Synthesis and properties of n-type triphenylpyridine derivatives and applications in deep-blue organic light-emitting devices as electron- transporting layer

Article abstract of DOI:10.1039/c1jm11898f

Two series of n-type triphenylpyridine derivatives with good thermal properties and efficient deep-blue emissions were designed, synthesized and systematically characterized. Most of them show high glass transition temperatures (T_g > 100 °C), relatively high electron mobilities, large ionization potentials (IP > 6.31 eV) and suitable electron affinities (EA > 2.93 eV) for facilitating efficient electron-injection. These attributes of the n-type triphenylpyridine derivatives favours their applications in organic light-emitting devices (OLEDs) as electron-transporting and hole-blocking materials (ETMs and HBMs). With these new materials, deep-blue OLEDs with a configuration of indium-tin oxide (ITO)/α- napthylphenylbiphenyl diamine (NPB)/9,10-di(2-naphthyl)anthracene (ADN)/triphenylpyridine derivative/LiF/MgAg were fabricated to investigate the properties of these triphenylpyridine derivatives as ETMs and HBMs. The devices show higher efficiency (2.54 cd A^-1), and better color purity (0.15, 0.10) compared to those of similarly-structured blue OLEDs using state-of-the-art ETMs. The large IP and deep-blue emission of the triphenylpyridine derivatives are considered to be key factors for the higher efficiencies and better color purity. Optical and other properties of the compounds are discussed in terms of their molecular structures.

Full text of DOI:10.1039/c1jm11898f

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PAPER
Synthesis and properties of n-type triphenylpyridine derivatives and
applications in deep-blue organic light-emitting devices as
electron-transporting layer†
a
b
a
a
a
b
Na Li, Shiu-Lun Lai, Weimin Liu, Pengfei Wang, Juanjuan You, Chun-Sing Lee and Zengtao Liub
*
*
Received 30th April 2011, Accepted 14th June 2011
DOI: 10.1039/c1jm11898f
Two series of n-type triphenylpyridine derivatives with good thermal properties and efficient deep-blue
emissions were designed, synthesizꢀed and systematically characterized. Most of them show high glass
transition temperatures (Tg > 100 C), relatively high electron mobilities, large ionization potentials
(IP > 6.31 eV) and suitable electron affinities (EA > 2.93 eV) for facilitating efficient electron-injection.
These attributes of the n-type triphenylpyridine derivatives favours their applications in organic light-
emitting devices (OLEDs) as electron-transporting and hole-blocking materials (ETMs and HBMs).
With these new materials, deep-blue OLEDs with a configuration of indium-tin oxide (ITO)/a-
napthylphenylbiphenyl diamine (NPB)/9,10-di(2-naphthyl)anthracene (ADN)/triphenylpyridine
derivative/LiF/MgAg were fabricated to investigate the properties of these triphenylpyridine
derivatives as ETMs and HBMs. The devices show higher efficiency (2.54 cd Aꢁ1), and better color
purity (0.15, 0.10) compared to those of similarly-structured blue OLEDs using state-of-the-art ETMs.
The large IP and deep-blue emission of the triphenylpyridine derivatives are considered to be key
factors for the higher efficiencies and better color purity. Optical and other properties of the
compounds are discussed in terms of their molecular structures.
the development of electron-transporting materials (ETMs) is
1. Introduction
lagging behind. It is thus important to design and synthesize
Organic light-emitting device (OLED) has been considered as
ETMs with improved performance.11,12 A good ETM should
a promising technology because of its high efficiency, wide
satisfy three essential requirements:13 (i) high electron mobility is
viewing angle, low cost and many other merits.1–3 Considerable
an important attribute of an ETM. It is well-known that hole-
progress has been made recently in flat panel displays (FPDs)
mobilities of many HTMs are several orders of magnitude higher
based on OLEDs. In the past two decades, various approaches
than the electron-mobilities of commonly used ETMs.14 As
have been attempted to enhance performance of OLEDs.4–6 For
a result, in many devices, the hole-electron recombination would
instance, chemists have designed and synthesized a large number
occur near the cathode instead of the middle of the EML. This
of organic materials to meet various needs for different func-
tional requirements.7,8 A classical example is the use of different
can affect the color purity as well as decrease the current effi-
ciency. (ii) An ETM should have a suitable electron affinity (EA)
organic materials in a device to function respectively as the light-
and a large ionization potential (IP). A suitable EA can minimize
emitting layer (EML), the electron-transporting layer (ETL) and
the electron-injection barrier and thus the turn-on/operating
the hole-transporting layer (HTL).9,10 Compared to the emitting
voltages; meanwhile a large IP can effectively block the hole from
materials (EMs) and the hole-transporting materials (HTMs),
entering the ETL. In fact, in many device structures, good elec-
tron-transporting and hole-blocking properties are simulta-
aKey Laboratory of Photochemical Conversion and Optoelectronic
neously required.14 (iii) A good ETM should also have a high
Materials Technical Institute of Physics and Chemistry, Chinese
glass transition temperature (Tg) and good thermal stability for
Academy of Sciences, Beiyitiao No. 2, Zhongguancun, Haidian District,
ensuring good operational stability.
