Tris(cyclometalated) iridium(III) phosphorescent complexes with 2-phenylthiazole-type ligands: Synthesis, photophysical, redox and electrophosphorescent behavior

Article abstract of DOI:10.1002/ejic.201300595

Tris(cyclometalated) iridium(III) phosphorescent emitters with 2-phenylthiazole-type ligands have been designed and synthesized. Their photophysical properties, electrochemical behavior and electroluminescent (EL) performance can be influenced by introduc

Full text of DOI:10.1002/ejic.201300595

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Thiazole-Based Phosphorescence
C. Yao, B. Jiao, X. Yang, X. Xu, J. Dang,
G. Zhou,* Z. Wu,* X. Lv,* Y. Zeng,
W.-Y. Wong* ................................... 1–11
Tris(cyclometalated) iridium(III) phos-
phorescent complexes with 2-phenylthi-
azole-type ligands have been developed.
Their photophysical, redox and electro-
phosphorescent properties were charac-
terized to address the light-emitting charac-
teristics of the triplet emitters bearing
thiazole-based ligands.
Tris(cyclometalated) Iridium(III) Phos-
phorescent Complexes with 2-Phenyl-
thiazole-Type Ligands: Synthesis, Photo-
physical, Redox and Electrophosphores-
cent Behavior
Keywords: Iridium
/ Phosphorescence /
Thiazole / Photochemistry / Organic light-
emitting diodes / Luminescence
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DOI:10.1002/ejic.201300595
Tris(cyclometalated) Iridium(III) Phosphorescent
Complexes with 2-Phenylthiazole-Type Ligands: Synthesis,
Photophysical, Redox and Electrophosphorescent Behavior
Chunliang Yao,[a] Bo Jiao,[b] Xiaolong Yang,[a] Xianbin Xu,[a]
Jingshuang Dang,[a] Guijiang Zhou,*[a] Zhaoxin Wu,*[b]
Xingqiang Lv,*[c] Yi Zeng,[d] and Wai-Yeung Wong*[d]
Keywords: Iridium / Phosphorescence / Thiazole / Photochemistry / Organic light-emitting diodes / Luminescence
Tris(cyclometalated) iridium(III) phosphorescent emitters
with 2-phenylthiazole-type ligands have been designed and
synthesized. Their photophysical properties, electrochemical
behavior and electroluminescent (EL) performance can be in-
fluenced by introducing fluorine atoms to the phenyl moiety
of the thiazole-based ligands. The phosphorescent emission
maxima can be shifted from 546 nm to 517 nm by increasing
the number of the fluorine atoms attached to the ligands of
the iridium(III) complexes. Furthermore, the HOMO levels
for these phosphorescent complexes exhibit a gradual de-
crease from –5.28 eV to –5.59 eV with the introduction of flu-
orine atoms. Owing to the character of their electronic struc-
tures, the phosphorescent emitters are preferentially excited
by means of a host-guest energy-transfer process in the or-
ganic light-emitting diodes (OLEDs). Accordingly, their EL
performance is strictly restricted by the triplet energy level
difference between the phosphorescent dopant and the host
materials. The thiazole-based cyclometalated iridium(III)
triplet emitters can exhibit maximum EL efficiencies with ηext
= 7.87%, ηL = 23.62 cdA–1 and ηp = 13.46 lmW–1
.
features, which would meet the requirements in the research
frontiers of organic light-emitting diodes (OLEDs),[1] or-
ganic photovoltaic cells,[2] chemosensors[3] etc. Owing to the
heavy-atom effect induced by the transition-metal centers,
the triplet excited states can be easily formed to induce trip-
let emission, i.e. phosphorescence, in these organometallic
Introduction
Recently, substantial research work has been carried out
on the design and synthesis of organometallic molecules of
RuII, OsII, PtII, ReI and IrIII for their unique photophysical
[a] MOE Key Laboratory for Nonequilibrium Synthesis and molecules.[4] Among them, IrIII complexes chelated with 2-
Modulation of Condensed Matter,
phenylpyridine (ppy) type anionic ligands exhibit unique
State Key Laboratory for Mechanical Behavior of Materials,
phosphorescent characteristics, such as high phosphores-
cent quantum yield (ΦP), tunable emission color and a rela-
tively short triplet-state lifetime (τP).[5] Hence, highly ef-
ficient monochromatic OLEDs and white OLEDs
(WOLEDs) have been constructed by employing these
phosphorescent emitters, indicating their great potential in
Institute of Chemistry for New Energy Materials,
Department of Chemistry, Faculty of Science, Xi’an Jiaotong
University
Xi’an 710049, P. R. China
E-mail: zhougj@mail.xjtu.edu.cn
Homepage: http://zhougj.gr.xjtu.edu.cn
[b] Key Laboratory for Physical Electronics and Devices of the
Ministry of Education,
Faculty of Electronic and Information Engineering, Xi’an bringing OLEDs into practical applications.[6]
Jiaotong University
Based on the photophysical and electroluminescent (EL)
Xi’an 710049, P. R. China
studies of the IrIII ppy-type triplet emitters, it can be clearly
E-mail: zhaoxinwu@mail.xjtu.edu.cn
Homepage: http://zhaoxinwu.gr.xjtu.edu.cn
seen that their properties such as emission color, ΦP, τP,
charge carrier injection/transporting properties etc. can be
[c] Shaanxi Key Laboratory of Degradable Medical Material,
Shaanxi Key Laboratory of Physico-Inorganic Chemistry,
Northwest University
greatly affected by the electronic structures of the chelated
organic ligands.[7] This can be ascribed to the fact that the
organic ligands have participated in the excited states of the
IrIII ppy-type phosphors, which has been confirmed by the
substantial contribution from the π orbitals of the organic
ligands to both the highest occupied molecular orbitals
(HOMOs) and the lowest unoccupied molecular orbitals
(LUMOs).[8] Hence, tuning the chemical structures of the
ppy-type organic ligands has become a prevalent strategy
Xi’an 710069, P. R. China
E-mail: lvxq@nwu.edu.cn
Homepage: http://mainpage.nwu.edu.cn/unit/uhgxy/?p=500
[d] Institute of Molecular Functional Materials,
Department of Chemistry and Institute of Advanced Materials,
Hong Kong Baptist University
Waterloo Road, Kowloon Tong, Hong Kong, P. R. China
E-mail: rwywong@hkbu.edu.hk
Homepage: http://chem.hkbu.edu.hk/rwywong
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300595.
