Naphthyl and thienyl units as bridges for metal-free dye-sensitized solar cells

Article abstract of DOI:10.1002/asia.201100972

A series of new organic dyes, comprising a naphthyl moiety as a π-conjugated bridge, different amines as donors, and a cyanoacrylic acid group as an electron acceptor and anchoring group, have been designed and synthesized for applications in dye-sensitized solar cells (DSSCs). One of the compounds was also characterized by single-crystal X-ray structural analysis. All of the dyes exhibited maximum absorptions in the range of 371-441 nm. The short-circuit photocurrent density, open-circuit voltage, and fill factor (FF) values of the devices are in the range of 6.13-10.90 mA cm^-2, 0.62-0.69 V, and 0.62-0.67, respectively, corresponding to an overall conversion efficiency of 2.76-4.55 %. The conversion efficiency reached 38-62 % of that of a N719-based device (7.31 %) fabricated and measured under similar conditions. Steric congestion between the naphthyl and aromatic moieties jeopardizes charge transfer from the donor to the acceptor. Insertion of an alkenyl entity between the naphthyl entity and the aromatic ring alleviates steric congestion and leads to longer wavelength electronic absorption spectra.

Full text of DOI:10.1002/asia.201100972

FULL PAPERS
DOI: 10.1002/asia.201100972
Naphthyl and Thienyl Units as Bridges for Metal-Free Dye-Sensitized Solar
Cells
Yung-Chung Chen,[a] Yo-Hua Chen,[b] Hsien-Hsin Chou,[a] Sumit Chaurasia,[a]
Yuh S. Wen,[a] Jiann T . Lin,*[a] and Ching-Fa Yao*[b]
Abstract: A series of new organic dyes,
comprising a naphthyl moiety as a p-
conjugated bridge, different amines as
donors, and a cyanoacrylic acid group
as an electron acceptor and anchoring
group, have been designed and synthe-
sized for applications in dye-sensitized
solar cells (DSSCs). One of the com-
pounds was also characterized by
single-crystal X-ray structural analysis.
All of the dyes exhibited maximum ab-
sorptions in the range of 371–441 nm.
The short-circuit photocurrent density,
open-circuit voltage, and fill factor
(FF) values of the devices are in the
range of 6.13–10.90 mAcmꢀ2, 0.62–
0.69 V, and 0.62–0.67, respectively, cor-
responding to an overall conversion ef-
ficiency of 2.76–4.55%. The conversion
efficiency reached 38–62% of that of
a N719-based device (7.31%) fabricat-
ed and measured under similar condi-
tions. Steric congestion between the
naphthyl and aromatic moieties jeop-
ardizes charge transfer from the donor
to the acceptor. Insertion of an alkenyl
entity between the naphthyl entity and
the aromatic ring alleviates steric con-
gestion and leads to longer wavelength
electronic absorption spectra.
Keywords: donor–acceptor
systems · dyes/pigments · solar cells ·
sensitizers
Introduction
are attracting more and more research efforts. Various or-
ganic dyes with acceptable stability and high cell efficiency
have been obtained in recent years.[5] A record efficiency of
up to 10% has been achieved for metal-free DSSCs.[6] Ex-
cellent temporal stability with a durable efficiency of 7–8%
at 608C for at least 1000 h was also demonstrated for organ-
ic dyes.[7]
Since the seminal report by OꢀRegan and Grꢁtzel in 1991,[1]
dye-sensitized solar cells (DSSCs) have received extensive
interest because of their low cost and relatively high solar
energy to electricity conversion efficiency (h).[2] The effi-
ciencies of DSSCs are affected by various factors of the sen-
sitizers, such as molecular structure, photophysical, and elec-
trochemical properties.[2,3] So far, ruthenium(II) polypyridyl
complexes still stand out as the most efficient and stable
sensitizers for DSSCs and the record high conversion effi-
ciency is ^11%.[4] Moreover, ruthenium-based dyes usually
exhibit higher open-circuit voltages (VOC) than metal-free
sensitizers. However, the expense and toxicity of these mate-
rials may limit their applications in DSSCs.
Sensitizers with higher molar extinction coefficients and
longer absorption wavelengths are advantageous for high
cell efficiency. In our studies on metal-free dyes for DSSCs
applications, we have developed a series of arylamine-based
compounds with various heteroaromatic rings in the conju-
gated spacers and achieved high-performance DSSCs by
using these dyes as sensitizers.[8] A common feature of these
molecules is the high intensity of their electronic absorption.
Heteroaromatic rings in these compounds should also play
an important role because of their higher tendency than the
phenyl entity to form a quinoid structure during charge-
transfer transitions. Conversion of the aromatic unit in the
conjugated chain into a quinoid structure will lead to
a small band gap, however, at the expense of stability due to
destruction of the aromaticity.[9] A more favorable quinoid
structure is therefore anticipated upon replacement of
a phen-1,4-diyl unit with naphtha-1,4-diyl or anthracen-9,10-
diyl units. Indeed, Pugh et al. observed a bathochromic shift
of the absorption wavelength in azoarenes when the phen-
1,4-diyl unit in the spacer was replaced by a naphtha-1,4-diyl
unit.[10] Anthracene-bridged metal-free sensitizers developed
by Sun et al. had high efficiencies of up to 7.03% despite
the large dihedral angle between the anthracenyl unit and
the neighboring phenyl ring.[11] Although the naphthyl-
bridged zinc porphyrin carboxylic acid investigated by Ima-
Compared with ruthenium(II) polypyridyl complex based
dyes, metal-free organic dyes possess larger molar extinction
coefficients and facile structure modification, therefore, they
[a] Dr. Y.-C. Chen, Dr. H.-H. Chou, Dr. S. Chaurasia, Y. S. Wen,
Prof. Dr. J. T. . Lin
Institute of Chemistry
Academia Sinica
11529, Nankang, Taipei (Taiwan)
Fax : (+886)783-1237
E-mail: jtlin@chem.sinica.edu.tw
[b] Y.-H. Chen, Prof. Dr. C.-F. Yao
Department of Chemistry
National Taiwan Normal University
117, Taipei (Taiwan)
E-mail: cheyaocfh@ntnu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100972.
1074
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 1074 – 1084

hori et al. indicated the exis-
tence of a large dihedral angle
between the naphthyl unit and
the porphyrin ring,[12] the steric
constraint between naphtha-1,4-
diyl and phen-1,4-diyl units
should be somewhat released
compared with that between
a anthracen-9,10-diyl unit and
a phen-1,4-diyl unit. Moreover,
appropriate deviation of a sensi-
tizer molecule from planarity
could avoid dye aggregation
and/or close contact of electro-
lyte molecules with the TiO2
surface. Therefore, we turned
our attention to metal-free dyes
containing
naphtha-1,4-diyl
units in the conjugated bridge.
Herein, we report new metal-
free dyes containing one or
more naphtha-1,4-diyl units in
the conjugated bridge. Their
photophysical properties and
the performance of the DSSCs
fabricated are also discussed.
