Published on the web March 17, 2012
363
Novel PhenyleneThiophene Oligomer Derivatives with Dibenzothiophene 5,5-Dioxide Core:
Synthesis, Characterization, and Applications in Organic Solar Cells
Zongfan Duan,* Xianqiang Huang, Shunjiro Fujii, Hiromichi Kataura,3,4 and Yasushiro Nishioka*1
1
2
3,4
1
College of Science and Technology, Nihon University, Narashinodai, Funabashi, Chiba 274-8501
2
College of Chemistry and Chemical Engineering, Liaocheng University, Shandong 252059, P. R. China
3
Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),
Higashi, Tsukuba, Ibaraki 305-8562
4
CREST, JST, Kawaguchi, Saitama 330-0012
(
Received December 14, 2011; CL-111193; E-mail: duanzf@xaut.edu.cn, nishioka@eme.cst.nihon-u.ac.jp)
Donoracceptordonor oligomers with dibenzothiophene
,5-dioxide core and end-cap phenylenethiophene substitutes
5
S
S
S
were newly synthesized. New oligomers were characterized by
UVvis absorption and photoluminescence spectroscopy, cyclic
voltammetry, and applied as electron donor in bulk-hetero-
junction solar cells. The results indicate that phenylene
thiophene oligomer derivatives containing dibenzothiophene
S
O
O
O
O
1
S
S
2
5
,5-dioxide moiety are appraised to be valuable electron donors.
Scheme 1. Molecular structures of oligomers 1 and 2.
Thiophene-containing oligomers with well-defined struc-
Br
Br
Br
Br
i
tures and monodispersed chain lengths, are easily modified by
the introduction of a variety of functional groups into the
aromatic backbone or through thienyl sulfur functionalization,
and have been widely used in organic electronics including
organic light-emitting diodes (OLEDs), organic field-effect
ii
S
S
O
O
3
4
S
ii
iii
Br
Br
1
,2
S
S
transistors (OFETs), and organic solar cells (OSCs). The
selective dearomatization of the thienyl rings of oligothiophenes
by the selective introduction of S,S-dioxides is a flexible and an
efficient methodology to decrease energy band gaps and to
improve electron affinities and stabilities of thiophene deriva-
O
O
O
O
5
6
7
iv
Br + (Bu)3Sn
S
S
S
v
3
tives. Recently, dibenzothiophene 5,5-dioxide moiety with
4
6
1
2
electron-transporting character as a central core has been
incorporated onto the backbone of electron donor units such
as fluorene, carbazole, arylamine, quinoxaline, or pyrazine
segments to afford highly efficient ambipolar charge-transport-
ing light-emitting materials.4 Phenylenethiophene oligomers
iv
(Bu)3Sn
8
,5
Scheme 2. Reaction conditions; i) Br , CH COOH, 25 °C, 2 h,
115 °C, 5 h; ii) H O , CH COOH, 110 °C, 4 h; iii) NBS, H SO ,
5 °C, 12 h; iv) DMF, [Pd(PPh ) ], 100 °C, 40 h; v) n-BuLi,
78 °C, 1 h, then, (Bu) SnCl, 25 °C, 12 h.
3
are promising electron-donor semiconducting materials with
2
3
improved oxidation stability.6,7 However, to our knowledge,
2
2
3
2
4
2
¹
3 4
there has been no paper reported on the synthesis of phenylene
thiophene oligomer derivatives containing dibenzothiophene
5,5-dioxide moiety and their applications in organic solar cells.
Herein, dibenzothiophene 5,5-dioxide groups with electron-
acceptor character were introduced into the framework of
phenylenethiophene oligomers via co-oligomerization, and
two novel donoracceptordonor thiophene-containing oligo-
mers 1 and 2 (Scheme 1) were obtained. Their characteristics
and applications in solar cells were also investigated.
pling of 4-bromo-n-hexylbenzene with 2-tri-n-butylstannylthio-
phene. 2-(4-n-Hexylphenyl)thiophene was readily lithiated using
butyllithium and then reacted with tributyltin chloride to give
6
the stannyl compound 8. The target oligomers 1 and 2 have
been systematically synthesized in good yields by Stille cross-
coupling reactions of 8 with dibromodibenzothiophene 5,5-
9
The strategy for preparation of the oligomers is outlined in
Scheme 2. Starting from the commercially available dibenzo-
thiophene, 2,8-dibromodibenzothiophene 5,5-dioxide (4) was
obtained by bromination in glacial acetic acid with bromine and
dioxides 4 and 6, respectively, in DMF at 100 °C for 40 h.
1
13
The spectral ( H, C NMR and MS) and elemental analysis
data for the newly synthesized oligomers are consistent with the
proposed structures. A single crystal of oligomer 2 was grown
9
8
followed by oxidation with H O according to the literature.
by slow evaporation from its benzene solution. Oligomer 2
2
2
ꢀ
3,7-Dibromodibenzothiophene 5,5-dioxide (6) could be attained
crystallizes in a triclinic crystal system with a P1 space group
1
0
by first oxidation of dibenzothiophene and then bromination
(Figure 1). The selected averaged bond lengths and angles of
4
9
with NBS. The key precursor 2-(4-n-hexylphenyl)thiophene (7)
2 are listed in Table S1. In the crystal structure, oligomer 2 is
was synthesized by the palladium-catalyzed Stille cross-cou-
composed of one dithienyl dibenzothiophene 5,5-dioxide core,
Chem. Lett. 2012, 41, 363365