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for optimizing the photophysical and EL properties of the
corresponding IrIII ppy-type phosphors. It is well accepted
that the thiazolyl group can be considered as a hybrid of
the electron-deficient pyridyl moiety and electron-rich thio-
phenyl ring,[9] and this provides a distinct electronic struc-
ture to the thiazole with respect to the traditional pyridyl-
type electron-deficient block in the ppy-type ligands. Hence,
introducing a thiazolyl group to the ppy-type ligands
should represent a new outlet for the development of IrIII
ppy-type phosphors with unique photophysical and EL fea-
tures. However, the potential of the thiazole ring in the
phosphorescent cyclometalate chemistry remains relatively
underexplored.[10] By combining a thiazolyl ring with tri-
phenylamine and naphthalene groups, we have successfully
obtained new thiazole-based ppy-type ligands, which can be
employed in the development of novel IrIII ppy-type phos-
phors with unique features, including high EL efficiencies,
tunable phosphorescent color, balanced charge carrier injec-
tion/transporting traits etc. All of these encouraging results
clearly indicate that the thiazole ring can play a critical role
in furnishing valuable properties in the IrIII ppy-type phos-
phors to advance the field of OLEDs. Bearing this in mind,
another series of IrIII ppy-type phosphorescent emitters
with relatively simple thiazole-based ligands has been devel-
Results and Discussion
Synthesis and Characterization
The synthetic procedure for the thiazole-based IrIII ppy-
type phosphors is shown in Scheme 1. The thiazole group
was introduced to the organic ligands L1, L2 and L3 by a
Suzuki cross-coupling reaction between 2-bromothiazole
and the corresponding phenylboronic acid with Pd(PPh3)4
as the catalyst. All the ligands can be easily obtained in
good yields (Ͼ 80%). All the phosphorescent IrIII com-
plexes were prepared according to the well-established two-
step strategy from the cyclometalation of IrCl3·nH2O with
the corresponding organic ligands to form, initially, the μ-
chloro-bridged dimers, followed by coordination of the
acetylacetone (acac) anion in the presence of Na2CO3.[11]
The complexes were purified by silica chromatography to
furnish IrIII complexes in high purity as air-stable orange
to yellow powders. Their well-defined chemical structures
were confirmed by NMR spectroscopy and FAB-MS.
Single Crystal Structure Analyses
Good-quality crystals of Ir-Tz2 were grown by slow dif-
oped for investigating their photophysical properties, elec- fusion of hexane into a CHCl3 solution of Ir-Tz2 at room
tronic structures and EL performance etc. The results pre- temperature. The geometry of Ir-Tz2 was characterized by
sented in this paper will provide not only valuable infor- X-ray crystallography (Figure 1). The crystal structure of
mation about the optoelectronic properties of these thi- Ir-Tz2 reveals that the iridium center is coordinated by two
azole-based phosphorescent IrIII complexes but also an as- anionic C N ligands of L2 and one chelating acac anion.
∧
sessment of the critical role that the thiazole group plays in The coordination around the IrIII center is distorted octahe-
phosphorescent emitters.
dral, with cis-O,O, cis-C,C and trans-N,N chelate disposi-
Scheme 1. Synthetic pathways for the thiazole-based phosphorescent IrIII complexes Ir-Tz1, Ir-Tz2 and Ir-Tz3.
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tions (Figure 1 and Table S1 in the Supporting Infor-
mation). Ir-Tz2 crystallizes in the P3221 space group with
half of the complex in the asymmetric unit. Hence, the two
Ir–N bonds show the same bond length of 2.035(3) Å in
Ir-Tz2. The same results were observed for the Ir–C bond
[1.990(4) Å] and the Ir–O bond [2.138(3) Å]. The N–Ir–N
bond angle is 173.4(2)°. Moreover, the involvement of elec-
∧
tron-withdrawing F atoms on the anionic C N ligand
strengthens the distortion of the coordination sphere of the
central IrIII ion, which can be shown from the dihedral
∧
∧
angles of 78.3(2)° between the two almost coplanar C N
ligands despite those of 89.2(2)–90.8(2)° between each C N
ligand and the acetylacetone anion ligand.
Figure 2. (a) 2D sheet network formed from intermolecular com-
plementary hydrogen bonds C12–H12···F1a [light orange dotted
lines, 3.543(6) Å and 130.6(2)°] in Ir-Tz2. (b) 3D pillared-layer net-
work formed from intermolecular complementary C12–H12···F1a
[light orange dotted lines, 3.543(6) Å and 130.6(2)°], C8–H8···O1b
(sea-green dotted lines, 3.472(8) Å and 168.0(2)°] hydrogen bonds
and π···π interactions (violet dotted lines, centroid-to-centroid dis-
tance 3.871(3) Å] in Ir-Tz2.
