Synthesis of metal-free organic dyes containing tris(dodecyloxy)phenyl and dithienothiophenyl units and a study of their mesomorphic and photovoltaic properties

Article abstract of DOI:10.1016/j.tet.2013.01.009

In this study we synthesized three metal-free organic dyes (Cpd11, Cpd16, and Cpd22) featuring 3,4,5-tris(dodecyloxy)phenyl and cyanoacrylic acid moieties as electron-donor and electron-acceptor/anchoring units, respectively, linked through various dithienothiophenyl conjugated spacers. Cpd16 exhibits mesomorphic properties, confirmed through polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction (XRD), due to the appropriate ratio of the lengths of its flexible chain to its rigid core. Molecular modeling of Cpd16, and its d-spacing determined from XRD data, verified the existence of a tilt angle in the SmC phase. Among these metal-free organic dyes, a dye-sensitized solar cell incorporating Cpd16 exhibited the best performance, presumably because of its better packing and its mesomorphic properties; the power conversion efficiency was 3.72% (V_oc=0.58 V; J_sc=9.98 mA cm^-2; FF=0.65) under simulated AM 1.5 irradiation (100 mW cm^-2).

Full text of DOI:10.1016/j.tet.2013.01.009

Tetrahedron 69 (2013) 2124e2130
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of metal-free organic dyes containing
tris(dodecyloxy)phenyl and dithienothiophenyl units and a
study of their mesomorphic and photovoltaic properties
Muthaiah Shellaiah a, Hsiao-Ping Fang a, Yu-Ling Lin a, Ying-Chan Hsu b,
,
Jiann-T’suen Lin b, Hong-Cheu Lin a
*
a Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30049, Taiwan, ROC
b Institute of Chemistry, Academia Sinica, Taipei, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study we synthesized three metal-free organic dyes (Cpd11, Cpd16, and Cpd22) featuring 3,4,5-
tris(dodecyloxy)phenyl and cyanoacrylic acid moieties as electron-donor and electron-acceptor/
anchoring units, respectively, linked through various dithienothiophenyl conjugated spacers. Cpd16
exhibits mesomorphic properties, confirmed through polarizing optical microscopy, differential scanning
calorimetry, and X-ray diffraction (XRD), due to the appropriate ratio of the lengths of its flexible chain to
its rigid core. Molecular modeling of Cpd16, and its d-spacing determined from XRD data, verified the
existence of a tilt angle in the SmC phase. Among these metal-free organic dyes, a dye-sensitized solar
cell incorporating Cpd16 exhibited the best performance, presumably because of its better packing and
Received 26 October 2012
Received in revised form 2 January 2013
Accepted 4 January 2013
Available online 11 January 2013
Keywords:
3,4,5-Tris(dodecyloxy)phenyl
Dye-sensitized solar cell
Dithienothiophene
its mesomorphic properties; the power conversion efficiency was 3.72% (Voc¼0.58 V; Jsc¼9.98 mA cmꢀ2
;
FF¼0.65) under simulated AM 1.5 irradiation (100 mW cmꢀ2).
Metal-free organic dye
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
functional groups to interact with the electron sink (TiO2). Metal-
free organic dyes featuring a donor/acceptor structural design
were synthesized have particularly wide absorption ranges for DSSC
applications.11e19 Liquid-crystalline (or mesomorphic) properties
can be introduced to organic dyes when 3,4,5-tris(dodecyloxy)
phenyl segments are incorporated, resulting in potential applica-
tions in flexible electronic materials.20e25 Kato et al.21 reported
conjugated oligothiophene-based polycatenar liquid crystal mate-
rials exhibiting electrochromism upon applying an oxidative po-
tential, with layered smectic and columnar structures capable of
enhancing hole mobilities up to 0.01 cm2 Vꢀ1 sꢀ1. Park et al.22 also
reported that the mesomorphic organization of a dicyanodistyr-
ylbenzene-based molecule could improve its fluorescence-
emitting and semiconducting properties. Therefore, great efforts
have been taken toward the preparation and characterization of
photo- and electro-active structures based on mesogenic units. The
The development of novel materials for use in organic opto-
electronic devices, such as dye-sensitized solar cells (DSSCs),1 has
become a popular research topic in the quest for low-cost, green
materials for sustainable use and a decrease in demand for fossil
fuels and nuclear power. DSSCs based on Ru-photosensitizers,2,3
such as cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarbox-
