New optoelectronic materials based on bitriazines: Synthesis and properties

Article abstract of DOI:10.1021/ol702698r

A series of bitriazine derivatives were synthesized for the first time by the self-coupling reactions of the monocholoro-triazines in the presence of nickel catalyst. Such bitriazines show excellent optoelectronic properties.

Full text of DOI:10.1021/ol702698r

ARTICLE
pubs.acs.org/JPCC
New Conjugated Triazine Based Molecular Materials for Application
in Optoelectronic Devices: Design, Synthesis, and Properties
Hongliang Zhong, Hua Lai, and Qiang Fang*
Key Laboratory of Organofluorine Chemistry and Laboratory for Polymer Materials, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People's Republic of China
ABSTRACT: Two novel triazine-based compounds with efficient
hole-blocking ability were designed and synthesized, which pro-
moted the performance of electroluminescent devices, especially
for PLED even when Al was applied as the cathode. The results
indicated that introducing electron-donating groups and extend-
ing π-conjugated were facile methodologies to improve the
optoelectronic properties of the triazine derivatives.
’ INTRODUCTION
Interestingly, the hole-confinement ability of FTRZ seems to
be weaker than that of TRZ because it shows the HOMO levels
in an order of FTRZ (-6.25 eV) > TRZ (-6.51 eV), whereas the
performance of devices based on them is in a contrary order.
Such paradoxical results are probably attributed to the difference
of LUMO levels and band gaps between the two compounds.
The higher LUMO level of TRZ (-2.70 eV) brings about a large
barrier to the electrons, moving from cathode to emitting layer,
meanwhile a large band gap of TRZ (3.71 eV) fetters the electron
jump or transporting, resulting in very low electron mobility.
Therefore, TAZ endows the devices with poor performance,
whereas FTRZ has a LUMO level of -2.95 eV and a band gap of
3.30 eV, which are suitable for blocking holes and transporting
electrons. Such facts imply that appropriate LUMO levels and
band gaps are benefit of 1,3,5-triazine derivatives on the premise
of keeping their HOMO levels sufficiently low.
Structurally, FTRZ possesses an extensive π-conjugated sys-
tem and electron-donating groups compared with TRZ, which
may be the source of the superior performance of FTRZ, imply-
ing that introducing electron-donating groups to a triazine ring
and extending the π-conjugation of the triazine derivative are
facile ways to improve the performance of the triazine-based
optoelectronic materials. Such an assumption motivated us to
design and synthesize two triazine conjugated molecules, M1 and
M2, whose structures are shown in Scheme 2. The results
showed that M1 and M2 can greatly increase the performance
both of OLED and PLED. We report the details in this work.
Organic conjugated molecular materials have received great
attention in recent decades in optoelectronic applications such
as organic (or polymeric) light-emitting diodes (OLED and
PLED),1,2 thin film transistors (TFT),3 organic photovoltaic
cells (OPV),4 and chemical/biological sensors,5 because they are
low cost and can easily be fabricated to obtain the devices. Within
the whole catalogue of the materials, 1,3,5-triazine derivatives
have become a focus due to their exceptional properties derived
from the unique structure of the 1,3,5-triazine units that show
strong electron-accepting ability and high symmetry.6 Con-
sequently, introducing a 1,3,5-triazine unit into the backbone
of the conjugated molecules can not only improve the abilities of
electron-injection and electron-transportation of the molecules,7
but also increase the heat resistance of the molecules.8 Undoubtedly,
from the point of view of the applications of the optoelectronic
materials, the organic molecules having both good electron-
injection/or transportation ability and high stability are desirable.
Thus, in recent years, there has been an increasing interest in
the synthesis and investigation of the chemical properties of
π-conjugated triazine based small molecules and polymers.9
Of the triazine based molecular materials used in OLED, 2,4,6-
triphenyl-1,3,5-triazine (TRZ) is popular and its application in
OLED as a hole-blocking material can trace back to the last
century.6a However, its use is limited because it shows poor
performance indevices.10 Muchefforthasbeenexertedto improve
the properties of the triazine-based materials, mainly concerning
introducing different substituents into a triazine ring. Examples of
the substituents include naphthalene,11 phenylnaphthalene,12
carbazole13 and pyridine groups.14 Recently, there are also reports
regarding the conjugated polymers containing triazine ring,15
which were expected to enhance the performance of devices.
Nevertheless, satisfactory triazine-based materials are very rare.
