Synthesis, photophysical and optoelectronic properties of quinazoline-centered dyes and their applications in organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2015.10.042

Quinazoline-centered derivatives with benzoimidazole, carbazole, and triphenylene moieties, were synthesized. Their relationships between electrochemical, photophysical, and optoelectronic properties and structure were discussed in detail. Efficient red p

Full text of DOI:10.1016/j.dyepig.2015.10.042

Accepted Manuscript
Synthesis, Photophysical and Optoelectronic Properties of Quinazoline-Centered
Dyes and their Applications in Organic Light-Emitting Diodes
Zhen Zhang, Jingwei Xie, Huibin Wang, Bowen Shen, Jianhua Zhang, Jian Hao, Jin
Cao, Zixing Wang
PII:
S0143-7208(15)00425-8
10.1016/j.dyepig.2015.10.042
DYPI 4989
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 August 2015
Revised Date: 25 October 2015
Accepted Date: 28 October 2015
Please cite this article as: Zhang Z, Xie J, Wang H, Shen B, Zhang J, Hao J, Cao J, Wang Z, Synthesis,
Photophysical and Optoelectronic Properties of Quinazoline-Centered Dyes and their Applications in
Organic Light-Emitting Diodes, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.10.042.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Highly thermal stable quinazoline-centered derivatives with tunable electron-only or bipolar
nature were employed as host materials to achieve high efficient red phosphorescent organic
light-emitting diodes.

ACCEPTED MANUSCRIPT
Synthesis, Photophysical and Optoelectronic Properties of
Quinazoline-Centered Dyes and their Applications in Organic Light-Emitting
Diodes
Zhen Zhang,Ɨ,ǂ § Jingwei Xie,Ɨ § Huibin Wang, Ɨ, ǂ Bowen Shen, Ɨ Jianhua Zhang, Ɨ Jian Hao, ǂ Jin Cao, Ɨ *
Zixing Wang Ɨ *
Ɨ
ǂDepartment of Chemistry, and Key Laboratory of Advanced Display and System Applications
Ministry of Education, Shanghai University, 99 Shangda Rd, Shanghai, 200444, China
*Corresponding Author: zxwang78@shu.edu.cn (Z. Wang), cj2007@shu.edu.cn (J. Cao)
Tel/Fax:+86-21-56333362;
§ These authors contributed equally to this work.
ABSTRACT: Quinazoline-centered derivatives with benzoimidazole, carbazole, and triphenylene moieties,
were synthesized. Their relationships between electrochemical, photophysical, and optoelectronic
properties and structure were discussed in detail. Efficient red phosphorescent organic light-emitting diodes
with low turn-on voltage were demonstrated by using them as host materials, and achieved maximum
external quantum efficiencies, current efficiencies, and power efficiencies of 19.2%, 18.3 cd/A, 21.7 lm/W
for 4-[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-2-[3-(tri-phenylen-2-yl)phen-3-yl]quinazoline, of
18.4%, 17.6 cd/A, 19.3lm/W for 4-(9-phenyl-9H-carbazol-3-yl)-2-[3-(triphenylen-2-yl)phenyl]quinazoline,
of 15.6%, 14.4 cd/A, 16.7 lm/W for 2,4-bis[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]quinazoline,
and of 17.4%, 16.7 cd/A, 15.7 lm/W for 2,4-bis(9-phenyl-9H-carbazol-3-yl)quinazoline, respectively.
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Moreover, improving the electron-injection/transport abilities of host materials could ideally improve the
performance of organic light-emitting diodes under low operation voltage, while enhancement of
hole-transporting abilities by using bipolar materials could balance the carriers to maintain high efficiency
°
under high operating voltage. These materials exhibited high glass-transition temperature of 146-154 C
and decomposition temperature of 400-447 °C.
Keywords: Synthesis, Photophysical Property, Organic light-emitting diodes, Quinazoline Dyes
1. INTRODUCTION
During the past two decades, organic light-emitting diodes (OLEDs) have been aroused intensive
interests due to their potential applications in solid-state lighting and flexible panel displays.1-7 Compared
to the conventional fluorescent OLEDs, great research efforts have been devoted to development of
phosphorescent OLEDs (PhOLEDs) because they can mostly approach 100% internal quantum efficiency
by harvesting both singlet and triplet excitons when they incorporate appropriate transition metal-centered
phosphorescent dopants as triplet emitters.8-11 Typically, in order to reduce the severe concentration
quenching and triplet-triplet annihilation (TTA), the emitters are usually doped with suitable host
materials.12-13 Carbazole-based derivatives were the most popular host materials for PhOLEDs due to their
intrinsic high triplet energy, attractive hole mobilities and good hole-transporting properties.14 On the
other hand, molecules with electron-transporting feature based on oxadiazole, triazine, phenanthroline, and
benzoimidazole units are used widely as functional groups that facilitate electron injection and transport.15
Up to now, numerous efforts have been carried out to develop efficient host materials for red PhOLEDs to
improve the efficiencies of EL devices.16-26 A balanced charge carrier recombination is required to achieve
high efficiency, while the electron mobilities are usually much lower than the hole mobilities in
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OLEDs.27-28 Thus, a host with bipolar features is highly desired to achieve both high and balanced charge
transporting ability. Benzoimidazole/carbazole hybrid bipolar material, CPhBzIm29 exhibited an excellent
bipolar charge transport ability, and was employed to achieve highly efficient electrophosphorescence and
two-color-based white OLEDs. Recently Wang's and Chen's groups co-reported a new simple
carbazole-N-benzoimidazole-based bipolar material, mNBICz30, which was utilized as a host for PhOLED
to demonstrate very high efficiencies too. However, mNBICz exhibits a low glass transition temperature
°
(Tg) only at 86 C mainly due to its low molecular weight. Adachi's group developed an efficient
triphenylene-based electron transport materials Bpy-TP2, which have both high electron mobility and
durability. Additional, Bpy-TP2-based OLED showed a significantly lower driving voltage, achieving
lower power consumption when compared with conventional electron transport materials (ETMs) such as
aluminium tris(quinolin-8-olate) (Alq3) and 1,3,5-tris(2-phenyl-1H-benzo[d]imidazol-1-yl)benzene
(TPBi).31 Pyridine is expected to be an electron deficient unit which has been used to construct novel
cyclometalated organic ligands and electron-transporting materials, as well as pyrimidine and quinazoline
derivatives.32 Su's group developed a series of pyridine-containing ETMs,33 and their energy levels of
relative materials could be tuned by introduction of pyridine rings with various of nitrogen atom orientation.
