Iridium complexes containing 2-aryl-benzothiazole ligands: Color tuning and application in high-performance organic light-emitting diodes

Article abstract of DOI:10.1039/c3tc30586d

Three 2-aryl-benzothiazole chromophores were designed and synthesized for use as major cyclometalating ligands of iridium complexes, in which the aryl groups were N-phenyl-3-carbazolyl, 2-(9,9-dioctyl)fluorenyl and N-phenyl-2-carbazolyl. The homoleptic tris-cyclometalated and heteroleptic bis-cyclometalated iridium complexes, 1-5, were synthesized using these ligands. By adjusting the chemical structures and then the electronic state of these complexes, we were able to continuously tune the phosphorescence from yellow to saturated red with peak wavelengths in the order of 1 < 2 < 3 < 4 < 5. The quantum chemical calculations and the electrochemical data clearly demonstrate the origin of the phosphorescence color tuning. The organic light-emitting diodes (OLEDs) containing these iridium complexes as doped emitters exhibited yellow to red electrophosphorescence with excellent performance. Particularly, the complex 1 based device produced high efficiencies of 75.9 cd A^-1, 48.2 lm W^-1, and 23.0% with CIE (0.46, 0.53), which represent the highest efficiencies for yellow OLEDs up to now. Furthermore, 1 was used to fabricate two-element white OLEDs in combination with a blue phosphor and high efficiencies of 57.9 cd A^-1 and 21.9% were achieved, which are among the best efficiencies for two-emitting-component white OLEDs reported so far. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3tc30586d

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Materials Chemistry C
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Iridium complexes containing 2-aryl-benzothiazole
ligands: color tuning and application in high-
performance organic light-emitting diodes†
Cite this: J. Mater. Chem. C, 2013, 1,
4171
*
Jiuyan Li, Renjie Wang, Ruixia Yang, Wei Zhou and Xin Wang
Three 2-aryl-benzothiazole chromophores were designed and synthesized for use as major cyclometalating
ligands of iridium complexes, in which the aryl groups were N-phenyl-3-carbazolyl, 2-(9,9-dioctyl)fluorenyl
and N-phenyl-2-carbazolyl. The homoleptic tris-cyclometalated and heteroleptic bis-cyclometalated iridium
complexes, 1–5, were synthesized using these ligands. By adjusting the chemical structures and then the
electronic state of these complexes, we were able to continuously tune the phosphorescence from
yellow to saturated red with peak wavelengths in the order of 1 < 2 < 3 < 4 < 5. The quantum chemical
calculations and the electrochemical data clearly demonstrate the origin of the phosphorescence color
tuning. The organic light-emitting diodes (OLEDs) containing these iridium complexes as doped emitters
exhibited yellow to red electrophosphorescence with excellent performance. Particularly, the complex 1
based device produced high efficiencies of 75.9 cd Aꢀ1, 48.2 lm Wꢀ1, and 23.0% with CIE (0.46, 0.53),
which represent the highest efficiencies for yellow OLEDs up to now. Furthermore, 1 was used to
fabricate two-element white OLEDs in combination with a blue phosphor and high efficiencies of
57.9 cd Aꢀ1 and 21.9% were achieved, which are among the best efficiencies for two-emitting-
component white OLEDs reported so far.
Received 29th March 2013
Accepted 1st May 2013
DOI: 10.1039/c3tc30586d
www.rsc.org/MaterialsC
tuned from 504 to 596 nm by varying the aryl moiety from
3-carbazolyl to 2-benzo[b]thiophenyl.4 Another typical strategy is
Introduction
Phosphorescent transition-metal complexes have drawn great
research and industry attention because of their excellent
performance when used as colorful emitters in organic light-
emitting diodes (OLEDs). Among them, cyclometalated IrIII
complexes are regarded as the most successful phosphorescent
material1 owing to their high photoluminescence efficiency,
relatively short lifetimes and exible color tunability.2 For both
the homoleptic and heteroleptic IrIII complexes, the cyclo-
metalating C^N ligands play essential roles in determining the
phosphorescent energy and emitting color of the complexes,
although occasionally the ancillary ligand is also important, like
to incorporate different substituents in either C-related arenes
or N-related heterocycles while xing the C^N ligand skeleton.
For example, incorporating different substituents at different
positions of the 2-phenylpyridine ligand framework has resul-
ted in a continuous tuning of the phosphorescence of their
homoleptic iridium complexes from sky blue (468 nm) to
orange-red (595 nm).5 All these methods are successful in
tuning the phosphorescence color since in most cases the
cyclometalating C^N ligands are involved in their lowest-energy
3
triplet excited state, either the ligand-centered p–p* state or
the metal-to-ligand-charge-transfer (3MLCT) triplet state, which
is responsible for the phosphorescence. For most of the IrIII
complexes, the highest occupied molecular orbital (HOMO)
consists of arene p- and Ir d-orbitals, while the lowest unoc-
cupied molecular orbital (LUMO) is located largely on the
N-related heterocycle that is usually electron-decient. In
general, the phosphorescence of the IrIII complex red shis
when the electron-donating substituent is introduced into the
HOMO-lying arene part and/or the electron-decient group is
incorporated in the LUMO-lying heterocycle moiety of the C^N
ligands. Whereas a hypsochromic shi will occur when the
electron-decient substituent is introduced into the HOMO-
lying arene and/or the electron-rich group is incorporated into
the LUMO-lying heterocycle.
in
iridum(III)bis[(4,6-di-uorophenyl)-pyridinato-N,C2]picoli-
nate (Firpic).3 Accordingly, two typical strategies have been
utilized to tune the emissive color of the iridium complexes.