ylato)-ruthenium(II) (N3)4 and related derivatives, have been ap-
plied very successfully with high power conversion efficiencies
(PCEs) of 9e12%.4e9 Recently, it has been demonstrated that DSSCs
can also be constructed from metal-free organic dyes.10 Because of
the high cost of rare Ru metal and the relatively low molar extinction
coefficients and tedious purification of Ru-photosensitizes,5 metal-
free organic sensitizers have become increasingly attractive and
widely developed.11,12 Nevertheless, the ability to reach higher ef-
ficiencies when using metal-free organic dyes remains a challenge,
although great progress has been made in this field.13e16 The key
characteristics fora dye to be used in a DSSC are high absorption over
a wide range of the solar spectrum with high molar extinction co-
efficients, efficient charge separation, redox stability, and suitable
use of various aromatic segments (e.g., fluorene,26,27 thiophene28,29
)
as spacers in organic dyes can improve the photocurrent generation
and intramolecular charge separation. Dithieno[3,2-b:20,30-d]thio-
phene (DTT)-conjugated dyes have also exhibited excellent DSSC
efficiencies.30 Inaddition, theDTTunitda sulfur-rich(three S atoms)
and electron-rich building blockdincreases the planarity of the dye,
resulting in longer
molecular rotation in fused-ring structures, such as DTT,
overlap in such conjugated molecules could be maximized to
p
-conjugation.31,32 Because of restricted intra-
-orbital
p
* Corresponding author. Tel.: þ886 3 5712121x55305; fax: þ886 3 5724727;
e-mail address: linhc@mail.nctu.edu.tw (H.-C. Lin).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.01.009

M. Shellaiah et al. / Tetrahedron 69 (2013) 2124e2130
2125
enhance intermolecular charge transport.33,34 In this study, we
prepared dyes featuring a 3,4,5-tris(dodecyloxy)phenyl unit21 and
a cyanoacrylic acid as electron donor and acceptor units, re-
spectively, with various spacers inserted as the conjugated bridge
(Fig.1). For example, we introduced a fused-DTT unit as a conjugated
spacer through which electrons could be transferred efficiently from
the donor to the acceptor. In addition to the DTT unit (as a main
structure of the conjugated spacer), we also employed bithiophene
and bithiazole units to extend the conjugated lengths and, thereby,
affect the electron mobilities and absorption spectra. Furthermore,
bithiophene unit enhanced the liquid crystallinity of the dye. To
increase solubility, we inserted alkyl chains onto bithiazole hetero-
cyclic rings. We were aware, however, that the presence of thio-
phene rings in the electron-rich segment might not result in
sufficient separation between the highest occupied molecular or-
bital (HOMO) and lowest unoccupied molecular orbital (LUMO); in
addition, too many alkyl chains would affect the packing between
the layers the molecule, induce molecular aggregation, increase
steric hindrance, and decrease charge transfer. If the structure had
too many rigid rings, however, its solubility would be impacted,
making it harder to dissolve in common solvents with low boiling
points and, thereby, complicating device fabrication. Therefore, we
also comparedthe effects of bithiophene35e37 and bithiazole38 units.
The design of the conjugated system can not only affect the ab-
sorption range but also further influence the electron transfer from
the excited state to the TiO2.39 In this study, we suspected that better
molecular arrangements and stacking would result if the donor/
acceptor molecules exhibited enhanced liquid crystallinity There-
fore, we prepared series of metal-free organic dyes containing DTT37
units with good coplanarity and investigated their mesomorphic
and photovoltaic properties.
Fig. 2. (a) UVevis absorption spectra and (b) normalized photoluminescence (PL)
spectra of metal-free organic dyes in THF solutions (10ꢀ5 M).
Table 1
Absorption, emission, and electrochemical properties of the tested dyes
labs (nm)a
lPL
Stokes
Shiftb
(nm)
Eg
Eox
HOMOe
(eV)
LUMOf
(eV)
a
c
d
Dye
[ε (Mꢀ1 cmꢀ1
]
(nm)
(eV)
(V)
Fig. 1. Chemical structures of the dyes Cpd11, Cpd16, and Cpd22.
Cpd11
Cpd16
443 (99,000)
386 (35,700)
476 (75,000)
374 (58,000)
473 (77,000)
557
624
114
149
2.45
2.19
1.15
0.71
ꢀ5.70
ꢀ5.26
ꢀ3.25
ꢀ3.07
2. Results and discussion
2.1. Optical properties
Cpd22
610
135
2.15
0.77
ꢀ5.32
ꢀ3.17
a
Absorption and PL emission wavelengths recorded in dilute THF solution
(10ꢀ5 M) at room temperature.
