Novel red-emitting thieno-[3,4-b]-pyrazine derivatives suitable for vacuum evaporation and solution method to fabricate non-doped OLEDs

Article abstract of DOI:10.1016/j.dyepig.2011.05.029

Two novel red-emitting thieno-[3,4-b]-pyrazine-cored molecules with phenyls (TP) or polyphenyls (Müllen type dendron, DTP) as peripheral groups were designed and synthesized. They have large Stokes shifts over 100 nm. DTP is thermally stable with decompos

Full text of DOI:10.1016/j.dyepig.2011.05.029

Dyes and Pigments 92 (2011) 674e680
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel red-emitting thieno-[3,4-b]-pyrazine derivatives suitable for vacuum
evaporation and solution method to fabricate non-doped OLEDs
,
Qing Li a, Jiuyan Li b, Ruixia Yang a, Lijun Deng b, Zhanxian Gao a, Di Liu a
*
a School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
b State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel red-emitting thieno-[3,4-b]-pyrazine-cored molecules with phenyls (TP) or polyphenyls
(Müllen type dendron, DTP) as peripheral groups were designed and synthesized. They have large Stokes
shifts over 100 nm. DTP is thermally stable with decomposition temperature up to 458 ꢀC. More
importantly, it is amorphous with a remarkably high glass transition temperature of 262 ꢀC. DTP can be
made into thin films either by solution method or vacuum evaporation. Red OLEDs were fabricated using
either spin coated or vacuum evaporated DTP film as emitting layer. The evaporated device exhibited
a maximum brightness of 1753 cd mꢁ2 and a luminous efficiency of 0.74 cd Aꢁ1, which are among the
best data ever reported for thieno-[3,4-b]-pyrazine derivatives so far. In contrary, TP failed to produce
satisfied red emission in its evaporated OLEDs.
Received 11 April 2011
Received in revised form
31 May 2011
Accepted 31 May 2011
Available online 15 June 2011
Keywords:
Thieno-[3,4-b]-pyrazine
Red
Organic light-emitting diodes
Solution processing
Vacuum evaporation
Large Stokes shift
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
light output. As far as we know, however, only few thieno[3,4-b]
pyrazine based materials have been developed for application as
Organic light-emitting devices (OLEDs) have been drawing
broad attentions due to their practical applications in both large-
area flat-panel displays and solid-state lighting. For light-emitting
materials used in OLEDs, to possess a large Stokes shift is usually
an essential merit if an efficient light output from the device is
desired, since otherwise the severe self-absorption due to spectral
overlap between the absorption and emission will definitely be
unfavorable to light output [1]. Among the three primary color
light-emitting materials and devices, the red one still lag behind in
terms of luminescent efficiency and lifetime [2e4]. It is strongly
desired that, therefore, highly efficient red emitters with good
merits such as a large Stokes shift are developed for application in
OLEDs. Thieno-[3,4-b]-pyrazine is a typical red fluorophore which
is characterized by pure and saturated red emission and large
Stokes shifts over 110 nm. There are two major absorption bands for
this chromophore, one narrow but strong band centered at 320 nm
and another broad but weak one covering from 450 to 530 nm. The
weak absorption at longer wavelength range has a tiny overlap with
the fluorescence, leading to negligible self-absorption and efficient
emitters in OLEDs to date [4].
The thieno-[3,4-b]-pyrazine chromophore has a nearly planar
conformation due to
p conjugation of the two aromatic rings. It is well
known that strong intermolecular interaction exists in the solid states
of organic molecules, especially of the planar molecules, which usually
results in unwanted effects such as fluorescence quenching and
spectral contamination. Therefore, in order to suppress the intermo-
lecular interaction to gain pure red emission with satisfactory
performance in the solid state, it is necessary to decorate the flat
thieno-[3,4-b]-pyrazine chromophore with bulky functional groups so
that it can act as efficient solid emitter. Based on the chemical structure
of thieno-[3,4-b]-pyrazine, the modification groups can be easily
grafted to its 2,3,5,7-positions. The molecules designed in this way
should have three-dimension non-planar conformation with the
planar emissive core encapsulated by the surrounding bulky groups.
In this paper, we report the design and synthesis of two novel
thieno-[3,4-b]-pyrazine based molecules, TP and DTP (Scheme 1).
For TP, Four phenyls were attached to the 2,3,5,7-positions of the
thieno-[3,4-b]-pyrazine emissive core as the periphery groups.
While for DTP, much larger tetraphenylphenylenes (i.e., polyphenyl,
Müllen type dendron) were introduced through a phenylene bridge
into the 2,3,5,7-positions of thieno-[3,4-b]-pyrazine core as bulky
arms, in order to protect the core via site-isolation effect, and to
* Corresponding author. Tel./fax: þ86 411 84986233.
E-mail address: liudi@dlut.edu.cn (D. Liu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.029

Q. Li et al. / Dyes and Pigments 92 (2011) 674e680
675
workstation (BAS100B, USA) at a scan rate of 50 mV sꢁ1. A glass carbon
working electrode, a Pt-wire counter electrode, and a saturated
calomel electrode (SCE) as reference electrode were used. All
measurements were made at room temperature on samples dissolved
S
S
in CH2Cl2, with 0.1 M tetra-n-ethylammonium tetrafluoroborate as
onset
N
N
N
N
supporting electrolyte. From the values of the onset oxidation (Eox
)
and reduction potentials (Eredonset) relative to the saturated calomel
electrode (SCE) electrode, the energies of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) as well as the electrochemical band gaps (EgCV) of DTP
were calculated according to the equations below (Eqs. (1)e(3))
[11e13]:
TP
À
Á
DTP
onset
ox
HOMO ¼ ꢁe E
þ 4:4 ½eVꢂ
(1)
(2)
(3)
Scheme 1. Chemical structures of TP and DTP.
