Bipolar hosts and non-doped deep-blue emitters (CIE: Y = 0.04) based on phenylcarbazole and 2-(2-phenyl-2 H -1,2,4-triazol-3-yl)pyridine groups

Article abstract of DOI:10.1039/c6tc05630j

We have designed and synthesized four bipolar materials, TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz, using 2-(2-phenyl-2H-1,2,4-triazol-3-yl)pyridine (TAZ) as an electron-withdrawing group and phenylcarbazole as an electron-donating group. Their rigid molecular structures improve their thermal stability with high glass transition temperatures: 99 °C for TAZ-2Cz, 100 °C for TAZ-3Cz and 103 °C for TAZ-4Cz. Employing TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz as hosts, green phosphorescent organic light-emitting devices (PhOLEDs) with fac-tris(2-phenylpyridine)iridium as an emitter display maximum external quantum efficiencies (EQEs) of 15.8, 15.5, 16.0 and 13.0%, respectively. And blue PhOLEDs hosted by TAZ-1Cz, TAZ-2Cz and TAZ-3Cz achieved maximum current efficiencies of 8.6, 11.0 and 10.6 cd A^-1, respectively. Furthermore, non-doped OLEDs using TAZ-1Cz and TAZ-4Cz as emitters exhibit electroluminescence (EL) peaks at 400 and 412 nm with CIE coordinates of (0.16, 0.04) and (0.15, 0.04), which are located in the deep-blue region.

Full text of DOI:10.1039/c6tc05630j

Journal of
Materials Chemistry C
View Article Online
View Journal
PAPER
Bipolar hosts and non-doped deep-blue emitters
(CIEy = 0.04) based on phenylcarbazole and
2-(2-phenyl-2H-1,2,4-triazol-3-yl)pyridine groups†
ab
Cite this:DOI: 10.1039/c6tc05630j
Huixia Xu,
*
Peng Sun,ab Kexiang Wang,ab Jie Li,ab Fang Wang,ab Yanqin Miao,ab
Hua Wang,ab Bingshe Xuab and Wai-Yeung Wongac
We have designed and synthesized four bipolar materials, TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz,
using 2-(2-phenyl-2H-1,2,4-triazol-3-yl)pyridine (TAZ) as an electron-withdrawing group and
phenylcarbazole as an electron-donating group. Their rigid molecular structures improve their thermal
stability with high glass transition temperatures: 99 1C for TAZ-2Cz, 100 1C for TAZ-3Cz and 103 1C for
TAZ-4Cz. Employing TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz as hosts, green phosphorescent
organic light-emitting devices (PhOLEDs) with fac-tris(2-phenylpyridine)iridium as an emitter display
maximum external quantum efficiencies (EQEs) of 15.8, 15.5, 16.0 and 13.0%, respectively. And blue
PhOLEDs hosted by TAZ-1Cz, TAZ-2Cz and TAZ-3Cz achieved maximum current efficiencies of 8.6,
11.0 and 10.6 cd Aꢀ1, respectively. Furthermore, non-doped OLEDs using TAZ-1Cz and TAZ-4Cz as
emitters exhibit electroluminescence (EL) peaks at 400 and 412 nm with CIE coordinates of (0.16, 0.04)
and (0.15, 0.04), which are located in the deep-blue region.
Received 28th December 2016,
Accepted 30th March 2017
DOI: 10.1039/c6tc05630j
rsc.li/materials-c
the low ET (r2.62 eV), while hosts for blue PhOLEDs with high
ET, whose emissions are located in the ultraviolet region, are also
Introduction
Phosphorescent organic light-emitting devices (PhOLEDs), which not suitable for use as blue fluorescent materials in non-doped
can reach theoretically 100% internal quantum efficiency, have OLEDs.6
attracted great attention.1 In PhOLEDs, phosphorescent hosts
Aimed at developing efficient bipolar hosts for green and
play a key role in achieving efficient energy transfer from the blue PhOLEDs, and using them as non-doped deep-blue fluorescent
hosts to emitters.2 Many organic molecules can be used as hosts emitters, we designed and synthesized four 2-(2-phenyl-2H-
for green and red PhOLEDs,3 while hosts for blue PhOLEDs are 1,2,4-triazol-3-yl)pyridine (TAZ)- and carbazole (Cz)-based bipolar
very rare due to the intrinsically high triplet level (ET) of blue materials – TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz. Carbazole
emitters.4
derivatives are widely utilized as donors in host materials due to
Recently, bipolar non-doped blue-emitting materials (CIEy r their high ET to prevent the back-energy transfer from the emitter
0.10) based on organic molecules with balanced carrier-transport to the host and sufficient hole-transport properties.7 Meanwhile,
properties have drawn much attention due to their facile synthesis 1,2,4-triazole has been used as an acceptor in bipolar hosts and
and low cost.5 However, these blue fluorescent materials are not ancillary ligands in metal complexes because of its excellent
suitable for use as hosts for green or blue PhOLEDs because of electron-transport characteristic.8 Our previous report demonstrated
that 9-(6-(3-phenyl-5-(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)hexyl)-9H-
carbazole (PPHCz) exhibited ultraviolet emission and a good
a Key Laboratory of Interface Science and Engineering in Advanced Materials,
bipolar ability.8b To shift the emission to the deep-blue region,
Ministry of Education, Taiyuan University of Technology, Taiyuan, Shanxi 030024,
the alkyl bridge between donors and acceptors was replaced by
P. R. China. E-mail: xuhuixia@tyut.edu.cn
b Shanxi Research Centre of Advanced Materials Science and Technology, Taiyuan,
conjugated phenyl groups in this paper. By attaching a carbazole
moiety to the different positions of the 2-(2-phenyl-2H-1,2,4-triazol-
3-yl)pyridine skeleton, the four novel bipolar materials were
synthesized (Fig. 1). In TAZ-1Cz and TAZ-2Cz compounds,
carbazoles are linked by the N-phenyl, while the carbazoles are
connected through C3- and C2-phenyl in TAZ-3Cz and TAZ-4Cz,
respectively. To reach high ET, the conjugation between the
acceptor and the donor was broken in TAZ-1Cz and TAZ-2Cz.
