Novel ZnII complexes of 2-(2-hydroxyphenyl)benzothiazoles ligands: Electroluminescence and application as host materials for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2jm34599d

A group of novel zinc complexes containing 2-hydroxyphenylbenzothiazole (BTZ) ligands were designed and synthesized, in which different substituents (OCH₃, CH₃, F, CF₃, COOCH₂CH ₃) were attached at th

Full text of DOI:10.1039/c2jm34599d

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PAPER
Novel ZnII complexes of 2-(2-hydroxyphenyl)benzothiazoles ligands:
electroluminescence and application as host materials for phosphorescent
organic light-emitting diodes
a
Renjie Wang, Lijun Deng, Min Fu, Jinling Cheng and Jiuyan Li
a
a
b
a
*
Received 13th July 2012, Accepted 21st September 2012
DOI: 10.1039/c2jm34599d
A group of novel zinc complexes containing 2-hydroxyphenylbenzothiazole (BTZ) ligands were
designed and synthesized, in which different substituents (OCH3, CH3, F, CF3, COOCH2CH3) were
attached at the 6-position of the benzothiazole ring in the BTZ ligands. Both photoluminescence (PL)
and electroluminescence (EL) behaviors of these zinc complexes were investigated. The emission colors
of these zinc complexes were readily tuned from bluish-green to yellow by simply varying the
substituent, with strong electron-withdrawing substituents being favorable for longer-wavelength
fluorescence. Efficient EL was obtained when these zinc complexes were used as non-doped emitting
layers in organic light-emitting diodes (OLEDs). Furthermore, these zinc complexes were proved to be
capable of acting as triplet hosts for iridium phosphor in red phosphorescent OLEDs. A high external
quantum efficiency of 17.5% was realized for the red phosphorescent OLED with the present zinc
complexes as hosts and tris(2-phenylisoquinoline)iridium as doped emitter, which is greatly enhanced
compared to that (12.6%) of the device with the traditional 4,40-bis(N-carbazoly)biphenyl (CBP) as
host. The present study successfully exploited novel zinc complexes as electron-transporting host
materials for phosphorescent OLEDs.
than the most famous electron-transporting material tris(8-
Introduction
hydroxyquinoline)aluminum (Alq3) in OLEDs.10,11 According to
the results of density functional theory (DFT) calculations per-
formed on Zn(BTZ)2,11 the highest occupied molecular orbital
(HOMO) of Zn(BTZ)2 is mainly distributed on the phenoxide
ring, while the lowest unoccupied molecular orbital (LUMO)
density is localized largely on benzothiazole and phenoxide rings,
with a small contribution from the metal atom. Thus, it seems
logical that substitution with appropriate functional group at
either benzothiazole or phenoxide ring would result in the vari-
ation of the HOMO–LUMO energy gap and thus effective color
tuning of Zn(BTZ)2 derivatives.
OLEDs have attracted a great deal of interest due to their
potential application in low-cost, more-efficient flat-panel
displays and solid-state lighting.1 Particularly, luminescent metal
complexes of Ir(III),2 Pt(II),3 Al(III)4 and Zn(II) seem attractive for
their electroluminescent (EL) application on the basis of a
precursory study by C. W. Tang and S. A. Van Slyke.5 In
comparison with the phosphorescent Ir and Pt complexes that
are usually doped in other matrixes or hosts due to possible
triplet–triplet annihilation in neat films and weak charge-trans-
portation ability, the Zn complexes are characterized by the non-
doped feature when used as emitters in OLEDs based on their
fluorophore nature and electron-transporting capability.6–19
Research on the application of zinc complexes in OLEDs is
mainly focused on designing new and facile ligands. Up to now,
several green-,20 blue-,16,21 red-,22 and white7,13,23-emitting Zn(II)
complexes with various ligands have been developed for OLEDs.
