Efficient organic dyes containing dibenzo heterocycles as conjugated linker part for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.tet.2012.12.074

Six new organic dyes, TBS1-TBS6, have been synthesized and characterized by optical, electrochemical, and theoretical studies. Among these dyes, DSSCs based on a dye containing N-hexylcarbazole and furan moieties (TBS4) showed the best performance, with light-to-electricity conversion efficiency up to 5.91% under AM 1.5 solar simulator, much higher than the reference dye TPS (4.24%). The high V_oc contributed to the good performance for TBS4, which may be ascribed to the combined effects of twisted structure and hexyl chain. Additionally, for TBS4 based DSSCs, under optimized conditions, the light-to-electricity conversion efficiency was enhanced up to 7.09%. The results suggest that dibenzo heterocycles containing dyes are promising candidates for application in DSSCs.

Full text of DOI:10.1016/j.tet.2012.12.074

Tetrahedron 69 (2013) 1970e1977
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Efficient organic dyes containing dibenzo heterocycles
as conjugated linker part for dye-sensitized solar cells
Shengyun Cai, Xiaohao Hu, Jinlong Han, Zhiyun Zhang, Xin Li, Chengyou Wang,
*
Jianhua Su
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Six new organic dyes, TBS1eTBS6, have been synthesized and characterized by optical, electrochemical,
and theoretical studies. Among these dyes, DSSCs based on a dye containing N-hexylcarbazole and furan
moieties (TBS4) showed the best performance, with light-to-electricity conversion efficiency up to 5.91%
under AM 1.5 solar simulator, much higher than the reference dye TPS (4.24%). The high Voc contributed
to the good performance for TBS4, which may be ascribed to the combined effects of twisted structure
and hexyl chain. Additionally, for TBS4 based DSSCs, under optimized conditions, the light-to-electricity
conversion efficiency was enhanced up to 7.09%. The results suggest that dibenzo heterocycles containing
dyes are promising candidates for application in DSSCs.
Received 6 November 2012
Received in revised form 23 December 2012
Accepted 31 December 2012
Available online 8 January 2013
Keywords:
DSSCs
Dibenzo
Heterocycles
Dibenzofuran
Dibenzothiophene
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
on triphenylamine donor generally can produce fluorescence
emission and the corresponding DSSCs exhibited unfavorable dye
1
€
Since the pioneering report by O’Regan and Gratzel in 1991,
dye-sensitized solar cells (DSSCs) have received extensive interest
because of their easy processing, low cost, high solar energy to
aggregation and fast charge recombination between the oxidized
dyes and the injected electrons.9,10 Many attempts were made to
increase the dihedron angles appropriately between the donor and
electricity conversion efficiency (
h
), and potential application in the
p spacer without hugely breaking the p conjugation of the dye
industry.2 Although the DSSCs based on ruthenium(II) polypyridyl
complexes exhibited a record high conversion efficiency over 11%:
N3, N719, and the black dye,3e5 there are many deficits to coun-
teract, such as high cost, the use of noble metal, and difficulties in
synthesis and purification.6 Recently, much attention is directed to
the development of metal-free organic dyes owing to the advan-
tages of environmental friendliness, higher structural flexibility,
lower cost, and easier preparation.2b,7 A large number of organic
dyes for use as molecular photovoltaics have been designed and
synthesized. Due to the effective photoinduced intramolecular
charge-transfer characteristics, most of the efficient organic sensi-
molecule. Previously, it was reported that the inserting of a fluorene
unit between triaryllamine unit and thiophene moiety could result
a relative large dihedron angle.10d Because dibenzo heterocycles,
such as carbazole, dibenzofuran, and dibenzothiophene have the
similar structure as fluorene, thus the inserting of dibenzo het-
erocycles units between triphenylamine unit and thiophene moiety
could also be expected to twist the dye molecule. Based on the
above ideas, we have developed six new dyes (Scheme 1) for DSSCs
using triphenylamine as donor, carbazole (or dibenzofuran or
dibenzothiophene) connected with thiophene or furan as p-spacer
and 2-cyno acetic acid as acceptor based on the following consid-
erations: (1) the direct linkage of carbazole unit with triphenyl-
amine was found to not only increase the electron-donating ability
but also reinforce the hole-transporting capabilities, which pro-
mote the charge separation.11,12 Besides, this kind of structure was
found to improve the morphological stability of the films in
OLEDs.11,12 Thus, similar effect could be expected to be produced in
DSSCs; (2) the insertion of carbazole (or dibenzofuran or dibenzo-
thiophene) unit between the triphenylamine unit and thiophene
(or furan) unit may twist the dye molecule thus result in appro-
priate deviation of a sensitizer molecule from planarity, which
tizers are modeled on the donore(
p
-spacer)e acceptor (De
peA)
system.8,9 Among others, the structure of the
p
spacer is one of the
crucial factors that influence the conversion efficiency of solar en-
ergy into electricity in DSSCs. Herein, we report our efforts on the
design of different
Triphenylamine is an excellent donor, however, due to the good
planarity between the donor and spacer, many sensitizers based
p conjugated linkers.
p
* Corresponding author. Tel./fax: þ86 21 64252288; e-mail address: bbsjh@
ecust.edu.cn (J. Su).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.12.074

S. Cai et al. / Tetrahedron 69 (2013) 1970e1977
1971
could avoid dye aggregation and/or close contact of electrolyte
molecules with the TiO2. As a result, a high Voc could be anticipated;
(3) dibenzofuran and dibenzothiophene are excellent charge car-
riers and electron transporters,12bef however, very few of sensi-
TBS1eTBS6 were obtained via Knoevenagel condensation with
cyanoacetic acid in the presence of a catalytic amount of piperidine
in THF. Compound TPS was synthesized by using a literature pro-
cedure.17 The purification of intermediates was performed by flash
column chromatography and subsequent HPLC. All the in-
termediates and target dyes were characterized by standard spec-
troscopic methods including 1H NMR, 13C NMR, and HRMS. Clearly,
all the intermediates and target compounds are stable.
