Ir(III) phosphors modified with fluorine atoms in pyridine-1,2,4-triazolyl ligands for efficient OLEDs possessing low-efficiency roll-off

Article abstract of DOI:10.1021/acs.organomet.6b00753

Five neutral heteroleptic Ir(III) complexes 1-5 using the same cyclometalated ligand and different pyridine-1,2,4-triazolyl derivatives as ancillary ligands with fluorine substituents attached, were rationally designed and prepared. Their photophysical, electrochemical, and thermal properties were studied, and theoretical calculations were performed to understand the emission behaviors as well. Introducing fluorine atoms has little effect on the photophysical and thermal properties, but the performances of the resulting devices can be fine-tuned. Among them, a heavy doping level device employing a phosphor with five fluorine atoms delivers superior device efficiencies with η_c = 32.6 cd A^-1 and η_p = 27.6 lm W^-1, respectively, which is higher than those of other counterparts. Importantly, such a device exhibits almost negligible roll-off in luminance efficiency. Despite nondoped devices achieving good EL performance, more fluorine atoms lead to a relatively higher efficiency roll-off. The results suggest that rational incorporation of fluorine atoms into the ancillary ligands can significantly improve the performance of devices with features of high efficiency and small roll-off.

Full text of DOI:10.1021/acs.organomet.6b00753

Article
pubs.acs.org/Organometallics
Ir(III) Phosphors Modified with Fluorine Atoms in Pyridine-1,2,4-
triazolyl Ligands for Efficient OLEDs Possessing Low-Efficiency Roll-
off
Hui-Ting Mao,† Chun-Xiu Zang,‡ Li−Li Wen,† Guo-Gang Shan,*,† Hai-Zhu Sun,† Wen-Fa Xie,*,‡
and Zhong-Min Su*,†
†Institute of Functional Material Chemistry and National & Local United Engineering Lab for Power Battery, Faculty of Chemistry,
Northeast Normal University, Changchun 130024, People’s Republic of China
‡State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun,
Jilin 130012, People’s Republic of China
S
* Supporting Information
ABSTRACT: Five neutral heteroleptic Ir(III) complexes 1−5
using the same cyclometalated ligand and different pyridine-
1,2,4-triazolyl derivatives as ancillary ligands with fluorine
substituents attached, were rationally designed and prepared.
Their photophysical, electrochemical, and thermal properties
were studied, and theoretical calculations were performed to
understand the emission behaviors as well. Introducing
fluorine atoms has little effect on the photophysical and
thermal properties, but the performances of the resulting
devices can be fine-tuned. Among them, a heavy doping level
device employing a phosphor with five fluorine atoms delivers
superior device efficiencies with ηc = 32.6 cd A−1 and ηp = 27.6
lm W−1, respectively, which is higher than those of other
counterparts. Importantly, such a device exhibits almost
negligible roll-off in luminance efficiency. Despite nondoped devices achieving good EL performance, more fluorine atoms
lead to a relatively higher efficiency roll-off. The results suggest that rational incorporation of fluorine atoms into the ancillary
ligands can significantly improve the performance of devices with features of high efficiency and small roll-off.
energy transfer and avoid self-quenching.4 5 However, effica-
b,
1. INTRODUCTION
cious doping often requires careful control of the doping
concentration, resulting in a restriction on their reproducibility
Organic light-emitting diodes (OLEDs) have drawn extensive
attention in recent years owing to their foreseeable effect in
in mass production, which needs high consistency for
both flat-panel displays and solid-state lighting.1 Among the
production quality.5c,6 In addition, the potential phase
related reports, phosphorescent OLEDs have continued to
attract significant research interest because of their high
intrinsic efficiency. Phosphorescent heavy-metal organic com-
plexes give an advantage over their fluorescent counterparts, in
separation occurring in blended systems usually leads to poor
energy transfer and device degradation over time.4 5c,7
b,
Recently, significant efforts have been made to develop high-
performance Ir(III)-based OLEDs with a nondoped and/or
which both the electrogenerated singlet and triplet excitons
heavy doping level emitting layer. Nondoped OLEDs have the
forming on charge recombination can be harvested and decay
benefit of being able to overcome the problems mentioned
above, as the EML is only composed of a single Ir(III)
radiatively, theoretically leading to 100% internal quantum
efficiency.1,2 Among these, Ir(III) complexes are of particular
complex; thus, the device performance apparently depends on
importance as a series of outstanding phosphorescent materials
due to appropriate exciton lifetimes, high luminescence
material performance.4 8 Additionally, heavy doping level
b,
OLEDs could also make a sophisticated control of the doping