Bis(2-(2-hydroxyphenyl)benzothiazolate)-zinc [Zn(BTZ)2] is
one of the best white electroluminescent materials,19,23 and has
been reported to exhibit better electron-transportation behavior
In the effort to investigate the efficacy of the substituent effect
and its possible use in color tuning, some progress has been seen
in the preparation of Zn(BTZ)2 derivatives with substituents on
the phenoxide ring.6–8 For example, it was reported8 that the
emission wavelengths of Zn(BTZ)2 derivatives can be easily
tuned from 505 nm to 583 nm by attaching different electron-
withdrawing and -donating aryl groups to the 5-position of the
phenoxide rings of the ligand. However, it should be noted that
few reports have been carried out on Zn(BTZ)2 derivatives with
substituents on the benzothiazole ring so far.6 A systematic
investigation of the substituent effect on the luminescent prop-
erties of Zn(BTZ)2 derivatives when the substituents are intro-
duced into the benzothiazole moiety is still absent. In addition,
most of the researches on Zn(BTZ)2 derivatives are only focused
aState Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, 2 Linggong Road, Dalian 116024, China.
E-mail: jiuyanli@dlut.edu.cn
bSchool of Chemistry, Dalian University of Technology, 2 Linggong Road,
Dalian 116024, China
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on their application as light-emitters in OLEDs, in which the
fluorescence of Zn(BTZ)2 derivatives are the output of the
devices. As far as we know, there was only one report to utilize
Zn(BTZ)2 as a host for phosphorescent dopants in host-dopant
type of OLEDs.9 Therefore, it is strongly desired to explore more
Zn(BTZ)2 derivatives as host materials for phosphorescent
dopants in high-efficiency OLEDs.
Scheme 2 Synthetic route to zinc complexes Zn(BTZ)2 and 1–5.
Here, we report a systematic study on the properties of a series
of Zn(BTZ)2 derivatives, 1–5 (Scheme 1), with different substit-
uents at the 6-position of benzothiazole ring. The substituent
groups are selected in such a way that the electron-withdrawing
ability gradually increases in the order of OCH3 < CH3 < F <
CF3 < COOCH2CH3. Our interest is to reveal how these simple
functional groups introduced into the benzothiazole ring affect
the photophysical, electrochemical and electroluminescent
properties of the corresponding zinc complexes. Finally, highly
efficient red-emitting phosphorescent OLEDs have also been
obtained by using 1–3 as the hosts and the iridium phosphor as
the dopant, which outperform similar devices with the traditional
4,40-N,N0-dicarbazolebiphenyl (CBP) host. To our best knowl-
edge, this is the first report to systematically study Zn(BTZ)2
derivatives as host materials for red phosphorescent OLEDs.
concentrated H2SO4. Finally, the desired zinc complexes 1–5
were obtained by treating ligands 1a–5a with zinc acetate in
ethanol in good yields of around 70%,6 as shown in Scheme 2.
The low solubility of these zinc complexes in common organic
solvents precludes NMR analysis in liquid solutions or purifi-
cation by conventional recrystallization and column chroma-
tography. All the complexes were fully purified by vacuum train
sublimation and characterized by MALDI-TOF mass spectros-
copy and elemental analysis. The thermal properties of the zinc
complexes were investigated by means of thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
measurements. The zinc complexes exhibited relatively high
decomposition temperatures (Td) of over 300 ꢀC. During the
DSC measurements, no melting or other phase transition was
observed before 300 ꢀC for the zinc complexes.
Results and discussion
Synthesis and characterization
Photophysical properties
The synthetic routes for the 2-hydroxyphenylbenzothiazole
derivative ligands 1a–5a and their zinc complexes 1–5 are out-
lined in Schemes 1 and 2, respectively. According to the general
procedures,24 the 4-substituted aniline was first treated with sal-
icylic acid in the presence of PCl3 at 120 ꢀC to generate the
corresponding amide 1b–5b, the hydroxy-protection of which
with tert-butyldimethylchlorosilane (TBDMSCl) followed by an
thionation reaction with Lawesson’s reagent produced the thio-
benzamide 1c–5c. Jacobson reaction of 1c–4c with potassium
ferricyanide in the presence of sodium hydroxide resulted in the
formation of ligands 1a–4a. The similar reaction of 5c followed
by further acidification with dilute HCl produced the interme-
diate 5d, which was then converted into the corresponding ligand
5a via subsequent reaction with ethanol in presence of
The photophysical properties of complexes Zn(BTZ)2 and 1–5
were studied by using UV-visible absorption and fluorescence
spectra and the pertinent data are summarized in Table 1. As
illustrated in Fig. 1a, these zinc complexes in thin films exhibit very
similar absorption features in the range of 300–500 nm. However,
by altering the nature of the substituents (OCH3, CH3, F, CF3, or
COOCH2CH3) on the benzothiazole ring of 2-hydrox-
yphenylbenzothiazole ligands, there is some difference in the
absorption maxima of the UV-vis absorption spectra. The
absorption maxima of the F-substituted complex 3 and the CF3-
substituted 4 shift to shorter wavelength by 10–12 nm, whereas
that of the COOCH2CH3-substituted complex 5 shifts by 3 nm to
longer wavelength, compared to the parent Zn(BTZ)2. The shifts
Scheme 1 Synthetic route to 2-hydroxyphenylbenzothiazole derivatives 1a–5a.
