Iridium(iii) complexes adopting 1,2-diphenyl-1H-benzoimidazole ligands for highly efficient organic light-emitting diodes with low efficiency roll-off and non-doped feature

Article abstract of DOI:10.1039/c3tc32092h

Two novel iridium(iii) complexes (pbi)₂Ir(mtpy) (1) and (pbi)₂Ir(pbim) (2) adopting 1,2-diphenyl-1H-benzoimidazole (Hpbi) as cyclometalated ligands were successfully synthesized and characterized. Strong emissions at 501 and 536 nm with high photoluminescence quantum yields of 48% and 91% in CH₂Cl₂ at 298 K were obtained for 1 and 2, respectively. The quantum chemical calculations and the photophysical properties indicated that the dominant ³MLCT (metal-to-ligand charge-transfer) state mixed with ³LLCT (ligand-to-ligand charge-transfer) and ³LC (ligand-centered ³π-π*) characters contributed to their phosphorescence emissions. Doped organic light-emitting diodes (OLEDs) based on 1 and 2 showed a peak current efficiency of 45.0 cd A^-1 and power efficiency of 47.9 lm W^-1 accompanied by very low efficiency roll-off values. In their non-doped OLEDs, high efficiencies of 24.4 cd A^-1 and 26.3 lm W^-1 were achieved as well. These appealing results reveal that complexes 1 and 2 open interesting perspectives for the development of high-performance OLEDs in the future.

Full text of DOI:10.1039/c3tc32092h

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ARTICLE TYPE
Iridium(III) complexes adopting 1,2-diphenyl-1H-benzoimidazole
ligands for highly efficient organic light-emitting diodes with low
efficiency roll-off and non-doped feature
Hongtao Cao,a Haizhu Sun,a Yongming Yin,b Xuemei Wen,b Guogang Shan,*a Zhongmin Su,*a Ronglin
5
Zhong,a Wenfa Xie,*b Peng Lia and Dongxia Zhua
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Two novel iridium(III) complexes (pbi)2Ir(mtpy) (1) and (pbi)2Ir(pbim) (2) adopting 1,2ꢀdiphenylꢀ1Hꢀ
benzoimidazole (Hpbi) as cyclometalated ligands were successfully synthesized and characterized. Strong
10 emissions at 501 and 536 nm with high photoluminescence quantum yields of 48% and 91% in CH2Cl2 at
298 K were obtained for 1 and 2, respectively. The quantum chemical calculations and the photophysical
properties indicated that the dominantly 3MLCT (metalꢀtoꢀligand charge transfer) state mixed with
3LLCT (ligandꢀtoꢀligand charge transfer) and 3LC (ligandꢀcentered 3π–π*) characters contributed to their
phosphorescence emissions. Doped organic lightꢀemitting diodes (OLEDs) based on 1 and 2 showed a
15 peak current efficiency of 45.0 cd A–1 and power efficiency of 47.9 lm W–1 accompanied by very low
efficiency rollꢀoff values. In their nonꢀdoped OLEDs, high efficiencies of 24.4 cd A–1 and 26.3 lm W–1
were achieved as well. These appealing results reveal that complexes 1 and 2 open interesting
perspectives for the development of highꢀperformance OLEDs in the future.
impede the application of phosphorescent OLEDs in next
generation solidꢀstate light source. As known, phosphorescent
OLEDs always suffer from serious efficiency rollꢀoff problem at
high luminance (1000 cd mꢀ2) due to a combination of triplet–
50 triplet annihilation and fieldꢀinduced quenching effects.37,38 In
addition, most phosphorescent OLEDs are usually fabricated
through doping the emitters into a chargeꢀtransporting host matrix
in a rather low and narrow concentration range because their
efficiencies are highly sensitive to emitter concentration.37,38
55 Thus, it is a challenge to synthesize appropriate phosphorescent
materials to fabricate OLEDs possessing appealing efficiency at
high luminance and relatively dopantꢀconcentrationꢀinsensitive
feature for the efficiency.
1,2ꢀdiphenylꢀ1Hꢀbenzoimidazole (Hpbi) has been used as
60 cyclometalated ligand or an important component to prepare
iridium(III) complexes that exhibit improved luminescence
properties and stability for applications, such as organic optical
devices and chemosensors.11,39–43 Further modification in these
iridium(III) complexes can be made through altering the ancillary
65 ligands to adjust their photophysical properties. Using Hpbi as
cyclometalated ligands and introducing different ancillary ligands
are effective for preparing novel iridium(III) complexes showing
intriguing photophysical properties. It is thus expected that this
strategy might help to develop appropriate materials for highly
70 efficient OLEDs with relatively dopantꢀconcentrationꢀinsensitive
characteristic. Herein, we reported the synthesis, crystal structure,
photophysical and EL properties of two novel iridium(III)
complexes adopting Hpbi as cyclometalated ligands, namely
(pbi)2Ir(mtpy) (1) and (pbi)2Ir(pbim) (2). The ancillary ligands
1. Introduction
20 Organic lightꢀemitting diodes (OLEDs) have aroused great
interest because of their potential application in future flatꢀpanel
displays and lighting techniques.1,2 Among many luminescent
materials used for OLEDs, iridium(III) complexes are drawing
extensive attention due to their very high luminescence
25 efficiencies, rather short phosphorescence lifetimes and easy
tunability of emission wavelength.3–10 These fascinating
characteristics make them effectively available to fabricate high
performance organic electroluminescence (EL) devices.
