Syntheses, photoluminescence, and electroluminescence of a series of iridium complexes with trifluoromethyl-substituted 2-phenylpyridine as the main ligands and tetraphenylimidodiphosphinate as the ancillary ligand

Article abstract of DOI:10.1021/ic302510p

Five bis-cyclometalated iridium complexes with tifluoromethyl-substituted 2-phenylpyridine (ppy) at different positions of its phenyl group as the main ligands and tetraphenylimidodiphosphinate (tpip) as the ancillary ligand, 2-6 (1 is a trifluoromethyl-f

Full text of DOI:10.1021/ic302510p

Article
pubs.acs.org/IC
Syntheses, Photoluminescence, and Electroluminescence of a Series
of Iridium Complexes with Trifluoromethyl-Substituted
2‑Phenylpyridine as the Main Ligands and
Tetraphenylimidodiphosphinate as the Ancillary Ligand
Qiu-Lei Xu, Cheng-Cheng Wang, Tian-Yi Li, Ming-Yu Teng, Song Zhang, Yi-Ming Jing, Xu Yang,
Wei-Nan Li, Chen Lin, You-Xuan Zheng,* Jing-Lin Zuo, and Xiao-Zeng You
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China
S
* Supporting Information
ABSTRACT: Five bis-cyclometalated iridium complexes with
tifluoromethyl-substituted 2-phenylpyridine (ppy) at different
positions of its phenyl group as the main ligands and
tetraphenylimidodiphosphinate (tpip) as the ancillary ligand,
2−6 (1 is a trifluoromethyl-free complex), were prepared, and
their X-ray crystallography, photoluminescence, and electro-
chemistry were investigated. The number and positions of
trifluoromethyl groups at the phenyl ring of ppy greatly
affected the emission spectra of Ir3+ complexes, and their
corresponding emission peaks at 533, 502, 524, 480, and 542
nm were observed at room temperature, respectively.
Constructed with complexes 2−6 as the emitters, respectively,
the organic light-emitting diodes (OLEDs) with the structure
of indium−tin oxide/1,1-bis[4-(di-p-tolylamino)phenyl]cyclohexane (30 nm)/Ir (x wt %):bis[3,5-bis(9H-carbazol-9-yl)phenyl]-
diphenylsilane (15 nm)/1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (45 nm)/LiF (1 nm)/Al (100 nm) showed good
performances. Particularly, device G4 based on 4-trifluoromethyl-substituted complex 4 with x = 8 wt % obtained a maximum
luminance of over 39000 cd m−2 and maximum luminance efficiency (ηL) and power efficiency (ηp) of 50.8 cd A−1 and 29.0 lm
W−1, respectively. The results suggested that all of the complexes 2−6 would have potential applications in OLEDs.
introduction of electron-withdrawing trifluoromethyl groups
improved the OLED performances effectively in several ways:4
(i) fluorinated compounds can usually be sublimed with ease
for thin film deposition; (ii) the bulky CF3 substituent can
affect the molecular packing, providing steric protection around
the metal, which can suppress the self-quenching behavior; (iii)
fluorination can enhance the electron mobility and result in a
better balance of charge injection and transfer. The effect of F
atoms on the photophysics of homoleptic complexes was
evaluated by various groups. Some groups also studied various
heteroleptic fluorinated iridium complexes, which suggested
that the fluorine and trifluoromethyl groups in different
positions of the ppy ligands showed different effects on the
emission properties.4,5 However, comprehensive and deeper
studies are still needed.
INTRODUCTION
■
Phosphorescent Ir3+ complexes play an important part in
efficient organic light-emitting diode (OLED) fabrication
because of the high quantum efficiency and short lifetime of
triplet excited states.1 These Ir3+ complexes have very strong
spin−orbit coupling, which introduces intersystem crossing to
mix the singlet and triplet excited states and change the spin-
forbidden radiative relaxation from the triplet excited state to be
allowed. As a result, both singlet and triplet excitons can be
harvested for light emission and the internal quantum efficiency
of these Ir3+ complexes can achieve 100% theoretically. In
particular, phosphorescent materials based on iridium(III) 2-
phenylpyridine (ppy) derivatives have drawn much more
attention and been successfully applied in OLED fabrication
because they are efficient phosphorescent materials emitting
lights in the region of red, green, and blue, which can be tuned
by modifying the ppy main ligands as well as by introducing
diverse ancillary ligands.2
Additionally, the emission energy of heteroleptic complexes
[Ir(C^N)2(LX)] can be fine-tuned by a combination of main
(C∧N) ligands and LX types of ancillary ligands (such as acac =
acetylacetone, pic = picolinate, sal = salicylimine, iq =
Different substituted groups at the different positions of the
ppy ring in Ir3+ complexes will result in various spectrum
character. Ir3+ complexes carrying fluorinate ppy ligands were
reported with good performances,3 which suggested that the
Received: November 16, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic302510p | Inorg. Chem. XXXX, XXX, XXX−XXX

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isoquinolinecarboxylate, and bpz = pyrazolylborate).6 Our
group has reported highly efficient green and blue-green
phosphorescent OLEDs by introducing tetraphenylimidodi-
phosphinate (tpip) as an ancillary ligand for the first time.7a
Compared with the acac ligand, tpip has a stronger polar PO
bond, which may improve the electron mobility of the Ir3+
complex and benefit its OLED performances,7 which suggests
that tpip is an actually useful ancillary ligand for the
phosphorescent Ir3+ complex. To investigate the effect of the
substituted position and number of the trifluoromethyl
substituent on the phenyl group of the ppy ring in the Ir3+
complex for its OLED performance and explore the application
of tpip ligands, we synthesized a series of Ir3+ complexes with
tpip as the ancillary ligand and studied their OLED character-
istics.
using SAINT and corrected for Lorentz and polarization effects.