Results and Discussion
Synthesis and Characterization
The structures of new metal-
free sensitizers are shown in
Scheme 1. The synthetic proto-
cols for compounds YO-1 to
YO-5 and YO-6 to YO-9 are il-
lustrated in Schemes 2 and 3,
respectively. The following key
steps are involved in the syn-
Scheme 1. Structures of compounds YO1–YO9 and reference compound L1.
thetic protocols: 1) a palladium-catalyzed Buchwald–Hart-
ꢀ
wig C N coupling reaction for incorporation of arylamine
(steps i and ii in Scheme 2 and step v in Scheme 3);[13] 2) a
palladium-catalyzed Stille coupling reaction between a stan-
nyl compound and an aryl bromide (steps iii, vii, ix, and x in
Scheme 2, and step i in Scheme 3);[14] 3) formylation of the
heteroaromatic ring (step iv in Scheme 2, and steps iii and vi
in Scheme 3); 4) a Sonogashira coupling reaction between
arylamine and thiophene bromide (step ix in Scheme 3);[15]
5) condensation of an aromatic aldehyde with cyanoacetic
acid with the formation of 2-cyanoacrylic acid (step v in
Scheme 2 and step ii in Scheme 3). The dyes are obtained as
yellow to dark red powders and are soluble in common or-
ganic solvents, such as tetrahydrofuran (THF), CH2Cl2, and
CHCl3. The structure of YO-8 was also confirmed by single-
crystal X-ray diffraction analysis and the ORTEP plot is
shown in Figure 1. Important bond lengths, bond angles, and
dihedral angles of the molecule are compiled in Table S1 in
Figure 1. ORTEP diagram of YO-8 with thermal ellipsoids shown at the
30% probability level.
the Supporting Information and more details are shown in
Figure S1. Although the thiophene ring is nearly coplanar
with the two neighboring vinyl entities, there is significant
deviation of the naphthyl unit from coplanarity with the
neighboring vinyl entity (dihedral angle=34.058). The car-
boxylic group and the thiophenyl ring are in mutual trans
Chem. Asian J. 2012, 7, 1074 – 1084
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1075

Metal-Free Dye-Sensitized Solar Cells
FULL PAPERS
Scheme 2. Synthesis of YO1–YO5.[a] Reagents and conditions: i) Ph2NH, tBuONa, [Pd
ACHTUNGTRENNUNG
ferrocene (dppf); ii) Ph2NH, tBuONa, PdACTHUNGTREN(NUNG OAc)2, PtBu3; iii) tributylthiophen-2-ylstannane, [PdACHTUNGTRENNNUG
AHCTUNGTRENNUNG
mosuccinimide (NBS); ix) 2,2’-bithiophenyl-5-yltributylstannane, [PdACTHNUTRGENN(UG PPh3)4]; x) [PdACHTUGNTREN(NUGN PPh3)4].
positions of the vinyl group.
Table 1. Electro-optical parameters of the dyes.
Compared with the cis isomer,
such a conformation may avoid
charge trapping at the cyano
moiety and facilitate electron
injection into TiO2.
[a]
[c]
[d]
Dyes
labs [nm]
(eꢃ10ꢀ4 [mꢀ1 cmꢀ1])[a]
lem
E1/2 (ox)[b]
[mV]
HOMO/LUMO
[eV]
E0–0
E0–0*
[eV]
G
E
[nm]
[eV]
YO-1
YO-2
YO-3
YO-4
YO-5
YO-6
YO-7
YO-8
YO-9
379 (3.22), 293 (3.17)
428 (3.99), 276 (9.10)
414 (1.36), 356 (1.67), 277 (10.6)
422 (2.56), 276 (7.43)
382 (2.80), 277 (6.05)
401 (1.04), 273 (5.29)
371 (3.64), 277 (9.28)
441 (3.89), 294 (3.37)
414 (2.86), 370 (2.45), 292 (2.31)
529
625
607
589
539
568
527
567
538
577 (85)
536 (73), 833 (86)
585 (88)
250 (91), 850 (86)
522 (79), 797 (86)
389 (78)
421 (86), 832 (86)
499 (66)
5.38/2.52
5.34/2.71
5.39/2.99
5.05/2.54
5.32/2.56
5.19/2.79
5.22/2.54
5.30/3.02
5.40/3.01
2.86
2.63
2.40
2.51
2.76
2.40
2.68
2.28
2.39
ꢀ1.48
ꢀ1.28
ꢀ1.01
ꢀ1.46
ꢀ1.44
ꢀ1.21
ꢀ1.46
ꢀ0.98
ꢀ0.99
Photophysical Properties
The absorption spectra of the
new dyes in THF are shown in
Figure 2a and the data are sum-
603 (125)
[a] Recorded in THF. [b] Recorded in CH2Cl2; scan rate=100 mVsꢀ1; electrolyte=(n-C4H9)4NPF6; potentials
marized in Table 1. The absorp- are quoted with reference to the internal ferrocene standard (E1/2 =+50 mV vs. Ag/AgNO3). [c] The band gap,
E0–0, was derived from the intersection of the absorption and emission spectra. [d] E0–0*: the excited-state oxi-
dation potential versus a normal hydrogen electrode (NHE).
tion band at a shorter wave-
length is attributed to a local-
ized p–p* transition. The broad
band at a longer wavelength,
ranging from 371 to 441 nm, is due to charge transfer mixed
with a more delocalized p–p* transition. Taking YO-3 as an
example, the charge-transfer character of the band is evi-
denced from the following observations: replacement of the
2-cyanoacrylic acid moiety in YO-3 by a formyl group (7 in
Scheme 2) and a hydrogen atom (2b in Scheme 2) leads to
a decrease in lmax by 30 (417 vs. 387 nm in THF) and 51 nm
(417 vs. 366 nm in THF), respectively.
The absorption wavelength of the compounds decreases
in the order of YO-8>YO-2 >YO-4 >YO-9 >YO-3 >YO-
6>YO-5 >YO-1 >YO-7. The large dihedral angle (see
below) between the naphthyl and neighboring aromatic ring
will jeopardize charge transfer. Therefore, it is not surprising
that YO-1 has a very small lmax value owing to the presence
of two sterically congested naphthyl entities. In comparison,
compound YO-8 has a longer lmax value than the others
1076
www.chemasianj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 1074 – 1084

J. T. Lin, C.-F. Yao et al.
Scheme 3. Synthesis of YO6–YO9.[a] Reagents and conditions: i) [Pd
HCl; iv) NBS; v) Ph2NH, tBuONa, Pd(OAc)2, PtBu3; vi) POCl3, DMF; vii) trimethylsilylacetylene, CuI, iPr2NH, [PdCl CHTUNGTRENNNUG
ACHTUNGTRENNUNG
G
viii) KF, MeOH; ix) 5-bromo-2-thiophenecarbaldehyde, CuI, iPr2NH, [PdCl2ACHTNUGTRNEUNG(PPh3)2].
owing to better coplanarity (see below) of the molecule.
L1 (444 nm, e=27490mꢀ1 cmꢀ1). Apparently the nonplanar
conformation of the spacer blocks YO-3 from forming a qui-
noid structure. A redshift of the longer wavelength tail from
YO-5 to YO-7 may also be attributed to impediment of the
quinoid structure owing to the more twisted conformation
in the former. A significant blueshift of the spectra of the
dyes on TiO2 surfaces (see Figure S2 in the Supporting In-
formation) may be caused by deprotonation of the carboxyl-
ic acid;[8d] this was also confirmed by measuring the absorp-
tion of the dyes in the presence of Et3N. A certain degree of
J aggregation[18] may also occur, as evidenced from the
prominent red tailing of the absorption spectra.