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Table 1 Photophysical and electrochemical data obtained experimentally for the zinc complexes
labs
[nm]
lem
[nm]
sb [ns]
HOMO/LUMO [eV]
c
Td (ꢀC)
d
Eg [eV]
Complex
FFa [%]
Zn(BTZ)2
412
413
413
390
392
415
480
479
480
514
526
536
19
14.1
—
12.9
7.5
2.1
24.1 (480 nm, 1.09)
25.2 (480 nm, 1.14)
—
23.9 (520 nm, 1.19)
23.4 (525 nm, 1.07)
66.2 (535 nm, 1.10)
357
359
—
340
352
340
ꢁ5.30/ꢁ2.53
ꢁ5.24/ꢁ2.47
ꢁ5.29/ꢁ2.51
ꢁ5.22/ꢁ2.46
ꢁ5.44/ꢁ2.75
ꢁ5.22/ꢁ2.57
2.77
2.77
2.78
2.76
2.69
2.65
1
2
3
4
5
a
b
Fluorescence quantum yields for powder samples were measured by an integrating sphere. In the parentheses: the data before comma are the
emission wavelengths for which the lifetime was measured, those after the comma are the corresponding CHISQ data. Decomposition
c
d
temperatures corresponding to 5% weight loss. Optical bandgaps were obtained by absorption edge technique.
Fig. 1 The UV-vis absorption spectra and PL spectra of 1–5 and
Zn(BTZ)2 films on quartz substrates.
Fig. 3 The current density–voltage–luminance (J–V–L) characteristics
of zinc complex-based fluorescent OLEDs.
a correlation between the PL properties and the electronic nature
of the attached substituents. As shown in Fig. 1b, a regular red-
shift of the emission maxima ranging from 479 nm (1) to 536 nm
(5) was observed with the substituent going from electron-
donating OCH3 to electron-withdrawing COOCH2CH3.
Similarly to the absorption spectra, the substitution on the
benzothiazole ring with CH3 and OCH3 groups does not have an
obvious influence on the fluorescence spectra features including
spectral profiles and peak wavelengths relative to the parent
Fig. 2 EL spectra of the zinc complex-based OLEDs.
Zn(BTZ)2. However, a greater substituent effect on the emission
maxima is achieved by attaching F, CF3, or COOCH2CH3
Table 2 Performance summary of the fluorescent OLEDs with zinc
complexes as emitting layer
groups to the benzothiazole ring. The fluorescent quantum yields
(FF) of the zinc complex powders were measured using an inte-
grating sphere at room temperature and the data are listed in
Table 1. The OCH3-substituted analogue 1 possesses a moderate
FF of 14%. However, with increasing electron-withdrawing
ability of the substituent, the FF values of the corresponding
complexes exhibit a regular decreasing trend. The fluorescence
lifetimes (s) of the zinc complexes, measured by time-resolved
fluorescence technique, are in the range of 23–25 ns for the parent
Zn(BTZ)2 and complexes 1–4. The COOCH2CH3-substituted
analogue 5 shows a remarkably long lifetime of 66.2 ns.
Lmax [cd hL
]
lEL
[nm] CIE (x, y) (at 8 V)
Compound Vturn-on [V] mꢁ2
[cd Aꢁ1
]
1
2
3
4
5
4.0
5.1
4.8
7.0
4.9
9137
10 010
4508
1377
3907
1.70
2.38
1.10
0.56
1.17
480 (0.27, 0.43)
488 (0.25, 0.40)
516 (0.26, 0.41)
530 (0.37, 0.50)
544 (0.38, 0.49)
in the absorption maxima for complexes 1 and 2 are negligible,
which suggests that the electronic nature of the lowest excited state
is very similar between Zn(BTZ)2 and 1 (or 2). The optical
bandgap (Eg) was determined by the absorption edge technique2c,2d
for each zinc complex and the data are provided in Table 1.
