Synthesis and spectroscopic properties of ring-fused thiophene bridged push-pull dyes and their application in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2012.04.052

Bisdimethylfluorenylamino-based push-pull dyes bearing one or two benzo[c]thiophene and its precursor bicyclo[2.2.2]octadiene (BCOD) fused thiophene as π bridge have been synthesized. The effect of fused-ring on thiophene bridge for the spectroscopic and

Full text of DOI:10.1016/j.tetlet.2012.04.052

Tetrahedron Letters 53 (2012) 3264–3267
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and spectroscopic properties of ring-fused thiophene bridged
push–pull dyes and their application in dye-sensitized solar cells
Quan Liu a, Fan-Tai Kong c, Tetsuo Okujima b, Hiroko Yamada b, , Song-Yuan Dai c, , Hidemitsu Uno b,
⇑
,
⇑
Noboru Ono b, Xiao-Zeng You a, Zhen Shen a,
⇑
a State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing 210093, PR China
b Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-0853, Japan
c Key Laboratory of Novel Thin Film Solar Cell, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 February 2012
Revised 4 April 2012
Accepted 12 April 2012
Available online 21 April 2012
Bisdimethylfluorenylamino-based push–pull dyes bearing one or two benzo[c]thiophene and its precur-
sor bicyclo[2.2.2]octadiene (BCOD) fused thiophene as
p bridge have been synthesized. The effect of
fused-ring on thiophene bridge for the spectroscopic and electrochemical properties of these dyes has
been investigated. Dye-sensitized solar cells fabricated with these dyes show moderate photovoltaic
properties.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Ring-fused thiophene
Push–pull
Dye-sensitized solar cell
Organic dye
Dimethylfluoreaniline
Introduction
by Ko’s group which brought the combination of high conversion
efficiency with excellent light and thermostability.17 A series of
Dye-sensitized solar cells (DSSCs) have attracted considerable
research interest in the past decades due to its advantages such
as low cost and easy fabrication.1 As one of the key components
of DSSC, Ru complex based sensitizers encountered the problems
of limited resource and high toxicity, although they hold the high-
est power conversion for a long time.2 The search for alternative
dyes is now ongoing. Metal-free organic dyes with the structure
novel dyes derived from dimethylfluoreaniline donor part were
hence developed.8,18–23 Recently we reported a series of push–pull
sensitizers in which benzo[c]thiophene and its precursor bicy-
clo[2.2.2]octadiene (BCOD) ring-fused thiophene have been intro-
duced as p-conjugated spacers, containing dihexyloxy-substituted
triphenylamine as the donor and cyanoacrylic acid as the accep-
tor.24 It was the first time that benzo[c]thiophene and its precursor
of donor–
p
–acceptor are considered to be the promising candidate
were applied as
priate extension in
p
bridge in dyes for DSSC and we found that appro-
conjugation significantly red-shifted the
because of their high molar extinction coefficient, easy modifica-
tion on structure, low cost, and clean to environment.3–5 In the past
10 years, great progresses have been achieved on organic dye
based DSSC.6–10 Some organic dyes exhibit good conversion
efficiency of about 10%11–14 and even higher.15,16 Triarylamine
and cyanoacrylic acid are most commonly employed as donor
p
absorption spectra and enhanced their performance in DSSC. Since
dimethyl-fluoreaniline exhibited excellent performance as donor
part of DSSC dyes, it motivated us to design and synthesis new dyes
in combination of dimethylfluoreaniline and ring-fused thiophene,
with the aim of enriching the alterations of dyes and finding robust
and efficient organic dyes for DSSC.
and acceptor, respectively, in the design of donor–p–acceptor
structure while thiophene and its derivatives are served as
p
bridge to provide conjugation and enhance light absorbance.