Some groups have done extensive investigations in the
30 utilization of appropriate iridium(III) complexes and optimization
of the device configurations to achieve high efficiency in OLEDs
displays.11–19 For example, Kido et al. adopted the most widely
used green phosphorescent dye Ir(ppy)3 as emitter doped in 4,4′ꢀ
N,N′ꢀdicarbazolebiphenyl (CBP) to fabricate highly efficient
35 OLED with high power efficiency (ηp) of 82.0 lm W–1.20
Simultaneously, Liao and other groups reported many iridium(III)
complexes that were mainly based on the cyclometalated ligand
2ꢀphenylpyridine (ppy) with an auxiliary ligand such as
acetylacetonate (acac) or picolinate (pic) for achieving highꢀ
40 performance OLEDs.21,22–28 In addition, significant efforts have
been made to exploit other types of iridium(III) complexes to
obtain promising OLEDs.29–35 For instance, Chang et al. have
utilized a new greenishꢀyellow emitter Ir(MDQ)2(Bpz) to get high
current efficiency (ηc) of 77.4 cd A–1 at 1000 cd m–2 in the
45 device.36 However, there remain a number of bottlenecks that
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Hmtpy and Hpbim were 2ꢀ(5ꢀmethylꢀ2Hꢀ1,2,4ꢀtriazolꢀ3ꢀ
yl)pyridine and 2ꢀ(pyridinꢀ2ꢀyl)ꢀ1Hꢀbenzoimidazole,
respectively. Complexes 1 and 2 exhibited strong emission at 501
(t, J = 7.5 Hz, 1H), 6.64–6.68 (m, 2H), 6.58 (t, J = 7.5 Hz, 1H),
6.53 (d, J = 8.0 Hz, 1H), 6.46–6.50 (m, 2H), 6.41 (d, J = 8.0 Hz,
1H), 6.11 (d, J = 8.5 Hz, 1H), 5.78 (d, J = 8.0 Hz, 1H), 2.32 (s,
and 536 nm with high photoluminescence quantum yields 60 3H), Anal. calcd for C46H33IrN8: C, 62.08; H, 3.74; N, 12.59.
5
(PLQYs) of 48% and 91% in CH2Cl2 at 298 K, respectively.
Adopting the doping technique, resultant OLEDs based on them
showed a peak ηc of 45.0 cd A–1 and ηp of 47.9 lm W–1. Even
more gratifying was that, very low efficiency rollꢀoff values were
achieved with increasing the brightness in their doped OLEDs,
Found: C, 66.13; H, 3.70; N, 12.66. ESIꢀMS: m/z 891.31 [M +
H]+.
2.2.2 Synthesis of (pbi)2Ir(pbim) (2)
The synthesis of complex 2 was similar to that of complex 1
65 except that the ancillary ligand Hmtpy was replaced by Hpbim.
The crude product was also purified by silica gel column
chromatography using an ethyl acetate–dichloromethane mixture
(10:1) and then a dichloromethane–methanol mixture (25:1) as
10 which still remained high efficiencies of 44.5 cd A–1 and 31.5 lm
W–1 at 1000 cd m–2. In addition, complexes 1 and 2 displayed a
certain dopantꢀconcentrationꢀinsensitive feature for the
efficiencies in their nonꢀdoped OLEDs, which processed a
maximum ηc of 24.4 cd A–1 and ηp of 26.3 lm W–1 accompanied
15 by small efficiency rollꢀoff, remaining 24.1 cd A–1 and 19.4 lm
W–1 at 1000 cd m–2.
1
eluent to afford complex 2 in 70% yield (0.22 mmol, 0.21 g). H
70 NMR (500 MHz, d6–DMSO, δ [ppm]): 8.65 (s, 1H), 8.32 (s, 1H),
8.12 (d, J = 5.0 Hz, 1H), 7.69–7.80 (m, 10H), 7.63–7.65 (m, 1H),
7.43–7.45 (m, 1H), 7.30 (s, 1H), 7.16–7.23 (m, 2H), 7.00–7.07
(m, 3H), 6.95 (s, 1H), 6.73–6.86 (m, 5H), 6.58–6.61 (m, 3H),
6.52 (d, J = 7.5 Hz, 1H). 6.21 (d, J = 8.0 Hz, 1H), 5.84 (d, J = 8.0
75 Hz, 1H), 5.77 (d, J = 8.0 Hz, 1H), Anal. calcd for C50H34IrN7: C,
64.92; H, 3.70; N, 10.60. Found: C, 66.95; H, 3.68; N, 10.64.
ESIꢀMS: m/z 926.20 [M + H]+.
2. Experimental section
2.1 General information
All reagents and solvents employed here were commercially
20 available. The solvents for syntheses were freshly distilled over
appropriate drying reagents. All experiments were performed
under
a nitrogen atmosphere by using standard Schlenk
techniques. Elemental analysis (C, H and N) was performed on a
Perkinꢀ Elmer 240C elemental analyzer. Complexes 1 and 2 were
1
25 verified through H NMR spectra on Bruker Avance 500 MHz
spectrometer with tetramethylsilane as internal standard. The
molecular weights of 1 and 2 were tested by using electrosprayꢀ
ionization mass spectroscopy (ESIꢀMS).
