Efficient red phosphorescent oleds using Ir(III) complexes based on bezoylphenylpyridine and the various ancillary ligands

Article abstract of DOI:10.1166/jnn.2014.8441

A series of red phosphorescent Ir(III) complexes 1-5 based on benzoylphenylpyridine and various ancillary ligands were synthesized and their photophysical properties were investigated. The EL efficiencies were quite sensitive to the structural features of the dopants in the emitting layers. Particularly, using complex 5 as a dopant in emitting layer, a high-efficiency orange-red OLED was fabricated, showing the maximum luminance of 16900 cd/m ² at 12 V, the luminous efficiency of 12.8 cd/A, the power efficiency of 4.48 lm/W, the external quantum efficiency of 9.65% at 20 mA/cm² and CIE x,y coordinates of (0.609, 0.390) at 12 V.Copyright

Full text of DOI:10.1166/jnn.2014.8441

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Nanoscience and Nanotechnology
Vol. 14, 6128–6132, 2014
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Efficient Red Phosphorescent OLEDs Using Ir(III)
Complexes Based on Bezoylphenylpyridine
and the Various Ancillary Ligands
Jin Soo Hwang1, Kum Hee Lee1, Seok Jae Lee2,
Young Kwan Kim2ꢀ∗, and Seung Soo Yoon1ꢀ∗
1Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Korea
2Department of Information Display, Hongik University, Seoul 121-791, Korea
A series of red phosphorescent Ir(III) complexes 1–5 based on benzoylphenylpyridine and various
ancillary ligands were synthesized and their photophysical properties were investigated. The EL
efficiencies were quite sensitive to the structural features of the dopants in the emitting layers.
Particularly, using complex 5 as a dopant in emitting layer, a high-efficiency orange-red OLED was
fabricated, showing the maximum luminance of 16900 cd/m2 at 12 V, the luminous efficiency of
12.8 cd/A, the power efficiency of 4.48 lm/W, the external quantum efficiency of 9.65% at 20 mA/cm2
and CIE xꢀy coordinates of (0.609, 0.390) at 12 V.
Delivered by Publishing Technology to: Florida State University, College of Medicine
Keywords: Red Phosphorescence, OLED, Iridium Complex, Benzoylphenylpyridine.
IP: 50.106.187.45 On: Mon, 02 Nov 2015 09:16:04
Copyright: American Scientific Publishers
1. INTRODUCTION
In this paper, a series of red phosphorescent Ir(III)
complexes 1–5 based on benzoylphenylpyridine ligands
were designed and synthesized. The electron donating
methyl group and the various ancillary ligands such
as acetylacetone, 1,3-diphenylpropane-1,3-dione, 2,2,6,6-
tetramethylheptane-3,5-dione were introduced to tune the
band gap of iridium complexes by the control of HOMO
or LUMO energy level.11–13 These types of ligand modifi-
cations would lead the change of electronic and structural
properties of the emitting materials of the devices, which
are key factors affecting OLED device performances. We
herein demonstrates highly efficient red OLEDs using
these Ir(III) complexes.
Phosphorescent OLEDs are of huge interest because the
high emission efficiency caused by the strong spin-orbit
coupling in the presence of heavy metals leads to the mix-
ing of singlet and triplet manifolds so that both singlet
and triplet excitons can be harvested.1ꢀ2 The design and
synthesis of efficient red emitters are intrinsically difficult
because their luminance quantum yields tend to decrease
with increasing emission wavelength in accordance with
the energy gap law.3 Therefore, a variety of phospho-
rescent materials based on iridium complexes have been
developed as emitting materials for the efficient phospho-
rescent OLEDs.4ꢀ5 Among those are the Ir(III) complexes
with carbonylated 2-phenylpyridine ligands.6–9 However,
their electroluminescent properties need to be improved
before they can be used in flat panel displays. For exam-
ple, an OLED device using (Bzppy)2 Ir(acac) as a dopant
in a CBP host showed the luminous efficiency of 7.7 cd/A
and the power efficiency of 1.18 lm/W at 20 mA/cm2 with
the CIExꢀy coordinates (x = 0ꢁ61, y = 0ꢁ38).10 To improve
the EL performances of devices using this material as the
emitting material, the optimization of the chemical and
device structures have been carried out in this lab.
