Enhanced light-harvesting capability by phenothiazine in ruthenium sensitizers with superior photovoltaic performance

Article abstract of DOI:10.1039/c1jm13178h

This paper reports on the design and synthesis of two new ruthenium dyes [Ru(dcbpy)(potip)(NCS)₂] (JF-3, dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine, potip = 2-(4-(10H-phenothiazin-10-yl)-5- octylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenant

Full text of DOI:10.1039/c1jm13178h

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PAPER
Enhanced light-harvesting capability by phenothiazine in ruthenium
sensitizers with superior photovoltaic performance
Jen-Fu Yin,ab Jian-Ging Chen,c Jiann-T’suen Lin,a Dibyendu Bhattacharya,a Ying-Chan Hsu,a
b
Hong-Cheu Lin, Kuo-Chuan Ho and Kuang-Lieh Lu
c
a
*
*
*
Received 8th July 2011, Accepted 28th September 2011
DOI: 10.1039/c1jm13178h
This paper reports on the design and synthesis of two new ruthenium dyes [Ru(dcbpy)(potip)(NCS)2]
(JF-3, dcbpy ¼ 4,40-dicarboxylic acid-2,20-bipyridine, potip ¼ 2-(4-(10H-phenothiazin-10-yl)-5-
octylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline, and [Ru(dcbpy)(dpotip)(NCS)2] (JF-4,
dpotip ¼ 2-(4-(N,N-diphenylamino)-5-octylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline)
that contain electron-donating phenothiazine-based or N,N-diphenylamino-based ancillary ligands.
The ruthenium dye JF-3, in which an excellent electron-donating phenothiazine is incorporated, shows
superior DSC performance (9.1%; compared to 8.8% for N3). A comparison of the electron-donating
phenothiazine and N,N-diphenylamino groups utilized in the molecular architecture of ruthenium dyes
demonstrates that the phenothiazine group efficiently increases the molar extinction coefficient of band
II and also broadens that band in the UV-vis absorption spectrum, reduces device resistance, increases
electron lifetime, and enhances power-conversion efficiency. This finding not only clarifies the
significance of multifunctionalized design in the molecular architecture of ruthenium dyes, but also
represents an alternative route for the introduction of an excellent electron-donating group for
improving the light-harvesting characteristics of ruthenium dyes as well as DSC performance.
photosensitizer, [Ru(dcbpy)2(NCS)2] (N3, where dcbpy ¼ 4,40-
1. Introduction
0
€
dicarboxylic acid-2,2 -bipyridine), by Gratzel and coworkers in
1993,10 various structural modifications have been proposed to
improve its power-conversion efficiency.11–25 Certain light-har-
vesting chromophores such as thiophene16,25 and carbazole17 that
are employed in producing multifunctionalized ruthenium dyes
are known to enhance their power-conversion efficiency. In
addition, the electron-donating phenothiazine was widely used to
enhance the light-harvesting capability of organic dyes.26,27 Its
structure is an analogous plane with a slight bending butterfly
conformation in the ground state, which can impede the molec-
ular aggregation and the formation of intermolecular excimers.26
However, issues related to how to select a beneficial chromo-
phore to improve the light-harvesting capability of ruthenium
dyes, and how it affects ruthenium dyes are still not very clear.
