Synthesis and characterization of novel phosphorescent host materials based on triphenylpyridine derivatives

Article abstract of DOI:10.1007/s11696-020-01063-y

Three novel phosphorescent host materials, 4′,4?-(4-phenylpyridine-2,6-diyl)bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine) (P1), 2,6-bis(4′-(naphthalen-1-yl)-[1,1′-biphenyl]-4-yl)-4-phenylpyridine (P2), and 2,6-bis(3′,5′-dimethyl-[1,1′-biphenyl]-4-yl)-4-phenylpyridine (P3), based on triphenylpyridine with a symmetrical molecular conformation were designed and synthesized from 2,6-bis(4-bromophenyl)-4-phenylpyridine (3) reacting with [4-(diphenylamino)phenyl]boronic acid (A1), [4-(naphthalen-1-yl)phenyl]boronic acid (A2), and (3,5-dimethylphenyl)boronic acid (A3) through Suzuki coupling reaction, respectively. The compounds obtained were characterized by elemental analysis, ¹H NMR, and ¹³C NMR spectroscopy. The thermal properties of these materials were studied by thermogravimetric analysis and differential scanning calorimetry measurements, which showed that they exhibited excellent thermal stability with glass transition temperatures over 94?°C. Their photophysical properties were also investigated, and the results indicated that all compounds possessed considerably high triplet energy level greater than 2.6?eV; they also emitted blue fluorescence in different solvents. Finally, the results of the density function theory calculations revealed that the compound P1 showed the well-separated frontier molecular orbital, which indicated that it possessed bipolar carrier transport ability for holes and electrons, whereas the compound P2 and P3 exhibited greater overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.

Full text of DOI:10.1007/s11696-020-01063-y

Chemical Papers
https://doi.org/10.1007/s11696-020-01063-y
ORIGINAL PAPER
Synthesis and characterization of novel phosphorescent host
materials based on triphenylpyridine derivatives
Xu Zhang1 · Qingqing Ye1 · Yingtao Fan1 · Xiaoyu Hu1 · Yingzhong Shen1
Received: 19 November 2019 / Accepted: 10 January 2020
© Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
Three novel phosphorescent host materials, 4′,4‴-(4-phenylpyridine-2,6-diyl)bis(N,N-diphenyl-[1,1′-biphenyl]-4-amine)
(P1), 2,6-bis(4′-(naphthalen-1-yl)-[1,1′-biphenyl]-4-yl)-4-phenylpyridine (P2), and 2,6-bis(3′,5′-dimethyl-[1,1′-biphenyl]-
4-yl)-4-phenylpyridine (P3), based on triphenylpyridine with a symmetrical molecular conformation were designed and
synthesized from 2,6-bis(4-bromophenyl)-4-phenylpyridine (3) reacting with [4-(diphenylamino)phenyl]boronic acid (A1),
[4-(naphthalen-1-yl)phenyl]boronic acid (A2), and (3,5-dimethylphenyl)boronic acid (A3) through Suzuki coupling reac-
tion, respectively. The compounds obtained were characterized by elemental analysis, 1H NMR, and 13C NMR spectroscopy.
The thermal properties of these materials were studied by thermogravimetric analysis and diferential scanning calorimetry
measurements, which showed that they exhibited excellent thermal stability with glass transition temperatures over 94 °C.
Their photophysical properties were also investigated, and the results indicated that all compounds possessed considerably
high triplet energy level greater than 2.6 eV; they also emitted blue fuorescence in diferent solvents. Finally, the results of
the density function theory calculations revealed that the compound P1 showed the well-separated frontier molecular orbital,
which indicated that it possessed bipolar carrier transport ability for holes and electrons, whereas the compound P2 and
P3 exhibited greater overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.
Keywords Phosphorescent material · Triphenylpyridine · Glass transition temperatures · Triplet energy level
Introduction
fuorescent OLEDs, which feature singlet excitons, phos-
phorescent organic light-emitting devices (PhOLEDs) can
reach 100% internal quantum efciency owing to their abil-
ity to simultaneously harvest both singlet and triplet excitons
for light emission (Baldo et al. 1998). However, the phos-
phorescent dopant cannot be used as a pristine emitter due
to the long lifetimes of phosphors, which can cause unde-
sired efects, such as triplet–triplet annihilation (TTA) and
concentration quenching (Yang et al. 2015). To overcome
these limitations, PhOLEDs are fabricated as host–guest
systems wherein the phosphorescent dopant is doped into
a host matrix to decrease the exciton density (Baldo et al.