À
Á
onset
red
LUMO ¼ ꢁe E
þ 4:4 ½eVꢂ
generate the thermally and morphologically stable molecule due to
the excellent stability of this type of group [5,6]. In addition, the
presence of these periphery groups helps to provide significant
molecular weight and good solubility, so that the resultant DTP is
capable of being processed by solution methods to form thin films
and OLEDs. To be suitable for solution processing is well accepted as
one essential merit for light-emitting materials since the solution
processing is the most favorable fabrication technique of OLEDs for
practical applications in low-cost, large-area flat-panel displays and
solid-state lighting [7,8]. In comparison with TP, the DTP molecule is
expected to have the advantageous features including high fluores-
cent quantum yield in solid state with large Stokes shift, being
suitable for solution processing, and good thermal stability for vapor
deposition for non-doped OLEDs. The photophysical, electro-
chemical and electroluminescent properties of TP and DTP are
investigated. The results show that DTP is a more efficient solid
emitter in comparison with TP. More importantly, DTP has the
flexibility to be processed either by solution method such as spin
coating or by vacuum evaporation to fabricate OLEDs. A brightness of
428 cd mꢁ2 was obtained for the spin coated OLEDs. While for the
OLED in which all organic layers were vacuum evaporated, the hole
transporting layer was utilized and the device performance was
greatly improved with a maximum brightness of 1753 cd mꢁ2 and
a luminous efficiency of 0.74 cd Aꢁ1. This performance is among the
best data ever reported for thieno-[3,4-b]-pyrazine derivatives so far.
À
EgCV ¼ e E
onset
Á
onset
red
ꢁ E
½eVꢂ
ox
2.3. OLED fabrication and measurements
The pre-cleaned ITO glass substrates were treated by UV-ozone
for 20 min. A 40 nm thick PEDOT:PSS film was first deposited on the
ITO glass substrates, and baked at 120 ꢀC for 40 min in air. In type I
device, the emitting layer were spin coated from the solution of
DTP in chlorobenzene on the top of PEDOT:PSS film. In type II
device, the DTP, TP film and other organic layers were deposited by
vacuum evaporation in a vacuum chamber with a base pressure less
than 10ꢁ6 Torr. Finally a thin layer of LiF (1 nm) and 100 nm of Al
were vacuum deposited on top of organic layers as cathode.
PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfo-
nate)) acts as the hole injecting and transporting layer, NPB (4,40-bis
[N-(1-naphthyl)-N-phenylamino]biphenyl) acts as the hole trans-
porting layer, TPBI (2,20,200-(1,3,5-benzenetriyl)tris(1-phenyl-1H-
benzimidazole))as the electron-transporting layer, ITO (indium tin
oxide) and LiF/Al as anode and cathode respectively. The film
thickness in above configuration is chosen after optimization. The
EL spectra, CIE coordinates, and currentevoltageeluminance
characteristics were measured with computer-controlled Spec-
trascan PR 705 photometer and a Keithley 236 source-measure-
unit. All the measurements were carried out at room temperature
under ambient conditions.
2. Experimental
2.1. Materials and instruments
2.4. Synthesis of compounds
All the chemicals and reagents for the synthesis are of analytical
grade and used as received from commercial sources without further
purification. 1H NMR spectra were recorded on a Bruker Avance II
(400 MHz) and Varian INOVA spectrometer (400 MHz). Mass spectra
were recorded on a GC–TOF-MS (Micromass, UK) mass spectrometer
for TOF-MS-EI, a MALDI micro MX (Waters, USA) for MALDI-TOF-MS
and HP 1100 LC-MSD (USA) mass spectrometer. The fluorescence
and UVevis absorption spectra measurements were performed on
a PerkineElmer LS55 fluorescence spectrometer and a PerkineElmer
Lambda 35 UVeVisible spectrophotometer, respectively. The fluores-
cence quantum yields were determined in CH2Cl2 or toluene solutions
2,5-dibromo-3,4-dinitrothiophene (1): Concentrated sulfuric acid
(100 ml) and fuming sulfuric acid (30% SO3, 100 ml) were combined
in a flask equipped with a mechanical stirrer and cooled with an ice
bath. 2,5-dibromothiophene (53.8 g, 0.222 mol) was then added at
a temperature below 20 ꢀC. Concentrated nitric acid (35 ml) was
then added carefully to keep the temperature under 30 ꢀC. Once the
addition was completed, the mixture was allowed to react for an
additional 3 h and then poured over 800 g of ice. Upon the melting
of the ice, the solid residue was recovered by vacuum filtration and
washed well with water to produce a light yellow powder. Recrys-
tallization with methanol gave 69.3 g (94%) of analytically pure
material; Mp. 135.8e136.7 ꢀC.