One method is to design different C^N ligand frameworks. For
this purpose, a exible combination of different arenes and
heterocycles has generated various C^N ligand frameworks. For
example, the phosphorescence peak wavelength of a group of
2-aryl-pyridine based homoleptic iridium complexes is readily
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian
University of Technology,
jiuyanli@dlut.edu.cn
2 Linggong Road, Dalian 116024, China. E-mail:
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc30586d
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Among the reported cyclometalating C^N ligands, 2-phenyl- transporting group,8 (2) its electron-donating feature will tune
benzothiazole and its derivatives have drawn increasing atten- the electronic state and then the emission color of the resulting
tion and many IrIII complexes containing this ligand framework iridium complexes. These 2-aryl-benzothiazole chromophores
have been developed as efficient electrophosphorescent emit- were used to prepare the homoleptic tris-cyclometalated
ters for OLED application.6 For example, a high EL efficiency of and heteroleptic bis-cyclometalated iridium complexes, 1–5
89 cd Aꢀ1 was obtained in our previous report when the (Scheme 2). The photophysical, electrochemical, and electro-
2-phenyl-benzothiazole ligand was decorated by CF3 at the luminescent properties of these iridium complexes were
6-position of a benzothiazole ring.6f A two-element white OLED investigated systematically. We demonstrated that the phos-
with a peak current efficiency of 68 cd Aꢀ1 was also fabricated phorescence of these benzothiazole-containing iridium
with the related derivative as an orange emitter.6d However,
most of the IrIII complexes containing 2-phenyl-benzothiazole
as major ligands, with a few exceptions,6b,g are limited to the
yellow-emissive range. The red-emissive analogues containing
benzothiazole moieties are still absent, which in turn are
identically important for application in both red and RGB white
OLEDs. As far as we know, besides 2-phenyl-benzothiazole, few
other benzothiazole-containing chromophores7 have been
developed as the cyclometalating ligand framework to prepare
corresponding IrIII complexes up to now.
Enlightened by the remarkable EL performance of the
current 2-phenyl-benzothiazole based iridium complexes, our
interest is to exploit more 2-aryl-benzothiazole chromophores
as cyclometalating C^N ligands to prepare novel IrIII complexes.
In this paper, we report the synthesis of three 2-aryl-benzo-
thiazole chromophores (Scheme 1), in which the aryl groups are
N-phenyl-3-carbazolyl (L1), 2-(9,9-dioctyl)uorenyl (L2), and
N-phenyl-2-carbazolyl (L3), respectively. It should be noted that
the uorene and carbazole rings have similar skeletons, but are
linked to the benzothiazole moiety via different sites, resulting
in a different extent of conjugation within these ligands.
Ligands L2 and L3 have identical linear conjugation between
the benzothiazole moiety and a biphenyl, which is longer than
that of the benzothiazole and a phenyl in ligand L1. Carbazole
has been selected as the important building block due to the
following reasons: (1) it is an excellent hole injecting and
Scheme 2 Synthetic routes for the cyclometalated iridium(III) complexes 1–5.
Scheme 1 Structures and synthetic routes for the ligands L1, L2, and L3.
4172 | J. Mater. Chem. C, 2013, 1, 4171–4179
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complexes are able to be tuned over a wide spectral range from
yellow to saturated red by adjusting the p-conjugation length of
the 2-aryl-benzothiazole ligand, introducing electron-donating
groups into the HOMO-locating aryl moiety of the ligands, or
incorporating high-eld-strength ancillary ligands. In addition,
high-performance monochromatic and white OLEDs were ach-
ieved using these novel benzothiazole-containing iridium
complexes as doped emitters.
Results and discussion
Preparation and characterization
Scheme 1 outlines the synthetic routes of the cyclometalating
ligands L1–L3. The important intermediates, including N-phenyl-
carbazole-3-carbaldehyde,9 2-(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)-9,9-dioctyluorene,10,11 and N-phenylcarbazole-2-boronic
acid,12 were synthesized according to the literature methods.