Beijing, P. R. China. E-mail: wangpf@mail.ipc.ac.cn; Fax: (+86) 10-
8254-3512
From the viewpoint of molecular design, n-type molecules
with electron deficient p-systems are generally considered to
have high electron mobilities. Typical examples include metal-
loles, oxadiazoles, pyridines, fluorinated aromatics etc. Among
these, pyridine derivatives have attracted much attention.15 In
this work, we designed and synthesized two groups of n-type
materials based on triphenylpyridine derivatives for applications
bCenter of Super-Diamond and Advanced Films (COSDAF), Department
of Physics and Materials Science, City University of Hong Kong, Tat Chee
Avenue, Kowloon, Hong Kong SAR, P. R. China. E-mail: apcslee@cityu.
edu.hk
† Electronic supplementary information (ESI) available: Absorption and
fluorescent emission spectra, molecular structures of the intermediate
products and electron mobility data for target compounds. See DOI:
10.1039/c1jm11898f
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as ETMs with hole-blocking ability in deep-blue OLEDs. These
triphenylpyridine derivatives are divided into two groups for
comparison: Group A: 2,4-diphenyl-6-p-tolylnicotinonitrile
(DPTNT), 4,40-(1,4-phenylene) bis(2-phenyl-6-p-tolylnicotino-
nitrile) (p-PPtNT),16 4,40-(1,3-phenylene) bis(2-phenyl-6-p-tol-
ylnicotinonitrile)
(m-PPtNT)
and
4,40-(1,3-phenylene)
bis(6-(4-methoxyphenyl)-2-phenylnicotinonitrile) (m-PmPNT);
and Group B: 4-(4-cyanophenyl)-2,6-diphenylnicotinonitrile
(CPPNN), 6,60-(1,4- phenylene) bis(2-phenyl-4-p-tolylnicotino-
nitrile) (p-PPtNN),16 6,60-(1,3-phenylene) bis(2-phenyl-4-p-tol-
ylnicotinonitrile) (m-PPtNN) and 6,60-(1,3-phenylene) bis
(4-(4-tert-butylphenyl)-2-phenylnicotinonitrile)
(m-PbPNN)
(Scheme 1). Synthesis procedures of these compounds are similar
to those described in literatures.17 Most of these compounds were
ꢀ
found to have high glass transition temperatures (Tg > 100 C),
relatively high electron mobilities (in the order of 10ꢁ5 cm2 Vꢁ1
sꢁ1), large ionization potentials (IP > 6.31 eV) and suitable
electron affinities (EA > 2.93 eV). High performances are realized
for the ADN based deep-blue OLEDs with these compounds as
ETMs and HBLs. Differences in electroluminescent (EL) prop-
erties of the compounds are discussed in terms of their molecular
structures.
2. Results and discussion
2.1 Thermal stability
Fig. 1 shows thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) measurements of the two groups of
Fig. 1 TGA and DSC data of the triphenylpyridine derivatives a) TGA
of Group A; b) DSC of Group A; c) TGA of Group B; d) DSC of
Group B).
triphenylpyridine derivatives. Td is defined as the temperature at
which 5% weight loss occurs during heating. Except DPTNT and
CPPNN, all the other compounds are thermally stable up to
370 ꢀC (Fig. 1(a) & (c) and Table 1). The higher Td are beneficial
to the thermal stability of these materials in OLED applications.
Tg and Tm values obtained from DSC measurements (Fig. 1(b) &
(d)) are tabulated in Table 1. The Tm values for p-PPtNT,
Scheme 1 The molecular structures of the two groups of triphenylpyr-
idine derivatives.
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m-PPtNT, m-PmPNT, p-PPtNN and m-PPtNN are in the range
of 308ꢂ363 ꢀC, while the Tg values for p-PPtNT, m-PPtNT,
m-PmPNT, p-PPtNN and m-PbPNN are in the range of
101.5ꢂ138.7 ꢀC. Comparing to a commonly used ETM with low
(LUMO) energy levels (i.e. larger in numerical values). At the
same time, electron-withdrawing (donating) groups linked to the
benzene ring can also lower (raise) the HOMO and LUMO
levels. These explain why the IPs of the compounds of Group A
are in the order of p-PPtNT > m-PPtNT > m-PmPNT. EA values
for them are in the same order. Their EA values (3.57ꢂ2.20 eV)
are suitable for minimizing the electron-injection barrier and
thus the turn-on/operating voltages. DPTNT has larger HOMO
and LUMO values mainly because of its asymmetric molecular
structure which has shorter p-conjugation length than the other
three triphenylpyridine derivatives with symmetric molecular
structures. From Table 1, it can be found that the corresponding
IP and EA values of p-PPtNN, m-PPtNN, and m-PbPNN of
Group B are almost the same. They are slightly larger than those
of Group A. It is considered that the sp-hybrid cyano group in
the para orientation in Group B has a higher electron with-
drawing ability and thus lowers the energy levels.
ꢀ
Tg (60 C), 4,7-diphenyl-1,10-phenanthroline (BPhen), with the
highest Tg of the five triphenylpyridine derivatives, allows
a wider range of applications in various organic electronic
devices. The differences in Td, Tm and Tg values of the two
groups of compounds can be attributed to their different struc-
tures. The molecule with para molecular orientation framework
is proved to have a bulky structure and higher polarity than that
with meta molecular orientation framework, which can increase
the thermal stability of the para molecular orientation frame-
work. This explains the highest temperature parameters of
p-PPtNT among all Group A molecules. For m-PPtNT and
m-PmPNT of Group A, the methoxyl group in m-PmPNT can
increase the molecular polarity by a larger extent than the methyl
group of m-PPtNT. Comparing with the above-mentioned three
compounds, DPTNT has relatively lower thermal parameters
due to its smaller molecular size and weight. So the Td, Tm and Tg
values of Group A compounds are in the order of p-PPtNT >
m-PmPNT > m-PPtNT > DPTNT. For Group B compounds,
p-PPtNN with para molecular orientation framework has the
2.3 Electron mobility
The electron mobilities of the two groups of triphenylpyridine
derivatives were investigated by transient electroluminescence
(EL) method.25 Table 2 and Figure S1† show that electron
mobilities of all the triphenylpyridine derivatives are in the order
of 10ꢁ5 cm2/Vs, which are one order of magnitude higher than
that of Alq3,26 two orders of magnitude higher than that of
BCP27 and comparable to that of TPBI.28 Their relatively high
electron mobilities are beneficial for achieving low driving
voltage and high efficiency. The compounds with meta molecule
orientation were observed to have higher electron mobilities than
ꢀ
highest Td and Tm, while Tg of m-PbPNN (138.7 C) is higher
than that of p-PPtNN (101.5 ꢀC) because the tert-butyl group in
m-PbPNN can increase Tg more than methyl group in p-PPtNN.
