Synthesis, structure, physical properties and OLED application of pyrazine-triphenylamine fused conjugated compounds

Article abstract of DOI:10.1039/c5ra12654a

In this paper, several donor-acceptor compounds using pyrazine and triphenylamine as building units have been synthesized and fully characterized. The physical properties and theoretical calculations were also investigated. In addition, the OLED devices u

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Synthesis, structure, physical properties and OLED application of
pyrazine–triphenylamine fused conjugated compounds
Liang Xu,1 Yongbiao Zhao,2 Guankui Long,1 Yue Wang,3 Jun Zhao,4 Dongsheng Li,4 Junbo Li,1 Rakesh
Ganguly,5 Yongxin Li, 5 Handong Sun, 3 Xiao Wei Sun,2, * and Qichun Zhang1,4 *
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
In this paper, several donor–acceptor compounds using pyrazine and triphenylamine as building units
have been synthesized and fully characterized. The physical properties and theoretical calculations were
also investigated. In addition, the OLED devices using these compounds as active elements have been
10 fabricated and all devices display very good performance. Specifically, the maximum EQE for 2 is 7.37
%, which is higher than the theoretical limit for fluorescent emitters (5 %), indicating that the mechanism
for compound 2 is different from traditional fluorescent dyes. Our results suggest that D-A molecules
could be a good approach to improve the performance of OLED devices.
45 transport material in OLED devices and memory devices.6 On the
other hand, pyrazine units have been widely demonstrated as the
electron acceptor to construct several electron-deficient
structures.9 Moreover, pyrazine units can be easy to be
functionalized by donor units, acceptor substituents, extended π–
50 conjugation species, or bulky groups to form new conjugated
materials.10 In this work, we are more interested in preparing a
series of pyrazine–triphenylamine fused conjugated compounds
for applications in OLEDs because (1) the LUMO and bandgap
of these D–A molecules could be tuned through choosing the
55 different electron–withdrawing abilities of the acceptor groups;
(2) the fluorescence efficiency could also be modified by
optimizing the intramolecular charge–transfer (ICT) strength; (3)
the introduction of fused aromatic rings into the push–pull system
could also lead to an enhanced carrier mobility and a lower band
60 gap.11 Here, we want to investigate how different groups such as
strong electron–acceptor cyano groups, moderate electron–
acceptor pyridine groups and bulky 9, 9-bis(2-ethylhexyl)
fluorine to affect the structures (molecular stacking), physical
properties and their performance in OLEDs.
15 1 Introduction
Organic light emitting diodes (OLEDs) have potential
applications in the field of flat-panel display due to their low
weight, low power consumption, self–emitting property, high
luminous efficiency, wide viewing angle and high contrast.1
20 OLEDs normally have three layers including hole–transporting
layer, light–emitting layer and electron–transporting layer, which
are sandwiched together between two electrodes. The ideal
compounds to approach better performance of OLEDs are dipolar
D–A molecules, which incorporate both hole–transport units and
25 electron–transport segments. There are two reasons for such
design: (1) the hole mobility in the D–A molecules is slightly
lower than that in
a single hole–transporting layer, thus
facilitating the exciton formation inside the layer; and (2) the
low-lying LUMO of D–A molecules is helpful to the capture of
30 electrons released from the cathode, which would improve the
performance.2 In fact, many emissive compounds incorporating
anthracene,3 pyrene,4 fluorine5 and triphenylamine6 segments
have been synthesized and all compounds displays good
performance in OLEDs. It is worthy to note that molecular
35 geometry has two-faced effects on the electroluminescent (EL)
efficiency: on one hand, planar geometry is very important to
help the delocalization of π−electrons and improve the EL
efficiency; on the other hand, non−planar geometry can provide
steric hindrance that reduces the molecular aggregation in the
40 solid state, thus increasing the EL efficiency.7
65 2 Experimental Section
2.1 Materials
Most reagents were purchased from Alfa Aesar or Aldrich and
used as received unless otherwise noted. All the solvents used in
photophysical measurements and electrochemical measurements
70 were of HPLC grade quality. All other solvents were obtained
commercially and purified using standard procedures.