The cyclometalating ligand L1 was obtained by the cyclizative
condensation of o-aminothiophenol with the corresponding
aldehyde precursor.13 Whereas Suzuki coupling of the commer-
cially available 2-bromobenzothiazole with the 2-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyluorene
and
N-
phenylcarbazole-2-boronic acid generated the ligands L2 and L3,
respectively, in high yields.11,12 The target iridium complexes 1–5
were synthesized following the traditional procedures as shown in
Scheme 2. First the cyclometalation of the iridium trichloride
hydrate with each cyclometalating ligand generated the corre-
sponding chloro-bridged dimmer [Ir(C^N)2Cl]2.14 Further reaction
of this dimer intermediate with the same ligand in reuxing
diglyme afforded the desired homoleptic tris-cyclometalated IrIII
complexes 1 and 3.5b On the other hand, the heteroleptic IrIII
complexes 2, 4, and 5 were obtained by reuxing the corre-
sponding chloro-bridged dimmer with acetylacetone in 2-ethox-
yethanol in the presence of Na2CO3.14 We tried to prepare the
desired homoleptic complex of ligand L3 by using the corre-
sponding chloro-bridged dimer, the heteroleptic complex 5, and
Ir(acac)3 as the starting materials. However, we nally failed to
obtain it aer trying all these typical pathways.5b,15 All the obtained
iridium complexes were well dissolved in common organic
solvents and thus puried by column chromatography over silica-
gel. They were fully characterized by 1H and 13C NMR, Maldi-TOF
or ESI-MS mass spectrometry, and elemental analysis.
Fig. 1 UV-vis absorption (a) and PL (b) spectra of complexes 1–5 in dilute
dichloromethane solution.
3p–p* states.17 In comparison with the homoleptic analogue,
the heteroleptic bis-cyclometalated iridium complex of each
ligand shows a regular bathochromic effect in their absorption
spectra, especially in the short wavelength range. For example, a
red-shi of 7 nm was detected in the absorption spectrum of
complex 2 relative to 1. In comparison with complexes 1 and 2,
complexes 3 and 4 revealed a red-shi of approximately 30 nm
in their absorption spectra, complex 5 showed a further red-
shi. This should be as a result of the extended p conjugation in
ligands L2 and L3 in comparison with ligand L1.
Fig. 1b shows the photoluminescence (PL) spectra of these
iridium complexes in dilute dichloromethane solution. Upon
photoexcitation at 400 nm, all these complexes show intense
room-temperature phosphorescence with vibronic ne struc-
tures in the PL spectra. With reference to the spectroscopic data
of other similar iridium complexes in the literature, all the
iridium complexes in the present study may possess dominantly
the 3p–p* lowest excited states. The vibronic splitting in the
Photophysical properties
Fig. 1 shows the absorption spectra and photoluminescence emission proles implies the phosphorescence may come
3
spectra of complexes 1–5 in dilute dichloromethane solution. mainly from the predominantly ligand-centered p–p* excited
The pertinent data of these IrIII complexes is summarized in states, rather than from the 3MLCT excited states, which are
Table 1. As shown in Fig. 1a, these complexes all displayed otherwise characterized by broad and featureless emission
characteristic absorption spectra of IrIII complexes, which are spectra.17 The emission wavelengths of these complexes
dominated by multiple absorption bands originating from the strongly depend on the structures of the cyclometalating
ligand-centered p–p* transitions and MLCT (metal-to-ligand- ligands. For the 3-carbazolyl-containing homoleptic tris-cyclo-
charge-transfer) transitions.16 The intense absorption bands in metalated complex 1, the phosphorescence is located in the
the short wavelength region appear to be the spin-allowed yellow range with a peak at 551 nm and a shoulder at 580 nm.
ligand-centered 1p–p* transitions. The weaker absorption With the cyclometalating ligand going from L1 to 2-uorene-
bands over 400 nm should correspond to the excitations to based L2, the triscyclometalated iridium complex 3 shows red-
1
3
the MLCT and MLCT, and the spin-orbit coupling enhanced shied phosphorescence at 587 nm with a shoulder at 634 nm
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Table 1 Photophysical and electrochemical data obtained experimentally for the iridium complexes
d [V]
ox
onset
HOMO/LUMOe [eV]
Eg [eV]
a
b
c
f
Compound
labs [nm]
lema [nm]
Fp
sp [ms]
E
1
2
3
4
5
322, 369, 399, 443
329, 379, 403, 465
358, 417, 465, 504
353, 422, 476, 506
361, 433, 502, 542
551, 580 sh
559, 588 sh
587, 634 sh
596, 643 sh
630
0.42
0.34
0.20
0.16
0.18
2.77
2.76
2.56
2.20
3.53
0.60
0.68
0.69
0.78
0.47
ꢀ5.00/ꢀ2.64
ꢀ5.08/ꢀ2.78
ꢀ5.09/ꢀ2.91
ꢀ5.18/ꢀ2.97
ꢀ4.87/ꢀ2.87
2.36
2.30
2.18
2.21
2.00
a
Measured in CH2Cl2 at 293 K with a concentration of 10ꢀ5 M, sh ¼ shoulder. b Measured in degassed CH2Cl2 relative to fac-[Ir(ppy)3] (Fp ¼ 0.4), lexc
¼ 400 nm. c Measured in degassed CH2Cl2 at a concentration of 10ꢀ5 M at 293 K. d 0.1 M [Bu4N]PF6 in CH2Cl2, scan rate 100 mV sꢀ1, versus SCE
couple. e HOMO ¼ ꢀe(E
+ 4.4), LUMO ¼ HOMO + Eg. f Estimated from the absorption edge (ledge) of solid lms by the equation Eg ¼ 1240/ledge
.