1999; Adachi et al. 2000). Therefore, the rational design of
a superior host material is critical for enhancing the EQEs
of devices (Sasabe et al. 2013; Seino et al. 2014; Fan et al.
2014).
Organic light-emitting diodes (OLEDs) have attracted con-
siderable attention owing to their superior inherent proper-
ties, such as fast response, low weight, and high luminous
efciency (Tao et al. 2011; Adachi et al. 2000; Baldo et al.
1999; Ma et al. 2017; Yeh et al. 2005). Both academia and
industry are devoting increasing efort to improving the
external quantum efciency (EQE) of OLEDs to enable
their practical application in displays and lighting devices
(Braveenth et al. 2018). In comparison with conventional
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01063-y) contains
supplementary material, which is available to authorized users.
* Yingzhong Shen
Among the reported host materials, bipolar host materials
have attracted intensive attention because they contain both
hole-transporting (p type) and electron-transporting units
(n type), which simultaneously results in a higher device
efciency for balancing charge transport (Su et al. 2011).
yz_shen@nuaa.edu.cn
1
Applied Chemistry Department, School of Material
Science and Engineering, Nanjing University
of Aeronautics and Astronautics, 210016 Nanjing,
People’s Republic of China
Vol.:(0123456789)
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Carbazole (Zhao et al. 2016; Li et al. 2015; Tao et al. 2010a,
b) and diphenylamine (Tao et al. 2009, 2010a, b) are com-
monly used as hole-transporting units (p type), whereas
pyrazole (Arbaciauskiene et al. 2010), triazole (Xue et al.
2017), phosphine oxide (Gong et al. 2014), oxazole (Xing
et al. 2017), and pyridine (Jia et al. 2018) are used as elec-
tron-transporting units (n type) in the design of bipolar host
materials. In comparison with the p-type units, which exhibit
excellent hole transport ability, few n-type units exhibit good
electron transport ability, potentially leading to a charge
imbalance in the emitting layer and to a decrease in the ef-
ciency of the device. Therefore, selecting and synthesizing
an appropriate n-type unit to match the p-type groups for
achieving good charge balance in the emitting layer is a criti-
cal challenge (Deng et al. 2013).
1H and 101 MHz for 13C nuclei using CDCl3 or DMSO-
d6 as solvent. Uncorrected melting points were observed
in sealed capillaries. Thermogavimetric analysis were per-
formed using a NETZCH STA 449C with a constant heat-
ing rate of 10 °C min−1 from 25 to 800 °C under a nitrogen
atmosphere. Diferential scanning calorimetry (DSC) meas-
urements were acquired using a NETZSCH DSC 214, at a
heating rate of 10 °C min−1 from 40 to 300 °C under a nitro-
gen atmosphere. The glass transition temperature (Tg) was
obtained from the second heating scan. UV–visible absorp-
tion and photo luminescence spectra were obtained using
UV-2250 UV–Vis spectrophotometer under room tempera-
ture, respectively. Phosphorescent spectra were obtained on
FLS920 fuorescence spectrophotometer. Molecualr simula-
tions were carried out with the help of Gaussian 16 pack-
age. B3LYP functional with 6-311(G) basis set were used
for all the ground and excited state calculations under gas
phase. Cyclic voltammetry (CV) was carried out on a CHI
660b voltammetric analyser at room temperature. A conven-
tional three-electrode confguration consists of a platinum
wire counterelectrode, an Ag/Ag+reference electrode, and
a glassy carbon electrode working electrode. The oxidation
scans were performed in degassed dichloromethane sol-
vent containing terabutylammonium hexafuorophosphate
(TBAPF6) (0.1 M) as the supporting electrolyte at scan rate
of 100 mV/s.
Triphenylpyridine has been widely used as an electron-
transporting/hole-blocking material in optoelectronic mate-
rials owing to its inherent electron defciency (Ran et al.
2016). Pyridine and aromatic heterocyclic units have several
excellent characteristics; they are good chromophores with
high electron afnity, high thermal and oxidative stability,
good charge-injection ability, and can function as trans-
port building blocks (Jia et al. 2018). In addition, integrat-
ing pyridinyl moiety into the polymer backbone not only
improves the electron afnity of the materials but also pro-
tect the fuorescence quenching factor because of the inter-
system crossing efect of the heavy atom. However, few
reports on triphenylpyridine-based conjugated fuorescent
materials with tunable bandgaps have been reported yet.
