A new multifunctional triazine-carbazole compound with high triplet energy for high-performance blue fluorescence, green and red phosphorescent host, and hybrid white organic light-emitting diodes

Article abstract of DOI:10.1002/ijch.201400055

A blue fluorescent compound, 9-[4-(4,6-diphenoxy-1,3,5-triazin-2-yl)phenyl] -9H-carbazole (POTC), the triplet energy level of which reaches 2.76 eV, has been designed and synthesized. POTC is an excellent blue emitter as well as host for green and red phosphors, and therefore, matches the requirements of the host for single-emitting-layer fluorescence and phosphorescence hybrid white organic light-emitting diodes (OLEDs). The blue, green, red, and white devices based on POTC show maximum external quantum efficiencies (EQEs) of 2.4, 22.4, 13.0, and 8.1 %, respectively. Even at a high brightness of 1000 cd m ^-2, these values maintain EQEs of 2.3, 22.1, 11.1, and 7.0 %, respectively, indicating less than 15 % roll-offs from the maxima.

Full text of DOI:10.1002/ijch.201400055

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DOI: 10.1002/ijch.201400055
A New Multifunctional TriazineꢀCarbazole Compound
with High Triplet Energy for High-Performance Blue
Fluorescence, Green and Red Phosphorescent Host, and
Hybrid White Organic Light-Emitting Diodes
Xiao-Ke Liu,[a, b] Cai-Jun Zheng,*[a] Jing Xiao,[d] Ming-Fai Lo,[c] Zhan Chen,[a, b] Chun-Sing Lee,*[c] Chuan-
Lin Liu,[a] Xue-Mei Ou,[a] Fan Li,[a] and Xiao-Hong Zhang*[a, e]
Abstract: A blue fluorescent compound, 9-[4-(4,6-diphenoxy-
1,3,5-triazin-2-yl)phenyl]-9H-carbazole (POTC), the triplet
energy level of which reaches 2.76 eV, has been designed
and synthesized. POTC is an excellent blue emitter as well
as host for green and red phosphors, and therefore, match-
es the requirements of the host for single-emitting-layer
fluorescence and phosphorescence hybrid white organic
light-emitting diodes (OLEDs). The blue, green, red, and
white devices based on POTC show maximum external
quantum efficiencies (EQEs) of 2.4, 22.4, 13.0, and 8.1%,
respectively. Even at a high brightness of 1000 cdmꢀ2, these
values maintain EQEs of 2.3, 22.1, 11.1, and 7.0%, respec-
tively, indicating less than 15% roll-offs from the maxima.
Keywords: electrochemistry · heterocycles · fluorescence · luminescence · organic light-emitting diodes
1 Introduction
Fluorescence and phosphorescence hybrid white organic
[a] X.-K. Liu, C.-J. Zheng, Z. Chen, C.-L. Liu, X.-M. Ou, F. Li,
light-emitting diodes (WOLEDs) have attracted consider-
able attention in recent years due to their unique advan-
tages of combining the excellent stability of blue fluoro-
phores and complete exciton utilization.[1] Thus, hybrid
WOLEDs are considered as the ideal candidates for the
next generation of solid-state lighting sources and full-
color, flat-panel displays with long lifetimes and high effi-
ciencies.[1a] Generally, hybrid WOLEDs can be realized
by adopting multiple emitting layer (EML)[1a–d,2] or single
EML[1e–g,3] device structures with different color emitting
materials. For multi-EML hybrid WOLEDs, the film
thicknesses need to be precisely controlled to effectively
separate the singlet and triplet excitons. Moreover, the
complicated multi-EML structure is impracticable to
obtain by a solution process, which is the most ideal pro-
cess for large-area, low-cost manufacturing.[4] Corre-
spondingly, single-EML hybrid WOLEDs undergo simple
manipulation of the EML and can also achieve high elec-
troluminescent (EL) performance.[1f,g] Therefore, the
single-EML architecture is the more appropriate strategy
for hybrid WOLEDs.