ox
onset
in comparison with complex 1. This red-shi of phosphores- shiing to red. This is reasonable since it is generally accepted
cence should be ascribed to the extended p-conjugation length that the smaller the energy band gap, the higher the possibility
within the cyclometalating ligand, since ligand L2 can be of non-radiative decay of the excited states.20 The relative high
regarded as p-conjugated between the benzothiazole ring and a phosphorescent quantum yields of these iridium complexes
biphenyl, which is denitely longer by one additional phenyl indicate that they should be promising phosphors for high-
than L1.
On the other hand, for each cyclometalating ligand, the cent lifetimes of these complexes were determined to be in the
heteroleptic bis-cyclometalated iridium complex shows range of 2.20–5.90 ms, which is indicative of strong spin-orbit
performance electrophosphorescent devices. The phosphores-
a
regular red shi of the phosphorescence in comparison with its coupling leading to efficient intersystem crossing from the
tris-cyclometalated analogue. For example, the phosphores- singlet to the triplet state.
cence of the heteroleptic complex 2 has a peak at 559 nm with a
shoulder at 588 nm, which was red shied by 8 nm relative to its
homoleptic analogue 1. A red shi by 9 nm was also observed
Electrochemical analysis
for the heteroleptic complex 4 in comparison to its homoleptic The electrochemical behavior of these complexes was investi-
analogue 3. The bathochromic effect in the phosphorescence of gated by means of cyclic voltammetry (CV) measurements.
the heteroleptic iridium complexes with the acac ancillary Upon the anodic sweep in deoxygenated dichloromethane
ligand was frequently observed and ascribed to the high-eld- solutions, one or two reversible peaks for all the complexes were
strength effect from the acetylacetonate anion.18 With the detected, which should be assigned to the metal-centered
cyclometalating ligand going from L2 to 2-carbazole based L3, IrIII/IrIV oxidation couple and/or the possible oxidation of the
the phosphorescence of the corresponding heteroleptic iridium uorene and carbazole groups on the cyclometalating ligands.21
complex 5 shows dramatic red-shi relative to that of 4. The These oxidation potentials are lower for the homoleptic
phosphorescence of complex 5 is in the saturated red region complexes 1 and 3 than their corresponding heteroleptic
with a peak at 630 nm. L2 and L3 have identical p-conjugation complexes 2 and 4, which is in accordance with the fact that
lengths between the benzothiazole and the biphenyl. Thus the the acac ligand with the stronger ligand-eld strength could
large red shi observed in the phosphorescence of complex 5 stabilize the HOMO level. Notably, by comparison with the
should be attributed to the additional electron-donating effect uorene-based complexes, the incorporation of 2-carbazole and
of the N moiety which is located at the para-site of the C–Ir bond 3-carbaozle in the ligands shis the oxidation potentials of the
and strongly activates the HOMO level of this complex. There- iridium complexes to less-positive values and the HOMO to
fore, through combination of the three strategies, i.e. expanding higher levels, indicating a greater ease for 1, 2, and 5 to loose an
the effective p-conjugation length of the cyclometalating ligand, electron and to accept the injected hole from the anode or other
introducing an electron-donating element into the aryl moiety functional layers in the OLEDs. On the other hand, the stabili-
of the ligands, and incorporating a high-eld-strength ancillary zation effect on the iridium complex caused by the extended
ligand, the phosphorescence of this series of iridium complexes p-conjugation length in the ligand frameworks can be veried
1–5 is continuously tuned from yellow to saturated red with by the lowering of both the HOMO and LUMO of complexes 3
peak wavelengths in the order of 1 < 2 < 3 < 4 < 5.
