19 V)
2.67
5.12
4.81
200
300
400
500
600
Wavelength (nm)
300
250
200
150
100
50
1.0
0.8
0.6
0.4
0.2
0.0
A
D
B
E
C
F
b
Complex A
Complex B
Complex C
Complex D
Complex E
Complex F
Mg:Ag/Ag
(150nm/10nm)
Alq3(20nm)
BCP(12nm)
PVK:PBD(40%):
Ir complex(6%)(70nm)
PEDOT:PSS(50nm) coated ITO
500
600
700
Wavelength/nm
800
Fig. 3. EL spectra of Complexes A–F. Insert shows the OLED device configuration.
0
450
550
650
750
850
form solutions using (ppy)2Ir(acac) as standard are 2.3%, 52%,
50%, 4.4%, 11%, and 12% for Complexes A–F, respectively [6,11]. It
is interesting to note that incorporation of an additional aromatic
segment on the 5-position of the pyridyl ring can largely increase
the PL quantum yields. For example, the PL quantum yields of
Complexes B and C are over 50%, while that of Complex A is only
2.3%. The same phenomena were observed on Complex D to Com-
plex F. Lin et al. have reported that Complex D showed low PL effi-
ciency and they explained that the possible reason for the low PL
Wavelength (nm)
Fig. 2. UV (a) and PL (b) spectra of Complexes A–F in chloroform solutions.
Tuning the conjugation length of the ligands will affect not only
the bandgaps of the complexes but also their energy levels. The
cyclic voltammetric measurements of the complexes in acetonitrile
with 0.10 M of tetrabutylammonium hexafluorophosphate
(Bu4NPF6) as the electrolyte under argon atmosphere indicated
that the HOMO of the complexes could be raised by about 0.3–
0.4 eV through increasing the conjugation length of the ligands.
The LUMOs can be elevated by about 0.2 eV, which are deduced
from the HOMO energy levels and the bandgaps obtained from
the UV absorption edges of the complexes. The CV measurement
results of the complexes are summarized in Table 1 and also drawn
in Fig. 5. It should be mentioned that none of the complexes
showed reversible oxidation in the cyclic voltammetry measure-
ment and no E1/2 values were available for these materials. The
PL quantum yields of the iridium complexes measured in chloro-
efficiency might be attributed to the intramolecular
p–p interac-
tion between the pyridyl ring in one ligand and one of the dangling
phenyl ring in the other in the complex, which was supported by
the single crystal X-ray diffraction study [30]. This explanation
may also be applied in Complex A. However, it will be a different
case for Complexes B, C, E, and F. In the later 4 complexes, the
incorporation of additional phenyl or fluorenyl group on pyridyl
ring at 5-position may twist the pyridyl ring and thus suppress
the intramolecular
p-p interaction between the pyridyl from
occurring. All the UV and PL data of the complexes are also summa-
rized in Table 1.
Table 1
Physical properties of Complexes A–F.
Materials
UV kpeak (MLCT) in CHCl3 (nm)
HOMO–LUMO gaps for
the complexes (eV)
PL kmax in CHCl3 (nm)
gPL in CHCl3 (%)
p-Doping (V)
Energy levels (eV)
Eonset
Ea
HOMO
LUMO
Complex A
Complex B
Complex C
Complex D
Complex E
Complex F
405
443
455
476
495
508
2.27
2.22
2.18
2.12
2.08
2.04
528
557
581
622
627
643
2.3
52
50
4.4
11
12
1.70
1.47
1.45
1.33
1.28
1.33
2.26
2.11
–
1.44/1.86
1.46/1.88
1.47/1.86
À6.10
À5.87
À5.85
À5.73
À5.68
À5.73
À3.83
À3.65
À3.67
À3.61
À3.60
À3.69

C. Huang et al. / Journal of Organometallic Chemistry 694 (2009) 1317–1324
1323
In order to understand the electroluminescent properties of
2.0
2.6
Complexes A–F, multi-layer OLED devices based on these materials
were fabricated and characterized with the configuration of ITO/
PEDOT: PSS (50 nm)/PVK:PBD (40%):Ir complex (6%) (70 nm)/BCP
(12 nm)/Alq3 (20 nm)/Mg:Ag (150 nm:10 nm) through solution
process. PVK was used as the host material for the device fabrica-
tion because its emission spectrum overlaps with the absorption
spectra of Complexes A–F. BCP (2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline) was used for hole and exciton blocker and PBD
(2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole) was doped to
the emission layer to enhance the electron injection and transport
ability [7,8]. The key device performance data were summarized in
Table 2 and the EL spectra, I–V–L curves, and efficiency-current
density curves are illustrated in Figs. 3 and 4, respectively.