X.-H. Zhang
Nano-organic Photoelectronic Laboratory and Key Laboratory of
Photochemical Conversion and Optoelectronic Materials
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing 100190 (P. R. China)
Tel : +86-10-82543510
Fax: +86-10-64879375
e-mail: xhzhang@mail.ipc.ac.cn
zhengcaijun@mail.ipc.ac.cn
[b] X.-K. Liu, Z. Chen
University of Chinese Academy of Sciences
Beijing 100190 (P. R. China)
[c] M.-F. Lo, C.-S. Lee
Center of Super-Diamond and Advanced Films (COSDAF)
& Department of Physics and Materials Sciences
City University of Hong Kong
Hong Kong (S.A.R. China)
e-mail: apcslee@cityu.edu.hk
[d] J. Xiao
College of Physics and Electronic Engineering
Taishan University
Shandong 271021 (P. R. China)
In single-EML hybrid WOLEDs, the blue fluorophore
has to double as the blue fluorescent emitter to utilize
the singlet excitons and the host for green and red phos-
phorescent dopants for triplet excitons, and therefore, has
become the key point of these devices. Since triplet exci-
tons are expected to be harvested by the green phosphor
[e] X.-H. Zhang
Functional Nano & Soft Materials Laboratory (FUNSOM)
Jiangsu Key Laboratory for Carbon-Based Functional Materials
& Devices and Collaborative Innovation Center of Suzhou
Nano Science and Technology, Soochow University
Suzhou, Jiangsu 215123 (P. R. China)
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1
to generate intense green emission, the triplet energy (T1)
of the blue fluorophore needs to be higher than 2.44 eV
(which is the T1 of commercial green phosphor Ir(ppy)3;
ppy=2-phenylpyridine)[5] to guarantee effective energy
transfer from the host to the guest. However, blue fluoro-
phores with such a high T1 are rarely reported, which
limits the development of single-EML hybrid WOLEDs.
Recently, we proposed a molecular design strategy for de-
veloping such efficient blue fluorophores with high T1,
and reported a multifunctional compound, 4-(4,6-diphe-
noxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA), for
high-performance single-EML hybrid WOLEDs accordin-
gly.[1g] However, the T1 of POTA is still no more than
2.44 eV and energy transfer from the guest back to the
host occurs, resulting in a low external quantum efficiency
(EQE) of 17.1% for the POTA-based green phosphores-
cent device. Thus, it is still challenging to develop blue
fluorophores with T1 higher than 2.44 eV.
ized by H and 13C NMR spectroscopy and mass spec-
trometry.
Room-temperature absorption and emission spectra of
POTC in ethyl acetate are shown in Figure 1. The broad
absorption spectrum with a peak surrounding 342 nm and
the emission band with a maximum at 457 nm are as-
cribed to the donorꢀacceptor charge-transfer transition.
The fluorescent quantum yield (Ff) of POTC was mea-
sured to be 0.48 in cyclohexane by using an integrated
sphere method. These results indicate that POTC is
a promising candidate as a blue emitter. The fluorescence
and phosphorescence spectra at 77 K of POTC were also
investigated (Figure 1). POTC has a fluorescence spec-
Herein, we report a new multifunctional blue emitter
with a high T1 of 2.76 eV. This compound, namely, 9-[4-
(4,6-diphenoxy-1,3,5-triazin-2-yl)phenyl]-9H-carbazole
(POTC), is an excellent blue emitter and phosphor host.
Despite the high T1 of 2.76 eV, stable blue emission can
still be achieved based on POTC. The blue device exhib-
its blue emission with CIE coordinates of (0.16, 0.16) and
a 2.3% EQE at a high brightness of 1000 cdcmꢀ2. The
Ir(ppy)3-doped green phosphorescent OLED based on
POTC shows a high current efficiency (CE) of 71.9 cdAꢀ1
and an EQE of 22.1% at 1000 cdcmꢀ2. A red phosphores-
cent OLED has also been constructed by using POTC as
the host and Ir(MDQ)2(acac) (MDQ=2-methyldibenzo-
[f,h]quinoxaline, acac=acetylacetonate) as the dopant.