A recent work in our group revealed that the fluorene
substituted triazine (FTRZ, Scheme 1) showed good properties
and could be used as a hole-blocking material in OLED.9d
’ EXPERIMENTAL SECTION
Reagents and Instruments. Commercially available reagents
were purchased and used without further purification. All solvents
Received: October 13, 2010
Revised:
December 6, 2010
Published: December 21, 2010
r
2010 American Chemical Society
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Scheme 1. Chemical Structure of TRZ and FTRZ
(20 mL), and the organic layer was separated, washed twice with
water, dried over anhydrous Na2SO4, and concentrated. The
residue was purified by flash chromatography on silica gel
(hexane/ethyl acetate 15:1), giving 3 (0.31 g, 0.96 mmol, 96%
yield) as a white solid. 1H NMR (300 MHz, CDCl3, δ):
7.81-7.74 (m, 6H), 7.57 (t, J = 7.2 Hz, 2H), 7.36 (br, 3H),
2.11-2.04 (m, 4H), 0.34 (t, J = 7.4 Hz, 6H). 13C NMR
(100 MHz, CDCl3, δ): 151.0, 150.3, 146.2, 142.3, 140.7,
137.9, 132.6, 127.7, 127.6, 127.1, 126.3, 123.0, 121.6, 120.2,
120.0, 119.1, 110.6, 56.3, 32.8, 8.6. EI-MS (m/z): Mþ 323. Calcd
for C24H21N: C 89.12; H 6.54; N 4.33; found: C 89.02, H 6.52,
N 4.21.
9,9-Diethyl-7-phenyl-9H-fluorene-2-carbonitrile (6). A similar
procedure for preparation of 3 was employed. The pure product
was obtained as a white powder (0.25 g, 0.77 mmol, 96% yield) after
flash chromatography on silica gel (hexane/ethyl acetate 15:1).
1H NMR (300 MHz, CDCl3, δ): 7.81 (t, J = 7.1 Hz, 2H),
7.69-7.58 (m, 6H), 7.49 (t, J = 7.5 Hz, 2H), 7.39 (t, J = 7.1 Hz,
1H), 2.13-2.06 (t, 4H), 0.34 (t, J = 7.2 Hz, 6H). 13C NMR (100
MHz, CDCl3, δ): 151.3, 150.9, 145.8, 142.2, 141.1, 138.8, 131.5,
128.9, 127.6, 127.3, 126.6, 126.5, 121.8, 121.1, 120.2, 119.9, 109.9,
55.7, 32.6, 8.5. MALDI-MS (m/z): Mþ 323.2. Calcd for C24H21N:
C 89.12; H 6.54; N 4.33; found: C 88.61, H 6.52, N 4.18.
Scheme 2. Procedure for the Preparation of M1 and M2
2,4,6-Tris(4-(9,9-diethyl-9H-fluoren-2-yl)phenyl)-1,3,5-triazine
(M1). A solution of 3 (0.29 g, 0.9 mmol) in anhydrous CH2Cl2
(10 mL) was added to a solution of trifluoromethanesulfonic acid
(1.5 g, 10 mmol) in anhydrous CH2Cl2 (5 mL). After being stirred
for 10 h at room temperature, an aqueous NaHSO3 solution was
added dropwise until the appearance of the solution is colorless.
The mixture was extracted with CHCl3 (30 mL) and the organic
phase was separated, washed twice with water, dried over anhy-
drous Na2SO4, and concentrated. The residue was purified by
flash chromatography on silica gel (hexane/ethyl acetate 20:1) to
afford to a white solid (0.27 g, 0.28 mmol, 93% yield). 1H NMR
(300 MHz, CDCl3, δ): 8.92 (d, J = 8.1 Hz, 6H), 7.94-7.70
(m, 18H), 7.35 (br, 9H), 2.14-2.10 (t, 12H), 0.41 (t, J = 7.2 Hz,
18H) 13C NMR (100 MHz, CDCl3, δ): 171.3, 150.7, 150.2, 145.6,
141.6, 141.0, 139.2, 135.0, 129.5, 127.3, 126.9, 126.2, 122.9, 121.6,
120.0, 119.8. FT-IR (KBr, cm-1): ν = 3067, 2962, 1606, 1568,
1512, 1371, 814. HRMS (MALDI, m/z): [M þ H]þ calcd for
C72H64N3, 970.5095; found, 970.5070.
