Novel pyrene derivatives: Synthesis, properties and highly efficient non-doped deep-blue electroluminescent device

Article abstract of DOI:10.1016/j.dyepig.2011.06.025

Two novel pyrene derivatives 1,6-bis(3,5-diphenylphenyl)pyrene (BDPP) and 1,6-bis(2-naphthyl)pyrene (BNP) were synthesized and characterized by ¹H NMR, ¹³C NMR, mass spectrum and elemental analysis. The compounds BDPP and BNP exhibit

Full text of DOI:10.1016/j.dyepig.2011.06.025

Dyes and Pigments 92 (2011) 732e736
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel pyrene derivatives: Synthesis, properties and highly efficient non-doped
deep-blue electroluminescent device
,
a
b
a
a,
*
Zhiqiang Wang a, Chen Xu a , Weizhou Wang , Xinming Dong , Bangtun Zhao , Baoming Ji
*
a College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, People’s Republic of China
b Department of Chemistry, Zhengzhou University, Zhengzhou 450052, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel pyrene derivatives 1,6-bis(3,5-diphenylphenyl)pyrene (BDPP) and 1,6-bis(2-naphthyl)pyrene
(BNP) were synthesized and characterized by 1H NMR, 13C NMR, mass spectrum and elemental analysis.
The compounds BDPP and BNP exhibit bright blue emission with high fluorescence quantum yields. The
quantum chemical calculations show that BDPP has a higher non-coplanar structure compared to BNP.
The electrochemical properties and energy levels of the compounds were investigated by cyclic vol-
tammetry. BDPP exhibits a high thermal stability with the decomposition temperature of 440 ꢀC and the
glass transition temperature of 139 ꢀC. The non-doped device based on BDPP achieves a very stable deep-
blue emission with a maximum efficiency of 3.26 cd/A. The 1931 CIE coordinates (0.15, 0.11) of this device
are very close to the National Television System Committee blue standard.
Received 21 February 2011
Received in revised form
8 June 2011
Accepted 20 June 2011
Available online 1 July 2011
Keywords:
Pyrene derivatives
Organic light emitting devices
Deep-blue emission
Luminescence
Ó 2011 Elsevier Ltd. All rights reserved.
Quantum chemical calculations
Thermal properties
1. Introduction
consumption of a full-color OLED is highly dependent upon the color
of blue emission, and the deeper the blue color (smaller CIE y -value)
Since the pioneering works on organic light emitting devices
(OLEDs) by the Kodak [1] and Cambridge groups [2], OLEDs have been
attracting considerable attention due to their potential application in
flat-panel displays, solid-state lighting sources and backlights for
liquid-crystal displays. For the full-color OLEDs, highly efficient and
stable three basic colors (red, green, and blue) emission are needed.
However, it is much more difficult to produce a high-performance
blue emission for its intrinsic characteristic of having a wide
bandgap irrespective of the type of materials [3]. Although the
electroluminescence (EL) efficiency can be significantly improved by
using dopant emitters, the addition of dopants implies additional
complexity and cost for the mass production of OLEDs. In addition to
efficiency and stability, color purity is another important essential for
OLEDs. However, the non-doped deep-blue OLEDs with the
Commission Internationale de l’Eclairage (CIE) coordinates matched
to the National Television System Committee (NTSC) standard (0.14,
0.08) are still rare [4]. In addition, it is well known that the power
is, the lower the power consumption of the device is [4]. Thus,
developing highly efficient and stable non-doped deep-blue OLEDs
continues to receive considerable attention [5e10].
Due to the large conjugated aromatic characteristic, pyrene not
only has high fluorescence efficiency, but also has high carrier
mobility. Recently, some pyrene derivatives have been used in blue
OLEDs in order to improve the hole-transporting ability for the
electron-rich characteristic of pyrene [11e13], but the perfor-
mances of those devices are not satisfactory in efficiency, stability
and color purity. In view of these findings and our continuous
interest in non-doped deep-blue OLEDs [4,14], we designed and
synthesized two novel pyrene derivatives BDPP and BNP. The
quantum chemical calculations were carried out to investigate
their stereostructures and electronic structures. The photophysical
properties, electrochemical properties and thermal properties of
the compounds were contrastively studied. The non-doped device,
ITO/PEDOT:PSS (30 nm)/NPB (30 nm)/BDPP (30 nm)/TPBI (30 nm)/
CsF (2 nm)/Mg:Ag, shows a stable deep-blue emission with high EL
efficiency. The 1931 CIE coordinates (0.15, 0.11) of this device are
very close to the National Television System Committee (NTSC)
blue standard.
* Corresponding authors. Tel./fax: þ86 379 65523821.
E-mail addresses: xubohan@163.com (C. Xu), lyhxxjbm@126.com (B. Ji).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.06.025

