Triphenylamine dendronized iridium(III) complexes: Robust synthesis, highly efficient nondoped orange electrophosphorescence and the structure-property relationship

Article abstract of DOI:10.1021/cm202732j

New triphenylamine dendronized homoleptic Ir(III) complexes, namely Ir-G1, Ir-G2, and Ir-G3, with six, eighteen, and up to forty-two triphenylamine units, respectively, are designed and efficiently synthesized through convergent strategy. Both linear enla

Full text of DOI:10.1021/cm202732j

Article
pubs.acs.org/cm
Triphenylamine Dendronized Iridium(III) Complexes: Robust
Synthesis, Highly Efficient Nondoped Orange
Electrophosphorescence and the Structure−Property Relationship
Minrong Zhu,† Jianhua Zou,‡ Xun He,† Chuluo Yang,*,† Hongbin Wu,*,‡ Cheng Zhong,† Jingui Qin,†
and Yong Cao‡
†Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072,
People’s Republic of China
‡Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South
China University of Technology, Guangzhou 510640, People’s Republic of China
S
* Supporting Information
ABSTRACT: New triphenylamine dendronized homoleptic Ir(III) complexes,
namely Ir-G1, Ir-G2, and Ir-G3, with six, eighteen, and up to forty-two
triphenylamine units, respectively, are designed and efficiently synthesized through
convergent strategy. Both linear enlargement of the dendritic arms and the “double-
dendron” strategy are applied to maximize the degree of site-isolation of the emissive
center. The relationship between the dendritic structures and their photophysical,
electrochemical, and electrophosphorescent performances is investigated. Phosphor-
escent organic light-emitting diodes (PhOLEDs) employing the dendrimers as
solution-processed emitters are fabricated. The nondoped devices with Ir-G1 and Ir-
G2 as emitters display very high efficiencies and small values of efficiency roll-off. For
example, a device with Ir-G1 as emitter exhibits the best results ever reported for
solution-processed orange phosphorescent devices with maximum luminous efficiency
of 40.9 cd A−1 and power efficiency of 39.5 lm W−1. Moreover, the maximum power
efficiency of the nondoped device is nearly three times higher than that of the doped
control device by doping Ir-G1 into the general polymer matrix. This indicates that incorporation of triphenylamine moieties
into the sphere of iridium(III) core is a simple and effective approach to develop highly efficient host-free dendritic phosphors.
KEYWORDS: iridium, dendrimer, synthesis, electrophosphorescence
Ir(III) dendrimers have been prepared through a convergent
strategy, which can give rise to a structure that is precisely
defined, is of high purity, and is without ill-defined end groups.8
However, the complexation of dendronized ligands with Ir(III)
salts in a typical reaction medium of glycerol usually results in
very discouraging yields in the range of 10−35% due to the stiff
and hardly soluble branching units.7a To improve the solubility
of the large dendronized ligands, surface groups such as alkyl or
alkoxy chains have to be attached, which could impair the
charge mobility and create difficulties during the reaction and
purification of the intermediate stages. Coming to the second-,
third-, and higher generations, the synthetic demands can grow
significantly with decreasing yields and increasing probability of
structural defects.7b Hence, preparation of high generation
Ir(III) dendrimers bearing dense dendrons remains a synthetic
challenge and major obstacle to application.
INTRODUCTION
■
Phosphorescent organic light-emitting diodes (PhOLEDs)
unfurl a bright future for the next generation flat-panel displays
and solid-state illumination sources due to their merit of high
quantum efficiencies compared with fluorescent OLEDs.1 The
most efficient PhOLEDs have the guest emitter blended in a
host matrix to prevent the luminescence quenching caused by
aggregation of the emissive species.2−4 However, the blending
systems intrinsically suffer from the physical phase separation
and hence deteriorate the device performance.5
Recently, the use of phosphorescent iridium-cored den-
drimers appears to be an effective approach to fabricate host-
free electrophosphorescent devices.6 With numerable func-
tional groups orderly attached to the periphery of the emissive
center, the interactions between iridium cores can be controlled
at the molecular level by the generation number and/or the
dendron number of the dendrimer.7 In particular, the “double-
dendron” materials, which have two dentrons per ligand of the
core, have enabled very efficient photoluminescence and
electroluminescence from the neat films.6e
In this contribution, we present the robust synthesis of the
triphenylamine dendronized homoleptic Ir(III) dendrimers
Received: September 13, 2011
Revised: November 27, 2011
Published: November 27, 2011
Considering the susceptibleness of the coordination between
Ir(III) and organic ligand in many reaction conditions, most
© 2011 American Chemical Society
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was determined from the second heating scan. The glass transition
temperature of Ir-G1 appears at 217.2 °C, and no obvious thermal
transition was observed for Ir-G2 and Ir-G3. Cyclic voltammetry
(CV) was carried out in nitrogen-purged dichloromethane (oxidation
scan) at room temperature with a CHI voltammetric analyzer.