Shanxi, 030024, P. R. China
c Department of Applied Biology and Chemical Technology,
The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R. China
† Electronic supplementary information (ESI) available: Synthetic routes, absorption
spectra in different solvents, phosphorescence spectra, electrochemical curves,
electroluminescence performances of blue and red PhOLEDs, EQE versus luminance
curves of green and red PhOLEDs hosted by CBP and electroluminescence spectra of
non-doped OLEDs. See DOI: 10.1039/c6tc05630j
This journal is ©The Royal Society of Chemistry 2017
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
Table 1 Photophysical properties of all bipolar materials
a
a
b
c
labs
Compound (nm)
lem
Eg
HOMO/
ET
(eV) Td/Tg (1C)
d
(nm) (eV) LUMO (eV)
TAZ-1Cz
TAZ-2Cz
TAZ-3Cz
TAZ-4Cz
290, 312 400
3.48 ꢀ5.71/ꢀ2.23 2.53 462/—
3.64 ꢀ5.74/ꢀ2.10 2.62 467/99
3.45 ꢀ5.63/ꢀ2.18 2.56 453/100
3.42 ꢀ5.71/ꢀ2.29 2.51 442/103
289, 342 402
259, 301 386
280, 315 400
a
Measured in CH2Cl2 solution of 1 ꢁ10ꢀ5 M at room temperature.
b
Calculated by the intersection of the UV-vis absorption and emission
c
spectra. Estimated from the highest vibronic peak phosphorescence
spectra in 2-MeTHF at 77 K. From TGA and DSC measurements.
d
for TAZ-3Cz and 103 1C for TAZ-4Cz, respectively. It is worth
noting that no glass transition is detected in the DSC curve for
TAZ-1Cz under the same conditions.
Fig. 1 Chemical structures of TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz.
Much higher Tg values than those of commonly used host
materials such as 1,3-bis(9H-carbazol-9-yl)benzene (mCP: 60 1C),
4,40-N,N0-dicarbazolebiphenyl (CBP: 62 1C)9 and some diphenyl-
phosphine oxide derivatives10 are observed, indicating that the
introduction of the rigid TAZ moiety increases their morphological
stability greatly.
The four bipolar materials possess twisted and rigid molecular
structures, which result in deep-blue emissions, good thermal
stabilities and carrier-transport abilities. They have not only
been employed as emitters in fabricating non-doped deep-blue
OLEDs, but have also been used as hosts to construct green and
blue PhOLEDs. The green and blue PhOLEDs hosted by TAZ-2Cz
exhibit maximum current efficiencies of 55.0 and 11.0 cd Aꢀ1
.
Fig. 3 shows the UV-vis absorption and emission spectra of
the synthesized bipolar materials in dichloromethane (CH2Cl2)
solution at room temperature. Although the p–p* transition
absorption character of donor and acceptor moieties is similar in
the range of 260–300 nm for all four materials, the long-wavelength
absorption bands are completely different. The absorption spectra
of TAZ-1Cz with a shoulder peak at 312 nm and the absorption peak
of TAZ-2Cz at 342 nm are attributed to the intramolecular charge-
transfer (ICT) transitions from the donor to acceptor. The absorption
spectrum of TAZ-3Cz displays a very weak ICT peak at 326 nm.
Interestingly, ICT transition peaks in TAZ-4Cz may be covered due to
the strong and broadened p–p* absorption at 315 nm. The emission
peaks of all bipolar materials in this paper are located at 395–400 nm
in CH2Cl2 solution. According to the intersection of the absorption
and emission bands in CH2Cl2 solution, the energy gaps (Egs) are
estimated to be 3.48, 3.64, 3.45 and 3.42 eV for TAZ-1Cz, TAZ-2Cz,
TAZ-3Cz and TAZ-4Cz, respectively (Table 1).
And the non-doped OLEDs using TAZ-4Cz as an emitter exhibit
deep-blue emission with CIE coordinates of (0.15, 0.04).
Results and discussion
The four bipolar materials were prepared by facile synthesis routes
as shown in Scheme S1 (ESI†). Compound 2 was obtained by the
Ullmann reaction between compound 1 and 1-iodobenzene, which
was reacted with the corresponding donors to produce the bipolar
materials by Suzuki coupling reactions. All of these materials
exhibit good solubility in most common organic solvents. The
molecular structures of the four bipolar materials were fully
1
characterized by H NMR, 13C NMR, and elemental analyses.
Fig. 2 shows the TGA and DSC curves of the four bipolar
materials and Table 1 summarizes the relevant data. The excellent
thermal stabilities of the synthesized bipolar materials are mani-
fested by their high decomposition temperatures (Td: corresponding
to 5% weight loss) in the range of 412–476 1C. Meanwhile, their
glass transition temperatures (Tg) are 99 1C for TAZ-2Cz, 100 1C
To better understand the photophysical properties of the four
compounds, the UV-vis and PL spectra in solvents with different
polarities were measured. The UV-vis absorption spectra seldom
Fig. 2 TGA and DSC (inset) curves of all bipolar materials.
Fig. 3 Normalized UV-vis and PL spectra in CH2Cl2 solution.
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2017

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 5 HOMO and LUMO distributions for the four synthesized bipolar
materials.