2,5-Bis-phenyl-3,4-dinitro-thiophene (2a) and 2,5-Bis-(4-bromo-
phenyl)-3,4-dinitro-thiophene (2b): To a mixture of compound 1
(5 g, 15 mmol), phenylboronic acid or 4-bromophenylboronic acid
(33 mmol) and [Pd(PPh3)4] (450 mg, 375 mmol) were added under
nitrogen 250 ml toluene, 50 ml methanol and 65 ml K2CO3 aqueous
(2 M) solution. The reaction mixture was refluxed for 8 h and then
against rhodamine B as the standard (
Melting points (Mp.) were recorded on a WRS-1B digital melting point
F
¼ 0.97 in ethanol) [9,10].
instrument (Shanghai Precision and Scientific Instrument Co.)
2.2. Cyclic voltammetry measurements
Electrochemical measurements were performed by using
a
conventional three-electrode configuration and an electrochemical

676
Q. Li et al. / Dyes and Pigments 92 (2011) 674e680
diluted with water. The organic layer was separated, and the
aqueous layer was extracted with CH2Cl2 (3 ꢃ 50 ml). After washed
with brine (1 ꢃ50 ml) and dried over MgSO4, the CH2Cl2 extraction
was combined with the original organic reaction solution. The
resulted organic solution was evaporated under reduced pressure
and the residue was purified by column chromatography over silica
gel with mixed petroleum ether and CH2Cl2 (2:1 v/v) as eluent to
give yellow solid. 2a: yield 37%, was used directly for synthesis of
3a.
7.37e7.33 (m, 8H, Ar-H); MS (TOF-MS-EI, m/z): Calcd. for C30H20N2S,
440.13; found 440.13. Elemental anal. calc. for C30H20N2S: C, 81.79; H,
4.58; N, 6.36. Found: C, 81.34; H, 4.63; N, 6.21.
4: yield 84.3%. 1H NMR (400 MHz, CDCl3):
d
¼ 8.13 (d, 4H,
J ¼ 7.6 Hz; Ar-H), 7.63 (d, 4H, J ¼ 8.0 Hz; Ar-H), 7.53 (d, 4H,
J ¼ 8.0 Hz; Ar-H), 7.40 (d, 4H, J ¼ 7.6 Hz; Ar-H); MS (TOF-MS-EI, m/
z): Calcd. for C30H16Br4N2S, 751.78; found 751.78.
2,3-Bis-(4-(trimethylsilylethynyl)phenyl)-5,7-bis-(4-(trimethylsilyl)
ethynylphenyl)-thieno[3,4-b]pyrazine (5): Compound 4 (500 mg,
0.67 mmol) was dissolved in a degassed mixture of triethylamine
(20 mL) and absolute THF (10 mL) under nitrogen. [PdCl2(PPh3)2]
(93 mg, 5 mol%), CuI (63 mg, 0.33 mmol), and PPh3 (86 mg,
0.33 mmol) were then added under a flow of nitrogen. After the
flask was sealed with a septum, trimethylsilylethyne (0.45 ml) was
injected. The reaction mixture was stirred at 45 ꢀC for 48 h, and
then poured into an equal volume of CH2Cl2 and filtered. The
solvent was removed and the crude product was purified by
column chromatography over silica gel with mixed petroleum
ether and CH2Cl2 (2:1 v/v) as eluent to give red solid (400 mg,
2b: yield 41.2%. 1H NMR (400 MHz, CDCl3):
d
¼ 7.65 (d, J ¼ 8.0 Hz,
4H; Ar-H), 7.39 (d, J ¼ 8.0 Hz, 4H; Ar-H); MS (TOF-MS-EI, m/z):
Calcd. for C16H8Br2N2O4S: 481.86; found 481.86.
2,5-Bis-phenyl-3,4-thiophene-3,4-diamine (3a) and 2,5-Bis-(4-
bromo-phenyl)-3,4-thiophene-3,4-diamine (3b): To a suspend of 2a
or 2b (10 mmol) and tin powder (9.4 g, 800 mmol) in ethanol
(150 ml) was added concentrated HCl acid and the mixture was
subsequently stirred under nitrogen at 50 ꢀC for 6 h. The homo-
geneous solution was poured into ice water and made alkaline with
aqueous NaOH. The organic layer was separated, and the aqueous
layer was extracted with CH2Cl2 (3 ꢃ 50 ml) and the dried extrac-
tion was combined with the original organic solution. The organic
mixture was evaporated under reduced pressure. The white solid
was recrystallized from CH2Cl2/petroleum ether (2:1 v/v) to yield
an analytically pure sample. 3a: yield 80.2%, not characterized.
3b: yield 85.2%. MS (TOF-MS-EI, m/z): Calcd. for C16H12Br2N2S,
421.91; found 421.91.
73.4%). 1H NMR (400 MHz, CDCl3):
d
¼ 8.23 (d, 4H, J ¼ 8.4 Hz; Ar-H),
7.56 (d, 4H, J ¼ 8.4 Hz; Ar-H), 7.46e7.41 (q, 8H; Ar-H), 0.278 (s, 18H;
CH3), 0.271(s, 18H; CH3); MS (MALDI-TOF, m/z): Calcd. for
C50H52N2SSi4, 824.29; found 824.18.
2,3-Bis-(4-ethynyl-phenyl)-5,7-bis(4-ethynyl-phenyl)-thieno[3,4-
b]pyrazine (6): Compound 5 (400 mg, 0.48 mmol) and NH4F
(143 mg, 3.9 mmol) were dissolved in THF (10 mL) under nitrogen.
A solution of n-Bu4NF (19 mg, 0.07 mmol) in THF (5 mL) was added
by injection. The mixture was stirred at room temperature for 2 h.