Absorption corrections were applied using SADABS12 supplied by
Bruker. The structures were solved by direct methods and refined by
full-matrix least squares on F2 using the program SHELXS-97.13a The
positions of the metal atoms and their first coordination spheres were
located from direct methods on E-maps; other non-H atoms were
found in alternating difference Fourier syntheses and least-squares
refinement cycles and, during the final cycles, refined anisotropically.
H atoms were placed in calculated positions and refined as riding
atoms with a uniform value of Uiso.
OLED Fabrication and Measurement. All OLEDs with an
emission area of 0.1 cm2 were fabricated on the prepatterned indium-
tin oxide (ITO)-coated glass substrate with a sheet resistance of 15 Ω
sq−1. The substrate was cleaned by ultrasonic baths in organic solvents
followed by ozone treatment for 20 min. All chemicals used for
electroluminescence (EL) devices were sublimed in vacuum (2.2 ×
10−4 Pa) prior to use. The 30 nm hole-transporting material of 1,1-
bis[4-(di-p-tolylamino)phenyl]cyclohexane (TAPC) was first depos-
ited on the ITO glass substrate. The phosphor (x wt %) and bis[3,5-
di(9H-carbazol-9- yl)phenyl]diphenylsilane (SimCP2) host were
coevaporated to form a 15 nm emitting layer from two separate
sources. Successively, 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)-
phenyl (TPBi; 45 nm), LiF (1 nm), and Al (100 nm) were
evaporated. The vacuum was less than 1 × 10−5 Pa during all material
deposition. The current density−voltage−luminance (J−V−L) char-
acteristics and current efficiency versus J curves of the devices were
measured with a computer-controlled Keithley 2400 source meter with
a calibrated silicon diode in air without device encapsulation. The EL
spectra were measured with a Hitachi F-4600 PL spectrophotometer.
On the basis of the uncorrected PL and EL spectra, the Commission
Internationale de l’Eclairage (CIE) coordinates were calculated using a
test program of the Spectra scan PR650 spectrophotometer.
EXPERIMENTAL SECTION
Materials and Measurements. All reagents and chemicals were
■
purchased from commercial sources and used without further
1
purification. H NMR spectra were measured on a Bruker AM 400
spectrometer or a Bruker AM 500 spectrometer. Mass spectrometry
(MS) spectra were obtained with an electrospray ionization (ESI)
mass spectrometer (LCQ Fleet, Thermo Fisher Scientific) or matrix-
assisted laser desorption ionization time-of-flight (MALDI-TOF) mass
spectrometer (Bruker Daltonic Inc.). Elemental analyses for C, H, and
N were performed on an Elementar Vario MICRO analyzer.
Absorption and photoluminescence (PL) spectra were measured on
a UV-3100 spectrophotometer and a Hitachi F-4600 PL spectropho-
tometer, respectively. The decay lifetimes were measured with an
Edinburgh Instruments FLS920P fluorescence spectrometer in
degassed CH2Cl2 solution and the solid state at room temperature
(RT). Cyclic voltammetry (CV) measurements were conducted on a
MPI-A multifunctional electrochemical and chemiluminescent system
(Xi’an Remex Analytical Instrument Ltd. Co., China) at RT, with a
polished platinum plate as the working electrode, platinum thread as
the counter electrode, and Ag-AgNO3 (0.1 M) in CH2Cl2 as the
reference electrode; tetra-n-butylammonium perchlorate (0.1 M) was
used as the supporting electrolyte, with Fc+/Fc as the internal
standard, and the scan rate was 0.1 V s−1.
Syntheses. General Syntheses of Ligands. The synthesis
procedures of ligands are listed in Scheme 1. Tetraphenylimidodi-
phosphinate acid (Htpip) and potassium tetraphenylimidodiphosphi-
nate (Ktpip) were prepared according to the literature.7,14 All CF3-
substituted ligands L2−L6 with a ppy core were synthesized by the
reaction of the corresponding 2-bromopyridine (21.1 mmol) and
arylboronic acids (25.5 mmol) using tetrakis(triphenylphosphine)-
palladium(0) (0.63 mmol) as the catalyst in 50 mL of tetrahydrofuran.