Fig. 4 PL spectra of TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz in
different solvents at room temperature.
and 2.51 eV for TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz,
respectively, according to the highest energy of the phosphorescence
peaks. The highest ET of compound TAZ-2Cz in all four bipolar
materials is due to the broken conjugated structure with the
change in terms of shape and position in various solvents N-linkage of carbazole. Remarkably, the ET of TAZ-2Cz is higher
(Fig. S1, ESI†), implying a rather small dipolar change in the ground than that of the blue emitter of iridium(III) bis[(4,6-difluorophenyl)-
0
state in different solvents and the electronic structures of ground pyridinato-N,C2 ]picolinate (FIrpic, ET = 2.6 eV), indicating that it
states are independent of the solvent polarity.
can be used to fabricate blue PhOLEDs.12
In contrast, the four bipolar materials exhibit distinct solvato-
The electrochemical behaviors were studied by cyclic voltammetry
chromic effects for the emission spectra as the solvent polarity (CV). Because the oxidation process occurred in the carbazole unit,
increases due to the different conjugation of the molecules all four bipolar materials show similar oxidation curves (see Fig. S3,
(Fig. 4). For TAZ-1Cz and TAZ-2Cz with carbazole N-linked to ESI†). The oxidation potentials of TAZ-Cz, TAZ-2Cz, TAZ-3Cz
the phenyl moieties, significant bathochromic shifts are observed and TAZ-4Cz are 1.19, 1.20, 1.08 and 1.23 V, respectively. The
from 354/372 to 417 nm and 326/370 to 430 nm from low-polarity corresponding EHOMO/ELUMO levels are estimated to be ꢀ5.71/
hexane to high-polarity acetonitrile.
ꢀ2.23, ꢀ5.74/ꢀ2.10, ꢀ5.63/ꢀ2.18, and ꢀ5.71/ꢀ2.29 eV, respectively,
Besides, the vibronic fine structures of emission spectra by using the formulae EHOMO = ꢀ4.8 ꢀ e(Eocx ꢀ Efox)V and ELUMO
=
gradually disappear, and broad emission spectra emerge with EHOMO + Eg (HOMO: highest occupied molecular orbital; LUMO:
the increase of the solvent polarity. These changes are consistent lowest unoccupied molecular orbital). The linkage modes of
with the change in the excited state from the locally excited (LE) carbazole and TAZ result in different electrochemical behaviors.
state to a strong charge-transfer (CT) excited state.11 For TAZ-3Cz To further study the electron structures of these bipolar materials,
and TAZ-4Cz with the C3- and C2-positions of carbazole linked to density functional theory (DFT) calculations were carried out at
the phenyl moieties, the PL behaviors were totally different. For the B3LYP/6-31G(d) level. When the carbazole is linked through
TAZ-3Cz, the vibrational structures with emission peaks at 362 the N-phenyl and C3-phenyl in TAZ-3Cz, TAZ-4Cz and TAZ-1Cz, the
and 380 nm remain almost unchanged with the increase of the HOMOs mainly disperse on the phenylcarbazole and partly on
solvent polarity, suggesting that the locally excited state remains 1,2,4-triazole rings, in Fig. 5. When the carbazole is linked through
due to the full conjugation. In the case of TAZ-4Cz, the LE-to-CT the C2-phenyl in TAZ-2Cz, the HOMOs distribute on the phenyl-
state conversion is observed in the emission spectra in solvents carbazole. For all the compounds, the LUMOs mainly locate at
from non-polar hexane to high-polarity N,N-dimethylformamide the electron-deficient TAZ moieties. The completely separated
(DMF). However, the bathochromic shift value (from 367/384 to distributions of the HOMOs and LUMOs and larger dihedral
400 nm) is much smaller than that of TAZ-1Cz and TAZ-2Cz, angle (561) between the donor unit and nearby phenyl in TAZ-2Cz
which could be ascribed to the limited conjugation of the molecules result in a higher ET and low-lying EHOMO. On the other hand, the
with the acceptor linked on the 2-position of the carbazole.
smaller dihedral angle (371) and better conjugation structure in
The low-temperature phosphorescence spectra of all bipolar TAZ-3Cz led to a high-lying EHOMO. The calculated EHOMO/ELUMO
materials, which were measured in 2-methyltetrahydrofuran levels are ꢀ5.35/ꢀ1.50, ꢀ5.24/ꢀ1.39, ꢀ5.13/ꢀ1.43 and ꢀ5.31/
(2-MeTHF) matrices at 77 K, showed fine vibronic structures, ꢀ1.45 eV for TAZ-Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz, respectively,
as seen in Fig. S2 (ESI†). ETs are estimated to be 2.53, 2.62, 2.56 which are in agreement with the experimental results (Table 1).