Figure 1. Perspective drawing of Ir-Tz2. Only selected atoms are
labeled, and all H atoms are omitted for clarity.
A detailed examination of the crystal packing in complex
Ir-Tz2 reveals that six sets of intermolecular complemen-
tary hydrogen bonds C12–H12···F1a [C12 from Me group
of acac, 3.543(6) Å and 130.6(2)°] in each molecule sur-
rounded by six others lead to the formation of a 2D layered
Thermal and Photophysical Properties
Under nitrogen, the thermal properties of the thiazole-
network, as shown in Figure 2a. On the other hand, as de- based phosphorescent IrIII complexes were characterized by
picted in Figure 2b, a bi-layer framework has been formed thermogravimetric analysis (TGA) and differential scanning
through weak π···π interactions [centroid-to-centroid dis- calorimetry (DSC). The TGA results indicate their good
tance 3.871(3) Å] between two adjacent 2D layers, and two thermal stability with the 5% weight-reduction temperature
adjacent bi-layer frameworks are closely aligned through (ΔT5%) of around ca 320 °C (Table 1). The DSC traces for
intermolecular complementary C8–H8···O1 [3.472(8) Å and the thiazole-based complexes revealed their high glass-tran-
168.0(2)°] hydrogen bonds resulting in the 3D network.
sition temperatures (Tg) in the range from 102 to 109 °C,
Table 1. Photophysical and thermostability data for the thiazole-based IrIII ppy-type phosphors.
[c]
[d]
Complex
Absorption (298 K)[a]
Emission (298 K)[b] ΦP
τP
[μs]
τr[e] ΔT5%/Tg
λabs [nm]
λem [nm]
[μs]
[°C]
Ir-Tz1
Ir-Tz2
Ir-Tz3
285 (4.67), 291 (4.67), 308 (4.57), 347 (4.14), 389 (3.92), 420 (3.83), 466 (3.54)
280 (4.55), 292 (4.59), 305 (4.55), 334 (4.13), 372 (3.91), 405 (3.81), 434 (3.65)
280 (4.75), 302 (4.68), 331 (4.21), 362 (4.10), 390 (4.01), 428 (3.31)
546
530
517
0.35 0.49 1.40
0.40 0.54 1.35
0.68 0.56 0.82
320/102
318/104
316/109
[a] Measured in CH2Cl2 at a concentration of 10–5 m with logε values shown in parentheses. [b] Measured in CH2Cl2 at a concentration
of 10–5 m. [c] In degassed CH2Cl2 relative to fac-[Ir(ppy)3] (ΦP = 0.40), λex = 360 nm. [d] Measured in degassed CH2Cl2 solutions at a
sample concentration of ca. 10–5 m with the excitation wavelength set at 355 nm for all the samples at 298 K. [e] The triplet radiative
lifetimes (τP) were deduced from τr = τP/ΦP.
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and this is desirable for high-performance OLEDs. Hence, (Figure 4 and Table 2). The DFT calculation results show
all the thiazole-based phosphorescent IrIII complexes pos- that the lowest-energy transitions correspond to
sess thermal properties good enough to guarantee the fabri- HOMOǞLUMO transitions with nonzero oscillator
cation of OLEDs by the vacuum-deposition method and strengths in these thiazole-based phosphors, and, hence, the
the operational stability of the devices prepared thereof.
S1 and T1 states are represented by the HOMOǞLUMO
All of the thiazole-based IrIII complexes exhibit two dis- excitation. Consequently, the characters for the T1 excited
tinct absorption bands in their UV/Vis spectra (Figure 3 states, which are responsible for the phosphorescence in
and Table 1). The strong UV absorption bands below these complexes, will be represented by the
325 nm were assigned to the spin-allowed S1ǟS0 transitions HOMOǞLUMO transitions. Accordingly, the frontier mo-
1
of the organic ligands, representing the π–π* bands. Con- lecular orbital (MO) compositions represent the features of
versely, the weaker and low-energy features located beyond both the S1 and T1 excited states in Ir-Tz1, Ir-Tz2 and Ir-
325 nm can be ascribed to both 1MLCTǟS0 and Tz3. As indicated by the noticeably different contribution
3MLCTǟS0 (MLCT = metal-to-ligand charge transfer) ex- to the HOMO and LUMO from the metal dπ orbitals
citations. The strong spin–orbit coupling effects induced by (Table 2 and Figure 4), the lowest-energy excited state T1
the IrIII center are indicated by the similar oscillator shows obvious MLCT features, which lead to the unstruc-
strengths for the two MLCT bands (Table 1).[1c] Obviously, tured features in the PL spectra of these thiazole-based IrIII
3
the energy level for their MLCT absorption bands falls in complexes as mentioned before (Figure 3). Similar to those
the order Ir-Tz1 (ca. 466 nm) Ͻ Ir-Tz2 (ca. 434 nm) Ͻ Ir- of their analogues in the literature, the HOMOs for Ir-Tz1,
Tz3 (ca. 428 nm).
Ir-Tz2 and Ir-Tz3 are mainly located on the dπ orbitals of
the IrIII center and the π orbitals of the phenyl ring. Hence,
introducing the strong electron-withdrawing fluorine atom
to the phenyl ring stabilizes the HOMOs, and this should
increase the energy associated with the HOMOǞLUMO
transitions. As mentioned above, the HOMOǞLUMO
transitions represent the characters of the T1 states showing
MLCT features. Therefore, Ir-Tz3 with two F atoms on the
Figure 3. UV/Vis and photoluminescence (PL) spectra for the new
thiazole-based phosphorescent complexes.
