Synthesis of some green dopants for OLEDs based on arylamine 2,3-disubstituted bithiophene derivatives

Article abstract of DOI:10.3390/molecules181114033

A series of green dopants based on 2,2-diphenylvinyl end-capped bithiophene and three different arylamine moieties (9-phenylcarbazole, triphenylamine, and N,N'-di-(ptolyl) benzeneamine) were successfully synthesized by the Suzuki and Wittig coupling react

Full text of DOI:10.3390/molecules181114033

Molecules 2013, 18, 14033-14041; doi:10.3390/molecules181114033
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Synthesis of Some Green Dopants for OLEDs Based on
Arylamine 2,3-disubstituted Bithiophene Derivatives
Mi-Seon Song, Quynh Pham Bao Nguyen, Chang-Hyun Song, Duckhee Lee †,* and
Kyu Yun Chai †,*
The Division of Bio-Nanochemistry, The College of Natural Sciences, the Wonkwang University,
Iksan City, Chonbuk, 570-749, Korea
†
These authors contributed equally to this work.
* Authors to whom correspondence should be addressed; E-Mails: geuyoon@wonkwang.ac.kr (K.Y.C.);
dl202@wonkwang.ac.kr (D.L.); Tel.: +82-63-850-6230 (K.Y.C.); Fax: +82-63-841-4893 (K.Y.C.).
Received: 20 September 2013; in revised form: 4 November 2013 / Accepted: 7 November 2013 /
Published: 13 November 2013
Abstract: A series of green dopants based on 2,2-diphenylvinyl end-capped bithiophene
and three different arylamine moieties (9-phenylcarbazole, triphenylamine, and N,N’-di-(p-
tolyl)benzeneamine) were successfully synthesized by the Suzuki and Wittig coupling
reactions. The photophysical properties of these compounds are reported. The strongest PL
emitting compound with the 9-phenylcarbazole moiety has been used for fabricating an
OLED device with good overall performance.
Keywords: green dopants; bithiophene; 2,2-diphenylvinyl; 9-phenylcarbazole;
triphenylamine; N,N'-di-(p-tolyl)benzeneamine
1. Introduction
Organic light-emitting diodes (OLEDs) have attracted increasing interest over the last few years
because of their potential to achieve low-cost, full-color and flat-panel displays [1]. One of the key
developments in OLED display technology can be attributed to the discovery of the guest-host doped
emitter system in which a single host material with optimized transport and luminescent properties
may be used together with a variety of highly fluorescent guest dopants, leading to its electro-luminescence
of desirable hues with very high efficiencies [2]. In addition, the doped emitter system enhances its

Molecules 2013, 18
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operational stability by transferring the electrogenerated exciton to the highly emissive and stable
dopant site, thus minimizing its possibility for nonradiative decay [3]. Consequently, many red, green,
and blue fluorescent dopants have been extensively studied to obtain full color displays [2]. For example,
green dopants based on coumarin, quinacridone, quinolone, quinnoxaline, and carbazole derivatives
with promising electroluminescent efficiencies have been exploited [3,4].
Oligothiophenes with well-defined structures have been widely exploited in the field-effect
transistors and light emitting diodes because they showed high conductivity and unique optical
properties which could be finely tuned by their chain length and terminal substituents [5–9]. The
strategy of introducing different groups to cap oligothiophenes has been used to diversify the structure
of a conjugated backbone, offering new possibilities of efficient electronic and optical properties. As
previously reported, oligothiophenes terminated with phenyl or biphenyl groups exhibit interesting
electroluminescent behavior and oligothiphones with terminal groups bearing diaryamino fuctional
groups have been used as efficient emitters and potential hole-transporting materials. Other
oligothiophene end-capping groups such as diaryboryl, pyridyl, diphenylphosphine, and charge-
transfer capable groups have also been documented [5,10–13].
Presented herein are the design, synthesis and characterization of a new class of the green dopants
based on arylamine 2,3-disubstituted bithiophene derivatives end-capped with the 2,2-diphenylvinyl
group (Figure 1). The arylamine substituents and diphenylvinyl group on the bithiophene core were
expected to enhance light emitting efficiency through the extended conjugated structures and to improve
the charge transport properties. In addition, the two phenyl rings as end-capping groups would force
the structures to twist due to steric hindrance and thus prevent self-quenching by molecular
aggregation [8,9,14–16].
Figure 1. Structures of designed green dopants 9.
N
N
N
S
S
S
S
S
S
N
N
N
9b
9a
9c
2. Results and Discussion
The designed green dopants 9 based on arylamine 2,3-disubstituted bithiophene derivatives end-capped
with the 2,2-diphenylvinyl group were prepared in four steps from the commercially available
bithiophene (1, Scheme 1). First, bithiophene (1) was formylated with the Vilsmeier reagent
(POCl3/DMF) in dichloromethane following a reported procedure to form compound 2 [17], which
was then brominated with Br2/NaHCO3 to produce compound 3 in 80% yield. Pd-catalyzed Suzuki
coupling reactions using Pd(PPh3)2Cl2/NaHCO3 in toluene between compounds 3 and 5 proceeded
very well to give 80%–85% yields of compounds 6. Finally, the designed green dopants 9 end-capped
with the 2,2-diphenylvinyl group were obtained in moderate yields (53%–58%) by the Wittig coupling