In this study, three novel triphenylpyridine-containing
materials P1–P3 were synthesized and characterized. The
compounds were designed as promising host materials, fea-
tured with good thermal properties and high triplet energy
values at the same time. In addition, the compound P1 pos-
sessed bipolar carrier transport ability for holes and electrons.
These excellent physical properties enable all the materials to
be potential candidates as host materials for OLEDs.
Synthesis of 2,6‑bis(4‑bromophenyl)‑4‑phenylpyridine (3)
A mixture of 1-(4-bromophenyl)ethanone (3.98 g,
20.00 mmol), benzaldehyde (1.06 g, 10.00 mmol), ammo-
nium acetate (12.00 g, 156.00 mmol) in acetic acid (50 mL)
was heated and reacted at 115 °C for 24 h. After cooling
down to 0 °C the precipitates were fltered. The obtained
crude product was recrystallized by ethanol to aford the
white solid 2.20 g. Yield: 46.0%. Mp: 196–198 °C. 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.06 (d, J=8.8 Hz, 4H), 7.89
(s, 2H), 7.75 (d, J=6.8 Hz, 2H), 7.66 (d, J=8.4 Hz, 4H),
7.50–7.57 (m, 3H). 13C NMR (101 MHz, CDCl3, δ, ppm):
156.5, 150.7, 138.7, 138.2, 131.9, 129.2, 128.7, 127.2,
123.7, 117.2. Anal. calcd. For C23H15Br2N: C, 59.39; H,
3.25; Br, 34.35; N, 3.01. Found: C, 59.35; H, 3.22; Br,
34.40; N, 3.01.
Experimental
Materials and methods
The starting materials, benzaldehyde, 1-(4-bromophenyl)
ethanone were purchased from Shanghai Aladdin Bio-Chem
Technology Co. LTD (China), arylboronic acids including
[4-(diphenylamimo)phenyl]boronic acid (A1), 4-(1-naphthy)
phenylboronic acid (A2) and 3,5-dimethylphenylboronic
acid (A3) were purchased from Hebei Delongtai Chemicals
Co. LTD (China). All the other solvents and reagents were
analytical grade and were used without further purifcation.
The NMR spectra were obtained on a Bruker Advance
400 spectrometer at resonant frequencies of 400 MHz for
Synthesis of 4′,4‴‑(4‑phenylpyridine‑2,6‑diyl)bis(N,N‑diph
enyl‑[1,1′‑biphenyl]‑4‑amine) (P1)
Under nitrogen atmosphere, the mixture containing inter-
mediate 3 (0.30 g, 0.645 mmol), [4-(diphenylamino)phe-
nyl]boronic acid (0.37 g, 1.29 mmol), potassium carbonate
(0.36 g, 2.58 mmol), tetrakis-(triphenylphosphino)palla-
dium(0) (0.015 g, 0.0129 mmol), toluene (15 mL) and water
(7.5 mL) was refuxed at 110 °C for 6 h. After being cooled
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to room temperature, the reaction mixture was extracted
using dichloromethane and distilled water. The organic
layer was dried with anhydrous sodium sulphate and then
fltered. The solvent was removed under reduce pressure,
the crude product was purifed through silica gel chroma-
tography using petroleum ether/dichloromethane (6:1, v/v)
as eluent and further purifed by recrystallization in ethanol/
chloroform to give white solids 0.30 g. Yield: 59%. Mp:
213–215 °C. 1H NMR (400 MHz, DMSO-d6, δ, ppm): 8.44
(d, J=8.4 Hz, 4H), 8.25 (s, 2H), 8.09 (d, J=8.8 Hz, 2H),
7.84 (d, J=8.8 Hz, 4H), 7.72 (d, J=8.8 Hz, 2H), 7.52–7.61
(m, 4H), 7.35 (t, J=7.6 Hz, 8H), 7.09 (d, J=8.0 Hz, 15H).
13C NMR (101 MHz, CDCl3, δ, ppm): 157.1, 150.4, 147.6,
147.5, 141.4, 139.0, 137.7, 134.4, 129.3, 129.2, 129.1,
127.8, 127.2, 126.9, 124.5, 123.8, 123.0, 117.0. Anal. calcd.
For C59H43N3: C, 89.25; H, 5.46; N, 5.29. Found C, 89.24;
H, 5.42; N, 5.29.
123.9, 115.9, 20.4. Anal. calcd. For C39H33N: C, 90.83; H,
6.45; N, 2.72. Found C, 90.75; H, 6.39; N, 2.71.