Tetrabutylammonium hexafluorophosphate (TBAPF6) (0.1 M) was
used as the supporting electrolyte. The conventional three-electrode
configuration consists of a platinum working electrode, a platinum wire
auxiliary electrode, and an Ag wire pseudoreference electrode with
ferrocenium−ferrocene (Fc+/Fc) as the internal standard. Cyclic
voltammograms were obtained at a scan rate of 100 mV s−1. The onset
potential was determined from the intersection of two tangents drawn
at the rising and background current of the cyclic voltammogram. The
calculation for Ir-G1 was performed at the density functional theory
(DFT) level with the B3LYP functional. A double-quality basis set
consisting of Hay and Wadt’s effective core potentials (LANL2DZ)
was employed for Ir atom and a 6-31G(d) basis for other elements.
The Ir-G2 was constructed based on Ir-G1 and then optimized with
Merck Molecular Force Field with the metal core frozen due to the
lack of force field parameter for Ir(III). The structure of Ir-G3 was
built on the optimized Ir-G2 structure with a molecular mechanics
method using Merck Molecular Force Field under the same procedure.
Then the closest contacts between dendrons were measured with the
threshold of 4.6 Å.
through traditional convergent strategy. The dendrimers are
denoted as Ir-G1, Ir-G2, and Ir-G3, with six, eighteen, and up
to forty-two triphenylamine units, respectively (Figure 1).
Figure 1. Ir(III)-cored dendrimers of first-generation (Ir-G1), second-
generation (Ir-G2), and third-generation (Ir-G3).
Device Fabrication and Measurement. Patterned indium tin
oxide (ITO)-coated glass substrates with a sheet resistance of 15−20
ohm/square underwent a wet-cleaning course in an ultrasonic bath,
beginning with acetone, followed by detergent, deionized water, and
isopropanol. After oxygen-plasma treatment, a 50 nm thick anode
buffer layer of PEDOT:PSS (Baytron P4083, Bayer AG) film was spin-
cast on the ITO substrate and dried by baking in vacuum oven at 80
°C overnight. The emitting layer was prepared by spin-coating from
chlorobenzene solution on top of the PEDOT layer and then annealed
at 100 °C for 20 min. TPBI (25 nm), Ba (4 nm), and Al (150 nm)
were evaporated with a shadow mask successively at a base pressure of
3 × 10−4 Pa. The thickness of the evaporated TPBI and cathode was
monitored by a quartz-crystal thickness/ratio monitor (Sycon model
STM-100/MF). The cross-sectional area between the cathode and
anode defined the pixel size of 19 mm2. Except for the spin coating of
the PEDOT layer, all the processes were carried out in the controlled
atmosphere of a nitrogen drybox (Vacuum Atmosphere Co.)
containing less than 1 ppm oxygen and moisture. All measurements
were carried out at room temperature under ambient conditions.
Materials and Synthesis. Starting chemicals and reagents were
purchased from commercial sources and used as received without
further purification. Solvents for synthesis were purified according to
standard procedures prior to use. All reactions were performed under
an inner argon atmosphere. Synthesis of 4-(diphenylamino)-
phenylboronic acid (1) was performed following the literature
procedure.12
Triphenylamine units possess a high HOMO level (ca. −5.2
eV) and sufficiently high triplet energy (ca. 2.9 eV), and hence
they can be used as good antenna for the charge transfer and/or
energy transfer to the emitting center.9 What is more, without
any solubilizing side groups, the dendritic triphenylamine
compounds have good solubility in common organic solvents.10
All these merits make triphenylamine groups highly appealing
as desirable branching units for construction of dendritic Ir(III)
phosphors. Unprecedentedly, we achieved very efficient
complexation between the iridium and the bulky dendritic
ligands by the optimization of reaction medium. To the best of
our knowledge, the third generation dendrimer (Ir-G3) is the
first report covering the highest loading functional dendrons
among the iridium phosphors.7d The relationship between the
dentritic structures and their photophysical, electrochemical,
and electrophosphorescent performances is discussed. The
nondoped device with Ir-G1 as emitter exhibits the best results
ever reported for solution-processed orange phosphorescent
devices. Moreover, the maximum power efficiency of the
nondoped device is nearly three times higher than that of the
doped control device.
Synthesis of LG1. To a well degassed solution of 2,4-
dibromopyridine (1.69 g, 7.15 mmol), 1 (5.40 g, 18.60 mmol), and
2 M Na2CO3 (45 mL, 22.50 mmol) in a mixed solvent of toluene (135
mL) and ethanol (45 mL) was added Pd(PPh3)4 (0.50 g, 0.43 mmol).