tizers have employed these structures as conjugated
p-spacers in
DSSCs, not to mention the systematic research on them. Here, by
changing carbazole unit for dibenzofuran or dibenzothiophene
moiety, thus the effects of the insertion of three kinds of moieties
on the performance of DSSCs based on these dyes could be sys-
tematically investigated; (4) thiophene could result in the bath-
chromic shift of the absorption spectrum, and furan can provide
more effective conjugation and lower the energy of the charge-
transfer transition, thus the photovoltaic performance for the
2.2. Optical properties
UV/Vis absorption spectra of the seven dyes in a diluted solution
of THF are shown in Fig. 1 and their absorption data are listed in
Table 1. In the UV/Vis spectra, the dyes TBS1eTBS6 and TPS all
exhibit one major and broad absorption band at 350e600 nm with
the maximum absorption at 422, 406, 414, 442, 384, 407, and
443 nm, respectively. The corresponding maximum molar extinc-
tion coefficients are 2.14ꢀ104, 2.86ꢀ104, 2.80ꢀ104, 2.70ꢀ104,
dyes with the two
p
-spacers could be compared with.9e,12a,13,14
Based on the above considerations, herein, we report the synthe-
sis, characterization, optical properties and electrochemical char-
acteristics of the six new organic dyes.
2.30ꢀ104, 2.02ꢀ104, and 2.51ꢀ104
M
ꢁ1 cmꢁ1. The absorption band
was ascribed to the intramolecular charge-transfer (ICT) between
the donor and the acceptor. It is obvious that TBS2 have higher
molar extinction coefficients and red-shifted absorption spectra
compared with TBS5, which may be due to the introduction of the
thiophene instead of furan moiety. The red-shifted phenomenon
may be ascribed to the different aromatic ability of the two five-
membered heteroaromatic bridges. The aromatic of furan is larger
than that of thiophene due to the stronger electronegative of oxy-
gen element than that of sulfur element. It could be seen that the
lmax values in the visible light region shift to a lower energy with
the decreased electronegativity of the heteroatoms, which is in
accordance with results reported previously.9e,13,14,18 The compar-
ison of TBS3 and TBS6 displayed the same phenomenon. Except
TBS4, all these dyes display obvious blue-shifted absorption spectra
compared with TPS, which could be explained by that the in-
troduction of dibenzo heterocycles moiety between thiophene (or
furan) and triphenylamine may twist the molecule thus lead to
larger dihedral angles between donor and thiophene moiety and
make less conjugation for the whole molecule. Among these dyes,
TBS4 exhibits the broadest absorption band in visible light region
and high molar extinction coefficient, thus more visible light could
be harvested and a high Jsc could be expected.
Scheme 1. Molecular structures of the dyes TBS1eTBS6 and TPS.
2. Results and discussion
2.1. Synthesis
The synthetic route to the six dyes TBS1eTBS6 is depicted in
Scheme 2. The synthesis of these compounds started from triphe-
nylamine, which was used to prepare (4-(diphenylamino)phenyl)
boronic acid according to previous report.17 The three key in-
termediates 4aec were easily obtained from commercially avail-
able materials carbazole, dibenzofuran and dibenzothiophene via
bromination reaction, respectively, according to the paper.15,16
Asymmetrical Suzuki coupling reaction of compound 4aec with
4-(diphenylamino)phenylboronic acid gave white compounds
5aec. Then Suzuki coupling of 5aec with 5-formylthiophene- 2-
boronic acid or 5-formylfuran-2-boronic acid, respectively, affor-
ded monoaldehyde-substituted precursors 6aef. The final products
The absorption spectra of TBS1eTBS6 on thin transparent films
(4 mm thick) after 12 h adsorption in THF solution are shown in
Fig. 2. The maximum absorption peaks for TBS1eTBS6 on the TiO2
films are located at 409, 392, 394, 410, 379, and 399 nm, corre-
spondingly blue-shifted by 13, 14, 20, 32, 5, 8 nm, respectively, from
Scheme 2. Synthetic routes of the dyes TBS1eTBS6. (i) NBS, CCl4; (ii) (a) BuLi, THF; (b)
B(OiPr)3; (c) 2 N HCl (aq); (iii) Pd(PPh3)4, K2CO3,THF; (iv) Pd(PPh3)4, K2CO3,THF; (v)
THF, 2-cyanoacetic acid, piperidine.
Fig. 1. UV/Vis absorption spectra of TBS1eTBS6 and TPS in THF (2ꢀ10ꢁ5 mol/L).

1972
S. Cai et al. / Tetrahedron 69 (2013) 1970e1977
Table 1
Optical and electrochemical data of TBS1eTBS6 and TPS
lmaxa/nm ε/104 Mꢁ1
cmꢁ1
lmaxb/nm
on TiO2
Ffa,c
E0-0
EHOMO
(V)
ELUMO
(V)
d
e
f
Dye
)
TBS1
TBS2
TBS3
TBS4
TBS5
TBS6
TPS
422(2.14)
406(2.86)
414(2.80)
442(2.70)
384(2.30)
407(2.02)
443(2.51)
409
392
394
410
379
399
414
0.028
2.38
2.60
2.48
2.36
2.70
2.52
2.38
0.89
1.00
0.91
0.88
0.98
0.89
1.04
ꢁ1.49
ꢁ1.60
ꢁ1.57
ꢁ1.48
ꢁ1.72
ꢁ1.63
ꢁ1.34
0.0076
0.0022
0.0098
0.0045
0.0065
0.54
a
Recorded in THF.
Absorption maximum on TiO2 transparent films.
Florescence quantum yields with rhodamine
b
c
B
(Ff¼0.70) in ethanol as
a standard.
d
E0-0 of dyes was estimated from the absorption thresholds from absorption
spectra of dyes adsorbed on the TiO2 film.
e
HOMOs were measured in CH3CN, scan rate¼100 mVsꢁ1
electrolyte¼(n-
;
C4H9)4NPF6; potentials were calibrated with ferrocene/ferrocenium (Fc/Fcþ) as an
external reference.
f
LUMO energy levels were estimated by subtracting E0ꢁ0 from the HOMO energy
level. NHE¼normal hydrogen electrode.