The lmax value of the charge-transfer band in YO-2 de-
creases as the solvent polarity increases, that is, toluene
(444)>1,4-dioxane (440)>THF (437)>acetone (415)ꢁ
MeOH (415)>DMF (407 nm) (see Figure S3 in the Sup-
porting Information). Negative solvatochromism may be at-
tributed to the strong interaction of polar solvent with the
Compound YO-9 absorbs at a shorter wavelength (lmax
=
414 nm) than that of YO-8 (lmax =441 nm), even though the
conjugated spacer of the former is much more planar than
that of the latter (see below). This can be attributed to the
poorer orbital match of the aromatic p orbital (naphthyl and
thienyl entities) with the alkynyl p orbital in energy.[16] Com-
pounds with the longest conjugated spacer between the
donor and the acceptor, YO-5 and YO-7, have short absorp-
tions, most likely the severe twist of the aromatic rings from
coplanarity hampers effective electronic delocalization and
charge transfer; however, saturation of conjugation in these
compounds cannot be ruled out. Elongation of the effective
conjugation length results in a redshift of the absorption
spectrum: lmax (YO-2)>lmax (YO-3). Compared with YO-2,
the absorption maximum of YO-4 is slightly blueshifted;
however, the long wavelength tailing of the latter is redshift-
ed, which is in agreement with the better donating power of
arylamine upon incorporation of alkoxy units at the phenyl
rings.
ꢀ
dyes, which weakens the O H bond of the carboxylic acid
and consequently decreases the electron-withdrawing nature
of the COOH group.[19] The compounds are weakly emissive
in the green to red region (Figure 2b); luminescence data
are presented in Table 1. It was noted that YO-1, YO-2, and
YO-7 had structured emission spectra. Therefore, excitation
spectra were measured for YO-2 (Figure 3). The excitation
spectrum obtained upon monitoring at 480 nm (the blue
side in the emission spectrum) was different from that ob-
One aim of this work was to study the possibility of qui-
noid structure of the naphthyl moiety in the spacer resulting
in a redshift of the electronic absorption. Compound YO-3
was therefore selected for comparison with its congener, L1
(Scheme 1),[17] in which the naphthyl unit in YO-3 was re-
placed with a phenyl-1,4-diyl entity. Compound YO-3 has
a blueshifted absorption with a lower intensity than that of
Chem. Asian J. 2012, 7, 1074 – 1084
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1077

Metal-Free Dye-Sensitized Solar Cells
FULL PAPERS
Electrochemical Properties
The electrochemical properties of the dyes were studied by
cyclic voltammetry (CV) and differential pulse voltammetry
(DPV). The relevant CV data are collected in Table 1 and
selected cyclic voltammograms are shown in Figure 4. Only
one quasi-reversible redox wave was observed for YO-1,
YO-3, YO-6, YO-8, and YO-9 owing to oxidation of the ar-
Figure 4. Cyclic voltammograms of YO-2, YO-4, YO-5, and YO-7 record-
ed in CH2Cl2.
ylamine. In addition to the first redox wave stemming from
oxidation of the arylamine, other compounds exhibited
a second quasi-reversible redox wave, most likely from oxi-
dation of the conjugated segment because of the presence of
two electron-rich thiophene rings. The first oxidation poten-
tial of the compounds decreases in the order of YO-9>YO-
3>YO-1 >YO-2 >YO-5 >YO-8 >YO-7 >YO-6 >YO-4.
The lower oxidation potential of YO-4 than YO-2 can be at-
tributed to the presence of an electron-donating alkoxy
group at the phenyl ring, when compared with similar com-
pounds reported in the literature.[20] As expected, compound
YO-6 was oxidized at a lower potential because of the direct
linkage of a thienyl moiety with the nitrogen atom of aryla-
mine.[21] Compound YO-7 has a lower oxidation potential
than that of YO-5. The smaller dihedral angle (see below)
of the phenyl/thienyl segment than the naphthyl/thienyl seg-
ment is likely to lead to a more stabilized cationic arylamine
as a result of better electronic interaction between the aryla-
mine and the thiophene ring. The energy levels of the
HOMOs of these materials were calculated with reference
to ferrocene (4.8 eV) and ranged from 5.05 to 5.40 eV. This
together with the HOMO/LUMO gap (E0–0, 2.28–2.86 eV)
obtained from the intersection of the absorption and emis-
sion spectra was utilized to derive the LUMO energy. The
excited-state oxidation potentials (E0–0*) of the sensitizers,
estimated from the difference between the first oxidation
potential at the ground state and E0–0, are more negative
(ꢀ0.98 to ꢀ1.48 V vs. NHE; see Table 1) than the conduc-
tion band edge of TiO2 (ꢀ0.5 V vs. NHE).[22] Therefore,
Figure 2. a) UV/Vis and b) photoluminescence (PL) spectra of YO-1–
YO-9 recorded in THF.
Figure 3. Absorption and emission spectra and excitation profiles of YO-
2 in THF.
tained from monitoring at 700 nm (the red side of the emis-
sion spectrum). The dual emission may be due to the pres-
ence of different excited structures owing to the restricted
rotation of the naphthyl entity.
1078
www.chemasianj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 1074 – 1084

J. T. Lin, C.-F. Yao et al.
electron injection from the excited dye to TiO2 should be
energetically favorable. Dye regeneration by the electrolyte
is also expected to be favorable because the first oxidation
potentials of the dyes are more positive (1.39–1.05 V vs.
ꢀ
NHE) than that of the Iꢀ/I3 redox couple (0.4 V vs.
NHE).[23]
Photovoltaic Properties
DSSCs were fabricated with the use of the new dyes and
nanocrystalline anatase TiO2. The cells had an effective area
of 0.25 cm2 and the electrolyte used was composed of 0.05m
I2/0.5m LiI/0.5m tert-butylpyridine (TBP) in acetonitrile. The
device performance statistics under standard global illumi-
nation from one sun (AM 1.5) are presented in Table 2. A
Figure 6. IPCE curves for the compounds.
Table 2. Photovoltaic properties of the cells.[a]
ces are in the range of 6.13–10.90 mAcmꢀ2, 0.62–0.69 V, and
0.62–0.67, respectively, corresponding to an overall conver-
sion efficiency of 2.76–4.55%. The DSSC of YO-8 has the
best power conversion efficiency (4.55%) of the compounds
tested, reaching 62% of the standard ruthenium dye N719-
based cell (conversion efficiency=7.31%) fabricated and
measured under similar conditions.
Dyes
Voc
[V]
JSC
[mAcmꢀ2
h
FF
Rct
[W]
G
]
[%]
YO-1
YO-2
YO-3
YO-4
YO-5
YO-6
YO-7
YO-8
YO-9
L1
0.68
0.66
0.64
0.66
0.67
0.65
0.69
0.64
0.62
0.66
0.74
6.13
2.76
4.45
3.04
3.99
3.26
3.58
3.90
4.55
3.30
4.29
7.31
0.66
0.63
0.67
0.64
0.65
0.66
0.62
0.65
0.67
0.65
0.65
50
30
50
31
42
24
24
20
33
27
18
10.80
7.16
9.47
7.49
8.33
9.19
10.90
7.97
9.97
15.20
The dye density on the TiO2 film was checked and de-
creased in the order of YO-8 (5.08ꢃ10ꢀ7)>YO-9 (4.95ꢃ
10ꢀ7)>YO-1 (4.68ꢃ10ꢀ7)>YO-7 (4.44ꢃ10ꢀ7)>YO-3 (4.30ꢃ
10ꢀ7)>YO-2 (4.21ꢃ10ꢀ7)>YO-4 (3.65ꢃ10ꢀ7)>YO-5 (3.52ꢃ
10ꢀ7)>YO-6 (2.59ꢃ10ꢀ7 molcmꢀ2). We suspect that the
higher dye density of YO-8 and YO-9 is due to the presence
of the vinyl or ethynyl entity, which may decrease steric con-
gestion of the dye molecules on TiO2. Several factors may
contribute to the higher efficiency of YO-8: 1) good light-
harvesting efficiency (long absorption wavelength and high
molar extinction coefficient); 2) high dye-loading density;
and 3) better electron collection because of lower electron
transport resistance (see below). However, the higher dark
current in the devices of YO-8 led to a lower VOC value and
counteracted the overall efficiency. The high performance of
the cell from YO-2 may also be rationalized by better light
harvesting and relatively high dye-loading density. The bi-
naphthyl entity in YO-1 seems to more effectively suppress
the dark current and increases the VOC value of the cell.