Z. Wang et al. / Dyes and Pigments 92 (2011) 732e736
733
2. Experimental
NMR (100 MHz, CDCl3)
d
(ppm): 139.01, 133.67, 132.76, 130.72,
129.53, 129.26, 129.10, 128.32, 128.20, 128.00, 127.97, 127.77, 126.58,
2.1. Chemicals and instruments
126.32, 125.57, 124.74; TOF-MS: m/z ¼ 454.48; Anal. Calcd. for
C
36H22: C, 95.12; H, 4.88; Found: C, 95.08; H, 4.86.
Commercially available reagents were used without purification
unless otherwise stated. 1H NMR and 13C NMR spectra were
recorded in CDCl3 solution on a Bruker Avance 400 spectrometer
with tetramethylsilane (TMS) as the internal standard. Elemental
analysis was performed on a Vario III elemental analyzer. Mass
spectrum was obtained on a Bruker Microflex spectrometer. The
absorption and photoluminescence spectra were recorded on
a Hitachi U-3010 UVeVis spectrophotometer and a Hitachi F-4500
fluorescence spectrophotometer, respectively. TGA and DSC
measurements were performed on a TA instrument TGA2050 and
TA instrument DSC2910, respectively, with a heating rate of 10 ꢀC/
min (0 ꢀC is corresponding to 273.15 K, 1 ꢀC ¼ 1 K) under the
nitrogen atmosphere. Cyclic voltammetry was performed on
a CHI620C electrochemical analyzer.
2.3. OLED fabrication and measurements
Indium-tin-oxide (ITO)-coated glass substrates were cleaned
with isopropyl alcohol and deionized water, then dried in an
oven over 393 K, and finally treated with UV-ozone. A 30 nm
(1 nm ¼ 1 ꢁ 10ꢂ9 m) thick film of poly (3, 4-ethylenedioxy
thiophene) doped with poly (styrene sulfonate) (PEDOT:PSS)
was spin-coated on the ITO glass substrates. After dried at 393 K
for 30 min under nitrogen atmosphere, the substrate was trans-
ferred to the vacuum deposition system with a base pressure of
<5 ꢁ 10ꢂ7 torr (1 torr ¼ 133.322 Pa). The device was fabricated by
evaporating organic layers onto the PEDOT layer sequentially
with an evaporation rate of 0.1e0.2 nm/s. The Mg:Ag alloy
cathode was prepared by co-evaporation of Mg and Ag at
a volume radio of 10:1. EL spectra and 1931 CIE color coordinates
were measured with a spectrascan PR650 photometer and the
current-voltage-luminance characteristics were measured with
a computer-controlled Keithley 2400 SourceMeter under ambient
atmosphere.
2.2. Synthesis
BDPP was synthesized by the Suzuki coupling reaction (Fig. 1). 1,6-
Dibromopyrene 1 (1.0 ꢁ 10ꢂ3 mol), 3,5-diphenylphenyl boronic acid 2
(2.2
ꢁ
10ꢂ3 mol) and tetrakis(triphenylphosphine)palladium
[Pd(PPh3)4] (0.12 ꢁ 10ꢂ3 mol) were mixed in toluene (20 ꢁ 10ꢂ6 m3),
then Na2CO3 (2 M, 5 ꢁ10ꢂ6 m3) and ethanol (5 ꢁ10ꢂ6 m3) were added
during stirring. The mixture was refluxed under the nitrogen atmo-
sphere for 24 h. After cooling, precipitate was collected by filtration
and purified by column chromatography (eluent ¼ dichloromethane/
hexane, 1:10 v/v). The product was recrystallized in toluene (yield:
3. Results and discussion
3.1. Quantum chemical calculations
To gain insight into the stereostructures and the electronic
structures of BDPP and BNP, quantum chemical calculations were
carried out by using the B3LYP/6-31G(d) method [15]. In BDPP,
the 3,5-diphenylphenyl substituents at the 1- and 6- positions
are twisted toward the pyrene backbone to a angle of 56.7ꢀ, and
the twist angles between adjacent benzene rings in 3,5-
diphenylphenyl group are 37.6ꢀ and 37.9ꢀ, respectively, which
indicates BDPP has a highly non-coplanar structure. The non-
coplanar structure can limit the intermolecular interactions and
facilitate the formation of stable amorphous thin films. In BNP,
the twist angle between 2-naphthyl substituents and pyrene
backbone is 54.9ꢀ, the lesser twist angle implies that BNP has
0.34 g, 52%). 1H NMR (400 MHz, CDCl3)
d
(ppm): 8.33 (2H, d, J ¼ 9.2),
8.25 (2H, d, J ¼ 7.6), 8.10 (4H, t, J ¼ 7.4), 7.95 (2H, s), 7.87 (4H, s), 7.77
(8H, d, J ¼ 7.6), 7.49 (8H, t, J ¼ 7.4), 7.40 (4H, t, J ¼ 7.2); 13C NMR
(100 MHz, CDCl3)
d (ppm): 142.48, 142.10, 141.19, 137.86, 130.79,
129.18, 129.08, 128.60, 128.02, 127.89, 127.78, 127.58, 125.55, 125.45,
125.21, 124.81; TOF-MS: m/z ¼ 658.74; Anal. Calcd. for C52H34: C,
94.80; H, 5.20; Found: C, 94.74; H, 5.22.
The synthesis of BNP is similar to that of BDPP, yield: 71%. 1H
NMR (400 MHz, CDCl3) d (ppm): 8.27 (2H, s), 8.25 (2H, s), 8.12 (2H,
s), 8.11 (1H, s), 8.09 (2H, s), 8.07 (2H, s), 8.04 (1H, s), 7.96e8.01 (4H,
m), 7.82 (1H, d, J ¼ 1.6), 7.78 (1H, d, J ¼ 1.7), 7.58e7.60 (4H, m); 13
C
Fig. 1. Synthetic route of BDPP and BNP.