The red device shows high EQEs of 13.0, 11.9, and
11.1% at maximum, 100, and 1000 cdmꢀ2, respectively.
Finally, a single-EML hybrid WOLED has been fabricat-
ed. The device has a forward-viewing CE of 17.8 cdAꢀ1
and an EQE of 7.0% at a brightness of 1000 cdmꢀ2, cor-
responding to total efficiencies[1a,2b,4] of 30.3 cdAꢀ1 and
11.9%, respectively.
Figure 1. Room-temperature UV/Vis absorption and photolumines-
cence (PL) spectra of POTC in ethyl acetate as well as fluorescence
and phosphorescence spectra in 2-MeTHF at 77 K.
trum with a single peak of about 412 nm. In contrast, the
phosphorescence spectrum exhibits an extra characteristic
vibrational structure for carbazole, indicating that the T1
3
state is a pp* state located mostly on the electron-donat-
3
ing moiety. For a pp* state, its zeroꢀzero energy is iden-
tified by the highest-energy peak of its emission.[6] Thus,
from the highest-energy phosphorescence peak, the T1 of
POTC is estimated at 2.76 eV. Such a high T1 is rare for
blue fluorophores. The high T1 can be attributed to the
high-T1 1,3,5-triazine and carbazole moieties as well as
short p conjunction over the molecule.
The thermal properties of POTC were determined by
thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC). The DSC and TGA curves are
displayed in Figure 2. The decomposition temperature
(Td, corresponding to 5% weight loss), melting point
(Tm), and glass transition temperature (Tg) are 357, 109
and 79 8C, respectively. The Tg of POTC is high enough
for the formation of a uniform amorphous film, which is
favorable for use in OLEDs.
2 Results and Discussion
The compound POTC was readily prepared in good yield
by using a palladium-catalyzed Suzuki cross-coupling re-
action of 4-(9H-carbazol-9-yl)phenylboronic acid with 2-
chloro-4,6-diphenoxy-1,3,5-triazine[1g] (Scheme 1). The
chemical structure of this new compound was character-
The electrochemical properties of POTC were investi-
gated in dimethylformamide (DMF) by using a three-
electrode cell with ferrocene (Fc) as a standard. As
shown in Figure 3, POTC shows a quasi-reversible reduc-
tive curve and irreversible oxidative wave, originating
from its electron-deficient 1,3,5-triazine moiety and elec-
Scheme 1. Synthetic route and molecular structure of POTC.
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To gain insight into the structureꢀproperty relationship
of POTC at the molecular level, density functional theory
(DFT) calculations were performed at the B3LYP/6-31G
(d) level (Figure 4). The HOMO of POTC is mainly lo-
Figure 4. Calculated spatial distributions of the LUMO and HOMO
of POTC.
cated at the electron-donating carbazole moiety and
partly in the benzene p bridge, whereas the LUMO sits
on the electron-deficient 1,3,5-triazine core and the ben-
zene p bridge. Such orbital distributions could lead to ex-
cellent hole- and electron-transporting properties. The in-
troduction of an oxygen atom interrupts p conjugation
between the 1,3,5-traizine ring and its adjacent benzene
ring. Such a design narrows the p-electron delocalization
zone of POTC and contributes to a high T1.[7] In addition,
separation between the HOMO and LUMO can be clear-
ly observed, which indicates an intramolecular charge-
transfer (ICT) transition from the donor to the acceptor.
ICT molecules commonly have small singletꢀtriplet split-
ting; in other words, ICT blue fluorophores may show
high T1.[1c] On the other hand, there is still overlap be-
tween the HOMO and LUMO, giving efficient ICT emis-
sion demonstrated by the Ff.
Figure 2. a) DSC and b) TGA curves of POTC.