2. EXPERIMENTAL DETAILS
2.1. Synthesis and Characterization
5-Benzoyl-2-bromopyridine,7 L2,14 and complex
2
[(Bzppy)2Ir(acac)]14 were synthesized as previously
reported.
Synthesis of L1. (85% yield). 1H NMR (300 MHz,
CDCl3ꢂ: ꢃ ppm 8.75 (dt, J = 1ꢁ2, 3.3 Hz, 1 H), 8.12
(d, J = 8ꢁ7 Hz, 2 H), 7.92 (d, J = 8ꢁ7 Hz, 2 H), 7.86–7.80
(m, 4 H), 7.64–7.58 (m, 1 H), 7.54–7.48 (m, 2 H), 7.33–
7.28 (m, 1 H). 13C NMR (125 MHz, CDCl3ꢂ: ꢃ ppm 196.6,
154.5, 150.2, 143.3, 138.0, 137.9, 137.1, 132.7, 130.8,
∗Authors to whom correspondence should be addressed.
6128
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doi:10.1166/jnn.2014.8441

Hwang et al.
Efficient Red Phosphorescent OLEDs Using Ir(III) Complexes Based on Bezoylphenylpyridine
130.3, 128. 6, 127.0, 123.1, 121.2. HRMS-EI+ calcd for
C18H13ON: 259.0997, found: 259.0997.
CDCl3ꢂ: ꢃ ppm 194.9, 193.0, 172.0, 151.7, 150.5, 143.9,
138.7, 137.1, 133.8, 133.3, 131.0, 130.2, 130.0, 129.0,
125.7, 121.1, 118.2, 90.5, 41.2, 27.8. HRMS-FAB+ calcd
for C41H31IrN2O4: 892.2852, found: 892.2847.
Synthesis of L3. (90% yield). 1H NMR (300 MHz,
CDCl3ꢂ: ppm 9.05 (dd, J = 0ꢁ8 Hz, 2.3 Hz, 1 H), 8.19
(dd, J = 2ꢁ3 Hz, 8.3 Hz, 1 H), 8.00 (d, J = 8ꢁ2 Hz,
2 H), 7.87–7.84 (m, 3 H), 7.66–7.61 (m, 1 H), 7.55–
7.50 (m, 2 H), 7.32 (d, J = 8ꢁ0 Hz, 2 H), 2.43 (s, 3 H).
13C NMR (500 MHz, CDCl3ꢂ: ppm 195.0, 160.6, 151.5,
140.6, 138.4, 137.3, 135.7, 133.2, 131.2, 130.2, 130.0,
128.8, 127.5, 119.8, 21.6. HRMS-EI+ calcd for C19H15ON
273.1154: found, 273.1154.
Synthesis of 5ꢁ (0.69 g, 41%). 1H NMR (300 MHz,
CDCl3ꢂ:ꢃ ppm 8.83 (d, J = 2ꢁ0 Hz, 2 H), 8.27 (dd, J =
2ꢁ0 Hz, 8.5 Hz, 2 H), 7.93 (d, J = 8ꢁ6 Hz, 2 H), 7.82–
7.79 (m, 4 H), 7.63–7.57 (m, 2 H), 7.55–7.47 (m, 6 H),
6.68 (dd, J = 1ꢁ6 Hz, 7.9 Hz, 2 H), 6.10 (s, 2 H), 5.28 (s,
1 H), 2.08 (s, 6 H), 1.55 (s, 6 H). 13C NMR (500 MHz,
CDCl3ꢂ: ꢃ ppm 193.1, 185.1, 172.4, 151.1, 150.7, 141.3,
140.9, 138.6, 137.3, 134.5, 133.0, 130.1, 129.9, 128.9,
125.8, 122.9, 118.0, 100.9, 28.9, 22.1. HRMS-FAB+ calcd
for C43H35IrN2O4 836.2226: found, 836.2228.