These materials had become the most popular ETMs to achieve highly efficient PhOLEDs.
Quinazoline-based derivatives have attracted our interest due to their application on a novel approach of
white OLED controlled by protonation34 and their promising photophysical properties with potential
application on colorimetric pH sensors.35 On the other hand, as a branch of pyridine-containing materials,
they are expected to exhibit potential applications on organic electronics.
In this paper we described synthesis of four new quinazoline-centered derivatives,
4-[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-2-[3-(tri-phenylen-2-yl)phen-3-yl]quinazoline
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(BITpQz),
4-(9-phenyl-9H-carbazol-3-yl)-2-[3-(triphenylen-2-yl)phenyl]quinazoline
(CzTpQz),
2,4-bis[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]quinazoline
(BBIQz),
and
2,4-bis(9-phenyl-9H-carbazol-3-yl)quinazoline (BCzQz) with benzoimidazole, carbazole, and triphenylene
moieties, respectively. Electrochemical, quantum calculation, photophysical, and optoelectronic properties
were discussed in detail as well as structure-properties relationship. The HOMO prefer to locate on
electron-donating segment, while the LUMO is mostly dispersed on quinazoline or on strong
electron-accepting unit. Different substituents resulted in their tunable and promising optoelectronic
properties. Comparatively, some important merits of multi-layer devices such as carrier balance, exciton
confinement, and energy-barrier reduction ensured the performance of devices. On the basis of these
materials as host, efficient red PhOLEDs with low turn on voltage were demonstrated.
2. EXPERIMENTAL SECTION
General methods.
The reagents and chemicals such as 2,4-dichloroquinazoline,
(4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)boronic
acid
(1),
9-phenyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole
(2),
and
4,4,5,5-tetramethyl-2-(3-(triphenylen-2-yl)phenyl)-1,3,2-dioxaborolane (5) were used as received unless
noted
otherwise.
The
auxiliary
materials
for
OLED
fabrication
such
as
N,N'-diphenyl-N,N'-bis(9-phenyl-9H-carbazol-3-yl)-[1,1'-biphenyl]-4,4'-diamine
(H04),
4,4',4''-tris(N-carbazolyl)triphenylamine (TCTA), TPBi were purchased from Yurui (Shanghai) Chemical
Co. Ltd. and Iridium(III)bis(4-methyl-2-(3,5-dimethylphenyl)quinolinato-N,C2') acetylacetonate
(Ir(mphmq)2acac) was purchased from Ningbo Intimechem Co. Ltd., and were purified further by vacuum
sublimation prior to use. Nuclear magnetic resonance (NMR) spectral data of 1H NMR and 13C NMR were
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recorded on Bruker AV-500 spectrometer with CDCl3 or dimethyl suloxide-d6 as deuterium solvent. Mass
spectra (MS) were obtained on a Finnigan/Thermo Quest MAT 95XL instrument. Elemental analyses (CHN)
were performed with Elemental Vario EL III instrument. UV-Vis absorption spectra were recorded on
UV-2501PC instrument, while photoluminescence (PL) spectra were taken using FLSP920 fluorescence
spectrophotometer, both in dichloromethane (CH2Cl2) solution and in solid state, respectively, and PL
quantum efficiency was measured by using a six inch integrating sphere. The glass transition temperatures
(Tg) of compounds were determined under a nitrogen atmosphere by differential scanning calorimetry
(DSC) on Netzstch STA409PC using a scanning rate of 10 ℃ /min with nitrogen flushing. The
decomposition temperature (Td) corresponding to 5% weight loss was conducted on a Perkin-Elmer Pyris 1
TGA thermal analyzer. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried
out using a CH Instrument 660C electrochemical analyzer and with Hg/Hg2Cl2 electrode as state reference
electrode, with tetra(n-butyl)ammonium hexa-fluorophosphate (TBAPF6) in DMF as supporting
electrolytes.
General procedure of Suzuki coupling reaction. 2,4-Dichloroquinazoline (5 g, 25 mmol), arylboronic
acid, palladium acetate (113 mg, 0.5 mmol), triphenylphosphine (264 mg, 1 mmol) and potassium
carbonate (10.4 g, 75.4 mmol) were suspended in a mixture of 1,4-dioxane (60 mL) and water (30 mL) in a
three-necked, round-bottomed flask with tightly sealed. The mixture was stirred and heated at 100 oC for 12
hours under nitrogen atmosphere. After cooling to room temperature, CH2Cl2 (50 mL) and water (50 mL)
were added. The aqueous layer was extracted with CH2Cl2 (3 × 20 mL). The combined organic layers were
washed by brine (3 × 30 mL), dried over anhydrous sodium sulfate and solvents were removed under
reduced pressure to yield the crude product as a light yellow solid, which was further purified by
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recrystallization (dichloromethane/ethanol) to give pure product.