Upon UV light irradiation at 360 nm, all the thiazole-
based IrIII complexes emit strong bluish-green to yellow
phosphorescence in CH2Cl2 (Figure 3 and Table 1). Their
photoluminescence (PL) spectra display unstructured line
shapes, indicating the predominant MLCT character for the
emissive lowest triplet excited states (T1) in all of them.
Figure 4. Plots of the LUMO (top) and HOMO (bottom) for each
of the thiazole-based IrIII complexes (a) Ir-Tz1, (b) Ir-Tz2 and
These results are clearly supported by the DFT calculations (c) Ir-Tz3.
Table 2. Contribution of the metal dπ orbitals to the HOMO and LUMO together with the TD-DFT calculation results.
Com-
pound
Contribution Contribution
Largest coefficient in the CI
expansion of the T1 state
(S0ǞT1 excitation energy)[a]
Largest coefficient in the CI
expansion of the S1 state
(S0ǞS1 excitation energy)[a]
Oscillator strength (f) of
the S0ǞS1 transition
of metal dπ
orbitals to
HOMO
of metal dπ
orbitals to
LUMO
Ir-Tz1
Ir-Tz2
Ir-Tz3
Ir
Ir
3.83%
Ir
3.80%
Ir
HǞL: 0.62354
(520 nm)
HǞL: 0.56978
(500 nm)
HǞL: 0.52981
(478 nm)
HǞL: 0.69814
(459 nm)
HǞL: 0.69585
(435 nm)
HǞL: 0.69534
(416 nm)
0.0523
0.0528
0.0503
57.71%
Ir
56.12%
Ir
57.10%
3.74%
[a] HǞL represents the HOMO-to-LUMO transition; CI stands for configuration interaction.
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3
phenyl ring shows higher MLCT energy than Ir-Tz2 with oxidation couple between 0.48 and 0.79 V (Table 3). With
one F atom. Also, Ir-Tz1, with no F atoms, possesses the the increase in the number of F atoms on the phenyl ring,
3
lowest MLCT energy. The time-dependent (TD) DFT cal- the oxidation potential moves markedly into the more posi-
culation results predicted the energy order of the T1 states tive direction due to the stabilization of the HOMOs in-
(3MLCT) as Ir-Tz1 Ͻ Ir-Tz2 Ͻ Ir-Tz3 (Table 2), which is duced by the F atom (vide infra). The reduction potentials
in good agreement with the absorption data (Figure 3 and for the complexes are spread over the narrow range of –2.55
Table 1). Due to the fact that phosphorescence of the IrIII to –2.60 V, showing the negligible effect of the F atom on
complexes comes from the 3MLCT states, the emission the reduction process of the complexes. The irreversibility
wavelength maxima for the thiazole-based complexes fol- for the reduction process of the complexes can be ascribed
lows the order Ir-Tz1 (546 nm) Ͼ Ir-Tz2 (530 nm) Ͼ Ir-Tz3 to the electron-rich character of the thiazole group, which
(517 nm) (Figure 3 and Table 1). As a result, the theoretical makes its reduction more susceptible to other environmen-
computational results properly support the photophysical tal factors.
properties of the thiazole-based IrIII complexes (Figure 3
and Table 1).
Table 3. Redox properties of the thiazole-based IrIII ppy-type com-
plexes.
Electrochemical Characterization
ox
red
Compound E1/2 [V] E1/2 [V] HOMO [eV] LUMO [eV]
–2.61[a]
–2.58[a]
–2.55[a]
–5.28
–5.43
–5.59
–2.19
–2.22
–2.25
The electrochemical properties of these thiazole-based
IrIII complexes were investigated by cyclic voltammetry
(CV) calibrated with ferrocene as the internal standard un-
Ir-Tz1
Ir-Tz2
Ir-Tz3
0.48
0.63
0.79
der nitrogen. All of the complexes show a quasi-reversible [a] Irreversible.
Figure 5. General configuration of electrophosphorescent OLEDs made from Ir-Tz1 to Ir-Tz3 and molecular structures of the relevant
compounds used in these devices together with energy level diagram.
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Electrophosphorescent Characterization
Due to their strong triplet emissions, the electrophospho-
rescent properties of Ir-Tz1 to Ir-Tz3 were characterized by
making OLEDs with the configuration of ITO/m-
MTDATA (40 nm)/NPB (20 nm)/x wt.-% Ir:CBP (20 nm)/
BCP (10 nm)/Alq3 (30 nm)/LiF:Al (0.5:100 nm). Figure 5
depicts the structures of the multi-layer OLEDs and the
chemicals involved in the fabrication of devices together
with the energy-level diagram. The layer of tris{4-[(3-meth-
ylphenyl)(phenyl)amino]phenyl}amine (m-MTDATA) was
firstly deposited on the pre-cleaned ITO surface to form a
buffer layer and facilitate hole-injection (HI) for its proper
HOMO level lying between that of ITO and NPB {NPB =
4,4Ј-bis[(1-naphthyl)(phenyl)amino]biphenyl}, while the
subsequent NPB layer acts as the hole-transport (HT) layer.
For its high triplet energy level and bipolar character, 4,4Ј-
bis(carbazol-9-yl)biphenyl (CBP) serves as a small-molecule
host material for the phosphorescent emitters. The hole-
Figure 6. EL spectra for the devices A4, B3 and C4 showing the
best EL performance.