Molecules 2013, 18
14035
reactions of compounds 6 and 8 in the present of t-BuOK. The three green dopants 9a–c were
identified and characterized by NMR, IR, UV-vis, PL spectroscopies and energy levels HOMO-LUMO
before the fabrication of OLED devices.
Scheme 1. Synthesis of green dopants 9.
R1
4
Br
O
O
O
B
n-BuLi
Br
Br
R1:
4a/5a/6a/9a
4b/5b/6b/9b
N
N
O
Br2/NaHCO3
CHCl3
POCl3
R1
+
B
H
H
S
S
S
S
S
S
O
DMF/CH2Cl2
reflux
O
+
O
5
1
2
3
O
R1
R1
P(OEt)2
R1
Pd(PPh3)2Cl2
K2CO3/toluene
t-BuOK
H
S
R1
S
S
THF/0oC-rt
S
O
9
8
6
4c/5c/6c/9c
180oC
N
P(OEt)3
Br
7
Figures 2 and 3 present the UV-vis and PL spectra of the green dopants 9 in dichloromethane with
the results summarized in Table 1. The UV-vis absorption peaks of 9a–c were observed at 402, 409,
and 376 nm, respectively. As seen in Figure 2, the absorption spectra of 9a–c overlapped with the
emission spectra of the common host material 9,10-di(2-napthyl)anthracene (ADN). Interestingly, the
absorption spectra of 9b overlapped more effectively with the emission spectra of ADN than that of 9a
and 9c.
Figure 2. UV spectra of green dopants 9.
These observations imply that 9a–c could accept energy from the ADN host material by a Förster-type
energy transfer and ADN acted as a good host in the OLED devices using 9a–c as dopants.
Particularly, a Förster-type singlet energy transfer from ADN host to 9b would be more effective than
that of 9a and 9c. The emission peaks of 9a–c were observed in the green region at 507, 491, and 502 nm,
respectively (Figure 3 and Table 1). Notably, with the same bithiophene core, compound 9b with

Molecules 2013, 18
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triphenylamine moiety turned out to have a slight blue shift, leading to emit the bluish green color and
compound 9a with 9-phenylcarbazole moiety exhibited extremely higher PL intensity than compounds
9b and 9c. The fluorescence quantum yields of 9a–c in chloroform were 0.94, 0.44, and 0.45,
respectively [18].
Figure 3. PL spectra of green dopants 9.
Table 1. Photophysical properties of green dopants 9.
Compound
UV λmax
(nm) a
PL λmax
(nm) a
E ox
(eV) c
HOMO
(eV) d
LUMO
(eV) d
Eg
e
Фf
(eV)
b
b
b
NPB
9a
-
-
0.55
0.76
0.58
0.49
5.40
5.61
5.43
5.34
2.50
2.97
2.88
2.58
2.90
2.64
2.55
2.76
-
402
409
376
507
491
502
0.94
0.44
0.45
9b
9c
a In dichloromethane (ca. 1 × 10−6 M); b Not determined; c Oxidation potential relative to Ag/AgCl electrode;
d HOMO = (Eox + 4.85) eV; LUMO = (HOMO − Eg) eV; e In chloroform with coumarin 153 as a standard.
In addition, from the cyclovoltammetric measurements and the bandgap energies (Eg) which were
estimated from the onset of absorption, the energy of the highest occupied molecular orbital (HOMO)
and that of the lowest unoccupied molecular orbital (LUMO) of the green dopants 9a–c were
determined and compared with those of N,N'-di(naphthalen-1-yl)-N,N’-diphenyl-benzidine (NPB), one
of the most widely used hole-transport material in OLEDs (Table 1). The HOMO/LUMO energy levels
of compounds 9a–c were 5.61/2.97, 5.43/2.88, and 5.34/2.58, respectively, and their calculated
HOMO-LUMO energy gaps (Eg) were 2.64, 2.55, and 2.76, respectively. For efficient device operation
it is necessary to have HOMO and LUMO levels of the emitter suitably aligned with the HOMO and
LUMO of hole-transport and electron-transport materials. As seen in Table 1, compounds 9a–c
satisfied the energy level conditions to be potentially used as green dopants in OLED devices.
Finally, compound 9a with the strongest PL emission was selected as a potential dopant for
fabricating an OLED device with the configuration of ITO/NPB (10 nm)/9a (30 nm)/Bebq2 (10 nm)/
LiF/Al. A yellowish green color (CIE = 0.42, 0.54) with a maximum emission peak at 568 nm was
emitted. The brightness, luminous efficiency and current density were shown in Figure 4. The device