Then the solvents were removed in vacuum, and the crude product
was purified by column chromatography over silica gel with mixed
petroleum ether and CH2Cl2 (3:1 v/v) as eluent to give a red solid 6
2,3-Bis-phenyl-5,7-bis-phenyl-thieno[3,4-b]pyrazine (TP) and 2,3-
Bis-(4-bromo-phenyl)-5,7-bis(4-bromo-phenyl)-thieno[3,4-b]pyrazine
(4): Compound 3 (1 mmol) and 1,2-bis-phenyl-ethane-1,2-dione or
1,2-bis-(4-bromo-phenyl)-ethane-1,2-dione (1 mmol) were dis-
solved in dry CHCl3 (60 ml) and a catalytic amount of p-toluene-
sulfonic acid was added. The mixture was stirred at room
temperature overnight. The red solid were collected by filtration and
dried under vacuum. Recrystallization with CHCl3/petroleum ether
(3:1 v/v) gave analytically pure material.
(255 mg, 98%). 1H NMR (400 MHz, CDCl3):
J ¼ 8.4 Hz; Ar-H), 7.61 (d, 4H, J ¼ 8.4 Hz; Ar-H), 7.48 (s, 8H; Ar-H),
3.19 (s, 2H; C^CH), 3.17(s, 2H; C^CH); MS (TOF-MS-EI, m/z):
Calcd. for C38H20N2S, 536.13; found 536.13.
d
¼ 8.27 (d, 4H,
TP: yield 65.1%. 1H NMR (400 MHz, CDCl3):
J ¼ 8.0 Hz; Ar-H), 7.56 (d, 4H, J ¼ 8.0; Ar-H), 7.51 (t, 4H; Ar-H),
d
¼ 8.3 (d, 4H,
DTP: Compound 6 (200 mg, 0.37 mmol) and 7 (1.79 mmol) in
10 ml o-xylene were stirred and refluxed for 72 h under nitrogen.
R
R
R
S
R
C
S
Br
Br
B
S
A
S
Br
Br
NH2
H2N
NO2
O2N
NO2
O2N
3a: R=H
3b: R=Br
2a: R=H
2b: R=Br
1
D
S
Si
Si
R
R
S
S
E
F
N
N
N
N
N
N
R
R
Si
Si
6
4 :Br=R
5
R=H: TP
O
7, G
DTP
7
Scheme 2. Synthetic routes for TP and DTP. Conditions and reagents; A) fuming H2SO4, conc. HNO3, r.t.; B) phenylboronic acid or 4-bromophenylboronic acid, Pd(PPh3)4, K2CO3,
toluene, MeOH, reflux, N2; C) Sn, conc. HCl, EtOH, 50 ꢀC, N2; D) p-toluenesulfonic acid, dry CHCl3, 1,2-diphenylethane-1,2-dione or 1,2-bis(4-bromophenyl)ethane-1,2-dione, r.t.; E)
NEt3, PdCl2(PPh3)2, PPh3, CuI, THF, trimethylsilylethyne, 45 ꢀC, N2; F) NH4F, n-Bu4NF, THF, r.t., N2; G) o-xylene, compound 7, reflux, N2.

Q. Li et al. / Dyes and Pigments 92 (2011) 674e680
677
matrix-assisted laser desorption ionization time-of-flight (MALDI-
TOF) mass spectrometry. Based on the good solubility and the
significant molecular weight of DTP, good-quality thin films (Fig. S1
in Supporting information) can be formed by spin coating its chlo-
robenzene solution, which makes it possible to use wet method for
DTP to fabricate OLEDs.
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
TP
DTP
3.2. Photophysical properties
The photophysical properties of TP and DTP were examined by
UVevis absorption and photoluminescence (PL) spectra in CH2Cl2
and toluene solutions. As shown in Fig. 1, they exhibit two major
300 350 400 450 500 550 600 650 700 750
electronic absorption bands:
pe
p* transition at 345e350 nm and
Wavelength (nm)
charge transfer (CT) transition at 515e520 nm [4,8]. It is obvious
that the absorption spectra of DTP exhibits a red shift of 30 nm
compared with that of the reference compound TP, which should
Fig. 1. UVevis absorption and photoluminescence (PL) spectra of TP and DTP in CH2Cl2
solutions (10ꢁ5 mol Lꢁ1
lexc ¼ 470 nm).
,
be assigned to the extended
p conjugation in DTP molecules. It
should be noted that the absorption intensity of the long-wave-
length band is much lower than the short wavelength range for
both thienopyrazine-based compounds. As illustrated by the
emission spectra in Fig. 1 and the data in Table 1, DTP transmits red
fluorescence in both non-polar solvents such as toluene and polar
solvents such as CH2Cl2 upon photoexcitation at room temperature.
The structureless emission spectra remains unaltered irrespective
of the excitation wavelength, which is possibly due to an efficient
relaxation to the lowest excited state [4]. This CT emission exhibits
a moderate positive solvatochromism. For instance, emission peak
of DTP moves from 613 nm in toluene to 623 nm in CH2Cl2. In dilute
solutions of CH2Cl2, the parent compound TP exhibits orange red
fluorescence with the emission peak at 605 nm, while the emission
spectra for DTP is red shifted with a peak at 623 nm. Furthermore,
the full width at half maximum (FWHM) in the PL spectra is 60 and
93 nm for DTP and TP, respectively. The observed spectra narrow-
ing for DTP should be attributed to the fact that the intermolecular
interaction is dramatically eliminated due to the site-isolation
effect of the bulky polyphenyl arms. It is evident that both the
red shift and the spectral narrowing due to the introduction of the
polyphenyl groups are definitely favorable to obtain the pure and
saturated red fluorescence for DTP. The parent compound TP
exhibits a moderate fluorescent quantum yield of 60.5% in toluene.