Fig. 3. Emission spectra of dyes in THF (1ꢀ10ꢁ5 mol/L).
calibrated using a ferrocene/ferrocenium (Fc/Fcþ) redox couple as
an external standard. The electrochemical data for the seven dyes
are shown in Fig. S2 (see in the Supplementary data) and sum-
marized in Table 1. HOMO levels of TBS1eTBS6 and TPS dyes
adsorbed on TiO2 films corresponding to their first redox potential
were determined to be 0.89,1.00, 0.91, 0.88, 0.98, 0.89, and 1.04 V vs
NHE, respectively, which were sufficiently more positive than the
iodine/iodide redox potential value (0.4 V), indicating that the ox-
idized dyes formed after electron injection into the conduction
band of TiO2 could thermodynamically accept electrons from Iꢁ
ions. It was easily found that inserting a furan unit instead of
a thiophene unit close to the acceptor group slightly promotes the
HOMO level, such as TBS4 and TBS1. The higher HOMO level of
TBS4 compared to TBS1 indicates that TBS4 is much easier to give
electrons. LUMO levels of TBS1eTBS6 and TPS dyes are much more
negative than that of the conduction band of TiO2 (ꢁ0.5 V vs NHE).
Generally, a minimal driving force of 0.2 V is sufficient to ensure fast
excited-state injection and regeneration of the oxidized dye.22
Thus, the excited-state electrons injection of all seven dyes are
guaranteed to be efficient. It is noteworthy that LUMO levels of
TBS1eTBS6 were much more negative than that of TPS, meaning
that TBS1eTBS6 have more driving force to inject electrons into the
conduction band of TiO2.
Fig. 2. UV/Vis absorption spectra of TBS1eTBS6 adsorbed on TiO2 films.
the solution spectra. The blue shift of the absorption spectra of
TBS1eTBS6 on TiO2 could be ascribed to deprotonation of car-
boxylic acid upon adsorption onto the TiO2 surface and formation of
H-aggregates.9e,14,19,20 Noted that compared with carbazole con-
taining dyes, the onset wavelengths of dibenzofuran (or dibenzo-
thiophene) containing dyes on TiO2 films are blue-shifted
obviously, which is consistent with the situation that the dyes in
THF solution. The emission spectra of TBS1eTBS6 and TPS in a di-
luted solution of THF are in Fig. 3 and the fluorescent quantum yield
data are listed in Table 1. It could be easily observed that the dyes
TBS1eTBS6 are almost non-emissive while TPS displayed obvious
fluorescence emission, suggesting that the inserting of carbazole
(or dibenzofuran or dibenzothiophene) twists the molecule and
suppresses the light emission thus decreases the energy loss.
2.4. Computational analysis
To gain insight into the molecular structure, the three-
dimensional geometries of the compounds were optimized using
density functional theory (DFT) at the B3LYP/6-31G* level of the-
ory.23 The Gaussian 03 program package was used to carry out the
calculations. As shown in Scheme 3, in the ground state the ge-
ometry of the dyes is not planar. For TBS1eTBS3, which contain
a thiophene moiety, the molecules are mainly twisted at two po-
sitions: One is located between the phenyl ring attached on the
nitrogen atom from the triphenylamine unit and the phenyl ring
from the carbazole (or dibenzofuran or dibenzothiophene) unit and
the dihedral angles are 36.8ꢂ, 35.5ꢂ, and 35.7ꢂ, respectively, the
other is located between the phenyl ring from the carbazole (or
dibenzofuran or dibenzothiophene) unit and thiophene moiety and
the corresponding dihedral angles are 25.3ꢂ, 23.5ꢂand 24.1ꢂ. How-
ever, for TBS4eTBS6, only one place is twisted, which is located
between the phenyl ring attached on the nitrogen atom from the
triphenylamine unit and the phenyl ring from the carbazole (or
dibenzofuran or dibenzothiophene) unit and the dihedral angles
2.3. Electrochemical properties
To estimate the feasibility of electron-transfer from the excited-
state of the dyes to the conduction band of TiO2 electrode, we
carried out cyclic voltammetry experiments in order to determine
the redox potentials. The redox potentials of the seven dyes,
TBS1eTBS6 and TPS were measured in CH3CN using 0.1 M tetra-
butylammonium hexafluorophosphate as the supporting electro-
lyte, Pt as the counter electrode and a saturated calomel electrode
(SCE) as reference electrode. The SCE reference electrode was

S. Cai et al. / Tetrahedron 69 (2013) 1970e1977
1973
are 37.0ꢂ, 35.5ꢂ, 36.4ꢂ. It is clear that inserting a thiophene unit
instead of a furan moiety close to the acceptor can twist the whole
molecule more thus decrease the conjugation of the molecule to
some extent. One explanation for the larger angles between car-
bazole (or dibenzofuran or dibenzothiophene) moiety and thio-
phene moiety compared to the angle between carbazole and furan
moieties is that the steric hindrance arose from larger sulfur atom is
more pronounced.9e Compared with TPS, the molecules of the
TBS1eTBS6 are twisted more, which facilitates them to prevent the
formation of excimer and/or aggregates. The non-planar shape of
these dyes due to the existence of carbazole (or dibenzofuran or
dibenzothiophene) may also reduce the degree of electronic reso-
nance between D and A.24 In the process of photoexcitation, one
key to increase the device efficiency is to reduce the rate of internal
charge recombination. The non-planar geometry between D and A
may help to slow down the rate, therefore higher Voc could be
expected.
value of 71.4% at 500 nm for TBS1, 76.3% at 410 nm for TBS2, 76.3%
at 400 nm for TBS3, 83.9% at 490 nm for TBS4, 75.2% at 400 nm for
TBS5, and 83.3% at 450 nm for TBS6, respectively. However, the
highest value of IPCE for TPS was only 63.0% at 470 nm, much lower
than that for TBS1eTBS6. Note that compared to the absorption
spectra of sensitized films, the IPCE spectra were broaden signifi-
cantly. This phenomenon could be ascribed to the scattering effect
of the large particles in the film and the reflective effect of the Pt
counter electrode. Moreover, the electrolyte could also pose in-
fluence on the IPCE values. The IPCE performances of the DSSCs
with carbazole containing dyes are better than that of the corre-
sponding dibenzofuran (or dibenzothiophene) containing dyes due
to their broader photocurrent action spectrum. The IPCE perfor-
mance of the TBS1 device is poorer than that the TBS4 device due to
its narrower photocurrent action spectrum and lower IPCE values,
which may be ascribed to the lower molar extinction coefficients.