quantum yields, and facile color tuning through control of
the ligand structure.3 Nevertheless, a number of challenges
process unnecessary, which is greatly beneficial for improving
the product yield of commercial mass production in the
remain; in particular, quenching of the luminescence caused by
future.8,9 In 2002, Chen et al. designed and synthesized a new
intermolecular interactions and typically poor carrier mobility
complex based on 2,3-(bis-N,N-1-naphthylphenylamino)-N-
result in substantial deficits, which prevent them from being
used in an undiluted form as the emission layer (EML).4
Therefore, most high-performance devices based on Ir(III)
phosphors have to disperse into a suitable host to improve the
Received: September 24, 2016
© XXXX American Chemical Society
A
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ethylmaleimide, giving a red emission nondoped device with
state of the art efficiency and luminance.10 Peng et al. reported
the nondoped phosphorescent Ir(III) complex (ppy)2Ir(dipig),
which realized a respectable external quantum efficiency (EQE)
up to 18%, accompanied by outstanding power efficiency as
high as 70 lm W−1.6c Our group and others have demonstrated
a series of excellent Ir(III) complexes containing 1,2-diphenyl-
H-benzoimidazole (Hpbi) for the purpose of understanding the
relationship between molecular structures and device perform-
ances, in which Hpbi-type ligands can be easily synthesized and
modified in relatively high yield.11 The fabricated OLEDs not
only exhibited promising performance but also displayed little
efficiency roll-off at high brightness.3f Despite the achieved
advances, constructing Ir(III) complexes capable of giving high-
performance OLEDs with nondoped and/or highly doped
features still remains a significant challenge to date.
Recently, phosphorescent emitters with the incorporation of
the simple electron-withdrawing character of fluorine atoms in
cyclometalated ligands were employed to tune the emission
colors to a high-energy region. The corresponding reports
indicated that fluorinated substituents in the cyclometalating
ligand may impose severe limitation on the long-term device
stability.12 However, such a design strategy can enhance the
photoluminescence (PL) efficiency of phosphors due to the
lower vibrational frequency of C−F bonds.13 Fluorine atoms
readily form strong inter-/intramolecular interactions with the
neighboring molecules, thereby controlling the solid-state
packing and intrinsic characters. In addition, adding fluorine
atoms indeed makes it possible to decorate with the ancillary
ligands reasonably. Nevertheless, such a design strategy with
the aim of tuning the properties of phosphorescent emitters to
furnish highly efficient devices has not yet been well
interpreted.
2-(3-(perfluorophenyl)-1H-1,2,4-tr-iazol-5-yl)pyridine, respec-
tively. Two contributing factors of the ancillary ligand are as
follows: (i) easy synthetic chemical accessibility and (ii)
exceedingly large ππ* energy gaps allowing the cyclometalated
ligand to tune the final emission color.11b,14 Thereby, it would
be ideal to understand the influence of the fluorine substituents
on the device performance while maintaining similar structures
and the same emission colors. Complexes 1−5 exhibited strong
emission at 498, 497, 495, 496, and 494 nm at room
temperature, respectively. Both heavy doping level (20%) and
nondoped devices employing 1−5 as emitting layers have been
fabricated. Increasing the numbers of fluorine substituents only
slightly affects the photophysical and thermal properties, but
the device performance has been significantly enhanced. A
heavy doping level (20%) device based on complex 5,
incorporating five fluorine substituents, reveals comparable
efficiencies with ηc = 32.6 cd A−1 and ηp = 27.6 lm W−1,
respectively. It is worth noting that the fabricated devices show
little roll-off in efficiency with increasing brightness.
2. EXPERIMENTAL SECTION
2.1. General Information. The reagents used in this work were
commercially available. The solvents were purified by a conventional
1
procedure and freshly distilled under argon before use. The H NMR
spectra were confirmed using a Bruker Avance 500 MHz instrument,
in which CDCl3 was used as the solvent and tetramethylsilane (TMS)
as the internal reference. The mass spectra of the emitters were
recorded on an electrospray ionization mass spectrometer (ESI-MS).
The absorption and photoluminescence spectra were observed with
a Cary 500 UV−vis−NIR spectrophotometer and FL-4600 FL
spectrophotometer, respectively. The thin films of 1−5 were prepared
by spin-coating in CH2Cl2 solution at a concentration at 20 mg mL−1.
The excited-state lifetimes (τ) and photoluminescence quantum yields
(Φp) in solution from the samples were recorded on a spectro-
fluorimeter (Edinburgh FLS-920). The thermal gravimetric analysis
(TGA) was carried out on a Perkin-Elmer TG-7 analyzer through
heating the samples from 50 to 800 °C under a nitrogen atmosphere.