However, it decreases to 28.4% in CH2Cl2. This dipolar quenching
phenomena is frequently observed for organic compounds due to
their interaction with a polar solvent [4]. While DTP shows a fluo-
rescent quantum yield of 43.6% in dilute toluene solution. It is
reasonable that the fluorescent quantum yield of DTP is slightly
decreased in comparison with TP, since the non-irridiative decay
efficiency is increased due to the presence of more chemical groups
within this molecule. The decrease in fluorescence quantum yield
for DTP will be acceptable if the comprehensive properties of this
material are taken into account.
After cooling to room temperature, the solvent was evaporated
under reduced pressure. The crude product was purified by column
chromatography over silica gel with mixed petroleum ether and
CH2Cl2 (3:2 v/v) as eluent and then recrystallization to give pure
product as red powder, yield 77.5%. 1H NMR (400 MHz, CDCl3):
d
¼ 8.05 (d, 4H, J ¼ 8.0 Hz; Ar-H), 7.62 (d, 4H, J ¼ 12.0 Hz; Ar-H),
7.25e7.23 (d, 4H, J ¼ 8.0 Hz; Ar-H), 7.17 (d, 20H, J ¼ 4.0 Hz; Ar-H),
7.09 (d, 4H, J ¼ 8.0 Hz; Ar-H), 6.97e6.92 (m, 20H; Ar-H), 6.91e6.82
(m, 32H; Ar-H), 6.80e6.77 (m, 12H; Ar-H); MS (MALDI-TOF, m/z):
Calcd. for
C150H100N2S, 1960.76; found 1961.1299, 1962.1105,
1963.1215. Elemental anal. calc. for C150H100N2S: C, 91.80; H, 5.14; N,
1.43. Found C, 91.42; H, 5.26; N, 1.35.
3. Results and discussion
3.1. Synthesis
The synthetic procedures for TP and DTP are shown in Scheme 2.
The thieno[3,4-b]-pyrazine emissive core was easily formed by
cyclizative condensation of 2,5-diaryl-3,4-diaminothiophene and
1,2-diarylethane-1,2-dione in the presence of p-toluenesulfonic
acid. The four terminal ethynyl groups were grafted at the periphery
of the key intermediate 6 to serve as the reaction sites to introduce
the polyphenyl arms. The polyphenyl groups were constructed
through DielseAlder cycloaddition of tetraphenylcyclopentadie-
nones 7 to the terminal ethynyl groups in 6. The DielseAlder
cycloaddition was carried out for a significantly long time under an
atmosphere of nitrogen to ensure the complete conversion of all
ethynyl groups and thus a high yield (77.5%) of the product. DTP is
well soluble in common organic solvents such as CH2Cl2, tetrahy-
drofuran, ethylacetate, so that it was easily isolated and purified by
column chromatography and recrystallization to reach an excellent
purity for OLEDs application. In contrast, TP is not as easily soluble
as DTP in same solvent. The chemical structure and the mono-
dispersity of DTP were verified using 1H NMR spectroscopy, and
As shown in Fig. 1, there is little overlap between the absorption
and emission spectra of these thieno-[3,4-b]-pyrazine derivatives.
Calculated from the positions of the long-wavelength absorption
maximum and the fluorescence maximum, the Stokes shifts for
Table 1
Photophysical and electrochemical data of TP and DTP.
labs (nm)
lem (nm)
F
(%)a
Stokes shift (nm)
Toluene/CH2Cl2
HOMO/LUMO (eV)
Egoptb (eV)
EgCVc (eV)
Toluene/CH2Cl2
Toluene/CH2Cl2
Toluene/CH2Cl2
TP
DTP
323, 488/320, 482
350, 519/346, 518
597/605
613/623
60.5/28.4
43.6/25.1
109/123
94/105
e/ꢁ3.18
ꢁ5.41/ꢁ3.42
2.04
1.99
a
Related to rhodamine B as the standard (
F
¼ 0.97 in ethanol).
Optical band gap, determined from absorption onset.
Electrical band gap, determined by subtracting LUMO energy from that of HOMO.
b
c

678
Q. Li et al. / Dyes and Pigments 92 (2011) 674e680
these compounds are as large as 105e123 nm in CH2Cl2. Such
a large Stokes shift is especially valuable for light-emitting mate-
rials used in non-doped OLEDs since each emissive molecule is
densely surrounded in the neat film and the absence of self-
absorption will definitely facilitate efficient light output from the
device.
TP
DTP
3.3. Thermal properties
The thermal properties of TP and DTP were examined by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) at a scanning rate of 10 ꢀC minꢁ1. The TGA and DSC
thermograms for DTP are illustrated in Figs. S2 and S3 in
Supporting information. In TGA measurement, the decomposition
temperature of DTP was determined as 506 ꢀC, indicating the
excellent thermal stability of this compound. This is an essential
merit for organic light-emitting materials especially when they are
used under high temperature. The thermal stability is suggested to
benefit from the presence of the polyphenyl groups, which is
famous for its excellent thermal and chemical stabilities [14]. The
morphological stability of DTP was monitored by DSC measure-
ment. When a powder of DTP obtained from organic solvent is
heated for the first run, an endothermic peak at 262 ꢀC was first
observed, which should be ascribed to the glass transition process.