2,4-Bis[4-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl]quinazoline (BBIQz) was synthesized according to the
general procedure from 2,4-dichloroquinazoline (5 g, 25 mmol) and 1 (55.4 mmol) in 71% isolated yield.
1H NMR (500 MHz, CDCl3, δ): 8.62 (d, J = 8.5 Hz, 2H), 8.12 (d, J = 8.5 Hz, 1H), 8.06 (d, J = 8.5 Hz, 1H),
7.93 (t, J = 7.0 Hz, 2H), 7.89 (t, J = 7.5 Hz, 1H), 7.85-7.81 (m, 4H), 7.74 (d, J = 9.5 Hz, 2H), 7.58-7.47 (m,
7H), 7.42-7.34 (m, 6H), 7.33-7.27 (m, 4H); 13C NMR (125 MHz, CDCl3, δ): 159.4, 152.1, 152.0, 151.5,
143.1, 143.1, 138.9, 138.3, 137.5, 137.4, 137.0, 136.9, 133.9, 131.8, 131.6, 130.2, 130.1, 130.0, 129.7,
129.5, 129.3, 128.9, 128.6, 128.5, 127.5, 127.5, 126.8, 123.7, 123.5, 123.3, 123.1, 121.6, 120.0, 120.0,
110.6, 110.5; MS (m/z, EI) Calcd for C46H30N6 666.25, found: 666.20. Anal. calcd. for C46H30N6: C 82.86,
H 4.54, N, 12.60; Found: C 82.74, H 4.53, N, 12.61.
[4-(1-Phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-2-chloroquinazoline (3) was synthesized according to the
general procedure from 2,4-dichloroquinazoline (5 g, 25 mmol) and 1 (25 mmol) in 79% isolated yield as a
1
white solid. H NMR (500 MHz, CDCl3, δ): 8.08 (d, J = 8.5 Hz, 1H), 8.04 (d, J = 8.5 Hz, 1H), 7.95-7.91
(m, 2H), 7.80 (d, J = 8.5 Hz, 2H), 7.75 (d, J = 8.5 Hz, 2H), 7.62 (t, J = 7.5 Hz, 1H), 7.57-7.50 (m, 3H),
7.39-7.36 (m, 3H), 7.32-7.27 (m, 2H); 13C NMR (125 MHz, CDCl3, δ): 172.1, 157.1, 153.2, 142.3, 141.6,
137.1, 134.6, 130.1, 128.4, 128.0, 127.8, 127.6, 127.2, 126.8, 123.7, 123.2, 123.1, 121.9, 120.7, 110.2,
110.0.
4-[4-(1-Phenyl-1H-benzo[d]imidazol-2-yl)phenyl]-2-[3-(tri-phenylen-2-yl)phen-3-yl]quinazoline
(BITpQz)
was synthesized according to the general procedure from 3 (5 g, 11.5mmol) and 5 (4.97g, 11.5 mmol) in
88% isolated yield as white solid (7.2 g). 1H NMR (500 MHz, CDCl3, δ): 9.10 (s, 1H), 8.94 (s, 1H), 8.78 (t,
J = 3.5 Hz, 1H), 8.72 (t, J = 7.5 Hz, 2H), 8.65 (m, 3H), 8.20 (d, J = 8.5 Hz, 1H), 8.09 (d, J = 7.5 Hz, 1H),
8.00 (dd, J = 8.5 Hz, J = 2.0 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.90 (m, 4H), 7.85 (d, J = 8.5 Hz, 2H), 7.66
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(m, 5H), 7.55 (t, J = 7.0 Hz, 1H), 7.53 (t, J = 8.0 Hz, 2H), 7.46 (t, J = 7.5 Hz, 1H), 7.41 (m, 3H), 7.32 (t, J
= 8.0 Hz, 1H), 7.28 (d, J = 8.0 Hz, 1H); 13C NMR (125 MHz, CDCl3, δ): 137.5, 136.9, 133.8, 131.5, 130.3,
130.1, 130.1, 130.0, 129.9, 129.8, 129.7, 129.5, 129.3, 129.2, 129.0, 128.9, 127.9, 127.6, 127.5, 127.3,
127.3, 127.3, 127.2, 126.8, 126.6, 123.9, 123.8, 123.5, 123.4, 123.4, 123.3,123.3, 122.0,121.7, 120.0,110.6;
MS (m/z, EI) Calcd for C51H32N4 700.26, found: 700.20. Anal. calcd. for C51H32N4: C 87.40, H 4.60, N,
7.99; Found: C 86.30, H 4.90, N, 7.63.
2,4-Bis(9-phenyl-9H-carbazol-3-yl)quinazoline (BCzQz) was synthesized according to the general
procedure from 2,4-dichloroquinazoline (5 g, 25 mmol) and 2 (55.4 mmol) in 67% isolated yield. 1H NMR
(500 MHz, CDCl3, δ): 9.59 (s, 1H), 8.90 (d, J = 8.5 Hz, 1H), 8.77 (s, 1H), 8.34 (d, J = 8.0 Hz, 2H), 8.29 (t,
J = 8.5 Hz, 2H), 8.22 (d, J = 8.5 Hz, 1H), 8.06 (d, J = 8.5 Hz, 1H), 7.90 (t, J = 7.0 Hz, 1H), 7.69-7.62 (m,
9H), 7.56-7.49 (m, 6H), 7.44 (m, 2H), 7.39-7.33 (m, 2H); 13C NMR (125 MHz, CDCl3, δ): 152.4, 142.6,
141.9, 141.6, 141.6, 137.6, 137.4, 133.3, 130.6, 130.1, 130.0, 129.8, 128.9, 128.5, 127.9, 127.6, 127.6,
127.2, 127.1, 126.5, 126.4, 126.1, 124.0, 123.8, 123.6, 123.5, 122.9, 121.9, 121.5, 120.9, 120.7, 120.5,
120.3, 110.2, 110.0, 109.9,109.7; MS (m/z, EI) Calcd for C44H28N4 612.23, found: 612.20. Anal. calcd. for
C44H28N4: C 86.25, H 4.61, N, 9.14; Found: C 85.88, H 4.56, N, 9.11.