Fig 2 displays the UVevis absorption and normalized photo-
luminescence (PL) spectra of Cpd11, Cpd16, and Cpd22 as solutions
in THF (10ꢀ5 M); Table 1 lists their corresponding data. The ab-
sorption spectra reveal that the signals for Cpd16 and Cpd22 were
red-shifted relative to those of Cpd11 after the insertion of the
bithiophene and bithiazole units, respectively, to lengthen the
conjugated linking structures. The maximum absorption peaks for
Cpd11, Cpd16, and Cpd22 at 443, 476, and 473 nm, respectively,
resulted from intramolecular charge transfer (ICT); that is, for the
transition from the 3,4,5-tris(dodecyloxy)benzene donor to the
cyanoacrylic acid acceptor. The spectra of the dyes Cpd16 and
Cpd22 both featured weak
374 nm, respectively, whereas that of Cpd11 featured only a single
intense band at 443 nm (see Table 1). As expected, the elongated
conjugations in Cpd16 and Cpd22 resulted in narrower * en-
ergy gaps and spectral red shifts for the * transitions. Because
the inserted bithiophene and bithiazole units extended the
b
Stokes shift calculated from the difference between labs and lPL
.
c
Optical band gap obtained from the equation Eogpt¼1240/lonset
.
d
e
Eox is the oxidation potential.
EHOMO¼[ꢀ(Eoxꢀ0.25)ꢀ4.8] eV, where 0.25 V is the value for ferrocene versus Ag/
Agþ and 4.8 eV is the energy level of ferrocene below the vacuum.
f
ELUMO¼EHOMOꢀEg.
conjugation lengths in Cpd16 and Cpd22, both dyes exhibited red-
shifted and broader absorptions than those of Cpd11.40 Since Cpd11
had the narrowest absorption wavelength (Fig. 2a), it also
displayed the worst photovoltaic performance among the tested
dyes. The molar extinction coefficients of Cpd16 (476 nm;
pep* transition bands, at 386 and
p-
ε¼7.50ꢁ104 Mꢀ1 cmꢀ1) and Cpd22 (473 nm; ε¼7.70ꢁ104 Mꢀ1 cmꢀ1
)
pe
p
at their maximum absorptions are lower than that of Cpd11
(443 nm; ε¼9.90ꢁ104 Mꢀ1 cmꢀ1), because the inserted bithiophene
and bithiazole units decreased the coplanarity of the acceptor and
pep

2126
M. Shellaiah et al. / Tetrahedron 69 (2013) 2124e2130
donor moieties and, therefore, decreased the degree of charge
transfer.41,42 Furthermore, in comparison with conventional ruthe-
nium complexes (e.g., N3; ε¼1.52ꢁ104 Mꢀ1 cmꢀ1),4 the molar ex-
tinction coefficients of the dyes are relatively large, indicating that
they have good light harvesting ability. The bathochromic shifts
upon proceeding from Cpd11 to Cpd16 (33 nm) and from Cpd11 to
might be arisen from the donor bithiophene linkage and hence to
achieve a higher PCE value. On the other hand, since the HOMO and
LUMO levels of Cpd16 obtained from CV measurements were found
to be higher than those of Cpd11 and Cpd22 in Table 3, and hence
a greater electron injection process was favorable to obtain a higher
PCE efficiency in later photovoltaic measurements.
Cpd22 (30 nm) presumably resulted from the extended
p-conju-
gations. Fig. 2b reveals that when THF solutions of Cpd11, Cpd16,
and Cpd22 were excited at 443, 475, and 475 nm, respectively, the
resulting PL spectra featured weak emissions with Stokes shifts in
the range 114e148 nm, with the PL emissions of dyes following
similar trends to those in their absorption spectra.
2.3. Mesomorphic properties
Table 2 lists the phase transition temperatures and enthalpies of
the dyes Cpd11, Cpd16, and Cpd22, as characterized using differ-
ential scanning calorimetry (DSC). The polarizing optical micros-
copy (POM) image in Fig. 4 reveals that Cpd16 possessed a tilted
2.2. Electrochemical properties
Table 2
The electrochemical properties of dyes can be obtained using
cyclic voltammetry (CV); Table 1 and Fig. 3 present the relevant CV
data and representative cyclic voltammograms, respectively, for
Cpd11, Cpd16, and Cpd22. We determined the HOMO energy levels
of these dyes from their corresponding irreversible oxidation peaks.