2.2. Synthesis of Ligands Used in This Work. The cyclo-
metalated ligand 2-(4-tert-butylphenyl)-1-phenyl-1H-benzo[d]-
imidazole (tbupbi) and the ancillary ligands L1−L5 (see Scheme S1
in the Supporting Information) were synthesized according to
previous reports.12,15
With this fundamental concern, we designed and synthesized
a series of phosphorescent Ir(III) complexes 1−5 (Scheme 1)
consisting of pyridine-1,2,4-triazolyl derivatives as ancillary
ligands modified with electron-withdrawing fluorine atoms. The
ancillary ligands L1−L5 are 2-(3-(3,5-dimethylphenyl)-1H-
1,2,4-triazol-5-yl)pyridine, 2-(3-phenyl-1H-1,2,4-triazol-5-yl)-
pyridine, 2-(3-(4-fluorophenyl)-1H-1,2,4-triazol-5-yl)pyridine,
2-(3-(3,5-difluorophenyl)-1H-1,2,4-triazol-5-yl)pyridine, and
2.3. Synthesis of Complexes 1−5. 2.3.1. Synthesis of Complex
1. One equivalent of IrCl3·3H2O (0.35 g, 1 mmol) and 2.2 equiv of
tbupbi (0.71 g, 2.20 mmol) were dissolved in 2-ethoxyethanol (30
mL) and water (10 mL), and the mixture was stirred under an argon
atmosphere for 24 h. After it was cooled, the mixture was washed with
water and ethanol and the formed precipitate was filtered, assumed to
be a chloro-bridged dimer. Without further purification, [Ir-
(tbupbi)2Cl]2 (0.24 g, 0.16 mmol) and ligand L1 (0.09 g, 0.40
mmol) were refluxed in a mixture of dichloromethane and ethanol for
12 h. After it was cooled to room temperature, the reaction mixture
was extracted with CH2Cl2 and then dried over anhydrous Na2SO4.
The product was obtained by silica gel column chromatography with
1/20 (v/v) ethyl acetate/dichloromethane as eluent to give complex 1
Scheme 1. Synthesis of Ir(III) Complexes 1−5
1
(0.20 g) in a yield of 67%. H NMR (500 MHz, CDCl3, δ [ppm]):
8.33 (d, J = 8.0 Hz, 1H), 7.82−7.87 (m, 4H), 7.57−7.68 (m, 8H), 7.37
(t, J = 8.5 Hz, 2H), 6.98−7.13 (m, 6H), 6.86−6.91 (m, 2H), 6.62−
6.67 (m, 2H), 6.57−6.60 (m, 2H), 6.50−6.54 (m, 3H), 5.98 (d, J = 8.0
Hz, 1H), 2.33 (s, 6H), 0.93 (s, 9H), 0.92 (s, 9H). MS: calcd m/z
1092.4 for [M]+ (C61H55IrN8), found m/z 1092.4. Anal. Calcd for
C61H55IrN8: C, 67.07; H, 5.07; N, 10.26. Found: C, 66.98; H, 5.12; N,
10.32.
2.3.2. Synthesis of Complex 2. The product was obtained by silica
gel column chromatography with a 1/20 (v/v) ethyl acetate/
dichloromethane mixture as eluent to give complex 2 (0.20 g) in a
yield of 69%. 1H NMR (500 MHz, CDCl3, δ [ppm]): 8.32 (d, J = 8.5
B
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Hz, 1H), 8.20 (d, J = 7.5 Hz, 2H), 7.81−7.88 (m, 2H), 7.55−7.70 (m,
8H), 7.36 (t, J = 7.5 Hz, 4H), 7.26 (s, 1H), 6.98−7.15 (m, 6H), 6.89
(t, J = 8.0 Hz, 1H), 6.63−6.67 (m, 2H), 6.60 (d, J = 8.5 Hz, 1H),
6.51−6.56 (m, 4H), 5.99 (d, J = 8.0 Hz, 1H), 0.92 (s, 9H), 0.92 (s,
9H). MS: calcd m/z 1064.4 for [M]+ (C59H51IrN8), found m/z 1064.4.
Anal. Calcd for C59H51IrN8: C, 66.58; H, 4.83; N, 10.53. Found: C,
66.65; H, 4.92; N, 10.60.
matrix least squares with the SHELXL-97 program.19 The non-
hydrogen atoms were located by successive difference Fourier
techniques. Hydrogen atoms were placed at calculated locations
without further refinement of the parameters. Selected parameters of
the molecular structure are given in Table S1 in the Supporting
Information.