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Z. Wang et al. / Dyes and Pigments 92 (2011) 732e736
larger
p-delocalization due to the more coplanar configuration
compared to BDPP. The highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) of
BDPP are almost concentrated on the pyrene backbone (Fig. 2a),
which implies that its emission and absorption are mostly
controlled by the pyrene unit. As shown in Fig. 2b, the 2-naphthyl
groups make a considerable contribution to the HOMO and
LUMO of BNP.
3.2. Photophysical properties
The absorption and photoluminescence (PL) spectra of BDPP and
BNP were measured in dilute dichloromethane solution (Fig. 3), the
corresponding data were summarized in Table 1. The absorption
spectra of BDPP and BNP exhibit the characteristic vibronic pattern
of the substituted pyrene group (lmax ¼ 361 nm for BDPP,
lmax
¼
365 nm for BNP). The additional absorption bands
(lmax ¼ 255 nm, 275 nm, 287 nm for BDPP, lmax ¼ 226 nm, 249 nm,
285 nm for BNP) can be assigned to the substituents at the 1- and 6-
positions of the central pyrene core. Upon excitation of the pyrene
moiety (at 361 nm for BDPP, at 365 nm for BNP), the PL spectra show
structureless emission peaks at 404 nm and 413 nm for BDPP and
BNP, respectively. It can be found that the absorption and PL spectra
of BNP show obvious red-shift compared to those of BDPP, which can
be attributed to the larger p-delocalization of BNP according to the
quantum chemical calculation results. The PL quantum yields in
dichloromethane solution were measured to be 0.82 for BDPP and
0.87 for BNP, respectively, by using 9,10-diphenylanthrancene as the
reference standard [16]. The PL spectra of the solid thin films red-
shifted 37 nm and 51 nm compared to those of dichloromethane
solution for BDPP and BNP, respectively. Whereas the fluorescence
emission of the films (lmax ¼ 441 nm for BDPP, lmax ¼ 464 nm for
BNP) are obvious shorter than that of the typical pyrene excimer
emission (480e500 nm) [17]. Thus, the red-shift is probably due to
the aggregation of pyrene moiety and the difference in dielectric
constant of the environment [13,18].
3.3. Electrochemical properties
Fig. 3. The absorption (--- in CH2Cl2 solution) and photoluminescence (-ꢃ- in CH2Cl2
solution, -:- solid thin film) spectra of BDPP (a) and BNP (b).
The electrochemical properties of BDPP and BNP were investi-
gated by cyclic voltammetry in CH2Cl2 solution containing tetra-
butylammonium hexafluorophosphate (0.1 M) with a scan rate of
0.05 V/s. The electrolytic cell was a conventional three-electrode
cell consisting of a Pt working electrode, a Pt wire counter
electrode and Ag/AgCl reference electrode. As shown in Fig. 4, both
BDPP and BNP exhibit reversible oxidation process with half-wave
oxidation potentials (Eo1x/2) of 1.29 V and 1.25 V (vs. Ag/AgCl),
respectively. The HOMO energy levels were calculated by using the
energy level value of ꢂ4.8 eV for ferrocene (Fc) with respect to zero
vacuum level [19e21]. The LUMO energy levels were estimated
according to the HOMO energy level values in combination with the
band gaps derived from the absorption band edges (Table 1).
Table 1
Key physical data of BDPP and BNP.
Compounds Absorption
(nm)
PL (nm)
HOMO LUMO Tg
(eV)
(eV)
(ꢀC)
Solutiona Filmb
BDPP
BNP
255, 275, 287,
361
226, 249, 285,
365
404
441
ꢂ5.62
ꢂ2.47
139
413
464
ꢂ5.58
ꢂ2.51
n.o.
n.o. ¼ Not observed.
a
In dilute dichloromethane solution.
Prepared by vacuum deposition (100 nm).
b
Fig. 2. Calculated stereostructures and HOMO and LUMO for BDPP (a) and BNP (b).