The cyclic voltammograms of the dyes Cpd11, Cpd16, and Cpd22
featured irreversible oxidation waves with oxidation potentials of
1.15, 0.71, and 0.77 V, respectively (Fig. 3). The HOMO energy level
of a dye must be more positive (>0.3 eV) than the electrolyte iodine
redox potential if it is to accept electrons effectively.43e46 The
HOMO energy levels of our dyes were in the range from ꢀ5.70 to
ꢀ5.26 eV with respect to the Iꢀ/Iꢀ3 redox couple (ꢀ4.60 eV vs vac-
uum). Because of the absence of reduction peaks, we could not
determine the LUMO energy levels of these dyes from the CV traces,
but we could elucidate them by subtracting the optical band gaps
from the HOMO energy levels. The resulting LUMO energy levels, in
the range from ꢀ3.07 to ꢀ3.25 eV, are higher than the conduction
band edge (ꢀ4.0 eV vs vacuum); therefore, the electron injection
process is energetically favorable. Relative to Cpd11 (containing
only a simple fused dithienothiophene spacer), the dyes Cpd16
(with one more bithiophene unit) and Cpd22 (with one more
bithiazole unit) both had smaller oxidation potentials (Eox).
Therefore, the shortest spacer in Cpd11 (possessing the shortest
conjugation length) resulted in it having the highest oxidation
potential and the largest optical band gap (Eg) among our tested
dyes. The optical band gaps of Cpd11, Cpd16, and Cpd22 were 2.45,
2.19, and 2.15 eV, respectively, which suggested that Cpd22 possibly
might have a higher PCE value. However, in contrast to Cpd11
(without a donor bithiophene linkage) and Cpd22 (with an ac-
ceptor bithiazole linkage), the improved electron injection of Cpd16
Phase transition temperatures and enthalpies of the dyes Cpd11, Cpd16, and Cpd22
Phase transition (ꢂC, [ H (J gꢀ1)])a,b
D
Cpd11
Cpd16
Cpd22
Heating Cr1 105.83[3.67] Cr2 155.7[6.01] Cr3 165.65[8.3]Iso
Cooling Iso 146.21[7.88] Cr3 130.46[1.9] Cr2 87.83[3.17] Cr1
Heating Cr1 182.99[4.9] Cr2 218.12[19.34] SmC 242.52[2.01]Iso
Cooling Isoc235.2 SmC 175.10[2.04] Cr2 142.78[1.54] Cr1
Heating Cr 202.91[4.7]Iso
Cooling Iso 194.97[3.3] Cr
a
Data determinedꢀth1 rough DSC from second heating/first cooling run at a scan-
ning rate of 5 ꢂC min
.
b
Abbreviations: Cr1ꢀ3, different crystalline modifications; SmC, tilted smectic
phase; Iso, isotropic liquid state.
c
Isotropic temperature determined through POM.
Fig. 4. (a) Optical texture of the nematic phase in the dye Cpd16 at 225 ꢂC (cooling),
observed through POM, and (b) XRD intensity plotted with respect to angle for the dye
Cpd16 at 225 ꢂC.
Fig. 3. Cyclic voltammograms of the dyes Cpd11, Cpd16, and Cpd22 (in THF), recorded
at a scan rate of 100 mV sꢀ1
.

M. Shellaiah et al. / Tetrahedron 69 (2013) 2124e2130
2127
smectic (SmC) phase, with a broken focal conic fan texture at
225 ꢂC. The mesomorphic properties of Cpd16 were confirmed
using powder X-ray diffraction (XRD); after thermal annealing at
225 ꢂC for 10 min, Cpd16 exhibited a primary diffraction feature in
the low angle region of Fig. 4, with a sharp peak at a value of 2q of
1.3ꢂ (corresponding to a d-spacing of 50.6 A). Fig. 5 presents
a possible packing motif (side-view) for Cpd16; this model suggests
that the dye molecules stacked with bilayer packing and may have
trivial interdigitated arrangements as a result of hydrogen bonding
interactions between terminal carboxyl (COOH) units. Using
Chemdraw software for simulation, we calculated the theoretical
ꢀ
ꢀ
molecular length of Cpd16 to be 38.44 A; its hydrogen-bonded
ꢀ
dimer would, therefore, have a length of w75 A. The d-spacing of
ꢀ
50.6 A for Cpd16, determined using XRD, suggested a tilted smectic
molecular arrangement (e.g., SmC phase) in Fig. 5. The broad peak
in the wide angle region at a value of 2q
of 15ꢂ in Fig. 4 corresponds
ꢀ
to a d-spacing of 4.6 A, which we assign to the lateral distance
between the conjugated backbones, as has been reported for other
similar
though this broad peak might also have contained some contri-
butions from the lateral
stacking of the dye planes.50 The
broad XRD halos in Fig. 4 suggest, however, that stacking in
p
-conjugated polymers presenting long pendants,47e49 al-
pep
pep
Cpd16 occurred only in very small areas; that is, it mainly possessed
an amorphous structure.51
Fig. 6. (a) IPCE plots and (b) IeV curves of DSSCs fabricated using the dyes Cpd11,
Cpd16, Cpd22, and N719.