2.3.3. Synthesis of Complex 3. The product was obtained by silica
gel column chromatography with 1/20 (v/v) ethyl acetate/dichloro-
methane and then 1/10 (v/v) ethyl acetate/dichloromethane as eluent
3. RESULTS AND DISCUSSION
3.1. Single-Crystal Structure. Figure 1 reports the X-ray
structure of 5. For complex 5, the Ir(III) ion exhibits a
1
to give complex 3 (0.22 g) in a yield of 73%. H NMR (500 MHz,
CDCl3, δ [ppm]): 8.30 (d, J = 7.5 Hz, 1H), 8.17 (s, 2H), 7.89 (d, J =
5.0 Hz, 1H), 7.83 (t, J = 7.5 Hz, 1H), 7.54−7.69 (m, 8H), 7.36 (t, J =
8.0 Hz, 2H), 6.98−7.15 (m, 8H), 6.89 (t, J = 8.0 Hz, 1H), 6.62−6.67
(m, 2H), 6.50−6.56 (m, 5H), 5.98 (d, J = 7.5 Hz, 1H), 0.92 (s, 9H),
0.92(s, 9H). MS: calcd m/z 1082.4 for [M]+ (C59H50FIrN8), found m/
z 1082.4. Anal. Calcd for C59H50FIrN8: C, 65.47; H, 4.66; N, 10.35.
Found: C, 65.41; H, 4.76; N, 10.43.
2.3.4. Synthesis of Complex 4. The product was obtained by silica
gel column chromatography with 1/20 (v/v) ethyl acetate/dichloro-
methane and then 1/10 (v/v) ethyl acetate/dichloromethane as eluent
1
to give complex 4 (0.21 g) in a yield of 70%. H NMR (500 MHz,
CDCl3, δ [ppm]): 8.30 (d, J = 8.0 Hz, 1H), 7.90 (d, J = 5.0 Hz, 1H),
7.84 (t, J = 7.5 Hz, 1H), 7.55−7.73 (m, 10H), 7.36 (t, J = 9.0 Hz, 2H),
7.09−7.16 (m, 3H), 6.99−7.05 (m, 3H), 6.90 (t, J = 7.5 Hz, 1H),
6.63−6.70 (m, 3H), 6.49−6.55 (m, 5H), 5.98 (d, J = 8.0 Hz, 1H), 0.92
(s, 9H), 0.92 (s, 9H). MS: calcd m/z 1100.4 for [M]+ (C59H49F2IrN8),
found m/z 1100.3. Anal. Calcd for C59H49F2IrN8: C, 64.40; H, 4.49; N,
10.18. Found: C, 64.47; H, 4.54; N, 10.25.
2.3.5. Synthesis of Complex 5. The product was obtained by silica
gel column chromatography with 1/4 (v/v) ethyl acetate/dichloro-
methane and then 1/1 (v/v) ethyl acetate/dichloromethane as eluent
to give complex 5 (0.22 g) in a yield of 70%.1H NMR (500 MHz,
CDCl3, δ [ppm]): 8.28 (d, J = 7.5 Hz, 1H), 7.91 (d, J = 5.5 Hz, 1H),
7.84 (t, J = 7.5 Hz, 1H), 7.56−7.70 (m, 8H), 7.38 (d, J = 7.5 Hz, 1H),
7.34 (d, J = 8.0 Hz, 1H), 7.12−7.20 (m, 4H), 7.05 (d, J = 8.0 Hz, 1H),
7.00 (d, J = 8.0 Hz, 1H), 6.96 (t, J = 7.5 Hz, 1H), 6.66−6.68 (m, 1H),
6.47−6.58 (m, 6H), 6.06 (d, J = 8.0 Hz, 1H), 0.92 (s, 9H), 0.87 (s,
9H). MS: calcd m/z 1154.3 for [M]+ (C59H46F5IrN8), found m/z
1154.3. Anal. Calcd for C59H46F5IrN8: C, 61.39; H, 4.02; N, 9.71.
Found: C,61.42; H, 3.93; N, 9.77.
2.4. Theoretical Calculations. The density functional theory
(DFT) calculations on electronic states of complexes 1−5 were
obtained by B3LYP methods.16 A double-ξ quality basis set containing
LANL2DZ was employed for Ir atoms, while a 6-31G* basis set was
used for other atoms. Solvent effects play a crucial role in predicting
the absorption and PL spectra with time-dependent DFT (TD-DFT)
calculations.17 All calculations were obtained from the Gaussian 09
software package.18
Figure 1. ORTEP drawing of complex 5. Thermal ellipsoids are drawn
at 30% probability. The solvent molecules and hydrogen atoms are
omitted for clarity.