The ligands Hpbi and Hmtpy were conveniently synthesized
30 according to early reported literatures,39,41,44 and Hpbim was
purchased from Aldrich Chem. Inc. Complexes 1 and 2 were
prepared with a similar synthetic procedure according to the
reported literature (see Scheme 1).45
2.2 Synthesis
35 2.2.1 Synthesis of (pbi)2Ir(mtpy) (1)
A mixture of IrCl3ꢁ3H2O (0.35 g, 1 mmol) and Hpbi (0.59 g, 2.20
mmol) in a mixture of 2ꢀethoxyethanol and water (20 mL, 3:1,
v/v) was refluxed under argon for 24 h. The reaction mixture was
then cooled down to room temperature and water (15 mL) was
40 added. The mixture was filtered and the residue was washed with
water (30 mL) and ethanol (30 mL). The crude dichloroꢀbridged
diiridium complex (0.52 g), [Ir(pbi)2Cl]2, was collected without
further purification after it was dried out. A solution of ligand
Hmtpy (0.06 g, 0.40 mmol) and the dichloroꢀbridged diiridium
45 complex [Ir(pbi)2Cl]2 (0.24 g, 0.16 mmol) in dichloromethane (30
mL) and ethanol (10 mL) was refluxed under argon for 12 h in
the dark. After cooling to room temperature, the mixture was
precipitated into hexanes (150 mL), while stirring. Then, the
precipitate was filtered and washed with hexanes. The crude
50 product was purified by silica gel column chromatography using
an ethyl acetate–dichloromethane mixture (10:1) and then a
dichloromethane–methanol mixture (25:1) as eluent to afford
complex 1 in 65% yield (0.21 mmol, 0.19 g). 1H NMR (500
MHz, d6–DMSO, δ [ppm]): 8.04–8.06 (m, 2H), 7.89 (d, J = 5.0
55 Hz, 1H), 7.70–7.82 (m, 8H), 7.62 (s, 2H), 7.41 (s, 1H), 7.23 (t, J
= 7.5 Hz, 2H), 7.02–7.10 (m, 3H), 6.98 (t, J = 8.0 Hz, 1H), 6.77
Scheme 1 Synthetic routes to prepare complexes 1 and 2.
80 2.3 Physical measurements
UVꢀvis absorption spectra were recorded on U3010 spectrometer
(Hitachi, Japan). Photoluminescence (PL) spectra of complexes 1
and 2 were using the Fꢀ7000 FL spectrophotometer. The neat
films of 1 and 2 were prepared through spinꢀcoating a solution of
85 them in CH2Cl2 (20 mg mL–1) on precleaned glass substrates at
2000 rpm. PLQYs (Φp) in neat films were measured in an
integrating sphere. The excitedꢀstate lifetimes and PLQYs in
solution were measured on
a transient spectrofluorimeter
(Edinburgh FLSP920) using timeꢀcorrelated singleꢀphoton
90 counting technique. The thermogravimetric analyses (TGA) were
performed on a Perkin–Elmer TGꢀ7 analyzer heated from 50 to
800 °C in flowing nitrogen. Differential scanning calorimetry
(DSC) was also carried out on a Perkin–Elmer TGꢀ7 analyzer in a
nitrogen atmosphere at a scanning rate of 10 °C min–1. The
95 poly(methylmethacrylate) (PMMA) films containing 1 and 2
were prepared through spinꢀcoating a solution of PMMA and the
2
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iridium(III) complex (1 or 2) (x wt %) in CH2Cl2 on precleaned
(HIL),
1,1ꢀbis[4ꢀ[N,N′ꢀdi(pꢀtolyl)amino]phenyl]cyclohexane
glass substrates at 2000 rpm.
(TAPC) acts as the holeꢀtransporting layer (HTL), 4,4′,4′′ꢀtrisꢀ(Nꢀ
carbazolyl)ꢀtriphenylamine (TCTA) acts as the electron/exciton
blocking layer (EBL), 4,7ꢀdiphenylꢀ1,10ꢀphenanthroline (Bphen)
60 acts as electron transporting layer (ETL) and LiF acts as the
electronꢀinjection layer (EIL). Luminanceꢀcurrentꢀvoltage
characteristics for the unpackaged devices were measured
simultaneously with a programmable Keithley 2400 source meter
and a Minolta luminance meter LSꢀ110. The spectra of the
65 devices were measured with an Ocean Optics Maya 2000ꢀPRO
spectrometer. All the measurements were carried out at room
temperature in air.
2.4 X-Ray crystallography
Single crystal of 1 was grown from slow vapor diffusion of
hexane to a mixed solution of dichloromethane and methanol.
The singleꢀcrystal Xꢀray diffraction data for complex 1 was
collected on a Bruker Apex CCD II areaꢀdetector diffractometer
with graphiteꢀmonochromated Mo Kα radiation (λ = 0.71069 Å)
at 293(2) K. Absorption correction was applied by using multiꢀ
10 scan technique. The structure of 1 was solved by Direct Method
of SHELXSꢀ97 and refined by fullꢀmatrix leastꢀsquare techniques
using the SHELXLꢀ97 program.46 Metal atom in 1 was located
from the Eꢀmaps and nonꢀhydrogen atoms were located in
successive difference Fourier syntheses and refined with
15 anisotropic thermal parameters on F2. Hydrogen atoms were
calculated in ideal geometries. The detailed crystallographic data
and structure refinement parameters are summarized in Table S1
(ESI†). Further details of the crystal structure determination have
been deposited to the Cambridge Crystallographic Data Centre as
20 supplementary publication. CCDC 966724 (1) contains the
supplementary crystallographic data for this paper.