Table 3
Cell performance of Cpd11, Cpd16, Cpd22, and N719-sensitized solar cells
DSSC dyea
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
h (%)
Cpd11
Cpd16
Cpd22
N719
0.57
0.58
0.60
0.70
ꢀ6.85
ꢀ9.98
0.70
0.65
0.70
0.65
2.69
3.72
2.82
7.04
Fig. 5. Molecular model of the dye Cpd16.
ꢀ6.77
ꢀ15.41
2.4. Photovoltaic properties of DSSCs
Measured under AM 1.5 irradiation, 100 mW cmꢀ2
.
a
Fig. 6 displays incident photon-to-current conversion efficiency
(IPCE) and photocurrentevoltage (IeV) curves of DSSCs based on
the dyes Cpd11, Cpd16, Cpd22, and N719. From Fig. 6b, we char-
acterized the photovoltaic parameters of the DSSCs (Table 3),
namely their open-circuit photovoltages (VOC), short-circuit pho-
tocurrent densities (JSC), fill factors (FFs), and solar-to-electrical
high short current intensity (Jsc¼9.98 mA cmꢀ2) and broadest and
most-intense IPCE spectrum (toward the longer wavelength re-
gion), both of which presumably resulted from the longer conju-
gated structure induced by this dye’s additional bithiophene linker.
The different PCE values of Cpd11, Cpd16, and Cpd22 may be at-
tributed to the following reasons; (i) Cpd16 and Cpd22 were
bridged through a donor bithiophene linkage and an acceptor
bithiazole linkage, respectively, in contrast to Cpd11; (ii) the steric
effect induced by the lateral alkyl chains of the bithiazole (a) unit
might affect the conjugation of Cpd22; (iii) as noticed in Fig. 4, the
greater packing nature of Cpd16 enhanced the electron injection to
TiO2 and also enhanced JSC to obtain a higher PCE value.
energy conversion efficiencies (
cies (PCEs, i.e., ) followed the trend Cpd16 (
¼2.82)>Cpd11 ( ¼2.69). The highest PCE was that of the DSSC
h
). The power conversion efficien-
h
h¼3.72)>Cpd22
(h
h
incorporating Cpd16, mainly because it had the highest short cur-
rent density (Jsc¼9.98 mA cmꢀ2), which reveals more electrons
were transferred from the excited state of the dye and injected into
the conduction band of TiO2; the DSSCs incorporating the three
dyes each had similar values of Voc and FF. The IPCE spectrum of
Cpd16 (Fig. 6a) featured the broadest response in the range
300e750 nm with a maximum IPCE value of 64%; this behavior is
3. Conclusion
consistent with its DSSC having the highest PCE (
h
¼3.72; with
We have synthesized three new metal-free organic dyes (Cpd11,
Cpd16, and Cpd22), each featuring a tris(dodecyloxy)phenyl moiety
(a common unit in liquid crystalline structures) as an electron
VOC¼0.58 V; JSC¼9.98 mA cmꢀ2; and FF¼0.65). Thus, the highest
PCE (h¼3.72) for the device incorporating Cpd16 resulted from its

2128
M. Shellaiah et al. / Tetrahedron 69 (2013) 2124e2130
donor, a cyanoacrylic acid moiety as an electron acceptor/anchoring
group, and a DTT-based spacer to bridge the donor and acceptor
moieties. To extend the length of conjugation, we appended
a bithiophene or bithiazole moiety to the DTT unit to enhance the
capacity for charge transfer and increase the range of absorption.
The dye Cpd16 exhibited mesomorphic properties, resulting from
the appropriate ratio of the lengths of its flexible chain to its rigid
core; molecular modeling of Cpd16, and its d-spacing value de-
termined using XRD, verified the existence of a tilt angle in the SmC
phase. In addition, among the tested dyes, the DSSC exhibiting the
best performance was that incorporating Cpd16, presumably be-
cause of its superior packing as a result of its mesomorphic prop-
erties. This DSSC exhibited a maximum PCE of 3.72% (Voc¼0.58 V;
Jsc¼9.98 mA cmꢀ2; FF¼0.65) under simulated AM 1.5 irradiation
(100 mW cmꢀ2).
(5-bromo-4-hexylthiazol-2-yl)-4-hexylthiazole-5-carbaldehyde
(7)26,43 were synthesized according to literature procedures.