Table 4. Maximum EL performance for all the electrophosphorescent OLEDs.
[a]
[a]
[a]
[b]
Device
Phosphorescent dopant
Vturnon Luminance L[a]
[V]
ηext
ηL
ηp
λmax
[cdm–2]
[%]
[cdA–1]
[lmW–1]
[nm]
A1
A2
A3
A4
A5
Ir-Tz1 (4 wt.-%)
Ir-Tz1 (6 wt.-%)
Ir-Tz1 (8 wt.-%)
Ir-Tz1 (10 wt.-%)
Ir-Tz1 (12 wt.-%)
5.2
5.0
4.1
3.9
4.8
16719 (14.3)
30826 (14.3)
35451 (13.6)
38549 (13.2)
29954 (13.9)
2.69 (9.2)
4.74 (9.2)
6.07 (8.2)
7.87 (6.5)
4.76 (8.2)
8.02 (8.8)
14.22 (8.8)
18.10 (7.8)
23.62 (6.4)
14.40 (8.2)
3.00 (7.8)
5.40 (7.8)
7.83 (7.1)
13.46 (4.8)
5.93 (6.4)
548, 584 (0.44, 0.52)
548, 584 (0.45, 0.52)
548, 584 (0.45, 0.52)
548, 584 (0.45, 0.52)
548, 584 (0.45, 0.53)
B1
B2
B3
B4
B5
Ir-Tz2 (4 wt.-%)
Ir-Tz2 (6 wt.-%)
Ir-Tz2 (8 wt.-%)
Ir-Tz2 (10 wt.-%)
Ir-Tz2 (12 wt.-%)
5.3
4.9
4.9
6.3
5.3
15491 (13.9)
22861 (14.6)
24193 (13.2)
20160 (14.3)
15594 (14.6)
2.60 (10.2)
3.61 (9.5)
4.59 (8.5)
3.55 (9.5)
2.61 (10.2)
8.14 (9.9)
11.33 (9.5)
14.05 (8.5)
11.20 (9.2)
8.30 (9.8)
2.77 (8.5)
4.11 (7.8)
5.74 (7.1)
4.21 (7.8)
2.16 (8.2)
528, 564 (0.40, 0.56)
532, 568 (0.42, 0.56)
528, 564 (0.40, 0.56)
528, 564 (0.40, 0.56)
532, 564 (0.40, 0.56)
C1
C2
C3
C4
C5
Ir-Tz3 (4 wt.-%)
Ir-Tz3 (6 wt.-%)
Ir-Tz3 (8 wt.-%)
Ir-Tz3 (10 wt.-%)
Ir-Tz3 (12 wt.-%)
4.2
5.3
3.9
4.9
4.9
8013 (14.6)
9200 (14.6)
11832 (13.9)
11907 (13.9)
10484 (14.3)
1.47 (10.2)
1.94 (9.8)
3.35 (7.5)
3.79 (7.1)
2.80 (8.5)
4.49 (10.2)
6.06 (9.8)
10.48 (7.5)
11.93 (7.1)
8.76 (8.5)
1.45 (9.5)
2.02 (8.5)
4.41 (7.5)
5.61 (6.1)
3.52 (7.1)
516, 548 (0.35, 0.56)
520, 548 (0.37, 0.56)
516, 548 (0.35, 0.56)
516, 548 (0.36, 0.57)
520, 548 (0.37, 0.56)
[a] Values in parentheses are the voltages at which the data were obtained. [b] CIE coordinates (x, y) are shown in parentheses.
Figure 7. Current density (J)–voltage (V)–luminance (L) curves for the devices A4, B3 and C4.
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blocking layer of the devices consists of 2,9-dimethyl-4,7- dopants Ir-Tz2 and Ir-Tz3 to the CBP host. Obviously, the
diphenyl-1,10-phenanthroline (BCP) due to its low LUMO undesired back energy transfer process would adversely af-
level. Tris(8-hydroxyquinolinato)aluminum (Alq3) acts as fect the EL performance of Ir-Tz2 and Ir-Tz3. Hence, Ir-
an electron-transporter and LiF as an electron-injection Tz1 exhibits much better EL properties than Ir-Tz2 and Ir-
layer. In order to optimize the EL efficiencies, doping-level Tz3. Based on both the triplet energy levels for the phos-
dependent experiments were also carried out in the range phorescent dopant and their EL performance mentioned
of 4–12 wt.-%.
before, it is clear that the closer the triplet energy level of
All the devices emit intense orange electrophosphoresc-
ence with maxima at ca. 550 nm for devices A1–A5, ca.
530 nm for devices B1–B5 and ca. 520 nm for devices C1–
C5 (Figure 6, Table 4 and Figure S1 in the Supporting In-
formation). The weak EL band centered at ca. 450 nm
should come from NPB due to the high LUMO levels of
the concerned phosphorescent emitters, which would in-
crease the chance of electron-leaking from the emission
layer to the NPB layer.[12] The EL band from NPB remains
relatively stable with variation of the doping level and be-
comes enhanced with an increase of the driving voltage ap-
plied to the devices. At higher driving voltages, more elec-
trons will enter the emission layer, which will promote the
leaking of electrons into the NPB layer.[12] Hence, the EL
band from NPB is enhanced at higher driving voltages. The
current density (J)–voltage (V)–luminance (L) curves for
the concerned devices are shown in Figures 7 and S2, while
the relationships between EL efficiencies and current den-
sity for the devices are presented in Figures 8 and S3. From
Table 4, it can be clearly seen that device A4 with 10 wt.-%
doping level shows the best EL ability of Ir-Tz1. The yel-
low-emitting device A4 doped with Ir-Tz1 exhibits impres-
sive EL performance with a low turn-on voltage of 3.9 V, a
maximum brightness (Lmax) of 38549 cdm–2 at 13.2 V, a
peak external quantum efficiency (ηext) of 7.87%, a lumi-
nance efficiency (ηL) of 23.62 cdA–1 and a power efficiency
(ηp) of 13.46 lmW–1 (Figures 7 and 8). The device B3 shows
the best EL properties among the devices doped with Ir-
Tz2. It can be turned on at ca. 4.9 V, and its light output
can reach 24193 cdm–2 at 13.2 V with peak EL efficiencies
of 4.59%, 14.05 cdA–1 and 5.74 lmW–1 (Figures 7 and 8).