3-(2-Chloroquinazolin-4-yl)-9-phenyl-9H-carbazole (4) was synthesized according to the general
1
procedure from 2,4-dichloroquinazoline (5 g, 25 mmol) and 2 (25 mmol) in 76 % isolated yield. H NMR
(500 MHz, CDCl3, δ): 8.64 (s, 1H), 8.30 (d, J = 8.5 Hz, 1H), 8.20 (d, J = 7.5 Hz, 1H), 8.05 (d, J = 8.5 Hz,
1H), 7.93 (t, J = 7.0 Hz, 1H), 7.87 (dd, J = 8.5 Hz, J = 2.0 Hz, 1H), 7.67-7.59 (m, 5H), 7.56-7.51 (m, 2H),
7.49-7.44 (m, 2H), 7.35 (t, J = 8.0 Hz, 1H); 13C NMR (125 MHz, CDCl3, δ): 170.7, 156.9, 153.1, 151.2,
143.0, 137.4, 136.7, 136.6, 135.1, 132.3, 130.2, 130.2, 129.7, 128.9, 128.2, 128.2, 127.4, 127.2, 123.9,
123.3, 121.5, 120.1, 110.6.
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4-(9-Phenyl-9H-carbazol-3-yl)-2-[3-(triphenylen-2-yl)phenyl]quinazoline (CzTpQz) was synthesized
1
according to the general procedure from 4 and 5 in 78% isolated yield. H NMR (500 MHz, CDCl3, δ):
9.22 (s, 1H), 8.99 (s, 1H), 8.83 (d, J = 8.0 Hz, 1H), 8.80 (dd, J = 7.5 Hz, J = 2.0 Hz, 1H), 8.77 (s, 1H), 8.70
(d, J = 8.5 Hz, 1H), 8.65 (m, 3H), 8.33 (d, J = 8.5 Hz, 1H), 8.24 (d, J = 8.0 Hz, 1H), 8.05 (dd, J = 4.5 Hz, J
= 1.5 Hz, 1H), 8.03 (dd, J = 4.5 Hz, J = 1.5 Hz, 1H), 7.93 (m, 2H), 7.72 (t, J = 7.5 Hz, 1H), 7.62 (m, 10H),
7.52 (tt, J = 7.0 Hz, J = 2.0 Hz, 1H), 7.47 (d, J = 3.5 Hz, 2H), 7.34 (m, 1H); 13C NMR (125 MHz, CDCl3,
δ): 137.4, 133.4, 130.1, 130.0, 129.9, 129.8, 129.7, 129.6, 129.5, 129.2, 129.2, 129.2, 128.9, 128.6, 128.0,
127.9, 127.7, 127.5, 127.3, 127.3, 127.1, 127.0, 126.6, 126.6, 123.9, 123.6, 123.5, 123.4, 123.3, 122.9,
122.1, 122.0, 120.7, 120.6, 110.2, 109.9; MS (m/z, EI) Calcd for C50H31N3 673.25, found: 673.10. Anal.
calcd. for C50H31N3: C 89.13, H 4.64, N, 6.24; Found: C 89.26, H 4.58, N, 6.26.
OLED fabrication and measurements. OLED devices were fabricated under high vacuum (~10-4 Pa)
chamber by thermal evaporation of organic layers onto a clean glass substrate precoated with a 150nm thick,
~10 ꢀ/sq indium tin oxide (ITO) layer. Prior to use, the substrate was degreased in an ultrasonic bath by the
following sequence: in detergent, de-ionized water, acetone, and isopropanol, and then cleaned in a
UV-ozone chamber for 15 min. The typical deposition rates, monitored by oscillating quartz, were 0.6, 0.1,
and 5.0Å/s for organic materials, lithium fluoride (LiF), and aluminum (Al), respectively. The device active
area defined by the overlap between the electrodes was 9 mm2 in all case. The organic layers consisted of
H04 as a hole-injection and -transporting layer (HIL, HTL), TCTA as an electron-blocking and
hole-transporting layer, these host materials doped with Ir(mphmq)2acac as emissive layer (EML), TPBi as
an electron-transporting and hole-blocking layer, 4,7-diphenyl-1,10-phenanthroline (BPhen) as an
electron-injection and transporting layer. The current-voltage characteristics of devices were measured by a
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Keithley 2400 electrometer in dark at room temperature and only the luminance from the front face of the
devices was collected with MinoltaL110 luminance meter. The electroluminescence (EL) spectra were
measured with the PR650 spectrometer. All measurements were carried out immediately under ambient
atmosphere without device encapsulation after the devices have been fabricated.
3. RESULTS AND DISCUSSION
Synthesis and characterization. The synthetic routes of BITpQz, CzTpQz, BBIQz, and BCzQz were
shown in Scheme 1. A classic and convenient Suzuki coupling reaction of 2,4-dichloroquinazoline with
aryl boronic acid under suitable condition could easily produce corresponding quinazoline-centered
materials. 2,4-Dichloroquinazoline reacted with 1 or 2 catalyzed by Pd(OAc)2 to afford BBIQz, 3, 4 and
BCzQz in 71%, 79%, 76% and 67% yields, respectively. Then 3 or 4 reacted with 5 to afford BITpQz or
CzTpQz in 88% and 78% yields, respectively. BITpQz, CzTpQz, BBIQz, and BCzQz were characterized
1
by H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. These host materials were
further purified by a five-zone thermal-gradient sublimator under a reduced pressure of 6 ×10-4 Pa before
devices fabrication.