Among the triarylamine derivatives, dimethylfluoreaniline (or
bisdimethylfluorenylamino group) was first introduced into DSSC
Experiment
The organic dyes OL7–OL10 were prepared following the
synthetic procedure illustrated in Scheme 1. 1 was prepared
through Ullmann reaction by aniline coupled with 2-iodo-dimeth-
ylfluorene in moderate yield. Compound 3 was achieved by
reacting 1 with NBS and then with trimethyl borate. Since
benzo[c]thiophene is very unstable, its introduction is achieved
⇑
Corresponding authors. Tel.: +86 025 83686679; fax: +86 025 83314502 (Z.S.).
E-mail address: zshen@nju.edu.cn (Z. Shen).
Present address: Graduate School of Material Science, Nara Institute of Science
and Technology and CREST, JST 8916-5, Takayama-cho, Ikoma, Nara 630-0192, Japan.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.04.052

Q. Liu et al. / Tetrahedron Letters 53 (2012) 3264–3267
3265
Br
B(OH)2
N
NH2
n-BuLi
B(OMe)3,H+
NBS
t-BuOK,bpy
CuI
N
I
+
N
2
1
3
COOH
NC
S
H
I
CHO
S
H
S
NC
COOH
S
Pd(PPh3)4
K2CO3
POCl3
DMF
N
n=1-2
N
+
N
n=1-2
piperidine
n=1-2
n=1-2
n=1 OL7
n=2 OL8
n=1
n=2
4
5
n=1
n=2
6
7
COOH
NC
S
CHO
n=1-2
CHO
270oC
in vacuo
S
S
NC
COOH
N
N
N
n=1-2
n=1-2
piperidine
n=1
n=2
6
7
n=1
n=2
8
9
n=1 OL9
n=2 OL10
Scheme 1. Synthetic procedure for dyes OL7–OL10.
through the transformation of appropriate precursor. Previously,
Ono’s group has reported the facile synthesis of isothianaphthene
oligomers using BCOD-thiophene as a precursor.25 Here we follow
this protocol and connect one or two BCOD-thiophene to the donor
part through Suzuki coupling reaction to obtain 4 and 5, respec-
tively. Target compounds OL7 and OL8 were synthesized through
Knoevenagel reaction from aldehydes 6 and 7 which were pre-
pared by Vilsmeyer reaction with 4 and 5, respectively. The trans-
formation of BCOD-thiophene precursor into benzo[c]thiophene
analog was carried out at the aldehyde stage to prevent potential
decarboxylation of the final product at 270 °C. OL9 and OL10 were
obtained in a way similar to OL7 and OL8 after the transformation.
Results and discussions
Spectroscopic properties of the ring-fused thiophene bridged
bisdimethylfluorenylamino-based push–pull organic dyes
Figure 1. UV–vis absorption spectra of dyes OL7–OL10 measured in CH2Cl2.
To inspect the effect of fused ring on thiophene for the spectro-
scopic properties of these dyes, the electronic absorption spectra of
dyes OL7–OL10 were measured in dichloromethane, as shown in
Figure 1 and the detailed data were listed in Table 1. All dyes exhib-
ited two peaks with maximum absorption bands mainly located at
around 340–370 nm and ranging from 470 to 640 nm, respectively,
and the latter could be attributed to the intrinsic charge transfer
transition from HOMO to LUMO. The absorption spectra are very
similar to their dihexyloxy-substitute triphenylamine analog,24 ex-
cept for OL10 with kmax further redshift to 645 nm. Comparing the
absorption spectra of OL7 and OL8, the additional BCOD thiophene
moiety has little effect on the charge transfer band but enhancing
the absorption in the ultraviolet region. The transformation of
BCOD thiophene into benzo[c]thiophene leading to the expansion
Table 1
Electrooptical and electrochemical properties of the dyes
Dye
kmax (nm)/log
e
EHOMO (eV)
E0–0 (eV)
ELUMO (eV)
OL7
OL8
OL9
OL10
485/4.32, 347/4.53
472/4.32, 367/4.68
569/4.37, 337/4.55
645/4.27, 345/4.41
À5.56
À5.50
À5.61
À5.50
2.28
2.42
2.01
1.67
À3.28
À3.08
À3.60
À3.83
and OL8 to OL10, the maximum absorption peak red-shifted for
84 and 73 nm, respectively. In addition from OL9 to OL10, a red
shift of 76 nm was observed. Among the four dyes, OL9 shows
of the
p-conjugation pathway, which notably bathchromic shifted
the highest absorption coefficient (
visible region. The absorption spectrum of OL10 is broadened with
e
= 23341 MÀ1 cmÀ1) in the
the absorption maximum in the visible region. From OL7 to OL9

3266
Q. Liu et al. / Tetrahedron Letters 53 (2012) 3264–3267
the tailed off at 850 nm. Such strong and broadening absorption in
the visible and even near infrared region indicates that the light
harvesting of device fabricated with these dyes may be improved
and hence enhance the generation of photocurrent.