The EL properties of POTC were investigated in
a device with a structure of indium tin oxide (ITO)/1,4-
bis-[(1-naphthylphenyl)amino]biphenyl (NPB; 30 nm)/
4,4’,4’’-tris(N-carbazolyl)triphenylamine (TCTA; 10 nm)/
EML (30 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)ben-
zene (TPBI; 30 nm)/LiF (1.5 nm)/Al. ITO and LiF/Al are
the anode and cathode, respectively; NPB is the hole-
transporting layer; TCTA is the exciton-blocking layer;
and TPBI serves as the electron-transporting layer, the
hole-blocking layer, and the exciton-blocking layer. The
blue fluorescent device (device A) was fabricated by
using POTC as the EML, while the EML of the green
and red phosphorescent devices (devices B and C) were
Figure 3. Cyclic voltammetry plots of POTC in DMF.
tron-rich carbazole moiety, respectively. From the onsets
of the oxidative and reductive curves, the HOMO and
LUMO energy levels were estimated to be ꢀ5.58 and
ꢀ2.80 eV, respectively. All physical properties data of
POTC are summarized in Table 1.
Table 1. Summary of the physical properties of POTC.
HOMO[e]
[eV]
LUMO[e]
[eV]
Tg/Tm/Td
[a]
[a]
[c]
[c]
[d]
lmax,abs
[nm]
lmax,em
[nm]
Ff[b]
lmax,fluo
[nm]
lmax,phos
[nm]
T1
[eV]
342
457
0.48
412
450, 473
2.76
ꢀ5.58
ꢀ2.80
79/109/357
[a] Absorption and emission maxima, measured in ethyl acetate at room temperature. [b] Measured in cyclohexane by using the integrated
sphere method. [c] Fluorescence and phosphorescence spectrum peaks measured in 2-MeTHF at 77 K. [d] Estimated from the phosphores-
cence maxima at 77 K. [e] Determined from the onset oxidation/reduction potentials of the cyclic voltammetry curve in DMF.
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POTC/6 wt% Ir(ppy)3 and POTC/4 wt% Ir(MDQ)2-
(acac), respectively. The single-EML hybrid WOLED
(device D) was constructed by controlling the concentra-
tions of the phosphorescent dopants, and the optimized
EML was POTC/0.2 wt% Ir(ppy)3/0.5 wt% Ir(MDQ)2-
(acac). Figure 5 demonstrates current densityꢀvoltage
and luminanceꢀvoltage plots of these devices. Figure 6
exhibits current efficiencyꢀluminance curves of these de-
vices and their EL properties data are summarized in
Table 2. Among all devices, device A shows the largest
current density versus the same voltage, while device B
shows the smallest one. This finding may be explained by
the ratios of excitons used. All singlet and triplet excitons
in device B are believed to be utilized for emission be-
cause the EQE of device B is greater than 20% (the the-
oretical limit). In contrast, device A shows approximately
one-tenth the EQE of device B, indicating that only
a small part of excitons is harvested. The unemployed ex-
citons mostly contribute to current density. The turn-on
voltages (observed at the brightness of 1 cdmꢀ2) of the
devices range from 3.2 to 3.5 V. The doped devices show
slightly lower turn-on voltages than that of device A due
to the charge-trap effect of phosphors.[8] The devices with
blue, green, red, and white color show maximum EQEs
of 2.4, 22.4, 13.0, and 8.1%, respectively. Even at a high
brightness of 1000 cdmꢀ2, these values are maintained at
2.3, 22.1, 11.1, and 7.0%, respectively, indicating less than
15% roll-offs from the maxima. The EQE of OLEDs can
be expressed by Equation (1):
Figure 5. a) Current densityꢀvoltage and b) luminanceꢀvoltage
plots of the devices.