Electrochemical and photophysical Properties. The electrochemical behaviours of BITpQz, CzTpQz,
BBIQz, and BCzQz were examined by using CV and DPV, respectively, the results were listed in Table 1.
Electrochemical reversibility was determined using CV, while all redox potentials were found using DPV
and reported relative to a ferrocenium/ferocene (Fc+/Fc) redox couple as an internal standard.36 All of the
four materials displayed one or two quasi-reversible reduction waves with peak potentials at -1.90 ~ -2.21
V, and irreversible oxidation waves with peak potentials above 0.91 V (Figure 1, Figure S1, in supporting
information). It is worth noting that CzTpQz and BCzQz showed a clear oxidation potential peak (Eox)
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contributed to N-phenyl-9H-carbazole moieties at 0.95 and 0.91 V respectively, and a reduction potential
peak (Ered) at -2.11 and -2.16 V respectively attributed to quinazoline components. However, BITpQz and
BBIQz didn't show any oxidation signal in testing range of DPV due to strong electron affinities of
benzoimidazole and quinazoline parts and resulted in pretty feeble irreversible oxidation waves and
increasing oxidation potentials.37-38 Moreover, BITpQz and BBIQz showed two quasi-reversible reduction
waves with peak potentials at -1.92, -2.21 V and -1.9, -2.19 V respectively, which suggests that quinazoline
or benzoimidazole moieties in these compounds undergo one or two electron reduction at certain potential.
Comparing with BCzQz and CzTpQz, the reduction potential around -1.9 V of BITpQz and BBIQz could
be assigned to the addition of benzoimidazole moiety.
UV-vis absorption, photoluminescence (PL) spectra of BITpQz, CzTpQz, BBIQz, and BCzQz
recorded at room temperature in CH2Cl2 solution and in solid film on quartz plates as well as their
phosphorescent spectra in 2-Methyl-THF solution at 77K are shown in Figure 2, and the data were
summarized in Table 1. The maximum absorption peaks at 337 nm for BITpQz, 365 nm for CzTpQz, 327
nm for BBIQz, and 341 nm for BCzQz, respectively can be assigned to the n–π* transitions, and the
absorption bands under 275 nm attributed to the π-π* transition of backbone. Compared to BBIQz and
BITpQz, replacement of benzoimidazole to carbazole group, red-shift absorbance were observed for both
CzTpQz and BCzQz due to a donor–acceptor (D–A) greatly increasing the ICT strength and red shifts of
their bands35. As expected, the absorbance bands of four compounds in thin film red-shifted substantially,
15, 8, 11, and 3 nm for BBIQz, BITpQz, CzTpQz and BCzQz respectively, probably due to the relatively
intermolecular interaction in thin film. Such a phenomenon had been commonly found in other known
organic materials.19
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Density functional theory (DFT) calculations were performed to better understand photophysical
properties and energy levels. The electron densities of the HOMO and the LUMO in vacuum state are
depicted in Figure 3 by using the most popular density functional basis sets of B3LYP /6-31G**. The
HOMO energy level is mainly localized on carbazole units and the LUMO energy level is dispersed on
quinazoline moieties for BCzQz and CzTpQz. There are two N-phenyl-9H-carbazol-3-yl groups in
BCzQz, while the HOMO energy level is prefer to localize on the 9-phenyl-9H-carbazol-3-yl unit at
2-position of quinazoline. The LUMO orbitals of BBIQz and BITpQz are distributed on
4-(N-phenylbenzoimidazol-2-yl-phenyl)quinazoline parts. However, the HOMO orbital is dispersed on
(N-phenylbenzoimidazol-2-yl-phenyl)quinazoline part at 2-position for BBIQz, and on
3-(triphenylen-2-yl)phenyl unit only for BITpQz, respectively, which could be contributed to the broken
conjugation between triphenylene and quinazoline by meta-substitution of phenyl. Such particular feature
resulted in their tunable and predictable photophysical properties.
The LUMO and the HOMO energy levels were calculated by the following equation:
HOMO (eV) = – (4.80 + Eox) or LUMO (eV) = – (4.8 + Ered)
HOMO (eV) = (LUMO – Eg)
Eg (eV) = 1239.81/λabs-onset
where λabs-onset (in nm) is the onset wavelength at UV-vis absorption spectra in thin solid film.39-40According
to reduction potentials, the LUMO values are -2.88 eV for BITpQz, -2.69 eV for CzTpQz, -2.90 eV for
BBIQz, and -2.64 eV for BCzQz, respectively. The HOMO values are -6.13 eV for BITpQz and -6.03 eV
for BBIQz calculated from Eg since they didn't show any clear oxidation potentials in DPV curves. The
HOMO values of CzTpQz (-5.75 eV from Eox, -5.78 eV from Eg ) and BCzQz (-5.71 eV from Eox, -5.69
eV from Eg) are estimated by using both two methods, and gave comparable data.
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Interestingly, all of these compounds exhibited very similar PL spectra both in CH2Cl2 solution and in
thin solid film, the maximum wavelength of emission (λPL) are around 450±5 nm at room temperature.
Unexpectedly, emission bands of these materials in thin solid films practically remain same as those in
solutions, which means that the intermolecular interactions were weak or negligible due to twisted bulky
aryl groups resulting in little intermolecular electronic communication between quinazoline and aryl groups.