distorted-octahedral geometry with cis-C,C and trans-N,N, as
found for previously reported cyclometalated Ir(III) com-
plexes.20 The corresponding crystallographic characteristics and
structure parameters are given in Table S1 in the Supporting
Information. The associated Ir−N distances on the ancillary
ligand (Ir1−N6 = 2.142(5) Å, Ir1−N5 = 2.136(6) Å) are
significantly longer than those on the tbupbi ligand (Ir1−N1 =
2.049(5) Å, Ir1−N3 = 2.053(5) Å) because of the stronger
electron-withdrawing abilities of triazole, confirming the
anticipated σ donation of the carbons. In addition, owing to
the steric hindrance of more fluorine atoms, the phenyl ring
twists to the opposite position with respect to the triazole ring
(N7−C53−C54−C55) with a value of 99.75°.
2.5. Electrochemical Measurements. The reference electrode
was an aqueous saturated calomel electrode, the working electrode was
a glassy-carbon electrode, and the counter electrode was a platinum
electrode. Cyclic voltammetric (CV) measurements were reported on
a BAS 100 W instrument in CH2Cl2 at a concentration of 10−3 M.
Ferrocene was used as the internal standard. The electrochemical data
were obtained under nitrogen in CH2Cl2.
2.6. Device Fabrication and Characterization. Before they
were loaded into a deposition chamber, glass substrates were cleaned
ultrasonically with water, dried using an oven, and coated with indium
tin oxide (ITO). Subsequently, various organic layers were deposited
on the ITO-coated glass substrates by thermal evaporation. The active
area of the device was set to 10 mm2, as defined by the shadow mask
used for the cathode. The current density−voltage−-luminance (J−V−
L) and current efficiency−luminance−power efficiency (ηc−L−ηp)
properties were obtained from both a programmable Keithley 2400
source meter and a Minolta LS-110 luminance meter.
3.2. Photophysical Properties. Figure 2a depicts the
absorption spectra of complexes 1−5 observed in CH2Cl2
solution (10−5 M) at room temperature. All complexes exhibit
similar spectral features. The intense absorptions below 380 nm
1
are the result of spin-allowed π−π* ligand-centered (1LC)
transitions, whereas the weaker absorptions in the low-energy
region (above 400 nm) can be attributed to 1MLCT (metal-to-
1
ligand charge transfer), LLCT (ligand-to-ligand charge trans-
1b,21
3
3
3
fer), and MLCT, LLCT, LC transitions.
The strong
spin−orbit coupling (SOC) endowed by the Ir(III) center
partially allows the triplet emission, enabling the enhancement
of phosphorescence efficiencies.21a
The PL spectra of 1−5 are shown in Figure 2b, and the
relevant data are collected in Table 1. Almost identical spectral
patterns with emission maxima at 498, 497, 495, 496, and 494
nm are observed at room temperature for 1−5, respectively.
The measured lifetimes of the excited state are on the order of
microseconds with values of 0.43, 0.50, 0.54, 0.45, and 0.59 μs,
respectively, which is the signature of phosphorescence for
2.7. X-ray Crystallography. The corresponding crystallographic
data for complex 5 were collected on a Bruker Apex CCD II area-
detector diffractometer. Absorption corrections were performed with
multiscan techniques. The structure of complex 5 was refined by full-
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Figure 2. (a) UV−vis absorption spectra of complexes recorded in CH2Cl2 solution. (b) PL spectra of Ir(III) complexes 1−5 at 298 K. (c) Low-
temperature (at 77 K) PL spectra in CH2Cl2 solution. (d) Neat film PL spectra of 1−5.
Table 1. Photophysical Properties and Thermodynamic Stabilities of Complexes 1−5
a c
ac
,
d
e
f
g
complex
λPL,max −, (nm)
Φp (%)
τ
(μs)
Eg (eV)
Eox (V)
HOMO (eV)
LUMO (eV)
Td (°C)
1
2
3
4
5
498, 488, 511
497, 487, 511
495, 486, 512
496, 487, 511
494, 484, 500
13
20
18
14
0.89, 0.43
0.89, 0.50
1.25, 0.54
1.14, 0.45
1.04, 0.59
2.23
2.22
2.22
2.23
2.22
0.41
0.38
0.38
0.44
0.47
−5.21
−5.18
−5.18
−5.24
−5.27
−2.98
−2.96
−2.96
−3.01
−3.05
407
391
422
398
394
13
b
a
c
d
e
Measured in CH2Cl2 solution at 298 K. Measured in CH2Cl2 solution at 77K. Measured in spin-coated thin film. Optical band gap. Measured
by CV using ferrocene as the internal standard. Calculated from the onset oxidation potentials of the compounds. Estimated by using the empirical
f
g
equation ELUMO = EHOMO + Eg.