5
3. Results and discussion
3.1 Single crystal structure
2.5 Theoretical calculations
The geometrical structures of the ground and the lowest triplet
states for 1 and 2 were fully optimized with C1 symmetry
25 constraints by using the restricted closedꢀshell and openꢀshell
PBE0 methods, respectively.47–49 The ‘‘Doubleꢀξ’’ quality basis
sets were employed for C, H, N atoms (6ꢀ31G*) and Ir atom
(LANL2DZ). The effective core potential (ECP) replaced the
inner core electrons of iridium leaving the outer core (5s)2(5p)6
30 electrons and the (5d)6 valence electrons of iridium(III). The
expectation values calculated for S2 was smaller than 2.05 in the
spinꢀunrestricted calculation. Timeꢀdependent density functional
theory (TDꢀDFT) calculation was performed within the solvent
effect.50,51 All calculations reported here were carried out with the
35 Gaussian 09 software package.52
70
Fig. 1 (a) Molecular structure and (b) crystal packing diagrams of
complex 1. Thermal ellipsoids are drawn at 30% probability. The solvent
molecules and H atoms have been omitted for clarity.
Fig. 1a depicts that complex 1 reveals distorted octahedral
75 geometries with two cyclometalated ligands (Hpbi) and an
ancillary ligand (Hmtpy) surrounding the iridium metal center. It
is notable that the two cyclometalated ligands adopt C,Cꢀcis, N,Nꢀ
trans configurations, agreeing well with those described for other
similarly constituted iridium(III) complexes.39,44,53,54 Selected
80 bond lengths and bond angles have been presented in Table S2
(ESI†). For 1, the Ir–N bond distances between the Ir center and
the ancillary ligand, Ir(1)–N(1) [2.177(4) Å] and Ir(1)–N(2)
[2.113(5) Å], are significantly longer than those between the Ir
center and cyclometalated ligands, Ir(1)–N(5) [2.033(4) Å] and
85 Ir(1)–N(7) [2.041(4) Å]. This phenomenon confirms the sigma
donation of the carbons (trans effect)44,55,56 in the iridium(III)
complexes. In addition, Fig. 1b also reveals that the distance
between two nearest benzene rings is 3.73 Å. This suggests that
there is no significant intermolecular π–π interaction between the
90 benzene rings, which can help depress undesired selfꢀquenching
problems in the solid state of 1.57–59
2.6 Electrochemical measurements
Cyclic voltammetry (CV) was performed on a BAS 100 W
instrument with a scan rate of 100 mV s–1 in CH3CN (10–3 M)
with the threeꢀelectrode configuration: a glassy carbon electrode
40 as the working electrode, an aqueous saturated calomel electrode
as the operating reference electrode and a platinum wire as the
counter electrode. A 0.1 M solution of tetrabutylammonium
tetrafluoroborate in CH3CN was used as the supporting
electrolyte and ferrocene was selected as the internal standard.
45 2.7 Device fabrication and characterization
Patterned indiumꢀtinꢀoxide (ITO)ꢀcoated glass substrates with the
sheet resistance of 20 ꢂ per square were subjected to a routine
cleaning process with rinsing in Decon 90, deionized water,
drying in an oven, and finally treating in a UVꢀozone chamber.
50 Organic layers and cathode were sequentially deposited on the
ITOꢀglass substrates without breaking vacuum (~5.0×10–4 Pa). A
shadow mask was used to define the cathode and to make four 10
mm2 devices on each substrate. The thickness of the organic
layers and metal were monitored in situ with quartz oscillator.
55 For the doped OLEDs, MoO3 acts as the hole injection layer
3.2 Photophysical properties
Fig. 2 displays UVꢀvis absorption and emission spectra of
complexes 1 and 2 in CH2Cl2 and neat film. It is found that the
95 high energy absorption bands of 1 and 2 below 380 nm can be
assigned to a ligandꢀcentered 1π–π* transition. The long tails
absorption occurring in lower energy region (longer than 400 nm)
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Table 1 Photophysical, electrochemical and thermal characteristics for complexes 1 and 2.
Complex
λPL, maxa,b,c (nm)
501, 493, 520
536, 517, 540
Φpa,c (%)
48, 24
τa,c (ꢃs)
0.33, 0.19
0.39, 0.31
Eoxd (V)
0.83
Ege (eV)
2.67
HOMOf (eV)
–5.15
LUMOg (eV)
Td (°C)
354
–2.48
–2.62
1
2
91, 38
0.92
2.62
–5.24
382
5
a Measured in CH2Cl2 (10–5 M) at 298 K and b 77 K. c Measured in the neat film. d Measured by CV with ferrocene as the standard e Estimated from UVꢀvis
absorption spectrum. f Calculated from onset oxidation potential. g Deduced from HOMO and Eg.
Fig. 2 Absorption and PL spectra of (a) 1 and (b) 2 in CH2Cl2 (10–5 M, 298 and 77 K) and neat films.
may be associated with 1MLCT (metalꢀtoꢀligand chargeꢀtransfer),
spectra tend to lose the broad and almost featureless emission
profiles, and show two major peaks accompanied by somewhat
10 3MLCT, 1LLCT (ligandꢀtoꢀligand chargeꢀtransfer), 3LLCT, and
ligandꢀcentered 3π–π* (3LC) transitions.22,60 This observation
blueꢀshift.