and 4 (Table 1) in comparison with complexes 1 and 2,
The photophysical properties of these complexes were respectively. Similar to the 2-uorene based complex 4, the
further probed by measuring their relative phosphorescent 2-carbazole based complex 5 shows a similar decrease of LUMO
quantum yields in dilute degassed dichloromethane solutions in comparison with complex 2 due to the extended p-conjuga-
at room temperature with fac-Ir(ppy)3 as a standard (FP ¼ 0.40, tion length. However, the HOMO of complex 5 is dramatically
Hppy ¼ 2-phenylpyridine).19 Most of these IrIII complexes raised to a higher level (ꢀ4.87 eV) than that (ꢀ5.18 eV) of
showed high Fp (Table 1). Especially, complex 1 showed the complex 4, resulting in a further smaller HOMO–LUMO band
highest Fp of 0.42 and even the red-emitting 5 showed a Fp as gap and further red-shi of phosphorescence in complex 5. This
high as 0.18. It is interesting that the Fp of these complexes further red-shi should be as a result of the electron-donating
shows a regular decreasing trend with the emission wavelength feature of the N atom located at the para-site of the C–Ir bond
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and its destabilization effect on the HOMO level. All these complex 5, is more extended to the 9-site N atom and the
electrochemical observations are consistent with the photo- appending phenyl ring. This should be because the N-phenyl
physical results. The measured oxidation potentials for these moiety is located in the para-site of the C–Ir bond in complex 5
complexes are in the range of 0.59–0.78 V vs. saturated calomel and its electron-donating character will have a stronger stabi-
electrode (SCE). On the basis of the onset potential of the rst lization effect on the HOMO level of this molecule than in
ox
onset
oxidation (E
) and the absorption edge data, the HOMO and complex 2. The LUMO orbitals of all these iridium complexes
LUMO energy levels can be estimated from the empirical cover the whole benzothiazole part and spread more and more
ox
formulae: EHOMO ¼ ꢀe(E
+ 4.4) and ELUMO ¼ EHOMO + Eg to the aryl moiety with the phosphor going from 2, to 4, to 5.
onset
(where Eg is the optical energy gap obtained from the absorption Since the ligands L2 and L3 have similar molecular skeletons,
threshold of the lm sample).22 The electrochemical and elec- the LUMO orbitals of 4 and 5 are identically located on the
tronic data of these complexes is collected in Table 1.
whole ligands including the benzothiazoles and the biphenyl
rings on the uorene and 2-carbazole moieties. It is obvious that
the 2-uorenyl and 2-carbazolyl rings effectively expand the
p-conjugation length of the ligand frameworks in comparison
with the 3-carbazolyl, which results in more delocalized and
stabilized LUMOs for 4 and 5 than 2. This is consistent with the
much lower LUMO levels obtained by experiment for 4 and 5. In
comparison with 2, the 2-uorenyl-based complex 4 possesses
both a stabilized HOMO and LUMO due to the extended
p-conjugation length. However, the much larger downwards
shi of the LUMO than the HOMO nally produces a further red
shi in its phosphorescence. In comparison with complex 4,
complex 5 has a more destabilized HOMO due to additional
contribution from the electron-donating carbazole and identi-
cally stabilized LUMO due to the same p-conjugation in the
ligand, and consequently exhibits further red-shi in phos-
phorescence relative to complex 4. Therefore, the phosphores-
cence peak wavelength of these heteroleptic complexes exhibits
a regular tunability in the order of 2 < 4 < 5.
Quantum chemical calculations
In order to gain further insight into the photophysical and
electrochemical behaviors of these phosphors and to under-
stand the origin of color tenability induced by substitution,
density functional theory (DFT) calculations were performed for
complexes 2, 4, and 5 using B3LYP hybrid functional theory
with Gaussian 03.23 These IrIII complexes have very similar
structures with octahedral geometry. Fig. 2 depicts the selected
molecular orbitals involved in the lowest-lying transitions of
these complexes.
As shown in Fig. 2, the HOMO orbital of each of these
complexes is located on both the central iridium atom and the
aryl group of the 2-arylbenzothiazole ligand, but with different
contributions from the aryl group. For the carbazole-containing
complexes, the HOMO orbital of complex 2 is mainly located on
the two benzene rings of the carbazole moiety, and that of
complex 5 is mainly distributed on the biphenyl part of the
carbazole moiety with additional contribution from the 9-site N
atom and the appending phenyl ring. However the HOMO
orbital of the uorenyl-containing complex 4 is only located on
the single phenyl ring that is directly bonded to the benzo-
thiazole. Apparently, the HOMO orbitals for these carbazole-
containing complexes 2, and 5 are more delocalized and
destabilized than 4. This is consistent with the electrochemical
result shown in Table 1 that the HOMO levels for 2, and 5 are
higher to some extent than that of 4 due to the stronger electron
donating character of carbazole groups than uorene. In
comparison with that of complex 2, the HOMO of its analogue,
Electroluminescent properties
To illustrate the electroluminescent properties of these
complexes, the monochromatic OLEDs A–E were fabricated using
these complexes as the doped emitter. With the exception of 5,
these complexes were signicantly stable against thermal subli-
mation and thus the OLEDs A–D were prepared with the emitting
layer deposited by the vacuum thermal evaporation approach.