The luminescence quantum efficiencies were calculated by a
comparison of the emission intensities (integrated areas) of a standard
sample [Ir(ppy)3] and the unknown sample according to eq 1.8
Scheme 1. Synthetic Routes of Ligands and Complexes
2
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞⎛
⎞
⎟
⎠
ηunk
Iunk Astd
⎜
⎜
⎟
⎟
Φ
= Φ
⎟⎜
unk
std
I
A
η
std ⎠⎝
unk
(1)
std
where Φunk and Φstd are the luminescence quantum yields of the
unknown sample and Ir(ppy)3, respectively. Iunk and Istd are the
integrated emission intensities of the unknown sample and Ir(ppy)3
solution, respectively. Aunk and Astd are the absorbances of the
unknown sample and Ir(ppy)3 solution at their excitation wavelengths,
respectively. The ηunk and ηstd terms represent the refractive indices of
the corresponding solvents (pure solvents were assumed). The Φstd
value of Ir(ppy)3 has been revalued to be 0.4.9
Density functional theory (DFT) calculations using the B3LYP10a
functional were performed. The basis set used for C, H, N, O, F, and P
atoms was 6-31G(d,p),10b while the LanL2DZ basis set were employed
for Ir atoms.10c The solvent effect of CH2Cl2 was taken into
consideration using the conductor-like polarizable continuum model
(C-PCM).10d All of these calculations were performed with Gaussian
09.10e
X-ray Crystallography. The single crystals of complexes were
carried out on a Bruker SMART CCD diffractometer using
monochromated Mo Kα radiation (λ = 0.71073 Å) at RT. Cell
parameters were retrieved using SMART software and refined using
SAINT11 on all observed reflections. Data were collected using a
narrow-frame method with scan widths of 0.30° in ω and an exposure
time of 10 s frame−1. The highly redundant data sets were reduced
B
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Table 1. Crystallographic Data and Structure Refinement for Complexes 1−3, 5, and 6
1
2
3
5
6
formula
fw
C46H36IrN3O2P2
916.92
C48H34F6 IrN3O2P2
1052.92
296(2)
C288H206F36 Ir6N18O13P12
6335.55
C50H32F12IrN3O2P2
1188.93
C50H32F12IrN3O2P2
1188.93
296(2) K
0.71073
monoclinic
P21/c
T (K)
296(2)
291(2)
293(2) K
0.71073
wavelength (Å)
cryst syst
space group
a (Å)
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
0.71073
monoclinic
C2/c
triclinic
P1
̅
15.5459(9)
11.1607(7)
23.5153(13)
90
9.7148(14)
20.775(3)
20.991(3)
90
41.6691(12)
15.2305(7)
23.3920(10)
90
10.7141(3)
11.7373(4)
18.7457(6)
79.9040(10)
84.4300(10)
87.9820(10)
2309.50(13)
2
10.9585(5)
17.7561(9)
24.2144(12)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
ρcalcd (g cm−3
μ(Mo Kα) (mm−1
106.3910
90
102.074(2)
90
113.056(2)
90
93.6330
90
7295(2)
4
4256.5(5)
4
13659.7(9)
2
4702.2(4)
4
)
1.556
1.688
1.540
1.710
1.679
)
3.535
3.372
3.069
3.053
2.999
F (000)
1824
2080
6260
1168
2336
range of transm factors (deg)
reflns collected
1.81−26.00
23203
1.96−26.00
24808
1.44−26.00
41301
1.76−25.00
13129
1.69−26.00
28259
unique reflns
7687
8134
13432
8108
9225
GOF on F2
0.956
1.036
1.018
1.080
1.008
a
b
R1, wR2 [I > 2σ(I)]
0.0323, 0.0632
0.0541, 0.0682
0.0235, 0.0521
0.0289, 0.0534
0.0478, 0.0998
0.0693, 0.1049
902472
0.0555, 0.1490
0.0571, 0.1505
902474
0.0455, 0.1413
0.05080, 0.1458
902473
a
b
R1, wR2 (all data)
CCDC
902470
902471
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑Fo|. wR2 = [∑w(Fo − Fc2)2/∑w(Fo )]1/2
.
After 30 mL of aqueous 2 N Na2CO3 was delivered, the reaction
mixture was heated at 70 °C for 1 day. The cooled mixture was poured
into water, extracted with CH2Cl2 (50 mL × 3), and then dried over
anhydrous magnesium sulfate. Finally, silica column purification (n-
hexane:EtOAc = 7:1 as the eluant) gave white solid products.
Complex 1. Yield: 40%. 1H NMR (500 MHz, CDCl3): δ 9.08 (d, J
= 5.4 Hz, 2H), 7.78 (dd, J = 12.3 and 6.5 Hz, 4H), 7.68 (d, J = 8.1 Hz,
2H), 7.55 (d, J = 7.5 Hz, 2H), 7.46−7.30 (m, 12H), 7.16 (t, J = 7.4 Hz,
2H), 6.99 (td, J = 7.6 and 2.9 Hz, 4H), 6.83 (t, J = 7.3 Hz, 2H), 6.68
(t, J = 7.9 Hz, 2H), 6.61 (t, J = 7.0 Hz, 2H), 6.17 (d, J = 7.6 Hz, 2H).
MALDI-TOF (M+): 918. Anal. Calcd for C46H36IrN3O2P2: C, 60.25;
H, 3.96; N, 4.58. Found: C, 60.47; H, 4.07; N, 4.49.
1
2-[2-(Trifluoromethyl)phenyl]pyridine (L2). Yield: 80%. H NMR
(CDCl3, 400 MHz): δ 7.2−7.6 (m, 4H), 7.49 (d, J = 7.2 Hz, 1H), 7.55
(d, J = 7.3 Hz, 1H), 7.65−7.8 (m, 1H), 8.60 (m, 1H). MS (ESI): m/z
223 [M+].