Obviously, with broadest spectra of IPCE and almost the highest
values of IPCE, TBS4 sensitized TiO2 electrode would generate
a higher conversion yield compared to other six dyes.
The photocurrentevoltage (JeV) plots of DSSCs fabricated with
these dyes, along with that of TPS for comparison, are shown in
Fig. 5. The detailed parameters, i.e., short circuit current (Jsc), open-
circuit photovoltage (Voc), fill factor (FF), and solar-to-electricity
conversion efficiency (h) measured under AM 1.5 solar light
(100 mW cmꢁ2) are summarized in Table 2. When THF was used as
the solvent for dye baths, the DSSCs based on TBS1eTBS6 and TPS
dyes exhibited inferior conversion efficiency of 5.29%, 3.03%, 3.84%,
5.91%, 3.36%, 4.21%, and 4.25%. From these data, it is clear that the
DSSCs based on TBS1eTBS6 dyes all had much higher Voc than that
based on TPS just as expected. Noticeably, DSSCs based on dyes
containing furan moiety correspondingly displayed lager Jsc and
thus higher conversion efficiency than based on dyes containing
thiophene moiety, such as TBS4 and TBS1, which could be ascribed
to broader spectra of IPCE and higher IPCE values. Clearly, DSSCs
based on TBS1 exhibited higher Voc, larger Jsc thus enhanced con-
version efficiency than that based on TPS, suggesting that inserting
a carbazole unit between the triphenylamine unit and thiophene
moiety close to the acceptor could significantly increase the Voc
without decreasing the Jsc. The performances of the DSSCs based on
the dyes containing dibenzofuran or dibenzothiophene moieties
were poorer than that the corresponding dyes containing the car-
bazole moieties. The lower Jsc and lower Voc should be responsible
for it. The lower Jsc for dibenzofuran (or dibenzothiophene) con-
taining dyes could be ascribed to the poorer IPCE performances
compared with the DSSCs based on the corresponding dyes
Scheme 3. Dihedral angles of the neighboring aromatic rings for TBS1eTBS6 and TPS.
2.5. DSSCs performance
The DSSCs were prepared by using dye-adsorbed TiO2 electrode,
Pt-coated glass as a counter electrode, and an acetonitrile solution
with iodine (0.05 M), lithium iodide (0.1 M), 1,2-dimethyl-3-
propylimidazolium iodide (0.6 M), and 4-tert-butylpyridine
(0.5 M) as electrolyte. Fig. 4 shows the action spectra of incident
photon-to-current conversion efficiency (IPCE) for DSSCs with
these dyes when THF was used as the solvent for dye loading. The
IPCE exceeds 60% in the range 395e525 nm for TBS1, 355e465 nm
for TBS2, 350e485 nm for TBS3, 375e545 nm for TBS4,
355e475 nm for TBS5, and 355e500 nm for TBS6 with the highest
Fig. 4. IPCE spectra of DSSCs based on TBS1eTBS6 and TPS when THF was used as the
solvent for dye baths.
Fig. 5. IeV curves of DSSCs based on TBS1eTBS6 and TPS with dye baths in THF.

1974
S. Cai et al. / Tetrahedron 69 (2013) 1970e1977
Table 2
Photovoltaic performance parameters of the DSSCs based on TBS1eTBS6 and TPS
Dye
Solventsa
Jsc/mA cmꢁ2
Voc/V
FF (%)
Eff (%)
TBS1
TBS2
TBS3
TBS4
TBS5
TBS6
TPS
TBS1
TBS1
TBS1
TBS4
TBS4
TBS4
TBS4b
THF
THF
THF
THF
THF
THF
THF
CH3CN
CH2Cl2
iPrOH
CH3CN
CH2Cl2
iPrOH
CH3CN
9.36
6.02
7.50
11.2
6.83
7.59
8.97
11.09
9.69
7.81
11.8
10.9
10.7
13.89
0.786
0.669
0.704
0.757
0.666
0.745
0.652
0.730
0.771
0.755
0.770
0.735
0.756
0.769
71.8
75.2
72.7
69.5
73.9
74.5
72.7
68.6
65.9
64.0
0.698
0.681
0.700
0.664
5.29
3.03
3.84
5.91
3.36
4.21
4.25
5.56
4.92
3.78
6.33
5.46
5.43
7.09
a
Solvents employed for dye baths for TiO2 sensitization.