The resulting mixture was stirred and heated to reflux at 110 °C for 48
h under argon atmosphere. After cooling to room temperature, the
solvent was evaporated under reduced pressure and taken up with
CH2Cl2. The organic layer was washed with brine and water
sequentially and dried over anhydrous Na2SO4. After having been
filtered, the solvent was evaporated to dryness and subjected to
column chromatography on silica gel with petroleum/ethyl acetate
(5:1, v/v) as the eluent to give the product (3.53 g) in a yield of 87%
as an off white solid. 1H NMR (300 MHz, CDCl3): δ 8.65 (d, J = 5.1
Hz, 1H), 7.91 (d, J = 8.1 Hz, 2H), 7.83 (s, 1H), 7.57 (d, J = 8.1 Hz,
2H), 7.36 (d, J = 5.1 Hz, 1H), 7.29−7.24 (m, 8H), 7.16 (d, J = 7.5 Hz,
12H), 7.09 (d, J = 7.8 Hz, 2H), 7.04 (d, J = 7.2 Hz, 2H); 13C NMR
(75 MHz, CDCl3): δ 162.80, 155.14, 154.25, 152.88, 152.65, 136.83,
134.81, 134.61, 133.22, 133.17, 130.35, 130.19, 128.92, 128.66, 128.33,
124.32, 122.72; MALDI-TOF: Calcd. for C41H31N3 565.70; Found
EXPERIMENTAL SECTION
■
General Information. 1H NMR and 13C NMR spectra were
measured on a MECUYR-VX300 spectrometer. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a Vario EL III
microanalyzer. MALDI-TOF mass spectra were performed on Bruker
BIFLEX III TOF mass spectrometer. Gel permeation chromatography
(GPC) measurements were recorded relative to a polystyrene standard
using a Waters 515 HPLC equipped with MZ gel SDplus linear 500 Å
column at 30 °C. THF was used as eluent at a flow rate of 1.0 mL
min−1. UV−vis absorption spectra were conducted on a Shimadzu UV-
2500 recording spectrophotometer. Photoluminescent spectra were
recorded on a Hitachi F-4500 fluorescence spectrophotometer. The
lifetimes of phosphorescence in toluene solution were measured by
exciting the materials with 350 nm from a Hydrogen lamp on a FLS
920 Combined steady-state and lifetime spectrometer. Differential
scanning calorimetry (DSC) was performed on a NETZSCH DSC
200 PC unit at a heating rate of 10 °C min−1 from room temperature
to 400 °C under a flow of nitrogen. The glass transition temperature
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a
Scheme 1. Synthesis of the Dendritic Ligands
a
Reagents and conditions: i) [Pd(PPh3)4], toluene, ethanol, 2 M base, reflux; ii) NBS, DMF, 0 °C, 80%; iii) 1-bromo-4-iodobenzene, CuI, ( )-trans-
1,2-diaminocyclohexane, KOtBu, 1,4-dioxane, 110 °C, 90%; iv) [Pd(dppf)Cl2] (dppf = 1,1′-bis(diphenylphosphanyl)ferrocene), bis(pinacolato)-
diboron, KOAc, 1,4-dioxane, 85 °C, 88%.
565.05; Anal. Calcd. for C41H31N3: C 87.05, H 5.52, N 7.43; Found: C
86.84, H 5.58, 7.33.
°C for 48 h under argon atmosphere. After having been cooled to
room temperature, the solvent was evaporated under reduced pressure
and taken up with CH2Cl2. The organic layer was washed with brine
and water sequentially and dried over anhydrous Na2SO4. After having
been filtered, the solvent was evaporated to dryness and subjected to
column chromatography on silica gel with petroleum/chloroform (2:1,
v/v) as the eluent to give the product (4.18 g) in a yield of 85% as a
Synthesis of 2. A solution of NBS (2.62 g, 14.74 mmol) in DMF
(30 mL) was slowly added to a mixture of LG1 (1.99 g, 3.51 mmol) in
DMF (30 mL) with several drops of acetic acid at 0 °C under argon
atmosphere. The resulting mixture was allowed to slowly warm up to
room temperature to stir for another 18 h. To the solution was added
H2O (300 mL), and then the solution extracted with CH2Cl2 (300
mL). The organic layer was washed with NaHCO3 saturated solution
and dried over anhydrous Na2SO4. After removal of the solvent, the
resulting solid was recrystallized from CH2Cl2 and ethanol twice to
1
white solid. H NMR (300 MHz, CDCl3): δ 7.40−7.16 (m, 10H),
7.13−6.84 (m, 26H); 13C NMR (75 MHz, CDCl3): δ 153.04, 152.34,
138.29, 135.18, 133.01, 131.56, 129.05, 128.62, 127.15; MALDI-TOF:
Calcd. for C48H37N3 655.83; Found 655.23.