4.1.2. Synthesis
4.1.2.1. 2-(3,4,5-Tris-n-dodecyloxyphenyl)dithieno[3,2-b:20,30-d]
thiophene (3). A solution of 1 (2.34 g, 1.00 mmol) and 2 (1.60 g,
3.29 mmol) in dry toluene (50 mL) in a 100-mL, three-neck, round-
bottom flask was deoxygenated with N2 for 30 min. Pd(PPh3)4
(110 mg, 0.02 mmol) was added and then the mixture was heated at
110 ꢂC for 2 days. The organic layer was extracted with DCM; the
extracts were dried over anhydrous MgSO4. Column chromatog-
raphy (SiO2; hexanes/CH2Cl2, 3:1) provided the title compound
(55.6%).
4.1.2.2. 6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30-a]
thiophene-2-carbaldehyde (4). A 250-mL three-neck flask contain-
ing anhydrous DMF (0.1 mL, 1 mmol) was cooled in an ice bath and
then POCl3 (0.1 mL, 5 mmol) was added dropwise over 30 min. A
solution of 3 (0.82 g, 0.1 mmol) in 1,2-dichloroethane (30 mL) was
added to the solution and then the mixture was heated at 90 ꢂC for
24 h. This solution was cooled to room temperature, poured into ice
water, and neutralized to pH 6e7 through dropwise addition of
saturated aqueous NaOH. The mixture was partitioned between
CH2Cl2 and water. The organic layer was dried (MgSO4) and con-
centrated under reduced pressure. The crude product was purified
through column chromatography SiO2; (CH2Cl2/hexane, 1:1) to give
a yellow solid (0.7 g, 82%).
4. Experimental section
4.1. General information
4.1.1. Materials. Chemicals and solvents were of reagent grade and
purchased from Aldrich, ACROS, TCI, or Lancaster Chemical. Tol-
uene, tetrahydrofuran (THF), dimethylformamide (DMF), and
dichloromethane (DCM) were dried and distilled prior to use. N-
Bromosuccinimide (NBS) was recrystallized from distilled water
and dried under vacuum. All other chemicals were used without
further purification. The synthetic routes for all dyes are presented
in Scheme 1. 1-Bromo-1,2,3-tris-n-dodecyloxybenzene (1),15 tri-n-
butyl(dithieno[3,2-b:20,30-d]thiophen-2-yl)stannane (2),32 5-(5-
bromothiophen-2-yl)thiophene-2-carbaldehyde (6),11,27 and 2-
4.1.2.3. 6-[3,4,5-Tris(dodecyloxy)phenyl]-2-tributylstannyl dithieno
[3,2-b:20,30-a]thiophene (5). n-BuLi (2.5 M in hexane, 0.59 mL,
1.5 mmol) was added over 1 h to a stirred solution of 3 (0.83 g,
0.1 mmol) in dry THF (20 mL) in a 250-mL flask at ꢀ78 ꢂC. The
mixture was then warmed slowly (1 h) to room temperature under
an ambient environment with stirring. After the mixture was had
been re-cooled to ꢀ78 ꢂC, Bu3SnCl (0.53 mL, 1.2 mmol) was added
slowly. The mixture was then stirred at ambient temperature for
18 h, followed by the addition of water (100 mL). The aqueous phase
was extracted with CH2Cl2 (200 mL); the combined organic phases
were dried (MgSO4) and concentrated under a reduced pressure. The
crude product was purified through column chromatography (SiO2;
CH2Cl2/hexane, 2:5) to give a pale-yellow oil.
C12H25
O
S
C12H25
O
S
i
C12H25
O
Br
+
C12H25
O
SnBu3
S
S
S
S
C12H25
O
C12H25
O
1
2
3
C12H25
O
S
ii
C12H25
O
CHO
S
S
4.1.2.4. 5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30-
a]thiophylthiophen-2-yl)thiophene-2-carbaldehyde (8). A 250-mL
two-neck flask containing 5 (1.1 mmol, 0.85 g), 6 (0.77 mmol,
0.21 g), and Pd(PPh3)4 (0.03 mmol, 0.026 g), in toluene (15 mL) was
heated at 90 ꢂC for 24 h. The mixture was the partitioned between
CH2Cl2 and water. The organic phase was dried (MgSO4) and con-
centrated under reduced pressure. The residue was purified
through column chromatography (SiO2; CH2Cl2/hexane, 1:1) to give
a red-orange solid (0.5 g, 64.5%).
