Novel thienyl-dibenzothiophene oligomers end-capped by hexylphenyl groups as potential organic semiconductor materials

Article abstract of DOI:10.1080/15421406.2012.702376

Novel thienyl-dibenzothiophene oligomers end-capped by hexylphenyl groups, 2,8-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (28HPTDBT) and 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (37HPTDBT), have been synthesized using Stille cross-coupling reactions. Newly synthesized oligomers were investigated by Ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopy, cyclic voltammogram, and thermal analysis. Strong intermolecular-interactions, appropriate energy band gaps and low HOMO energy levels imply that the two oligomers are promising candidate materials for durable organic electronic devices.

Full text of DOI:10.1080/15421406.2012.702376

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Novel Thienyl-dibenzothiophene
Oligomers End-capped by Hexylphenyl
Groups as Potential Organic
Semiconductor Materials
a
b
a
Zongfan Duan , Zhigang Yang , Daiki Hoshino , Toshio Hirao
c
c
a
a
,
Mitsumasa Taguchi , Hirokuni Ohuch , Yuichiro Yanagi
&
a
Yasushiro Nishioka
a
College of Science and Technology , Nihon University,
Narashinodai , Funabashi , Chiba , Japan
b
School of Pharmacy , Nihon University, Narashinodai , Funabashi ,
Chiba , Japan
c
Japan Atomic Energy Agency , 12333 Watanuki, Takasaki , Gunma ,
Japan
Published online: 17 Sep 2012.
To cite this article: Zongfan Duan , Zhigang Yang , Daiki Hoshino , Toshio Hirao , Mitsumasa Taguchi ,
Hirokuni Ohuch , Yuichiro Yanagi & Yasushiro Nishioka (2012) Novel Thienyl-dibenzothiophene
Oligomers End-capped by Hexylphenyl Groups as Potential Organic Semiconductor Materials,
Molecular Crystals and Liquid Crystals, 567:1, 28-33, DOI: 10.1080/15421406.2012.702376
To link to this article: http://dx.doi.org/10.1080/15421406.2012.702376
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Mol. Cryst. Liq. Cryst., Vol. 567: pp. 28–33, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.702376
Novel Thienyl-dibenzothiophene Oligomers
End-capped by Hexylphenyl Groups as Potential
Organic Semiconductor Materials
1
2
1
ZONGFAN DUAN, ZHIGANG YANG, DAIKI HOSHINO,
3
3
TOSHIO HIRAO, MITSUMASA TAGUCHI, HIROKUNI
OHUCH, YUICHIRO YANAGI, AND YASUSHIRO
1
1
NISHIOKA1
,∗
1College of Science and Technology, Nihon University, Narashinodai, Funabashi,
Chiba, Japan
2
School of Pharmacy, Nihon University, Narashinodai, Funabashi, Chiba, Japan
Japan Atomic Energy Agency, 12333 Watanuki, Takasaki, Gunma, Japan
3
Novel thienyl-dibenzothiophene oligomers end-capped by hexylphenyl groups, 2,8-
bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (28HPTDBT) and 3,7-bis[5-(4-n-
hexylphenyl)-2-thienyl]dibenzothiophene (37HPTDBT), have been synthesized using
Stille cross-coupling reactions. Newly synthesized oligomers were investigated by
Ultraviolet-visible (UV-vis) and photoluminescence (PL) spectroscopy, cyclic voltam-
mogram, and thermal analysis. Strong intermolecular π-π interactions, appropriate
energy band gaps and low HOMO energy levels imply that the two oligomers are
promising candidate materials for durable organic electronic devices.
Keywords Dibenzothiophene; phenylene-thiophene oligomers; organic
semiconductor
1.Introduction
There has been growing interest in π-conjugated polymers and oligomers over the last
few years because of their unique photo-electronic properties and potential applications in
different electronic devices, such as organic light emitting diodes (OLEDs), solar cells and
organic field effect transistors (OFETs) [1]. Phenylene (P)-thiophene (T) oligomers with
various P:T ratios are a promising class of semiconductor materials, and many of them
have been employed in OLEDs [2], solar cells [3], and p-channel OFETs [4,5]. Previous
work with OFETs fabricated from phenylene-thiophene oligomers suggests that phenylene
substitutes introduced at the 2-position or within the oligothiophene backbone can result in
a small lowering of the EHOMO level, and provide stability against oxidation and polymer-
ization while minimizing steric interactions [4–6]. Dibenzothiophene unit with coplanar
∗
Address correspondence to Yasushiro Nishioka, Department of Precision Machinery Engineer-
ing, College of Science and Technology, Nihon University, 7-24-1 Narashinodai, Funabashi-shi, Chiba
2
74-8501, Japan. Tel.: +81-47-469-6482; Fax: +81-47-469-6482. E-mail: nishioka@eme.cst.nihon-
u.ac.jp
[230]/28