Results and discussion
Synthesis
The procedure used to synthesize P1–P3 is shown
in Scheme 1. The important intermediate 2,6-bis(4-
bromophenyl)-4-phenylpyridine (3) was synthesized through
a Chichibabin pyridine synthesis reaction according to the
method reported elsewhere (Ran et al. 2016). P1–P3 were
synthesized via the Suzuki cross-coupling reaction between
intermediate 3 and corresponding boronic acid. The chemi-
cal structures of P1–P3 were fully characterized and con-
frmed through 1H and 13C NMR spectroscopy (Figs. S1–S8)
and elemental analysis. Moreover, P1–P3 were prepared in
yields of 59–70% and could be further purifed by column
chromatography and recrystallization to attain high-purity
products for study and applications.
Synthesis of 2,6‑bis(4′‑(naphthalene‑1‑yl)‑[1,1′‑biphenyl]‑4‑
phenylpyridine (P2)
The same approach for synthesizing P1 was followed. P2
was prepared from intermediate 3 (0.30 g, 0.645 mmol)
and [4-(naphthalene-1-yl)phenyl]boronic acid (0.32 g,
1.29 mmol). The crude product was purifed through silica
gel chromatography using petroleum ether/dichloromethane
(5:1, v/v) as eluent to obtain desire product as white solid
0.28 g. Yield: 61%. Mp: 203–205 °C. 1H NMR (400 MHz,
DMSO-d6, δ, ppm): 8.54 (d, J=8.0 Hz, 4H), 8.34 (s, 2H),
8.13 (d, J = 7.2 Hz, 2H), 7.99–8.03 (m, 12H), 7.93 (d,
J=8.4 Hz, 2H), 7.53–7.66 (m, 15H). 13C NMR (101 MHz,
DMSO-d6, δ, ppm): 156.0, 149.6, 140.3, 139.3, 138.9, 138.5,
137.9, 137.6, 133.4, 130.7, 130.4, 129.3, 129.0, 128.3,
127.7, 127.6, 127.3, 126.9, 129.8, 126.7, 126.4, 125.9,
125.5, 125.2, 116.6. Anal. calcd. For C55H37N: C, 92.57; H,
5.24; N, 1.97. Found C, 92.70; H, 5.19; N, 1.97.
Thermal properties
The thermal properties of P1–P3 were investigated by the
means of thermogravimetric analysis (TGA) and diferential
scanning calorimetry (DSC) under a nitrogen atmosphere;
the results are shown in Fig. 1. The thermal stability of the
host materials is critical for device fabrication because these
materials can restrict aggregation and phase separation. The
TGA data show that the thermal decomposition tempera-
ture (Td corresponding to 5% weight loss) is 446 °C for P1,
410 °C for P2, and 408 °C for P3. All these compounds
exhibited very similar weight loss rates from 50 to 400 °C,
which can be attributed to their common triphenylpyridine
units. In addition, the DSC analysis shows that compounds
P1–P3 exhibited substantially high glass transition tempera-
tures (Tgs) of 112 °C, 108 °C, and 94 °C, respectively, which
are higher than those of widely used host materials, such as
4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP, 62 °C) (Kautny
et al. 2016) and 1,3-bis(N-carbazolyl)benzene (mCP, 60 °C)
(Wu et al. 2007). Overall, all the compounds exhibited high
Tds and Tgs, demonstrating that they are sufciently stable
for thermal evaporation and can maintain an amorphous
state when used in OLEDs.
Synthesis of 2,6‑bis(3′‑5′‑dimethyl‑[1‑1′‑biphenyl]‑4‑yl)‑4‑p
henylpyridine (P3)
The same synthetic method for P1 was followed, P3 was
prepared from intermediate 3 (0.30 g, 0.645 mmol) and
(3,5-dimethylphenyl)boronic acid (0.19 g, 1.29 mmol). The
crude product was purifed through silica gel chromatog-
raphy using petroleum ether/dichloromethane (6:1, v/v)
as eluent to obtain desired product as white solid 0.23 g.