1
give a light yellow powder (2.47 g) in a yield of 80%. H NMR (300
Synthesis of 4. Potassium tert-butoxide (0.97 g, 8.60 mmol) was
added to a 100 mL round-necked flask containing 3 (3.78 g, 5.76
mmol), 1-bromo-4-iodobenzene (1.96 g, 6.92 mmol), and CuI (0.12 g,
0.58 mmol) under a flow of argon, and then ( )-trans-1,2-
diaminocyclohexane (137 μL, 1.14 mmol) and 1,4-dioxane (40 mL)
were added with syringe in sequence. The mixture was heated to 110
°C with good stirring overnight under argon atmosphere. After
cooling, H2O (100 mL) was added, and the mixture was extracted with
CH2Cl2. The organic layer was combined and washed with H2O and
then dried over anhydrous Na2SO4. After the solvent was evaporated,
the residue was purified by column chromatography on silica gel with
petroleum/chloroform (2:1, v/v) to give a gray solid (4.20 g) with a
MHz, CDCl3): δ 8.67 (d, J = 5.1 Hz, 1H), 7.94 (d, J = 8.7 Hz, 2H),
7.82 (s, 1H), 7.59 (d, J = 8.4 Hz, 2H), 7.40−7.35 (m, 9H), 7.15 (d, J =
8.7 Hz, 4H), 7.00−6.97 (m, 8H); 13C NMR (75 MHz, CDCl3): δ
162.07, 154.71, 153.52, 152.94, 151.06, 150.85, 138.95, 137.51, 137.43,
133.08, 131.00, 130.81, 128.64, 124.27, 122.58, 121.35, 121.01;
MALDI-TOF: Calcd. for C41H27Br4N3 881.29; Found 881.65.
Synthesis of 3. To
a well degassed solution of 4-
(diphenylamino)phenylboronic acid (5.46 g, 18.80 mmol) and bis(4-
bromophenyl)amine (2.46 g, 7.50 mmol) and 2 M Na2CO3 (30 mL,
15.0 mmol) in toluene (90 mL) was added Pd(PPh3)4 (0.26 g, 0.23
mmol). The resulting mixture was stirred and heated to reflux at 110
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yield of 90%. 1H NMR (300 MHz, CDCl3): δ 7.45 (d, J = 7.8 Hz, 8H),
7.26−7.21 (m, 14H), 7.11−7.00 (m, 12H), 6.99−6.86 (m, 6H); 13C
NMR (75 MHz, CDCl3): δ 152.46, 151.71, 150.60, 139.26, 137.04,
134.07, 132.16, 129.19, 128.81, 127.70; MALDI-TOF: Calcd. for
C54H40BrN3 810.82; Found 811.10.
CDCl3): δ 7.89 (s, 3H), 7.67 (s, 3H), 7.59 (d, J = 9.0 Hz, 6H), 7.51−
7.43 (m, 28H), 7.25 (d, J = 6.9 Hz, 80H), 7.20−7.11 (m, 46H), 7.04−
6.91 (m, 58H), 6.81 (d, J = 8.7 Hz, 16H), 6.25−6.20 (m, 6H); 13C
NMR (75 MHz, CDCl3): δ 147.97, 147.18, 146.57, 146.13, 136.08,
135.23, 134.66, 129.52, 127.79, 127.63, 127.42, 126.96, 125.33,
124.61,124.39, 124.22, 123.15, 122.83; MALDI-TOF: Calcd. for
C339H246IrN21 4805.94; Found 4805.80; Anal. Calcd. for
C339H246IrN21: C 84.72, H 5.16, N 6.12; Found: C 84.47, H 5.31, N
6.11. Mw/Mn = 1.01.
Synthesis of 5. 4 (3.09 g, 3.82 mmol), bis(pinacolato)diborane
(1.27 g, 5.0 mmol), and KOAc (1.37 g, 14.0 mmol) were mixed
together in a 100 mL flask. After degassing, dioxane (40 mL) was
added to the mixture under flow of argon. Afterward, [Pd(dppf)Cl2]
(50 mg) was added. The reaction mixture was kept at 85 °C overnight
under argon atmosphere and then cooled to room temperature. The
solvent was concentrated, and the inorganic salt was dissolved
completely after addition of water. After having been extracted with
CH2Cl2, the combined organic layer was washed with brine and dried
over anhydrous Na2SO4. The solvent was evaporated under reduced
pressure, and the residue was purified through column chromatog-
raphy with petroleum/CH2Cl2 (1:1, v/v) as the eluent to give the
Preparation of Ir-G3. Ir-G3 was prepared following the
procedure described for Ir-G1. The product was obtained as a yellow
1
solid (0.25 g) with a yield of 50%. H NMR (300 MHz, CDCl3): δ
7.80−7.50 (m, 15H), 7.46−7.35 (m, 126H), 7.18−7.14 (m, 123H),
7.02−6.91 (m, 288H), 6.34−6.12 (m, 6H); 13C NMR (75 MHz,
CDCl3): δ 137.23, 133.16, 129.77, 127.22, 125.15, 122.67; MALDI-
TOF: Calcd. for C771H558IrN45 10645.20; Found 10649.04; Anal.
Calcd. for C771H558IrN45: C 86.99, H 5.28, N 5.92; Found: C 87.03, H
5.12, N 5.55. Mw/Mn = 1.05.