DSSCs fabricated with 16 mm thick 13 nm sized TiO2 nanoparticles (T/SP).
b
Fig. 6. Nyquist plots of impedance spectra for the DSSCs based on these dyes with dye
baths in THF.
containing carbazole moieties. The lower Voc may be indicative of
more severe charge recombination between the electrolytes and
the oxidized dyes compared with the DSSCs based on the corre-
sponding dyes containing carbazole moieties. The DSSCs based on
TBS1 displayed poorer performance than that based on TBS4,
which could be attributed to the much lower Jsc. The DSSCs based
on TBS4 displayed the best performance with high Voc, large Jsc and
relatively low FF corresponding to an overall efficiency of 5.91%. In
different solvents, dyes exhibit diversified interactions with the
solvents and could cause changes of the physical and chemical
properties between the dyes and semiconductor surface.9a,21,24,25
To investigate the effect of solvents on dye baths for TiO2 sensiti-
zation, TBS1 and TBS4 in different solvents were employed to
sensitize TiO2. The photovoltaic performance for DSSCS based on
TBS1 and TBS4 were tested and shown in Table 2. The table reveals
that the performances of TBS1 based DSSCs were all lower than that
TBS4 based DSSCs when used the same solvents for dye baths,
which is in accordance with the situation that employed THF as the
solvent for dye baths. It could be seen that the two dyes based
DSSCs both displayed best when CH3CN was used as the solvent for
dye baths. Noted that TBS4 based DSSCs fabricated with dye baths
Fig. 7. Bode phase plots of the impedance spectra for the DSSCs based on these dyes
with dye baths in THF.
in CH3CN exhibit the best performance with
h enhanced up to
6.33%. The enhanced Jsc is responsible for the good device perfor-
mance, which may be resulted from the increased amount of dyes
adsorbed on TiO2 films because of the interaction between the dyes
and the solvent, what’s more, with dye baths in CH3CN, the DSSCs
made of thicker films of 13 nm sized TiO2 nanoparticles (T/SP)
exhibited the solar-to-electricity conversion efficiency of 7.09%
(see Fig. S4 and Fig. S5 in the Supplementary data) due to boomed
Jsc. The enhancement of Jsc could be also ascribed to the increased
amount of dyes adsorbed on TiO2 films because of thicker films
of TiO2.
surface (Rrec), it can be deduced by fitting curves from a range of
intermediate frequencies using Z-view software. It is related to the
charge recombination rate, e.g., a smaller Rrec indicates a faster
charge recombination and therefore a larger dark current (shown in
Fig. S3 in the Supplementary data). The radius of the biggest
semicircle
Rrec
values
decreased
in
the
order
of
TBS1zTBS4>TBS6>TBS3>TBS5>TBS2zTPS. The result appears
to be roughly consistent with the increase of Voc in the DSSCs based
on TBS1eTBS6 and TPS. In Bode phase plots, the peak position of
the middle frequency is related to the electron lifetime, for exam-
ple, a shift to low frequency corresponds to a longer electron life-
time. Thus, according to Fig. 7, the order of the corresponding
electron lifetimes TBS1zTBS4>TBS6>TBS3>TBS2zTBS5>TPS
further approximately support the order of the Voc of DSSCs based
on these dyes. From these data, it could be concluded that the
DSSCs based on the dyes of TBS1eTBS6 with the inclusion of car-
bazole (or dibenzofuran or dibenzothiophene) show slower charge
recombination and longer electron lifetime, which contributes to
much higher Voc. Noted that carbazole containing dyes exhibited
much higher Voc than the dibenzofuran or dibenzothiophene con-
taining dyes. The possible reason is that the attachment of hexyl on
the carbazole moiety may suppress the dye aggregation and in-
termolecular charge recombination further.
2.6. Electrochemical impedance spectroscopy
To further elucidate the photovoltaic properties of these dyes,
electrochemical impedance spectroscopy (EIS) was performed. EIS
analysis of the DSSCs made with these sensitizers were carried out
in the dark under a forward bias of ꢁ0.70 V. The Nyquist plots of the
DSSCs based on the seven dyes under a forward bias of ꢁ0.70 V
with a frequency range of 0.1 Hze100 kHz are shown in Fig 6. The
Bode phase plots are also shown in Fig. 7. Two semicircles were
observed in the Nyquist plots. The smaller semicircle at high fre-
quency is assigned to the redox charge-transfer response at the Pt/
electrolyte interface. The large one at the intermediate frequency
represents the electron-transfer impedance at the TiO2/dye/elec-
trolyte interface. The charge recombination resistance at the TiO2

S. Cai et al. / Tetrahedron 69 (2013) 1970e1977
1975
3. Conclusion
UVevis absorption spectra of the dyes in solution and adsorbed on
TiO2 films were obtained from a Varian Cary 500 spectrophotom-
eter. The cyclic voltammograms of dyes were measured with
a Versastat II electrochemical workstation (Princeton applied re-
search) using a normal three-electrode cell with a Pt working
electrode, a Pt wire counter electrode, and a regular calomel ref-
erence electrode in saturated KCl solution. The electrochemical
impedance spectroscopy (EIS) measurements of all the DSSCs were
performed using a Zahner IM6e Impedance Analyzer (ZAHNER-
Elektrik GmbH & CoKG, Kronach, Germany).
Photovoltaic measurements were carried out by an AM 1.5 solar
simulator equipped with a 300 W xenon lamp (Model No. 91160,
Oriel). The power of the simulated light was calibrated to
100 mW cmꢁ2 using a Newport Oriel PV reference cell system
(Model 91160V). Cell active area was tested with a mask of
0.25 cmꢁ2. The photocurrent action spectra were measured with an
IPCE test system consisting of a model SR830 DSP Lock. In Amplifier
and a model SR540 Optical Chopper (Stanford Research Corpora-
tion, USA), a 7IL/PX150 xenon lamp and power supply, and
a 7ISW301 spectrometer.
In summary, three different dibenzo heterocycles units (carba-
zole, dibenzofuran, and dibenzothiophene) are introduced into
spacer section and six new metal-free organic dyes (TBS1eTBS6)
based on De eA structure were successfully synthesized and
p-
p
demonstrated as efficient sensitizers for DSSCs. The results showed
that the inserting of carbazole (or dibenzofuran or dibenzothio-
phene) appropriately twists the molecule thus decreases the en-
ergy loss caused by fluorescence emission. More importantly, the
twisted structure slowed down intermolecular charge re-
combination, as a result, high Voc were obtained. It is noteworthy
that the inserting of carbazole (or dibenzofuran or dibenzothio-
phene) moiety elevated the HOMO levels of the dyes so that all the
dyes have strong donating abilities. Among these seven dyes, the
DSSCs based on TBS4 exhibited the best performance with an
overall efficiency of 5.91%, much higher than that TPS (4.24%) under
the same condition. The good device performance could be inter-
preted by reinforced donating ability, low emission, high electron
injection efficiency and low intermolecular charge recombination
rate. Additionally, with an optimized dye baths for TiO2 sensitiza-
tion and thicker film of TiO2 nanoparticles (T/SP), the Jsc was ob-
4.3. Synthesis
viously enhanced, as a result,
h of the DSSCs based on TBS4 was
improved to 7.09%. The increase of the Jsc may be resulted from the
increase of amount of dyes adsorbed on TiO2 films caused by the
interaction between the dyes and solvents and thicker films.