According
to
previous
reports,65–67
the
implies that, owing to the strong spinꢀorbit coupling endowed by 45 phosphorescence spectra of iridium(III) complexes always suffer
the iridium atom, the spinꢀforbidden 3MLCT, 3LLCT and 3LC
transitions have gained considerable intensity via mixing with the
15 higherꢀlying spinꢀallowed 1MLCT transitions.22,60
a rigidochromic effect on going from 298 to 77 K and their
phosphorescence from a polar excited state (such as 3MLCT
state) can suffer a rigidochromic shift much larger than that from
a less polar excited state (3LC state). Thus, the lowest excited
Complexes 1 and 2 show emissions of 501 and 536 nm in
CH2Cl2 at 298 K, respectively. This difference in emission color 50 states of 1 and 2 are likely to be the dominantly 3MLCT state, as
between 1 and 2 suggests that increasing the conjugation length
of the ancillary ligands from Hmtpy to Hpbim has a strong effect
20 on their energy levels. In addition, their PL spectra display broad
and almost featureless emission profiles, indicating that the
emissive excited states of 1 and 2 may have predominantly
characters of 3MLCT or 3LLCT other than 3LC, as the latter
always shows vibronically structured emission spectra. In
25 degassed CH2Cl2, 1 reveals a PLQY of 48% and an excitedꢀstate
lifetime (τ) of 0.33 ꢃs. For 2, the PLQY is as high as 91% with an
excitedꢀstate lifetime of 0.39 ꢃs. These excitedꢀstate lifetimes on
the order of microseconds are the signature of phosphorescence
for iridium(III) complexes. Compared with their solutions, the PL
30 spectra of spinꢀcoated neat films of 1 and 2 exhibit various
degrees of redꢀshift (19 and 4 nm, respectively) resulting from
aggregation and triplet–triplet interaction.61,62 These neat films
still remain satisfactory PLQY values of 24% for 1 and 38% for
2. Such observed PLQY difference between complexes 1 and 2
35 may be attributed to their different lowest excited states (T1)
characters, and further support of this viewpoint has been shown
in theoretical calculations. The corresponding excitedꢀstate
lifetimes in neat films are found to be 0.19 ꢃs for 1 and 0.31 ꢃs
for 2. These relatively short lifetimes are thus expected to delay
40 the rollꢀoff of EL efficiency to some extent.31,37,63,64
supported by the theoretical calculation results (vide infra). The
relevant data described here have been tabulated in Table 1.
3.3 Theoretical calculations
The quantum chemical calculations were then performed (see
55 Experimental section) to gain better insight to the nature of the
emissive excited states involved in the emission processes of
complexes 1 and 2. We first validated our theoretical method
through comparing the representative bond lengths and bond
angle of the DFTꢀoptimized geometries to the corresponding
60 experimental values of complex 1. Table S2 (ESI†) suggests that
the theoretical results show small differences (below 3%)
compared with those of experimental geometric parameters. Very
close match is found for the bond length Ir(1)ꢀC(28) and bond
angle C(27)ꢀIr(1)ꢀN(5). Such agreement ensures
a proper
65 description of the nature of the lowest excited states for 1 and 2.
The excitation energies and molecular orbitals involved in the
excitations of 1 and 2 are summarized in Table S3 (ESI†) and
Fig. 3, respectively. The calculation results suggest that the
highest occupied molecular orbital (HOMO) of 1 mainly resides
70 on the phenylbenzimidazole moiety of one cyclometalated ligand
and the iridium atom, along with small distribution on triazole
moiety of the ancillary ligand. While the lowest unoccupied
molecular orbital (LUMO) mostly locates on one of the 2ꢀ
Upon cooling CH2Cl2 solutions of 1 and 2 to 77 K, their PL
4
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phenylbenzimidazole groups. TDꢀDFT calculation implies that
spectra.70 These values can help to design the structure of OLEDs
the T1 of 1 mainly originates from HOMO→LUMO (82%).
Accordingly, its T1 state has significant characters of 3MLCT
associated with 3LLCT and 3LC components.
based on complexes 1 and 2.
45 3.5 Thermal properties
The TGA results suggest that 1 and 2 exhibit good thermal
stability properties with high decomposition temperatures (Td,
corresponding to 5% weight loss) of 354 and 382 °C,
respectively. These high values are required and important for
50 fabrication of OLEDs based on them by evaporation method. In
addition, DSC measurement shows that no endothermic or
exothermic peaks are observed for them at the region around 285
°C, which suggests that solid 1 and 2 are amorphous states
without determined glass transition temperatures (Tg).