Molecules 2013, 18
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exhibited good overall performance with a maximum brightness and luminous efficiency of
5,100 cd/m2 and 2.56 cd/A, respectively.
Figure 4. Device characteristics of the OLED with the configuration of ITO/NPB
(10 nm)/9a (30 nm)/Bebq2 (10 nm)/LiF/Al.
(a)
(b)
(c)
3. Experimental
3.1. General Procedures
All reagents and solvents were obtained from commercial suppliers (Aldrich, Seoul, Korea and TCI
1
13
Chem. Co., Seoul, Korea) and were used without further purification. H- and C-NMR spectra were
recorded on a JEOL JNM-ECP FT-NMR spectrometer operating at 500 and 125 MHz, respectively. IR
spectra were measured on a Shimadzu Prestige-21 FT-IR spectrophotometer. The samples were
prepared as a KBr pellet and scanned against a blank KBr pellet background at a wave number
ranging from 4000 to 400 cm−1. UV-vis absorption spectra were recorded on a Scinco S-3100
spectrophotometer while photoluminescence (PL) spectra were measured on a CARY Eclipse Varian
fluorescence spectrophotometer. The HOMO levels were calculated from the oxidation potentials,
while the LUMO levels were calculated based on the HOMO levels and the lowest-energy absorption
edges of the UV-vis absorption spectra.

Molecules 2013, 18
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3.2. Synthesis
Compounds 2, 5 and 8 were known and synthesized by following the methods previously reported
in the corresponding literature [17,19,20].
3.2.1. Synthesis of 5-(4,5-dibromothiophen-2-yl)thiophene-2-carbaldehyde (3)
To a stirred solution of 5-(thiophen-2-yl)thiophene-2-carbaldehyde (2, 1 g, 5.15 mmol) in
chloroform (30 mL) under an Ar atmosphere was added Br2 (2.4 g, 5.15 mmol) and NaHCO3 (0.43 g,
5.15 mmol). The reaction mixture was refluxed for 4h, cooled to room temperature, filtered, and then
evaporated to remove the solvent. The obtained crude product was purified by column chromatography
to give the requisite product 3. Yield: 80%; yellow solid; m.p: 124–126 °C; FT-IR: ʋmax 1672, 1449,
1
1422, 1264, 730 cm−1; H-NMR (CDCl3) δ 9.82 (s, 1H), 7.65 (s, 1H), 7.32 (d, J = 5.0 Hz, 1H), 7.09
(d, J = 5.0 Hz, 1H); 13C-NMR (CDCl3) δ 181.6, 140.9, 140.1, 134.9, 130.5, 128.8, 116.3, 108.5.
3.2.2. Typical Procedure for Synthesis of Compounds 6
To stirred solutions of compound 3 (1 g, 2.8 mmol) and compounds 5 (7 mmol) in toluene (30 mL)
were added Pd(PPh3)2Cl2 (0.1 g, 0.14 mmol) and K2CO3 (3 g, 21 mmol) in water (10 mL). The
reaction mixtures were refluxed for overnight, cooled to room temperature, and extracted with
chloroform. The organic layers were washed with brine, dried with MgSO4 and then evaporated. The
obtained crude products were purified by column chromatography to give the requisite products 6.
5-(4,5-bis(4-(9H-Carbazol-9-yl)phenyl)thiophen-2-yl)thiophene-2-carbaldehyde (6a). Yield: 80%;
yellow solid; m.p: 140–145 °C; FT-IR: ʋmax 1592, 1495, 1489, 1264, 737 cm−1; 1H-NMR (acetone-d6)
δ 10.03 (s, 1H), 7.29-8.30 (m, 27H); 13C-NMR (acetone-d6) δ 205.5, 205.3, 183.1, 141.1, 131.4, 129.8,
127.6, 127.3, 126.3, 120.3, 110.0.
5-(4,5-bis(4-(Diphenylamino)phenyl)thiophen-2-yl)thiophene-2-carbaldehyde (6b). Yield: 85%;
orange solid; m.p: 75–80 °C; FT-IR: ʋmax 1424, 1265, 896, 730 cm−1; 1H-NMR (CDCl3) δ 9.86 (s, 1H),
13
7.66 (s, 1H), 7.38 (d, J = 11.0 Hz, 1H), 7.28 (d, J = 11.0 Hz, 1H), 6.90–7.27 (m, 28H); C-NMR
(CDCl3) δ 182.7, 148.1, 148.0, 147.5, 147.4, 146.7, 142.2, 139.9, 139.6, 139.2, 133.4, 130.4, 129.5,
129.4, 129.1, 128.6, 127.4, 126.6, 124.9, 124.8, 124.7, 123.6, 123.5, 123.3, 123.2, 122.5.
5-(4,5-bis(4-(di-p-Tolylamino)phenyl)thiophen-2-yl)thiophene-2-carbaldehyde (6c). Yield: 82%;
orange solid; m.p: 76–80 °C; FT-IR: ʋmax 1507, 1265, 896, 730 cm−1; 1H-NMR (CDCl3) δ 9.84 (s, 1H),
7.65 (s, 1H), 6.90–7.35 (m, 26H), 2.31 (s, 6H), 2.28 (s, 6H); 13C-NMR (CDCl3) δ 182.7, 148.6, 148.4,
146.9, 145.1, 144.9, 142.3, 139.7, 138.3, 133.1, 133.0, 132.9, 130.2, 130.1, 130.0, 128.9, 126.5, 125.1,
124.9, 122.4, 122.2, 122.0, 20.9.
3.2.3. Typical Procedure for Synthesis of Compounds 9
To stirred solutions of compounds 6 (1.47 mmol) and compound 8 (0.7 g, 2.20 mmol) in THF
(30 mL) under an Ar atmosphere were added t-BuOK (0.3 g, 2.20 mmol) at 0 °C. The reaction
mixtures were warmed to room temperature, stirred for 30 min and then extracted with chloroform.