This journal is ©The Royal Society of Chemistry 2017
J. Mater. Chem. C

View Article Online
Paper
To investigate their bipolar charge-transport characteristics,
Journal of Materials Chemistry C
Devices G1, G3 and G4 show close turn-on voltage values
we fabricated hole-only (HODs) and electron-only devices (EODs). (Von, voltage at 1.0 cd mꢀ2) of 3.3, 3.2 and 3.3 V. The highest Von
The configurations of the HODs and EODs are ITO/MoO3 (3 nm)/ of 3.6 V in device G2 may be due to the large barrier to electron
TCTA (10 nm)/bipolar materials (40 nm)/TCTA (10 nm)/Al injection from TPBi to the host of TAZ-2Cz (the energy difference is
(200 nm), and ITO/TPBi (10 nm)/bipolar materials (40 nm)/TPBi 0.6 V between TPBi and TAZ-2Cz). Device G1 hosted by TAZ-1Cz
(10 nm)/LiF (1 nm)/Al (200 nm), respectively. Tris(4-carbazoyl-9- exhibits a maximum current efficiency (Zc,max) of 55.0 cd Aꢀ1, a
ylphenyl)amine (TCTA) in HODs and 1,3,5-tris(2-phenylbenzimid- maximum power efficiency (Zp,max) of 41.3 lm Wꢀ1 and a maximum
azolyl)-benzene (TPBi) in EODs were used to block the electron- external quantum efficiency (EQEmax) of 15.5%. And device
injection from Al and the hole-injection from ITO. MoO3 G2 hosted by TAZ-2Cz shows a performance with Zc,max of
(molybdenum oxide) and LiF (lithium fluoride) serve as hole- 53.0 cd Aꢀ1, Zp,max of 40.0 lm Wꢀ1 and EQEmax of 16.0%. In
and electron-injecting layers. The current density–voltage ( J–V) comparison, when TAZ units are linked to the C3- and C2-phenyl
curves of the HODs and EODs are shown in Fig. 6. The J–V of carbazole in TAZ-3Cz and TAZ-4Cz, their hosted devices G3 and
characteristics of the hole- and electron-only devices illustrate G4 show inferior performance with maximum efficiencies of
that all four compounds have bipolar charge-transport capacities 47.8 cd Aꢀ1, 35.1 lm Wꢀ1, and 13.9% for device G3 and 46.1 cd Aꢀ1
,
as evidenced by the considerable hole/electron current density 35.3 lm Wꢀ1, and 13.0% for device G4. The much better EL
with similar magnitudes. The hole current density is slightly performance of device G2 than that of the other devices can be
higher than that of electrons, which can be attributed to the high- ascribed to high ET and large Eg. Furthermore, it is found that
lying LUMO energy level of the compound. The carrier-transport devices G1, G2, G3 and G4 show EQEs of 12.4, 10.9, 11.8 and
balance of TAZ-1Cz and TAZ-3Cz is superior to that of TAZ-2Cz 4.8% at a luminance of 1000 cd mꢀ2, respectively. The percentage
and TAZ-4Cz.
drops in EQE at a given field (F) and luminance (L) with reference
To evaluate their electroluminescence (EL) properties, green to EQEmax were calculated to be 20, 31.8, 15 and 63% using the
phosphorescent OLEDs using fac-tris(2-phenylpyridine)iridium
(Ir(ppy)3) (ET = 2.4 eV) as a dopant were fabricated with the
EQEmax ꢀ EQEðF ;LÞ
equation EQEroll-off
¼
ꢁ 100% at a luminance
EQEmax
following configuration: ITO/MoO3 (3 nm)/NPB (30 nm)/TCTA of 1000 cd mꢀ2. The most balanced charge-transporting
(10 nm)/Host: Ir(ppy)3 8 wt% (20 nm)/TPBi (40 nm)/LiF (1 nm)/ character of host TAZ-3Cz results in the lowest EQEroll-off and
Al (200) (hosts: TAZ-1Cz for device G1, TAZ-2Cz for device G2, the maximum luminance (Lmax) (68 330 cd mꢀ2) of device G3 in
TAZ-3Cz for device G3 and TAZ-4Cz for device G4). The energy all devices.
level diagrams of the devices and all molecular structures are
shown in Fig. S4 (ESI†).
Considering the high ET and performances in the green
devices of TAZ-1Cz, TAZ-2Cz and TAZ-3Cz, the blue and red
The current density–voltage–luminance ( J–V–L) characteristics, PhOLEDs using FIrpic (ET = 2.6 eV) and bis(1-phenyl-isoquino-
current density–efficiency and external quantum efficiency (EQE)– line)(acetylacetonato)iridium(III) (Ir(piq)2acac, ET = 2.25 eV) as
luminance curves for the green devices are shown in Fig. 7, and dopants were prepared with the structures of ITO/MoO3 (3 nm)/
key device performance data are listed in Table 2. Fig. 7a shows the NPB (30 nm)/TCTA (10 nm)/hosts:Firpic 15 wt% (20 nm)/TPBi
normalized EL spectra of these green devices. The EL spectra of (40 nm)/LiF (1 nm)/Al (100 nm) (host: TAZ-1Cz for device B1,
devices G1–G4 are similar to the phosphorescent emitter Ir(ppy)3, TAZ-2Cz for device B2 and TAZ-3Cz for device B3) and ITO/
indicating that the emission originates from the Ir(ppy)3 dopant. MoO3 (3 nm)/NPB (30 nm)/TCTA (10 nm)/hosts:Ir(piq)2acac
Therefore, the triplet energy transfers from the host to the dopant 4 wt% (20 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm) (host:
are complete. In addition, the EL spectra of all devices remain TAZ-1Cz for device R1, TAZ-2Cz for device R2, TAZ-3Cz for
quite stable under different voltages.
device R3 and TAZ-4Cz for device R4). The key data are also
listed in Table 2 and the performances are displayed in Fig. S5
and S6 (ESI†). The EL spectra of blue PhOLEDs are consistent
with the PL spectra of FIrpic in Fig. S5a (ESI†), suggesting that a
complete energy transfer from the host to FIrpic can be
achieved. Similarly, the TAZ-2Cz hosted device B2 exhibits the
best performances with maximum Lmax, Zc, Zp and EQE values
of 5412 cd mꢀ2, 11.0 cd Aꢀ1, 7.3 lm Wꢀ1 and 5.0%, respectively.
The EL peaks of red PhOLEDs are located at 624 nm with
shoulders at about 670 nm. Device R1 hosted by TAZ-1Cz shows
better performance with maximum Lmax, Zc, Zp and EQE values
of 27 670 cd mꢀ2, 12.1 cd Aꢀ1, 9.1 lm Wꢀ1 and 16.2%,
respectively. Furthermore, the luminescence efficiencies of all
red PhOLEDs are superior to that of the device using CBP as a
host (Fig. S7, ESI†).
Because the emissions of TAZ-1Cz and TAZ-4Cz with excellent
carrier-transport properties are located in the deep-blue region,
we fabricated non-doped devices with the following structures:
Fig. 6 J–V characteristics of electron-only and hole-only devices.
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2017

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 7 (a) EL spectra, (b) J–V–L characteristics, (c) current efficiency–current density–power efficiency curves (Zc–J–Zp) and (d) external quantum
efficiency (EQE) versus luminance for green devices G1–G4.