2,4,6-Tris(9,9-diethyl-7-phenyl-9H-fluoren-2-yl)-1,3,5-triazine
(M2). M2 was prepared by using the analogous procedure for
preparation of M1. The crude product was purified by flash
chromatography on silica gel (hexane/ethyl acetate 20:1), giving
M2 (0.29 g, 0.30 mmol, 90% yield) as a white powder. 1H NMR
(300 MHz, CDCl3, δ): 8.91 (d, J = 7.8 Hz, 3H), 8.80 (s, 3H), 7.98
(d, J = 7.8 Hz, 3H), 7.92 (d, J = 7.5 Hz, 3H), 7.73 (d, J = 7.5 Hz,
6H), 7.67 (m, 6H), 7.51 (t, J = 7.4 Hz, 6H), 7.40 (t, J = 7.1 Hz,
3H), 2.27 (q, 12H), 0.48 (t, J = 7.1 Hz, 18H). 13C NMR
(100 MHz, CDCl3, δ): 171.7, 151.8, 150.6, 145.6, 141.4, 141.1,
140.0, 135.2, 128.8, 127.3, 127.2, 126.2, 123.2, 121.8, 120.8,
119.8, 56.4, 32.8, 8.7. FT-IR (KBr, cm-1): ν = 3064, 3026,
2963, 1608, 1577, 1516, 1372, 817. HRMS (MALDI, m/z):
[M þ H]þ calcd for C72H64N3, 970.5095; found, 970.5083.
Device Fabrication. Indium tin oxide (ITO) with a sheet
resistance of about 15 ohm/square was cleaned with isopropyl
alcohol, acetone and deionized water then treated with UV/
ozone. Then, 20 nm NPB (N,N0-bis(naphthalen-1-yl)-N,N0-bis-
(phenyl)benzidine) and 40 nm Alq were thermal evaporated
sequentially on ITO for OLED. For PLED, a layer of PEDOT:
PSS (Baytron AI 4083, Bayer) film with thickness of 50 nm was
were dehydrated and distilled under an inert atmosphere before use.
1H and 13C NMR spectra were recorded on Bruker DRX 400 or
Varian Mercury 300 spectrometer using CDCl3 as solvent and TMS
as internal standard. Mass spectra were taken with an HP 5989A
or Finnigan 4021. High resolution mass spectra (HRMS) were
obtained by MALDI/DHB. Elemental analysis was performed with
a Carlo Erba 1106 elemental analyzer. UV-vis and PL spectra were
obtained with a Hitachi UV-2810 and a Hitachi F-4500 spectro-
photometer, respectively. Cyclic voltammetry was performed in an
acetonitrile solution containing [n-Bu4N]ClO4 (0.10 M) with
ferrocene as standard using (0.10 M AgNO3)/Ag and platinum
disk electrode as reference and counter electrodes, respectively.
A CHI 600B analyzer was used for the cyclic voltammetry.
Electroluminescent Properties were recorded on a photometer
PR 705 at room temperature in air.
Synthesis. As can be seen from Scheme 1, intermediates
1, 2, 4, and 5 were synthesized following previously reported
procedures.16 The precursors, 3 and 6, were prepared by the
standard Suzuki coupling reaction, and then cyclization of the
two precursors gave M1 and M2 in a high yield.
4-(9,9-Diethyl-9H-fluoren-2-yl)benzonitrile (3). An aqueous
Na2CO3 (2 mL, 2M) solution containing toluene (5 mL) was
added to a mixture of 2-(9,9-diethyl-9H-fluoren-2-yl)-1,3,2-
dioxaborinane (0.31 g, 1.0 mmol), 4-bromobenzonitrile (0.18 g,
1.0 mmol) and Pd(PPh3)4 (0.058 g, 0.05 mmol) under argon.
The resulting solution was kept at reflux for 48 h, and then cooled
to room temperature. To the reaction system was added CHCl3
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Figure 2. CV curves of TRZ, FTRZ, M1, and M2.
Figure 1. UV-vis and PL spectra of TRZ, FTRZ, M1, and M2.