Molecules 2013, 18
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The organic layers were washed with brine, dried with MgSO4 and then evaporated. The obtained
crude products were purified by column chromatography to give the requisite products 9.
9-(4-(3-(4-(9H-Carbazol-9-yl)phenyl)-5-(5-(2,2-diphenylvinyl)thiophen-2-yl)thiophen-2-yl)phenyl)-
9H-carbazole (9a). Yield: 55%; yellow solid; m.p: 135–140 °C; FT-IR: ʋmax 1506, 1264, 730 cm−1;
1H-NMR (CDCl3) δ 6.90–8.20 (m, 38H); 13C-NMR (CDCl3) δ 140.1, 139.9, 131.0, 130.0, 129.8,
128.9, 128.6, 127.5, 127.2, 127.0, 126.1, 126.0, 124.8, 124.2, 123.5, 120.5, 120.4, 120.1, 109.9.
4-(2-(4-(Diphenylamino)phenyl)-5-(5-(2,2-diphenylvinyl)thiophen-2-yl)thiophen-3-yl)-N,N-diphenyl-
benzenamine (9b). Yield: 53%; orange solid; m.p: 77–83 °C; FT-IR: ʋmax 1599, 1502, 1453, 1264, 1231,
835, 700 cm−1; 1H-NMR (CDCl3) δ 6.90–7.52 (m, 42H); 13C-NMR (CDCl3) δ 147.7, 147.5, 144.0, 138.5,
134.8, 130.7, 130.3, 129.4, 129.3, 128.1, 127.5, 126.4, 124.6, 124.5, 123.8, 123.7, 123.2, 123.0, 122.0.
4-(2-(4-(di-p-Tolylamino)phenyl)-5-(5-(2,2-diphenylvinyl)thiophen-2-yl)thiophen-3-yl)-N,N-di-p-tolyl-
benzenamine (9c). Yield: 58%; orange solid; m.p: 78–83 °C; FT-IR: ʋmax 1592, 1495, 1489, 1264,
1
13
730 cm−1; H-NMR (CDCl3) δ 6.90–7.40 (m, 37H); C-NMR (CDCl3) δ 147.8, 147.5, 145.3, 144.9,
144.2, 138.8, 132.9, 132.6, 131.5, 130.8, 130.1, 130.0, 129.8, 128.5, 128.1, 127.2, 126.3, 124.9, 124.7,
123.5, 122.5, 121.8, 20.9.
4. Conclusions
In summary, a series of green dopants based on 2,2-diphenylvinyl end-capped bithiophene and
three different arylamine moieties, namely 9-phenylcarbazole, triphenylamine, and N,N’-di-(p-
tolyl)benzeneamine, were successfully synthesized by Suzuki and Wittig coupling reactions. The
strongest PL emitting compound with the 9-phenylcarbazole moiety has been used for fabricating an
OLED device with yellowish green emission (CIE = 0.42, 0.54), maximum brightness and luminous
efficiency of 5,100 cd/m2 and 2.56 cd/A, respectively.
Acknowledgments
This work was financially supported by the Ministry of Education, Science Technology (MEST)
and Korea Industrial Technology Foundation (KOTEF) through the Human Training Project for
Regional Innovation.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds 9a–c are available from the authors.
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