Table 2 Device performances in this work
Device
Host
lmax (nm)
CIEa (x, y)
Von (V)
Lmax (cd mꢀ2
)
Zc (cd Aꢀ1
b
)
Zp (lm Wꢀ1
c
)
EQEmax (%)
R1
R2
R3
R4
G1
G2
G3
G4
B1
B2
B3
DB1
DB2
TAZ-1Cz
TAZ-2Cz
TAZ-3Cz
TAZ-4Cz
TAZ-1Cz
TAZ-2Cz
TAZ-3Cz
TAZ-4Cz
TAZ-1Cz
TAZ-2Cz
TAZ-3Cz
624, 670
628, 670
624, 670
628, 670
512, 540
512, 540
512, 540
512, 540
476
472
476
400
412
(0.67, 0.32)
(0.67, 0.32)
(0.67, 0.32)
(0.67, 0.32)
(0.31, 0.60)
(0.31, 0.59)
(0.30, 0.59)
(0.31, 0.60)
(0.13, 0.34)
(0.16, 0.33)
(0.16, 0.34)
(0.16, 0.04)
(0.16, 0.04)
3.8
4.1
3.5
3.8
3.3
3.6
3.2
3.3
3.3
3.9
3.6
3.9
3.9
27 670
8008
12.1
11.4
11.9
11.9
53.2
55.0
47.8
46.1
8.6
11.0
10.6
0.53
0.68
9.1
7.9
9.1
16.2
16.1
16.3
16.1
15.5
16.0
13.9
13.0
3.8
5.0
4.8
1.65
2.48
21 590
14 750
55 170
42 710
68 330
17 210
2922
5412
3664
784
1478
8.9
40.0
41.3
35.2
35.3
7.0
7.3
8.6
0.43
0.58
a
b
c
The CIE coordination at a voltage of 8 V. The maximum current efficiency. The maximum power efficiency.
ITO/MoO3 (3 nm)/NPB (30 nm)/TCTA (10 nm)/TAZ-1Cz or TAZ-4Cz the EL spectra remain almost unchanged under different driving
(30 nm)/TPBI (35 nm)/LiF (1 nm)/Al (100 nm) (TAZ-1Cz for voltages from 5 to 10 V (Fig. S8, ESI†). Device DB1 using TAZ-1Cz
device DB1, TAZ-4Cz for device DB2). The EL spectra, J–V–L as an emitter exhibits a turn-on voltage of 3.9 V, maximum Zc of
characteristics, Zc–J–Zp and EQE–luminance curves are shown 0.53 cd Aꢀ1, Zp of 0.43 lm Wꢀ1 and EQE of 1.65%, respectively.
in Fig. 8 and the corresponding EL data are also listed in Device DB2 employing TAZ-4Cz as an emitter exhibits superior
Table 2. Both devices DB1 and DB2 exhibit deep-blue emission performance to that of device DB1 with a Von of 3.9 V, maximum
with CIE coordinates of (0.16, 0.04) and (0.15, 0.04). And their Zc of 0.68 cd Aꢀ1, Zp of 0.58 lm Wꢀ1 and EQE of 2.48% due to its
EL peaks are located at 400 and 412 nm, respectively. The conjugation structure. It is worth noting that the EQE roll-offs
CIEy values are smaller than the NTSC (National Television for non-doped deep-blue devices are negligible, indicating a
Standards Committee) blue standard (CIEy = 0.08). In addition, well-balanced charge-transport property.
This journal is ©The Royal Society of Chemistry 2017
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
Fig. 8 (a) EL spectra, (b) J–V–L characteristics, (c) Zc–J–Zp and (d) external quantum efficiency (EQE) versus luminance curves of non-doped deep-blue devices.
Conclusions
with an Elemental Vario EL elemental analyzer. UV-vis absorption
spectra were measured using a Lambda Bio 40. The photo-
luminescence (PL) spectra were recorded using a HORIBA
FluoroMax-4 spectrophotometer. The low-temperature phos-
phorescence spectra at 77 K in 2-methyltetrahydrofuran (2-MeTHF)
were recorded on an Edinburgh F-980 spectrometer. Thermo-
gravimetric analysis (TGA) curves were obtained using a Netzsch
TG 209F3 under a dry nitrogen atmosphere at a heating rate of
10 1C minꢀ1 from room temperature to 800 1C. Differential
scanning calorimetry (DSC) was performed on a DSC Q2000 at a
heating rate of 10 1C minꢀ1 from 20 to 300 1C, then cooling to
room temperature rapidly, and heating to 300 1C at a heating
rate of 10 1C minꢀ1 again, from which the melting temperature
(Tm) was obtained from the first heating scan and the glass
transition temperature (Tg) was determined from the second
heating scan.
Cyclic voltammetry (CV) was carried out in CH2Cl2 solution
with chromatographic purity at room temperature using a CHI
660E voltammetry analyzer. Tetrabutylammonium hexafluoro-
phosphate (TBAPF6) (0.1 M) was used as a supporting electrolyte.
A platinum wire was used as a working electrode. A platinum
electrode is a counter and a calomel electrode is a reference with
ferrocenium–ferrocene (Fc+/Fc) as an internal standard. The scan
In summary, four bipolar materials TAZ-1Cz, TAZ-2Cz, TAZ-3Cz
and TAZ-4Cz, using the 1,2,4-triazole derivative as an acceptor
and carbazole as a donor, have not only been used as non-
doped deep-blue fluorescent emitters in OLEDs, but also as
host materials to fabricate blue, green and red PhOLEDs. The
different linkage modes of donors and acceptors result in
various photophysical properties. TAZ-1Cz and TAZ-2Cz with
N-linked carbazole exhibit obvious conversion from LE to CT
state transition. Compounds TAZ-2Cz and TAZ-3Cz possess
higher ETs of 2.56 and 2.62 eV. PhOLEDs hosted by TAZ-2Cz
show excellent performances among all devices with efficiencies of
55.0 cd Aꢀ1 and 41.3 lm Wꢀ1 and maximum EQE of 16% in green
devices and 11.0 cd Aꢀ1, 7.3 lm Wꢀ1 and 5.0% in blue devices. The
non-doped deep-blue device using TAZ-4Cz as an emitter exhibits a
maximum EQE of 2.48%. Our results demonstrate that the
bipolar host materials TAZ-1Cz and TAZ-4Cz may also be excellent
deep-blue fluorescent emitters.