5
However, for complex 2, the calculation results indicate that its
HOMO mainly resides on the benzimidazole groups of ancillary
ligand and the iridium atom, while the LUMO mostly locates on
the ancillary ligand and phenylbenzimidazole moiety in one
cyclometalated ligand. The LUMO+1 mostly resides on the
10 phenylbenzimidazole moiety in one cyclometalated ligand and
the ancillary ligand. It also suggests that the T1 of 2 originates
predominately
from
HOMO→LUMO+1
(45%)
and
HOMO→LUMO (43%), which indicates that the transition
incorporation of 3MLCT associated with 3LLCT and 3LC
15 components is responsible for the phosphorescence emission of
this complex. The emission spectra indicacte that T1 character of
2 shows somewhat different from that of 1. The calculation
results show that the contributions of 3MLCT are 26.5% for 1 and
29.5% for 2. For heavyꢀmetal containing complexes, the
55 3.6 Electroluminescence properties
3.6.1 Doped devices based on complexes 1 and 2
We altered triplet hosts from 4,4′ꢀN,N′ꢀdicarbazolebiphenyl
(CBP)
to
2,7ꢀbis(diphenylphosphoryl)ꢀ9ꢀ(4ꢀ
(POAPF) for
diphenylamino)phenylꢀ9'ꢀphenylꢀfluorene
3
60 complexes 1 and 2 to fabricate OLEDs named as A to B and C to
D, respectively. Chemical structures of the materials used here
are shown in Fig. S2 (ESI†). These devices have a traditional
configuration of indium tin oxide (ITO)/MoO3 (3nm)/TAPC (30
nm)/TCTA (5 nm)/Host:Ir (10%) (30 nm)/Bphen (30 nm)/LiF
65 (0.5 nm)/Mg:Ag (Fig. 4a). Moreover, we compared the EL
properties of 1 and 2 with the traditional greenꢀemitting Ir(ppy)3
in doped OLEDs. The corresponding devices with the same
configuration were also fabricated through incorporating Ir(ppy)3
(10%) into CBP (device E) and POAPF (device F). The relevant
70 EL performances of these devices are summarized in Table 2.
Devices A–D present intense green emissions with maximum
at 508–513 nm. No additional emission signals from CBP or
POAPF was observed, suggesting an efficient energy transfer
from the host to iridium(III) complexes. The EL spectra at 4.5 V
75 for 1 and 2 (selected OLEDs B and D) have been exhibited in
Fig. 4b, while those for devices A and C are shown in Fig. S3
(ESI†). Fig. S4 (ESI†) shows that Ir(ppy)3 displays typical green
emissions at 515 nm. It can be observed from Fig. 4c that A–D
display low turnꢀon voltages (defined as the bias at a brightness
80 of 1 cd m–2) of 2.5 to 3.2 V. However, it is noted that the POAPFꢀ
based devices exhibit lower turnꢀon voltages (2.5 V for B and D)
than those of CBPꢀbased devices (3 V for A and 3.2 V for C),
which can be attributed to the facilitation of both hole and
electron injection when using bipolar POAPF as the host
85 material.71 In addition, devices A–D reveal promising luminance
up to 96550 cd m–2. This value along and their low turnꢀon
voltages are comparable to those of devices E and F (2.5 V,
51340 cd m–2, Fig. S5, ESI†) as well as many other green
OLEDs.72–74
20 decreased MLCT character in T1 state will reduce the ln(knr) (knr
represents nonradiative decay rate). Moreover, 1 shows lager
emission energy than 2, which further decreases the radiative
decay rates, causing a decline in PLQYs.68 This viewpoint may
account for the observed PLQY difference between 1 and 2 to
25 some extent. The differences in electron density between T1 and
the ground state S0 of 1 and 2 exhibited in Fig. S1 (ESI†) clearly
confirm the transition characters mentioned above for them.
According to the photophysical characterizations of 1 and 2, it is
thus believed that, their emissions at room temperature should
3
3
30 originate from the dominantly MLCT state mixed with LLCT
and 3LC characters.
Fig. 3 Molecular orbitals involved in the excitations of 1 and 2.
3.4 Electrochemical properties
90
As revealed in Fig. 4d that presents the current efficiency–
luminance–power efficiency characteristics for these devices, a
maximum ηc of 45.0 cd A–1 at 501 cd m–2 (4.2 V) for C and a
peak ηp of 47.9 lm W–1 for B are successfully achieved here.
These values are much higher than those of devices based on
35 CV measurement indicates that these two complexes reveal onset
oxidation peaks at 0.83 and 0.92 V, respectively, which are
assigned to the oxidation of the iridium metal cationic site
[Ir(III)→Ir(IV)].69 According to the onset potentials of their
oxidation processes, the HOMOs of 1 and 2 are estimated to be
40 approximately –5.15 and –5.24 eV, respectively.70 Subsequently,
the corresponding LUMOs are calculated to be –2.48 eV for 1
and –2.62 eV for 2, referring to their HOMOs and absorption
95 Ir(ppy)3 (35.2 cd A–1 and 26.8 lm W–1, Fig. S6, ESI†). The device
B exhibits higher efficiency than D although 1 displays lower
PLQY. This is probably due to the better recombination
efficiency of holes with electrons in device B and other
influential factors, such as their different capture of injected
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Table 2 Summary of electroluminescence performance for OLEDs A–N2.