General procedure for the synthesis of 1. To a flask
containing IrCl3 ·3H2O (0.30 g, 1 mmol) and L1 (0.57 g,
2.2 mmol) was added a 3:1 mixture of 2-ethoxyethanol
and water (12 mL). The mixture was refluxed for 24 h
and cooled to room temperature. A colored precipitate
was filtered off and washed with water, methanol, and
hexane. The crude product was pumped dry to give
crude (L1)2Ir(ꢄ-Cl)2Ir(L1)2. Crude (L1)2Ir(ꢄ-Cl)2Ir(L1)2
was mixed with Na2CO3 (0.29 g, 2.7 mmol) in a two-
neck flask. 2-Eethoxyethanol (10 mL) and acetylacetone
(0.14 mL, 1.4 mmol) were added and the mixture was
heated at 100 ꢀC for 6 h. The solution was cooled to room
temperature and the red solid was collected by filtration.
The orange solid washed with water, methanol, and hex-
ane. The crude product was purified by column chromatog-
raphy using 0–50% ethyl acetate in hexane as eluent. 1 was
2.2. Device Fabrication and Measurements
OLEDs using red dopants in the emitting layers were fabri-
cated by vacuum (5ꢁ0×10−7 torr) thermal evaporation onto
pre-cleaned ITO coated glass substrates. The structure was
as follows: ITO/2-TNATA (60 nm)/NPB (20 nm)/CBP: 8%
Ir(III) complexes (30 nm)/BCP (10 nm)/Alq3 (20 nm)/Liq
(2 nm)/Al. All of the optical and electrical properties of
OLEDs such as the current density, luminance, luminous
efficiency and CIE coordinate characteristics were mea-
sured with Keithley 236, LS-50B, and MINOLTA CS-
100A, respectively.
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obtained as a orange solid (0.44 g, 54%) after recrystalliza-
IP: 50.106.187.45 On: Mon, 02 Nov 2015 09:16:04
3. RESULTS AND DISCUSSION
Copyright: American Scientific Publishers
tion from CH2Cl2/Methanol. 1H NMR (300 MHz, CDCl3ꢂ:
Scheme 1 shows the structure and synthetic route for
ꢃ ppm 8.51–8.49 (m, 2 H), 7.86 (d, J = 8ꢁ1 Hz, 2 H),
7.68 (dd, J = 1ꢁ5, 7.5 Hz, 2 H), 7.63 (d, J = 8ꢁ4 Hz,
2 H), 7.57–7.54 (m, 4 H), 7.49–7.44 (m, 2 H), 7.34–
7.28 (m, 6 H), 7.16–7.11 (m, 2 H), 6.59 (d, J = 1ꢁ5 Hz,
2 H), 5.25 (s, 1 H), 1.80 (s, 6 H). 13C NMR (125 MHz,
CDCl3ꢂ: ppm 197.3, 185.1, 167.5, 149.5, 148.6, 146.7,
138.4, 137.4, 136.8, 135.0, 131.7, 130.2, 127.9, 123.4,
123.4, 122.9, 119.7, 100.8, 28.9. HRMS-FAB+ calcd for
C41H31IrN2O4: 808.1913, found: 808.1918.
the Ir(III) complexes 1–5. The ligands were synthesized
in moderate yields via the Stille and Suzuki cross cou-
pling reactions. The Ir(III)-complexes were obtained by
cyclometalation of the ligands with IrCl3 and subse-
quent reaction with acetylacetone, 1,3-diphenylpropane-
1,3-dione, 2,2,6,6-tetramethylheptane-3,5-dione.
Figure 1 shows the UV-vis absorption and photo-
luminescense spectra of these phosphors in dilute dichloro-
methane solution. A summary of photophysical data of the
red emitters were given in Table I. For all complexes, the
absorption band below 350 nm can be assigned to the spin-
Synthesis of 3. (0.45 g, 25%). 1H NMR (300 MHz,
CDCl3ꢂ: ꢃ ppm 9.00 (s, 2 H), 8.27 (dd, J = 2ꢁ1, 8.5 Hz,
2 H), 7.99 (d, J = 8ꢁ5 Hz, 2 H), 7.69–7.65 (m, 6 H), 7.56
(dd, J = 1ꢁ3, 8.5 Hz, 4 H), 7.34 (d, J = 7ꢁ3 Hz, 2 H),
7.22–7.13 (m, 10 H), 6.91 (td, J = 1ꢁ2, 7.3 Hz, 2 H), 6.81
(td, J = 1ꢁ3, 7.4 Hz, 2 H), 6.62 (s, 1 H), 6.41 (d, J =
7ꢁ6 Hz, 2 H). 13C NMR (75 MHz, CDCl3ꢂ: ꢃ ppm 192.6,
180.2, 171.9, 150.6, 149.9, 144.0, 141.3, 139.0, 136.8,
133.6, 132.9, 131.1, 130.5, 130.4, 129.6, 128.6, 128.4,
126.6, 125.9, 121.6, 118.7, 96.0. HRMS-FAB+ calcd for
C41H31IrN2O4: 932.2226, found: 932.2223.
allowed ꢅ–ꢅ∗ transitions of the cyclometalated ligands.