2.2 IP and EA characterization
The IP and EA values of the two groups of triphenylpyridine
derivatives are also listed in Table 1. Phenylpyridine moiety has
been shown to contribute to good electron-transporting ability
and large IP for hole-blocking ability.18–20 It was found that the
IP of DPTNT (7.05 eV) and p-PPtNT (6.81 eV) of Group A and
CPPNN (7.10 ev), p-PPtNN (6.87 eV), m-PPtNN (6.93 eV) and
m-PbPNN (6.90 eV) of Group B are all larger than those of
typical hole-blockers, such as, 2,9-dimethyl-4,7-diphenylphena-
throline (BCP, IP ¼ 6.70 eV),21 1,3,5-tris (phenyl-2-benzimida-
zolyl) benzene (TPBI, IP ¼ 6.70 eV),22 BPhen (IP ¼ 6.40 eV)23
and another new electron-transporting material reported by
Kido et al. (IP ¼ 6.67 eV).24 In general, a para molecular
orientation framework would give a more extended p-conjuga-
tion comparing to a corresponding meta orientation. These
would generally result in deeper highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
those with para molecular orientation (e.g. m-PPtNT
>
p-PPtNT, m-PPtNN > p-PPtNN). This can also be attributed to
the bulky structures of compounds with para molecule orienta-
tion, which leads to more steric hindrance that makes packing or
aggregating of molecules more difficult. For the same molecule
orientation, compounds from Group A exhibit slightly higher
mobilities than those from Group B (e.g. p-PPtNT > p-PPtNN
and m-PPtNT > m-PPtNN). This might be caused by the fact
that the cyano groups in Group B compounds reduce the
molecular planarity and thus the mobility.
2.4 Absorption and emission spectroscopy
Absorption and fluorescent spectral data of the two groups of
triphenylpyridine derivatives in different solvents at room
Table 1 Summary of thermal and spectral data of the two groups of triphenylpyridine derivatives
a
Compound
Td/Tm/Tg [ꢀC]
IP/EA [eV]
lab.
[nm]
lem.
b [nm]
max.
ETc [eV]
max.
DPTNT
250/177/46.6
7.05/3.63
6.81/3.57
6.31/2.93
5.42/2.20
7.10/3.70
6.87/3.69
6.93/3.52
6.90/3.47
281/335
294/330
281/337
283/352
280/333
291/355
280/328
280/328
380
407
403
421
389/475
427
387
3.07
3.04
3.14
3.04
2.76
2.99
2.75
2.77
p-PPtNT
m-PPtNT
m-PmPNT
CPPNN
p-PPtNN
m-PPtNN
m-PbPNN
388/363/138.5
372/308/130.2
384/313/131.3
278/No Tm & Tg
375/383/101.5
374/317/No Tg
374/219/138.7
383
a
lab.max.: maximum wavelength of absorption (deposited on the quartz substrates). b lem.max.: maximum wavelength of emission (vacuum deposited on
quartz substrates). ET: triplet energy level (in 2-methyltetrahydrofuran solution at 77 K).
c
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Table 2 Electron mobilities of triphenylpyridine derivatives measured by transient electroluminescence (EL) method
Electric field
V cmꢁ1
ETMs
Electron mobility cm2 Vꢁ1 sꢁ1
Reference
DPTNT
p-PPtNT
m-PPtNT
m-PmPNT
CPPNN
p-PPtNN
m-PPtNN
m-PbPNN
TPBI
2.8 ꢃ 10ꢁ5ꢂ3.7 ꢃ 10ꢁ5
4.6 ꢃ 10ꢁ5ꢂ5.6 ꢃ 10ꢁ5
9.2 ꢃ 10ꢁ5ꢂ9.9 ꢃ 10ꢁ5
5.5 ꢃ 10ꢁ5ꢂ6.1 ꢃ 10ꢁ5
5.2 ꢃ 10ꢁ5ꢂ6.3 ꢃ 10ꢁ5
2.4 ꢃ 10ꢁ5ꢂ4.0 ꢃ 10ꢁ5
4.4 ꢃ 10ꢁ5ꢂ4.8 ꢃ 10ꢁ5
6.7 ꢃ 10ꢁ5ꢂ8.5 ꢃ 10ꢁ5
4.4 ꢃ 10ꢁ5ꢂ7.1 ꢃ 10ꢁ5
3.3 ꢃ 10ꢁ5ꢂ8.0 ꢃ 10ꢁ5
1.4 ꢃ 10ꢁ6
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
5.6 ꢃ 105ꢂ10 ꢃ 105
4.7 ꢃ 105ꢂ7.0 ꢃ 105
4.0 ꢃ 105
This work
This work
This work
This work
This work
This work
This work
This work
This work
28
TPBI
Alq3
BCP
26
27
6.0 ꢃ 10ꢁ7
7.0 ꢃ 105
temperature are listed in Table S1 and corresponding spectra are
shown in Figure S2.† The two groups of triphenylpyridine
derivatives possess a strong absorption band with a maximum at
approximately 270 or 280 nm and a shoulder at 310ꢂ340 nm. As
mentioned, a para molecular orientation framework would give
a more extended p-conjugation compared to a corresponding
suitable for applications as ETMs in deep-blue OLEDs except
CPPNN which has another emission peak of longer wavelength
at 475 nm.
