Synthesis of a novel tris-cyclometalated iridium(III) complex containing triarylamine unit and its application in OLEDs

Article abstract of DOI:10.1016/j.ica.2009.08.007

The synthesis of the tris-cyclometalated iridium(III) complex [Ir(DCP)₃] (HDCP = 1-(N,N-diphenyl-amino)-4-(4-chlorophenyl)-phthalazine) from hydrated iridium(III) chloride and the ligand HDCP under mild reaction conditions was described. The ph

Full text of DOI:10.1016/j.ica.2009.08.007

Inorganica Chimica Acta 362 (2009) 4985–4990
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Inorganica Chimica Acta
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Synthesis of a novel tris-cyclometalated iridium(III) complex containing
triarylamine unit and its application in OLEDs
Yuan Fang a, Sujun Hu b, Yuezhong Meng a, Junbiao Peng b, Biao Wang a,
*
a State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Sun Yat-Sen University,
Guangzhou 510275, People’s Republic of China
b Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of the tris-cyclometalated iridium(III) complex [Ir(DCP)3] (HDCP = 1-(N,N-diphenyl-
amino)-4-(4-chlorophenyl)-phthalazine) from hydrated iridium(III) chloride and the ligand HDCP under
mild reaction conditions was described. The photophysical, electrochemical and electrophosphorescent
properties of this complex were investigated. Organic light-emitting diodes (OLEDs) using the complex
as a dopant and a blend of poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxa-
diazol (PBD) as a host exhibited bright red emission at 620 nm with the Commission Internationale de
l’Eclairage (CIE) coordinate of (0.67, 0.32). A maximum external quantum efficiency of 13.6% photos/elec-
tron with a luminous efficiency of 7.4 cd/A at a current density of 0.73 mA/cm2, and a maximum lumi-
nance of 2941 cd/m2 at 99 mA/cm2 were obtained in the device at 4 wt% doping concentration.
Ó 2009 Elsevier B.V. All rights reserved.
Received 5 January 2009
Received in revised form 30 July 2009
Accepted 3 August 2009
Available online 9 August 2009
Keywords:
Phosphorescence
Tris-cyclometalated iridium(III) complex
Cyclometalating ligand
Triarylamine
Organic light-emitting diodes
1. Introduction
emission efficiency. It has been well demonstrated that modifica-
tions of the molecular frameworks and the substituent groups on
Phosphorescent heavy metal complexes play a very important
role in organic light-emitting diodes (OLEDs), because they can
fully utilize both singlet and triplet excitons through the strong
spin–orbital coupling caused by heavy metal ions in complexes
[1]. Compared with other heavy metal complexes, iridium com-
plexes have been recognized as one of the best phosphorescent
material candidates because of their intense phosphorescence at
room temperature and relatively short excited state lifetime which
minimize quenching of triplet emissive state [2]. Recently, various
types of iridium(III) complexes have been developed [3–5], such
as tris-cyclometalated complexes Ir(C^N)3, bis-cyclometalated
neutral complexes Ir(C^N)2(L^X) and cationic complexes Ir((C^N)2
(L^X)+Yꢀ (C^N = cyclometalating ligand, L^X = bidentate ancillary
ligand, Y = anion). The synthesis of Ir(C^N)2(L^X) and Ir((C^N)2
(L^X)+Yꢀ can be carried out under mild conditions. In contrast,
the preparations of Ir(C^N)3, especially for facial isomers, generally
involve treating Ir(III) organic complexes, such as (C^N)2Ir(O^O)
cyclometalating ligands can afford significant color tuning and effi-
ciency improving of electrophosphorescence. Recently, we and Gao
et al. have respectively reported the strong phosphorescent tris-
cyclometalated iridium(III) complexes with similar ligands owning
the feature of an sp2-hybrid N-atom adjacent to the chelating N-
atom [7–10]. These results inspired us to do more research about
iridium(III) complexes bearing analogous ligands.
In this article, a novel ligand 1-(N,N-diphenyl-amino)-4-(4-
chlorophenyl)-phthalazine was synthesised. Triarylamine unit in
the ligand should affect the hole-transporting/trapping properties,
and it is also beneficial to extend the
p-electron delocalization of
the aromatic ligand chromophore [11]. In addition, the synthesis
of tris(1-(N,N-diphenyl-amino)-4-(4-chlorophenyl)-phthalazine)
iridium(III) [Ir(DCP)3] is exceptional. Different from above men-
tioned conditions, Ir(DCP)3 was synthesised in one-step by reaction
of the ligand with IrCl3ꢁ3H2O at 80 °C for 20 h. The synthesis and
electrochemical, photophysical and electrophosphorescent proper-
ties of this complex were investigated.
(O^O = b-diketonate anion), (C^N)2Ir(l-Cl)2Ir(C^N)2 or Ir(O^O)3,
with excess cyclometalating ligands at high temperature
(>200 °C) [6]. Nevertheless, tris-cyclometalated complexes as
phosphor dopants of OLEDs have attracted particular attention,
owing to good chemical and photochemical stability and high
2. Experimental
2.1. General information
All commercially available starting materials were purchased
from Aldrich and Alfa Aesar, and used without further purification,
* Corresponding author. Tel./fax: +86 20 84115692.
E-mail address: wangbiao@mail.sysu.edu.cn (B. Wang).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.08.007