1
Also, in the absorption spectra of all complexes, there was
a weak but well-resolved peak near 400 nm with lower
extinction coefficients, which can be interpreted as aris-
ing from a spin-orbit coupling, together with an admix-
1
3
ture of MLCT and MLCT states.6 The Ir(III) complexes
1–5 show absorption peak over 450 nm region at 474,
523, 513, 528, and 517 nm, respectively. It means that
3MLCT/3ꢅ–ꢅ∗ transition occurs in these complexes due
to strong spin-orbit coupling from Ir(III). The difference
of the spectra between L2 and corresponding iridium com-
plexes (2, 3, and 4) implied that ligand has been coor-
dinated successfully with iridium ion. In PL spectra at
278 K, the maximum emission peaks for complexes 1–5
were located from 596 to 630 nm in the red region of the
Synthesis of 4. (0.40 g, 68%). 1H NMR (500 MHz,
CDCl3ꢂ: ꢃ ppm 8.72 (d, J = 1ꢁ8 Hz, 2 H), 8.19 (dd, J =
1ꢁ9, 8.5 Hz, 2 H), 7.96 (d, J = 8ꢁ5 Hz, 2 H), 7.77 (d, J =
7ꢁ2 Hz, 4 H), 7.65 (d, J = 7ꢁ5 Hz, 2 H), 7.58 (d, J = 7ꢁ5 Hz,
2 H), 7.50–7.45 (m, 4 H), 6.88 (t, J = 7ꢁ4 Hz, 2 H),
6.77 (t, J = 7ꢁ4 Hz, 2 H), 6.44 (d, J = 7ꢁ5 Hz, 2 H),
5.46 (s, 1 H), 0.65 (s, 18 H). 13C NMR (75 MHz,
J. Nanosci. Nanotechnol. 14, 6128–6132, 2014
6129

Efficient Red Phosphorescent OLEDs Using Ir(III) Complexes Based on Bezoylphenylpyridine
Table I. Optical properties of complexes 1–5.
Hwang et al.
O
N
O
O
O
Compound
UV [nm]a
1
2
3
4
5
O
O
O
Sn(Bu3
)
Ir
2
ii
285,
474
289,
330,
523
619
587
285,
304,
513
617
578
283,
320,
528
630
586
283,
330,
517
618
585
i
iii
iii
N
1
2
N
Br
O
N
L1
O
O
PL [nm]a
PL [nm]b
FWHM
HOMO [eV]c
LUMO [eV]
Eg
596
570
84
−5.59
−3.23
2.36
2.17
0.06
O
O
Ir
2
B(OH)2
76
93
94
84
O
−5.56
−3.34
2.22
2.11
0.29
−5.54
−3.33
2.21
2.14
0.31
−5.47
−3.11
2.36
2.11
0.22
−5.72
−3.45
2.27
2.12
0.25
O
O
ii
O
N
iv
N
iii
ET
ꢆ [%]d
O
O
Ir
2
L2
O
Notes: aMeasured in CH2Cl2 (1 × 10−5 M at 278 K); bMeasured in 2-Me THF
(1 × 10−5 M at 77 K); cThe energy levels were measured with a low-energy
photo-electron spectrometer (Riken-Keiki, AC-2); dDegassed CH2Cl2 relative to
fac-[Ir(ppy)3] (ꢆp = 0ꢁ40), ꢇex = 470 nm.