Upon further heating beyond this glass transition temperature (Tg),
an exothermal peak was detected at 288 ꢀC (Tc), which is assigned
to the crystallization. With further increasing the temperature, the
crystal state melts into the isotropic liquid at 444 ꢀC (Tm). The
isotropic liquid was rapidly cooled into the glassy state. In the
second heating run, the glass transition was repeated at the same
temperature. The detection of the glass transition in the first
heating run implies that DTP is quite ready to form the glassy state
even when it is obtained directly from organic solvents. This is in
contrast to most organic compounds that usually crystallize when
they separate out from liquid solvents. Such a high Tg of 262 ꢀC
indicates a high amorphous stability of DTP in solid state, which
should benefit from the non-planar configuration of this molecule
due to the presence of the bulky polyphenyl groups [15]. A high Tg is
one essential merit for light-emitting materials used in OLEDs since
a morphologically stable amorphous organic layer usually leads to
a longer lasting OLED [12]. In principle, all the organic layers
forming the OLED devices should have Tg as high as possible. The
individual layer that has the lowest Tg is likely to limit the thermal
stability of the OLED. In comparison with DTP, the reference
compound TP has a much lower melting point (Tm) of 263 ꢀC.
Evidently the introduction of the polyphenyl arms in DTP molecule
resulted in the excellent thermal and amorphous stability of this
material. Due to the excellent thermal stability, it is observed that
DTP is suitable for vacuum evaporation to form good-quality thin
films at a proper temperature without decomposition.
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
Potential (V, vs SCE)
Fig. 2. Cyclic voltammograms of TP and DTP (scan rate 50 mV Sꢁ1, in CH2Cl2).
safe to assign the reduction to the thieno-[3,4-b]-pyrazine core and
onset
the oxidation to the polyphenyl groups. From the values of Eox
and Eredonset, the energies of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) as
CV
well as the electrochemical band gaps (Eg
) of DTP were
calculated.
The optical band gap (Egopt) of DTP was also determined by
opt
absorption edge technique for its film. The Eg was calculated as
CV
2.04 eV, which is quite close to the electrochemical band gap Eg
.
The consistence of this parameter obtained from two different
methods implies the high accuracy of these experimental methods.
All the electronic data are listed in Table 1.
3.5. Electroluminescence
In order to evaluate the electroluminescent properties, TP and
DTP were used as the neat emitting layer (EML) to fabricate non-
doped OLEDs. Based on the excellent solubility of DTP in common
organic solvents, high quality neat films without pinholes can be
obtained by spin coating its solution in chlorobenzene. Therefore,
two types of devices were fabricated by spin coating the DTP layer:
ITO/PEDOT:PSS (40 nm)/DTP (25 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF
(1 nm)/Al (100 nm) (device A), and ITO/PEDOT:PSS (40 nm)/DTP
(25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm) (device B). In device A,
PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrene sulfo-
nate)) was used as the hole injecting layer, BCP (2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline) as hole-blocking layer, Alq3 (tris(8-
hydroxyquino) aluminum) as the electron-transporting layer. For
performance optimization purpose, TPBI (1,3,5-tris[N-(phenyl)
benzimidazole]-benzene) was used as both hole-blocking and
electron-transporting layer in device B. These two OLEDs were
fabricated by spin coating the PEDOT:PSS and DTP layers and then
thermally evaporating other organic layers and cathode in a vacuum
chamber.
Both device A and B exhibited saturated red electrolumines-
cence (EL) with emission peak at 646 nm. The EL spectra of both
devices are identical to each other regardless of the difference in
device configuration. This indicates that both BCP and TPBI layers
played essential role to well confine holes and excitons within the
DTP emitting layer and the red EL is obtained exclusively from the
DTP molecules without contamination from any interface emission.
As shown in Fig. 3, the EL spectra are red shifted and broadened if
3.4. Electrochemical properties
The redox behavior of these thieno[3,4-b]pyrazine derivatives
was investigated by means of cyclic voltammetry measurements.
As shown by the cyclic voltammograms in Fig. 2, the reference
compound TP exhibits two reversible reduction waves, which
should be assigned to two step one-electron reductions of the
thieno-[3,4-b]-pyrazine core. No oxidation was observed for it. The
detection of only reduction processes indicates the electron-
deficient and n-type feature of the thieno-[3,4-b]-pyrazine chro-
mophore, despite that most thiophene-containing species usually
reveal electron-donating and p-type nature [4,16,17]. With intro-
duction of the polyphenyl arms, multiple oxidation waves were
detected in addition to the reduction waves for DTP. It would be

Q. Li et al. / Dyes and Pigments 92 (2011) 674e680
679
2000
1.0
0.8
0.6
0.4
0.2
0.0
300
250
200
150
100
50
EL
PL of film
A
B
C
1600
1200
800
0
0
2
4
6
8
10
Voltage (V)
400
0
0
2
4
6
8
10
500
550
600
650
700
750
800
Voltage (V)
Wavelength (nm)
Fig. 4. Luminanceevoltage (LeV) curves of OLEDs A, B, and C. Insert: current densi-
tyevoltage (JeV) curves of the three OLEDs.
Fig. 3. The PL spectrum of DTP film and the EL spectrum of DTP based OLEDs.
compared to the PL spectra of the DTP film. This is frequently
observed in OLEDs due to the effect of the electrical field on the
excited states of organic molecules. The EL spectra correspond to
coordinates on the 1931 CIE chromaticity diagram of (0.65, 0.33).