1
desired compound as a white solid (2.82 g) with a yield of 88%. H
NMR (300 MHz, CDCl3): δ 7.68 (d, J = 7.5 Hz, 2H), 7.46 (d, J = 8.0
Hz, 8H), 7.26−7.25 (m, 12H), 7.18−7.11 (m, 14H), 7.04 (t, J = 7.5
Hz, 4H), 1.33 (s, 12H); 13C NMR (75 MHz, CDCl3): δ 151.77,
150.94, 150.18, 140.02, 139.63, 138.65, 133.34, 131.64, 129.11, 128.42,
128.12, 126.94, 126.24, 87.68, 28.97; MALDI-TOF: Calcd. for
C60H52BN3O2 857.88; Found 857.30.
Synthesis of LG2. According to the similar procedure for the
preparation of LG1, LG2 was synthesized by the Suzuki coupling of 5
with 2,4-dibromopyridine (or the 4-fold Suzuki coupling of
tetrabromide 2 with 1 using Cs2CO3 as base) to give a yield of 80%
(86%). 1H NMR (300 MHz, CDCl3): δ 8.67 (d, J = 5.4 Hz, 1H), 7.97
(d, J = 9.0 Hz, 2H), 7.87 (s, 1H), 7.62 (d, J = 8.4 Hz, 2H), 7.52−7.45
(m, 16H), 7.39 (d, J = 5.1 Hz, 1H), 7.28−7.20 (m, 32H), 7.14 (d, J =
9.0 Hz, 20H), 7.04 (t, J = 7.5 Hz, 8H); 13C NMR (75 MHz, CDCl3): δ
153.84, 152.61, 151.90, 150.81, 140.92, 139.57, 134.23, 132.96, 132.35,
130.14, 129.94, 129.38, 129.01, 128.13, 127.95; MALDI-TOF: Calcd.
for C113H83N7 1538.92; Found 1538.33; Anal. Cacld. for C113H83N7: C
88.19, H 5.44, N 6.37; Found: C 87.70, H 5.50, N 6.33.
RESULTS AND DISCUSSION
■
Preparation and Characterization. The synthetic route
for the dendritic chelating ligands is outlined in Scheme 1. The
first-generation triphenylamine-based ligand (LG1) was facilely
prepared through the Suzuki-coupling reaction of 4-
(diphenylamino)phenylboronic acid (1) with commercially
available 2,4-dibromopyridine.12a Initially, the stepwise prep-
aration of second-generation counterpart (LG2) commenced
with the synthesis of 3, which was then subjected to a modified,
mild Buchwald cross-coupling methodology to generate 4 in
90% yield.12b In this step, the selective amination of the aryl
iodide in 1-bromo-4-iodobenzene was performed by using a
powerful catalyst system comprised of CuI and racemic trans-
1,2-cyclohexanediamine in the presence of KOtBu. Subsequent
borylation using bis(pinacolato)diboron afforded the desired
pinacol boronic ester 5 in 88% yield. The Suzuki−Miyaura
cross-coupling of building block 5 with 2,4-dibromopyridine
gave the LG2 in 80% yield. Alternatively, direct 4-fold Suzuki
coupling of 1 with aryl tetrabromide 2, which was converted by
treating LG1 with NBS (4 equivalent), also furnished the ligand
bearing six triphenylamine units successfully. Following the
same strategy, the third-generation analogue (LG3) was
obtained in a yield of 50% by the Suzuki coupling of 5 with
2. This efficient approach rendered it suitable for building up
high generation dendrons without structural defect. The final
step in the preparation of Ir-G(1-3) is the complexation of
large all-aromatic ligands with an iridium source.13 For the
convenience and retrenchment of ligands, the standard one-pot
procedure favors the preparation of homoleptic Ir(III)
complexes, which involves treating Ir(acac)3 (acac = 2,4-
pentanedionate) with 3.2 equiv of free ligand in glycerol at
refluxing temperature. Considering the moderate solubility of
polytriphenylamine ligands in glycerol and high temperature
required for the formation of desired facial configuration, o-
dichlorobenzene and 2-(2-methoxyethoxy)ethanol were added
as cosolvents proportionally. Consequently, the complexations
for Ir-G1 and Ir-G2 were accomplished in nearly quantitative
yields. Moreover, 50% of yield was obtained for Ir-G3 bearing
42 triphenylamine branching units without difficulty from steric
crowding. We assume that this robust coordination could be
contributed from the decreased viscosity in the mixed solvent
systems as well as the inherent good solubility of the
triphenylamine-based ligands. This judicious choice of dendron
source and reaction medium renders the gram-scale preparation
Synthesis of LG3. The compound was prepared by the 4-fold
Suzuki coupling of 5 with 2 similar to the procefure for LG2 with a
1
yield of 50%. H NMR (300 MHz, CDCl3): δ 8.64 (s, 1H), 7.97 (s,
2H), 7.88 (s, 1H), 7.60−7.44 (m, 51H), 7.40−7.23 (m, 62H), 7.21−
7.11 (m, 40H), 7.05−7.00 (m, 30H); 13C NMR (300 MHz, CDCl3): δ
147.97, 147.07, 146.65, 135.34, 134.86, 129.57, 127.62, 124.61, 124.37,
123.14; MALDI-TOF: Calcd. for C257H187N15 3485.34; Found
3486.09; Anal. Calcd. for C257H187N15: C 88.56, H 5.41, N 6.03;
Found: C 88.29, H 5.52, N 5.86.