Nevertheless, compound YO-1 has the lowest cell efficiency
because of its inefficient light harvesting. The much lower
cell efficiency of YO-9 than that of YO-8 can be largely at-
tributed to the shorter absorption wavelength of the former,
as evidenced from the IPCE plots. Although the dye density
of L1[17] reaches only about 50% (2.08ꢃ10ꢀ7 molcmꢀ2) of
that of YO-3, the cell efficiency of L1 is nearly 40% higher
than that of YO-3. Apparently light harvesting plays a very
important role in cell performance. Better electron collec-
tion (see below) and dark current suppression were also
found in the cell of L1.
N719
[a] Experiments were conducted by using TiO2 photoelectrodes approxi-
mately 18 mm thick and with a 0.25 cm2 working area on the fluorine tin
oxide (FTO) substrates.
DSSC fabricated from the aforementioned sensitizer L1[17]
was also prepared for comparison. The photocurrent–volt-
age (J–V) curves and dark current plots are shown in
Figure 5, and incident monochromatic photo-to-current con-
version efficiency (IPCE) plots of the cells are shown in
Figure 6. The short-circuit photocurrent density (JSC), open-
circuit voltage (VOC), and fill factor (FF) values of the devi-
The dark currents of all DSSCs were also measured
(Figure 5) and their VOC data are collected in Table 2. The
VOC data of DSSCs decreases in the order of N719>YO-7 >
Figure 5. J–V curves and dark currents for the compounds.
Chem. Asian J. 2012, 7, 1074 – 1084
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1079

Metal-Free Dye-Sensitized Solar Cells
FULL PAPERS
YO-1 >YO-5 >YO-2 ꢁYO-4 >YO-6 >YO-3 ꢁYO-8 >YO-
9. Large dark currents of the cells from YO-9, YO-3, and
YO-8 are consistent with their lower VOC values. Electro-
chemical impedance measurements under illumination are
shown in Figure 7. In general, electrochemical impedance
Figure 8. Electrochemical impedance spectra (Nyquist plots) of DSSC for
YO-3, L1, and N719 dyes measured in the dark under ꢀ0.55 V bias.
Theoretical Approach
Quantum chemistry calculations were conducted to gain fur-
ther insight into the correlation between molecular structure
and physical properties as well as device performance. The
computed frontier orbitals of the compounds and their cor-
responding energy states are included in Figures S5 and S6
in the Supporting Information, and the results for theoreti-
cal computation are listed in Table S2. The molecules were
divided into several segments: the arylamine (or alkylamine)
group (Am); the naphthyl, phenyl, and thiophene units
(Nap, Ph, T); and the 2-cyanoacrylic acid (Ac). The Mullik-
en charges for the S0!S1 and S0!S2 transitions were also
calculated as a projection from the time-dependent density
functional theory (TD-DFT) results. Differences in the Mul-
liken charges in the excited and ground states were calculat-
ed and grouped into several segments in the molecules to
estimate the extent of charge separation upon excitation
(Figure S7 in the Supporting Information). For all com-
pounds, the lowest energy transition (S0!S1, >99%
HOMO!LUMO transition) has very prominent charge
separation from arylamine to 2-cyanoacrylic acid; however,
the oscillator strength (f) varies. For YO-1 and YO-5, in
which the spacer is less coplanar (see below), the f values
are only 0.01 and 0.10, respectively. This is consistent with
the low short-circuit currents and conversion efficiencies for
the two devices. Except for YO-9, compound YO-8 has the
largest f value for the S0!S1 transition because of a more
planar spacer. Moreover, the S0!S2 transition (mainly
HOMO-1!LUMO transition) also has a large f value. A
cascade transition from S2 to S1 will also assist electron in-
jection. Despite of the largest f value for the S0!S1 transi-
tion, compound YO-9 suffers from a short absorption wave-
length and less efficient dark current suppression, and there-
fore, results in low cell performance (see above). Although
YO-4 has very similar structural features to YO-2, the
former has an absorption spectrum extending to a longer
wavelength. Moreover, DSSCs based on the two dyes have
comparable electron transport resistances. Therefore, the in-
ferior performance of the DSSC based on YO-4 is likely to
Figure 7. Electrochemical impedance spectra (Nyquist plots) of DSSCs
for dyes measured under illumination (AM j1.5).
measured under illumination provides information on elec-
tron injection and transport inside TiO2.[24] Upon illumina-
tion with 100 mWcmꢀ2 under open-circuit conditions, the
radius of the intermediate frequency semicircle in the Ny-
quist plot (Figure 7) represents the electron transport resist-
ance. The electron transport resistance (Rct; Table 2) de-
creased in the order of YO-1 (50)ꢁYO-3 (50)>YO-5
(42)>YO-9 (33)>YO-4 (31)>YO-2 (30)>YO-6 (24)ꢁ
YO-7 (24)>YO-8 (20)>N719 (18 W). Lower electron trans-
port resistance will assist electron collection and improve
cell efficiency. The smallest Rct value for YO-8 is consistent
with it having the highest cell efficiency. In contrast, com-
pounds YO-1 and YO-3, with higher electron transport re-
sistance, exhibited the lowest efficiency of all of the devices.
The cell of YO-3 had a larger electron transport resistance
(50 W) than that of L1 (27 W), in accordance with its lower
power conversion efficiency. The results obtained herein are
roughly consistent with the trend observed for cell perfor-
mance (Table 2).
Nyquist plots of DSSCs under a forward bias of ꢀ0.55 V
in the dark are shown in Figure S4 in the Supporting Infor-
mation. The second semicircle can be used to derive the
charge recombination resistance on the TiO2 surface (Rrec
)
by fitting curves using Z-view software. A larger Rrec value
implies a smaller dark current. The Rrec value is roughly con-
sistent with the trend of the dark current. Three Rrec values
of YO-3, L1, and N719 are shown in Figure 8 for compari-
son. The Rrec value decreases in the order of N719 (760)>
L1 (362)>YO-3 (186 W), thus suggesting that the YO-3 has
more facile charge recombination. This is consistent with
the lower VOC value and cell efficiency of YO-3.
1080
www.chemasianj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 1074 – 1084

J. T. Lin, C.-F. Yao et al.
Scheme 4. Structures and dihedral angles of all compounds.
be a result of the lower dye density on TiO2 and lower
molar extinction coefficient.