The emission spectral data for all the zinc complexes in the
solid state are summarized in Table 1. In this series, we observed
Electrochemistry
The electrochemical properties of the zinc complexes were
investigated by cyclic voltammetry (CV) in CH3CN solutions
containing 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6) as electrolyte. A conventional three-electrode config-
uration was used, which was composed of an Ag/AgCl reference
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Table 3 Performance of the red phosphorescent OLEDs with zinc complexes and CBP as hosts in emitting layer
Lmax [cd
mꢁ2
hL
[cd Aꢁ1
hP
[lm Wꢁ1
lem
[nm]
Host
Vturn-on [V]
]
]
]
hext [%]
CIE (x, y) (8 V)
1
2
3
CBP
3.9
4.0
3.9
4.3
24 870
15 570
19 400
11 710
13.85
10.33
10.30
9.97
7.25
5.41
5.39
5.22
17.51
13.06
13.02
12.61
622
622
622
622
(0.67, 0.32)
(0.67, 0.32)
(0.67, 0.32)
(0.67, 0.33)
electrode and a Pt-wire counter electrode. Due to the poor
solubility of the zinc complexes, they were deposited on ITO
substrates to act as the working electrode. Ferrocene was used as
the internal standard, and all potentials were quoted with refer-
ence to the ferrocene–ferrocenium (Fc/Fc+) couple. During the
anodic sweep at a scan rate of 100 mV sꢁ1, all the zinc complex
films deposited on ITO electrodes underwent an irreversible one-
electron oxidation reaction. Similar irreversible redox behavior
has ever been reported when the active substance was in the form
of solid film rather than being dissolved in liquid solution.6 The
observed irreversible oxidation for these zinc complex films are
probably because the partially oxidized zinc complex film was
easily separated from the ITO substrate into the solvent and
difficult to return to the electrode for the subsequent reduction.
No discernable reduction wave was observed for the fresh films of
these zinc complexes during the cathodic sweep. The HOMO
Fig. 4 The current density–voltage–luminance (J–V–L) characteristics
of 1–3 and CBP based red phosphorescent OLEDs.
energy levels of these complexes were determined by the onset
ox
potential of the oxidation wave (E
ox
) according to the equation:
onset
HOMO (eV) ¼ ꢁ(4.8 + E
). The LUMO energy levels were
onset
estimated from the HOMO energy level and the optical bandgap
(Eg) using the equation of LUMO (eV) ¼ HOMO + Eg. All the
electronic data for these complexes are summarized in Table 1.
It is found that these new complexes show similar HOMO
energy levels in the range of ꢁ5.22 to ꢁ5.29 eV, while their
HOMO–LUMO gaps (Eg) and the LUMO energy levels are
more affected by the attached substituents. For example, the
strong electron-withdrawing COOCH2CH3 substituted complex
5 has a relatively low LUMO level at ꢁ2.57 eV, while complexes
1–3 with weak electron-withdrawing F group and electron-
donating CH3 and OCH3 group have somewhat higher LUMO
energy levels at about ꢁ2.46 to ꢁ2.51 eV. The CF3 substituted
complex 4 does not follow the trend in the series, which has
simultaneously much more stabilized HOMO and LUMO
energy levels than other complexes. Both the photophysical and
electrochemical data of the zinc complexes combine to show that
there is an electronic communication between the appended
substituent and the 2-hydroxyphenylbenzothiazole fluorophores,
and the electronic properties and emission colors of the corre-
sponding zinc complexes can be effectively tuned by the variation
of the substituents on benzothiazole ring.
Fig. 5 The plots of luminance efficiency versus current density for 1–3
and CBP based red phosphorescent OLEDs.