Slight blue-shifts of λPL in thin solid film than those in solution could attribute to the Stokes shifts in
solution. The triplet energies (ET) of BITpQz, CzTpQz, BBIQz, and BCzQz determined by peaks of the
highest-energy vibronic sub-band of phosphorescent spectra in 2-methyl-THF at 77K are 2.40, 2.40, 2.46
and 2.36 eV, respectively. ET values of them are sufficiently high to host a red phosphorescent emitters
(lower than 2.1 eV) as a result of the effective prevention of reverse triplet energy transfer. Due to different
feature of substituents carrying out difference of the HOMO and LUMO energy levels, they might exhibit
interesting carrier-transporting properties as well as in PhOLEDs.
Thermal Analysis.
These materials showed good thermal stabilities likely because of their high
molecular weights and the rigidities of molecules. Thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) were used to explore the decomposition temperature (Td, corresponding to 5%
o
weight loss) and glass transition temperature (Tg) under a nitrogen atmosphere at a heating rate of 10 C
min-1 Figure 4. All compounds exhibited excellent morphological and thermal stabilities in terms of their
high Tg (146-154 oC) and Td (400-447 oC) respectively. The Tg and Td exhibit a slight increase when a
benzoimidazole or carbazole was replaced by triphenylene moieties, like as BBIQz and BITpQz. In
addition, no distinct crystallization and melting point were detected during heating.
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Carrier-transporting properties. To investigate the carrier-transporting properties of these materials,
hole-only and electron-only devices of BITpQz, CzTpQz, BBIQz, and BCzQz were fabricated. The hole
only device consists of the layers ITO/host (100 nm)/MoO3 (5 nm) /Al (80 nm), while the electron-only
devices have the configuration of ITO/ LiF (1 nm) /host (100 nm) / LiF (1 nm) /Al (80 nm), where the LiF
and Al served as a electron-injection layer and cathode. As shown in Figure 5, plots of the current density
vs. applied voltage of these materials revealed different electron and hole transporting abilities. The
electron mobilities of BITpQz, CzTpQz, BBIQz, and BCzQz are around 1.55×10-8, 1.35×10-9, 0.81×10-8,
and 1.58×10-9 cm2 V-1 s-1, while the hole mobilities are around 1.52×10-11, 1.00×10-9, 1.91×10-11, and
2.82×10-8 cm2 V-1 s-1, respectively (Table 1), which were calculated by fitting the current-voltage
characteristics to the space-charge limited current (SCLC) model.41 The electron mobilities of BITpQz
and BBIQz were about three orders of magnitude higher than their hole motilities due to the electron
affinities of benzoimidazole and quinazoline units. On the other hand, when benzoimidazole moieties were
replaced to N-phenylcarbazole units, CzTpQz exhibited its bipolar feature with almost same hole and
electron motility. Moreover, BCzQz affords the greatest hole mobility among these materials, it was ten
times higher than its electron mobility. Highly planar triphenylene moiety could usually improve
carrier-transporting properties. Comparing with BBIQz, the electron mobility of BITpQz was almost twice
than that of BBIQz. Similar result was observed between BCzQz and CzTpQz. However, addition of
triphenylene moiety decreased the hole mobility.
Electroluminescent properties. To investigate the performances of BITpQz, CzTpQz, BBIQz, and
BCzQz based PhOLEDs, several red PhOLEDs were fabricated and as well as a reference device using
4,4'-Bis(9H-carbazol-9-yl)biphenyl (CBP) as host. Ir(mphmq)2acac was chosen as the red phosphorescent
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dopant since it already showed its promising saturated red emission in red PhOLEDs.42 The devices were
fabricated with a typical structure consisting of ITO (indium tin oxide)/H04 (60 nm)/TCTA (10 nm)/host:
dopant 8 wt% (25 nm)/TPBi (10 nm)/Bphen (30 nm)/LiF (1 nm)/Al (80 nm). To improve the hole injection
and transporting from the anode, H04 was spun onto the precleaned ITO substrate to form HIL and HTL.
Then the electron-blocking layer consisting of a 10 nm-thick layer of TCTA43-44 was implemented, due to
its good transporting capability with limited intermolecular interactions in the solid states. After that, a 25
nm-thick emissive layer was evaporated, which consisted of 8 wt% phosphors doped into certain host
material. To further confine the holes or generated excitons within the emissive layer, a 10 nm-thick TPBi,
which possesses the low HOMO/LUMO energy levels, was used to benefit hole blocking and facilitate
electron injection and transport, and a 30 nm-thick 4,7-diphenyl-1,10-phenantroline (BPhen) was selected
as the electron-transport layer. LiF and Al were served as the electron-injection layer and cathode,
respectively.
The performances of these devices is summarized in Table 2, while the relative energy levels of these
materials and chemical structure of Ir(mphmq)2acac are displayed in Figure 6. The current
density-voltage-luminance (J-V-L) characteristics of these devices are shown in Figure 7. All PhOLEDs
exhibited low turn-on voltages of 2.3-3.0 V, and the corresponding current densities and luminance
displayed sustained increase upon increasing driving voltage. Particularly, BITpQz-based OLED exhibited
the lowest turn on voltage at 2.3 V, which is 0.4 V lower than that of BBIQz-based devices. Similarly,
CzTpQz-based OLED showed turn on voltage at 2.5 V, which is 0.5 V lower than that of BCzQz-based
OLED at 3.0 V as same as that of CBP-based devices. This unique feature could attribute to introduction of
triphenylene group due to its planar structure resulting in higher carrier transport properties. High
luminance values close to 20000 cd m-2 were demonstrated at driving voltage above 9.2 V for all OLEDs.