As a strong electron donor, triphenylamine has a unique
propeller–like structure, and its distorted geometry diminishes the
intermolecular interactions and favors efficient fluorescence.8
Thus, triphenylamine has been considered as an excellent hole
2.2 Characterization
Using CDCl3 as a solvent and tetramethylsilane (TMS) as the
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1
internal standard, H NMR and 13C NMR spectra were measured
Synthesis of compound 1: 4,4'-bis(N,N-diphenylamino)benzil
on a Bruker Advance 300 MHz NMR spectrometer at ambient 40 (540 mg, 1.0 mmol) and diaminomaleonitrile (108 mg, 1.0 mmol)
temperature. UV/Vis absorption spectra were carried out at room
temperature with a Shimadzu UV-3600 spectrophotometer. High
resolution mass spectra (HRMS) were recorded on a Waters Q-
Tof premierTM mass spectrometer. Cyclic voltammetry
were dissolved in anhydrous acetic acid (50 mL) with catalytic
amount of 2-iodoxybenzoic acid (IBX). The solution was
refluxed for 24 hours under Argon atmosphere. After that the
crude product was concentrated in vacuo and purified by column
5
measurements were carried out on a CHI660 electrochemical 45 chromatography (CH2Cl2/petroleum ether=1/2), affording
analyzer. Glassy carbon (diameter: 1.6 mm; area 0.02 cm2) was
used as a working electrode, platinum wires were used as counter
compounds 1 (542 mg, yield 88%).
1H NMR (300 MHz, CDCl3) δ 7.50 (d, 4 H), 7.34 - 7.29 (m, 8 H),
7.16 - 7.12 (m, 12 H), 6.96 - 6.93 (d, 4 H).13C NMR (75 MHz,
CDCl3) δ 154.09, 150.78, 146.55, 130.87, 129.76, 128.33,
10 electrode and reference electrode, respectively. Fc+/Fc was used
as an internal standard. Potentials were recorded versus Fc+/Fc in
a solution of anhydrous dichloromethane (DCM) with 0.1M 50 127.54, 126.09, 124.80, 120.32, 113.77. HRMS (M+1) =
tetrabutylammonium hexafluorophosphate (TBAPF6) as
supporting electrolyte at a scan rate of 100 mV s-1.
a
617.2440 (calc. 617.2454).
Synthesis of compound 2: The procedure was the same with
above except that diaminomaleonitrile was replaced by 2,3-
diaminopyridine (109 mg, 1.0 mmol). This reaction gave
55 compounds 2 (549 mg, yield 89%).
15 2.3 Fabrication of the OLED devices
The device structure is ITO/MoO3(2 nm)/NPB(50 nm)/CBP: X
wt.% Dye (20 nm)/Bphen (50 nm)/LiF (1 nm)/ Al (150 nm),
where ITO is anode, MoO3 is hole injection layer, NPB is hole
transporting layer, CBP: X wt.% Dye is the emission layer,
20 Bphen is the electron transporting layer, LiF is the electron
injection layer and Al is the cathode.12 The doping concentrations
of dye in CBP are 100% (neat dye layer), 50 %, 10 % and 3 %,
respectively.
1H NMR (300 MHz, CDCl3) δ 9.10 - 9.09 (m, 1 H), 8.47 - 8.44
(m, 1 H), 7.67 - 7.64 (m, 1 H), 7.59 (d, 2 H), 7.49 (d, 2 H), 7.31 -
7.27 (m, 7 H), 7.15-7.13 (m, 8 H), 7.10-7.08 (m, 3 H) ), 7.07-7.06
(m, 2 H) ), 7.04-7.00 (m, 3 H). 13C NMR (75 MHz, CDCl3) δ
60 156.15, 154.56, 153.25, 149.73, 149.39, 149.18, 147.27, 147.24,
138.16, 136.00, 131.85, 131.45, 131.24, 130.86, 129.56, 125.45,
125.32, 124.76, 123.91, 123.85, 121.99, 121.41. HRMS (M+1) =
618.2673 (calc. 618.2658).
2.4 Synthesis
Synthesis of compound 3: The procedure was the same with
65 above except that 2,3-diaminopyridine was replaced by 2,3,6,7-
tetraamino-9,9-bis(2-ethylhexyl) fluorene (596 mg, 1.0 mmol).
This reaction gave compounds 3 (787 mg, yield 80%).