The device conguration was ITO/PEDOT:PSS (40 nm)/NPB
(20 nm)/CBP:IrIII phosphor (x wt%, 30 nm)/TPBI (45 nm)/LiF
(1 nm)/Al, in which 4,40-N,N0-dicarbazolebiphenyl (CBP) served as
a host material for the iridium complexes, poly(3,4-ethyl-
enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the
hole injecting layer, and 4,40-bis[N-(1-naphthyl)-N-phenylamino]-
biphenyl (NPB) and 1,3,5-tris[N-(phenyl)benzimidazole]-benzene
(TPBI) as the hole-transporting and electron-transporting mate-
rials, respectively. Since complex 5 was found to decompose
before sublimation, its OLED was fabricated by spin coating a
mixed solution of 5 in small molecular host materials as the
emitting layer. The solution-processed OLED E containing 5 has
the conguration of ITO/PEDOT:PSS (40 nm)/5 (5 wt%): m-TDATA
(50 wt%): PBD (45 wt%) (30 nm)/TPBI (45 nm)/LiF (1 nm)/Al, in
which m-TDATA (4,40,400-tris[(3-methylphenyl)phenylamino]tri-
phenylamine) acts as the hole-transporting host and PBD
(2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole) as the
electron transporting one. For the purpose of optimization, the
doping level of each iridium phosphor was varied in the range
Fig. 2 The distribution of the HOMOs and LUMOs for complexes 2, 4, and 5.
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5 to 10 wt%. Table 2 summarizes the best performance data for
each device at the optimized doping level of the Ir phosphor.
Fig. S1† shows the normalized electroluminescence (EL)
spectra of these devices. Essentially, a wide EL color range
spanning from yellow to red (550 of 1 to 638 nm of 5) was
observed for these devices. Fig. S2† illustrates the EL spectra
under different luminance for complex 2 based device B as an
example. Apparently, the EL spectra were independent of the
driving voltage and the luminance, indicating a high stability of
the EL spectra for these phosphors. In addition, the same color
tunability was obtained in EL as in PL with simply varying the
organic ligands. Highly efficient electrophosphorescent OLEDs
have been demonstrated with these new complexes and they
exhibited low turn-on voltages of 3.2–4.9 V at a brightness of
1 cd mꢀ2. The luminance–voltage–current density (L–V–J)
characteristics of OLEDs A–D and their efficiency curves are
provided in Fig. 3 and 4, and S3,† respectively. The best
performance was achieved for the yellow-emitting device A
doped with 1 at the doping level of 7 wt%, with a maximum
luminance (Lmax) of 51 220 cd mꢀ2 at 11.5 V, a forward viewing
maximum luminance efficiency hL of 75.9 cd Aꢀ1 at 5.5 V, a
maximum power efficiency hp of 48.2 lm Wꢀ1 and a maximum
external quantum efficiency (hext) of 23.0%. For device A, the EL
spectrum at 8 V has a maximum peak at 550 nm and
Commission International de I'Eclairage (CIE) coordinates of
(0.46, 0.53), which falls in the yellow region of the CIE chro-
maticity diagram. Indeed, the efficiency of 75.9 cd Aꢀ1 (48.2 lm
Wꢀ1, 23.0%) is much higher than the highest value ever
reported for yellow OLEDs24 and is even comparable with the
vacuum-deposited green OLEDs.25 This high performance for
device A should be predominantly ascribed to the high phos-
phorescence quantum yield of 1 (0.42). Furthermore, the hole-
transporting carbazole moiety that brings about a more
balanced electron and hole transportation and recombination
in CBP host layer is critical in improving the EL efficiency. The
performance of its heteroleptic analogue 2-doped device B is
also remarkable with Lmax of 55 350 cd mꢀ2, peak hL of 56.3 cd
Aꢀ1, hp of 35.4 lm Wꢀ1 and hext of 18.7% at a doping concen-
tration of 7 wt%. Although these efficiencies are slightly lower
than device A, they are still among the highest for yellow-
emitting OLEDs.
Fig. 3 The luminance–voltage–current density (L–V–J) characteristics for OLEDs
A–D.
32 830 cd mꢀ2, peak hL of 33.9 cd Aꢀ1, hp of 21.9 lm Wꢀ1 and hext
of 14.2%. In particular, the device D containing dopant 4
exhibited an almost saturated red emission and good perfor-
mance with a maximum efficiency of 19.6 cd Aꢀ1, 10.3 lm Wꢀ1
and 13.4% with CIE coordinates of (0.63, 0.37). To the best of
our knowledge, these EL efficiencies of device C are some of the
highest ever reported for orange-red emitting OLEDs based on
vacuum-sublimated Ir dopants.26
Complex 5 based device E emitted a saturated red light with a
peak wavelength at 638 nm and a shoulder at 688 nm, which
correspond to the CIE coordinates of (0.67, 0.32). In comparison
with all the aforementioned vacuum deposited OLEDs, the
performance of the partially solution-processed device E are not
so remarkable (Fig. S4†), with a maximum luminance of 1706 cd
mꢀ2 at 10 V, and peak hL of 5.4 cd Aꢀ1, hp of 4.4 lm Wꢀ1 and hext
of 8.3%. The reduced performance in device E should be
understandable if considering the device fabrication approach
and the relatively lower phosphorescent quantum yield of
complex 5. It is usually accepted that the solution-processed
devices exhibit an inferior performance than those vacuum
deposited ones due to the relatively worse lm quality caused by
the wet methods.