EQE ¼ gchPLhoc
ð1Þ
in which g is the recombination efficiency of injected
holes and electrons, c is the fraction of excitons that can
potentially radiatively decay, hPL is the intrinsic PL effi-
ciency of the EML, and hoc is the light out-coupling
factor, which is about (20ꢁ2)%.[9] Based on the maxi-
mum EQE (22.4% for device B) and the estimated out-
coupling factor ((20ꢁ2)%), g, c, and hPL can reach unity
for device B. This result demonstrates balanced hole and
electron flows and complete energy transfer from the
POTC host to Ir(ppy)3 dopant. Since the charge balance,
charge confinement, and optical out-coupling effects by
ITO and organic materials are similar in devices A, B, C,
and D, the parameters g and hoc are approximately equal
Figure 6. Current efficiencyꢀluminance curves of the devices.
for these devices. Thus, devices A, C, and D show bal-
anced hole and electron flows as well, which are responsi-
ble for the high efficiencies and slight efficiency roll-offs.
Table 2. EL properties of the devices.
Device
Von [V][a]
CE[b]
2.9/2.6/2.8
72.2/68.4/71.9
25.4/23.4/21.4
20.4/19.9/17.8
PE[b]
1.8/1.7/1.5
54.3/53.8/47.7
22.7/18.7/14.1
16.5/15.1/11.0
EQE[b]
CIE[c]
A
B
C
D
3.5
3.3
3.2
3.3
2.4/2.1/2.3
22.4/21.7/22.1
13.0/11.9/11.1
8.1/7.9/7.0
(0.16, 0.16)
(0.26, 0.62)
(0.58, 0.41)
(0.43, 0.45)
[a] Turn-on voltage (at the luminance of 1 cdmꢀ2). [b] CE (current efficiency), PE (power efficiency), and EQE (external quantum efficiency) at
maximum, 100, and 1000 cdmꢀ2. [c] At 1000 cdmꢀ2
.
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Figure 7 exhibits the EL spectra of devices A and D
under different luminances. The CIE coordinates of
device A is (0.16, 0.16), which scarcely changes under dif-
ferent luminances. For the warm white emission of device
dopants, shows a forward-viewing CE of 17.8 cdAꢀ1 and
an EQE of 7.0% at a brightness of 1000 cdmꢀ2, corre-
sponding to total efficiencies of 30.3 cdAꢀ1 and 11.9%,
respectively.
4 Experimental Section
4.1 General Information
Commercially available reagents were used without pu-
rification. NMR spectra were recorded on a Bruker Ad-
vance-400 spectrometer with chemical shifts reported as
ppm. Mass spectra were obtained with a Finnigan 4021C
GC-MS spectrometer. Elemental analysis (C, H, N) was
carried out with an Elementar Vario ELIII element ana-
lyzer. Absorption and PL spectra were recorded with
a Hitachi UV/Vis spectrophotometer U-3010 and a Hita-
chi fluorescence spectrometer F-4500, respectively. Quan-
tum fluorescence yield was measured by the integrating
sphere method with an Edinburgh Instruments FLS920
spectrometer. TGA and DSC measurements were per-
formed on a TA instrument TGA2050 and TA instrument
DSC2910, respectively, with a heating rate of 108Cminꢀ1
under a nitrogen atmosphere. Cyclic voltammetry was
performed on a CHI660E electrochemical analyzer with
0.1 m Bu4NPF6 as a supporting electrolyte, a saturated cal-
omel electrode (SCE) as a reference electrode, a Pt disk
as a working electrode, and a scan rate of 100 mVsꢀ1.
4.2 Device Fabrication and Measurement
Figure 7. EL spectra of a) device A and b) device D under different
luminances.