25 As shown in scheme 1, the starting material 4, 4'–bis (N, N–
diphenylamino) benzyl was prepared from TPA and oxalyl
chloride through Friedel–Crafts acylation.13 And compounds 1-3
are synthesized by condensing 4, 4'–bis (N, N–diphenylamino)
benzyl and different kinds of diamines and tetraamines including
30 diaminomaleonitrile, 2,3-diaminopyridine and 2,3,6,7-tetraamino-
9,9-bis(2-ethylhexyl) fluorene in acetic acid with a catalytic
amount of IBX.14 Note that the 2,3,6,7-tetraamino-9,9-bis(2-
ethylhexyl) fluorene cannot react with two equivalents of 4,4'-
bis(N,N-diphenylamino)benzil to form two pyrazine units due to
35 the steric hindrance. In fact, it only reacts with one equivalent of
4,4'-bis(N,N-diphenylamino)benzil and one equivalent of acetic
acid (solvent) to form compound 3 containing a pyrazine unit and
oxazole segment.15
1H NMR (300 MHz, CDCl3) δ 8.45 (s, 1 H), 8.13 (s, 1 H), 8.09
(s, 1 H), 7.72-7.69 (m, 1 H), 7.54-7.52 (m, 1 H), 7.51-7.44 (m, 5
70 H), 7.29 (d, 6 H), 7.14 (d, 8 H), 7.09-7.04 (m, 8 H). 13C NMR (75
MHz, CDCl3) δ 167.90, 164.67, 152.15, 149.17, 148.58, 147.46,
143.96, 141.77, 136.57, 132.62, 131.02, 129.50, 128.95, 125.11,
123.59, 122.32, 117.76, 111.66, 106.33, 68.31, 54.93, 46.33,
38.89, 34.92, 34.87, 33.66, 33.32, 32.07, 30.52, 29.84, 29.59,
75 29.51, 29.40, 29.24, 29.08, 28.05, 26.95, 26.67, 26.62, 24.90,
23.91, 23.13, 22.84, 22.77, 14.87, 14.26, 14.19, 13.98, 13.95,
11.11, 10.26. HRMS (M+1) = 984.5613 (calc. 984.5508).
80 Scheme 1 Synthesis route for compounds 1-3. Conditions: anhydrous acetic acid, Ar2, 2-iodoxybenzoic acid, 120°C, 24 h.
2
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3.2 Optical and electrochemical properties
3 Results and discussion
The maximum absorption wavelengths of compounds 1-3
show a trend of 3<2<1, reflecting the substituent nature. Fluorene
is a weak electron donor, while pyridine is a moderate electron
30 acceptor and cyano group is a strong electron acceptor. With the
enhancement of the role of the electron acceptor, the efficiency of
intramolecular charge-transfer (ICT) is improved, resulting a
bathochromic absorption shift. This trend is also manifested in
the emission characteristics. The emission intensity decreases
35 significantly with a concomitant red shift in the emission
wavelength from compound 3 to 1, which suggests that dipolar
interactions have a great contribution to the non-radiative decay
in these compounds.16 The lifetime of films of 1, 2 and 3 are
3.1 Crystal structures
The structures and molecular packing of compounds 1-3 are
shown in Figure 1 (CCDC number for compounds 1-3 are
5
1401901, 1401902, and 1401903, respectively. For compound 1,
the length of C– N bond in cyano group is 1.13–1.14 Å, which is
a typical C– N triple bond. The length of C–C bond connecting
cyano group to pyrazine ring (1.45 Å) is between the average
bond length of C– C single bond (1.54 Å) and aromatic C– C
10 bond (double bond 1.33 Å and aromatic 1.40 Å). For compounds
2 and 3, the length of C –C bond connecting pyrazine ring to
triphenylamine (1.48 Å) are also between the average bond length
of C –C single bond and aromatic C –C bond while the length of
C –N bond in triphenylamine unit (1.41– 1.43 Å) is close to the
15 average aromatic C –N bond length (C – N single bond 1.47 Å;
double bond 1.34 Å; aromatic 1.43 Å). These phenomena indicate
that strong donor-acceptor interactions exist in these molecules.