The CT action is also observed in the PL spectra of the triazine
molecules. For example, TRZ without a donor-acceptor struc-
ture is not photoluminescent, whereas its analogues, FTRZ, M1,
and M2, show photoluminescence. As seen from Figure 1 and
Table 1, the maximum emission peaks are in an order of FTRZ
(385 nm) < M2 (412 nm) < M1 (424 nm) in solution. In the
solid state, their emission maxima show a slight red shift, and the
values of red-shifting maintain an order of FTRZ > M1=M2,
indicating the existence of partial π-staking structure in the three
compounds. However, the degree of π-staking in M1 and M2 is
lower than that in FTRZ, suggesting the attachment of more
groups with large steric-hindrance can suppress the molecular
aggregation. It is noted that M1 shows a Stokes shift of 74 nm,
relative to 33 nm for FTRZ and 42 nm for M2. This result
indicates that the Donator-space-Acceptor molecular struc-
ture of M1 produces greater conformational relaxation and more
conformational disorder in the excited state.18
Table 1. Optical and Electrochemical Properties of TRZ,
FTRZ, M1, and M2
λmax abs(nm)a
λmax em(nm)b LUMO HOMO Eg
opt
compounds solution film
solution film
(eV) c (eV) d (eV) e
TRZ
FTRZ
M1
270, 279
352, 358
350, 349
370, 372
-, -
-2.80 -6.51 3.71
-2.95 -6.25 3.30
-2.96 -6.10 3.14
-2.94 -6.07 3.13
385, 417
424, 436
412, 424
M2
a Maximum UV-vis absorption peaks. b Maximum PL emission peaks.
c Evaluated from their reduction onset potentials versus ferrocene (Fc)
as the internal standard. d HOMO = LUMO - Egopt. e Optical band gap,
estimated from the onset wavelength of the absorption spectra in the
solid film.
The redox behavior of TRZ, FTRZ, M1, and M2 was
characterized by cyclic voltammetry (CV) at a platinum disk
electrode versus a (0.1 M AgNO3)/Ag reference electrode in
0.1 M (n-Bu4N)ClO4/acetonitrile and the results are described
in Figure 2. From their reduction onset potentials, the LUMO
levels of M1, M2, FTRZ, and TRZ are estimated as -2.96,
-2.94, -2.95, and -2.80 eV, respectively. On the basis of the
LUMO levels and the optical band gap, HOMO levels of M1,
M2, FTRZ, and TRZ are evaluated with the value of -6.10,
-6.07, -6.25, and -6.51 eV, respectively. In comparison with
TRZ and FTRZ, M1, and M2 achieved the appropriate band gaps
and adequately low HOMO energy levels, meeting the require-
ment of the original molecular design.
To confirm the above-mentioned assumption regarding the
relationship between the molecular structure and the properties,
five electroluminescent devices were designed and fabricated
with the following configuration: ITO/NPB/Alq/compounds
containing triazine units/LiF/Al, where a 40 nm thickness of Alq
was used as the emitting layer and TRZ, FTRZ, M1, and M2 were
used as the electron-transporting/hole-blocking materials with a
thickness of 20 nm. Considering that Alq is also an effective
electron-transporting material,1a a comparative device was fabri-
cated with a configuration of ITO/NPB/Alq/Alq/LiF/Al, and
the thickness of the Alq used as electron-transporting layer was
the same as that of the triazine compounds, e.g., 20 nm.
spin-coated from its aqueous dispersion and dried at 180 °C for
10 min. MEH-PPV (Mn 150,000-250,000, Aldrich) layer was
formed by spin-coating at 3000 rpm from its solution in
cyclohexanone (10 mg/mL), and then by heating at 60 °C for
2 h in a glovebox. Finally, triazine compounds (20 nm), LiF
(2 nm) and Al (100 nm) were deposited sequentially by thermal
evaporation at a pressure below 10-4 Pa for both OLED and
PLED. The control devices without triazine compounds were
also fabricated using an aforementioned analogous procedure.
’ RESULTS AND DISCUSSION
The basic photophysical characteristics of M1 and M2 were
investigated by UV-vis and photoluminescent (PL) spectra, and
the data are shown in Figure 1 and Table 1. For comparison, the
optical data of TRZ and FTRZ are also listed. Because fluorene
is an electron-donating group and shows an expansion of the
π-conjugation system, compared with phenyl group, FTRZ
shows a UV-vis peak red-shifted by about 80 nm from
270 nm of TRZ. Analogously, M1 and M2 also show the red
shift compared with TRZ. Interestingly, the UV-vis peak of M2
red-shifts about 20 nm, compared with 350 nm of M1. Such a
result may be attributed to the presence of a charge-transferred
(CT) electronic state in M2, caused by the action between
electron-donating fluorene group and electron-accepting triazine
group.17 In contrast to M2, the presence of a phenyl space
between fluorene group and triazine ring in the molecule may
weaken the CT action and results in the UV-vis peak of M1 is
located at lower wavelength.