Yield: 70%. Mp: 198–199 °C. 1H NMR (400 MHz, DMSO-
d6, δ, ppm): 8.43 (d, J=8.4 Hz, 4H), 8.25 (s, 2H), 8.08 (d,
J = 7.2 Hz, 2H), 7.83 (d, J = 8.4 Hz, 4H), 7.25–7.62 (m,
3H), 7.38 (s, 4H), 7.04 (s, 2H), 2.37 (s, 12H). 13C NMR
(101 MHz DMSO-d6, δ, ppm): 155.6, 149.0, 140.5, 138.9,
137.4, 137.2, 137.1, 128.7, 128.6, 128.5, 126.8, 126.4,
DFT calculations
To investigate the electronic structure of these three hosts,
we conducted DFT calculations. The frontier molecu-
lar orbital distributions of these materials are shown in
Fig. 2. The highest occupied molecular orbital (HOMO)
of P1 was mainly localized on the two electron-donating
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Scheme 1 Synthetic routes of P1, P2 and P3
Fig. 1 a Thermogravimetric
analysis (TGA) and b dif-
ferential scanning calorimetry
(DSC) thermograms of P1–P3,
as recorded at a heating rate of
10 °C min−1
triphenylamine units and partly extended to the attached
phenyl rings. On the contrary, the lowest unoccupied
molecular orbital (LUMO) was dispersed over the electron-
accepting triphenylpyridine unit. The separation between
the HOMO and LUMO suggests the occurrence of ICT,
which could be indicated by a solvatochromic efect, and
provides an independent pathway for the migration of holes
and electrons. The calculation results imply that P1 may
possess bipolar transport ability, enabling it to maintain bal-
anced charge transport and increasing the opportunity for
holes and electrons to recombine in the emission layer. This
leads to a high EQE and enhancement of the electrolumines-
cence (EL) performance of PhOLEDs in which this material
is incorporated. In contrast to the HOMOs and LUMOs of
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Fig. 2 Theoretically calculated
spatial distributions of P1, P2
and P3
P1, those of P2 and P3 exhibit greater overlap. The HOMOs
were mainly distributed on the whole molecule except the
phenyl ring attached at the pyridine group’s para-position,
whereas the LUMOs were dispersed over the whole mol-
ecule. The calculated HOMO and LUMO levels were −1.62
and − 5.14 eV for P1, − 1.72 and − 5.79 eV for P2, and
−1.59 and −5.88 eV for P3, respectively.
assigned to π–π* transition from HOMO to LUMO from
diferent states. P2 spectrum shows a broad absorption
band at approximately 302 nm. Since the electronic dis-
tribution of P2 is similar to that of P3, the strong absorp-
tion can be assigned to the π–π* transition from HOMO
to LUMO. Because of the π-conjugation enhancement of
the naphthalene unit in P2, its absorption peaks exhibit a
weaker red-shift (approximately 25 nm) than those of P3.
The bandgap energy (Eg) values of P1–P3 were
obtained from the absorption edges and the formula
Eg = 1240/λonset (Bresas et al. 1983), were calculated to
be 3.05, 3.35, and 3.37 eV, respectively. The PL emission
spectra of all three materials show bell-shaped emission
curves. The maximum emission exhibited purple–blue
fuorescence with emission peaks at 379 and 376 nm for
P2 and P3, respectively. However, the emission of P1
exhibited wathet-blue fuorescence at longer wavelengths
of approximately 468 nm and showed a larger red-shift
(90 nm) than the other two materials because of the strong
electron-donating ability of the triphenylamine unit of P1.
The triplet energy values of P1, P2, and P3 calcu-
lated from the formula ET = 1240/λph (Bresas et al. 1983)
and the onset point in the phosphorescence spectra were
determined to be 2.61, 2.75, and 2.91 eV, respectively.
The triplet energy is a critical factor for host materials
because a relatively high triplet energy level can lead to
a smooth energy transfer from the host to the dopant and
large triplet–triplet energy diference between the host and
dopant can prevent energy back-transfer, maintaining the
efciency of the OLEDs (Tao et al. 2011; Lyu et al. 2009;
Photophysical properties
The absorption and photoluminescence (PL) spectra of
P1–P3 diluted in dichloromethane (1.0 × 10−5 M) at room
temperature and phosphorescence spectra of P1–P3 in
toluene at 77 K are shown in Fig. 3 and are summarized
in Table 1. The absorption spectrum of P1 shows peaks
at approximately 270 nm; these peaks are assigned to the
n–π* transition of a triphenylamine moiety (Tao et al.
2010a, b), whereas the longer wavelength absorption at
368 nm is attributed to intramolecular charge transfer
(ICT) between the electron-rich triphenylamine moiety
and electron-defcient triphenylpyridine moiety (Liu et al.
2015). P3 spectrum possesses a strong absorption band at
approximately 275 nm and a weaker absorption at 326 nm.