Experimental
Materials and measurements
1H-NMR and 13C-NMR spectra were measured on a Switzerland rate for CV curves is 100 mV sꢀ1. The highest occupied molecular
Bruker DRX600. C, H, and N microanalyses were carried out orbital level (EHOMO) was calculated according to the equation
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2017

View Article Online
Journal of Materials Chemistry C
Paper
EHOMO = ꢀ4.8 ꢀ e(Ecox ꢀ Efox), where Eocx is the first oxidation The product was then obtained by column chromatography on
peaks measured from CV curves and Eof x is the oxidation peak of silica gel with petroleum ether/ethyl acetate (3 : 1) as an eluent to
ferrocene. The lowest unoccupied molecular orbital level (ELUMO
was calculated from the equation ELUMO = EHOMO + Eg, and Eg is (dd, J = 4.7, 1.6 Hz, 1H), 8.41–8.38 (m, 2H), 8.16 (dt, J = 7.8, 0.9 Hz,
the intersection of the absorption and emission spectra. 2H), 7.98 (dt, J = 7.9, 1.0 Hz, 1H), 7.92–7.89 (m, 2H), 7.83–7.80 (m,
)
yield a white solid. Yield: 64%. 1H NMR (600 MHz, CDCl3, d): 8.54
Theoretical calculations were performed using the Gaussian 3H), 7.69–7.66 (m, 2H), 7.50–7.42 (m, 9H), 7.35–7.33 (m, 1H), 7.31
03 package. Geometry optimization was performed using density (ddd, J = 7.9, 7.0, 1.1 Hz, 2H). 13C NMR (150 MHz, CDCl3, d): 164.44,
functional theory (DFT) in B3LYP/6-31G(d) basis sets. The molecular 156.39, 152.38, 150.52, 144.02, 143.78, 142.66, 141.81, 140.01,
orbitals were visualized using GaussView 5.0.
139.64, 132.95, 131.86, 131.58, 131.38, 130.27, 130.19, 130.17,
128.91, 128.49, 127.36, 127.30, 126.39, 123.24, 122.92, 112.77. Anal.
calcd for C37H25N5: C, 82.37; H, 4.64; N, 12.99. Found: C, 82.44; H,
OLED fabrication and measurements
PhOLEDs and non-doped devices with an area of 3 ꢁ 3 mm2 4.67; N, 12.78 (%).
were fabricated by vacuum deposition onto an indium tin oxide
TAZ-2Cz, TAZ-3Cz and TAZ-4Cz were prepared via similar
(ITO) glass substrate. The ITO glass substrate was cleaned in procedures to compound TAZ-1Cz.
order with deionized water, acetone and ethanol. The electro-
TAZ-2Cz. Yield: 71%. 1H NMR (600 MHz, CDCl3, d): 8.54–
luminescence (EL) spectra and CIE coordinates were measured 8.52 (m, 1H), 8.37–8.34 (m, 2H), 8.17 (d, J = 7.7 Hz, 2H), 7.95 (d,
by a PR-655 spectrophotometer. The current density–voltage– J = 7.8 Hz, 1H), 7.88 (s, 1H), 7.81–7.76 (m, 4H), 7.70 (t, J = 7.8 Hz,
luminance (J–V–L) characteristics of PhOLEDs and OLEDs were 1H), 7.57 (dt, J = 7.9, 1.4 Hz, 1H), 7.50–7.41 (m, 9H), 7.34–7.29
recorded using a Keithley 2400 Source Meter and an ST-900M (m, 3H). 13C NMR (150 MHz, CDCl3, d): 164.35, 156.37, 152.38,
Spot Brightness Meter. The EQE values were calculated according 150.47, 145.53, 143.86, 143.85, 141.76, 141.22, 139.64, 133.24,
to previously reported methods.11a All measurements were carried 133.15, 131.86, 131.58, 130.21, 130.17, 129.05, 128.93, 128.63,
out at room temperature under ambient conditions.
128.47, 127.37, 127.29, 126.34, 123.25, 122.89, 112.74. Anal.
calcd for C37H25N5: C, 82.37; H, 4.64; N, 12.99. Found: C, 82.94;
H, 4.64; N, 12.97 (%).
Synthesis
1
The starting reagents were purchased from commercial companies
TAZ-3Cz. Yield: 66%. H NMR (600 MHz, CDCl3, d): 8.54 (d,
and used without further purification. Solvents for synthesis were J = 4.8 Hz, 1H), 8.44 (dd, J = 1.9, 0.6 Hz, 1H), 8.38–8.36 (m, 2H),
purified according to standard procedures prior to use. All 8.23 (dt, J = 7.7, 1.0 Hz, 1H), 7.97 (d, J = 7.9 Hz, 1H), 7.86–7.83
reactions were performed under nitrogen atmosphere. The (m, 2H), 7.81 (td, J = 7.7, 1.8 Hz, 1H), 7.74 (dd, J = 8.5, 1.8 Hz,
synthetic routes of all host materials are presented in Scheme 1H), 7.66–7.58 (m, 4H), 7.51–7.42 (m, 9H), 7.33 (tdd, J = 7.9, 3.9,
S1 (ESI†). And the synthesis of 2-(5-phenyl-4H-1,2,4-triazol-3- 2.4 Hz, 2H). 13C NMR (150 MHz, CDCl3, d): 164.70, 156.29,
yl)pyridine (1) has been reported.13
152.34, 150.62, 145.83, 144.32, 143.47, 141.86, 140.59, 139.60,
Synthesis of 2-(5-(4-bromophenyl)-2-phenyl-2H-1,2,4-triazol- 135.82, 132.82, 131.82, 131.49, 130.44, 130.24, 130.04, 130.00,
3-yl)pyridine (2). Compound 1 (2.28 g, 7.6 mmol), iodobenzene 129.06, 128.50, 128.31, 127.38, 127.22, 126.87, 126.43, 123.34,
(2.45 g, 12 mmol), 1,10-phenanthroline (0.96 g, 4.8 mmol), 123.03, 121.69, 112.96, 112.84. Anal. calcd for C37H25N5: C,
copper(I) iodide (0.48 g, 2.5 mmol) and caesium carbonate 82.37; H, 4.64; N, 12.99. Found: C, 81.16; H, 4.58; N, 12.72 (%).