Besides their high efficiencies, another attractive feature of
our present IrIII complexes is the less efficient roll-off with
increasing driving voltage (Fig. 4 and S2†). Taking complex 1 as
an example, the luminance efficiency hL and external quantum
efficiency hext at a brightness of 1000 cd mꢀ2 are still as high as
75.3 cd Aꢀ1 and 22.6%, respectively. The efficiency roll-off ratios
Device C fabricated with 10 wt% of the orange-red complex 3
also showed an impressive performance with an Lmax of
Table 2 EL performance of the IrIII complexes-doped monocharomatic OLEDs (A–E) and white-emitting device W
Doping level
[wt %]
Lmax
Device
Dopant
Vturn-on [V]
[cd mꢀ2
]
hexta [%]
hLa [cd Aꢀ1
]
hPa [lm Wꢀ1
]
lem [nm]
CIE (x, y)
A
B
C
D
E
1
2
3
4
7
7
10
10
5
3.6
3.2
4.9
4.3
2.9
4.0
51 220
55 350
26 130
19 070
1706
23.0, 22.6
18.7, 18.5
16.0, 15.0
13.4, 12.4
8.3
75.9, 75.3
56.3, 56.1
32.9, 31.2
19.6, 18.1
5.4
48.2, 42.1
35.4, 33.6
15.9, 12.9
10.3, 8.1
4.4
550, 584 sh
563, 595 sh
585, 637 sh
598, 651 sh
638, 688 sh
472, 545
(0.46, 0.53)
(0.50, 0.50)
(0.59, 0.41)
(0.63, 0.37)
(0.67, 0.32)
(0.31, 0.45)
5
W
1, Firpic
1.5, 8
23 180
21.90, 18.61
57.91, 48.82
36.37, 21.55
a
The numbers before the commas are the maximum values of the devices, those aer the comma are values collected at a brightness of
1000 cd mꢀ2
.
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Journal of Materials Chemistry C
respectively. With increasing the driving voltage, the CIE coor-
dinates varied only slightly from (0.31, 0.45) at 6 V, (0.29, 0.44) at
8 V to (0.28, 0.43) at 10 V. The white emission slightly shied
with more contribution from the blue emitter when increasing
driving voltage and luminance, as indicated by the spectra
change in Fig. S5.† This is reasonable since the higher-energy
blue component in the device will be more electrically excited at
higher electric eld than at low eld. However, the slight CIE
variation still indicates the relatively high stability of the
emission color in this WOLED.
The L–V–J characteristics and the efficiency plots are shown
in Fig. 6. As can be seen, the threshold voltage of the device was
4.0 V and the luminance reached a maximum of 23 180 cd mꢀ2
Fig. 4 The plots of luminance efficiency versus luminance for device A–D.
at 12 V. The peak efficiencies of this WOLED reached hext
¼
21.9%, hL ¼ 57.9 cd Aꢀ1, and hP ¼ 36.4 lm Wꢀ1, respectively. The
gradual efficiency roll-off was observed in device W with
increasing the current density, which can be typically attributed
to a combination of triplet–triplet annihilation and eld-
induced quenching effects in electrophosphorescent devices.18a
However, at a practical brightness of 1000 cd mꢀ2, the effi-
ciencies were maintained at 18.6%, 48.8 cd Aꢀ1, and 21.6 lm
Wꢀ1. Apparently, there is still room for improvement of the
efficiency and color purity for our present WOLED by opti-
mizing the device conguration parameters and/or combining
the present orange phosphors with more suitable blue emitters.
In recent years, many encouraging results of two-emitting-
component WOLEDs have been reported. Wu et al. realized
highly efficient WOLEDs with a luminance efficiency hL of
42.9 cd Aꢀ1 by doping yellow and blue phosphorescent dyes into
a poly(N-vinylcarbazole) (PVK) host. Ma et al. reported a
from the peak hL (75.9 cd Aꢀ1) and hext (23.0%) values to those at
a brightness of 1000 cd mꢀ2 are only 0.8% and 1.56%,
respectively.
The remarkable performance of 1 in its monochromatic
OLED prompted us to explore its application in a two-emitting-
component white OLED. The WOLED had the structure of ITO/
PEDOT:PSS (40 nm)/CBP:1 (x wt%) (10 nm)/mCP:Firpic (8 wt%)
(20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (device W), in which Firpic
and mCP represent iridium(III)bis(4,6-(diuorophenyl)pyr-
0
idinato-N,C2 )picolinate and N,N0-dicarbazolyl-3,5-benzene,
respectively. The 10 nm-thick CBP doped with 1 served as the
yellow-emitting layer and 20 nm-thick mCP doped with 8 wt%
Firpic as the sky-blue-emitting layer. In order to avoid the
undesired dopant–dopant energy transfer if both the yellow and
blue iridium phosphors were co-doped in one host layer, each of
these two dopant emitters were arranged in individual emitting
layers. mCP with a higher triplet energy was selected as the host
for blue-emitting Firpic to ensure efficient host-to-dopant
energy transfer and to avoid the unwanted back energy transfer.