Z. Wang et al. / Dyes and Pigments 92 (2011) 732e736
735
Fig. 6. The relative energy level alignments of the BDPP based device.
Fig. 4. The cyclic voltammograms of BDPP (solid line) and BNP (dot line).
3.4. Thermal properties
The thermal properties of BDPP and BNP were investigated using
thermal gravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) with a heating rate of 10 ꢀC/min under the nitrogen
atmosphere. Both the two compounds exhibit high thermal stability,
the decomposition temperatures (Td) which correspond to a 3%
weight loss upon heating during TGA, are around 440 ꢀC and 391 ꢀC
for BDPP and BNP, respectively. DSC measurements were performed
from 25 ꢀC to 350 ꢀC under the nitrogen atmosphere. BDPP melted at
331 ꢀC (Tm) upon the first heating process, and then was rapidly
cooled by liquid nitrogen to form a amorphous glassy state. When
the glassy sample was heated again, a glass transition occurred at
139 ꢀC (Tg) (Fig. 5a). The high Tg of BDPP is presumably due to its non-
coplanar structure. Upon further heating beyond Tg, an exothermic
crystallization was observed at 210 ꢀC (Tc), and then an exothermic
solidesolid phase transition was observed at 286 ꢀC. For BNP, the Tm
appeared at 307 ꢀC, however, no glass transition was observed
(Fig. 5b).
3.5. Electroluminescent properties
For the higher performance of BDPP compared to BNP, we fabri-
cated the non-doped device based on BDPP with the configuration of
Fig. 5. The DSC and TGA (insert) curves of BDPP (a) and BNP (b).
Fig. 7. The EL spectra of the BDPP based device at different applied voltages.

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Z. Wang et al. / Dyes and Pigments 92 (2011) 732e736
0.11) and a maximum efficiency of 3.26 cd/A. The high efficiency
and stability of the device indicate that BDPP is a promising emit-
ting material for non-doped deep-blue OLEDs.
Acknowledgment
This work was supported financially by the National Natural
Science Foundation of China (NO.20872057, 20902043 and
20872058).
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ITO/PEDOT:PSS (30 nm)/NPB (30 nm)/BDPP (30 nm)/TPBI (30 nm)/CsF
(2 nm)/Mg:Ag. ITO (indium-tin-oxide) and CsF/Mg:Ag are the anode
and the cathode, respectively; PEDOT:PSS is the hole injection layer;
4,40-bis[N- (1-naphthyl)-N-phenyl amino] biphenyl (NPB) is the hole-
transporting layer; 1,3,5-tris(N-phenylbenzimidazol)benzene (TPBI)
is used as the electron transporting and hole blocking layer; BDPP was
used as the emitting layer. The relative energy level alignments of the
device are shown in Fig. 6.
Fig. 7 shows the normalized EL spectra of this device at different
applied voltages. The emission peaks are all at 451 nm, and the EL
spectra show no emission at longer wavelength from the exciplex
species, which could be attributed to the non-coplanar structure of
BDPP because of introducing the bulky 3,5-diphenylphenyl
substituents. The 1931 CIE coordinates (0.15, 0.11) of this device
are very close to the NTSC blue standard. While increasing the
applied voltage from 5 V to 10 V, the EL spectra and the CIE coor-
dinates show very little change, which is highly desirable for OLEDs.
The current density-voltage-luminance characteristics of the
device are shown in Fig. 8. The turn-on voltage (at a luminance of
1.0 cd/m2) is about 4.5 V, and the maximum luminance is 5184 cd/
m2 at a voltage of 11 V with a current density of 473 mA/cm2. The
device achieves a maximum efficiency of 3.26 cd/A at 6 V with
a current density of 24.5 mA/cm2 and a luminance of 798 cd/m2.
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OLEDs based on pyrene derivatives [13,14]. The high EL efficiency
may be attributed to the high fluorescence quantum yield and non-
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barriers, 0.22 eV at NPB/BDPP interface and 0.23 eV at BDPP/TPBI
interface (Fig. 6), can facilitate the electron and hole injection into
the emitting layer and result in a high EL efficiency.
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4. Conclusions
In conclusion, two novel pyrene derivatives BDPP and BNP were
synthesized and characterized. Both the compounds BDPP and BNP
exhibit bright blue emission with high fluorescence quantum
yields. The quantum chemical calculations, photophysical proper-
ties and thermal properties show BDPP has a higher performance
compared to BNP. The non-doped device based on BDPP exhibits
very stable deep-blue emission with the CIE coordinates of (0.15,