3
O
O
O
N
N
iii
Br
O
O
Ir
2
B(OH)2
O
N
4
5
visible spectrum. Compared with complex 2, complex 1
showed a 16 nm blue shift of the maximum peak in the
emission spectra, while the maximum emission peak of
complex 4 red-shifted by 11 nm. This observation means
that HOMO/LUMO energy levels are very sensitive to the
subtle change of substituents in ligands, due to the dif-
ferences in electron donating/withdrawing properties, the
conjugation effects and field strength of ligand.12ꢀ13 To
determine the triplet energy levels, the PL of 1–5 were
measured at 77 K. As shown in Table I, triplet energies
O
O
O
N
iv
ii
iii
O
O
Ir
2
L3
Scheme 1. Synthetic route to Ir(III) complexes 1–5. Conditions:
(i) Pd(PPh3ꢂ4, LiCl, Toluene; (ii) IrCl3 · H2O, H2O, 2-ethoxyethanol;
(iii) Na2CO3, 2-ethoxyethanol; (iv) Pd(PPh3ꢂ4, LiCl, Toluene.
(a)
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of 1–5 (2.11∼2.17 eV) were lower than that of host mate-
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1.0
0.8
0.6
0.4
0.2
0.0
1
rial (CBP: T1 = 2ꢁ60), which indicated that energy trans-
Copyrig2ht: American Scientific Publishers
fer from host (CBP) to red Ir-complexes 1–5 would be
3
effective. The emission quantum yields of complexes 1–5
were 0.06, 0.29, 0.31, 0.22, and 0.25 respectively, as deter-
mined with Ir(ppy)3 (0.40), used as a reference. Interest-
ingly, the position electron-withdrawing benzoyl group in
ligand of iridium complexes affected greatly the quantum
yields (ꢆ = 0ꢁ06 for 1 ver ꢆ = 0ꢁ29 for 2).
4
5
L2
The HOMO energy levels were measured with an AC-
2 photoelectron spectrometer and the LUMO energy lev-
els calculated by subtracting the corresponding optical
band gap energies from the HOMO values. The HOMO
and LUMO energy levels for complexes 1–5 varied from
−5ꢁ47 to −5ꢁ72 eV and −3ꢁ11 to −3ꢁ45 eV, respectively.
To explore the electroluminescent properties of these
molecules, devices were fabricated with a structure of
ITO/2-TNATA (60 nm)/NPB (20 nm)/CBP: 8% red Ir(III)
complexes 1–5 (30 nm)/BCP (10 nm)/Alq3 (20 nm)/Liq
(2 nm)/Al and their electroluminescent performance char-
acteristics of devices are summarized in Table II. As shown
in Figure 2, the EL spectra of devices A–E were red emis-
sions with the maximum peaks at 587–614 nm, respec-
tively. Interestingly, the maximum emission peaks of 1–5
in devices A–E were blue-shifted as compared to those in
CH2Cl2 solution. Iridium complexes 1–5 were surrounded
with CBP host in devices A–E, while those were solvated
with CH2Cl2 in solution states. These differences in sol-
vation between devices and CH2Cl2 solution states would
300
400
500
600
Wavelength [nm]
(b)
1: 77 K
2: 77 K
3: 77 K
4: 77 K
5: 77 K
1: 293 K
2: 293 K
3: 293 K
4: 293 K
5: 293 K
1.0
0.8
0.6
0.4
0.2
0.0
550
600
650
700
750
800
Wavelength [nm]
Figure 1. (a) The absorption and (b) emission spectra of Ir(III) com-
plexes (1–5).
6130
J. Nanosci. Nanotechnol. 14, 6128–6132, 2014

Hwang et al.
Efficient Red Phosphorescent OLEDs Using Ir(III) Complexes Based on Bezoylphenylpyridine
Table II. EL performance characteristic of the devices A–E.
Device
Dopant
EL (nm)
Voan (V)
Lb (cd/m2ꢂ
LEc/d (cd/A)
PEc/d (lm/W)
EQEc/d (%)
CIE (xꢀy)
A
B
C
D
E
1
2
3
4
5
587
606
598
614
606
4.0
4.0
4.5
3.5
4.0
4501
5994
5805
7088
16900
10.4/5.32
16.7/11.0
12.8/8.80
8.52/7.43
16.3/12.8
8.32/1.76
11.3/3.10
9.26/2.87
7.36/2.71
11.9/4.48
5.02/2.54
10.5/6.55
6.57/4.33
7.16/5.54
13.0/9.65
(0.557, 0.438)
(0.617, 0.383)
(0.595, 0.402)
(0.633, 0.364)
(0.609, 0.390)
Notes: aTurn-on voltage at 1 cd/m2; bMaximum luminance; cMaximum values; dAt 20 mA/cm2; eCIE values are measured at 12 V.