Preparation of Ir-G1. LG1 (0.96 g, 1.70 mmol) and Ir(acac)3
(0.25 g, 0.50 mmol) were precisely weighted up and added to a 50 mL
round-necked flask. Thereafter, distilled o-dichlorobenzene (5 mL)
was first added to the mixture. After the ligand was completely
dissolved, 2-(2-methoxyethoxy)ethanol (10 mL) and glycerol (20 mL)
were added to the flask. The mixture was refluxed at 230 °C for 24 h.
After completion, o-dichlorobenzene was removed under reduced
pressure. The mixture was poured into H2O and extracted with
CH2Cl2. The organic phase was washed with brine and dried over
anhydrous Na2SO4. The solvent was evaporated under reduced
pressure, and the residue was purified through column chromatog-
raphy with petroleum/CH2Cl2 (1:1, v/v) as eluent to afford Ir-G1
1
(0.95 g) as a red powder with a yield of 95%. H NMR (300 MHz,
CDCl3): δ 7.85 (s, 3H), 7.61 (s, 3H), 7.52 (d, J = 8.0 Hz, 6H), 7.38 (d,
J = 9.0 Hz, 3H), 7.29 (d, J = 9.0 Hz, 14H), 7.16−7.09 (m, 28H), 6.98
(s, 6H), 6.85−6.80 (m, 16H), 6.64 (s, 5H), 6.25 (d, J = 7.8 Hz, 6H);
13C NMR (75 MHz, CDCl3): δ 166.57, 149.19, 148.44, 147.91,
147.49, 147.28, 138.06, 131.42, 130.61, 129.67, 128.81, 127.85, 125.77,
125.01, 124.62, 123.85, 123.06, 122.37, 118.51, 114.85; MALDI-TOF:
Calcd. for C123H90IrN9 1886.31; Found 1886.01; Anal. Calcd. for
C123H90IrN9: C 78.32, H 4.81, N 6.68; Found: C 78.28, H 4.88, N
5.86. Mw/Mn = 1.02.
Preparation of Ir-G2. Ir-G2 was prepared following the
procedure described for Ir-G1. The product was obtained as an
orange-red powder (0.84 g) with a yield of 98%. 1H NMR (300 MHz,
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of sophisticated iridium dendrimers feasible and contributes to
their potential applications in plastic electronics.
3. The photophysical data are summarized in Table 1. The
absorptions around 300 and 380 nm for the dendrimers can be
assigned to triphenylamine-centered π−π* and spin-allowed
intraligand π−π* transitions, respectively, which are signifi-
cantly enhanced in intensity with the increasing number of
triphenylamine units. The weak features in the range of 380−
500 nm come from the metal-to-ligand charge transfer
(MLCT) states of the complexes.7b
The good solubility of the dendrimers in common solvents
facilitates their purification and structural characterization. The
molecular ion peaks in MALDI-TOF mass spectra verified the
presence of the desired dendrimers. Furthermore, gel
permeation chromatography (GPC) data exhibited a stepwise
decrease of elution time as the generation grew (Figure 2). A
The three dendrimers exhibit almost the same emission peak
at ca. 561 nm in toluene solution, but Ir-G3 displays a reduced
emission red-shift of 2 nm from solution to film compared to
10 nm of Ir-G1 and 6 nm of Ir-G2, which is indicative of a
significantly reduced interaction between emissive cores with
the increasing generation number.14 Furthermore, the solution
PL quantum yields (PLQYs) were measured in N2-saturated
toluene solutions by a relative method using fac-Ir(ppy)3 (ΦFL
= 40%, in toluene) as reference,15 and the relative PLQYs of
dendrimers increase from 29% for Ir-G1, 40% for Ir-G2, to
44% for Ir-G3, respectively, indicating improving chromophore
separation with the increasing generation number.