2.5 Phosphorescence spectroscopy and ET
meta orientation. These would generally result in longer lab.
Phosphorescence spectra of the triphenylpyridine derivatives
were also measured in 2-methyltetrahydrofuran solutions at
77 K. The ET values of the compounds are in the range of 3.04–
3.14 eV (Group A) and 2.75–2.99 eV (Group B) (Fig. 3 and
Table 1). The values are larger than those of the most typical
phosphorescent emitters, including the blue phosphorescent
max.
(wavelength of absorption maximum). So the lab.max. of p-PPtNT
and p-PPtNN are the longest. Peak wavelengths of the fluores-
cent spectra of the eight triphenylpyridine derivatives shown in
Table S1 are red-shifted from several nanometres to several
tenths of nanometres with increasing solvent polarity.† More-
over, 4em. (emission quantum yield) for all the compounds
increase with increasing solvent polarity from non-polar to
moderately polar solvents, then decrease with further increase of
solvent polarity. This indicates the co-existence of both ‘‘nega-
tive’’ and ‘‘positive’’ solvatokinetic effects. The negative sol-
vatokinetic effect may be attributed to ‘‘proximity effects’’;
whereas, the positive solvatokinetic effect may be caused by the
interaction between the molecule and the surrounding polar
solvent (acetonitrile & methanol), which can reduce bandgap,
and result in more non-radiative decay.
Fig. 2 shows absorption and emission spectra of the two
groups of triphenylpyridine derivatives in solid films deposited
on quartz substrates. The compounds show two absorption
peaks at about 280ꢂ294 and 328ꢂ355 nm (Fig. 2 and Table 1).
They show photoluminescent (PL) peaks in the range of 380 to
427 nm (Fig. 2 and Table 1). Exceptionally, the PL spectrum in
solid film for CPPNN has a longer wavelength peaked at 475nm,
while only one peak at 400 nm was observed in solution (shown
in Figure S1).† It may be attributed to the formation of inter-
molecular aggregates in solid film, which is often found in
a variety of organic films.29,30 It is well-known that color purity of
deep-blue OLEDs has been a challenge for years, as emissions
from other layers besides the EML that often contribute to the
electroluminescence. The triphenylpyridine derivatives employed
here are expected to improve the color purity of the devices for
two reasons. Firstly, their large IPs would effectively block the
hole from entering them and lead to a better confinement of the
recombination in the EML. On the other hand, as they are deep-
blue emitters themselves, even if recombination takes place
within these ETMs, the emission color will not be severely
affected. Hence, these materials with short PL wavelength are
Fig. 2 Absorption and fluorescent emission spectra of the two groups of
triphenylpyridine derivatives in solid film (deposited on quartz
substrates).
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material iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C20]
picolinate (FIrpic, 2.65 eV) and green phosphorescent material
fac-tris(2-phenylpyridine) iridium (Ir(PPy)3, 2.55 eV). The high
ET levels can effectively confine the triplet excitons in the phos-
phorescent emissive layer for application as ETL in phospho-
rescent OLEDs of blue and other colors. ET values of Group B
compounds are slightly smaller than those of Group A
compounds. It may be attributed to the lower band gap in Group
B compounds as good p-overlap can extend the conjugation
length. For Group A compounds, the cyano group is located
such that there is a shorter conjugation length. Moreover the
location of the cyano in Group A can cause the bridge to twist
out of plane due to unfavorable steric interaction thus further
shortening the conjugation length resulting in higher ET.
the voltage required to give a luminance of 1 cd mꢁ2) for the
devices are 3.5 V (DPTNT), 4.1 V (p-PPtNT), 3.5 V (m-PPtNT)
and 3.9 V (m-PmPNT), respectively. The values are all lower
than those of the corresponding devices based on Alq3, TPBI
and BCP/Alq3.31–36 The lower turn-on voltages are closely related
to their smaller electron-injection barriers, which enable efficient
electron-injection from the cathode. Moreover, the sequence of
J–V curves for devices respectively based on the four triphe-
nylpyridine derivatives is consistent with that of their L–V
curves. In other words, the current density and the luminous are
both in the order of DPTNT > m-PPtNT > m-PbPNT >
p-PPtNT at the same voltage. The current efficiency (hL)–current
density (J)–power efficiency (hP) characteristics of the four
devices are shown in Fig. 4(b). The maximum current efficiencies
of devices are 1.51 cd Aꢁ1 (DPTNT), 2.54 cd Aꢁ1 (p-PPtNT),
2.6 EL Performance
To confirm the electron-transporting and hole-blocking abilities
of the triphenylpyridine derivatives, we fabricated devices with
a configuration of ITO/NPB [60 nm]/ADN [30 nm]/a triphe-
nylpyridine derivative [30 nm]/LiF [1 nm]/MgAg [100 nm, 10 : 1].
In the device, ITO patterned on glass is used as the anode; NPB is
used as the HTL; ADN is used as the blue EML; the two groups
of triphenylpyridine derivatives are used as ETLs and hole-
blocking layers (HBLs); and the LiF/MgAg bilayer is used as
a composite cathode.
Fig. 4(a) shows the current density (J)–voltage (V)–lumine-
cence (L) curves of the devices based on the Group A
compounds. It can be seen that the turn-on voltages (defined as
Fig. 4 EL characteristics of devices based on the triphenylpyridine
derivatives of Group A as ETL. Device structure: ITO/NPB [60 nm]/
ADN [30 nm]/a triphenylpyridine derivative [30 nm]/LiF [1 nm]/MgAg
[100 nm 10 : 1]. (a): Current density (J)–Voltage (V)–Luminecence (L).