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Y. Fang et al. / Inorganica Chimica Acta 362 (2009) 4985–4990
unless otherwise stated. HPLC-grade dimethylformamide (DMF)
was distilled from CaH2 immediately before use. 1H NMR spectra
were measured on a Varian Inova500NB spectrometer. Elemental
analyzes of carbon, hydrogen, and nitrogen were performed on a
Vario EL microanalyzer. UV–Vis absorption spectra were recorded
on a Shimadzu UV-2501 PC spectrophotometer. Photolumines-
cence spectra were recorded on a Shimadzu RF-5301PC fluores-
cence spectrophotometer. Luminescence lifetime was determined
on an Edinburgh FL920 time-correlated pulsed single-photon-
counting instrument. Electron ionization (EI) mass spectrum were
recorded on a Shimadzu GC–MS-QP2010 PLUS mass spectrometer,
and electron spray ionization (ESI) mass spectra on a Thremo LCQ
DECA XP mass spectrometer. Cyclic voltammetry (CV) was carried
out on a Solartron SI 1287 voltammetric analyzer at room temper-
ature in nitrogen-purged anhydrous CH2Cl2 with tetrabutylammo-
208 °C. 1H NMR (CDCl3, 500 MHz): d 8.02–8.00 (d, J = 8.3 Hz, 1H),
7.90–7.88 (d, J = 8.4 Hz, 1H), 7.76–7.72 (m, 3H), 7.65–7.62 (m,
1H), 7.54–7.52 (m, 2H), 7.30–7.25 (m, 4H), 7.12–7.07 (m, 6H). EI-
MS (m/z): 406 (100%), 407 (27%), 408 (33%), 409 (10%).
2.5. Synthesis of tris(1-(N,N-diphenyl-amino)-4-(4-chlorophenyl)-
phthalazine) iridium(III) [Ir(DCP)3]
HDCP (0.416 g, 1.02 mmol) and hydrated iridium(III) chloride
(0.1 g, 0.284 mmol) were added in a mixture of 2-ethoxyethanol
(12 mL) and distilled water (4 mL). The mixture was stirred at
80 °C for 20 h under nitrogen. Cooled to room temperature, the
precipitate was collected by filtration and washed with water, eth-
anol and hexane. The crude product was purified by column chro-
matography over silica gel using dichloromethane as the eluent to
give Ir(DCP)3 as a red solid in a yield of 37%. 1H NMR (CDCl3,
500 MHz): d 8.44–8.41 (d, J = 8.9 Hz, 1H), 7.86–7.83 (d, J = 8.6 Hz,
1H), 7.68–7.65 (t, J = 8.0 Hz, 2H), 7.50–7.46 (t, J = 7.5 Hz, 1H),
7.13–7.10 (d, J = 2.1 Hz, 1H), 6.92–6.89 (dd, J = 8.5, 2.2 Hz, 1H),
6.67–6.64 (m, 2H), 6.57–6.52 (m, 8H). ESI-MS (m/z): [M++H] Calc.
for C78H52N9Cl3Ir 1410.30 (32.5%), 1411.30 (30%), 1412.30 (100%),
1413.30 (83%), 1414.29 (99%), 1415.30 (67.5%), 1416.30 (45%),
1417.29 (24%); Found: 1410.30 (29%), 1411.33 (52.5%), 1412.26
(100%), 1413.26 (96%), 1414.23 (99%), 1415.22 (77%), 1416.25
(41%), 1417.21 (24%). Anal. Calcd. for C78H51N9Cl3Ir: C, 66.31; H,
3.64; N, 8.92. Found: C, 66.18; H, 3.83; N, 8.71%.
nium hexafluorophosphate as
a supporting electrolyte at a
scanning rate of 100 mV/s. A Pt disk, Pt wire, and SCE (saturated
calomel electrode) were used as the working, counter, and refer-
ence electrodes, respectively. Oxidation potential was calibrated
with that of ferrocene. The energy gap between the highest occu-
pied molecular orbital (HOMO) and the lowest unoccupied molec-
ular orbital (LUMO) was estimated from the UV–Vis absorption
spectrum edge.
2.2. Synthesis of 4-(4-chlorophenyl)-1-(2H)-phthalazinone (2)
A
mixture of 2-(4-chlorobenzoyl)-benzoic acid (1) (13 g,
50 mmol) and hydrazine hydrate (80%) (5 mL) in ethanol
(120 mL) was refluxed for 16 h. After cooled, the resulting white
precipitate was filtered off, washed with cold water, and dried to
give 2 in a yield of 95%. m.p.: 272–274 °C. 1H NMR (DMSO-d6,
500 MHz): d 12.90–12.87 (s, 1H), 8.38–8.32 (m, 1H), 7.94–7.87
(m, 2H), 7.70–7.65 (m, 1H), 7.65–7.60 (m, 4H). EI-MS (m/z): 255
(90%), 256 (100%), 257 (44%), 258 (32%).
2.6. OLED fabrication and measurements
The pre-cleaned ITO glass substrate was treated with ozone for
20 min. Then, a 45 nm thick PEDOT:PSS film was spin-coated onto