Conclusion
The dihedral angles between successive units of the mole-
cules are shown in Scheme 4. It is noteworthy that the opti-
mized geometry of the molecule YO-8 is in agreement with
the molecular structure obtained from single-crystal X-ray
diffraction analysis (Figure S1 in the Supporting Informa-
tion), confirming the validity of the computation. The dihe-
dral angle (>408) between the naphthyl entity and the
neighboring aromatic ring is large in all compounds, and it
even reaches about 758 between the two naphthyl units of
YO-1. In accordance with the electronic spectra and cell
performances, large dihedral angles in these compounds
hamper coplanarity of the spacers and efficient charge trans-
fer from the donor to the acceptor. To facilitate formation
of the quinoid structure from the naphthyl moiety for better
light harvesting, incorporation of alkenyl entities on both
sides of the naphthyl moiety for alleviation of steric conges-
tion is therefore needed. Compared with the compounds in
this study, the dyes with a 2,6-naphthyl entity in the spacer
have smaller twist angles.[25] Consequently, the 2,6-naphthyl
compounds have more efficient charge transfer, leading to
better light harvesting and cell performance, although loss
of resonance energy is expected to be more serious for
a 2,6-naphthyl entity than for a 1,4-naphthyl entity in form-
ing the quinoid structure during the charge-transfer transi-
tion.
We synthesized a series of dyes containing a naphthyl-based
conjugated spacer between an arylamine donor and a 2-cya-
noacrylic acid acceptor. Significant deviation of the naphthyl
unit from coplanarity with the neighboring aromatic rings
hampered formation of the quinoid structure by the naph-
thyl unit. The performance of DSSCs with these dyes as the
sensitizers exhibited efficiencies ranging from 2.76 to 4.55%
under illumination from one sun (AM 1.5). The best perfor-
mance of the device was achieved with the use of a dye con-
taining an alkenyl unit between the naphthyl and aromatic
moieties, and the cell efficiency reached 62% of that of an
N719-based DSSC (7.31%) fabricated and measured under
similar conditions. Alleviation of steric congestion upon in-
sertion of the alkenyl unit apparently results in longer wave-
length absorption of the dye and increases the overall power
efficiency of DSSCs.
Experimental Section
General Information
1H and 13C NMR spectra were recorded on Bruker AMX-400 or Bruker
AV-400
spectrometers
by
using
CDCl3,
[D6]acetone,
[D6]dimethylsulfoxide (DMSO), or [D8]THF as the solvent. CV measure-
ments were performed on a BAS-100 instrument by using CH2Cl2 as sol-
vent. UV/Vis spectra were recorded on a Varian Cary 50 spectrophotom-
eter. PL spectra were recorded on a Hitachi F-4500 spectrophotometer.
Elemental analysis was performed on a Perkin–Elmer 2400 instrument.
Fast-atom bombardment mass spectrometry (FABMS) analysis was per-
formed on a JEOL Tokyo Japan JMS-700 mass spectrometer equipped
Chem. Asian J. 2012, 7, 1074 – 1084
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1081

Metal-Free Dye-Sensitized Solar Cells
FULL PAPERS
with a standard FAB source. The photoelectrochemical characterizations
on the solar cells were carried out by using an Oriel Class A solar simula-
tor (Oriel 91195A, Newport Corp.). Photocurrent–voltage characteristics
of the DSSCs were recorded with a potentiostat/galvanostat (CHI650B,
CH Instruments, Inc.) at a light intensity of 100 mWcmꢀ2 calibrated by
an Oriel reference solar cell (Oriel 91150, Newport Corp.). The mono-
chromatic quantum efficiency was recorded through a monochromator
(Oriel 74100, Newport Corp.) under short-circuit conditions. The intensi-
ty of each wavelength was in the range of 1–3 mWcmꢀ2. Electrochemical
impedance spectra were recorded for DSSCs under illumination at an
open-circuit voltage (VOC) or dark at a potential of ꢀ0.55 V at room tem-
perature. The frequencies explored ranged from 10 mHz to 100 kHz. The
TiO2 nanoparticles and the reference compound, N719, were purchased
from Solaronix, S.A., Switzerland.
J=8.0 Hz, 1H), 7.27 (t, J=8.0 Hz, 4H), 7.04 (d, J=8.0 Hz, 4H),
7.00 ppm (d, J=8.0 Hz, 2H); 13C NMR ([D6]DMSO, 125 MHz): d=
163.5, 147.7, 146.3, 145.3, 143.8, 142.6, 141.3, 135.3, 134.1, 132.3, 130.8,
129.6, 129.4, 128.9, 128.8, 127.5, 127.3, 126.9, 126.5, 125.7, 125.2, 124.2,
122.1, 121.6, 116.4, 98.2 ppm; MS (FAB): m/z: 554.1 [M+]; elemental
analysis calcd (%) for C34H22N2O2S2: C 73.62, H 4.00, N 5.05; found: C
73.22, H 4.19, N 5.19.
Compound YO-3
Yield: 52%; 1H NMR (400 MHz, CDCl3): d=8.39 (s, 1H), 8.16 (d, J=
8.4 Hz, 1H), 8.02 (d, J=8.4 Hz, 1H), 7.91 (d, J=4.0 Hz, 1H), 7.58 (d, J=
8.0 Hz, 1H), 7.49 (t, J=7.6 Hz, 1H), 7.41 (d, J=4.0 Hz, 1H), 7.38 (t, J=
7.6 Hz, 1H), 7.31 (d, J=8.0 Hz, 1H), 7.21 (t, J=7.6 Hz, 4H), 7.03 (d, J=
7.6 Hz, 4H), 6.96 ppm (t, J=7.6 Hz, 2H); 13C NMR ([D8]THF,
100 MHz): d=164.1, 152.3, 149.7, 146.8, 146.6, 145.4, 138.9, 137.6, 134.2,
132.5, 130.1, 130.0, 129.7, 127.7, 127.6, 127.4, 127.3, 126.8, 126.0, 122.9,
116.6, 100.9 ppm; MS (FAB): m/z: 472.1 [M+]; elemental analysis calcd
(%) for C30H20N2O2S: C 76.25, H 4.27, N 5.93; found: C 75.92, H 4.55, N
6.04.
Devices Fabrication
The photoanode used was a TiO2 thin film (12 mm of 20 nm particles as
the absorbing layer and 6 mm of 400 nm particles as the scattering layer)
coated on an FTO glass substrate with dimensions of 0.5ꢃ0.5 cm2.[26] The
film thickness was measured by a profilometer (Dektak3, Veeco/Sloan
Instruments Inc., USA). Platinized FTO produced by thermopyrolysis of
H2PtCl6 was used as a counter electrode. The TiO2 thin film was dipped
into a 3ꢃ10ꢀ4 m solution of the dye sensitizers in THF for at least 12 h.
After rinsing with THF, the photoanode adhered to a polyester tape
60 mm thick with a square aperture of 0.36 cm2 was placed on top of the
counter electrode and tightly clipped together to form the cell. Electro-
lyte was then injected into the space and then the cell was sealed with
Torr Seal cement (Varian, MA, USA). The electrolyte was composed of
0.5m LiI, 0.05m I2, and 0.5m TBP that was dissolved in acetonitrile.