Electrochemical Characterization. The electrochemical
property of the dendrimers was probed by cyclic voltammetry
(CV), using tetrabutylammonium hexafluorophosphate
(TBAPF6) as the supporting electrolyte and ferrocene as the
internal standard. The highest occupied molecular orbital
(HOMO) energy levels of the dendrimers were determined
from the onset of the oxidation potentials with regard to the
energy level of ferrocene (4.8 eV below vacuum), and the
lowest occupied molecular orbital (LUMO) levels were
deduced from the HOMO energy levels and energy gaps
determined by the onset of absorption. The HOMO and
LUMO levels for Ir-G1 were estimated to be −4.72 and −2.39
eV, respectively, and the HOMO levels of Ir-G2 and Ir-G3
were estimated to be higher than −5.0 eV but lower than that
of Ir-G1. It would be ambiguous to obtain exact values of
HOMO and LUMO levels for Ir-G2 and Ir-G3 since the first
oxidation potential around 0.10 eV that corresponds to the
oxidation of iridium core became too weak to be recognized
(Figure S1, see the Supporting Information).6d Extra oxidation
waves at higher potentials can be ascribed to the oxidation of
the periphery dendrons, indicating that the polytriphenylamine-
based dendrons could directly participate in the charge
transport process.7a
Figure 2. GPC traces for Ir-cored dendrimers in THF.
narrow polydispersity index (PDI) (1.01−1.05) indicates their
purity. Also, the dendrimers were fully characterized by 1H, 13
NMR spectroscopy and elemental analysis.
C
Photophysical Properties. The absorption spectra of the
dendrimers in toluene and the photoluminescence (PL) spectra
of the dendrimers in toluene and films are presented in Figure
Figure 3. Electronic absorption spectra of dendrimers in toluene and
their emission spectra recorded both in toluene (open) and film state
(solid). The emission scale is arbitrary, and the spectra have been
normalized and offset for comparison.
Nondoped Orange Electrophosphorescence. To eval-
uate the performance of the three dendrimers as self-host
phosphorescent emitters, nondoped PhOLEDs were fabricated
with the following configurations: ITO/poly-
Table 1. Photophysical Properties and Electroluminescent Characteristics of Phosphorescent Dendrimers
a
b
c
d
e
f
g
h
I
dendrimer λmax [nm] (ε*10−5
)
λem [nm] λem [nm]
τ
(ns)
Vturn‑on [V] ηc. max [cd/A)] ηp. max [lm/W] ηext. max [%]
CIE x, y
Ir-G1
Ir-G2
Ir-G3
304 (0.7),
376 (1.2).
308 (2.3),
370 (3.2).
310 (3.5),
371 (6.7).
561
562
562
571
568
564
10.5
10.3
2.8
3.0
3.9
40.9
35.7
38.9
32.1
7.6
39.5
31.6
38.3
28.8
5.4
16.4
14.6
15.6
12.9
3.0
0.50, 0.48
0.52, 0.47
0.48, 0.50
16.7, 36.4
4.8
1.2
1.8
a
Measured in toluene solution with concentrations of 3.39, 1.27, and 0.43 *10−5 M for Ir-G1, Ir-G2, and Ir-G3, respectively. Extinction coefficients
b
c
are given in parentheses. Measured in toluene at 298 K with a concentration of 10−5 M and the excitation wavelength of 380 nm. Neat film data
d
e
measured at 298 K with the excitation wavelength of 380 nm. Measured in toluene at a concentration of 10−5 M at 298 K. At a brightness of 1 cd
f
g
h
m−2. Maximum current efficiency, then data at 1000 cd m−2. Maximum power efficiency, then data at 1000 cd m−2. Maximum external quantum
I
efficiency, then data at 1000 cd m−2. CIE at 7 V.
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Chemistry of Materials
Article
(ethylenedioxythiophene):poly(styrene sulfonic acid) (PE-
DOT:PSS, 50 nm)/Ir-G1, Ir-G2, or Ir-G3 (80 nm)/1,3,5-
tris(2-N-phenylbenzimidazolyl)benzene (TPBI, 25 nm)/Ba (4
nm)/Al (150 nm) (Figure S2, see the Supporting Information).
In this sandwich geometry, TPBI acts as an electron-
transporting and hole-blocking material. The electrolumines-
cence (EL) devices display the consistent spectra with their PL
counterparts, without additional emission from the aggregation
or the dendrons. The current−voltage−luminance (J−V−L)
characteristics and efficiency versus current density curves are
depicted in Figure 4, and the main device data are collected in
high as 35.7 cd A−1 or 24.2 cd A−1 with Commission
Internationale de L’Eclairage (CIE) coordinates of (0.50, 0.48).