Moreover, reports of alternate electron-donating groups that can
be used for the structural modification of ruthenium dyes are also
rare. In this paper, we address these issues. We introduced an
excellent electron-donating phenothiazine for enhancing the
light-harvesting capability of ruthenium dyes and applied
the concept of multifunctionalized molecular design to the
preparation of ruthenium dyes. A ruthenium dye with an elec-
tron-donating phenothiazine shows excellent photovoltaic
performance greater than that with an N,N-diphenylamino
group (Scheme 1). This up and down adjustment for the opti-
mization of DSC efficiency is of fundamental importance. The
Humans require energy for almost any type of activity that they
perform. The world annual energy consumption is ca. 4.7 ꢀ 1020
J and is expected to grow by about 2% each year for the next 25
years.1,2 However, fossil fuel reserves, such as oil, coal and gas,
are rapidly becoming depleted. The development of alternative
energy sources to meet our needs has become a significant topic
in recent years.3–5 Energy from the sunlight that strikes the earth
in 1 hour exceeds all of the energy consumed by humans in an
entire year. Furthermore, the energy supplied from the sun is
clean and sustainable.6 Photovoltaic technology is one of the
most favorable ways to convert solar energy into some forms of
usable energy.7 Dye-sensitized solar cells (DSCs) have attracted
widespread interest owing to the fact that they are relatively cost-
effective, easy to manufacture, and can be readily shaped with
flexible substrates to satisfy the demands of various applica-
tions.8,9 DSC performance is deeply affected by three crucial
components: porous nanocrystalline TiO2 film, photosensitizers,
and electrolyte. Since the report of the ruthenium-based
aInstitute of Chemistry, Academia Sinica, Taipei, 115, Taiwan. E-mail:
lu@chem.sinica.edu.tw
bDepartment of Materials Science and Engineering, National Chiao Tung
University, Hsinchu, 300, Taiwan
cDepartment of Chemical Engineering and Institute of Polymer Science and
Engineering, National Taiwan University, Taipei, 106, Taiwan
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6-dione30 and ammonium acetate, which is used as the source of
ammonia, in glacial acetic acid, according to protocols reported
by Steck and Day.31 Scheme 2 depicts the synthetic routes used in
preparing the potip and dpotip ligands. The ruthenium dyes, JF-
3 and JF-4 (Fig. 1), were prepared by treating a ruthenium dimer
with each of the ancillary ligands, potip and dpotip, in DMF
under an argon atmosphere using a typical one-pot synthesis.32
The one-pot synthesis consists of three different temperature
regimes: [RuCl2(p-cymene)]2 reacts with the ancillary ligand at 80
ꢁC, resulting in the formation of a mononuclear Ru(II) complex;
the cymene ligand from tꢁhe Ru(II) coordination sphere is
replaced with dcbpy at 160 C under dark conditions; and the
desired ruthenium dye product is obtained by adding excess
Scheme 1 The up and down adjustment of power-conversion efficiencies
of the DSC with JF-2, JF-3, and JF-4.
ꢁ
ammonium thiocyanate at 130 C.
origins of these effects can be explained by spectral data, theo-
retical studies, and device properties.
2.2. Photophysical properties
Fig. 2 shows UV-vis absorption spectra of the free ligands,
potip, dpotip and otip (where otip, 2-(5-octylthiophen-2-yl)-1H-
imidazo[4,5-f][1,10]phenanthroline, was used as ancillary ligand
for JF-2 in our previous study,18 Fig. 1), in DMF solution. All
ligands display an intense absorption band with a shoulder in
the UV-vis region. The absorption maxima (lmax) for all ligands
are in the range of 270–290 nm. The lsh of dpotip (375 nm) is
red-shifted by 35 nm compared to that (340 nm) of otip, and the
molar extinction coefficient (3) of the shoulder for dpotip is
reduced to half that for otip. However, the 3 value of potip is
doubled at lmax and increased slightly at lsh compared with that
of otip, indicating that the phenothiazine group strengthens the
molar extinction coefficient in the UV-vis absorption spectra of
otip. The UV-vis absorption spectra of JF-2, JF-3, and JF-4
show three main features, assigned as bands I, II (shoulder), and
III with increasing energy order (Fig. 3). The UV-vis absorption