However, the performances of the DSSCs based on the dyes con-
taining dibenzofuran or benzothiophene moieties were poorer
than that the TPS based DSSCs. This could be explained by that the
narrower absorption spectra resulted in the corresponding poorer
IPCE performances, which contributed to lower Jsc. On the other
hand, the absence of attachment of a hexyl chain on the di-
benzofuran (or dibenzothiophene) moiety contributed to more
severe charge recombination between the electrolyte and the oxi-
dized dyes, thus resulted in lower Voc compared with carbazole
containing dyes based DSSCs. By the insertion of electron-deficient
4.3.1. 5-(7-(4-(Diphenylamino)phenyl)-9-hexyl-9H-carbazol-3-yl)
thiophene-2-carbaldehyde (6a). In a 100 mL three-necked round-
bottom flask, compound 5a obtained above (1.2 g, 2.0 mmol), K2CO3
(1.38 g, 10 mmol), Pd(PPh3)4 (50 mg, 0.05 mmol), water (5 mL), and
THF (10 mL) were heated to 45 ꢂC for 1 h under an argon atmo-
sphere. Then, (4-formylphenyl)boronic acid (468 mg, 3.0 mmol)
dissolved in 10 mL THF was added and the reaction was stirred at
45 ꢂC for 4 h. The mixture was washed with water and extracted
with chloroform. The combined organic layer was dried over an-
hydrous MgSO4 and concentrated using a rotary evaporator. The
residue was purified by column chromatography on silica (petro-
leum ether/dichloromethane¼1:2, v/v) to yield the product as
a yellow powder (360 mg, 30%). 1H NMR (400 MHz, CDCl3): 9.89 (s,
1H), 8.44 (d, J¼1.5 Hz, 1H), 8.33 (s, 1H), 7.83e7.76 (m, 2H), 7.74 (d,
J¼8.5 Hz, 1H), 7.61 (d, J¼8.5 Hz, 2H), 7.50e7.42 (m, 3H), 7.30 (d,
J¼8.1 Hz, 4H), 7.20 (d, J¼8.5 Hz, 2H), 7.17 (d, J¼7.7 Hz, 4H), 7.04 (t,
J¼7.3 Hz, 2H), 4.34 (t, J¼7.2 Hz, 2H), 1.95e1.86 (m, 2H), 1.36e1.28
p
spacer, it will improve the conjugation extent of the dye molecule
and would be a good try to enhance the device performance for the
DSSCs based on these dyes.
4. Experimental section
(m, 6H), 0.87 (t, J¼7.0 Hz, 3H). 13C NMR (DMSO-d6, 100 MHz),
d:
182.63, 156.32, 147.82, 146.62, 141.48, 141.22, 140.27, 137.95, 136.01,
132.67, 129.29, 127.93, 125.63, 124.63, 124.57, 124.41, 124.26, 124.15,
123.57, 123.16, 118.62, 118.56, 109.43, 109.37, 43.44, 31.57, 29.02,
26.98, 22.56. HRMS (m/z): [M]þ calcd for C41H37N2OS, 605.2627;
found, 605.2631.
4.1. Materials and reagents
Optically transparent FTO conducting glass (fluorine doped tin
oxide overlayer, transmission >90% in the visible, sheet resistance
15
U
squareꢁ1), Titania pastes of Ti-Nanoxide T/SP and Ti-Nanoxide
300 were obtained from Geao Science and Educational Co. Ltd. of
China. Tetra-n-butylammonium hexafluorophosphate (TBAPF6), 4-
tert-butylpyridine (TBP), 2.4 M BuLi solution in hexane and the
reaction catalyst Pd(PPh3)4 were obtained from J&K. Lithium iodide,
1,2-dimethyl-3-propylimidazolium iodide were from Fluka and
iodine, 99.999%, was from Alfa Aesar. Acetonitrile with HPLC purity
was purchased from SK Chemicals. THF was pre-dried over sodium
and distilled under argon atmosphere from sodium benzophenone
ketyl immediately prior to use. All other chemicals were used as
received without further purification. The starting materials 3,6-
4.3.2. Compound 6bef. Compound 6bef was synthesized by the
same procedure as described for 6a. Compound 6b: 1H NMR
(400 MHz, CDCl3): 9.91 (s, 1H), 8.27 (d, J¼1.7 Hz, 1H), 8.17 (d,
J¼1.5 Hz, 1H), 7.79 (d, J¼8.5 Hz, 1H), 7.71 (dd, J¼8.5,1.5 Hz, 1H), 7.63
(d, J¼8.6 Hz, 3H), 7.56 (d, J¼8.5 Hz, 2H), 7.47 (d, J¼3.9 Hz, 1H), 7.29
(t, J¼7.8 Hz, 5H), 7.20 (d, J¼8.5 Hz, 2H), 7.16 (d, J¼8.1 Hz, 4H), 7.05 (t,
J¼7.2 Hz, 2H). 13C NMR (DMSO-d6, 100 MHz),
d: 182.72, 154.36,
147.63, 147.54, 147.44, 142.37, 141.17, 138.46, 137.92, 137.51, 136.31,
135.55, 134.68,129.67,129.35,128.05,126.50,125.02, 124.50, 124.16,
119.68, 119.29. HRMS (m/z): [M]þ calcd for C35H24NO2S, 522.1528;
found, 522.1528. Compound 6c: 1H NMR (400 MHz, CDCl3): 9.92 (s,
1H), 8.47 (d, J¼1.5 Hz, 1H), 8.37 (d, J¼1.4 Hz, 1H), 7.91 (dd, J¼8.3,
1.9 Hz, 2H), 7.81e7.70 (m, 3H), 7.60 (d, J¼8.6 Hz, 2H), 7.53 (d,
J¼3.9 Hz,1H), 7.30 (t, J¼7.9 Hz, 4H), 7.19 (dd, J¼16.0, 8.1 Hz, 6H), 7.06
dibromo-9-hexyl-9H-carbazole,
2,8-dibromodibenzo[b,d]furan,
and 2,8-dibromodibenzo[b,d]thiophene were synthesized by the
previous reported procedures.