Compound YO-4
Yield: 46%; 1H NMR (400 MHz, [D6]acetone): d=8.47 (s, 1H), 8.33 (d,
J=8.4 Hz, 1H), 8.12 (d, J=8.4 Hz, 1H), 7.99 (d, J=4.0 Hz, 1H), 7.73 (d,
J=4.0 Hz, 1H), 7.67 (d, J=8.0 Hz, 1H), 7.60–7.55 (m, 2H), 7.44 (t, J=
8.0 Hz, 1H), 7.25 (d, J=8.0 Hz, 1H), 6.92 (d, J=8.0 Hz, 4H), 6.84 (d, J=
8.0 Hz, 4H), 3.95 (t, J=6.6 Hz, 4H), 1.80–1.71 (m, 4H), 1.48–1.45 (m,
4H), 1.35–1.34 (m, 8H), 0.92–0.88 ppm (m, 6H); 13C NMR ([D8]THF,
125 MHz): d=164.1, 156.0, 147.3, 147.2, 146.5, 145.2, 143.8, 140.2, 137.0,
135.9, 134.3, 132.0, 130.0, 129.6, 129.2, 127.8, 127.6, 126.9, 126.8, 126.3,
125.6, 125.5, 125.0, 116.7, 116.0, 100.0, 68.9, 68.1, 32.8, 30.5, 26.9, 26.5,
25.0, 23.7, 14.5 ppm; MS (FAB): m/z: 754.3 [M+]; elemental analysis
calcd (%) for C46H46N2O4S2: C 73.18, H 6.14, N 3.71; found: C 73.37, H
6.45, N 3.90.
Quantum Chemistry Calculations
Calculations were performed with Q-Chem 3.0 software.[27] Geometry op-
timization of the molecules were performed by using a hybrid B3LYP
functional and 6-31G* basis set. For each molecule, a number of possible
conformations were examined and the one with the lowest energy was
used. The same functional was also applied for the calculation of excited
states by TD-DFT. A number of previous works employed TD-DFT to
characterize excited states with charge-transfer character.[28] In some
cases, underestimation of the excitation energies was seen.[27,29] There-
fore, in the present work, we used TD-DFT to visualize the extent of
transition moments as well as their charge-transfer characters, and avoid-
ed drawing conclusions from the excitation energy.
Compound YO-5
Yield: 20%; 1H NMR (400 MHz, [D6]acetone): d=8.57 (s, 1H), 8.56–
8.55 (m, 1H), 8.48 (d, J=8.4 Hz, 1H), 8.36–8.33 (m, 1H), 8.16 (d, J=
4.0 Hz, 1H), 8.10 (d, J=8.4 Hz, 1H), 7.85–7.78 (m, 3H), 7.74–7.71 (m,
2H), 7.64–7.60 (m, 2H), 7.55–7.49 (m, 3H), 7.43 (d, J=7.6 Hz, 1H),
7.29–7.24 (m, 4H), 7.06–7.04 (m, 4H), 7.01–6.98 ppm (m, 2H); 13C NMR
([D8]THF, 125 MHz): d=149.7, 146.9, 145.4, 143.7, 139.1, 132.5, 130.3,
130.1, 129.8, 129.4, 129.2, 129.0, 128.5, 128.2, 128.0, 127.7, 127.5, 127.4,
127.3, 126.7, 123.2, 123.0, 119.3, 116.7, 101.0 ppm; MS (FAB): m/z: 681.2
[M+]; elemental analysis calcd (%) for C44H28N2O2S2: C 77.62, H 4.15, N
4.11; found: C 77.38, H 4.17, N 4.60.
Synthesis
Compounds YO-2 to YO-9 were synthesized from appropriate aldehydes
in the same manner as that of YO-1. Details of intermediates are de-
scribed in the Supporting Information.
Compound YO-6
Compound YO-1
Yield: 44%; 1H NMR (400 MHz, [D6]acetone): d=9.13 (s, 1H), 8.54 (d,
J=8.0 Hz, 1H), 8.29 (d, J=7.6 Hz, 1H), 8.21 (d, J=8.0 Hz, 1H), 7.78–
7.70 (m, 3H), 7.40–7.36 (d, J=7.6 Hz, 4H), 7.28 (d, J=4.0 Hz, 1H), 7.25
(d, J=7.6 Hz, 4H), 7.12 (t, J=7.6 Hz, 2H), 6.86 ppm (d, J=4.0 Hz, 1H);
13C NMR ([D8]THF, 100 MHz): d=163.9, 154.5, 152.2, 149.0, 138.5,
135.2, 133.6, 133.5, 132.5, 130.3, 129.5, 128.6, 128.5, 128.4, 128.1, 128.0,
127.7, 124.6, 124.4, 124.2, 124.0, 123.9, 123.7, 121.6, 116.1, 108.1 ppm; MS
(FAB): m/z: 473.1 [M+]; elemental analysis calcd (%) for C30H20N2O2S:
C 76.25, H 4.27, N 5.93; found: C 75.88, H 4.52, N 5.65.
Precursor 4 (0.80 g, 1.5 mmol), NH4OAc (0.03 g, 0.3 mol%), and 2-cyano-
acetic acid (0.17 g, 2.0 mmol) were dissolved in acetic acid (10 mL) in
a 100 mL round-bottomed flask. The resulting solution was heated to
reflux for 10 h. The solution was extracted with dichloromethane and
water, and the organic extracts collected were dried over MgSO4. The
crude product was further purified by column chromatography on silica
gel by eluting with CH2Cl2/MeOH (20:1 by v/v) to give YO-1 (80%,
0.72 g). 1H NMR (400 MHz, [D6]acetone): d=8.55 (s, 1H), 8.37 (d, J=
8.0 Hz, 1H), 8.36–8.11 (m, 2H), 7.89 (d, J=7.2 Hz, 1H), 7.70–7.62 (m,
4H), 7.56–7.35 (m, 6H), 7.30 (t, J=8.0 Hz, 4H), 7.12 (d, J=8.0 Hz, 4H),
7.01 ppm (t, J=8.0 Hz, 1H); MS (FAB): m/z: 598.2 [M+]; elemental
analysis calcd (%) for C40H26N2O2S: C 80.24, H 4.38, N 4.68; found: C
80.22, H 4.66, N 5.00.
Compound YO-7
Yield: 77%; 1H NMR (400 MHz, [D6]acetone): d=8.57 (s, 1H), 8.49–
8.47 (m, 1H), 8.33–8.31 (m, 1H), 8.15 (d, J=4.0 Hz, 1H), 7.80–7.76 (m,
2H), 7.71–7.67 (m, 4H), 7.62 (d, J=4.0 Hz, 1H), 7.54 (d, J=4.0 Hz, 1H),
7.38 (d, J=3.6 Hz, 1H), 7.35 (t, J=8.0 Hz, 4H), 7.14–7.08 ppm (m, 8H);
13C NMR (100 MHz, [D8]THF): d=164.3, 152.2, 148.7, 146.8, 146.2,
140.7, 139.0, 137.8, 135.2, 133.3, 132.9, 132.2, 130.3, 130.1, 129.4, 128.9,
128.3, 128.2, 127.9, 127.5, 127.4, 126.6, 125.6, 124.6, 124.2, 123.9, 116.7,
101.0 ppm; MS (FAB): m/z: 631.1 [M+]; elemental analysis calcd (%) for
C40H26N2O2S2: C 76.16, H 4.15, N 4.44; found: C 75.92, H 4.45, N 4.37.
Compound YO-2
Yield: 82%; 1H NMR (400 MHz, [D6]acetone): d=8.42 (s, 1H), 8.37 (d,
J=8.4 Hz, 1H), 8.08 (d, J=8.4 Hz, 1H), 7.94 (d, J=4.0 Hz, 1H), 7.74 (d,
J=8.0 Hz, 1H), 7.71 (d, J=4.0 Hz, 1H), 7.60 (t, J=8.4 Hz, 1H), 7.58 (d,
J=4.0 Hz, 1H), 7.48 (t, J=7.6 Hz, 1H), 7.43 (d, J=4.0 Hz, 1H), 7.40 (d,
1082
www.chemasianj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 1074 – 1084

J. T. Lin, C.-F. Yao et al.
Compound YO-8
[2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem.