Electrochemical properties of the ring-fused thiophene bridged
bisdimethylfluorenylamino-based push–pull dyes
The electrochemical properties of these dyes were studied by
cyclic voltammetry (CV) in CH2Cl2 solution containing tetrabutyl-
ammonium hexafluorophosphate (0.1 M) as a supporting electro-
lyte with a scan rate of 100 mV/s and ferrocene was taken as
reference. Comparing with the HOMO level of electrolyte I3À/IÀ re-
dox couple (À4.85 eV vs vacuum),3 it can be found that all the dyes
could meet the requirements of regeneration after the photoin-
duced electron injection. Beside OL10, the LUMO level of OL7–
OL9 is higher enough than the conduction band edge of TiO2
(À4.00 eV vs vacuum),26 as an offset of ca. 0.2 eV is required for
the driving force of electron injecting.27 Comparing with their
dihexyloxy-substitute triphenylamine analog,24 it can be found
that their LUMO level is almost the same (except for OL3 and
Figure 2. ORTEP plot of OL7 with thermal ellipsoids shown at 50%. Disordered
ethano group and CH2Cl2 are omitted for clarity.
OL9,
DLUMO is 0.11 eV), while HOMO level of dimethylfluoreani-
line derivatives maintains somewhat lower (ranging from À0.03
to À0.09 eV). This may be attributed to the fluorene moiety which
disperses the electron density distribution and thus stabilized the
HOMO. To evaluate the effect of introducing benzo[c]thiophene
Table 2
Photovoltaic performance of dyes OL7–OL9 and N719
Dye
Voc/mV
Jsc/mA cmÀ2
FF
g/%
as a
dyes. From OL7 and OL8 to OL9 and OL10 the change in energy le-
vel
p bridge, we compared the HOMO and LUMO levels of the
OL7
OL8
OL9
N719
465
500
405
5
0
5
14.32 0.08
13.08 0.04
11.86 0.06
19.38 0.54
0.66 0.01
0.69 0.00
0.65 0.01
0.61 0.01
4.37 0.06
4.54 0.01
3.13 0.06
6.22 0.04
D
HOMO is À0.05 and 0 eV, respectively, and
D
LUMO is À0.32
and À0.75 eV, respectively, indicating that fused benzo[c]thio-
phene can stabilize the LUMO level remarkably while has little af-
fect on the HOMO level. This character may be applicable for fine
tuning the energy level of organic push–pull dyes to match the
conduction band edge of TiO2 and the sunlight spectra for better
performance in DSSC.
535 15
X-Ray analysis of OL7
For further understanding the relationship between the struc-
ture and performance of the designed dyes, we tried to obtain sin-
gle crystals of these dyes, but unfortunately only the single crystal
of OL7 was obtained by slow evaporation of a mixed solution of
CH2Cl2 and hexane. Its structure was subjected to X-ray analysis.