3.61
3.60
3.65
5.87
3.67
5.85
3.69
3.83
5.5
5.68
5.73
5.73
6.10
6.2
Complex F
PVK Complex A Complex B Complex C
Complex E
PBD
ComplexD
Fig. 5. Energy levels of the complex materials.
High performance OLED devices have been demonstrated with
these complex materials. According to the EL spectra and CIE coor-
dinates of the devices, the emission light of the complexes could be
well tuned by changing the conjugation of the ligands. The turn-on
voltages of the devices are in the range of 4.5 and 8.7 V for the six
complexes, which are very common for PVK hosted Ir complex-
based OLED devices [6,7]. The maximum emission wavelengths
of the EL spectra of the devices are from 549 nm to 640 nm. The
maximum brightness of the devices are of 5701 cd/m2 (20 V),
50867 cd/m2 (20 V), 30543 cd/m2 (18 V), 3176 cd/m2 (20 V),
6138 cd/m2 (19 V), and 5697 cd/m2 (19 V) for Complexes A–F
respectively. The maximum EL efficiencies for Complexes A–F are
4.31 cd/A (1.23%), 34.0 cd/A (10.3%), 30.9 cd/A (10.9%), 2.67 cd/A
(2.12%), 5.12 cd/A (5.12%), 4.81 cd/A (5.60%), respectively. The EL
efficiencies of Complexes B and C are the highest among those
PVK based phosphorescent OLED devices [6,7,20], while the EL effi-
ciencies of 5 cd/A (>5%) for saturated red emission materials of
Complexes E and F are also comparable to most of the reference
reported results for solution processed devices [9,22]. From the de-
vice energy level diagram in Fig. 5, we can find that it is difficult for
holes to be injected from PVK (À5.5 eV) [35] to Complexes A–F due
to their unmatched HOMO energy levels. However, the LUMO en-
ergy levels (À3.6 to À3.8 eV) of the complexes are about 1 eV lower
than that of PBD (À2.6 eV) [36]. Therefore, electrons could be read-
ily trapped by the complex materials then holes may subsequently
hop onto the negative charged phosphorescent molecules to form
singlet/triplet excitons [6–8,37,38]. The much lower EL efficiencies
of Complexes A and D than the others may be mainly due to their
lower PL efficiencies, which is in good agreement with the reported
results by Lin et al. [30]. It is worth mentioning that the high EL
efficiencies of the four complexes are stably sustained over a large
range of current density, which could be ascribed to the efficacy of
polyphenylphenyl dendron groups in the complexes to suppress
the triplet–triplet annihilation. The slightly high turn-on voltage
of the devices is very common for PVK hosted Ir complexes based
OLED devices [6,7].
In order to further improve device performance, two devices
using 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) as
the block layer and PVK:PBD:Complex B, PVK:PBD:Complex
C
as the emissive zone have been fabricated. The I–V–L curves of
the two devices are shown in Fig. 6. Both current efficiency and
external quantum efficiency for the two devices have been largely
100
10
100000
10000
1000
100
450
400
350
300
250
200
150
100
50
A (I-V)
B (I-V)
C (I-V)
A (V-L)
B (V-L)
C (V-L)
A
B
C
1
10
1
0
0.1
0
2
4
6
8
10 12 14 16 18 20 22
100
0.1
1
10
1000
Voltage / V
Current density / mA.cm-2
450
400
350
300
250
200
150
100
50
10000
1000
100
10
10
1
D (I-V)
E (I-V)
F (I-V)
D (V-L)
E (V-L)
F (V-L)
D
E
F
0.1
0
1
100
0.1
1
10
1000
0
2
4
6
8
10 12 14 16 18 20 22
Voltage / V
Current density / mA.cm-2
Fig. 4. Device performance of Complexes A–C (top) and Complexes D–F (bottom).

1324
C. Huang et al. / Journal of Organometallic Chemistry 694 (2009) 1317–1324
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20
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Graetzel, J. Am. Chem. Soc. 125 (2003) 8790.
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I-V (B)
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L-V (B)
10
1
0
2
4
6
8
10
12
14
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enhanced. The maximum current efficiencies and external
quantum efficiencies for Complexes B and C based devices are
46.3 cd/A (13.9%) and 42.5 cd/A (12.8%), respectively. In compari-
son to devices using BCP as the block layer, the efficiencies were in-
creased by about 40%. The improvement of device performance
should be ascribed to the better blocking effect of TPBI over BCP
to holes/excitons.
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4. Conclusion
In summary, a series of novel iridium-based organic phospho-
rescent materials with ligand bearing polyphenylphenyl dendron
groups have been developed through simple synthesis and purifi-
cation procedures. Good OLED device performance with tunable
emission colors from yellow to saturated red has been demon-
strated by doping the complex materials into PVK matrices. These
solution processed OLED devices demonstrated quite stable EL effi-
ciencies over a large range of current density, which indicated that
the polyphenylphenyl dendron structure in the complexes could
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