(b): Current efficiency (hL)–J–power efficiency (hP). (c): EL spectra.
Fig. 3 Phosphorescent spectra of the triphenylpyridine derivatives in
2-methyltetrahydrofuran solutions at 77 K.
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2.15 cd Aꢁ1 (m-PPtNT) and 2.12 cd Aꢁ1 (m-PmPNT), respec-
tively. The results, except that of DPTNT, are considerably
higher than those of devices based on Alq3 (1.60 cd Aꢁ1), TPBI
(1.30 cd Aꢁ1) and BCP (as HBL)/Alq3 (1.30 cd Aꢁ1) reported
previously.31–36 Power efficiencies of the devices are also higher
than corresponding values reported in the literature.31–36 On one
hand, the high efficiencies for devices based on the present tri-
phenylpyridine derivatives can be attributed to their relatively
high electron-transporting ability. On the other hand, these
compounds with large IPs can more effectively confine electrons
in the EML. This increases the recombination of electron and
charge within the EML and thus contributes to the improved
efficiency. It is worthy to note that the p-PPtNT based device has
a better performance than the m-PPtNT based device, though the
latter has a higher electron mobility than the former. As
p-PPtNT has a lager IP than m-PPtNT, p-PPtNT would better
confine the exciton in the EML. These show that performances of
the present devices are controlled by both the carrier mobility
and hole blocking ability of the ETL. EL spectra of the devices
are depicted in Fig. 4(c). EL peak positions (and Commission
Internationale de L0Eclairage (CIE) coordinates) of the devices
based on DPTNT, p-PPtNT, m-PPtNT and m-PmPNT are
440 nm (0.15, 0.08), 452 nm (0.15, 0.12), 452 nm (0.15, 0.11) and
448 nm (0.15, 0.12), respectively. The deep-blue emission of EL
spectra for devices based on the four triphenylpyridine deriva-
tives can be attributed to the emission of ADN. The devices
based on these triphenylpyridine derivatives exhibit better color
purity than the corresponding devices based on other ETMs
reported previously.31–36 The CIE coordinates are closer to the
National Television System Committee (NTSC) standards of
(0.14, 0.08)37 for blue emission. The EL spectra for devices based
on these triphenylpyridine derivatives manifest these compounds
with large IPs confine electrons in the ADN layer triumphantly.
The IP value of m-PmPNT is smaller (5.42 eV); however, the
color purity of the device based on m-PmPNT is better. There-
fore, it can be deduced that the deep-blue emission of m-PmPNT
may contribute to the better color purity.
Different EL properties of the Group A compounds can be
interpreted in terms of their different IPs. As discussed above, the
IPs of the compounds are in the order of p-PPtNT > m-PPtNT >
m-PmPNT > DPTNT. A larger IP value implies a better hole-
blocking ability which ensures a better confinement of the elec-
tron-hole recombination in the EML. This not only enhances the
color purity, but also leads to better efficiency as the EML is
typically optimized. Thus, among the devices based on these
ETMs, the efficiency is in the order of p-PPtNT > m-PPtNT > m-
PmPNT. Lower efficiency for a device based on DPTNT might
be attributed to its lower Tg and poor film-forming ability.
Fig. 5 shows EL characteristics of Group B compounds. As
revealed in Fig. 5(a), the maximum current efficiencies of the
devices based on CPPNN, p-PPtNN, m-PPtNN and m-PbPNN
are 2.61, 2.18, 2.11 and 2.46 cd Aꢁ1 respectively. These are higher
than the maximum current efficiencies of 1.60, 1.30 and
1.30 cd Aꢁ1 reported in similarly-structured devices using
respectively Alq3 (as ETL), TPBI (as ETL) and BCP (as HBL)/
Alq3 (as ETL).31–36 Efficiencies of the devices based on CPPNN,
m-PPtNN and m-PbPNN are especially high. The higher effi-
ciencies might be attributed to their higher electron-transporting
ability, as p-PPtNN has the lowest electron mobility among
Fig. 5 EL characteristics of devices based on the triphenylpyridine
derivatives of Group B as ETL. Device structure: ITO/NPB [60 nm]/
ADN [30 nm]/a triphenylpyridine derivative [30 nm]/LiF [1 nm]/MgAg
[100 nm 10 : 1]. (a): Current density (J)–voltage (V)–luminecence (L). (b):
Current efficiency (hL)–J–power efficiency (hP). (c): EL spectra.
Group B. In particular, device based on CPPNN has the highest
efficiency as CPPNN has the largest IP among Group B. In
addition, it can be seen that the current efficiencies show only
mild decreases as the current density increases. It can be seen
from Fig. 5(b) that turn-on voltages (defined as the voltage
required to deliver a luminance of 1 cd mꢁ2) for the devices are in
the range of 3.0 to 3.7 V only. These turn-on voltages are among
the lowest values reported in similarly-structured blue emitting
devices using other ETMs. Fig. 5(c) depicts EL spectra of the
four devices using Group B compounds. EL spectra of the device
respectively based on m-PPtNN and m-PbPNN show a peak at
452 nm, while those of the device based on CPPNN and
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p-PPtNN respectively show peaks at 460 and 464 nm. lFWHM
(full spectral widths at half maxima) of the four devices are
respectively 87, 94, 67 and 71 nm. lFWHM of 67 nm for the device
based on m-PbPNN is among the narrowest values31–36 reported
so far and the CIE coordinates of (0.15, 0.10) are the nearest to
the coordinates of NTSC standards of (0.14, 0.08)37 for blue
emission. The better color purity is attributed to the good hole-
blocking properties of the triphenylpyridine derivatives which
help to confine the emission zone within the ADN Layer. EL
spectra of the device based on CPPNN are red-shifted to the sky
blue region. This is possibly due to the 475 nm emission peak of
CPPNN. lFWHM of the device based on p-PPtNN is obviously
broader than those of the others. It may be attributed to the
formation of exciplex in the ADN/p-PPtNN interface which can
lead to red shift and EL peak broadening, because of the more
deficient-electron molecular structure of p-PPtNN.