According to the electronic distribution maps in Fig. 2, the
electronic cloud of P3 in HOMO and LUMO is formed
majorly from π electrons, owing to a strong conjugation
in the structure, and the electronic cloud delocalized over
the entire molecular network. Hence, the strong absorp-
tion band can be assigned to π–π* transition from HOMO
to LUMO, and the weaker absorption at 326 nm could be
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Fig. 3 Fluorescence and
phosphorescence spectra of a
P1, b P2, and c P3 in dichlo-
romethane at room temperature
(RT) and in toluene at 77 K,
respectively; the absorption
spectra of these compounds in
dichloromethane at RT are also
shown (d)
Table 1 Physical properties of
Compounds Tg/Tad (oC) UV–Visb (nm) PLbmax (nm) Egc (eV) EdT (eV) HOMOe (eV) LUMOe (eV)
P1–P3
P1
P2
P3
112/446 270,368
108/410 275,302
468
379
376
3.05
3.35
3.37
2.61
2.75
2.91
−5.42
−5.98
−6.00
−2.37
−2.63
−2.63
94/408
275,326
aObtained from DSC and TGA thermograms
bObtained from the absorption and PL spectra of samples at a concentration of 10−5 M in dichloromethane
cOptical bandgap determined from the onset absorption wavelength of samples in dichloromethane solution
dObtained from the PL spectra of samples at a concentration of 10−5 M in toluene at 77 K
eMeasured by cyclic voltammetry
Shih et al. 2007; Hsu et al. 2009). To investigate the rela-
tion between molecular structure and luminescence prop-
erties, we recorded the PL spectra of P1–P3 in solvents
with diferent polarities.
These compounds showed a slight red-shift with increasing
solvent polarity, indicating no strong ICT upon photoexci-
tation. In summary, all three compounds showed a triplet
energy level greater than 2.60 eV, which can transfer energy
to the dopant efciently. P1 also exhibited a smaller optical
energy gap and ICT character, which were attributed to the
introduction of the electron-rich triphenylamine unit.
As shown in Fig. S9, the absorption spectra of P1–P3
were recorded with the samples in solvent with diferent
polarities. The spectrum of P1 shows large solvatochromism,
which can be attributed to its strong charge-transfer charac-
teristics. The emission spectra show peaks at 400 nm for
P1 in hexane and 496 nm for P1 in dimethyl formamide
(DMF), demonstrating a large red-shift (96 nm). However,
the emission spectra of the other two materials did not show
strong solvatochromism: P2 emitted at 378 nm when dis-
solved in hexane and 396 nm when dissolved in DMF, and
P3 emitted at 375 nm in hexane and at 381 nm at DMF.
Electrochemical properties
To investigate the electrochemical behavior of P1–P3,
cyclic voltammetry (CV) measurements were performed in
degassed dichloromethane solution, as shown in Fig. 4. The
HOMO energy levels of P1, P2, and P3 were obtained from
the onset potential of oxidation vs Fc+/Fc according to the
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greater overlap, which meant they can be used as electron
transport host materials. Based on these preliminary results,
we believe that these new triphenylpyridine-containing
materials have potential applications for OLEDs.
Acknowledgements This study was funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions (Grant
PAPD).
Compliance with ethical standards
Conflict of interest The authors declare that they have no confict of
interest.
Fig. 4 Cyclic voltammograms of P1–P3 in dichloromethane for oxi-
dation
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higher than those of the other two materials because of the
strong electron-donating ability of the triphenylamine unit
of P1. In addition, P1 also showed a reversible oxidation
process, which could be assigned to the oxidation of a triph-
enylamine moiety. The LUMO energy levels of P1, P2, and
P3 were obtained as −2.37, −2.63, and −2.63 eV, respec-
tively, according to the equation LUMO = HOMO + Eg.
The LUMO energy levels were very similar to each other,
possibly indicating that the reduction should occur on their
common n-type triphenylpyridine moieties. Moreover, their
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In summary, three novel triphenylpyridine-based host
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bromophenyl)-4-phenylpyridine reacting with correspond-
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chemical structure of all compounds was characterized by
elemental analysis, 1H NMR, and 13C NMR spectroscopic
techniques. The physical properties of these compounds
were also studied. All three materials exhibited not only
good thermal stability, with Tgs greater than 94 °C, but also
signifcant triplet energy, greater than 2.60 eV. Additionally,
the HOMO and LUMO of P1 demonstrated signifcant spa-
tial separation. These features revealed that P1 can be used
as an efcient bipolar host material for OLEDs. In contrast to
the HOMO and LUMO of P1, those of P2 and P3 exhibited
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