Device
A
Vturnꢀona (V) Lmaxb (cd m–2,V)
ηcb,c (cd A–1)
31.8, 31.7
43.7, 41.5
45.0, 44.5
39.7, 39.1
31.4, 31.1
35.2, 34.6
24.4, 24.1
22.4, 21.4
ηpb,c (lm W–1)
24.1, 20.5
47.9, 30.2
36.9, 31.1
40.9, 31.5
22.7, 17.0
26.8, 22.1
26.3, 19.4
23.4, 16.0
ηextb,c (%)
10.1, 10.0
14.1, 13.4
13.9, 13.7
12.3, 12.1
9.2, 9.1
λEL (nm)
509
CIE [(x, y), V]
(0.34, 0.58), 4.5
(0.32, 0.57), 4.5
(0.35, 0.58), 4.5
(0.35, 0.58), 4.5
(0.32, 0.61), 6.0
(0.33, 0.61), 7.0
(0.37, 0.57), 5.0
(0.39, 0.57), 5.0
3.0
2.5
3.2
2.5
3.3
2.5
2.5
2.6
38780, 9.6
21920, 8.8
96550, 9.6
43460, 8.6
38510, 10.8
51340, 9.8
29910, 7.6
25910, 7.8
508
B
511
C
513
D
514
E
10.4, 10.3
7.7, 7.6
515
F
539
N1
N2
7.0, 6.7
544
5
a Defined as the bias at a brightness of 1 cd m–2. b Maximum values of the devices. c Values of the devices at 1000 cd m–2. Values after the comma are the
voltages at which the data are collected.
Fig. 4 (a) The general structure for OLEDs (A, B, C and D) based on 1 and 2. (b) PL spectra in the neat films and EL spectra for selected OLEDs (B and
D) of 1 and 2. (c) Current density–voltage–luminance and (d) current efficiency–luminance–power efficiency characteristics for OLEDs A, B, C and D.
10 charges and radiative decay of excitons, etc.17,75 Fig. S7 (ESI†)
suggests that 1 and 2 are electronꢀtransport dominated, but 2
shows higher electron mobility than 1. CBP is a holeꢀtransport
favorable ηp of 31.5 lm W–1 at 1000 cd m–2. Devices A and B also
keep high ηc greater than 30 cd A–1 and ηp over 20 lm W–1 at this
high luminance. These excellent efficiencies at 1000 cd m–2 are
dominated host while POAPF is electronꢀtransport dominated. 25 also superior to those of Ir(ppy)3ꢀbased devices (34.6 cd A–1 and
Thus altering of the host from CBP to POAPF may cause
15 somewhat unbalance of carriers transport and it will be worse to
that of 2, leading to the lowerr ηc in 2ꢀbased device D. Even more
gratifying is that, devices A–D possess low efficiency rollꢀoff
22.1 lm W–1, Table 2). Device B still exhibits a peak ηext of
14.1% and 13.7% at 1000 cd m–2. Such promising character could
partly profit from the low turnꢀon voltages and stable carrier
transport balance within the emitter layers in these devices.57,58
values with increasing the brightness (see Table 2). At a high 30 Additionly, the excitedꢀstate lifetimes of Ir(ppy)3 were measured
luminance of 1000 cd m–2, device C retains an excellent ηc of
20 44.5 cd A–1, and its ηc rollꢀoff value is only 0.5 cd A–1. For device
D, it exhibits a rather low rollꢀoff value of 9.4 lm W–1, retaining a
to be 0.56 and 0.42 ꢃs in CH2Cl2 and neat film, respectively,
which were longer than those of 1 and 2. Thus, the lower
efficiency rollꢀoff in devices based on 1 and 2 may also be related
6
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5
Fig. 5 PL spectra of (a) 1 and (b) 2 doped in PMMA films (1–100 wt %).
Fig. 6 (a) The EL spectra for OLEDs N1 and N2 (at 5 V). (b) Current efficiency–luminance–power efficiency characteristics for N1 and N2.
10
to their relatively short excitedꢀstate lifetimes and the weak
intermolecular interaction.58,74 These results reveal the potential
from 520 to 498 nm with reducing its concentration to 1 wt % in
the films, accompanied by vibronically structured emission
application of complexes 1 and 2 as fascinating candidates for 35 character. Similarly, a prodigious blueꢀshift of 27 nm is observed
highly efficient green OLEDs.
for 2 (Fig. 5b) when its concentration is reduced to 1 wt % in the
film. The emission peak of 2 shifts from 540 to 530 nm with
unchangeable profile when its concentration is decreased from
100 to 50 wt % in the PMMA film. However, the PL spectrum
40 displays vibronically structured emission for 2 with diminishing
its concentration to 10 wt % or even lower. Photophysical
measurements and theoretical calculations have suggested that
the phosphorescence emissions of 1 and 2 originate from the
dominantly 3MLCT state, and they suffer the rigidochromic effect
45 from 298 to 77 K. Furthermore, according to the previous works,
a blueꢀshift can be observed for a chargeꢀtransfer emission in a
more rigid matrix due to a perturbed dipolar relaxation of the
chargeꢀtransfer transition state.65,76 Thus, the PMMA (rigid
matrix) films and photophysical results presented in these films
50 (coincident with those in CH2Cl2 from 298 to 77 K) lead us to
suppose that the blueꢀshifts observed here are probably relate to
the rigidochromic effect in 1 and 2. Accordingly, the blueꢀshifts
in EL spectra for 1 and 2 may be ascribed in part to the
rigidochromic effect shown in their photophysical properties.
15 3.6.1 Possible mechanism for the blue-shift in EL spectra
It should be noted from Fig. 4 (b) that, the EL spectra for 1 and 2
show obvious blueꢀshifts of 12 and 27 nm, respectively,
compared to their PL spectra of the neat films. Thus, it can be
seen that complex 2 exhibits green emission in doped devices C
20 and D even though it reveal yellowishꢀgreen emitting in the neat
film. Nevertheless, this obvious blueꢀshift is rare although there
are some common blueꢀshifts in many reported OLEDs based on
iridium(III) complexes.13,57 To better understand this
phenomenon, PL measurements of 1 and 2 doped in PMMA films
25 were then carried out. This doping constructs analogous
environment in comparison with those in CBP and POAPF of
OLEDs. The corresponding films with different doping
concentrations (1–100 wt %) were marked as a–d for 1 and e–i
for 2, respectively, and Table S4 (ESI†) collects the
30 photophysical data for them.