Electroluminescent properties
To illustrate the electroluminescent properties of zinc complexes
1–5, the OLEDs were fabricated using these complexes as emit-
ters by vacuum thermal evaporation approach. The device
configuration is ITO/NPB (35 nm)/1–5 (30 nm)/Alq3 (40 nm)/LiF
(1 nm)/Al, in which NPB {4,40-bis[N-(1-naphthyl)-N-phenyl-
amino]-biphenyl} and Alq3 [tris(8-hydroxyquino)aluminum]
Fig. 6 The current density–voltage curves for the electron-only devices
based on the zinc complexes or Alq3.
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serve as hole-transporting and electron-transporting materials,
respectively. It has been reported that the unusually broad EL
spectra of OLEDs containing Zn(BTZ)2 as the emitting layer
were attributed to optical interference effects and light emission
originating from both fluorescence and phosphorescence.11 The
EL spectra were also greatly dependent on the thickness of the
Zn(BTZ)2 layer. Usually, the increase in the thickness of
Zn(BTZ)2 layer would result in a systematic red-shift and
broadening of the EL spectra.13 Thus, in our research, a relatively
thin (30 nm) layer of zinc complex was chosen with the aim to
remove the aforementioned optical interference effect.
developing new host materials other than CBP or PVK is quite
important. Here, the zinc complexes 1–3 were employed as hosts
in red OLEDs based on their relatively high excited state energies
that would better match with the energy of red phosphor
dopants. These red OLEDs were fabricated with a structure of
ITO/PEDOT:PSS (40 nm)/NPB (20 nm)/Ir(piq)3:1–3 (5 wt%,
30 nm)/Alq3 (40 nm)/LiF (1 nm)/Al. Ir(piq)3 [tris(2-phenyl-
isoquinoline)iridium]29 was selected as the red-emitting phosphor
and doped in the host matrix in these OLEDs. PEDOT:PSS
[poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)] was
used as a hole-injecting material due to its relatively high HOMO
level (ꢁ5.2 eV)30 and spin coated from its aqueous dispersion on
the anode. Then the following organic layers were deposited by
vacuum evaporation technique. A control device using CBP as
the host for Ir(piq)3 dopant in the same device architecture was
also prepared for comparison. Complexes 4 and 5 were similarly
utilized as hosts for red phosphorescent OLEDs. However,
incomplete energy transfer was observed and the device perfor-
mance was not ideal, probably due to their insufficient triplet
energies, although phosphorescence was not detected even at
77 K for all the zinc complexes.
Fig. 2 shows the normalized EL spectra of these devices. In
accordance with their PL spectra, wide EL color range spanning
from bluish-green (480 nm for 1) to yellow (544 nm for 5) was
simply obtained for these devices. In addition, the EL spectra of
these devices are almost invariant of the applied voltages, and it
is clear that the EL spectra of each complex matched well with
their PL in terms of spectral profile and position, confirming that
no emission signal from electroplex or excimer/exciplex has
appeared in these devices under electroexcitation.
The device performance data for the complexes are listed in
Table 2. The bluish-green-emitting complexes 1 and 2 that
contain electron-donating groups show better performance than
complexes 3–5 that have electron-withdrawing substituents. For
example, the complex 1 based device shows a low turn-on voltage
(to deliver a brightness of 1 cd mꢁ2) of 4.0 V and a maximum
luminance (Lmax) of 9137 cd mꢁ2 at 10 V. This OLED realized a
maximum luminance efficiency (hL) of 1.70 cd Aꢁ1. For the
complex 2 based device, the overall performance is increased in
comparison with that of complex 1, which has a Lmax ¼ 10 010 cd
The EL performance data of the red OLEDs with four
different hosts were systematically compared in Table 3. Fig. 4
and 5 depict the current density–voltage–luminance (J–V–L)
characteristics and current efficiency curves of these devices.
Both the luminance and efficiency of the 1–3 based devices are
superior to those of the CBP based device. For instance, the 1-
based device shows the best performance, which exhibits a
maximum luminance of 24 870 cd mꢁ2 at 12 V, high maximum
efficiencies of hL ¼ 13.85 cd Aꢁ1 and hp ¼ 7.25 lm Wꢁ1, and CIE
coordinates of (0.67, 0.32). The forward viewing external
quantum efficiency (hext or EQE) was calculated using the
luminance efficiency, the EL spectra, and the human photopic
sensitivity. A peak hext of 17.5% was obtained for the complex 1-
based device. Under identical conditions, the CBP based device
m
ꢁ2, and a maximum hL ¼ 2.38 cd Aꢁ1. The OLED performance
declined for complex 3, which exhibits a maximum luminance of
4508 cd mꢁ2 and maximum luminance efficiency of 1.10 cd Aꢁ1
.