Complex 2. Yield: 60%. 1H NMR (500 MHz, CDCl3): δ 9.17 (d, J
= 5.6 Hz, 2H), 8.18 (d, J = 8.5 Hz, 2H), 7.83−7.67 (m, 4H), 7.51−
7.30 (m, 12H), 7.26 (d, J = 7.5 Hz, 2H), 7.17 (t, J = 8.0 Hz, 2H), 7.01
(td, J = 7.6 and 3.0 Hz, 4H), 6.67 (t, J = 7.7 Hz, 2H), 6.61 (t, J = 7.0
Hz, 2H), 6.21 (d, J = 7.6 Hz, 2H). MALDI-TOF (M+): 1054. Anal.
Calcd for C48H34F6IrN3O2P2: C, 54.75; H, 3.25; N, 3.99. Found: C,
54.94; H, 3.32; N, 4.01.
1
2-[3-(Trifluoromethyl)phenyl]pyridine (L3). Yield: 75%. H NMR
(CDCl3, 400 MHz): δ 7.1 (m, 1H), 7.44 (t, J = 7.8 Hz, 1H), 7.59−
7.71 (m, 3H), 8.06 (d, J = 7.8 Hz, 1H), 8.3 (s, 1H), 8.62 (dd, J = 4.7
and 0.9 Hz, 1H). MS (ESI): m/z 223 [M+].
1
2-[4-(Trifluoromethyl)phenyl]pyridine (L4). Yield: 76%. H NMR
(CDCl3, 400 MHz,): δ 7.1 (ddd, J = 8.6, 4.7, and 2.1 Hz, 1H), 7.55 (d,
J = 7.5 Hz, 4H), 7.93 (d, J = 8 Hz, 2H), 8.56 (d, J = 4.4 Hz, 1H). MS
(ESI): m/z 223 [M+].
Complex 3. Yield: 55% 1H NMR (500 MHz, CDCl3): δ 9.06 (d, J =
5.1 Hz, 2H), 7.77 (dd, J = 17.4 and 8.7 Hz, 8H), 7.51 (td, J = 8.0 and
1.4 Hz, 2H), 7.43−7.30 (m, 10H), 7.17 (t, J = 8.0 Hz, 2H), 7.00 (td, J
= 7.7 and 3.0 Hz, 4H), 6.88 (d, J = 9.2 Hz, 2H), 6.70 (t, J = 7.1 Hz,
2H), 6.24 (d, J = 8.1 Hz, 2H). MALDI-TOF (M+): 1054. Anal. Calcd
for C48H34F6IrN3O2P2: C, 54.75; H, 3.25; N, 3.99. Found: C, 54.70; H,
3.28; N, 4.01.
2-[3,5-Bis(trifluoromethyl)phenyl]pyridine (L5). Yield: 70%. 1H
NMR (CDCl3, 400 MHz): δ 8.73 (d, J = 8 Hz, 1H), 8.47 (s, 2H), 7.90
(s, 1H), 7.81 (dd, J = 8 and 2 Hz, 2H), 7.33 (dd, J = 8 and 2 Hz, 1H).
MS (ESI): m/z 292 [M+].
2-[2,4-Bis(trifluoromethyl)phenyl]pyridine (L6). Yield: 80%. 1H
NMR (CDCl3, 400 MHz): δ 8.71 (d, aromatic, J = 8 Hz, 1H), 8.02 (s,
1H), 7.88 (d, J = 8 Hz, 1H), 7.79 (dd, J = 8 and 2 Hz, 1H), 7.66 (d, J =
8 Hz, 1H), 7.44 (d, J = 8 Hz, 1H), 7.37 (dd, J = 8 and 2 Hz, 1H). MS
(ESI): m/z 292 [M+].
General Syntheses of Complexes. The synthesis of complex 4 was
reported in our group,7 and complexes 1−3, 5, and 6 were synthesized
according to similar procedures.7,15 A mixture of IrCl3·3H2O (1
mmol) and L (2.5 mmol) in 2-ethoxyethanol and water (20 mL, 3:1,
v/v) was refluxed for 24 h. After cooling, the yellow solid precipitate
was filtered to give the crude cyclometalated Ir3+ chloro-bridged dimer.
Then a slurry of the crude chloro-bridged dimer (0.2 mmol) and Ktpip
(0.5 mmol) in 2-ethoxyethanol (20 mL) was refluxed for 24 h. After
the mixture was cooled to RT, the solvent was evaporated at low
pressure. The crude product was washed by water and then
chromatographed using CH2Cl2 to give complexes 1−6, which were
further purified again by sublimation in vacuum.
Complex 4. Yield: 70%. 1H NMR (500 MHz, DMSO): δ 8.92 (d, J
= 5.5 Hz, 1H), 8.19 (d, J = 8.1 Hz, 1H), 7.95 (d, J = 8.1 Hz, 1H), 7.77
(t, J = 7.8 Hz, 1H), 7.66 (dd, J = 11.7 and 7.3 Hz, 2H), 7.48−7.36 (m,
3H), 7.27−7.14 (m, 3H), 7.11 (d, J = 8.0 Hz, 1H), 7.02 (dt, J = 12.9
and 6.2 Hz, 3H), 6.11 (s, 1H). MALDI-TOF (M+): 1054. Anal. Calcd
for C48H34F6IrN3O2P2: C, 54.75; H, 3.25; N, 3.99. Found: C, 54.68; H,
3.16; N, 4.07.