It can be observed from Fig. 5a and Table S4 (ESI†) that, the
PL spectra for 1 doped in PMMA films show obvious blueꢀshift
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DOI: 10.1039/C3TC32092H
Besides, the dispersion of the luminescence complexes in a
addition,
1
and
2
exhibit
a
certain dopantꢀconcentrationꢀ
host matrix can avoid their selfꢀquenching, causing somewhat 60 insensitive feature for the efficiencies in their nonꢀdoped OLEDs.
blueꢀshifts in the doped devices13,57 and, hence, the blueꢀshifts
observed in the EL spectra for 1 and 2 may also be related to the
doping technique. As a result, herein, we tentatively attribute the
blueꢀshifted EL spectra of 1 and 2 to the rigidochromic effect and
the doping technique adopted in this work.
A maximum ηc of 24.4 cd A–1 and ηp of 26.3 lm W–1 also
accompanied by small efficiency rollꢀoff (retaining 24.1 cd A–1
and 19.4 lm W–1 at 1000 cd m–2) are achieved for the nonꢀdoped
OLEDs. These appealing results reveal that complexes 1 and 2
65 open interesting perspectives for the development of highꢀ
performance OLEDs in the future.
5
3.6.3 Non-doped devices based on complexes 1 and 2
To evaluate whether complexes 1 and 2 process dopantꢀ
10 concentrationꢀinsensitive feature in the efficiencies of their
OLEDs, nonꢀdoped devices with the traditional configurations of
ITO/MoO3 (3nm)/TAPC (35 nm)/TCTA (5 nm)/1 or 2 (20
nm)/Bphen (40 nm)/LiF (0.5 nm)/Mg:Ag, N1 and N2, were
prepared respectively. The carrier injection and transportation
15 layers used here are identical to those in devices A–F.
As depicted in Fig. 6a, the EL spectra of devices N1 and N2
reveal intense emissions with CIE coordinates of (0.37, 0.57) and
(0.39, 0.57), respectively, which are identical to their PL profiles
in the neat films (see Fig. 2), suggesting that their emissions
20 occur from the iridium(III) complexes. The redꢀshifts observed
here compared with the EL spectra in doped devices based on 1
and 2 are inevitable and agreeing well with those of previously
reported examples,57 which can be ascribed to the annihilation
effect in iridium(III) complex.37,38 The current density–voltage–
25 luminance characteristics for N1 and N2 are given in Fig. S8
(ESI†). It can be found that N1 and N2 reveal low turnꢀon
voltages of 2.5 and 2.6 V, respectively. This feature is also the
prerequisite for achieving highꢀperformance in nonꢀdoped
OLEDs.
Acknowledgements
The authors gratefully acknowledge the financial support from
NSFC (21273030, 21303012, 51203017, 21203019, 21131001
70 and 60937001), 973 Program (2009CB623605), the Science and
Technology Development Planning of Jilin Province
(20130522167JH and 20130204025GX) and the Fundamental
Research Funds for the Central Universities (12QNJJ012).
Notes and references
a
75 College of Chemistry, Northeast Normal University, Changchun, Jilin,
130024,
P.
R.
China.
Eꢀmail:
shangg187@nenu.edu.cn,
zmsu@nenu.edu.cn
b
State Key Laboratory on Integrated Optoelectronics, College of
Electronic Science and Engineering, Jilin University, Changchun, Jilin,
80 130012, P. R. China. Eꢀmail: xiewf@jlu.edu.cn
† Electronic Supplementary Information (ESI) available: Crystal data and
structure refinements for complex 1, comparison between theory and
experiment for selected bond lengths and bond angles in 1, calculated
triplet states for 1 and 2 by a TDꢀDFT approach, photophysical data for 1
85 and 2 doped in PMMA films, difference electron density computed by
subtracting the electron densities of the T1 and S0 states for 1 and 2,
chemical structures of materials used in OLEDs, EL spectra for devices A
and C at 4.5 V, and for E (at 6 V) and F (at 7 V), the current density–
voltage–luminance characteristics for devices E and F, the current
90 efficiency–luminance–power efficiency characteristics for devices E, F,
N1 and N2. See DOI: 10.1039/b000000x/
30
Besides the low turnꢀon voltages, devices N1 and N2 also
display good current, power and external quantum efficiencies
(Fig. 6b). A maximum ηc of 24.4 cd A–1 and ηp of 26.3 lm W–1
are successfully achieved for device N1. At 1000 cd m–2, it shows
small efficiency rollꢀoff values, still remaining high ηc of 24.1 cd
35 A–1 and ηp of 19.4 lm W–1. For device N2, it reveals a maximum
ηc of 22.4 cd A–1 and ηp of 23.4 lm W–1. Similarly, it can hold
good efficiencies of 21.4 cd A–1 and 16.0 lm W–1 at 1000 cd m–2.
Obviously, the appealing efficiencies presented here for
complexes 1 and 2 are also inseparable from their favorable
40 photophysical properties. Meanwhile, this promising quality also
suggests a certain dopantꢀconcentrationꢀinsensitive feature shown
in their OLEDs.
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