the ITO glass and dried at 80 °C for 12 h under vacuum. A film of
PVK and PBD, containing different amounts of Ir(DCP)3, was spin-
coated on top of the PEDOT:PSS using chloroform as the solvent;
the assembly was then dried for 3 h at 60 °C under vacuum. The
TPBI layer was grown by thermal sublimation under vacuum
(3 ꢂ 10ꢀ6 Torr). Subsequently, a layer of Ba (4 nm) and a layer of
Al (100 nm) were vacuum-evaporated on the top of the EL polymer
layer. Current–voltage characteristics were recorded with a Keith-
ley 236 source meter. EL spectra were collected by a PR 705 pho-
2.3. Synthesis of 1-chloro-4-(4-chlorophenyl)phthalazine (3)
To a 250 mL round-bottom flask, compound 2 (10.3 g, 40 mmol)
and POCl3 (5.5 mL, 60 mmol) were added all at once in 100 mL of
CHCl3. The mixture was refluxed for 16 h. After the reaction was
completed, CHCl3 was removed under reduced pressure. The mix-
ture was then cooled and poured into ice water. A 20% Na2CO3
solution was added to adjust the pH to 7. The resulting precipitate
was filtered off and purified by silica gel column chromatography
using dichloromethane as the eluent to give 3 in a yield of 73%.
m.p.: 180–183 °C. 1H NMR (CDCl3, 500 MHz): d 8.42–8.39 (d,
J = 7.88 Hz, 1H), 8.06–8.01 (m, 2H), 7.98–7.93 (m, 1H), 7.70–7.66
(d, J = 8.61 Hz, 2H), 7.58–7.54 (d, J = 8.59 Hz, 2H). EI-MS (m/z):
273 (100%), 274 (51%), 275 (75%), 276 (34%).
tometer (Photo Research). Luminance was measured by
a
calibrated silicon diode and calibrated by a PR 705 photometer.
The external quantum efficiencies were determined by a Si photo-
diode with calibration in an integrating sphere (IS080, Labsphere).
3. Results and discussion
3.1. Synthesis and characterization
2.4. Synthesis of 1-(N,N-diphenyl-amino)-4-(4-chlorophenyl)-
phthalazine (HDCP)
Scheme 1 outlines the synthetic routes toward the ligand and
the complex. The synthesis of Ir(DCP)3 is surprising for its simple
reaction conditions. Generally, the reactions of ligands with
A 60.5% suspension of sodium hydride (1.0 g, 25 mmol) in par-
affin oil was added slowly to a solution of diphenylamine (4.23 g,
25 mmol) in anhydrous DMF (80 mL). After stirred for 1 h at room
temperature under nitrogen as a protective gas, this mixture was
added dropwise to a solution of compound 3 (6.88 g, 25 mmol)
in anhydrous DMF (70 mL), while stirring and cooling with an ice
bath, and the mixture was stirred at room temperature for 12 h.
After reaction, the resulting mixture was poured into water and ex-
tracted with chloroform. The extracts were combined, washed
with brine, dried over anhydrous magnesium sulfate, and filtered.
The solvent was completely removed and the residue was purified
by column chromatography over silica gel using chloroform as the
eluent to give HDCP as a yellow solid in a yield of 76%. m.p.: 207–
IrCl3ꢁ3H2O give chloride-bridged diiridium complexes (C^N)2Ir(
l-
Cl)2Ir(C^N)2 at 80 °C for 20 h. Specially, at the above conditions,
the treatment of HDCP with IrCl3ꢁ3H2O affords the tris-cyclometa-
lated iridium(III) complex Ir(DCP)3. The 1H NMR data indicate that
Ir(DCP)3 is formed exclusively as facial isomer, because the three
ligands surrounding the iridium atom are magnetically equivalent
[6,12]. Since the yield of the complex is not high, we employed
electron spray ionization mass spectrometry (ESI-MS) to investi-
gate what side product formed. The crude mixture was prepared
by reaction of IrCl3ꢁ3H2O with an excess of HDCP in 2-ethoxyetha-
nol/water mixed solvent at 80 °C for 20 h. After cooled, this crude
mixture was diluted with CH2Cl2, and then characterized with
mass spectrometer without purification. As shown in Fig. 1, the