Complex 5. Yield: 68%. 1H NMR (500 MHz, CDCl3): δ 8.81 (d, J
= 5.6 Hz, 2H), 8.01 (s, 2H), 7.74−7.57 (m, 6H), 7.50 (s, 2H), 7.40−
7.27 (m, 8H), 7.24 (dd, J = 12.6 and 7.4 Hz, 6H), 7.10 (td, J = 7.4 and
3.0 Hz, 4H), 6.49 (t, J = 6.6 Hz, 2H). MALDI-TOF (M+): 1188. Anal.
Calcd for C50H32F12IrN3O2P2: C, 50.51; H, 2.71; N, 3.53. Found: C,
50.20; H, 2.60; N, 3.51.
1
Complex 6 . Yield: 72%. H NMR (500 MHz, CDCl3): δ 9.13 (s,
2H), 8.24 (d, J = 8.2 Hz, 2H), 7.76 (d, J = 8.2 Hz, 4H), 7.54 (d, J =
26.4 Hz, 4H), 7.39 (s, 10H), 7.18 (s, 2H), 7.02 (s, 4H), 6.74 (s, 2H),
6.32 (s, 2H). MALDI-TOF (M+): 1188. Anal. Calcd for
C
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Inorganic Chemistry
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Figure 1. ORTEP diagrams of complexes 1−3, 5, and 6 with atom-numbering schemes. H atoms are omitted for clarity. Ellipsoids are drawn at the
30% probability level.
Figure 2. Absorption (a) and relative emission (b) spectra of complexes 1−6 in degassed dichloromethane (2 × 10−5 M) at RT.
C50H32F12IrN3O2P2: C, 50.51; H, 2.71; N, 3.53. Found: C, 50.39; H,
2.68; N, 3.53.
rings in the complexes. The deformation is caused by steric
hindrance originating from repulsion between the trifluor-
omethyl group located on the phenyl rings and pyridine. The
X-ray data show large dihedral angles for complexes 2 [C8−
C7−C1−C6, −13.4(5)°] and 6 [C15−C14−C23−C24,
−11.6(1)°], which suggests that the CF3 group in position 2
(close to the pyridine ring) will cause larger molecular
deformation. For complex 1 without the CF3 group, the
dihedral angle [N3−C17−C11−C5] between two rings is only
−0.3(6) °, indicating that the two pyridine rings are almost
coplanar.
RESULTS AND DISCUSSION
■
X-ray Crystallography. All single crystals of the five
complexes 1−3, 5, and 6 were grown from sublimation and
characterized by X-ray crystallography. Selected parameters of
the molecular structures and tables of atomic coordinates are
collected in Table 1, and bond lengths and bond angles for each
complex are given in the Supporting Information (Tables S1−
S5). Figure 1 shows the Oak Ridge thermal ellipsoidal plot
(ORTEP)13b diagrams given by X-ray analysis of the five
complexes. It should be noted that the sterically crowded
ligands result in large deformation with regard to the phenyl
The Ir−N bond lengths observed ranging from 2.017(5) to
2.039(4) Å are similar to values reported for heteroleptic
complexes. The average Ir−C [2.006(0) Å] bond length for
D
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Inorganic Chemistry
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Table 2. Photophysical Data of Ir3+ Complexes in the CH2Cl2 Solution
emission (λmax, nm)
lifetime (μs)
a
b
b
b
c
complex Tm/Td (°C)
absorption [λ, nm (ε, ×103 M−1 cm−1)]
298 K (L/S)
77 K
Φ
em(%)
7.1
τL
τS
Eox (eV) HOMO/LUMO (eV)
1
2
3
4
5
6
348/381
320/375
311/344
327/361
334/358
266/361
269 (41.9), 345 (8.7), 408 (4.2), 465 (3.5)
268 (43.1), 299 (27.2), 356 (9.1), 418 (4.2)
268 (39.3), 293 (24.2), 342 (7.1), 454 (2.3)
270 (55.4), 359 (10.2), 408 (4.1), 458 (3.4)
267 (44.7), 288 (41.0), 342 (12.4), 383 (7.2)
517/541
533/567
502/514
524/564
480/487
542/548
506
518
494
521
484
536
1.77
2.26
2.14
1.86
1.83
3.61
1.90
2.12
1.85
2.28
2.69
3.88
0.76
0.98
1.04
0.99
1.26
1.16
−5.39/−2.9
−5.61/−3.18
−5.78/−3.25
−5.44/−2.98
−5.92/−3.29
−5.82/−3.46
25.1
7.0
23.3
0.88
11.0
267 (42.6), 297 (27.2), 365 (7.1), 418 (4.3)
a
b
c
Tm: melting temperature. Td: decomposed temperature. L: liquid. S: solid. HOMO/LUMO means that the value was calculated from
experimental data.