The yellowish-green-emitting device based on complex 4 has a
higher turn-on voltage of 7.0 V, and a lower Lmax ¼ 1377 cd mꢁ2
.
This may be ascribed to its much lower HOMO energy levels
(ꢁ5.44 eV) than the other complexes and thus the higher hole
injection barrier at the NPB/emitting layer interface, which
finally leads to less efficient charge recombination in the emitting
layer. The device based on complex 5 shows a yellow EL with a
Lmax of 3907 cd mꢁ2 and a maximum hL of 1.17 cd Aꢁ1. The
current density–voltage–luminance (J–V–L) characteristics for
the complex 1–5 based OLEDs are illustrated in Fig. 3.
exhibited peak efficiencies of hL ¼ 9.97 cd Aꢁ1 and hext
¼
12.61%, which has already outperformed the best vacuum-
deposited device with the Ir(piq)3–CBP emitting layer (hext
¼
10.3%, CIE (0.68,0.32)).29 Evidently, out present red phospho-
rescent device with zinc complex 1 as host represents a tremen-
dous increase of about 39% in both hL and hext values with
reference to the CBP-based device. To our knowledge, the peak
hext of 17.5% is one of the highest for saturated red OLEDs with
such a close ultimate limit of pure red CIE values.31,32 The 2- and
3-based OLEDs show similar performances, which display peak
hL ¼ 10.33 cd Aꢁ1, hp ¼ 5.41 lm Wꢁ1 and hext ¼ 13.06% for 2 and
hL ¼ 10.30 cd Aꢁ1, hp ¼ 5.39 lm Wꢁ1 and hext ¼ 13.02% for 3.
Although the efficiencies of both devices do not seem to be much
improved compared to the CBP-based device, the turn-on volt-
ages of both devices are lower than the latter. The turn-on
voltages for the 1–3 based devices were 3.9, 4.0 and 3.9 V
respectively, whereas for the CBP-based device the turn-on
voltage was 4.3 V. This decline in driving voltages should be
ascribed to the better energy matching in the HOMOs of 1–3 and
NPB, and also in the LUMOs of 1–3 and Alq3. In addition, the
electron transport properties of these zinc complexes may also
play an important role in the EL process. It should be noted that
there are few literature examples of red phosphorescent OLEDs
using zinc complexes as host material.9 It is evident that our
In addition to the intrinsic electrofluorescence of these novel
zinc complexes, we also explored the possibility to use them as
triplet hosts for iridium phosphor in phosphorescent OLEDs.
Red phosphorescent organic light-emitting devices have been
widely investigated due to the advantage of their high efficiency
and their practical applications in full-color displays and white
OLED lighting. However, the current host materials used in red
phosphorescent OLEDs are limited to organic materials, such as
the carbazole-containing materials including the typical poly-
(vinylcarbazole) (PVK) and the small molecular 4,40-bis(N-car-
bazolyl)biphenyl (CBP). These host materials are indeed widely
used in red phosphorescent OLEDs and usually give good
performance.25,26 However, relatively high driving voltages are
usually observed for OLEDs based on these materials due to
their relatively low HOMO levels (CBP ꢁ5.9 eV, PVK
ꢁ5.8 eV)27,28 and thus the high hole injection barriers.9 Hence,
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present Zn(BTZ)2 derivatives 1–3 are capable of acting as host
materials for iridium phosphors in high-performance red phos-
phorescent OLEDs, and are superior to the traditional CBP host
in terms of device performance.
Max-4 (TCSPC) instrument. Thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) measurements were
carried out using a Perkin-Elmer thermogravimeter (Model
TGA7) and a Netzsch DSC 204 at a heating rate of 10 ꢀC minꢁ1
under a nitrogen atmosphere, respectively. Cyclic voltammo-
grams of the zinc complexes were recorded on an electrochemical
workstation (BAS100B, USA) at room temperature in a 0.1 M
[Bu4N]PF6 solution under nitrogen gas protection. The deposited
films of the zinc complexes on ITO substrates were used as the
working electrode, and Ag/AgCl reference electrode and Pt-wire
counter electrode were used to compose a three-electrode
configuration. Ferrocene was used as the internal standard, and
all potentials reported were quoted with reference to the ferro-
cene–ferrocenium (Fc/Fc+) couple.