complexes 1−5.22 As illustrated in Figure 2b, vibronic
structures are clearly observed for complexes 1−5 at room
temperature, indicating that their emissive excited states should
contain the 3LC excited state. Further comparison of the
spectra at different temperatures shows that, in frozen glassy
solvents at 77 K, the vibronic structure becomes better resolved
and emission maxima exhibit apparent rigidochromism (9−10
nm) relative to those at room temperature (Figure 2c). In light
of the shift in 77 K emission spectra, we speculate that both the
3
3MLCT and LLCT characters are also involved in the excited
state of complexes 1−5.23 On the basis of the above analysis, it
is believed that their emission should originate from the
3
3
3
admixture of MLCT, LLCT, and LC characters.
Figure 2d displays the PL spectra in neat films of complexes
1−5, of which manifested obvious red shifts on going from the
solutions to the neat films. This phenomenon may arise from
molecular aggregation and triplet−triplet interaction in the
solid state.24 The PLQYs of complexes 1−5 were determined
to be 13%, 20%, 18%, 14%, and 13%, respectively.
Figure 3. Electron distributions in HOMOs and LUMOs for
complexes 1−5, together with the HOMO−LUMO band gaps.
3.3. Theoretical Calculations. In order to understand the
nature of the excited states of these phosphors, density
functional theory (DFT) and time-dependent density func-
tional theory (TD-DFT) calculations were carried out using
B3LYP hybrid functional theory.16 Figure 3 shows the energy
levels and electron density distributions of the highest-occupied
molecular orbitals (HOMO) and the lowest-unoccupied
molecular orbitals (LUMO) for complexes 1−5. Almost
identical HOMOs and LUMOs with regard to the orbital
character are obtained from the DFT calculations. For complex
1, the HOMO is comprised of Ir dπ orbitals and the
phenylbenzimidazole fragment of one main ligand, there
being a small distribution from the ancillary ligand. The
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LUMO resides on one of the cyclometalated ligands. According
to the orbital distributions, the lowest triplet states (T1) of
complex 1 originate from HOMO−LUMO (77%), possessing a
mixture of 3MLCT, 3LLCT, and 3LC transitions. These
observations strongly indicate the existence of a rigidochromic
shift and fine vibronic progression of the emission spectra at 77
K, consistent with the results of photophysical studies.
Analogous conclusions are obtained for complexes 2−5. In
addition, theoretical calculations predict identical HOMO−
LUMO energy gaps for complexes (3.33 eV for 1, 3.34 eV for 2,
3.34 eV for 3, 3.36 eV for 4, and 3.37 eV for 5), further
supporting their similar emission wavelengths in experiments.
3.4. Electrochemical and Thermal Properties. The
electrochemical properties of complexes 1−5 were measured by
cyclic voltammetry (CV). All of complexes exhibited
irreversible oxidation process in CH2Cl2 solution (Figure S6
in the Supporting Information). Their onset oxidation
potentials are 0.41, 0.38, 0.38, 0.44, and 0.47 eV, respectively.
The HOMO energy levels of complexes 1−5 were in the region
of −5.18 to −5.27 eV. The LUMO levels were in the region of
− 2.96 to −3.05 eV, which were deduced from HOMO levels
and optical band gaps.25 As expected, complexes 1−5 have
similar optical characteristics, and the electrochemical gaps
found for 1 (2.23 eV), 2 (2.22 eV), 3 (2.22 eV), 4 (2.23 eV),
and 5 (2.22 eV) are in accordance with the HOMO and
LUMO energy gaps obtained from DFT calculations presented
in the last section.
Thermal stabilities of complexes 1−5 were evaluated using
TGA under nitrogen. From the TGA curves, 1−5 exhibit good
thermal stability with decomposition temperatures (Td) as high
as 407, 391, 422, 398, and 394 °C, respectively, which is
desirable for the fabrication of state of the art OLEDs by a
vacuum deposition approach. Notably, introducing fluorine
substitutions has only a relatively small influence on the thermal
stability, which is in good agreement with previous reports.12,26
3.5. Electroluminescence Properties. Doped OLEDs
employing complexes 1−5, namely A−E, were fabricated. The
devices have a configuration as follows: ITO/MoO3 (3 nm)/
TAPC (35 nm)/TCTA (5 nm)/DCZppy:Ir (20%) (20 nm)/
Bphen (40 nm)/LiF (0.5 nm)/Mg:Ag (120 nm, 15:1), in which
MoO3 and LiF serve as the hole-injecting layer (HIL) and
electron-injecting layer (EIL). TAPC is 1,1-bis[4-[N,N′-bis(p-
tolyl)amino]phenyl]cyclohexane, TCTA is 4,4′,4″-tris(N-
carbazolyl)triphenylamine, and Bphen is 4,7-diphenyl-1,10-
phenanthroline. They act as the hole-transporting layer
(HTL), electron blocking layer (EBL), and electron trans-
porting layer (ETL), respectively. The energy level diagrams of
the device and molecular structures of the compounds used are
displayed in Figure 4. Complexes 1−5 were doped into
DCZppy as the emitting layer, mainly due to the well-matched
HOMO and LUMO energy levels of complexes 1−5 with that
of DCZppy, enabling an efficient energy transfer from the host
materials to the phosphor dopants. As far as we know, lower
energy barrier between emitters and adjacent layers could make
holes/electrons inject smoothly, which is helpful for optimizing
the efficiency of devices. In this study, the HOMO/LUMO
levels of TCTA are −5.7/2.3 eV, which areall in the middle of
those of TAPC (−5.6/2.0 eV) and DCZppy (−6.05/2.56 eV).