Figure 3. Normalized emission spectra of complexes 1−6 in the CH2Cl2 solution at RT and 77 K and in the solid state.
complex 5 is slightly longer than that of other complexes [Ir−C
withdrawing CF3 group in position 3, resulting in weaker
1.980(1) Å], which is most likely due to the electron-
adjacent Ir−C bonding and stronger corresponding Ir−O
E
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bonding. Furthermore, the C−C and C−N bond lengths and
angles are within normal ranges and are in agreement with the
corresponding parameters described in other similarly con-
stituted complexes.16
solvent relaxation occurs, resulting in rigidochromic effects on
the emission spectra. Among them, complexes 1−3 and 6 show
obvious blue shift, while complex 4 shifts very slightly [only 3
nm (110 cm−1)]. The most surprising result is that complex 5
even shows some slight red shifting. As we know, the
phosphorescence lifetime (τp) is the crucial factor that
determines the rate of triplet−triplet annihilation in the
OLEDs. A longer τp of the material usually causes more severe
triplet−triplet annihilation.19 The lifetimes of complexes 1−6
are in the range of microseconds in the CH2Cl2 solution (1.77−
3.61 μs) and in the solid state (1.85−3.88 μs) at RT (Table 2
and Figures S1−S12 in the Supporting Information), which are
indicative of the phosphorescent origin for the excited states in
each case.
Electronic Spectroscopy. The UV−vis absorption spectra
of complexes 1−6 in CH2Cl2 at 2 × 10−5 M are shown in
Figure 2a, and the electronic absorption data are listed in Table
2. The absorption of these complexes shows intense bands with
extinction coefficients on the order of 104 M−1 cm−1 below 320
nm, which were assigned to the spin-allowed intraligand
1LC(3π→π*) transition of cyclometalated ppy derivatives and
tpip ligands. The band around 400 nm can be assigned to spin-
allowed metal−ligand charge-transfer (1MLCT) bands with
extinction coefficients on the order of 103 M−1 cm−1. The spin-
forbidden 3MLCT transition bands around 450 nm indicate an
efficient spin−orbit coupling that is prerequisite for phosphor-
escent emission.
Compared with complex 1 in CH2Cl2 with a weak emission
band at 517 nm at RT, complexes 2, 4, and 6 show much
stronger red-shifted emission at 533, 524, and 542 nm, whereas
3 and 5 are blue-shifted to 502 and 480 nm, respectively
(Figure 2b). Especially, complex 5 exhibits the weakest PL
intensity and lowest quantum yield (0.88%). The results
suggest that the position and number of the trifluoromethyl
group on the phenyl ring affect the emitting properties of Ir3+
complexes. The trifluoromethyl group on position 3 shows a
hypsochromic shift of the emission of the complex, and the
trifluoromethyl group on positions 2 and 4 shows a
bathochromic shift of the emission of the complex. Addition-
ally, complex 2 has a stronger bathochromic effect and a higher
quantum yield than those of complex 4. Compared to the
unsubstituted complex 1, in the case of complex 5, where CF3
groups are in positions 3 and 5, there is a strong hypsochromic
effect (λmax = 480 nm), indicating that those positions and
numbers of CF3 groups are the most effective in tuning the
emission toward the blue region. Conversely, in comparison
with complex 1, complex 6 with two CF3 groups in the meta
position of the coordinating C shows a bathochromic shift in
the emission maximum of 25 nm (892 cm−1). This behavior is
consistent with the inductive-only nature of the CF3 group,
which renders the meta positions the less electron deficient on
the aromatic ring.5b
For most known Ir3+ complexes, their emission in the solid
state is very weak because of serious triplet−triplet annihilation,
but they show efficient light in solution, although there is a
solvent effect. However, for complexes 5 and 6, bright
luminescence in the solid state under UV excitation can be
observed and their emissions in solution are relatively weak,
especially for 5. This phenomenon, showing weak phosphor-
escence in solution and enhanced phosphorescence emission in
the solid state (EPESS), also called “aggregation-induced
phosphorescent emission”, has been reported in the literature.20
In the CH3CN/H2O (1:5, v/v) solution, efficient PL can be
observed for complexes 5 and 6 (Figure 4), which should
originate from molecular microaggregates and is almost 20
times stronger than that in pure CH3CN.
According to the previous work,17 phosphorescence spectra
from the LC(3π→π*) state display vibronic progressions, while
those from the 3MLCT state are broad and featureless. Figure 3
shows the phosphorescence spectra of the Ir3+ complexes
measured at RT (in solution and in the solid state) and at 77 K.
Compared with Ir(ppy)3, which was assumed to be essentially
dominated by the excited triplet energy of ppy-centered
3MLCT, we conclude that complexes 1−6 gain larger
contributions from the MLCT state and the lowest excited
Figure 4. PL spectra of complexes 5 and 6 in the CH3CN/H2O
solution in different v/v ratios with a concentration of 5.0 × 10−4 mol
L−1.
Electrochemistry. To investigate the electronic effects
caused by trifluoromethyl substituents on the pyridine rings,
CV experiments of complexes 1−6 were carried out using
ferrocene as the internal standard (Figure 5). The highest
occupied molecular orbital (HOMO)/lowest unoccupied
molecular orbital (LUMO) energy levels are listed in Table
2. During the anodic scan in CH2Cl2, each of the Ir3+ complexes
exhibits a reversible oxidation with the redox potential in the
regions of 0.76 and 1.26 V; this positive oxidation potential is
attributed to the metal-centered Ir3+/Ir4+ oxidation couple, in
3
states are likely to dominate the MLCT excited state.