(3.72 g, 11 mmol) were dissolved in DMF (40 mL). The mixture
TAZ-4Cz. Yield: 64%. 1H NMR (600 MHz, CDCl3, d): 8.53
was stirred vigorously for 10 min at room temperature and (ddd, J = 4.8, 1.8, 1.0 Hz, 1H), 8.33–8.30 (m, 2H), 8.22–8.19 (m,
subsequently refluxed under a nitrogen atmosphere for 24 h. 1H), 8.17 (dd, J = 7.7, 1.0 Hz, 1H), 7.95 (dt, J = 7.8, 1.0 Hz, 1H),
After being cooled to RT, the mixture was poured into water 7.79 (td, J = 7.7, 1.8 Hz, 1H), 7.77–7.74 (m, 2H), 7.66–7.60 (m,
(50 mL) and extracted with DCM (200 mL). The organic extracts 6H), 7.50 (tt, J = 6.9, 1.8 Hz, 1H), 7.48–7.40 (m, 7H), 7.33–7.29
were washed with water and dried over anhydrous MgSO4. The (m, 2H). 13C NMR (150 MHz, CDCl3, d): 164.58, 156.31, 152.35,
product was then obtained by column chromatography on 150.57, 145.80, 144.48, 144.44, 141.84, 141.82, 140.56, 139.60,
silica gel with petroleum ether/ethyl acetate (2 : 1) as an eluent 134.68, 132.92, 132.35, 131.82, 131.51, 131.14, 130.55, 130.54, 130.20,
to yield a white solid (yield: 75%). 1H NMR (600 MHz, CDCl3, d): 129.96, 128.94, 128.49, 127.37, 127.23, 126.06, 125.76, 123.51, 123.28,
8.53 (dd, J = 4.7, 1.4 Hz, 1H), 8.18–8.10 (m, 2H), 7.91 (d, J = 123.01, 122.53, 112.73, 111.06. Anal. calcd for C37H25N5: C, 82.37; H,
7.9 Hz, 1H), 7.81–7.76 (m, 1H), 7.63–7.58 (m, 2H), 7.44 (s, 5H), 4.64; N, 12.99. Found: C, 81.83; H, 4.59; N, 13.00 (%).
7.32 (dd, J = 7.4, 4.9 Hz, 1H).
Synthesis of TAZ-1Cz. Compound 2 (3.76 g, 10 mmol), 9-(4-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole
(4.61 g, 12 mmol), tetrakis(triphenylphosphine)palladium(0)
Acknowledgements
(Pd(PPh3)4) (170 mg, 0.15 mmol), and sodium carbonate (3.18 g, This work was financially supported by the International Science
30 mmol) were dissolved in tetrahydrofuran (THF) (40 mL). The and Technology Cooperation Program of China (2012DFR50460),
mixture was heated to reflux for 24 h under a nitrogen atmosphere. the National Natural Scientific Foundation of China (21071108,
After being cooled to room temperature, the mixture was poured 60976018, 61274056 and 61205179), the Natural Science Foundation
into water (50 mL) and extracted with CH2Cl2 (200 mL). The organic of Shanxi Province (2015021070), the Program for Changjiang
extracts were washed with water and dried over anhydrous MgSO4. Scholar and Innovation Research Team in University (IRT0972),
This journal is ©The Royal Society of Chemistry 2017
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
(c) W. Li, D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu,
B. Yang and Y. Ma, Adv. Funct. Mater., 2012, 22, 2797;
(d) J. Jayabharathi, A. Prabhakaran, V. Thanikachalam and
M. Sundharesan, RSC Adv., 2016, 6, 62208.
and the Shanxi Provincial Key Innovative Research Team in
Science and Technology (201513002-10).
6 (a) Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki and
C. Adachi, J. Am. Chem. Soc., 2012, 134, 14706; (b) Y.-M. Xie,
L.-S. Cui, F.-S. Zu, F. Igbari, M.-M. Xue, Z.-Q. Jiang and L.-S. Liao,
Org. Electron., 2015, 26, 25; (c) Y.-K. Wang, Y.-L. Deng, X.-Y. Liu,
X.-D. Yuan, Z.-Q. Jiang and L.-S. Liao, Dyes Pigm., 2015, 122, 6.
7 (a) L. Wang, W.-Y. Wong, M.-F. Lin, W.-K. Wong, K.-W. Cheah,
H.-L. Tam and C. H. Chen, J. Mater. Chem., 2008, 18, 4529;
(b) C. S. Oh, J. Y. Lee, C. H. Noh and S. H. Kim, J. Mater. Chem. C,
2016, 4, 3792; (c) X. Lv, B. Wang, B. Pan, Z. Huang, S. Xiang,
J. Tan, W. Yi, H. Huang and L. Wang, RSC Adv., 2016, 6, 46775;
(d) W. C. Lin, H. W. Lin, E. Mondal and K. T. Wong, Org.