Novel Thienyl-dibenzothiophene Oligomers
[231]/29
Scheme 1. Structures of 28HPTDBT and 37HPTDBT
benzene rings has excellent hole-injection/transportation character and dibenzothiophene
derivatives are potential high mobility semiconductor materials [7,8].
Bearing in mind the above factors, we have designed, synthesized and characterized two
novel thienyl-dibenzothiophene oligomers 28HPTDBT and 37HPTDBT (Scheme 1). Their
intrinsic potentials as organic materials for electronics and photonics were also assessed.
2. Experimental
The synthetic pathways to 28HPTDBT and 37HPTDBT are outlined in Scheme 2. Start-
ing from the commercially available dibenzothiophene, 2,8-dibromodibenzothiophene was
obtained by bromination [9], 3,7-dibromodibenzothiophene can be attained by three steps
[10]. The key precursor 2-(tri-n-butylstannyl)-5-(4-n-hexylphenyl)thiophene was synthe-
sized according to the literature [4]. The desired oligomers 28HPTDBT and 37HPTDBT
were prepared by Stille cross-coupling reactions. All compounds were fully characterized
1
with H NMR, MS (EI-HR) and elemental analysis and found to be in good agreement with
their structures.
Synthesis of 2,8-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (28HPT-
DBT). To a 100 ml three-neck round-bottomed flask were added 2,8-dibromodibenz
othiophene (0.342g, 1 mmol), 2-(tri-n-butylstannyl)-5-(4-n-hexylphenyl)thiophene (1.60g,
3
mmol), and dry DMF (30 ml). The mixture was deoxygenated with nitrogen for 30 min. Af-
ter tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (46 mg, 0.04 mmol) was added,
◦
the mixture was stirred at 110 C for 48 hours under nitrogen. The resulting large amount
of yellow solid was collected by filtration and washed with aqueous 8% aqueous KF
(100 ml), 5% aqueous HCl (100 ml), 10% aqueous Na2CO3 (100 ml), water (200 ml) and
methanol (100 ml) successively. The residue was purified by column chromatography using
hexane/dichloromethane (10:1) as eluent. After recrystallization from ethyl acetate, pale
◦
1
yellow solid was obtained (0.434 g, 65%). mp 175–177 C. H NMR (600 MHz, CDCl3):
δ 8.38 (d, 2H, J = 1.4 Hz), 7.82 (d, 2H, J = 8.2 Hz), 7.73 (dd, 2H, J = 8.2 Hz, 1.7 Hz),
7
4
.59 (d, 4H, J = 8.3 Hz), 7.41 (d, 2H, J = 3.8 Hz,), 7.30 (d, 2H, J = 3.8 Hz), 7.22(d,
H, J = 8.3 Hz), 2.64 (t, 4H, J = 7.7 Hz), 1.66-1.63 (m, 4H), 1.37-1.32(m, 12H), 0.90 (t,
Scheme 2. Synthetic pathways to oligomers 28HPTDBT and 37HPTDBT

30/[232]
Z. Duan et al.
Figure 1. Normalized UV-Vis absorption spectra: 28HPTDBT in chloroform (28HPTDBT-S) and
in thin film (28HPTDBT-F); 37HPTDBT in tetrachloroethane (37HPTDBT-S) and in thin film
(
37HPTDBT-F).
6
1
1
H, J = 6.9 Hz). 13C NMR (CDCl3, 150MHz, ppm): 143.9, 142.9, 142.6, 138.9, 135.9,
31.7, 131.2, 129.0, 125.6, 124.9, 124.2, 123.6, 123.2, 118.4, 35.7, 31.7, 31.4, 29.0, 22.6,
+
4.1. MS (EI-HR), m/z: [M] calcd. for C44H44S3, 668.2605; found, 668.2610. Elemental
Analysis calcd. for C44H44S3: C, 78.99; H, 6.63. Found: C, 79.25; H, 6.34.
Synthesis of 3,7-bis[5-(4-n-hexylphenyl)-2-thienyl]dibenzothiophene (37HPT-
DBT). Applying the same procedure as described for 28HPTDBT, green-yellow solid
◦
1
◦
was obtained with a yield of 66%. mp>300 C. H NMR (600 MHz, C2D2Cl4, 100 C): δ
8
7
.14 (d, 2H, J = 7.9 Hz), 8.11 (d, 2H, J = 1.7 Hz), 7.76 (dd, 2H, J = 8.3 Hz, 1.7 Hz),
.59 (d, 4H, J = 7.9 Hz), 7.40 (d, 2H, J = 3.8 Hz), 7.31 (d, 2H, J = 3.4 Hz), 7.24(d, 4H,
J = 7.9 Hz), 2.68 (t, 4H, J = 7.9 Hz), 1.71–1.69 (m, 4H), 1.44–1.38(m, 12H), 0.95 (t,
+
6
H, J = 7.2 Hz). MS (EI-HR), m/z: [M] calcd. for C44H44S3, 668.2605; found, 668.2606.
Elemental Analysis calcd. for C44H44S3: C, 78.99; H, 6.63. Found: C, 78.75; H, 6.44.
3. Results and Discussion
3.1. Photo-physical Properties
The UV-vis absorption and PL spectra of two oligomers in solutions and in thin films
prepared by drop-casting of tetrachloroethane solutions on quartz glasses are shown in
Figure 1 and 2, respectively, and the photo-physical data are summarized in Table 1. The
absorption maxima of 28HPTDBT and 37HPTDBT in solutions peak at 340 and 387 nm,
respectively, and there is a substantial red shift (ꢀλ = 47 nm) of the absorption maxima
upon variation of substituted situations of 4-n-hexylphenyl-2-thienyl group at the aryl ring
of dibenzothiophene, which is attributed to the asymmetric destabilization of the HOMO
and the LUMO energy levels leading to a decrease in the energy gap [11]. In thin films,
the absorption bands of two oligomers become relatively broader and less structured.
The maximum absorption of 28HPTDBT is red-shifted about 12 nm, while the UV-vis
spectra of 37HPTDBT display a large blue-shift more than 30 nm relative to those of
their corresponding solutions. The energy band gaps of 28HPTDBT and 37HPTDBT were
determined from the absorption onset and were found to be 2.95 and 2.72 eV, respectively