350
contribute the blue-shifts effect in EL spectra compared to
PL spectra of 1–5. Among devices A–E, device D using
complex 4 showed the deepest red emissions in the EL
spectrum, which is well compatible with the trends of PL
studies of 1–5 in CH2Cl2 solution. The CIE coordinates of
device D were (0.633, 0.364) with the luminous efficiency
of 7.43 cd/A, the external quantum efficiency (EQE) of
5.54% at 20 mA/cm2.
(a)
Current density
Device 1
10000
1000
100
10
300
250
200
150
100
50
Device 2
Device 3
Device 4
Device 5
Luminance
Device 1
Device 2
Device 3
Device 4
Device 5
The current density–voltage–luminance (J–V –L) and
luminous efficiency (LE) characteristics of devices A–E
are shown in Figure 3. The turn-on voltages for devices
A–E were 4.0, 4.0, 4.5, 3.5, and 4.0 V, respectively, which
indicated that the carrier injection into the emitting layer
of device D was more effective than in devices A, B, C
and E. Compared to the other complexes, complex 4 has the
lowest HOMO energy level. Thus the direct hole-trapping
process from NPB to dopant in device D is more effective
0
1
0
2
4
6
8
10
12
Voltage [V]
18
16
14
12
10
8
(b)
A
B
C
D
E
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than those in the other devices. This would lead to the low
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turn-on voltage of device D, compared to the other devices.
Copyright: American Scientific Publishers
4
Among devices A–E, device E showed the extraordi-
2
0
nary luminance and displayed excellent EL performances
with a LE, PE, and EQE of 12.8 cd/A, 4.48 lm/W and
9.65% at 20 mA/cm2, respectively. Hence, compared to
device B using complex 2, device E using complex 5 with
the additional methyl group in ligand possessed improved
EL efficiencies. Interestingly, upon the changes of ancil-
lary ligands of iridium complexes from acetylacetone
to 1,3-diphenylpropane-1,3-dione and 2,2,6,6-tetramethyl-
heptane-3,5-dione, the EL efficiencies of devices using
them as dopants were decreased greatly. For example,
compared to device B, the EQE of devices C and D at
0
50
100
150
Current density [mA/cm2]
Figure 3. (a) J–V –L curve and (b) LE versus current density of devices
A–E.
20 mA/cm2 were reduced by 33.9 and 15.4%, respec-
tively. Also, the position electron-withdrawing benzoyl
group in ligand of iridium complexes affected greatly the
external quantum efficiencies of devices using them as
dopants (EQE = 2ꢁ54% for device A vs. EQE = 6ꢁ55%
for device B). These observations reflect the complicated
aspects on the nature of electroluminescences in phospho-
rescent dopant/host emitting systems in the amorphorus
solid states. Presumably, the changes of electronic and
structural properties of the emitting materials by the mod-
ification of ligands of iridium complexes would affect the
physical properties of those such as the quantum yields,
HOMO/LUMO energy levels and the carrier mobilities,
which play important roles in OLED device performances.
1.0
A
B
0.8
0.6
0.4
0.2
0.0
C
D
E
500
550
600
650
700
750
4. CONCLUSION
Red phosphorescent Ir(III) complexes 1–5 based on
benzoylphenylpyridine derivative units and the various
Wavelength [nm]
Figure 2. EL spectra of devices A–E.
J. Nanosci. Nanotechnol. 14, 6128–6132, 2014
6131

Efficient Red Phosphorescent OLEDs Using Ir(III) Complexes Based on Bezoylphenylpyridine
Hwang et al.
ancillary ligands such as pentan-1,3-dione, 1,3-diphenyl-
propane-1,3-dione and 2,2,6,6-tetramethylheptane-3,5-
dione were synthesized and their electroluminescent
properties were investigated. Multilayered OLEDs employ-
ing the benzoylphenylpyridine derivatives as the dopant
materials were found to have highly efficient EL perfor-
mance. Also, this study suggests that the subtle structural
modification of ligands of iridium complexes would lead
the great improvement of the EL performances of OLED
devices using them.
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Received: 20 May 2011. Accepted: 27 July 2012.
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