Y. Fang et al. / Inorganica Chimica Acta 362 (2009) 4985–4990
4987
Fig. 1. ESI mass spectra of the crude mixture prepared by reaction of IrCl3ꢁ3H2O with an excess of HDCP.
Fig. 2. ESI mass spectra of side products in reaction.
most abundant peaks are 408.3 and 1414.0, which correspond to
the molecular masses of HDCP and Ir(DCP)3, respectively. Medium
signals were obtained at 1040.5, 1054.6 and 1068.4m/z, and the
corresponding molecule might be formulated as Ir(DCP)2Cl. How-
ever, due to the limitation of measurement range, we cannot ob-
tain the signals above 2000m/z. For comparison, we used

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chromatographic column to separate side products from Ir(DCP)3
and HDCP, and the polarity of side products is much higher than
that of Ir(DCP)3. Fig. 2 shows ESI mass spectra of side products.
The abundant peaks have values 1038.8, 1054.3 and 1068.4m/z,
which are in accord with Fig. 1, considering the isotopes of Cl
and Ir in the mass spectra.
3.4. Electroluminescent OLEDs characterization
To illustrate the electroluminescent properties of Ir(DCP)3, the
devices using the complex as a dopant were fabricated with the
following structures: ITO/PEDOT:PSS(45 nm)/x wt%Ir(DCP)3:
PVK + 30%PBD(80 nm)/TPBI(30 nm)/Ba(4 nm)/Al(100 nm).
Poly
(3,4-ethylenedioxythiophene) (PEDOT) doped with poly (styrene
sulfonic acid) (PSS) was used as a hole-injecting material. As a good
hole-transporter, PVK was blended with PBD, which is an electron-
transport material, to enable the host to transport both electrons
and holes [17]. 1,3,5-tris(N-phenylbenzimidazol-2-yl)-benzene
(TPBI) acted as both a hole-blocker and an electron-transporter.
The electroluminescence (EL) spectra of OLEDs at 2 and 4 wt% dop-
ing concentrations are shown in Fig. 4. The devices emit red light
with an maximum at around 620 nm and its corresponding shoul-
der at 670 nm. There is no residual emission from the hosts at the
low doping concentrations, indicating that energy and charge
transfer from the host exciton to the phosphor is efficient under
electrical excitation [18]. The Commission Internationale de l’Eclai-
rage (CIE) color coordinate is (0.67, 0.32) which is almost identical
to the standard red values of (0.67, 0.33) demanded by the National
Television System Committee (NTSC).
3.2. Photophysical properties
The UV–Vis absorption and photoluminescence (PL) spectra of
Ir(DCP)3 and HDCP in CH2Cl2 are shown in Fig. 3. Compared with
the absorption spectra of free ligand, the intense absorption band
*
around 290 nm are assigned to the spin-allowed
p–p transitions
from the cyclometalating ligand. According to the previous report
[13], the band around 395 nm can be attributed to spin-allowed
metal–ligand charge transfer band (1MLCT). The moderately in-
tense and weak absorption at k > 450 nm correspond to the for-
mally spin-forbidden 3MLCT transition. The PL spectra of Ir(DCP)3
show the maximum emission at 614 nm. The lifetime of Ir(DCP)3
was measured in degassed CH2Cl2 at 298 K and well fitted to a sin-
gle-exponential decay. The value is 1.62 ls. By comparison with
those of known tris-cyclometalated iridium(III) complexes [14],
this shorter lifetime is beneficial to suppress the triplet–triplet
annihilation.
Figs. 5 and 6 present current density and luminance vs. voltage
curves of Ir(DCP)3-doped OLEDs at 2 and 4 wt% doping concentra-
3.3. Electrochemical properties
Cyclic voltammetry experiment was also carried out to investi-
gate the electrochemical properties of Ir(DCP)3. The complex
showed reversible oxidation processes during the anodic scan in
CH2Cl2, while the oxidation potential was observed to be 0.36 V
which is mainly assigned to the oxidation of Ir metal cationic site
(IrIII ? IrIV) [15]. However, no clear reversible reduction process
was observed through many tests. The HOMO and LUMO energy
levels of Ir(DCP)3 which are estimated with regard to the energy le-
vel of the ferrocene reference (4.8 eV below the vacuum level) and
UV–Vis absorption spectrum edge are ꢀ5.16 and ꢀ3.07 eV, respec-
tively. In order to get efficient OLEDs, the HOMO and LUMO levels
of iridium(III) complex should fall within the band gap of the hosts,
such as PVK (HOMO: ꢀ5.8 eV; LUMO: ꢀ2.2 eV), PBD (ꢀ6.2,
ꢀ2.6 eV) or 4,40-N,N0-dicarbazolebiphenyl (CBP) (ꢀ6.0, ꢀ2.9 eV)
[16]. As a guest, Ir(DCP)3 with the lower triplet excited state energy
and the narrow HOMO-LUMO energy gap has well energy level
match to the above hosts.
Fig. 4. EL spectra of the devices containing Ir(DCP)3 as a dopant at 12 V.
Fig. 3. UV–Vis absorption of Ir(DCP)3 (a) and HDCP (b), and photoluminescence
Fig. 5. Current density vs. voltage curves of devices at different doping
spectra of Ir(DCP)3 (c) in CH2Cl2 at 293 K.
concentrations.