By xing the concentration of FIrpic at 8 wt%, the doping
concentrations of the 1 in CBP varied from 0.5 to 2 wt% in order
to tune the purity of the white spectrum, as well as the
device efficiency. Fig. 5 shows the EL spectrum of device W with
1.5 wt% of 1 in CBP, which was selected as the optimized
doping concentration and exhibits a balanced white light
emission with CIE coordinates of (0.31, 0.45). Two intense
emission peaks at 472 nm and 545 nm were observed and
assigned to the emission from Firpic and complex 1,
Fig. 6 The luminance–voltage–current density (L–V–J) characteristics (a) and the
plots of efficiencies versus current density (b) for the white OLED.
Fig. 5 EL spectrum of the WOLED at 6 V.
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Paper
vacuum-deposited phosphorescent WOLED exhibiting a peak electrochemical workstation (BAS100B, USA) at room tempera-
hL of 52.8 cd Aꢀ1 and a hP of 42 lm Wꢀ1. In our own recent ture in a 0.1 M [Bu4N]PF6 solution under nitrogen gas
publication, an even higher efficiency of 68.6 cd Aꢀ1 and hP of protection.
34 lm Wꢀ1 was achieved for two-element WOLEDs. Apparently,
even without any outcoupling enhancement, the maximum
Quantum chemical calculations
luminance efficiency of 57.9 cd Aꢀ1 and peak power efficiency of
Density functional theory (DFT) calculations using the B3LYP
36.4 lm Wꢀ1 in the present study are still among the best values
functional were performed. The basis set used for C, H, O, N,
ever reported for two-emitting-component WOLEDs.24,27
and S atoms was 6-31G while the LanL2DZ basis set was
employed for the Ir atoms. There were no imaginary frequencies
for both optimized structures. All these calculations were per-
formed with Gaussian 03.
Conclusions
In conclusion, ve novel homoleptic and heteroleptic iridium
complexes were designed and synthesized using three 2-aryl-
benzothiazole derivatives as major cyclometalating ligands. By
OLED fabrication and measurements
The pre-cleaned ITO glass substrates (30 U ,ꢀ1) were treated by
expanding the effective p-conjugation length of the cyclo-
UV-ozone for 20 min. For all devices, a 40 nm thick PEDOT:PSS
metalating ligand framework, introducing an electron-donating
lm was rst deposited on the ITO glass substrates, and baked
at 120 ꢁC for 30 min in air. For monochromatic OLEDs A–E and
element in the HOMO-locating moiety within the cyclo-
metalating ligands, or incorporating a high-eld ancilliary
white device W, all the organic layers were deposited on top of
ligand, the phosphorescence peak of the iridium complex was
PEDOT:PSS lm by vacuum evaporation in a vacuum chamber
successfully shied to red. By these means, the phosphores-
with a base pressure less than 10ꢀ6 torr. For device E, the
cence color of these iridium phosphors was regularly tuned
emitting layer was deposited on the PEDOT layer by spin
from yellow to saturated red. Amongst these strategies, it seems
coating the mixed solution of complex 5 in m-TDATA and
that the p-conjugation length of the cyclometalating ligand
PBD hosts. The EL spectra, CIE coordinates, and current–
voltage–luminance characteristics were measured with
computer-controlled Spectrascan PR 705 photometer and a
framework is the predominent factor over others to determine
the phosphorescence energy and color of the iridium phos-
phors. High-performance monochromatic and white OLEDs
source-measure-unit Keithley 236 under ambient conditions.
were achieved by using these iridium phosphors as doped
The forward viewing external quantum efficiency (hext) was
emitters. Notably, the yellow-emitting OLEDs containing
calculated by using the luminance efficiency, EL spectra and
complex 1 exhibited an excellent performance with 75.90 cd
human photopic sensitivity.
Aꢀ1, 48.19 lm Wꢀ1, and 23.00%, which represent the highest
The details of the compound’s syntheses can be found in
efficiencies for yellow electrophosphorescence up to now. The
ESI.†
2-carbazolyl-benzothiazole ligand based heteroleptic complex 5
exhibited standard red electrophosphorescence with CIE
Acknowledgements
coordinates of (0.67, 0.32) and an efficiency of 8.3%. The two-
emitting-component white OLED fabricated with the yellow 1
and another blue phosphor also produced high-efficiencies of
We thank the National Natural Science Foundation of China
(21072026, 20923006, and 21274016) and the Fundamental
57.91 cd Aꢀ1, 36.37 lm Wꢀ1, and 21.90%, which are also among
Research Funds for the Central Universities (DUT12ZD211 and
the best values for two-element WOLEDs ever reported so far.
DUT13LK06) for nancial support of this work.
Given the ease of synthesis and color tunability, as well as
performance advantages, these iridium phosphors have signif-
icant potential for applications in multicolor displays and
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