Scheme 2 Synthetic routes for DPTNT.
Taking DPTNT as an example, benzaldehyde (1.06 g,
10 mmol) was first added to an alcohol solution of p-methyl
acetophenone (2.00 g, 15 mmol) under stirring. A 10% NaOH
solution (10 ml) was then dropped into the alcohol solution
under rapid stirring. Stirring was continued for another 12 h.
Solid powder was then collected, washed with H2O, recrystal-
lized with alcohol. White solid powder (DPTNT I) (1.92 g, 85%
yield) were collected. The crystal (DPTNT I) (1.11 g, 5 mmol),
benzoylacetonitrile (0.73 g, 10 mmol) and anhydrous ammonium
acetate (0.5 g, 6.5 mmol) were added into a three-necked flask.
Glacial acetic acid (50 ml) was then injected in under a nitrogen
atmosphere. The mixture was refluxed for 24 h under stirring.
The whole process was completed under nitrogen atmosphere.
After cooling to room temperature, the reaction mixture was
filtered and crystallized with CH2Cl2/petroleum ether to obtain
a white solid powder (DPTNT). p-PPtNT and m-PPtNT were
obtained by recrystallization with 1,2-diclorobenzene. m-
PmPNT was obtained via silicon gel column chromatography
with CH2Cl2/petroleum ether (3 : 1) as the eluent.
3. Conclusion
We have designed and synthesized two series of n-type materials
based on triphenylpyridine derivatives, namely Group A:
DPTNT, p-PPtNT, m-PPtNT and m-PmPNT; Group B:
CPPNN, p-PPtNN, m-PPtNN and m-PbPNN. The triphe-
nylpyridine derivatives with deep-blue emission show excellent
thermal stabilities with high Tg and Tm. More importantly, these
materials exhibit good electron-transporting abilities, large IPs
and suitable EAs, which will be in favor of the electron trans-
porting, hole blocking and electron injection in OLEDs. The
deep-blue OLEDs were successfully fabricated by using the tri-
phenylpyridine derivatives as the electron-transporting and hole-
blocking materials. The maximum current efficiency of these
devices is above 2.10 cd Aꢁ1, which is much higher than that of
the devices with a similar device structure using the state-of-the-
art ETMs. The CIE coordinates of these devices are very close to
the NTSC standard for blue emission. These results indicate that
the triphenylpyridine derivatives are promising n-type ETMs,
which could be used not only in different OLEDs, including
fluorescent, phosphorescent, white emission devices, but also in
organic thin-film transistors (OTFTs), etc. The relevant investi-
gations are in progress in our laboratory.
(E)-3-Phenyl-1-p-tolylprop-2-en-1-one (DPTNT I). 1H NMR
(400 MHz, CDCl3, d): 7.95 (d, J ¼ 8.16 Hz, 2H), 7.83 (d,
J ¼ 15.72 Hz, 1H), 7.66–7.63 (m, 2H), 7.56 (d, J ¼ 15.68 Hz, 1H),
7.43–7.41 (m, 3H), 7.31 (d, J ¼ 7.96 Hz, 2H), 2.44 (s, 3H).
4. Experimental section
2,4-Diphenyl-6-p-tolylnicotinonitrile (DPTNT). 1H NMR
(400 MHz, CDCl3, d): 8.10 (d, J ¼ 8.16 Hz, 2H), 8.05–8.02 (m,
2H), 7.79 (s, 1H), 7.69–7.67 (m, 2H), 7.58–7.54 (m, 6H), 7.33 (d,
J ¼ 8.04 Hz, 2H), 2.43 (s, 3H); 13C NMR (400 MHz, CDCl3, d):
162.5, 159.3, 155.5, 141.1, 138.3, 137.1, 134.9, 130.2, 130.0, 129.9,
129.5, 129.1, 128.8, 128.6, 127.6, 118.4, 118.0, 104.1, 21.6. HRMS
(EI, m/z): [M+] calcd for C25H18N2: 346.1470; found 346.1437
(Calcd. m/z 346.1470).
4.1 General
Chemical structures of the two groups of triphenylpyridine
derivatives are shown in Scheme 1. They were prepared using
methods similar to those described in the literature.17 As all the
compounds are made using a similar reaction sequence, a repre-
sentative synthetic route for DPTNT is outlined in Scheme 2.
Firstly, DPTNT I, p-PPtNT I, m-PPtNT I, m-PmPNT I, CPPNN
I, p-PPtNN I, m-PPtNN I and m-PbPNN I (Scheme S1†) were
respectively prepared by Claisen reactions between correspond-
ing aromatic ketone and aromatic aldehyde. DPTNT, p-PPtNT,
m-PPtNT, m-PmPNT, CPPNN, p-PPtNN, m-PPtNN and
m-PbPNN were then obtained by cyclocondensation of corre-
sponding intermediate products with benzoylacetonitrile in the
presence of ammonium acetate. These triphenylpyridine deriva-
(2E,20E)-3,30-(1,4-Phenylene)bis(1-p-tolylprop-2-en-1-one) (p-
PPtNT I). 1H NMR (400 MHz, CDCl3, d): 7.96 (d, J ¼ 8.12 Hz,
4H), 7.82 (d, J ¼ 15.68 Hz, 2H), 7.69 (s, 4H), 7.60 (d, J ¼ 15.68
Hz, 2H), 7.33 (d, J ¼ 7.96 Hz, 4H), 2.45 (s, 6H).