Y. Fang et al. / Inorganica Chimica Acta 362 (2009) 4985–4990
4989
Fig. 6. Luminance vs. voltage curves of devices at different doping concentrations.
Fig. 7. External quantum efficiency vs. current density curves of devices.
tions. The device at 2 wt% doping concentration gave a maximum
external quantum efficiency ( ext) of 12.1% corresponding to a
induced quenching effects [19,20]. When the doping concentration
increased to 4 wt%, the maximum external quantum efficiency and
luminance were enhanced correspondingly. The external quantum
efficiency reached 13.6% with a luminous efficiency of 7.4 cd/A, at a
practical current density of 0.73 mA/cm2. The maximum lumi-
nance of 2941 cd/m2 was observed at a current density of 99 mA/
cm2, corresponding to an external quantum efficiency of 5.5%. With
the double increase in the doping concentration, the improvement
of the device performance was slight. It is associated with the
g
luminous efficiency of 6.48 cd/A at a current density of 0.86 mA/
cm2. When the current density increased to 17.7 mA/cm2, gext de-
creased to 9.2% with a loss of 24.2%. At 106 mA/cm2, the value of
gext was 4.8% and the device showed a maximum luminance of
2750 cd/m2. The efficiency started to decrease with increasing cur-
rent density (Fig. 7), which can be attributed to the increasing trip-
let–triplet annihilation of the phosphor-bound excitons and field-
Scheme 1. Synthesis of of Ir(DCP)3.

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Y. Fang et al. / Inorganica Chimica Acta 362 (2009) 4985–4990
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In summary, we have synthesised the tris-cyclometalated irid-
ium(III) complex Ir(DCP)3 from hydrated iridium(III) chloride and
the ligand 1-(N,N-diphenyl-amino)-4-(4-chlorophenyl)-phthal-
azine in one-step at 80 °C for 20 h. The devices based on Ir(DCP)3
exhibited a peak electrophosphorescence wavelength of 620 nm
along with a shoulder around 670 nm. For the device at doping
2 wt% concentration, a maximum external quantum efficiency of
12.1% was obtained at a current density of 0.86 mA/cm2, and a
maximum luminance reached 2750 cd/m2 at 106 mA/cm2. When
the doping concentration increased to 4 wt%, the maximum
external quantum efficiency and luminance rose to 13.6% and
2941 cd/m2, respectively.
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Acknowledgements
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This work was supported by the National Natural Science Foun-
dation of China (10732100, 10572155), Guangdong Science and
Technology Bureau (2006A11001002), and the Ministry of Educa-
tion PRC Special Scientific Research Fund for Doctor Subjects in
Colleges and Universities (20060558070).
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