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The dramatically increased luminance and current densities indicated excellent carrier injection and
transport properties of these materials, which are consistent with our design strategy of balance hole and
electron densities in suitable combination zone. The external quantum efficiencies (EQE), current
efficiencies (CE), and power efficiencies (PE) of these devices plotted with respect to the luminance are
shown in Figure 7. These OLEDs achieved maximum EQE, CE, and PE of 19.2%, 18.3 cd/A, 21.7 lm/W
for BITpQz, of 18.4%, 17.6 cd/A, 19.3lm/W for CzTpQz, of 15.6%, 14.4 cd/A, 16.7 lm/W for BBIQz,
and of 17.4%, 16.7 cd/A, 15.7 lm/W for BCzQz, respectively, which were much higher than that of
reference OLED based on CBP at 10.3%, 9.9 cd/A, 7.8 lm/W, improving over 90%.
It is well documented that the magnitude of current density under an identical voltage is not only relate
to energy level of the HOMO and LUMO, but also controlled by carrier transport properties of host
materials. The electroluminescence spectra of these devices are shown in Figure 7a. Nevertheless, a small
emission band around 460 nm were observed for CBP-based OLED (Figure 7a, insert picture), which
might contribute to exciplex emission of CBP/TPBI.45 Usually the electron mobility of CBP is much lower
than its hole mobility resulting in recombination zone of CBP-based PhOLEDs locating the interface of
host and TPBI/Bphen layer. Due to the electron affinity of quinazoline units, all other PhOLEDs based on
BITpQz, CzTpQz, BBIQz, and BCzQz showed completion of energy transfer from hosts to dopant. The
recombination zone located only in emissive layer due to their higher electron injection/transporting
properties than their hole transport abilities, which is consistent with our results obtained from the hole- and
electron-only devices above.
There's an interesting results shown in J-V-L curves that when the driving voltage was higher than 6.5 V,
the current density and intensity of luminance of CzTpQz-based device increased dramatically and was the
highest one in all devices. As we mentioned above, the electron transporting of Bphen will increase fast as
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increasing of driving voltage due to effect of high electric field. CzTpQz with N-phenylcarbazole unit
could efficiently improve hole transporting and balance carrier recombination under high operating voltage
resulting in high efficiency. Similar results were demonstrated that when the driving voltage is high than
8.2 V, BCzQz-based device showed higher current density and intensity of luminance than those of
BBIQz- , BITpQz- and CBP-based devices. After all, improving the electron-injection/transport abilities
of host materials could ideally improve the performance of OLEDs under low operation voltage, while
enhancement of hole-transporting abilities could balance the carriers to maintain high efficiency under high
operating voltage. All devices also showed low efficiency roll-off at high current densities. The CE and
EQE at 500 cd/m2 and at 2000 cd/m2 were still high as 16.9 cd/A (EQE = 17.8% ) and 14.6 cd/A (EQE =
15.2% ) for BITpQz, 16.9 cd/A (EQE = 17.8% ) and 15.0 cd/A (EQE = 15.5% ) for CzTpQz, 15.3 cd/A
(EQE = 15.9 % ) and 14.6 cd/A 15.0 cd/A (EQE = 13.7% ) and for BCzQz, and 12.9 cd/A (EQE = 14% )
and 11.9 cd/A (EQE = 12.6 % ) for BBIQz, respectively. Further devices' architecture are fully expected to
achieve excellent devices for applications in organic electronics.
4. CONCLUSION
Four quinazoline-centered materials with benzoimidazole, carbazole, and triphenylene moieties,
BITpQz, CzTpQz, BBIQz, and BCzQz were synthesized via a simple synthetic route. These materials
°
exhibited outstanding morphological and thermal stabilities due to high Tg of 146-154 C and Td of
400-447 °C. Quinazoline unit showed the lowest reduction potential around -2.1 V, while
1-phenyl-1H-benzo[d]imidazole moiety showed the reduction potential around -1.9 V relative to Fc+/Fc.
Since quinazoline system has an efficient electron affinity, further addition of electron-donating/accepting
moieties could tune optoelectronic properties of its derivatives easily with electron transporting only or
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bipolar nature. Highly efficient red PhOLEDs by using them as host materials were demonstrated, and
achieved maximum EQE, CE, and PE of 19.2%, 18.3 cd/A, 21.7 lm/W for BITpQz, of 18.4%, 17.6 cd/A,
19.3lm/W for CzTpQz, of 15.6%, 14.4 cd/A, 16.7 lm/W for BBIQz, and of 17.4%, 16.7 cd/A, 15.7 lm/W
for BCzQz, respectively, which were much higher than that of reference OLED based on CBP at 10.3%,
9.9 cd/A, 7.8 lm/W. Interestingly, BITpQz-based red PhOLED showed the lowest turn on voltage at 2.3 V,
and the highest maximum efficiencies under low driving voltage, while CzTpQz-based devices exhibited
higher luminescence and lower roll-off properties due to better electron/hole balance under high operating
voltage. According to their tunable and promising photophysical, electrochemical and charge carrier
properties, we strongly believe that these materials could give us more opportunities to achieve ideal
materials for organic electronic applications.
Supporting Information
The DPV curves are shown in Figure S1. 1H NMR and 13C NMR spectra of synthesized compounds.
Acknowledgments
This work is financially supported by National Natural Science Foundation of China (No.21302122),
the National Basic Research Program of China (973 program 2015CB655005), Science and Technology
Commission of Shanghai Municipality (13ZR1416600, 13DZ0500302), and Ningbo Science and
Technology Innovation (NO. 2013B70050)
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Scheme 1. Synthesis and chemical structures of BBIQz, BITpQz, CzTpQz, and BCzQz.