The small values of efficiency roll-off should be attributed to
the sufficiently depressed concentration quenching of emissive
core and the good charge-transporting ability of the branching
units. The device from Ir-G2 also exhibits high performances
with ηc. max of 38.9 cd A−1, ηp. max of 38.3 lm W−1, and ηext. max of
15.6%. We note that the EL efficiencies enabled by Ir-G1 and
Ir-G2 are the highest ever reported for solution-processed
orange emissive devices and not far from those of the
evaporated PhOLEDs (ηc. max of 57.8 cd A−1, ηp. max of 51.9
lm W−1, and ηext. max of 20.5%) we recently reported.16
Nevertheless, the device with Ir-G3 as emission layer
suffered a severe drop in performances with ηc. max of 7.6 cd
A−1, ηp. max of 5.4 lm W−1, and ηext. max of 3.0%. The decreased
efficiencies could be rationalized by the following two factors: i)
according to the molecule modeling, the directional energy
transfer from the outer-dendrons to the emissive center
becomes less efficacious as the molecular radius increases
from 14 Å for Ir-G1 to 20 Å for Ir-G2 and 26 Å for Ir-G3; ii)
the triplet excitons on triphenylamine units may annihilate as
the distance between the two triphenylamine units is within
4−5 Å.17 The amounts of such contacts (4−5 Å) among the
outer dendrons increase sharply from 30 units in Ir-G2 to
nearly 300 units in Ir-G3 (Figure S3, see the Supporting
Information), thus much more annihilation of triplet excitons
on triphenylamine units may occur in Ir-G3. The increased
close contacts also make against the energy transfer from the
outer-dendrons to the emissive core. This assumption is
consistent with the PL experimental results that Ir-G3 exhibits
nonmonoexponential decay profiles (Figure 5) and relative
Figure 5. Transient photoluminescence decay of the dendrimers.
long PL lifetime (16.7 and 36.4 ns, Table 1) compared with Ir-
G1 and Ir-G2 with monoexponential decay profiles and relative
short PL lifetimes of ca. 10 ns (Table 1).6e
Figure 4. The J−V−L characteristics (a), current/power efficiency
versus current density curves (b), and EL spectra of dendrimers at a
driving voltage of 7 V (c).
Doped Orange Electrophosphorescence. For compar-
ison, we also prepared a control device under the identical
conditions except using Ir-G1 as guest blended with the general
polymer matrix containing poly(N-vinylcarbazole) (PVK) and
electron-transport material 2-tert-butylphenyl-5-biphenyl-1,3,4-
oxadiazol (PBD) with the ratio of PVK (72 wt %):PBD (21 wt
%):Ir-G1 (7 wt %) as emissive layer. The device acquires ηc. max
of 41.7 cd A−1 and ηext. max of 16.7% at a brightness of 253 cd
m−2 (Figure 6), which are comparable with those of the self-
hosted device from Ir-G1. Similarly, when the luminance
reaches 1000 cd m−2, the efficiencies are still high: ηc of 38.3 cd
Table 1. The low turn-on voltages in a range of 2.8−3.9 eV are
realized due to the reduced energy barrier between anode and
emission layer, as evidenced by the fact that HOMO levels of
the three dendrimers are close to the work function of the hole-
injection layer of PEDOT (−4.80 eV). The device from Ir-G1
achieves a maximum current efficiency (ηc. max) of 40.9 cd A−1, a
maximum power efficiency (ηp. max) of 39.5 lm W−1, and a
maximum external quantum efficiency (ηext. max) of 16.4%
photons per electron at a brightness of 179 cd m−2; even at a
high brightness of 1000 cd m−2 or 10000 cd m−2, ηc is still as
A
−1 and ηext of 15.3%. However, its maximum power efficiency
of 13.6 lm W−1 is much lower than the latter (39.5 lm W−1)
due to its high turn-on voltage of 7.2 V. We believe that the
179
dx.doi.org/10.1021/cm202732j | Chem. Mater. 2012, 24, 174−180

Chemistry of Materials
Article
Science Foundation of China (61177022); J. Zou thanks China
Postdoctoral Science Foundation (201104352) for financial
support.
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CONCLUSIONS
■
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In conclusion, we have developed a convenient and robust
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preparation of large dendritic ligands without structural defect
as well as thereafter efficient complexation with iridium source
renders the iridium dendritic phosphors feasible in real
application of organic electronics. Bearing excellent hole-
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good spacers isolating the Ir complex core but also participate
straightforwardly in charge injection, transport, and energy
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double-dendron Ir dendritic array is structurally enough for
solution-processable host-free phosphors. This demonstrates
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dendron and generation numbers are excellent self-host−guest
system at the molecular level. With the best performances
recorded so far for solution-processed orange PhOLEDs, we
believe that this powerful synthetic strategy and efficient
triphenylamine-dendron system would have great potential to
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colors covering the whole visible region for use in full-color
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ASSOCIATED CONTENT
■
(13) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.;
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S
* Supporting Information
Figures S1−S3.This material is available free of charge via the
Internet at http://pubs.acs.org.
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Goddard, W. A.; Thompson, M. E. J. Am. Chem. Soc. 2009, 131, 9813.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: clyang@whu.edu.cn (C.Y.); hbwu@scut.edu.cn
(H.W.).
■
ACKNOWLEDGMENTS
■
C. Yang thanks the National Science Fund for Distinguished
Young Scholars of China (No. 51125013), the National Natural
Science Foundation of China (90922020), the National Basic
Research Program of China (973 Program 2009CB623602),
and the Fundamental Research Funds for the Central
Universities of China. H. Wu thanks the National Natural
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