C12H25
O
4
C12H25
O
S
iii
C12H25
O
SnBu3
S
S
C12H25
O
5
CHO
S
Br
C12H25
C12H25
C12H25
O
S
S
4.1.2.5. 5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30-
a]thiophenyl-2-(4-hexylthiazol-2-yl)-4-hexylthiazole-2-
carbaldehyde) (9). A 250-mL, two-neck flask containing 5 (0.85 g,
1.1 mmol), 7 (0.21 g, 0.77 mmol), and Pd(PPh3)4 (0.026 g,
0.03 mmol), in toluene (15 mL) was heated at 90 ꢂC for 24 h. The
mixture was partitioned between CH2Cl2 and water and then the
organic phase was dried (MgSO4) and concentrated under reduced
pressure. The residue was purified through column chromatogra-
phy (SiO2; EtOAc/hexane, 1:10) to give a red-orange solid (0.5 g,
64.5%).
CHO
S
6
O
S
S
S
iv
O
8
C6H13
Br
CHO
N
S
S
C6H13
C12H25
O
S
N
N
S
C6H13
CHO
C6H13
7
C12H25
O
S
S
S
N
v
C12H25
O
9
vi
Cpd11, Cpd16, Cpd22
4, 8, 9
4.1.2.6. 3-{6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30-a]
thiophene-2-yl}-2-cyanoacrylic acid (Cpd11). A mixture of 4 (0.70 g,
Scheme 1. Synthetic routes of dyes. (i, iv, v) Pd(PPh3)4, toluene, reflux. (ii) POCl3, DMF,
DCE. (iii) n-BuLi, THF, ꢀ78 ꢂC, SnBu3Cl. (vi) NCCH2COOH, NH4OAc, CH3COOH.

M. Shellaiah et al. / Tetrahedron 69 (2013) 2124e2130
2129
0.08 mmol), cyanoacetic acid (0.12 g, 1.41 mmol), ammonium ace-
tate (0.063 g, 0.08 mmol), and glacial AcOH (60 mL) was heated
overnight at 110 ꢂC with efficient stirring. The red solution was
cooled to induce a precipitate, which was filtered off and thor-
oughly washed with water and MeOH, to give a red solid (79.8%).
method. Polyester tape (3 M) was used as an adhesive on two edges
of the FTO glass. The tape was removed after the TiO2 paste had
been spread on the FTO using a glass rod and then the TiO2 paste
was dried in air at room temperature for 1 h. The TiO2-coated FTO
was heated to 500 ꢂC at a heating rate of 10 ꢂC minꢀ1 and then
maintained at that temperature for 30 min before cooling to room
temperature. After repeating the procedure above to control the
thickness of the TiO2 film, the final coating was performed using
TiO2 pastes containing different sizes (300 and 20 nm; 30 and
70 wt %, respectively) of light-scattering TiO2 particles; the samples
were then heated at 500 ꢂC. The thicknesses of the TiO2 films were
4.1.2.7. 3-{5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-b:20,30-
a]thiophylthiophen-2-yl)thiophenyl}-2-cyanoacrylic acid (Cpd16). Pre-
pared, using the same procedure as that described for Cpd11, as
a dark-brown solid (73.2%).
4.1.2.8. 3-{5-(5-6-[3,4,5-Tris(dodecyloxy)phenyl]dithieno[3,2-
b:20,30-a]thiophyl}-2-(4-hexylthiazol-2-yl)-4-hexylthiazole-2-
cyanoacrylic acid) (Cpd22). Prepared, using the same procedure as
that described for Cpd11, as a dark-brown solid (73.2%).
measured using a profilometer (Dektak3, Veeco/Sloan In-
struments). The density of each adsorbed dye was calculated from
the concentration difference of each solution before and after TiO2
film immersion. The TiO2 electrode with a geometric area of
0.25 cm2 was immersed overnight in a MeCN/tert-butanol (1:1, v/v)
solution of 3ꢁ10ꢀ4 M cis-di(thiocyanato)bis(2,20-bipyridyl-4,40-
dicarboxylato)-ruthenium(II) bis(tetrabutylammonium) (N719,
Solaronix SA) or in a THF solution containing 3ꢁ10ꢀ4 M organic
sensitizers. Thermally platinized FTO was used as the counter
electrode; its active area was controlled to 0.36 cm2 using adhered
4.1.3. Measurement and characterizations. 1H NMR spectra were
recorded using a Varian unity 300 MHz spectrometer, with DMSO-
d6 and CHCl3 as solvents. Elemental analyses were performed using
a HERAEUS CHNeOS RAPID elemental analyzer. UVevis absorption
spectra of dilute THF solutions (10ꢀ5 M) were recorded using an HP
G1103A spectrophotometer; photoluminescence (PL) spectra were
recorded using a Hitachi F-4500 spectrophotometer. Cyclic vol-
tammetry (CV) was performed at room temperature using a BAS
100 electrochemical analyzer, a standard three-electrode electro-
polyester tape having a thickness of 60 mm. After rinsing with MeCN
or THF, the photoanode was placed on top of the counter electrode
and tightly clipped together to form a cell. Electrolyte was injected
into the space and then the cell was sealed with Torr Seal cement
(Varian). The electrolyte comprised 0.5 M LiI, 0.05 M I2, and 0.5 M 4-
tert-butylpyridine (TBP) in MeCN. The photovoltage transients of
the assembled devices were recorded using a digital oscilloscope
(LeCroy, WaveSurfer 24Xs). Pulsed laser excitation was applied
using a Q-switched Nd:YAG laser (Continuum, model Minilite II)
operated at 532 nm, with a 1 Hz repetition rate and a 5-ns pulse
width at half-height. The beam size was slightly larger than
0.5ꢁ0.5 cm2 to cover the area of the device. The photovoltage of
each device was adjusted by incident pulse energy to be 40 mV.