The device C4 emits at 4.9 V, with an Lmax of 11907 cdm–2
at 13.9 V and maximum EL efficiencies of 3.79%,
11.93 cdA–1 and 5.61 lmW–1 (Figures 7 and 8). By compar-
ing their EL performances (Table 4 and Figure 8), it is clear
that the EL properties generally fall in the order of Ir-Tz1
Ͼ Ir-Tz2 Ͼ Ir-Tz3. From the energy level diagram (Fig-
ure 5), we can see that the HOMO level of the CBP (ca.
–6.0 eV) host matches well with that of NPB (ca. –5.5 eV),
and its LUMO (ca. –2.9 eV) level is very close to that of
both BCP and Alq3 (ca. –3.0 eV). However, the LUMO
levels for the phosphorescent emitters (ca. –2.2 to –2.3 eV)
are located notably above that of CBP. Such information
shows that the phosphorescent emitters are preferably ex-
cited by the host-guest energy-transfer process. The triplet
energy levels for Ir-Tz1, Ir-Tz2 and Ir-Tz3 are 2.27 eV,
2.34 eV and 2.40 eV, respectively, while that for CBP is
2.56 eV. This result indicates that the energy transfer from
the CBP host to Ir-Tz2 and Ir-Tz3 might be inefficient due
to the smaller difference between their triplet energy levels,
Figure 8. Relationship between EL efficiencies and current density
which would lead to endothermic back energy transfer from for the devices A4, B3 and C4.
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volution of the luminescence decay profile with the instrument re-
sponse function by using the software package provided by Edin-
burgh Instruments. DSC was performed with a Netzsch DSC 200
PC unit under a nitrogen flow at a heating rate of 10 °Cmin–1.
TGA was conducted with a Netzsch STA 409C instrument under
nitrogen with a heating rate of 20 °Cmin–1. Electrochemical mea-
surements were performed by using a Princeton Applied Research
2273A potentiostat. Cyclic voltammetry of the sample solutions
was performed at a scan rate of 100 mVs–1 by using a glassy carbon
working electrode, a platinum counter electrode and a platinum-
the dopant to that of the CBP host, the poorer the EL per-
formance will be. This also indicates the validity of ascrib-
ing the EL behavior of Ir-Tz1, Ir-Tz2 and Ir-Tz3 to the back
energy transfer process. Recently, some carbazole-based
IrIII phosphorescent emitters have shown attractive EL effi-
ciencies of 40 cdA–1 and 10 lmW–1 in solution-processed
devices with bipolar host materials of poly(N-vinylcarb-
azole) (PVK) and 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-
1,3,4-oxadiazole (PBD).[6f,6g] As mentioned before, Ir-TZ1
can achieve EL efficiencies of 23.62 cdA–1 and wire reference electrode. The solvent was deoxygenated THF, and
13.46 lmW–1 in a traditional CBP host. Generally, phos-
the supporting electrolyte was 0.1 m [nBu4N][BF4]. Ferrocene was
added as a calibrant after each set of measurements, and all poten-
phorescent OLEDs can show much better EL performance
tials reported are quoted with reference to the ferrocene/ferrocen-
by employing a bipolar host to maintain a more balanced
ium couple (E1/2 = +0.27 V relative to the reference electrode). The
charge carrier injection/transport.[6c] So, we would expect
oxidation (Eox) and reduction (Ered) potentials were used to deter-
mine the HOMO and LUMO energy levels by using the equations
EHOMO = –(Eox + 4.8) eV and ELUMO = –(Ered + 4.8) eV, which
better EL performance for our thiazole-based IrIII phos-
phorescent emitters by doping them in bipolar host materi-
als.
were calculated by using the internal standard ferrocene value of
–4.8 eV with respect to the vacuum level.[14]
General Procedure for the Synthesis of L1, L2 and L3: Under nitro-
gen, 2-bromothiazole and the corresponding phenylboronic acid
(1.2 equiv.) were added to toluene and 2 m Na2CO3 (1:1, v/v).
Pd(PPh3)4 (5 mol.-%) was then added to the solution. The resultant
Conclusions
Three thiazole-based cyclometalated IrIII phosphorescent
emitters have been successfully prepared and characterized.
mixture was stirred at 110 °C for 24 h and was then cooled to room
By introducing fluorine atoms to the phenyl ring of their
cyclometalating ligand, the photopysical properties, electro-
chemistry and EL performance of the concerned metal
complexes can be tuned significantly. Owing to the charac-
ters of their electronic structures, the phosphorescent emit-
ters are preferably excited through host-guest energy-trans-
fer processes in the EL devices with CBP as the host mate-
rial, and their EL performances depend on the triplet en-
ergy level of the novel phosphorescent emitters. The thi-
azole-based cyclometalated IrIII triplet emitters exhibit
temperature and extracted with CH2Cl2. The combined organic
phases were dried with anhydrous MgSO4. The solvent was then
removed, and the residue was purified by column chromatography
by eluting with CH2Cl2/hexane (2:1, v/v). All the ligands were ob-
tained in high yields of ca. 85%.