The lengths of C– N bond of pentagon in compound 3 are 1.281
and 1.400 Å, separately. Note that compound 2 forms dimer via
20 intermolecular N–H hydrogen bond. The adjacent molecules in
compound 1 adopt head-to-tail arrangement to reduce the electron
coupling effect due to the large molecule dipole moment.
measured by
a streak camera system (Table S2). The
40 experimental details are described in reference17. We can see that
the PL decay rate increases from 1, 2 to 3, and the lifetime (the
time at which the initial PL intensity (I0) decreases to the (I0/e))
are extracted to be 4.25, 3.27 and 2.12 ns for 1, 2 and 3,
respectively.
45
Figure 1 The crystal structures of compounds 1- 3 (a, c, e) and the crystal
25 packing diagrams of compounds 1-3 (b, d, f).
Figure 2 The UV-Vis (a) and fluorescence (b) spectra of compounds 1-3.
They are measured in dichloromethane with the concentration of 10-5M
and 10-6M, respectively.
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Table 1. The UV-Vis and fluorescence data of compounds 1-3
Abs λmax(nm) ε(M-1·cm-1)
Fluor λmax(nm)
фf1
1
2
3
468
430
428
39540
23540
46900
593
568
528
0.02
0.09
0.99
1
1
2
2
3
3
-2
-2
-1
-1
0
1
2
2
0
1
Figure 4 The dipole moment and molecular orbitals of compounds 1-3.
Table 3. The calculated data of compounds 1-3
Potential (V)
Potential (V)
Figure 3 The CV spectra of compounds 1-3.
HOMO (eV)
-5.25
LUMO (eV)
-2.49
Gap (eV)
2.76
Dipole moment (D)
11.92
5
As shown in Figure 3, for compound 1, one reversible
reduction wave and one irreversible reduction wave were
investigated, while for compound 2, only one reversible reduction
wave was found. Because the electron affiliation of cyano group
is stronger than that of pyridine, the reduction potentials of
1
2
3
-4.95
-2.04
2.91
1.0259
-4.85
-1.86
2.99
0.9586
10 compounds 1 are higher than that of compound 2.
Table 2. The CV data of compounds 1-3
30 3.4 OLED performance
Eox1/V Eox2/V Ered1/V Ered2/V HOMO LUMO
Eg (eV)
3.15
As shown in Figure 5, the turn-on voltages for the devices range
from 2.9 V to 3.6 V, which are within the normal range for
OLEDs. For doped emission layers, maximum brightness levels
around or higher than 10,000 cd/m2 were achieved. All the dyes
35 show good efficiencies (with EQE above 2 %). Among them,
compounds 1 and 2 show better performance. The maximum
EQE for compounds 1 and 2 are 4.01 % and 7.37 %, respectively,
both achieved at low doping concentration (less than 10 wt.%).
Especially for compound 2, the EQE is higher than the theoretical
40 limit for fluorescent emitters (5 %),18 indicating that the
1
0.52
1.50
-0.29
-1.62
-5.32
-2.17
2
3
0.53
0.35
1.01
0.89
-1.74
-
-
-
-5.33
-5.15
-1.92
-1.85
3.41
3.30
3.3 Theoretical calculation
The optimization for geometric structures of compounds 1-3 is
carried out by Gaussian 09 program at the B3LYP/6-31G* level.
15 The electronic distributions in the HOMO and LUMO for these
compounds are shown in Figure 4. The HOMOs of compounds 1
and 2 are delocalized over the whole molecule but the HOMO of
compound 3 are delocalized over the triphenylamine groups. The
LUMOs are delocalized on the cyano group, pyridine or
20 quinoxaline. With the acceptor strength increasing in the order
from compound 3 to compound 1, the donor-acceptor interactions
are enhanced gradually, which induce a higher dipole moment
and stabilize the HOMO and LUMO energy level. Since the
LUMO is more sensitive than the HOMO in our case, the band
25 gap becomes narrower.
mechanism for compound
2 is different from traditional
fluorescent dyes. The mechanism is still under investigation. The
concentration dependence of device EQE for the dyes also shows
some difference. For compounds 1 and 2, the concentration
45 quenching effect is much stronger than compound 3. This can
also inferred from the concentration dependent electroluminance
(EL) spectrum. (Figure S3-2) For compounds 1 and 2, their EL
spectra shows larger redshift compared with that of compound 3.