Figure 1. Cyclic voltammograms of BITpQz, CzTpQz, BBIQz, and BCzQz in DMF solution
Figure 2. UV-vis absorption and PL spectra of BITpQz, CzTpQz, BBIQz, and BCzQz in CH2Cl2
solutions (Abssol and PLsol) and in neat films (Absfilm and PLfilm) at room temperature, and phosphorescent
spectra (Ph) in 2-Methyl-THF solutions at 77K.
Figure 3. The HOMO and LUMO levels distribution of BITpQz, CzTpQz, BBIQz, and BCzQz from
DFT calculation
Figure 4. TGA thermograms of BITpQz, CzTpQz, BBIQz, and BCzQz. Insert: DSC thermograms
Figure 5. The current density versus voltage curves of the electron-only and hole-only devices for
BITpQz, CzTpQz, BBIQz, and BCzQz.
Figure 6. Relative energy diagram of HOMO/LUMO levels for materials used in the EL devices;
Chemical structure of Ir(mphmq)2acac with HOMO/LUMO values of -5.0/-3.0 eV (dot line)
Figure 7. (a) EL spectra; (b) Current density-voltage-luminance (J-V-L) characteristics; (c) CE and PE
versus luminance curves; (d) EQE versus current density curves for BBIQz, BITpQz, CzTpQz, BCzQz and
CBP-based devices
Table 1. Photophysical, Electrochemical, and Thermal properties of the BITpQz, CzTpQz, BBIQz,
and BCzQz
Table 2. Electroluminescence Properties of devices with BITpQz, CzTpQz, BBIQz, BCzQz, and CBP
as hosts.
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Table 1. Photophysical, Electrochemical, and Thermal properties of the BITpQz, CzTpQz, BBIQz,and BCzQz.
a
a
b
c
d
e
h
λabs Sol. /film
λPL Sol. /film
λPh
Eox/Ered
Eg
ET
HOMO/LUMO
[eV]
-6.13g/-2.88 f
-5.75 f/-2.69 f
-6.03g/-2.90 f
-5.71 f/-2.64 f
Tg/Td
hole mobilityg
(cm2 v-1 s-1)
electron mobilityg
(cm2 v-1 s-1)
[nm]
[nm]
[nm]
516
517
[V]
[eV] (eV)
[oC]
na/-1.92
2.40
337/345
365/376
455/453
456/448
3.246
154/425
153/447
BITpQz
CzTpQz
1.52×10-11
1.00×10-9
1.91×10-11
1.58×10-9
1.55×10-8
1.35×10-9
0.81×10-8
2.82×10-8
0.952/-2.11
na/-1.90
2.40
3.092
2.46
327/342
341/344
455/448
462/454
505
526
3.131
146/400
148/438
BBIQz
BCzQz
0.905/-2.16
2.36
3.054
c
a λabs Sol., measured in CH2Cl2; λabs film, measured in thin solid film. b Measured in 2-Methyl-THF solutions at 77K Eox = oxidation potential; Ered = reduction
potential; na = none appear d The value of Eg was calculated from the absorption onset of solid film. e The value of ET was estimated from the peak values of
phosphorescence spectra (λPh). f The HOMO and LUMO values were determined using the following equations: EHOMO (eV) = -(Eox + 4.8), ELUMO (eV) = - (Ered
4.8); g EHOMO (eV) = ELUMO + Eg; h Obtained from DSC and TGA measurements. g hole and electron mobilities were obtain by SCLC method.
+

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Table 2 Electroluminescence Properties of devices with BITpQz, CzTpQz, BBIQz, BCzQz, and CBP as hosts.
λmaxd (nm)
b
Devicea
R1
Host
Vd (V)
2.3
Lmaxc (cd/m2, V)
19250, 9.2
20980, 8.2
21570, 9.2
19750, 8.7
16800,9.4
EQEmaxc (%) CEmaxc (cd/ A)
PEmaxc (lm/W)
618
619
620
618
618
19.2
18.4
15.6
17.4
10.3
18.3
17.6
14.4
16.7
9.9
21.7
19.3
16.7
15.7
7.8
BITpQz
CzTpQz
BBIQz
BCzQz
CBP
R2
2.5
R3
2.7
R4
3.0
R5
3.0
a
Device configuration: ITO (indium tin oxide)/H04 (60 nm)/TCTA (10 nm)/host: dopant 8 wt% (25 nm)/TPBi (10
nm)/Bphen (30 nm)/LiF (1 nm)/Al (80 nm). b Vd: the operating voltage at a brightness of 1 cd m-2. c Lmax: maximum
luminance; EQEmax: maximum external quantum efficiency; CEmax: maximum current efficiency; PEmax: maximum
power efficiency. d λmax: the wavelength where the EL spectrum has the maximum intensity.

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Scheme 1. Synthesis and chemical structures of BBIQz, BITpQz, CzTpQz, and BCzQz.

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Figure 1. Cyclic voltammograms of BITpQz, CzTpQz, BBIQz, and BCzQz in DMF
solution.

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Figure 2. UV-vis absorption and PL spectra of BITpQz, CzTpQz, BBIQz, and BCzQz in
CH2Cl2 solutions (Abssol and PLsol) and in neat films (Absfilm and PLfilm) at room temperature,
and phosphorescent spectra (Ph) in 2-Methyl-THF solutions at 77K.

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Figure 3. The HOMO and LUMO levels distribution of BITpQz, CzTpQz, BBIQz, and
BCzQz from DFT calculation

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Figure 4. TGA thermograms of BITpQz, CzTpQz, BBIQz, and BCzQz. Insert: DSC
thermograms