4,40-(1,4-Phenylene)bis(2-phenyl-6-p-tolylnicotinonitrile)
(p-
1
1
tives have been identified with H NMR, 13C NMR and high-
PPtNT). H NMR (400 MHz, CDCl3, d): 8.14 (d, J ¼ 8.16 Hz,
resolution mass spectrometry (HRMS).
4H), 8.08–8.05 (m, 4H), 7.90 (s, 2H), 7.88 (s, 4H), 7.58–7.56
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(m, 6H), 7.35 (d, J ¼ 8.08 Hz, 4H), 2.44 (s, 6H); 13C NMR
(400 MHz, CDCl3, d): 162.6, 159.6, 154.4, 141.3, 138.4, 138.2,
134.7, 130.3, 130.0, 129.6, 128.7, 127.7, 118.4, 118.0, 103.9, 21.6.
HRMS (EI, m/z): [M+] calcd for C44H30N4: 614.2470; found:
614.2466.
(2E,20E)-1,10-(1,3-Phenylene)bis(3-p-tolylprop-2-en-1-one) (m-
1
PPtNN I). H NMR (400 MHz, CDCl3, d): 8.63 (s, 1H), 8.23–
8.21 (m, 2H), 7.87–7.83 (m, 2H), 7.67–7.63 (m, 1H), 7.59–7.52
(m, 6H), 7.26–7.23 (m, 4H), 2.41 (s, 6H).
6,60-(1,3-Phenylene)bis(2-phenyl-4-p-tolylnicotinonitrile)
(m-
(2E,20E)-3,30-(1,3-Phenylene)bis(1-p-tolylprop-2-en-1-one) (m-
PPtNT I). 1H NMR (400 MHz, CDCl3, d): 7.97 (d, J ¼ 8.16 Hz,
4H), 7.87 (s, 1H), 7.84 (d, J ¼ 15.68 Hz, 2H), 7.69–7.67 (m, 2H),
7.60 (d, J ¼ 15.72 Hz, 2H), 7.50–7.48 (m, 1H), 7.33 (d, J ¼ 8.00
Hz, 4H), 2.45 (s, 6H).
PPtNN). 1H NMR (400 MHz, CDCl3, d): 8.90 (s, 1H), 8.31–8.29
(m, 2H), 8.07–8.04 (m, 4H), 7.88 (s, 2H), 7.69–7.65 (m, 1H), 7.61
(d, J ¼ 8.08, 4H), 7.57–7.54 (m, 6H), 7.39 (d, J ¼ 7.92 Hz, 4H),
2.46 (s, 6H); 13C NMR (400 MHz, CDCl3, d): 162.6, 158.7, 155.9,
140.5, 138.7, 138.1, 133.9, 130.3, 129.9, 129.8, 129.7, 129.6, 128.8,
128.7, 126.7, 118.9, 117.9, 104.9, 21.6. HRMS (EI, m/z): [M+]
calcd for C44H30N4: 614.2470; found: 614.2455.
4,40-(1,3-Phenylene)bis(2-phenyl-6-p-tolylnicotinonitrile)
(m-
1
PPtNT). H NMR (400 MHz, CDCl3, d): 8.13 (d, J ¼ 8.16 Hz,
4H), 8.07–8.05 (m, 4H), 8.01 (s, 1H), 7.95 (s, 2H), 7.90–7.87 (m,
2H), 7.79–7.77 (m, 1H), 7.59–7.55 (m, 6H), 7.33 (d, J ¼ 8.08 Hz,
4H), 2.43 (s, 6H); 13C NMR (400 MHz, CDCl3, d): 162.6, 159.6,
154.4, 141.3, 138.2, 137.7, 134.7, 130.3, 130.2, 129.9, 129.6, 129.4,
128.7, 127.7, 118.5, 118.1, 103.9, 21.6. HRMS (EI, m/z): [M+]
calcd for C44H30N4: 614.2470; found: 614.0865.
(2E,20E)-1,10-(1,3-Phenylene)bis(3-(4-tert-butylphenyl)prop-2-en-
1-one) (m-PbPNN I). 1H NMR (400 MHz, CDCl3, d): 8.62 (s, 1H),
8.23–8.21 (m, 2H), 7.88 (d, J ¼ 15.68 Hz, 2H), 7.66 (m, 1H), 7.63
(d, J ¼ 8.36 Hz, 4H), 7.56 (d, J ¼ 15.68 Hz, 2H), 7.47 (d, J ¼ 8.40
Hz, 4H), 1.35 (s, 18H).
6,60-(1,3-Phenylene)bis(4-(4-tert-butylphenyl)-2-phenylnicotino-
nitrile) (m-PbPNN). 1H NMR (400 MHz, CDCl3, d): 8.88 (s, 1H),
8.31–8.29 (m, 2H), 8.07–8.05 (m, 4H), 7.89 (s, 2H), 7.69–7.55 (m,
15H), 1.39 (s, 18H); 13C NMR (400 MHz, CDCl3, d): 162.6,
158.6, 155.7, 153.5, 138.7, 138.1, 133.8, 130.2, 129.8, 129.6, 126.7,
126.2, 119.0, 117.9, 104.7, 35.0, 31.4. HRMS (EI, m/z): [M+]
calcd. for C50H42N4: 698.3409; found: 698.2701.
(2E,20E)-3,30-(1,3-Phenylene)bis(1-(4-methoxyphenyl)prop-2-en-
1
1-one) (m-PmPNT I). H NMR (400 MHz, CDCl3, d): 8.07 (d,
J ¼ 8.80 Hz, 4H), 7.87 (s, 1H), 7.84 (d, J ¼ 15.68 Hz, 2H), 7.68–
7.66 (m, 2H), 7.61 (d, J ¼ 15.68 Hz, 2H), 7.49–7.46 (m, 1H), 7.01
(d, J ¼ 8.76, 4H), 3.90 (s, 6H).