data for dyes, JF-2, JF-3 and JF-4, are listed in Table 1.18 The 3
values (0.99–1.05 ꢀ 104 Mꢂ1 cmꢂ1) and positions (520 nm) of the
lowest metal-to-ligand charge transfer (MLCT) band I for all
dyes are nearly identical. However, the 3 values of band II are
increased in the order JF-4 < JF-2 < JF-3. The order of the
absorption band II wavelength for all dyes is consistent with
that of lsh in their corresponding ancillary ligands, indicating
that band II is an ancillary ligand-localized transition. More-
over, the ancillary ligand with an electron-donating phenothia-
zine group can efficiently increase the molar extinction
2. Results and discussion
2.1. Synthesis
2-Bromo-3-(10H-phenothiazin-10-yl)thiophene and 2-bromo-3-
(N,N-diphenylamino)thiophene were synthesized through
a cross-coupling reaction28,29 of 3-bromothiophene with pheno-
thiazine and N,N-diphenylamine, respectively, followed by
bromination with N-bromosuccinimide (NBS) in dime-
thylformamide (DMF). 3-(10H-Phenothiazin-10-yl)-2-octylth-
iophene and 3-(N,N-diphenylamino)-2-octylthiophene were
obtained by forming the dianion of their 2-bromo-3-substituted
thiophene precursors with 1.2 equiv. of n-butyllithium, followed
by reaction with 1-bromooctane. 4-(10H-Phenothiazin-10-yl)-5-
octylthiophene-2-carbaldehyde and 4-(N,N-diphenylamino)-5-
octylthiophene-2-carbaldehyde were then prepared using
a synthetic procedure similar to that described above, except that
N-formylpiperidine was used instead of 1-bromooctane. Even-
tually, the ancillary ligands, 2-(4-(10H-phenothiazin-10-yl)-5-
octylthiophen-2-yl)-1H-imidazo[4,5-f][1,10]phenanthroline
(potip) and 2-(4-(N,N-diphenylamino)-5-octylthiophen-2-yl)-
1H-imidazo[4,5-f][1,10]phenanthroline (dpotip), were obtained
by coupling the corresponding 4-(10H-phenothiazin-10-yl)-5-
octylthiophene-2-carbaldehyde and 4-(N,N-diphenylamino)-5-
octylthiophene-2-carbaldehyde with 1,10-phenanthroline-5,
Scheme 2 Preparation of ancillary ligands, potip and dpotip: (a) Pd(dba)2, P(tBu)3, NaOtBu, toluene, reflux 1.5 h; (b) NBS, DMF, 0 ꢁC; (c) n-
butyllithium, 1-bromooctane, THF, ꢂ78 ꢁC; (d) n-butyllithium, N-formylpiperidine, THF, ꢂ78 ꢁC; (e) NH4OAc, CH3COOH, reflux 2 h.
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Fig. 1 Molecular structures of JF-2, JF-3, and JF-4.
Fig. 3 UV-vis absorption spectra of ruthenium dyes, JF-2, JF-3 and JF-
4, in DMF.
Fig. 2 UV-vis absorption spectra of ancillary ligands, otip, potip and
dpotip, in DMF.
coefficient of band II and hence result in superior light-har-
vesting characteristics.
Fig. 4 shows the UV-vis absorption spectra of JF-2, JF-3 and
JF-4, when anchored onto the surface of 2.4 mm thick trans-
parent nanocrystalline TiO2 films. The lowest energy MLCT
bands of JF-2/TiO2 (513 nm), JF-3/TiO2 (470 nm) and JF-4/TiO2
(510 nm) are blue-shifted by 6, 50 and 10 nm, respectively,
compared with their spectra in DMF solution (Fig. 3). This blue-
shift of the spectra on TiO2 films can be attributed to p-stacking
interactions or the formation of aggregates on the TiO2 surface.33
In addition, one can see the broader absorption spectrum of JF-3
compared to JF-4 (and also JF-2), suggesting a better molecular
planar configuration between thiophene and phenothiazine
groups in JF-3 than that of thiophene and the N,N-diphenyla-
mino group in JF-4 (vide infra).
Fig. 4 UV-vis absorption spectra of ruthenium dyes, JF-2, JF-3 and JF-
4, anchored onto 2.4 mm thick transparent nanocrystalline TiO2 films.
a sufficient driving force for efficient dye regeneration, and the
avoidance of geminate charge recombination. Furthermore, the
energy levels of the LUMO of JF-2, JF-3 and JF-4 at ꢂ3.62,
ꢂ3.65 and ꢂ3.60 eV are more negative than the conduction band
of TiO2, ensuring a sufficient driving force for electron injection
from the excited dyes to TiO2.
2.4. Molecular modeling
2.3. Electrochemical behaviors
The optimized molecular structures of JF-3 and JF-4 as well as
their frontier molecular orbitals of their calculated HOMO and
LUMO were computed by density functional theory (DFT), and
the data are shown in Fig. 6. The HOMOs of JF-3 and JF-4 are
populated from Ru-t2g to the NCS-p orbital, while their LUMOs
are delocalized homogeneously on the anchoring ligand (dcbpy).