1
L1: Yield: 86%. H NMR (400 MHz, CDCl3): δ = 7.95–7.80 (m, 2
H, Ar), 7.86 (d, J = 3.2 Hz, 1 H, Ar), 7.44–7.42 (m, 3 H, Ar), 7.31
(d, J = 4.8 Hz, 1 H, Ar) ppm. 13C NMR (100 MHz, CDCl3): δ =
168.38, 143.63, 133.55, 129.94, 128.91, 126.54, 118.76 ppm. FAB-
MS: m/z = 161 [M]+. C9H7NS (161.03): calcd. C 67.05, H 4.38, N
8.69; found C 66.95, H 4.56, N 8.58.
maximum EL efficiencies with ηext = 7.87%, ηL
=
1
23.62 cdA–1 and ηp = 13.46 lmW–1, indicating their promis-
L2: Yield: 83%. H NMR (400 MHz, CDCl3): δ = 7.96–7.93 (m, 2
H, Ar), 7.84 (d, J = 3.2 Hz, 1 H, Ar), 7.31 (d, J = 3.2 Hz, 1 H,
Ar), 7.15–7.11 (m, 2 H, Ar) ppm. 13C NMR (100 MHz, CDCl3): δ
= 167.17, 165.03, 162.54, 143.65, 129.95, 129.92, 128.48, 128.40,
118.76, 116.10, 115.88 ppm. FAB-MS: m/z = 179 [M]+. C9H6FNS
(179.02): calcd. C 60.32, H 3.37, N 7.82; found C 60.02, H 3.05, N
7.68.
ing potential in the field of OLEDs.
Experimental Section
General Information: All reactions were conducted under nitrogen,
and no special precautions were required during workup. Solvents
were carefully dried and distilled from appropriate drying agents
prior to use. Commercially available reagents were used without
further purification unless otherwise stated. All reactions were
monitored by TLC with Merck precoated aluminum plates. Flash
column chromatography was carried out by using silica gel (200–
300 mesh). FAB mass spectra were recorded with a Finnigan MAT
SSQ710 system. NMR spectra were measured in CDCl3 with a
Bruker AXS 400 MHz spectrometer with the chemical shifts
quoted relative to those of tetramethylsilane.
1
L3: Yield: 85%. H NMR (400 MHz, CDCl3): δ = 8.33–8.27 (m, 1
H, Ar), 7.92 (s, 1 H, Ar), 7.45–7.44 (m, 1 H, Ar), 7.03–6.93 (m, 2
H, Ar) ppm. 13C NMR (100 MHz, CDCl3): δ = 164.76, 164.63,
162.24, 162.12, 161.32, 161.20, 159.92, 159.80, 158.79, 158.67,
142.55, 130.06, 130.02, 129.97, 129.93, 120.00, 119.92, 118.10,
112.36, 112.33, 112.15, 112.11, 104.67, 104.41, 104.15 ppm. FAB-
MS: m/z = 197 [M]+. C9H5F2NS (197.20): calcd. C 54.81, H 2.56,
N 7.10; found C 54.63, H 2.75, N 6.95.
General Procedure for the Synthesis of Iridium Complexes: Under
nitrogen, each appropriate organic ligand and IrCl3·nH2O
(0.5 equiv.) were heated to 110 °C in a mixture of 2-ethoxyethanol
and water (3:1, v/v) for 16 h. The reaction mixture was then cooled
to room temperature, and water was added. The cyclometalated
IrIII μ-chlorido-bridged dimer was formed as a precipitate, which
was collected and dried under vacuum. Subsequently, the precursor
complex, acetylacetone (5 equiv.) and Na2CO3 (10 equiv.) were
added to 2-ethoxyethanol under nitrogen, and the mixture was
heated to 110 °C for 12–16 h. After cooling to room temperature
and the addition of water, the colored precipitate was collected by
Physical Measurements: UV/Vis spectra were recorded with a Shi-
madzu UV-2250 spectrophotometer. The photoluminescent (PL)
properties of the complexes were measured with a Hitachi F-4500
fluorescence spectrophotometer. The phosphorescence quantum
yields were determined in degassed CH2Cl2 solutions at 293 K
against the fac-[Ir(ppy)3] standard (ΦP = 0.40).[13] The lifetimes
were measured with a single photon counting spectrometer from
Edinburgh Instruments (FLS-920) with a nanosecond pulse LED
excitation source. The data analysis was conducted by iterative con-
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filtration and washed with water and dried. The crude product was
purified on a silica column by using an appropriate eluent to pro-
duce a pure sample of each of the title iridium complexes after
solvent evaporation and drying.