Due to the dual functions of Alq, the maximum luminance of
the control device Alq/Alq is more than 19 000 cd m-2
,
meanwhile device Alq/FTRZ (16000 cd m-2), device Alq/M1
(13 000 cd m-2) and device Alq/M2 (18 000 cd m-2) also keep
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Table 2. Electroluminescent Properties of the Devices
ηmax
(cd A-1
Vmax
(V) b
Bmax
(cd m-2
ηBmax
(cd A-1
)
a
c
d
devices
Alq/Alq
)
)
3.1
13
19 040
1740
2.1
Alq/TRZ
1.3
21
0.4
Alq/RTRZ
Alq/M1
4.5
13
16 100
12 860
18 290
1358
3.8
4.0
15
4.0
Alq/M2
4.9
16
4.4
MEH-PPV
0.25
0.97
0.97
1.37
10
0.23
0.64
0.68
0.79
MEH-PPV/FTRZ
MEH-PPV/M1
MEH-PPV/M2
12
2683
12.5
2327
10.5
2871
a Maximum current efficiency. b Breakdown voltage. c Maximum luminance.
d Current efficiency at maximum luminance.
Figure 3. Efficiency curves of OLEDs.
of ITO/PEDOT:PSS/MEH-PPV/hole-blocking layer/LiF/Al.
Here, triazine molecules were applied as hole-blocking materials
and a stable and low-cost cathode Al was employed as cathodes.
Meanwhile, a device without a hole-blocking layer was also
fabricated for comparison. The utilization of Al is based on the
following facts: the common cathode in PLED is Ca or Ba, which
can usually improve the performance of PLED, however, their
utilization brings out the complication of the encapsulation
procedure and raises the cost due to their air-sensitivity.
As shown in Figure 4 and Table 2, the maximum luminance of
device FTRZ, M1 and M2 are promoted from 1358 cd m-2
(comparative device MEH-PPV) to 2683, 2327, and 2871 cd m-2
,
respectively. Furthermore, Devices FTRZ, M1, and M2 show
maximum current efficiencies of 0.97, 0.97, and 1.37 cd A-1
,
respectively, which are more than four times higher than 0.25 cd A-1
of device MEH-PPV. These results suggest that these triazine
derivatives possess excellent hole-blocking capacity. Compared
with device FTRZ, devices M1 and M2 exhibit good perfor-
mance, conforming that the new triazine molecular materials can
also improve the performance of PLED.
Figure 4. Efficiency curves of PLEDs.
high luminance. But the maximum luminance of the device Alq/
TRZ is only 1700 cd m-2
.
Alternatively, as shown in Figure 3, the maximum current
efficiencies of the devices containing FTRZ or M1 or M2 are
remarkably improved from 3.1 cd A-1 of the comparative device
to 4.5, 4.0, and 4.9 cd A-1, respectively. However, device TRZ
shows poor performance with a maximum current efficiency of
1.2 cd A-1. These results indicate that the application of FTRZ,
M1, and M2 enables the devices to improve the balance between
hole and electron transporting. In particular, the promotion is
superior to the utilization of Alq, which has been proven to be an
available electron transporting material.
The excellent performance of M1 and M2 in OLED inspires
us to investigate the probability of application of the compounds
in PLED. Recently, although there are many reports about
introducing small molecules to PLED for improvement of the
performance, the results basically are not satisfactory.19 The main
reason is as follows: (1) doping small molecules to PLED usually
brings about phase separation and unfavorable interfacial effect
that may decline the performance of devices;20 (2) although
depositing small molecules on the surface of PLED is a good way,
the choice of the small molecular materials is limited. On the
basis of the aforementioned situation, we chose the depositing
way to study the behavior of the triazine molecules in PLED.
Because MEH-PPV is a famous PLED material, and its perfor-
mance should be improved, thus, it was used as a light emitting
layer in our case. The devices were fabricated with the configurations
’ CONCLUSIONS
In summary, two new conjugated triazine-containing molec-
ular materials, M1 and M2, were synthesized and used to
improve the performance of OLED and PLED. The results
showed that M1 and M2 can greatly increase the performance
both of OLED and PLED, which supports a concept for molec-
ular design that introducing more electron-donating groups and
extended π-conjugation into a triazine ring can turn the optimal
energy levels and band gaps of the molecules, resulting in the
improvement of the optoelectronic properties of the molecules.
Such a concept for molecular design may useful for developing
and exploring new 1,3,5-triazine derivatives with excellent opto-
electronic properties.
’ AUTHOR INFORMATION
Corresponding Author
*Tel: þ86 21 5492 5337. Fax: þ86 21 5492 5337. E-mail:
qiangfang@mail.sioc.ac.cn.
’ ACKNOWLEDGMENT
Financial support from the Natural Science Foundation of
China (NSFC, No 20872171) is gratefully acknowledged.
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