This spatially directed separation of HOMO and LUMO is an
ideal condition for DSCs, which is good for dye regeneration and
interfacial electron injection from the excited dye to the
conduction band of TiO2. On closer inspection, the dihedral
angle between the planes of thiophene and phenothiazine is
80.357ꢁ, and the phenothiazine group in the structure of JF-3 is
The electrochemical behaviors of dyes, JF-3 and JF-4, were
investigated by square-wave voltammetry (Fig. 5) and their
oxidation potentials are listed in Table 1. The redox potentials of
dyes, JF-3 and JF-4, were unequivocally determined to be 0.35
and 0.33 (V vs. Fc/Fc+). The highest-occupied molecular orbital
(HOMO) and lowest-unoccupied molecular orbital (LUMO)
energy levels of JF-2, JF-3 and JF-4 were calculated from their
oxidation potentials and their absorption edges obtained from
UV-vis absorption spectra (listed in Table 1). The energy levels of
the HOMO of JF-2, JF-3 and JF-4 at ꢂ5.50, ꢂ5.45 and ꢂ5.43 eV
are more positive than the iodide electron donor. This provides
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spectra from 420 to 700 nm are not different. However, the
intensity of the calculated absorption spectra between 340 and
400 nm for JF-3 is obviously higher than that of JF-4. Overall,
the experimental and predicted electronic spectra are in good
agreement. Both the calculated results and experimental
absorption spectra are consistent with the higher molar extinc-
tion coefficient of band II in JF-3 than that in JF-4.
2.5. Photovoltaic performance
As shown in Fig. 8, the short-circuit photocurrent density (Jsc),
open-circuit voltage (Voc), and fill factor (ff) of the JF-3-sensi-
tized solar cell under AM 1.5 sunlight (100 mW cmꢂ2) are 17.1
mA cmꢂ2, 0.74 V and 0.72, respectively, yielding an overall
power-conversion efficiency (h) of 9.1% which is higher than that
of JF-2 (8.3%)18 and JF-4 (7.9%). The photophysical data for the
ruthenium dyes in solution showed that the 3 values for band II
(Fig. 3 and Table 1) are increased in the order JF-4 < JF-2 < JF-
3. Moreover, the UV-vis absorption spectra of the dyes on the
TiO2 film (Fig. 4) showed that the 3 values are in the order JF-4
z JF-2 < JF-3, which basically follow the same trend as that in
the solution state. These results are consistent with the findings
relative to photovoltaic performance, which is increased in the
order of JF-4 < JF-2 < JF-3. The incident photon-to-current
conversion efficiency (IPCE) of the JF-3-sensitized solar cell
exceeds 80% in the spectral range 410–610 nm, reaching
Fig. 5 Square-wave voltammograms of ruthenium dyes, JF-3 and JF-4.
an analogous plane with a slight bending, however, the N,N-
diphenylamino group in the structure of JF-4 is twisted and non-
planar (Fig. 6). Therefore, the phenothiazine group is more
highly conjugated than the N,N-diphenylamino group.
The calculated absorption spectra of JF-3 and JF-4 obtained
by computing their optimized structures from time-dependent
density functional theory (TDDFT) calculations are shown in
Fig. 7. For both dyes, JF-3 and JF-4, the calculated absorption
Table 1 Optical, electrochemical data of JF-3 and JF-4
3/ꢀ104 Mꢂ1 cmꢂ1
Eox of
RuIII/II (V vs. Fc/Fc+)b
Dye
p–p*
p–p* or 4d–p*
4d–p*
EHOMOc/eV
ELUMOc/eV
JF-2
JF-3
JF-4
4.76 (298)a
3.59 (298)
4.95 (296)
2.17 (358)
3.02 (342)
1.56 (373)
0.99 (519)
1.05 (520)
1.02 (520)
0.38
0.35
0.33
ꢂ5.50
ꢂ5.45
ꢂ5.43
ꢂ3.62
ꢂ3.65
ꢂ3.60
a
b
Absorption maxima, lmax (nm). The Ag/AgNO3 reference electrode was calibrated with a ferrocene/ferrocinium (Fc/Fc+) redox couple. The
c
electrochemical experiments were carried out in 0.1 M tetrabutylammonium tetrafluoroborate/DMF solution. The values of EHOMO and ELUMO
were calculated with the following formula: HOMO (eV) ¼ Eox ꢂ EFc/Fc+ + 5.1; LUMO (eV) ¼ HOMO ꢂ Eg. Eg is the absorption onset estimated
from the UV-vis absorption spectra of the sensitizers.
Fig. 6 Optimized molecular structures and the frontier orbitals of JF-3 and JF-4 along with isodensity plots for the HOMO and LUMO orbitals.
Atoms in yellow, gray, red, cyan and blue color correspond to sulfur, carbon, oxygen, ruthenium and nitrogen, respectively.