Therefore, TCTA not only acts as a buffer gradient to facilitate
hole injection but also blocks electron drift out of the host
material, making holes and electrons effectively confined within
the light-emitting layers and thus improving device perform-
ance.
Figure 4. Energy levels and molecular structures of compounds used
in devices.
The device configuration and other characteristics are shown
in Figure 5. To fabricate the high doping concentration devices
herein, the dopant concentration of devices A−E was kept at
20%. Figure 5b depicts the electroluminescence (EL) spectra of
these devices with peaks centered at 502 nm for 1, 502 nm for
2, 501 nm for 3, 498 nm for 4, and 497 nm for 5. Coincident
with the data in the PL study, the EL spectral profile of each
device is nearly identical with that of the PL counterparts,
indicating the success in the OLED structure design for
efficient energy transfer from the host material to the
phosphors. In addition, this suggests that the emissions indeed
originate from the triplet states of the phosphors regardless of
the nature of the host materials.
Figure 5c,d describes current density−voltage−luminance
(J−V−L) characteristics and efficiencies versus luminance
curves of devices A−E. The resulting devices emit bright
green light at a low turn voltage of 3.0 V. Device A based on
complex 1 without any fluorine atoms achieves a good current
efficiency (ηc) of 24.5 cd A−1 and power efficiency (ηp) of 19.3
lm W−1. Gratifyingly, device E employing complex 5 with five
fluorine atoms as the emitting-layer exhibits much better EL
performance with ηc = 32.6 cd A−1 and ηp = 27.6 lm W−1,
respectively. The results suggest that the modification of Ir(III)
complexes using the appropriate number of fluorine groups can
construct more efficient OLEDs to some extent. The strong
intermolecular interactions occur in the host material and
emitters, caused by the added fluorine atoms, facilitating energy
transfer from the triplet of DCZppy to the Ir dopants. In
addition, it is worth noting that all devices exhibit negligible
drop-off with increasing brightness. For example, at the
practical brightness of 1000 cd m−2, a small roll-off in
luminance efficiency was observed and the power efficiency
E
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Article
Figure 5. (a) Device configurations with doped EML at a concentration of 20%. (b) EL spectra of devices A−E. (c) Current density and luminance
as a function of voltage for devices A−E. (d) Current efficiency and power efficiency versus luminance curves for devices A−E.
Figure 6. (a) Configuration of nondoped devices. (b) EL spectra of nondoped devices N1−N5. (c) Current density and luminance as a function of
voltage for N1−N5. (d) Current efficiency and power efficiency versus luminance curves for devices N1−N5.
Table 2. Summary of the EL Performance of Various Devices
a
bc
ηc (cd A−1
,
bc
ηp (lm W−1
,
b
b
device
Vturn‑on (V)
λEL (nm)
)
)
EQE (%)
Lmax (cd m−2
)
CIE ((x, y), V)
A
3.0
3.0
3.0
3.0
3.0
2.5
2.5
2.5
2.5
2.5
502
502
501
498
497
511
512
509
507
24.5, 23.5
26.8, 26.0
24.4, 24.3
26.2, 25.6
32.6, 32.6
14.8, 14.7
14.4, 14.3
16.2, 16.1
16.7, 16.3
16.1, 14.2
19.3, 15.9
24.6, 19.3
21.6, 17.1
20.2, 18.3
27.6, 23.0
13.3, 11.8
13.7, 11.7
15.2, 13.4
13.9, 13.2
8.0
8.6
7.8
8.6
10.7
4.6
4.6
5.2
5.3
5.7
29610
41500
39900
36720
32710
30390
32540
33670
34680
21450
(0.32,0.57), 5.0
(0.32,0.57), 5.0
(0.32,0.57), 5.0
(0.31,0.56), 5.0
(0.31,0.56), 5.0
(0.36,0.57), 5.0
(0.35,0.57), 5.0
(0.34,0.57), 5.0
(0.34,0.57), 5.0
(0.34,0.57), 5.0
B
C
D
E
N1
N2
N3
N4
N5
506
16.6, 10.5
a
b
c
Voltages estimated at 1 cd m−2. Maximum values of the various devices. Measured at 1000 cd m−2.