This is close to (0.64, 0.33), which are the coordinates of the stan-
dard red color of the National Television System Committee (NTSC)
[9]. Moreover, the EL spectra and CIE coordinates of both device A
and B almost keep unchanged with increasing driving voltage,
which offers better device operation compared to red OLEDs with
dopants in which the color changes with voltage [18]. Both device A
and B turned on (to deliver a brightness of 1 cd/m2) at 5.5 V. A
maximum brightness of 362 cd mꢁ2 and a peak luminance effi-
of NPB layer in device C greatly improved the device current. As
a result, the brightness at a given voltage of the device C is
substantially higher than other two devices. A maximum bright-
ness of 1753 cd mꢁ2 (at 10 V) was obtained for device C. Fig. 5
compares the luminance efficiencies curves for these three types
of OLEDs. It is apparent that the device C is much superior to other
two devices and a maximum luminance efficiency of 0.74 cd Aꢁ1 at
20 mA cmꢁ1 was obtained. The luminance efficiency almost keeps
constant even up to a high current density of 270 mA cmꢁ1. The
maximum external quantum efficiency was calculated as 0.44%.
The substantially improved emission efficiency in device C indi-
cates that the introduction of the NPB hole transporting layer
improved the positive and negative charge balance and conse-
quently favored the efficient charge recombination and light
emission. It should be noted that the vacuum evaporated DTP film
probably have better film quality than the spin coated one, which
may be a possible reason for the improved device performance of
device C than device B.
ciency of 0.26 cd Aꢁ1 and 428 cd mꢁ2 and 0.30 cd Aꢁ1 were
,
obtained for device A and B, respectively. Obviously the perfor-
mance of these two OLEDs containing spin coated DTP layer are not
as satisfied as expected. This is probably due to the absence of hole
transporting layer and the non-balance of hole and electron
charges in the emitting layers in these devices.
In order to optimize the EL performance of DTP based device,
the third type of OLED was fabricated in which a hole transporting
layer was introduced between the anode and the DTP emitting
layer. The configuration of device C is as follows: ITO/NPB (40 nm)/
DTP (25 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). NPB (4,40-bis[N-
(1-naphthyl)-N-phenylamino]biphenyl) was used as the hole
transporting layer to increase the hole current and thus to improve
the charge balance in the emitting layer. Device C was fabricated by
thermal evaporating all organic layers and cathode in vacuum. As
shown by the data in Table 2, the EL spectra and CIE coordinates of
device C are identical to those of device A and B. Fig. 4 illustrates the
curves of current density and luminance versus voltage for these
three OLEDs. Device C turned on at a much lower voltage (4.5 V)
than both device A and B. Furthermore, the current at a given
voltage is higher for device C than device B, especially at high
driving voltage range. The only difference in device configuration
between device B and C should be responsible for their different
current density. It would be safe to conclude that the introduction
As stated in literatures, the thieno-[3,4-b]-pyrazine derivative in
previous report [4] could reach a maximum luminance efficiency of
0.34 cd Aꢁ1 and brightness of 1766 cd mꢁ2 with CIE (0.65, 0.33). A
higher efficiency of 0.65 cd Aꢁ1 was possible but at the expense of
the emission color shifting to orange (0.59, 0.33). Evidently DTP has
higher emission efficiency than the reported counterparts on the
premise of saturated red emission. To the best of our knowledge,
a luminance efficiency of 0.74 cd Aꢁ1 of our present red OLEDs is the
0.8
0.6
0.4
0.2
0.0
A
B
C
Table 2
Device performance of DTP based OLEDs A, B and C.
Devices Von Lmax
(V) (cd/m2)
hmax
h
(cd/A)
lmax CIE(x, y)
(cd/A)
[at 100 mA/cm2] (nm)
[at the voltage (V)] [at mA/cm2]
0
50
100
150
200
250
300
Current density (mA/cm2)
A
B
C
5.5 362 (11)
5.5 428 (10.5)
4.5 1753 (10)
0.26 (31.0) 0.23
0.30 (17.9) 0.28
0.74 (16.5) 0.70
646 0.65, 0.33
646 0.65, 0.33
646 0.65, 0.33
Fig. 5. Plots of the luminance efficiency versus current density for OLEDs A, B and C.

680
Q. Li et al. / Dyes and Pigments 92 (2011) 674e680
best efficiency ever reported for thieno-[3,4-b]-pyrazine derivatives
so far.
Appendix. Supplementary data
In contrast to the good performance of DTP, the OLEDs con-
taining the evaporated TP emitting layer with similar structure to
device A always produced a mixed emission from both TP and Alq3
(Fig. S4 in Supporting information) with poor intensity, although
great efforts were taken to tune the thickness of each functional
layer. Alternatively the TP based OLEDs with similar configuration
to device B produced extremely weak emission. This may result
from the unbalanced charge transporting processes within the
devices and the strong unwanted intermolecular interaction of
more planar TP molecules in solid states. Once again it implies that
the bulky polyphenyl groups in DTP molecules indeed play essen-
tial role to improve its emission performance.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2011.05.029.
References
[1] Bordeau G, Lartia R, Teulade-Fichou M-P. meta-Substituted triphenylamines
as new dyes displaying exceptionally large stokes shifts. Tetrahedron Lett
2010;51:4429e32.