OL7 belongs to triclinic space group P-1 and its structure is
shown in Figure 2. The two fluorene moieties are highly coplanar,
respectively (mean deviation from plane is 0.0206 Å of C1–C13 and
0.0294 Å of C16–C28). The dihedral angle between C16–C28 and
C1–C13 rings is 62°. The dihedral angles between the benzene ring
and two fluorene planes are 64.3° and 64.5°, respectively. Also the
thiophene ring is distorted with the benzene ring in 34.5°. It is
noteworthy that thiophene and cyanoacrylic acid group are not
coplanar, torsion angle between C44 and C47 is 13.4°, and carbox-
ylate group plane twist 2.9° from the cyano group. Due to the exis-
tence of carboxylate group, there is hydrogen bonding between the
neighboring molecules in the crystal lattice in a head-to-tail
fashion.
Figure 3. The incident photon-to-current conversion efficiency spectra for DSSCs
based on OL7–OL9 and N719.
as it was not optimized in this work. Among all the dyes, OL8
exhibited the highest light to power conversion efficiency of
4.54% (Voc = 500 mV, Jsc = 13.08 mA cmÀ2, FF = 0.69), while OL7
Photovoltaic performance of the ring-fused thiophene bridged
bisdimethylfluorenylamino-based push–pull dyes
had a maximum short circuit current density of 14.32 mA cmÀ2
.
Although OL9 exhibited broader and stronger absorption in the vis-
ible region, its conversion efficiency was 3.13% with less short cir-
cuit current density and open circuit voltage, which was lower
than OL7 and OL8. Under the same condition, N719 sensitized
DSSC showed a conversion efficiency of 6.22%. Since OL10 was
unstable in solution and gradually decomposed with color fading
from original blue to yellow, we could not obtain reliable photovol-
taic data under the same condition of other dyes.
The dye-sensitized solar cells fabricated with these dyes and
N719 as a reference were measured and their photovoltaic perfor-
mance are listed in Table 2. It should be noted that the solar cell
performance for the new and reference dyes was compared under
the same but not optimal conditions and the efficiency for N719
listed in Table 2 was much lower than the reported highest value,

Q. Liu et al. / Tetrahedron Letters 53 (2012) 3264–3267
3267
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (Nos. 20971066, 21021062, 21003130), the Na-
tional Basic Research Program of China (Nos. 2011CB808704,
2011CBA00700), Program for New Century Excellent Talents in
University (No. NCET-08-0272), Grant-in-Aids for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science
and Technology, Japan (No. 22350083 to H.Y.), JST, CREST (to H.Y.).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.04.
052.
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In the incident photo-to-current conversion efficiency (IPCE)
spectra of these dyes (Fig. 3), OL7 and OL8 showed very similar
spectrum response with highest IPCE value of 85% and 82%, respec-
tively. Their IPCE values were over 70% at the wavelength between
400 and 550 nm and cut off at 700 nm. As a comparison, OL9 has
highest value of 58% and above 50% between 450 and 600 nm.
But it is interesting that the spectrum response of OL9 was ex-
tended to 800 nm, which is similar to that of N719.
The J–V curves of these dyes are shown inFigure 4. From the dark
current we could find that the charge recombination trend follows
the order of OL9 > OL7 > OL8 in these DSSCs. This may explain the
relative poor performance of OL9. Another reason for the lower per-
formance may be the poor solubility.28 Considering the good planar-
ity and
p
conjugation of fluorene and benzo[c]thiophene, it is
stacking which
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possible that the poor solubility is caused by the
p–p
prevents the efficient electron injection from the dye into the TiO2
conducting band. Structural modification with more space hin-
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of OL9 and the corresponding work is now undergoing.
16. Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C. Y.; Chandiran, A. K.; Nazeeruddin, M. K.;
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In summary, we have introduced benzo[c]thiophene and its pre-
cursor as
p bridge to bisdimethylfluorenylamino-based push–pull
organic dyes; their spectroscopic and electrochemical properties
have been studied. Fusion with benzene ring on thiophene bridge
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significantly expands
p-conjugation thus red-shift the spectrum
response and greatly stabilized the LUMO level of dyes. Dye-sensi-
tized solar cells fabricated with these dyes are measured and mod-
erate power conversion is obtained compared with that of N719,
suggesting that these dyes may be good candidate for sensitizers.
Further modifications of the dye structure and device optimization
to improve the performance of these dyes are now in progress.
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