4.2. Instruments and characterization
(t, J¼7.3 Hz, 2H). 13C NMR (DMSO-d6, 100 MHz),
d: 182.75, 157.21,
156.10, 154.52, 147.68, 147.22, 142.28, 137.63, 136.48, 134.90, 129.34,
128.25, 128.07, 127.01, 125.96, 125.31, 124.45, 123.01, 118.84, 118.74,
112.50, 112.03. HRMS (m/z): [M]þ calcd for C35H24NOS2, 538.1299;
found, 538.1296. Compound 6d: 1H NMR (400 MHz, CDCl3): 9.64 (s,
The new compounds were characterized by NMR and Mass.
NMR spectra were obtained on a Brucker AM 400 spectrometer and
Mass spectra were recorded on an ESI mass spectrometer. The

1976
S. Cai et al. / Tetrahedron 69 (2013) 1970e1977
1H), 8.64 (d, J¼1.4 Hz, 1H), 8.37 (d, J¼1.5 Hz, 1H), 7.93 (dd, J¼8.6,
1.6 Hz, 1H), 7.74 (dd, J¼8.5, 1.7 Hz, 1H), 7.61 (d, J¼8.6 Hz, 2H), 7.46 (t,
J¼8.3 Hz, 2H), 7.37 (d, J¼3.7 Hz, 1H), 7.31e7.26 (m, 4H), 7.19 (dd,
J¼13.8, 8.1 Hz, 6H), 7.04 (t, J¼7.3 Hz, 2H), 6.86 (d, J¼3.7 Hz, 1H), 4.34
(t, J¼7.2 Hz, 2H), 1.96e1.86 (m, 2H), 1.46e1.30 (m, 6H), 0.87 (t,
J¼1.4 Hz, 1H), 8.51 (s, 1H), 8.16e8.06 (m, 3H), 7.97 (d, J¼4.0 Hz, 1H),
7.87 (dd, J¼8.4, 1.8 Hz, 1H), 7.84e7.77 (m, 3H), 7.35 (t, J¼7.9 Hz, 4H),
7.14e7.06 (m, 8H). 13C NMR (DMSO-d6, 100 MHz),
d: 153.01, 147.03,
146.73, 141.29, 140.31, 137.72, 136.82, 136.35, 136.00, 133.91, 129.58,
129.16, 128.09, 126.00, 125.41, 125.03, 124.01, 123.51, 123.45, 123.18,
120.20, 119.74, 116.54. HRMS (m/z): [M]þ calcd for C38H25N2O2,
605.1357; found, 605.1354. Compound TBS4: 1H NMR (400 MHz,
DMSO-d6): 13.58 (s, 1H), 8.88 (d, J¼1.4 Hz, 1H), 8.41 (s, 1H),
8.11e8.04 (m, 2H), 7.84e7.77 (m, 2H), 7.71 (dd, J¼11.5, 8.7 Hz, 3H),
7.61 (d, J¼3.8 Hz, 1H), 7.38e7.31 (m, 5H), 7.12 (d, J¼8.6 Hz, 2H),
7.08e7.06 (m, 6H), 4.47 (t, J¼6.8 Hz, 2H), 1.80 (d, J¼6.9 Hz, 2H),
1.37e1.23 (m, 6H), 0.8 (t, J¼7.1 Hz, 3H). 13C NMR (DMSO-d6,
J¼7.0 Hz, 3H). 13C NMR (DMSO-d6, 100 MHz),
d: 176.67, 161.28,
151.58, 147.83, 146.62, 141.61, 140.22, 135.98, 132.71, 129.28, 127.90,
125.52, 124.40, 124.27, 123.49, 123.44, 120.06, 118.73, 117.96, 109.34,
109.26, 106.15, 43.43, 31.55, 29.02, 26.97, 22.55, 14.02. HRMS (m/z):
[M]þ calcd for C41H37N2O2, 589.2855; found, 589.2858. Compound
6e: 1H NMR (400 MHz, CDCl3): 9.67 (s, 1H), 8.50 (d, J¼1.6 Hz, 1H),
8.20 (d, J¼1.6 Hz, 1H), 7.92 (dd, J¼8.6, 1.8 Hz, 1H), 7.72 (dd, J¼8.6,
1.8 Hz, 1H), 7.63 (dd, J¼8.6, 4.8 Hz, 2H), 7.57 (d, J¼8.6 Hz, 2H), 7.37
(d, J¼3.7 Hz, 1H), 7.32e7.27 (m, 5H), 7.20 (d, J¼8.6 Hz, 2H), 7.17 (d,
J¼7.6 Hz, 4H), 7.05 (t, J¼7.3 Hz, 2H), 6.90 (d, J¼3.7 Hz, 1H). 13C NMR
100 MHz), d: 164.21, 160.67, 147.19, 146.83, 145.98, 141.23, 139.86,
137.44, 135.28, 131.28, 131.58, 129.52, 127.60, 125.20, 124.03, 123.79,
122.94, 122.58, 119.52, 118.08, 117.97, 117.06, 110.28, 110.21, 108.29,
95.36, 42.55, 30.91, 28.53, 26.06, 21.97, 13.80. HRMS (m/z): [M]þ
calcd for C44H38N3O3, 656.2913; found, 656.2909. Compound TBS5:
1H NMR (400 MHz, DMSO-d6): 8.69 (d, J¼1.1 Hz, 1H), 8.32 (s, 1H),
8.03 (dd, J¼8.7, 1.6 Hz, 1H), 7.91 (s, 1H), 7.79 (dd, J¼12.1, 5.1 Hz, 2H),
7.74 (d, J¼8.6 Hz, 1H), 7.67 (d, J¼8.6 Hz, 5H), 7.35 (d, J¼8.0 Hz, 3H),
7.32 (d, J¼2.6 Hz, 2H), 7.26 (d, J¼3.6 Hz, 1H), 7.10 (d, J¼3.3 Hz, 2H),
(DMSO-d6, 100 MHz),
d: 177.02, 159.72, 157.31, 156.05, 151.92,
147.68, 147.20, 136.46, 134.85, 129.34, 128.02, 126.92, 125.17, 124.98,