Rev. 2010, 110, 6595–6663.
Yield: 55%; 1H NMR (400 MHz, [D6]DMSO): d=8.47 (s, 1H), 8.43 (d,
J=8.5, 1H), 8.09–8.13 (m, 1H), 7.97–8.40 (m, 2H), 7.91 (d, J=8.3, 1H),
7.59–7.69 (m, 3H), 7.47 (t, J=7.5, 1H), 7.34 (d, J=7.7, 1H), 7.24 (t, J=
7.5, 4H), 6.90–7.00 ppm (m, 6H); 13C NMR (CDCl3, 125 MHz): 206.5,
163.7, 151.8, 147.8, 146.3, 143.8, 141.0, 134.5, 132.4, 131.4, 130.6, 129.4,
128.4, 127.4, 126.9, 126.8, 126.7, 124.7, 124.4, 124.2, 123.4, 122.1, 121.6,
116.6, 98.3, 30.7 ppm; MS (FAB): m/z: 498 [M+]; elemental analysis
calcd (%) for C32H22N2O2S (YO-8·C3H6O): C 75.52, H 5.07, N 5.03;
found: C 75.52, H 5.06, N 5.66. The compound was recrystallized from
acetone and diethyl ether, and one equivalent of acetone was found to
cocrystallize with the compound.
[3] Y. Ooyama, Y. Harima, Eur. J. Org. Chem. 2009, 2903–2934.
[4] a) M. Grꢁtzel, Inorg. Chem. 2005, 44, 6841–6851; b) Y. Cao, Y. Bai,
Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao, P. Wang, J. Phys. Chem. C
2009, 113, 6290–6297; c) C. Y. Chen, M. Wang, J. Y. Li, N. Pootra-
kulchote, L. Alibabaei, C. Ngoc-le, J. D. Decoppet, J. H. Tsai, C.
Grꢁtzel, C. G. Wu, S. M. Zakeeruddin, M. Grꢁtzel, ACS Nano 2009,
3, 3103–3109.
[5] a) A. Mishra, M. K. R. Fischer, P. Bauerle, Angew. Chem. 2009, 121,
2510–2536; Angew. Chem. Int. Ed. 2009, 48, 2474–2499; b) B. S.
Chen, D. Y. Chen, C. L. Chen, C. W. Hsu, H. C. Hsu, K. L. Wu, S. H.
Liu, P. T. Chou, Y. Chi, J. Mater. Chem. 2011, 21, 1937–1945; c) D.
Zhou, N. Cai, H. Long, M. Zhang, Y. Wang, P. Wang, J. Phys. Chem.
C 2011, 115, 3163–3171; d) W. Zhu, Y. Wu, S. Wang, W. Li, X. Li, J.
Chen, Z. S. Wang, H. Tian, Adv. Funct. Mater. 2011, 21, 756–763;
e) Z. Ning. Y. Fu, H. Tian, Energy Environ. Sci. 2010, 3, 1170–1181;
f) Z. Ning, Q. Zhang, H. Pei, J. Luan, C. Lu, Y. Cui, H. Tian, J.
Phys. Chem. C 2009, 113, 10307–10313.
Compound YO-9
Yield: 90%; 1H NMR (400 MHz, [D6]DMSO): d=8.32 (d, J=8.4 Hz,
1H), 7.93 (d, J=8.5 Hz, 1H), 7.69–7.73 (m, 2H), 7.53–7.59 (m, 1H),
7.32–7.39 (m, 2H), 7.13–7.30 (m, 6H), 6.92–7.13 ppm (m, 6H); 13C NMR
(100 MHz, CDCl3): d=87.5, 97.8, 117.3, 122.6, 122.8, 125.1, 126.1, 126.7,
127.1, 127.6, 129.4, 130.7, 131.8, 132.7, 133.0, 134.9, 137.1, 138.2, 145.9,
148.5, 182.5 ppm; MS (FAB): m/z: 496 [M+]; HRMS: m/z calcd for
C32H20N2O2S: 496.1245; found: 496.1245; elemental analysis calcd (%)
for C32H22N2O2S: C 77.40, H 4.06, N 5.64; found: C 77.90, H 4.28, N 5.59.
[6] W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang, C.
Pan, P. Wang, Chem. Mater. 2010, 22, 1915–1925.
[7] G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, P. Wang, Chem.
Commun. 2009, 2198–2200.
Crystal Structure Analysis
[8] a) C. H. Chen, Y. C. Hsu, H. H. Chou, K. R. Justin Thomas, J. T.
Lin, C. P. Hsu, Chem. Eur. J. 2010, 16, 3184–3193; b) J. T. Lin, P. C.
Chen, Y. S. Yen, Y. C. Hsu, H. H. Chou, M. C. P. Yeh, Org. Lett.
2009, 11, 97–100; c) Y. S. Yen, Y. C. Hsu, J. T. Lin, C. W. Chang,
C. P. Hsu, D. J. Yin, J. Phys. Chem. C 2008, 112, 12557–12567;
d) K. R. Justin Thomas, Y. C. Hsu, J. T. Lin, K. M. Lee, K. C. Ho,
C. H. Lai, Y. M. Cheng, P. T. Chou, Chem. Mater. 2008, 20, 1830–
1840.
[9] a) Y. J. Cheng, S. H. Yang, C. S. Hsu, Chem. Rev. 2009, 109, 5868–
5923; b) M Alicia, A. M. Fraind, J. D. Tovar, J. Phys. Chem. B 2010,
114, 3104–3116.
[10] I. D. L. Albert, J. O. Morley, D. Pugh, J. Phys. Chem. 1995, 99,
8024–8032.
[11] C. Teng, X. Yang, C. Yang, S. Li, M. Cheng, A. Hagfeldt, L. Sun, J.
Phys. Chem. C 2010, 114, 9101–9110.
[12] M. Tanaka, S. Hayashi, S. Eu, T. Umeyama, Y. Matano, H. Imahori,
Chem. Commun. 2007, 2069–2071.
[13] a) J. F. Hartwig, Angew. Chem. 1998, 110, 2154–2177; Angew. Chem.
Int. Ed. 1998, 37, 2046–2067; b) J. P. Wolfe, S. Wagaw, J.-F. Marcoux,
S. L. Buchwald, Acc. Chem. Res. 1998, 31, 805–818.
[14] J. K. Stille, Angew. Chem. 1986, 98, 504–519; Angew. Chem. Int. Ed.
Engl. 1986, 25, 508–524.
An orange prism-like specimen (dimensions 0.18ꢃ0.1ꢃ0.08 mm3) ob-
tained by slow diffusion of Et2O into a solution of the compound in ace-
tone was used for the X-ray crystallographic analysis. The single-crystal
X-ray diffraction experiments were carried out at 100.0(1) K by using
graphite-monochromated MoKa radiation (l=0.71073 ꢄ) on a Bruker
SMART CCD diffractometer. A total of 676 frames were collected. The
frames were integrated with the CrysAlisPro, Oxford Diffraction, Version
1.171.34.40 (released 27-08-2010 CrysAlis171 NET; compiled Aug 27
2010, 11:50:40) software package using a narrow-frame algorithm. Inte-
gration of the data using a triclinic unit cell yielded a total of 24934 re-
flections to a maximum q angle of 29.488 (0.80 ꢄ resolution), of which
6903 were independent and 4587 (99.8%) were greater than 2s(F2). Final
cell constants of a=9.2816(5), b=9.3435(4), c=17.8317(9) ꢄ; a=
97.474(4), b=98.226(4), g=113.725(4)8; and V=1370.78(12) ꢄ3 are
based upon the refinement of 5411 reflections with 5.748<2q<58.978.