Electron., 2015, 17, 1.
References
1 (a) H. Uoyama, K. Goushi, K. Shizu, H. Nomura and
C. Adachi, Nature, 2012, 492, 234; (b) G. Xie, X. Li,
D. Chen, Z. Wang, X. Cai, D. Chen, Y. Li, K. Liu, Y. Cao
and S.-J. Su, Adv. Mater., 2016, 28, 181; (c) B. Wang, G. Mu,
X. Lv, L. Ma, S. Zhuang and L. Wang, Org. Electron., 2016,
34, 179; (d) J. W. Sun, J. Y. Baek, K.-H. Kim, J.-S. Huh,
S.-K. Kwon, Y.-H. Kim and J.-J. Kim, J. Mater. Chem. C, 2017,
5, 1027; (e) P. Tao, W.-L. Li, J. Zhang, S. Guo, Q. Zhao,
H. Wang, B. Wei, S.-J. Liu, X.-H. Zhou, Q. Yu, B.-S. Xu and
W. Huang, Adv. Funct. Mater., 2016, 26, 881.
2 (a) C. Yao, Z. Xue, M. Lian, X. Xu, J. Zhao and G. Zhou,
J. Organomet. Chem., 2015, 784, 31; (b) Y. Wu, D. Yu,
W.-Y. Wong, B. Liu, J. Zhao, C. Luo, F. Lu, S. Tao and
Q. Tong, J. Mater. Chem. C, 2016, 4, 2003; (c) M. Zhu, Y. Li,
J. Miao, B. Jiang, C. Yang, H. Wu, J. Qin and Y. Cao, Org.
Electron., 2014, 15, 1598.
8 (a) S. Y. Lee, T. Yasuda, H. Nomura and C. Adachi, Appl.
Phys. Lett., 2012, 101, 093306; (b) H. Xu, P. Sun, K. Wang,
Y. Miao, T. Yang, H. Wang, B. Xu and W.-Y. Wong, Tetra-
hedron, 2016, 72, 4408; (c) C. Adachi, Jpn. J. Appl. Phys., 2014,
53, 060101; (d) D. Liu, D. Li, M. Wang and W. Li, J. Mater.
Chem. C, 2016, 4, 7260.
3 (a) Y. Nagai, H. Sasabe, S. Ohisa and J. Kido, J. Mater. Chem.
C, 2016, 4, 9476; (b) J. Zhao, Y. Yu, X. Yang, X. Yan,
9 A. Wada, T. Yasuda, Q. Zhang, Y. S. Yang, I. Takasu,
S. Enomoto and C. Adachi, J. Mater. Chem. C, 2013, 1, 2404.
H. Zhang, X. Xu, G. Zhou, X. Ran and W.-Y. Wong, ACS 10 M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su, C.-C. Wu,
Appl. Mater. Interfaces, 2015, 7(44), 24703; (c) S. K. Jeon,
C. S. Oh, M. Kim, K. S. Yooka and J. Y. Lee, J. Mater. Chem. C,
A. Matoliukstyte, J. Simokaitiene, S. Grigalevicius, J. V.
Grazulevicius and C.-P. Hsu, Adv. Mater., 2007, 19, 862.
2016, 4, 1606; (d) Y. Chen, W. Liang, W. H. Choi, J. Huang, 11 (a) H. Liu, Q. Bai, L. Yao, H. Zhang, H. Xu, S. Zhang, W. Li,
Q. Dong, F. Zhu and J. Su, Dyes Pigm., 2015, 123, 196; (e) L. Duan,
J. Qiao, Y. Sun and Y. Qiu, Adv. Mater., 2011, 23, 1137.
Y. Gao, J. Li, P. Lu, H. Wang, B. Yang and Y. Ma, Chem. Sci.,
2015, 6, 3797; (b) Y. Gao, S. Zhang, Y. Pan, L. Yao, H. Liu,
Y. Guo, Q. Gu, B. Yang and Y. Ma, Phys. Chem. Chem. Phys.,
2016, 18, 24176; (c) X. Tang, Q. Bai, Q. Peng, Y. Gao, J. Li,
Y. Liu, L. Yao, P. Lu, B. Yang and Y. Ma, Chem. Mater., 2015,
27, 7050; (d) H. Liu, Q. Bai, W. Li, Y. Guo, L. Yao, Y. Gao,
J. Li, P. Lu, B. Yang and Y. Ma, RSC Adv., 2016, 6, 70085;
(e) X. Ouyang, X. Zhang and Z. Ge, Dyes Pigm., 2014, 103, 39.
4 (a) Y. Zhao, C. Wu, P. Qiu, X. Li, Q. Wang, J. Chen and D. Ma,
ACS Appl. Mater. Interfaces, 2016, 8, 2635; (b) K. S. Yook and
J. Y. Lee, Chem. Rec., 2016, 16, 159; (c) S. M. Kim, J. H. Kim,
S. K. Jeon and J. Y. Lee, Dyes Pigm., 2016, 125, 274; (d) H. Li,
R. Bi, T. Chen, K. Yuan, R. Chen, Y. Tao, H. Zhang, C. Zheng
and W. Huang, ACS Appl. Mater. Interfaces, 2016, 8, 7274.
5 (a) H. Liu, Q. Bai, W. Li, Y. Guo, L. Yao, Y. Gao, J. Li, P. Lu, 12 K. S. Yook and J. Y. Lee, Chem. Rec., 2016, 16, 159.
B. Yang and Y. Ma, RSC Adv., 2016, 6, 70085; (b) W.-C. Chen, 13 H. Xu, Y. Yue, L. Qu, Y. Hao, H. Wang, L. Chen and B. Xu,
C.-S. Lee and Q.-X. Tong, J. Mater. Chem. C, 2015, 3, 10957;
Dyes Pigm., 2013, 99, 67.
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2017