As shown in Figure 3, the phosphorescence spectra of the
Ir3+ complexes suffer the rigidochromic effect: the phosphor-
escence bands shift to blue on going from RT to 77 K. The
rigidochromic effect on some kinds of transition-metal
complexes has been previously reported.18 Because of the low
viscosity of the medium at RT, solvent molecules in the vicinity
of the excited-state molecule readily undergo reorientation by
the dipole−dipole interaction within the lifetime of the excited
state, resulting in the formation of a fully relaxed excited state.
Thus, the emission at RT occurs from a fully relaxed excited
state. On the other hand, the excited state at 77 K emits before
accordance with the reported cyclometalated Ir3+ systems.21
A
reversible oxidation is observed for complex 1 at 0.76 V. It is
noteworthy that complexes 2−4 show higher oxidation
potentials than that of 1, which can be ascribed to the great
F
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Figure 5. Cyclic voltammograms of complexes 1−6.
electron-withdrawing ability of the trifluoromethyl substituent
on the phenyl rings of ppy, making the loss of electrons in
complexes more difficult. Therefore, the two trifluoromethyl
groups contained in complexes 5 and 6 show even higher
oxidation potentials. The HOMO levels of the complexes were
calculated from the oxidation potentials and the LUMO levels
were calculated from the HOMO and band gap obtained from
UV−vis spectra.22 From Table 2, it can be observed that both
the HOMO and LUMO energy levels have been lowered by
introduction of the CF3 group, and complexes 5 and 6 have the
lowest values.
Theoretical Calculations. DFT calculations for complexes
1−6 were carried out using the Gaussian 09 computer program
to study the orbital distribution employing B3LYP with 6-
31G(d,p) basis sets. The effect of substitution on the relative
energies of HOMO and LUMO and its impact on the orbital
distribution are summarized in Figure 6 and Table S6 in the
Figure 7. Selected MOs for complexes 1, 2, and 6.
potentials: the lower (more negative) EHOMO and ELUMO
energies correspond to the higher (more positive) Eox and
Ered potentials, respectively. At the same time, the larger
calculated HOMO−LUMO energy gap corresponds to a larger
electrochemical gap.
Thermal Properties. To investigate the thermal stability of
complexes 1−6, thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) were conducted. If a
complex is suitable for application in OLEDs, the decom-
position temperature (Td) should be high enough to guarantee
that the complex could be deposited onto the solid face without
any decomposition under reduced pressure by sublimation. As
shown in Figures S13−S18 in the Supporting Information, the
TGA curves of complexes 1−6 exhibit good thermal stability up
to 380 °C. For complex 1, no loss of weight was observed up to
350 °C, while the decomposition temperature, which is defined
as a 5 wt % loss of weight, appeared at 389 °C. Furthermore, it
can be vacuum-evaporated at 280 °C without any decom-
position. The introduction of a trifluoromethyl group into the
Ir3+ complexes will reduce their melting point (Tm) and
decomposition temperature to a certain extent. Correspond-
ingly, complexes 2−6 are much easier to vacuum-evaporate at
lower temperature, 250 °C for 2−5, and 210 °C for 6 at a
vacuum of 2.2 × 10−4 Pa, which suggests that all of the
complexes have good film-forming ability and make purification
by sublimation available.
OLED Performances. To evaluate the EL performances,
the OLEDs named G2−G6 using Ir3+ complexes 2−6 as the
emitters were fabricated and investigated with the structure of
ITO/TAPC (30 nm)/Ir complexes (x wt %):SimCP2 (15
nm)/TPBi (45 nm)/LiF (1 nm)/Al (100 nm). TAPC and
SimCP2 were employed as the hole-transporting layer and
bipolar host material, respectively. TPBi was used as an
electron-transporting and hole-blocking layer. The optimized
Ir3+ complexes with 8 wt % doped concentration in SimCP2
was used as the emitting layer for G2−G4 and G6, but for G5,
the best doped concentration was 20 wt %. Scheme 2 shows the
energy diagram of the devices, as well as the molecular
structures of the materials used.
Figure 6. Theoretical (black) and experimental (red, determined by
CV) HOMO and LUMO energy levels of complexes 1−6.
Supporting Information, and selected molecular orbitals (MOs)
for complexes 1, 2, and 6 are shown in Figure 7. For all of the
complexes, the HOMO corresponds to a mixture of phenyl
groups attached to the pyridyl ring and Ir d orbitals with minor
contribution from the tpip ligand, while the LUMO was mainly
localized on the ppy ligand with minor contributions from Ir d
orbitals and the tpip ligand. Therefore, modification of the
phenyl ring changes both the HOMO and LUMO energy. The
incorporation of an electron-withdrawing CF3 group results in a
net increase in the HOMO−LUMO gap, resulting from
stabilization of both frontier MOs. The trends of the calculated
HOMO and LUMO energies of 1−6 are basically in agreement
with the trends of the measured oxidation and reduction
The EL spectra, power efficiency, and current efficiency
versus J curves and voltage−luminance (V−L) characteristics of
each device are shown in Figure 8, and the key EL data are
summarized in Table 3. For all of the devices, typical emission
G
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Scheme 2. Energy Level Diagram of the HOMO and LUMO Levels (Relative to the Vacuum Level) for Materials Investigated in
This Work and Their Molecular Structures
Figure 8. Characteristics of devices with configuration ITO/TAPC (30 nm)/2−6:SimCP2 (15 nm, x wt %)/TPBi (45 nm)/LiF (1 nm)/Al (100
nm) (x is 20 wt % for device G5 and 8 wt % for the others): (a) EL spectra at different current densities; (b) power efficiency (ηp) as a function of
the current density (J); (c) current efficiency (ηc) as a function of the current density (J); (d) luminance versus voltage (L−V) for G2−G6.