Novel Thienyl-dibenzothiophene Oligomers
[233]/31
Figure 2. Normalized photoluminescence (PL) spectra: 28HPTDBT in chloroform (28HPTDBT-S)
and in thin film (28HPTDBT-F); 37HPTDBT in tetrachloroethane (37HPTDBT-S) and in thin film
(
37HPTDBT-F).
[3]. These energy band gaps are significantly higher than that of pentacene, implying the
greater stabilities of 28HPTDBT and 37HPTDBT against photo-degradation [12]. The
pronounced changes in the absorption spectra are a result of the delocalization of the
exciton within co-facial stacks induced by the π-π interactions, which is also evidenced
by a related red-shift of the PL spectra. 28HPTDBT and 37HPTDBT show strong blue
fluorescence in solutions, while emission color of 37HPTDBT in the solid state is observed
in the green region. The remarkable shifts of UV-vis and PL spectra in thin films relative to
the solutions correspond to the extremely high intermolecular π-π interactions in the film
state.
3.2. Electrochemical Properties
The cyclic voltammograms of 28HPTDBT and 37HPTDBT, performed by using a three-
electrode system, show quasi-reversible and irreversible oxidation waves, respectively.
The onset oxidation potentials of 28HPTDBT and 37HPTDBT are 0.96 and 0.77 eV,
respectively. Considering the onset oxidation potential of ferrocene/ferrocenium was 0.26V
with respect to Ag/AgCl reference electrode and assuming that the energy level of the
Table 1. Optical and electrochemical data for 28HPTDBT and 37HPTDBT.
Electrochemical
data
abs
max
pl
λmax (nm)
λ
(nm)
opt
Eg
∗
ELUMO
EHOMO
Oligomers
Solution
Film
(eV)
Solution
Film
(eV)
(eV)
2
3
8HPTDBT
7HPTDBT
340
387
352
356
2.95
2.72
418
434
448
511
−5.50
−5.31
−2.55
−2.59
∗
opt
LUMO levels were determined by adding Eg and HOMO energy levels.

32/[234]
Z. Duan et al.
Figure 3. Differential scanning calorimetry (DSC) trace for 28HPTDBT and 37HPTDBT.
ferrocene/ferrocenium reference is 4.8 eV below vacuum. The HOMO energy levels of
8HPTDBT and 37HPTDBT were calculated to be −5.50 and −5.31 eV, respectively
2
(
Table 1). Compared to that of sixthiophene (−4.99 eV) [12], the lower HOMO energy levels
of 28HPTDBT and 37HPTDBT suggest that the introduction of the phenylene ring into
oligothiophene conjugated chain can really decrease the HOMO energy levels, and therefore
increase the oxidative stabilities of the corresponding oligomers. The calculated LUMO
energy levels of 28HPTDBT and 37HPTDBT are −2.55 eV and −2.59 eV, respectively.
Such energy levels may provide a closer match to the work function of Ca (−2.76 eV)
when they are used as electron-transporting materials in the OLEDs.
3.3. Thermal analysis
Two oligomers were found to exhibit good thermal stabilities, losing less than 5% of
◦
weight upon heating up to 400 C in nitrogen as determined by thermal gravimetric analysis
◦
(
TGA). 28HPTDBT shows only one endothermal peak at 176 C and exothermal peak at
45 C on the differential scanning calorimeter (DSC) curve (Figure 3). Interestingly, for
◦
1
oligomer 37HPTDBT with a high phase transition temperature, sublimation is complete at
the temperature much lower than its melting point.
4. Conclusions
In a word, straightforward syntheses, unique photo-electronic properties and durability
make oligomers 28HPTDBT and 37HPTDBT very attractive for low-cost organic electron-
ics applications.
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