ITO-coated glass with a sheet resistance of 30 W per
square was used as the substrate. The substrates were
first cleaned with isopropanol and deionized water, then
dried in an oven at 1208, treated with UVꢀozone, and fi-
nally transferred to a deposition system with a base pres-
sure of about 1ꢀ10ꢀ6 torr. Thermally evaporated organic
materials were sequentially deposited at a rate of 1–
2 ꢁsꢀ1 onto the ITO substrates. The cathode was com-
pleted by thermal deposition of LiF at a deposition rate
of 0.1 ꢁsꢀ1, and then capped with Al metal deposited at
a rate of 10 ꢁsꢀ1. EL luminescence, spectra and CIE
color coordinates were measured with a Spectrascan
PR650 photometer and the currentꢀvoltage characteris-
tics were measured with a computer-controlled Keithley
2400 SourceMeter under an ambient atmosphere. EQE
was calculated from the current density, luminance, and
EL spectrum, assuming a Lambertian distribution.
D, the CIE coordinates change slightly from 100 to
10000 cdmꢀ2, with (0.42, 0.48) at 100 cdmꢀ2, (0.43, 0.45)
at 1000 cdmꢀ2, and (0.43, 0.44) at 10000 cdmꢀ2. In the
spectra, the blue emission is strengthened with increasing
luminance, whereas the green emission is weakened, pri-
marily resulting from T1–T1 interactions at high current
densities.
3 Conclusions
A new multifunctional triazineꢀcarbazole hybrid com-
pound, POTC, has been designed and synthesized. POTC
shows efficient blue emission, a high T1 of 2.76 eV, and
can be used as a blue emitter as well as the host for green
and red phosphors. Devices with blue, green, and red
colors show maximum EQEs of 2.4, 22.4, and 13.0%, re-
spectively. Moreover, these values are maintained at 2.3,
22.1, and 11.1%, respectively, at a high brightness of
1000 cdmꢀ2, indicating less than 15% roll-offs. Further-
more, a single-EML hybrid WOLED, in which POTC
doubles as the blue emitter and host for green and red
4.3 Synthetic Details
9-[4-(4,6-Diphenoxy-1,3,5-triazin-2-yl)phenyl]-9H-carbazole
(POTC)
Toluene (12 mL), ethanol (6 mL), and 2 m aqueous
Na2CO3 (9 mL) were added to a mixture of 2-chloro-4,6-
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diphenoxy-1,3,5-triazine (0.598 g, 2 mmol), 9-phenyl-9H-
carbazol-3-ylboronic acid (0.574 g, 2 mmol), and tetrakis(-
triphenylphosphine)platinum (0.234 g, 0.2 mmol). With
stirring, the suspension was heated at 908 for 15 h under
a nitrogen atmosphere. When cooled to room tempera-
ture, the mixture was extracted with CH2Cl2 and dried
over Na2SO4. After the solvent had been removed under
reduced pressure, the residue was purified by column
chromatography on silica gel using CH2Cl2/petroleum
ether (4 :1) as the eluent to give a white solid (0.820 g,
81%). 1H NMR (400 MHz, CDCl3): d=8.52 (d, J=
8.5 Hz, 2H), 8.14 (d, J=7.7 Hz, 2H), 7.67 (d, J=8.4 Hz,
2H), 7.49–7.40 (m, 8H), 7.33–7.28 ppm (m, 8H); 13C NMR
(100 MHz, CDCl3): d=175.09, 173.07, 152.07, 142.40,
140.37, 133.21, 130.92, 129.51, 126.54, 126.18, 125.98,
123.93, 121.62, 120.57, 120.42, 109.81 ppm; HRMS (EI):
m/z calcd for C33H22N4O2: 506.1743; found: 506.1749; ele-
mental analysis calcd (%) for C33H22N4O2: C 78.25, H
4.38, N 11.06; found: C 78.29, H 4.41, N 11.01.
Acknowledgements
We acknowledge financial support from the National Nat-
ural Science Foundation of China (nos. 51373190,
51033007, and 51103169), the Instrument Developing
Project of the Chinese Academy of Sciences (grant
no. YE201133), and the Collaborative Innovation Center
of Suzhou Nano Science and Technology .
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Received: February 26, 2014
Accepted: March 15, 2014
Published online: May 23, 2014
Isr. J. Chem. 2014, 54, 952 – 957
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www.ijc.wiley-vch.de
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