These results indicate that the introduction of bulk substituted
50 groups can reduce the aggregation caused quenching (ACQ)
effect by sacrificing the efficiency or raising the turn-on-
voltage.19
4
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Figure 5 The current density-voltage-luminance curves (a, c, e) and external quantum efficiency curves (b, d, f) of compounds 1-3
Table 4 OLED device characteristics of compounds 1-3
Max efficiency
Efficiency @ 100 cd/m2
Efficiency @ 1000 cd/m2
X
Von
Lmax
CIE (x, y)
(wt. %) (V)
(cd/m2)
EQE(%) CE (cd/A) PE (lm/W) EQE (%) CE (cd/A) PE (lm/W) EQE (%) CE (cd/A) PE (lm/W)
100
50
10
3
3.6
3.4
3.3
3.3
3.0
2.7
3.0
3.3
3.0
2.9
3.3
3.6
3270
4775
0.89
1.81
3.12
4.01
0.60
1.24
7.37
7.16
1.36
2.04
2.89
2.09
1.48
3.64
9.23
16.12
2.04
4.43
26.65
24.66
3.93
6.24
7.98
5.13
1.23
3.39
8.05
15.35
1.10
4.13
27.90
23.47
3.51
5.64
8.35
4.13
0.57
0.87
1.31
1.30
0.50
1.23
2.60
2.10
1.22
2.02
2.05
1.81
1.03
1.75
2.20
4.57
1.69
4.41
9.41
7.20
3.51
6.18
5.68
4.45
0.63
1.02
3.93
2.51
1.09
3.30
6.16
4.01
2.40
4.60
3.50
2.63
0.22
0.48
0.78
0.84
0.60
1.18
1.24
1.22
0.61
1.64
1.42
1.33
0.60
0.97
2.31
2.95
2.03
4.23
4.50
4.15
1.78
5.02
3.93
3.27
0.36
0.40
0.93
1.14
0.96
2.21
1.90
1.70
0.75
2.51
1.64
1.37
(0.59,0.40)
(0.56,0.43)
(0.49,0.50)
(0.40,0.57)
(0.41,0.56)
(0.39,0.58)
(0.33,0.61)
(0.29,0.61)
(0.24,0.55)
(0.22,0.58)
(0.19,0.51)
(0.17,0.44)
1
2
3
8202
11550
7916
100
50
10
3
10520
12650
12860
2378
100
50
10
3
11860
11120
10200
5 Von is turn-on voltage (the voltage when luminance reaches 1 cd/m2); Lmax is maximum luminance; EQE is external quantum efficiency; CE is current
efficiency; PE is power efficiency.
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50 W. Sun, Q. Zhang, J. Org. Chem. 2015, 80, 3030-3035; c) J. Li, Y. Zhao,
J. Lu, G. Li, J. Zhang, Y. Zhao, X. Sun, Q. Zhang, J. Org. Chem., 2015,
80, 109-113
4 Conclusion
In summary, using one–step reaction between 1, 2-diamine and 1,
2-diketones,
a
series of D–A molecules containing
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triphenylamine segment and pyrazine unit have been synthesized
and fully characterized. The modification of the pyrazine unit
with different substituted groups such as strong electron–acceptor
cyano groups, moderate electron–acceptor pyridine groups and
large steric functional group 9,9-bis(2-ethylhexyl) fluorine gave
different patterns of D–A compounds with different photoelectric
10 properties. The OLED devices based on these compounds exhibit
good efficiencies. In particular, CBP-doped devices using
compounds 1 and 2 as active layers show maximum external
quantum efficiency of 4.01 % and 7.37 %, respectively. These
results indicate that these compounds have good applications in
15 OLED devices.
5
65 5 F. Polo, F. Rizzo, M. Veiga-Gutierrez, L. De Cola, S. Quici, J. Am.
Chem. Soc., 2012, 134, 15402-15409.
Acknowledgements
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Q.Z. acknowledges financial support from AcRF Tier 1 (RG 16/12 and
RG 133/14) and Tier 2 (ARC 20/12 and ARC 2/13) from MOE, and the
CREATE program (Nanomaterials for Energy and Water Management)
20 from NRF.
Notes and references
1 School of Materials Science and Engineering, Nanyang Technological
University, Singapore 639798, Singapore.
* Correspondence to Q. Zhang, E-mail: qczhang@ntu.edu.sg
25 2School of Electrical and Electronic Engineering, Nanyang Technological
University, Singapore 639798, Singapore.
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3 Division of Physics and Applied Physics, School of Physical and
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