chemical cell, and
a
0.1
M
tetrabutylammonium hexa-
fluorophosphate (TBAPF6) solution (in THF), with a scanning rate of
100 mV sꢀ1. During CV measurements, the solutions were purged
with N2 for 30 s. In each case, a carbon coating rod was the working
electrode, a platinum wire was the counter electrode, and a silver
wire was the quasi-reference electrode; a Ag/AgCl (3 M KCl) elec-
trode served as the reference electrode for all potentials quoted
herein. The redox couple of ferrocene/ferrocenium ion (Fc/Fcþ) was
used as an external standard. The corresponding HOMO and LUMO
energy levels were calculated from the onset oxidation potential
(Eox/onset) and UVevis absorption edge (Eogpt), respectively. Meso-
phasic textures were characterized through POM using a Leica
DMLP equipped with a hot stage.
4.1.3.4. Device measurements. A 0.6ꢁ0.6 cm2 cardboard mask
was clipped onto the device to constrain the illumination area.
Photoelectrochemical characterization of the DSSCs was performed
using an Oriel Class A solar simulator (Oriel 91195A, Newport). The
photocurrentevoltage characteristics of the DSSCs were recorded
using a potentiostat/galvanostat (CHI650B, CH Instruments) at
a light intensity of 1.0 sun, calibrated using an Oriel reference solar
cell (Oriel 91150, Newport). The monochromatic quantum effi-
ciency was recorded through a monochromator (Oriel 74100,
Newport) under short-circuit conditions. The intensity of each
4.1.3.1. XRD characterization. Synchrotron powder XRD was
performed at beamline BL17A of the National Synchrotron Radia-
tion Research Center (NSRRC), Taiwan; the wavelength of the
ꢀ
X-rays was 1.33366 A. The powder samples were packed into
a capillary tube and heated with a heat gun, the temperature
controller of which was programmed by a PC with a PID feedback
system. The scattering angle,
silver behenate and silicon.
q, was calibrated using a mixture of
wavelength was in the range from 1 to 3 mW cmꢀ2
.
Acknowledgements
4.1.3.2. TiO2 paste preparation. The TiO2 precursor and the
electrode were fabricated using previously reported procedures34
with an autoclave temperature of 240 ꢂC. The precursor solution
was prepared according to the following procedure: 0.1 M HNO3
(430 mL) under vigorous stirring was slowly combined with
Ti(C3H7O)4 (72 mL). After hydrolysis, the mixture was heated at
85 ꢂC in a water bath and stirred vigorously for 8 h to achieve
peptization. The mixture was cooled to room temperature and the
resultant colloid was filtered; the filtrate was then heated in an
autoclave at 240 ꢂC for 12 h to grow the TiO2 particles. The
colloid was cooled to room temperature and vibrated ultrasonically
for 10 min. The TiO2 colloid was concentrated to 13 wt %,
followed by the addition of 30 wt % (with respect to TiO2 weight) of
poly(ethylene glycol) (PEG; MW¼20,000 g molꢀ1) to prevent the
film from cracking while drying.
The powder XRD measurements are supported by beamline
BL17A1 (charged by Dr. Jey-Jau Lee) of the National Synchrotron
Radiation Research Center (NSRRC) in Taiwan. The financial sup-
ports of this project provided by the National Science Council of
Taiwan (ROC) through NSC 99-2113-M-009-006-MY2, NSC 99-
2221-E-009-008-MY2, and National Chiao Tung University through
97W807 are acknowledged.
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