Data were corrected for absorption effects by using the multiscan
method. The ratio of minimum to maximum apparent transmission was
0.993. The calculated minimum and maximum transmission coefficients
(based on crystal size) were 0.9927 and 1.0000. The structure was solved
and refined by using the SHELXS-97 (Sheldrick, 1997) software package,
using the space group Pꢀ1, with Z=2 for the formula unit C35H28N2O3S.
The final anisotropic full-matrix least-squares refinement on F2 with 377
variables converged at R1=5.58%, for the observed data and wR2=
11.74% for all data. The goodness-of-fit was 1.064. The largest peak in
the final difference electron density synthesis was 0.739 eꢀ ꢄ3 and the
largest hole was ꢀ0.3868 eꢀ ꢄ3 with a root-mean-square (RMS) deviation
of 0.067 eꢀ ꢄ3. On the basis of the final model, the calculated density was
[15] K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett. 1975, 16,
4467–4470.
[16] a) N. S. Baek, J. S. Yum, H. K. Kim, M. K. Nazeeruddin, M. Grꢁtzel,
Energy Environ. Sci. 2009, 2, 1082–1087; b) A. Bhaskar, G. Ramak-
rishna, Z. Lu, R. Twieg, J. M. Hales, T. Goodson III., J. Am. Chem.
Soc. 2006, 128, 11840–11849; c) D. H. Lee, M. J. Lee, H. M. Song,
B. J. Song, K. D. Seo, M. Pastore, C. Anselmi, S. Fantacci, F. De An-
gelis, M. K. Nazeeruddin, M. Grꢁtzel, H. K. Kim, Dyes Pigm. 2011,
91, 192–198.
ACHTUNGTRENNUNG
1.349 gcmꢀ3 and F(000) was 584 eꢀ. The compound crystallized with one
molecule of acetone. Crystal data and structure refinement parameters
are given in the Supporting Information.
[17] D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura, P. Qin,
G. Boschloo, T. Brinck, A. Hagfeldt, L. Sun, J. Org. Chem. 2007, 72,
9550–9556.
CCDC 856091 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[18] a) K. Hara, T. Sato, R. Katoh, A. Furube, Y. Ohga, A. Shinpo, S.
Suga, K. Sayama, H. Sugihara, H. Arakawa, J. Phys. Chem. B 2003,
107, 597–606; b) K. Hara, Y. Tchibana, Y. Ohga, A. Shinpo, S.
Suga, K. Sayama, H. Sugihara, H. Arakawa, Sol. Energy Mater. Sol.
Cells 2003, 77, 89–103; c) S.-L. Li, K.-J. Jiang, K.-F. Shao, L.-M.
Yang, Chem. Commun. 2006, 2792–2794.
Acknowledgements
We acknowledge the support of the Academia Sinica (A.C.) and NSC
(Taiwan), and the Instrumental Center of Institute of Chemistry (A.C.).
[19] M. Velusamy, Y. C. Hsu, J. T. Lin, C. W. Chang, C. P. Hsu, Chem.
Asian J. 2010, 5, 87–96.
[20] a) S.-J. Moon, J.-H. Yum, R. Humphry-Baker, K. M. Karlsson, D. P.
Hagberg, T. Marinado, A. Hagfeldt, L. Sun, M. Grꢁtzel, M. K. Na-
zeeruddin, J. Phys. Chem. C 2009, 113, 16816–16820; b) L.-Y. Lin,
[1] B. OꢀRegan, M. Grꢁtzel, Nature 1991, 353, 737–740.
Chem. Asian J. 2012, 7, 1074 – 1084
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1083

Metal-Free Dye-Sensitized Solar Cells
FULL PAPERS
C.-H. Tsai, K.-T. Wong, T.-W. Huang, C.-C. Wu, S.-H. Chou, F. Lin,
S.-H. Chen, A.-I. Tsai, J. Mater. Chem. 2011, 21, 5950–5958.
[21] M. Velusamy, K. R. Justin Thomas, J. T. Lin, Y.-C. Hsu, K.-C. Ho,
Org. Lett. 2005, 7, 1899–1902.
[22] J. He, W. Wu, J. Hua, Y. Jiang, S. Qu, J. Li, Y. Long, H. Tian, J.
Mater. Chem. 2011, 21, 6054–6062.
[23] A. Hagfeldt, M. Grꢁtzel, Chem. Rev. 1995, 95, 49–68.
[24] a) Q. Wang, J. E. Moser, M. Grꢁtzel, J. Phys. Chem. B 2005, 109,
14945–14953; b) M. Adachi, M. Sakamoto, J. Jiu, Y. Ogata, S. Isoda,
J. Phys. Chem. B 2006, 110, 13872–13880.
[25] C. Olivier, F. Sauvage, L. Ducasse, F. Castet, M. Grꢁtzel, T. Tou-
pance, ChemSusChem 2011, 4, 731–736.
[26] Y. S. Yen, Y. C. Chen, Y. C. Hsu, H. H. Chou, J. T. Lin, D. J. Yin,
Chem. Eur. J. 2011, 17, 6781–6788.
[27] Y. Shao, L. F. Molnar, Y. Jung, J. Kussmann, C. Ochsenfeld, S. T.
Brown, A. T. B. Gilbert, L. V. Slipchenko, S. V. Levchenko, D. P.
OꢀNeill, R. A. DiStasio, Jr., R. C. Lochan, T. Wang, G. J. O. Beran,
N. A. Besley, J. M. Herbert, C. Y. Lin, T. V. Voorhis, S. H. Chien, A.
Sodt, R. P. Steele, V. A. Rassolov, P. E. Maslen, P. P. Korambath,
R. D. Adamson, B. Austin, J. Baker, E. F. C. Byrd, H. Dachsel, R. J.
Doerksen, A. Dreuw, B. D. Dunietz, A. D. Dutoi, T. R. Furlani,
S. R. Gwaltney, A. Heyden, S. Hirata, C.-P. Hsu, G. Kedziora, R. Z.
Khalliulin, P. Klunzinger, A. M. Lee, M. S. Lee, W. Z. Liang, I.
Lotan, N. Nair, B. Peters, E. I. Proynov, P. A. Pieniazek, Y. M. Rhee,
J. Ritchie, E. Rosta, C. D. Sherrill, A. C. Simmonett, J. E. Subotnik,
H. L. Woodcock III, W. Zhang, A. T. Bell, A. K. Chakraborty, Phys.
Chem. Chem. Phys. 2006, 8, 3172–3191.
[28] a) H. M. Vaswani, C. P. Hsu, M. Head-Gordon, G. R. Fleming, J.
Phys. Chem. B 2003, 107, 7940–7946; b) Y. Kurashige, T. Nakajima,
S. Kurashige, K. Hirao, Y. Nishikitani, J. Phys. Chem. A 2007, 111,
5544–5548.
[29] A. Dreuw, M. Head-Gordon, J. Am. Chem. Soc. 2004, 126, 4007–
4016.
Received: November 30, 2011
Published online: March 1, 2012
1084
www.chemasianj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 1074 – 1084