124.45, 123.00, 118.91, 117.92, 112.39, 112.00, 107.23. HRMS (m/z):
[M]þ calcd for C35H24NO3, 506.1756; found, 506.1741. Compound 6f:
1H NMR (400 MHz, CDCl3): 9.69 (s, 1H), 8.69 (s, 1H), 8.45 (d,
J¼1.5 Hz, 1H), 7.91 (d, J¼8.2 Hz, 3H), 7.74 (s, 1H), 7.63 (d, J¼8.6 Hz,
2H), 7.39 (d, J¼3.7 Hz, 1H), 7.32e7.28 (m, 4H), 7.20 (dd, J¼16.8,
8.1 Hz, 6H), 7.07 (d, J¼7.3 Hz, 2H), 6.97 (d, J¼3.7 Hz, 1H). 13C NMR
7.09 (d, J¼0.6 Hz, 4H), 7.06 (s, 2H). 13C NMR (DMSO-d6, 100 MHz),
d:
156.32, 156.00, 155.27, 148.29, 147.03, 146.58, 135.50, 134.08, 129.57,
127.92, 124.59, 124.35, 124.16, 124.04, 123.77, 123.48, 123.16, 118.68,
117.51, 112.44, 112.04, 108.52. HRMS (m/z): [M]þ calcd for
C38H25N2O4, 573.1814; found, 573.1816. Compound TBS6: 1H NMR
(400 MHz, DMSO-d6): 9.03 (d, J¼1.0 Hz, 1H), 8.66 (d, J¼1.2 Hz, 1H),
8.17 (d, J¼8.4 Hz, 1H), 8.10 (d, J¼8.4 Hz, 1H), 8.07e8.00 (m, 2H), 7.83
(dd, J¼8.4, 1.6 Hz, 1H), 7.76 (d, J¼8.6 Hz, 2H), 7.48 (s, 2H), 7.35 (t,
(DMSO-d6, 100 MHz),
d: 177.06, 159.65, 152.00, 147.66, 147.41,
141.45, 138.30, 137.98, 136.20, 135.68, 134.68, 129.34, 128.11, 126.41,
125.51, 124.48, 124.01, 123.81, 119.92, 118.27, 107.68. HRMS (m/z):
[M]þ calcd for C35H24NO2S, 522.1528; found, 522.1524.
4.3.3. 5-(4-(Diphenylamino)phenyl)thiophene-2-carbaldehyde
(6g). The procedure is similar as the procedure for the synthesis of
6a. The yield: 45%.
J¼7.9 Hz, 4H), 7.14e7.06 (m, 8H). 13C NMR (DMSO-d6, 100 MHz),
d:
163.94, 157.55, 147.89, 147.02, 146.84, 140.03, 137.72, 136.81, 135.70,
135.34, 133.71, 129.60, 127.86, 125.98, 125.65, 124.13, 123.85, 123.42,
119.37, 118.52, 117.85, 109.50. HRMS (m/z): [M]þ calcd for
C38H25N2O3S, 589.1586; found, 589.1582.
4.3.3.1. (Z)-2-Cyano-3-(5-(7-(4-(diphenylamino)phenyl)-9-hexyl-
9H-carbazol-3-yl)thiophen-2-yl)acrylic acid (TBS1). A mixture of 6a
(300 mg, 0.5 mmol), 2-cyanoacetic acid (160 mg, 2 mmol) and 2
drops of piperidine in 20 ml THF was refluxed for 4 h under N2
atmosphere. After cooled to rt, the solvent was removed in vacuo.
The residual was dissolved in 200 ml dichloromethane and washed
with redistilled water for three times. The combined organic layer
was dried over anhydrous MgSO4 and then filtered. The filtrate was
concentrated using rotary evaporator. The crude product was
chromatographed on silica gel and firstly using pure dichloro-
methane as the eluent to exclude the impurities and then using THF
and methanol mixture (1:1) as the eluent to obtain the pure
product as an orange solid (301 mg, 90%). 1H NMR (400 MHz,
DMSO-d6): 8.67 (d, J¼1.6 Hz, 1H), 8.61 (d, J¼1.5 Hz, 1H), 8.19e8.09
(m, 1H), 7.88e7.81 (m, 1H), 7.81e7.74 (m, 4H), 7.72e7.65 (m, 3H),
7.37e7.31 (m, 4H), 7.13e7.04 (m, 8H), 4.57e4.32 (m, 2H), 1.91e1.71
(m, 2H), 1.35e1.20 (m, 6H), 0.81 (t, J¼7.1 Hz, 3H). 13C NMR (DMSO-
Supplementary data
Supplementary data related to this article can be found online at
http://dx.doi.org/10.1016/j.tet.2012.12.074.
References and notes
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1H NMR (400 MHz, DMSO-d6): 8.64 (d, J¼1.8 Hz, 1H), 8.56 (d,
J¼1.6 Hz, 1H), 8.50 (s, 1H), 8.03 (d, J¼4.1 Hz, 1H), 7.94 (dd, J¼8.7,
1.9 Hz, 1H), 7.80 (dt, J¼7.4, 2.5 Hz, 3H), 7.72 (dd, J¼14.0, 8.6 Hz, 3H),
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