In order to verify the electron-transporting feature of these zinc
complexes, the electron-only devices were fabricated with device
structure of ITO/TPBI (20 nm)/zinc complex or Alq3 (40 nm)/
TPBI (20 nm)/LiF (1 nm)/Al (200 nm). It is apparent that the hole
injection at the ITO/TPBI interface is negligible due to too high a
hole barrier (1.5 eV). As shown by the current density–voltage
curves in Fig. 6, all zinc complex based devices exhibited
remarkably higher current density than the Alq3 device.
Although the different electron injection efficiency at the TPBI–
zinc complex or TPBI–Alq3 interface due to different electron-
injecting barriers may have an influence on the current density of
the overall device, the absolutely higher current densities in all the
zinc complex based devices unambiguously confirmed their better
or at least comparable electron-transporting ability versus the
widely used electron transporting material Alq3.
OLED fabrication and measurements
The pre-cleaned ITO glass substrates (30 U Yꢁ1) were treated by
UV-ozone for 20 min. For the red phosphorescent devices, a
40 nm thick PEDOT:PSS film was first deposited on the ITO
ꢀ
glass substrates, and baked at 120 C for 30 min in air. All the
Conclusion
organic layers were deposited by vacuum evaporation in a
vacuum chamber with a base pressure less than 10ꢁ6 torr. The
emitting area of each pixel is determined by overlapping of the
two electrodes as 9 mm2. The EL spectra, CIE coordinates, and
current–voltage–luminance characteristics were measured with a
computer-controlled Spectrascan PR 705 photometer and a
source-measure-unit Keithley 236 under ambient conditions. The
forward viewing external quantum efficiency (hext) was calcu-
lated by using the luminance efficiency, EL spectra and human
photopic sensitivity.
In conclusion, we have reported the synthesis and luminescent
properties of a group of novel zinc complexes containing
2-hydroxyphenylbenzothiazole ligand frameworks, in which
different substituents were attached at the 6-position of the
benzothiazole ring. Color-tunable fluorescence was achieved for
these zinc complexes by varying the electronic properties of the
substituent, with strong electron-withdrawing groups leading to
a bathochromic shift. The zinc complexes exhibited good
performance when used as emitting layer in non-doped OLEDs,
for example with a maximum luminance of 10 010 cd mꢁ2 and a
maximum luminance efficiency of 2.38 cd Aꢁ1 for the methyl-
substituted complex 2. More importantly, some of the zinc
complexes (1–3) were successfully tested as host materials for red
phosphorescent OLEDs and displayed greatly improved
performance with decreased driving voltage and enhanced effi-
ciency compared to the traditional CBP host. In particular, the
methoxy-substituted complex 1 based device exhibited a high
luminance of 24 870 cd mꢁ2 and a maximum external quantum
efficiency of 17.51%, which are greatly superior to those (11 710
cd mꢁ2 and 12.6%) of the CBP hosted device. The present study
reveals that these novel zinc complexes are excellent phospho-
rescent host materials possessing electron-transporting charac-
teristics, and some of them may be alternatives to the traditional
hole-transporting host materials such as CBP.
Materials synthesis
The synthesis of ligands 1a–4a was described in our previous
publication.24 Compound 5c is synthesized by a similar proce-
dure to those of 1c–4c.