maxima of the complexes 2−6 at 534, 503, 521, 480, and 543
nm, respectively, are shown at different current densities,
suggesting that the energy can be transferred from SimCP2 to
the emitter. However, it is noted that there is a little residual
emission from the host SimCP2 for each device, which means
that the energy and/or charge transfer from the host exciton to
the phosphor is not complete upon electrical excitation. The
CIE color coordinates are x = 0.34, y = 0.60 for G2, x = 0.23, y
= 0.61 for G3, x = 0.17, y = 0.41 for G5, and x = 0.39, y = 0.58
for G6 at 100 mA cm−2.
All devices display good performances. The turn-on voltages
(Vturn‑on) for devices G2−G6 are 3.5−3.9 V. The maximum
luminance efficiency (ηL) and power efficiency (ηp) for device
G2 are 43.5 cd A−1 and 23.08 lm W−1, respectively, with a
maximum luminance of over 31522 cd m−2 at a driving voltage
of 11.8 V. The driving voltages to reach the practical brightness
H
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Table 3. EL Performances of the Devices G2 (x = 8 wt %), G3 (8 wt %), G4 (8 wt %), G5 (x = 20 wt %), and G6 (x = 8 wt %)
a
b
c
d
e
f
g
Vturn‑on
(V)
Lmax (cd m−2), voltage
ηp,max (lm W−1), voltage
ηc,max (cd A−1), voltage
ηc,100 /ηc,1000 (cd
A−1
L100 (cd
m−2
h
device
(V)
(V)
(V)
)
)
CIE (x, y)
G2
G3
G4
G5
G6
3.5
3.6
3.9
3.7
3.6
38115, 13.3
38005, 10.6
39286, 11.7
24863, 11.3
23411, 14.2
23.08, 5.1
21.16, 6.4
29.01, 5.6
20.58, 5.4
43.5, 6.5
43.13, 6.4
50.8, 5.5
39.67, 6.8
33.1/42.4
11.3/35.7
21.7/43.0
27.4/37.6
15.5/34.8
23604
27417
35130
18228
17348
(0.34, 0.60)
(0.23, 0.61)
(0.29, 0.63)
(0.17, 0.41)
19.60, 6.0
38.68, 6.3
(0.39, 0.58)
a
b
c
d
e
Vturn‑on: turn-on voltage recorded at a brightness of 1 cd m−2. Lmax: maximum luminance. ηp: power efficiency. ηc: current efficiency. ηc,100
:
f
g
current efficiency at the brightness of 100 cd m−2. ηc,1000: current efficiency at the brightness of 1000 cd m−2. L100: luminance measured at a current
h
density of 100 mA cm−2. Taken at 100 mA cm−2.
of 100 and 1000 cd m−2 are 4.9 and 6.4 V, respectively. For G3
Notes
device, the turn-on voltage is 3.6 V and the maximum
luminance of over 38000 cd m−2 is achieved at a driving
voltage of 10.6 V. The maximum luminance efficiency and
power efficiency are 43.13 cd A−1 and 21.16 lm W−1,
respectively. The driving voltages to get the practical brightness
of 100 and 1000 cd m−2 are 4.4 and 5.4 V, respectively. Among
all devices, G4 fabricated with complex 4 as the guest material
displays the best performance. The maximum luminance for
device G4 is over 39000 cd m−2, and the maximum luminance
efficiency and power efficiency are 50.8 cd A−1 and 29.0 lm
W−1, respectively. For both devices G3 and G4, it can be
observed that the maximum luminance values can be retained
during a driving voltage range, suggesting that they exceed the
measurement limitation for our equipment.
As discussed, complexes 5 and 6 show weak phosphor-
escence in solution and the EPESS phenomenon. The
maximum luminance data are 24863 and 23411 cd m−2 for
G5 and G6, respectively. The maximum power/current
efficiency values are 20.58 (lm W−1)/39.67 (cd A−1) and
19.60 (lm W−1)/38.68 (cd A−1), respectively.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Major State Basic Research
Development Program (Grants 2013CB922101 and
2011CB808704) and the National Natural Science Foundation
of China (Grants 20971067 and 21021062).
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CONCLUSION
■
In conclusion, five interesting bis-cyclometalated iridium
complexes with tpip as the ancillary ligand have been
successfully prepared and applied in the OLEDs. The
tifluoromethyl substituents at different positions on the phenyl
rings of ppy in iridium complexes can affect their emission color
and intensity obviously. The devices ITO/TAPC (30 nm)/Ir
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̈
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Corresponding Author
*Tel: 0086 25 83596775. Fax: 0086 25 83314502. E-mail:
yxzheng@nju.edu.cn.
■
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