charge from The Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
Theoretical Computation Details: Density functional theory (DFT)
calculations by using B3LYP were performed for all the IrIII com-
plexes, and their geometries were obtained by theoretical optimiza-
tion during the computation. The basis set used for the C, H, F,
N and O atoms was 6-31G, while effective core potentials with a
LanL2DZ basis set were employed for S and Ir atoms.[18] The ener-
gies of the excited states of the complexes were computed by time-
dependent (TD) DFT (TD-DFT) based on all the ground-state
geometries. All calculations were carried out by using the
Gaussian 09 program.[19] Mulliken population analyses were per-
formed by using MullPop.[20] Frontier molecular orbitals obtained
from the DFT calculations were plotted by using the Molden 3.7
program.[21]
1
Ir-Tz1: Yield: 33%. H NMR (400 MHz, CDCl3): δ = 7.67 (d, J =
3.2 Hz, 2 H, Ar), 7.48 (dd, J = 7.6, 1.2 Hz, 2 H, Ar), 7.36 (d, J =
3.6 Hz, 2 H, Ar), 6.80–6.76 (m, 2 H, Ar), 6.71–6.67 (m, 2 H, Ar),
6.30 (d, J = 7.2 Hz, 2 H, Ar), 5.22 (s, 1 H, acac), 1.82 (s, 6 H, acac)
ppm. 13C NMR (100 MHz, CDCl3): δ = 184.89 (acac), 178.35,
145.29, 141.20, 139.35, 133.77, 129.39, 124.04, 120.76, 115.94 (Ar),
100.35, 28.41 (acac) ppm. FAB-MS: m/z
=
612 [M]+.
C23H19IrN2O2S2 (612.05): calcd. C 45.16, H 3.13, N 4.58; found C
45.00, H 3.32, N 4.38.
1
Ir-Tz2: Yield: 30%. H NMR (400 MHz, CDCl3): δ = 7.61 (d, J =
3.5 Hz, 2 H, Ar), 7.47 (dd, J = 8.4, 5.6 Hz, 2 H, Ar), 7.37 (d, J =
3.5 Hz, 2 H, Ar), 6.55–6.50 (m, 2 H, Ar), 5.88 (dd, J = 9.8, 2.5 Hz,
2 H, Ar), 5.24 (s, 1 H, acac), 1.83 (s, 6 H, acac) ppm. 13C NMR
(100 MHz, CDCl3): δ = 185.14 (acac), 177.42, 164.06, 161.54,
148.36, 139.17, 137.51, 125.65, 125.55, 120.10, 119.93, 115.96,
108.48, 108.24 (Ar), 100.54, 28.35 (acac) ppm. 19F NMR
(376 MHz, CDCl3): δ = –109.34 (1 F) ppm. FAB-MS: m/z = 648
[M]+. C23H17F2IrN2O2S2 (648.03): calcd. C 42.65, H 2.65, N 4.32;
found C 42.48, H 2.92, N 4.13.
Supporting Information (see footnote on the first page of this arti-
cle): Crystal data for Ir-Tz2; EL spectra for the phosphorescent
OLEDs made from the thiazole-based triplet emitters; J–V–L
curves for the concerned phosphorescent OLEDs; EL efficiency-
luminance curves for all the phosphorescent OLEDs.
Acknowledgments
This work was supported by the Tengfei Project from Xi’an Jiao-
tong University, a Research Grant from Shaanxi Province (No.
2009JQ2008), the National Natural Science Foundation of China
(No. 20902072), the Program for New Century Excellent Talents in
University, and the Ministry of Education of China (NECT-09-
0651). W.-Y. W. acknowledges the financial support from the Hong
Kong Baptist University (FRG2/11-12/156), the Hong Kong Re-
search Grants Council (HKBU202410 and HKUST2/CRF/10), the
Areas of Excellence Scheme from the University Grants Committee
of HKSAR, China (Project No. AoE/P-03/08), and the National
Basic Research Program of China (973 Program) (2013CB834702).
1
Ir-Tz3: Yield: 29%. H NMR (400 MHz, CDCl3): δ = 7.63 (dd, J
= 3.6, 1.7 Hz, 2 H, Ar), 7.49 (d, J = 3.6 Hz, 2 H, Ar), 6.38–6.33
(m, 2 H, Ar), 5.66 (dd, J = 9.0, 2.2 Hz, 2 H, Ar), 5.26 (s, 1 H,
acac), 1.84 (s, 6 H, acac) ppm. 13C NMR (100 MHz, CDCl3): δ =
185.28 (acac), 172.27, 172.21, 164.76, 164.65, 162.22, 162.11,
160.41, 160.28, 157.85, 157.71, 148.82, 148.75, 138.52, 118.02,
117.96, 116.19, 116.16, 116.01, 115.98 (Ar), 100.67 (acac), (96.84,
96.61, 96.56, 96.33) (Ar), 28.31 (acac) ppm. 19F NMR (376 MHz,
CDCl3): δ = –106.40 (1 F), –108.42 (1 F) ppm. FAB-MS: m/z =
684 [M]+. C21H15F4IrN2O2S2 (684.01): calcd. C 40.40, H 2.21, N
4.10; found C 40.29, H 2.52, N 4.23.
OLED Fabrication and Measurements: The precleaned ITO glass
substrates were treated with ozone for 20 min. The m-MTDATA
layer was then deposited on the surface of ITO glass to form a
40 nm thick hole-injection layer. A 20 nm thick hole-transporting
layer of NPB was then constructed. The 20 nm doped emission
layer was deposited by co-evaporation of the IrIII complex and the
CBP host. After that, BCP (15 nm), Alq3 (40 nm), LiF (0.5 nm)
and Al cathode (100 nm) were successively evaporated. All the
evaporation processes were conducted at a base pressure less than
10–6 Torr. The EL spectra and CIE coordinates were measured with
a PR650 Spectra colorimeter. The J–V–L curves of the devices were
recorded by using a Keithley 2400/2000 source meter, and the lumi-
nance was measured by using a PR650 SpectraScan spectrometer.
All the experiments and measurements were carried out under am-
bient conditions.
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Received: May 8, 2013
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