4,40-(1,3-Phenylene)bis(6-(4-methoxyphenyl)-2-phenylnicotinonitrile)
(m-PmPNT). 1H NMR (400 MHz, CDCl3, d): 8.21 (d, J ¼ 8.72
Hz, 4H), 8.07–8.04 (m, 4H), 8.01 (s, 1H), 7.91 (s, 2H), 7.89–7.87
(m, 2H), 7.79–7.77 (m, 1H), 7.59–7.54 (m, 6H), 7.04 (d, J ¼ 8.72
Hz, 4H), 3.88 (s, 6H); 13C NMR (400 MHz, CDCl3, d): 162.5,
162.1, 159.2, 154.3, 138.2, 137.7, 130.3, 130.1, 129.9, 129.8, 129.5,
129.4, 128.7, 118.3, 118.0, 114.6, 103.3, 55.6. HRMS (EI, m/z):
[M+] calcd for C44H30N4O2: 646.2369; found: 646.2191.
4.2 Thermal and spectral measurements
Thermal properties were characterized with TGA measurement
at a ramping rate of 20 ꢀC minꢁ1 and DSC measurement at
a ramping rate of 10 ꢀC minꢁ1 under nitrogen. Their IP were
determined by using ultraviolet photoelectron spectroscopy
(UPS). EA of individual compound was estimated by subtracting
its optical transfer gap, as determined from UV-visible optical
absorption spectroscopy, from the corresponding IP. The triplet
energy (ET) levels were estimated from the initial peak position of
corresponding phosphorescent spectra.
1
(E)-4-(3-Oxo-3-phenylprop-1-enyl)benzonitrile (CPPNN I). H
NMR (400 MHz, CDCl3, d): 8.03–8.02 (m, 2H), 7.80–7.76 (m,
1H), 7.75–7.70 (m, 4H), 7.62–7.58 (m, 2H), 7.55–7.51 (m, 2H).
4-(4-Cyanophenyl)-2,6-diphenylnicotinonitrile (CPPNN). 1H
NMR (400 MHz, CDCl3, d): 8.19–8.17 (m, 2H), 8.05–8.00 (m,
2H), 7.88–7.86 (m, 2H), 7.82–7.78 (m, 3H), 7.76–7.52 (m, 6H);
13C NMR (400 MHz, CDCl3, d): 162.7, 159.8, 153.5, 141.3, 137.7,
137.2, 132.9, 131.1, 130.5, 129.7, 129.5, 129.2, 128.8, 127.7, 118.3,
118.2, 117.3, 114.0, 104.0. HRMS (EI, m/z): [M+] calcd for
C25H15N3: 357.1266; found: 357.1236.
4.3 Electron mobility measurements
Bilayer devices with the triphenylpyridine derivatives as the
electron-transporting layers (ETLs) and NPB as the hole-trans-
porting layer (HTL) were fabricated. The OLEDs have
a configuration of ITO/NPB [60nm]/one of these triphenylpyr-
idine derivatives [60nm]/MgAg [100nm, 10 : 1]. Using a pulse
generator as an electrical switch in a high-current driver circuit,
voltage pulses of 3–12V with a rise time less than 20 ns was
obtained. The OLEDs were driven by the voltage pulses with
duration of 2–25 ms. The luminance response from the device was
detected by a silicon photo-diode attached to a high-speed
receiver. The resulting transient photocurrent was analyzed with
a storage oscilloscope (HP 54810A). For the interpretation of the
luminance delay time, a voltage supply source was employed to
give a trigger signal to start the collection of photocurrent. The
transient responses of EL of all the devices were measured by
averaging the data taken from 100 consecutive signals averaging.
(2E,20E)-1,10-(1,4-Phenylene)bis(3-p-tolylprop-2-en-1-one) (p-
PPtNN I). 1HNMR(400MHz, CDCl3, d):8.11(s, 4H),7.84(d, J¼
15.68Hz, 2H), 7.57 (d, J ¼ 8.08Hz, 4H), 7.51(d, J ¼ 15.72 Hz, 2H),
7.26(d, J¼ 8.48Hz, 4H),2.41(s, 6H);13CNMR(400MHz, CDCl3,
d):190.3, 146.0, 141.6, 141.5, 132.0, 129.9, 128.7, 121.0, 21.7.
6,60-(1,4-Phenylene)bis(2-phenyl-4-p-tolylnicotinonitrile)
(p-
PPtNN). 1H NMR (400 MHz, CDCl3, d): 8.32 (s, 4H), 8.06–8.04
(m, 4H), 7.87 (s, 2H), 7.62 (d, J ¼ 8.08 Hz), 7.59–7.51 (m, 6H),
7.40 (d, J ¼ 7.88 Hz, 4H), 2.47 (s, 6H); HRMS (EI, m/z): [M+]
calcd for C44H30N4: 614.2470; found: 614.2466.
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4.4 OLED fabrication and measurements
The EL devices were fabricated on ITO coated glass substrates
with a sheet resistance of 30 U ,ꢁ1. They were cleaned with
Decon 90, rinsed in de-ionized water, dried in an oven and finally
exposed to UV-ozone for about 25 min. The ITO substrates were
then transferred into a deposition chamber with a base pressure
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Acknowledgements
This work is financially supported by the National Natural
Science Foundation of China (Grant No. 60778033), the
National High Technology Research and Development Program
of China (863 Program) (Grant No. 2008AA03A327,
2011AA03A110), General Research Fund of the Research Grant
Council of Hong Kong (CityU 101508).
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