Preparation of ligand 5a. To the thiobenzamide 5c (0.01 mol,
3.68 g) wetted with small amount of ethanol beforehand was
added 30% aqueous sodium hydroxide (0.08 mol, 3.2 g). The
mixture was diluted with water to provide a final suspension of
10% aqueous sodium hydroxide. The diluted sample was added
within 10 minutes to a stirred solution of potassium ferricyanide
(0.04 mol, 13.2 g) in water (20 wt%) at 80–90 ꢀC. After stirring for
3 h and then cooling to room temperature, the reaction solution
was poured into water and neutralized with dilute aqueous HCl
to give the crude product of intermediate 5d. The precipitate 5d
was then filtered, washed with water and dried. To a mixture of
10 mL of ethanol and the above 5d (4 mmol, 1.02 g) at 90 ꢀC was
added dropwise 5 mL of concentrated H2SO4. After stirring for
2 h, the solution was diluted with water, neutralized with 10%
Na2CO3, and extracted with dichloromethane. The extracted
organic layer was removed to give the crude product, which was
purified by silica gel column chromatography using petroleum
ether (30–60 ꢀC) and dichloromethane (3 : 2) as eluent to yield the
pure compound 5a as white solid (0.82 g, 68.5%). Mp 189 ꢀC. 1H
NMR (400 MHz, CDCl3): d ¼ 8.63 (s, 1H), 8.20–8.18 (d, J ¼ 8.0
Hz, 1H), 8.03–8.01 (d, J ¼ 8.0 Hz, 1H), 7.73–7.71 (d, J ¼ 8.0 Hz,
Experimental section
Instruments and methods
1H NMR spectra were recorded on a 400 MHz Varian Unity
Inova spectrophotometer. Mass spectra were taken on MALDI
micro MX and HP1100LC/MSD MS spectrometers. The pho-
toluminescence and UV-vis absorption spectra measurements
were performed on a Perkin-Elmer LS55 spectrometer and a
Perkin-Elmer Lambda 35 spectrophotometer for the films of the
zinc complexes on quartz substrates, respectively. The fluores-
cence lifetimes and quantum yields of the powder samples of the
zinc complexes were measured on a Horiba Jobin Yvon Fluoro
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 23454–23460 | 23459

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J. Leonhardt, G. Diener, L. De Cola and C. A. Strassert, Chem.
Mater., 2011, 23, 3659.
1H), 7.44–7.40 (m, 1H), 7.13–7.11 (d, J ¼ 8.0 Hz, 1H), 7.01–6.97
(m, 1H), 4.47–4.42 (m, 2H), 1.46–1.43 (m, 3H). MALDI-TOF-
MS (m/z): 299 [M]+.
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General procedure for synthesis of the zinc complexes (1–5). A
solution of zinc acetate (0.49 mmol) in ethanol (2 mL) was added
dropwise to a solution of ligands 1a–5a (1 mmol) in dried ethanol
(10 mL) at 40 ꢀC. The resulting mixture was stirred at 70 ꢀC for
24 h. The yellow or green precipitate was collected by filtration
and washed with ethanol. The zinc complexes were all purified by
vacuum sublimation.
1: Yield 81%. MALDI-TOF-MS (m/z): 576.0 [M]+. Anal. calcd
for C28H20N2O4S2Zn: C, 58.18; H, 3.49; N, 4.85. Found: C,
58.35; H, 3.52; N, 4.62.
2: Yield 80%. MALDI-TOF-MS (m/z): 567.0 [M + Na]+. Anal.
calcd for C28H20N2O2S2Zn: C, 61.59; H, 3.69; N, 5.13. Found: C,
61.38; H, 3.83; N, 5.26.
3: Yield 57%. MALDI-TOF-MS (m/z): 551.9 [M]+. Anal. calcd
for C26H14F2N2O2S2Zn: C, 56.37; H, 2.55; N, 5.06. Found: C,
56.54; H, 2.37; N, 5.30.
4: Yield 62%. MALDI-TOF-MS (m/z): 651.9 [M]ꢁ. Anal.
calcd for C28H14F6N2O2S2Zn: C, 51.43; H, 2.16; N, 4.28. Found:
C, 51.55; H, 1.98; N, 4.47.
5: Yield 69%. MALDI-TOF-MS (m/z): 660.0 [M]+. Anal. calcd
for C32H24N2O6S2Zn: C, 58.05; H, 3.65; N, 4.23. Found: C,
58.26; H, 3.54; N, 4.12.
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Acknowledgements
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We thank the National Natural Science Foundation of China
(21072026, 20923006, and U0634003), the Ministry of Education
for the New Century Excellent Talents in University (Grant
NCET-08-0074), and the Fundamental Research Funds for the
Central Universities (DUT12ZD211) for financial support of this
work.
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