F
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Article
of devices E still remains 23.0 lm W−1 without significant decay.
If the excited-state characters of Ir(III) complexes are capable
of being well controlled, introducing the fluorine substitutions
in ancillary ligands favors of the efficiency of doped OLEDs, in
spite of a small effect on their photophysical and electro-
chemical behaviors and thermal stabilities. To the best of our
knowledge, reports concerning devices with high efficiency and
small roll-off value for high doping concentration attainable
with phosphorescent Ir(III) complexes are relatively rare.
In order to further probe their nondoped EL properties,
devices N1−N5 were fabricated with the multilayer config-
uration of ITO/MoO3 (3 nm)/TAPC (35 nm)/TCTA (5
nm)/Ir (20 nm)/Bphen (40 nm)/LiF (0.5 nm)/Mg:Ag (120
nm, 15:1) (as depicted in Figure 6a). Figure 6b displays the EL
spectra recorded at 5 V. Devices N1−N5 exhibit emissions at
506−512 nm, which resembled the PL spectra of these
phosphors. Figure 6c shows the J−V−L characteristics for the
nondoped devices N1−N5, and the relevant EL data are
collected in Table 2. All of them display low turn-on voltages of
2.5 V and the maximum value of luminance (Lmax) reaches
>20000 cd m−2. N5 shows a relatively lower Lmax value in
comparison to other nondoped devices. The phenomenon
could be partially attributed to lower Φp of the emitter. In
addition, strong intermolecular interactions among the emitters
caused by more fluorine atoms in the ligand may induce self-
quenching effects, leading to inferior luminance.
showed relatively higher efficiency roll-off, though achieving
decent performances. The results suggest that rational
incorporation of fluorine atoms in tuning the properties of
materials can significantly improve device performances
possessing high efficiency and small roll-off.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00753.
Supplemental schemes, figures, and X-ray structure data
for complex 5 (PDF)
Crystallographic data for 5 (CIF)
Calculated structures (MOL)
AUTHOR INFORMATION
Corresponding Authors
*G.-G.S.: fax, +86-431-85684009; tel, +86-431-85099108; e-
mail, shangg187@nenu.edu.cn.
■
*W.-F.X.: tel, +86-13844907598; e-mail, xiewf@jlu.edu.cn.
*Z.-M.S.: e-mail, zmsu@nenu.edu.cn.
ORCID
Zhong-Min Su: 0000-0002-3342-1966
Notes
The ηc and ηp versus luminance curves of devices N1−N5 are
illustrated in Figure 6d. Devices N1 and N2 exhibit maximum
ηc values of 14.8 and 14.4 cd A−1 and ηp values of 13.3 and 13.7
lm W−1, respectively. In contrast, complexes 3−5 with fluorine
atoms attached deliver superior device efficiencies. The ηc and
ηp values of devices N3−N5 are in the range of 16.1−16.7 cd
A−1 and 13.9−16.6 lm W−1, respectively. It is noted that all
nondoped devices except N5 exhibit little efficiency roll-off. For
instance, the efficiencies are still as high as 16.1 cd A−1 and 13.4
lm W−1 for device N3 and 16.3 cd A−1 and 13.2 lm W−1 for
device N4 at a brightness of 1000 cd m−2. The remarkable
feature of the small efficiency roll-off may be due to the
balanced charge transportation in these devices at high
brightness.27 Similar to the case for N3 and N4, device N5
based on complex 5 also reveals comparable efficiencies.
Different from the doped device based on complex 5 as well as
other nondoped counterparts, N5 shows relatively higher
efficiency roll-off values. The phenomenon indicates that the
greater number of fluorine atoms in the ligand may induce the
strong intermolecular interactions among the emitters, leading
to significant roll-off value at high luminance for the nondoped
devices.12a,28 That is to say, the ingenious modification of the
number of fluorine atoms in the ancillary ligands will be an easy
and feasible way to optimize EL devices possessing high
performance and small efficiency roll-off.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (21303012,
21273030, 51203017, 21131001, and 60937001), 973 Program
(2013CB834801), and the Science and Technology Develop-
ment Planning of Jilin Province (20130204025GX).
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