[2] Wu WC, Yeh HC, Chan LH, Chen CT. Red organic light-emitting diodes with
a non-doping amorphous red emitter. Adv Mater 2002;14:1072e5.
[3] Wang JL, Zhou Y, Li YF, Pei J. Solution-processable gradient red-emitting
p-
conjugated dendrimers based on benzothiadiazole as core: synthesis, char-
acterization, and device performances. J Org Chem 2009;74:7449e56.
[4] J-Thomas KR, Lin JT, Tao YT, Chuen C-H. Star-shaped thieno-[3,4-b]-pyrazines:
a new class of red-emitting electroluminescent materials. Adv Mater 2002;14:
822e6.
[5] Li JY, Liu D. Dendrimers for organic light-emitting diodes. J Mater Chem 2009;
19:7584e91.
[6] Liu D, Ren HC, Li JY, Tao Q, Gao ZX. Novel perylene bisimide derivative with
fluorinated shell: a multifunctional material for use in optoelectronic devices.
Chem Phys Lett 2009;482:72e6.
4. Conclusions
We have demonstrated that attaching bulky polyphenyl
groups at the periphery of the planar thieno-[3,4-b]-pyrazine
chromophore produced the efficient solid-state emitter. The
generated thieno-[3,4-b]-pyrazine derivative, DTP, has good
solubility, excellent thermal and amorphous stabilities. It is
characterized by the flexibility to be made into thin films and
OLEDs either by spin coating or vacuum evaporation technique.
Non-doped OLEDs containing either spin coated or vacuum
evaporated DTP film as emitting layer were fabricated and they
all exhibited saturated red emission with CIE coordinates of
(0.65, 0.33). In the vacuum evaporated OLED, due to the utiliza-
tion of the hole transporting layer and the improved DTP film
quality by vacuum deposition, the device performance are much
superior to those of the spin coated devices. A maximum
brightness of 1753 cd mꢁ2 and a peak luminance efficiency of
0.74 cd Aꢁ1 were obtained for the vacuum evaporated red OLEDs.
This report provides a practical strategy to decorate the highly
efficient but planar luminophor to be efficient solid emitter
suitable for application in non-doped OLEDs.
[7] Hamada Y, Kanno H, Tsujioka T, Takahashi H, Usuki T. Red organic light-
emitting diodes using an emitting assist dopant. Appl Phys Lett 1999;75:
1682e4.
[8] Yang Y, Zhou Y, He QG, He H, Yang CH, Bai FL, et al. Solution-processable red-
emission organic materials containing triphenylamine and benzothiadiazole
units: synthesis and applications in organic light-emitting diodes. J Phys Chem
B 2009;113:7745e52.
[9] Karstens T, Kobs K. Rhodamine B and rhodamine 101 as reference substances
for fluorescence quantum yield measurements. J Phys Chem 1980;84:1871e2.
[10] Yuan WZ, Lu P, Chen SM, Lam JWY, Wang ZM, Liu Y, et al. Changing the
behavior of chromophores from aggregation-caused quenching to
aggregation-induced emission: development of highly efficient light emitters
in the solid state. Adv Mater 2010;22:2159e63.
[11] Chen S, Xu XJ, Liu YQ, Yu G, Sun XB, Qiu WF, et al. Synthesis and character-
ization of n-type materials for non-doped organic red-light-emitting diodes.
Adv Funct Mater 2005;15:1541e6.
[12] Tonzola CT, Alam MM, Kaminsky W, Jenekhe SA. New n-type organic semi-
conductors: synthesis, single crystal structures, cyclic voltammetry, photo-
physics, electron transport, and electroluminescence of
a series of
diphenylanthrazolines. J Am Chem Soc 2003;125:13548e58.
[13] Zhu Y, Alam MM, Jenekhe SA. Regioregular head-to-tail poly(4-alkylquinoline)
s: synthesis, characterization, self-organization, photophysics, and electrolu-
minescence of new n-type conjugated polymers. Macromolecules 2003;36:
8958e68.
[14] Casalbore-Miceli G, Esposti AD, Fattori V, Marconi G, Sabatini C. A correlation
between electrochemical properties and geometrical structure of some tri-
arylamines used as hole transporting materials in organic electroluminescent
devices. Phys Chem Chem Phys 2004;6:3092e6.
Acknowledgments
[15] Carroll WR, Pellechia P, Shimizu KD. A rigid molecular balance for measuring
face-to-face areneearene interactions. Org Lett 2008;10:3547e50.
We thank the National Natural Science Foundation of China
(20704002, 21072026 and U0634003), the Ministry of Education
for the New Century Excellent Talents in University (Grant NCET-
08-0074), the NKBRSF (2009CB220009), the Fundamental Research
Funds for the Central Universities (DUT10LK16), and the Key
Laboratory of Photochemical Conversion and Optoelectronic
Materials (TIPC, CAS) for financial support of this work.
[16] Roncali J. Synthetic principles for bandgap control in linear
p-conjugated
systems. Chem Rev 1999;97:173e205.
[17] Shahid M, Ashraf RS, Klemm E, Sensfuss S. Synthesis and properties of novel
low-band-gap thienopyrazine-based poly(heteroarylenevinylene)s. Macro-
molecules 2006;39:7844e53.
[18] Kwong RC, Sibley S, Dubovoy T, Baldo M, Forrest SR, Thompson ME. Efficient,
saturated red organic light emitting devices based on phosphorescent plati-
num(II) porphyrins. Chem Mater 1999;11:3709e13.