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Fig. 9 Incident photon-to-current conversion efficiency spectra of JF-3-
and JF-4-sensitized solar cells under AM 1.5 simulated sunlight (100 mW
cmꢂ2) illumination (thickness of TiO2: 12 mm; cell active area: 0.16 cm2).
Fig. 7 Calculated absorption spectra of ruthenium dyes, JF-3 and JF-4,
in DMF.
Table 2 Photovoltaic performance and parameters for JF-3 and JF-4
Dye
Jsc/mA cmꢂ2
Voc/V
ff
ha (%)
s/ms
Ln/mm
JF-3
JF-4
17.1 (16.0)b
16.1 (15.1)
0.74
0.71
0.72
0.69
9.1
7.9
6.4
4.3
29.2
31.6
a
The cell performance data of JF-3 and JF-4 are the average of four
measurements, and the power-conversion efficiency of N3-sensitized
solar cell (where N3 is [Ru(dcbpy)2(NCS)2]) measured by the same
b
device fabrication process is 8.8%. The values in parentheses were
calculated by integration of the IPCE with the AM 1.5G solar spectrum.
the impedance Z0 when sweeping the frequency). The radius of
the middle semicircle in the Nyquist plot for JF-3 is smallest
amongst the three dyes (Fig. 10). This indicates that the overall
resistance at the TiO2/electrolyte interface and the TiO2
network of the JF-3-sensitized solar cell is smaller and follows
the following trend: JF-4 < JF-2 < JF-3. We were also able to
clarify two important factors from EIS data, namely, electron
lifetime (s) and effective diffusion length (Ln), both of which
affect DSC performance.39–43 The electron lifetime was calcu-
lated by s ¼ 1/(2pumin), where umin is the angular frequency at
the midfrequency peak in the EIS Bode plot, see Fig. 11, and
the s value expresses the electron recombination in TiO2 films.
The effective diffusion length was calculated by Ln ¼ L(Rct/
Rt)1/2, where L is the thickness of TiO2 films, Rct and Rt are the
charge transfer resistance at the dye/TiO2/electrolyte interface
and are related to electron recombination and electron trans-
port resistance in the TiO2 film, respectively, and Ln describes
the competition between charge collection and recombination.
The values of Ln for JF-3 and JF-4 are 29.2 and 31.6 mm,
respectively. The effective diffusion lengths for both dyes in the
cells are larger than the thickness of the TiO2 films, indicating
that all photogenerated electrons will be collected efficiently.
Moreover, the values of s for the JF-2-, JF-3- and JF-4-
sensitized solar cells are 6.4, 6.4 and 4.3 ms, respectively,
indicating that the electron transport efficiency follows the
order JF-3 z JF-2 > JF-4. Based on the above analysis of the
Nyquist plots and Bode plots, we can conclude that the EIS
Fig. 8 Current density–voltage characteristics of the JF-3- and JF-4-
sensitized solar cells under AM 1.5 simulated sunlight (100 mW cmꢂ2
illumination (thickness of TiO2: 12 mm; cell active area: 0.16 cm2).
)
a maximum of 88% at 521 nm (Fig. 9). The calculated Jsc from
the overlap integral of the IPCE spectrum of the JF-3-sensitized
solar cell is 16.0 mA cmꢂ2, and the mismatch factor34 in our
device fabrication for the calculated and measured photocurrent
density is less than 1.07. Data related to the photovoltaic
performance for JF-3- and JF-4-sensitized solar cells are
summarized in Table 2.
2.6. Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) provides useful
information for the realization and characterization of the
photovoltaic parameters in DSCs. EIS was mainly applied to
DSCs to study the resistance of the charge transfer at the
counter-electrode, the resistance at the dye-adsorbed TiO2/
electrolyte interface together with electron transport in the
TiO2 network, and the resistance of the triiodide to diffusion in
the electrolyte.35–38 Fig. 10 shows EIS data for JF-3- and JF-4-
sensitized solar cells in the form of a Nyquist plot (i.e., minus
the imaginary part of the impedance ꢂZ00 vs. the real part of
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4. Experimental
Materials and measurements
All reactants and solvents were purchased from commercial
sources and were used as received. 1H NMR spectra were
recorded on a Bruker AMX400 or AV400 spectrometer with
tetramethylsilane as the internal standard. Elemental analyses
were determined on a Perkin-Elmer 2400 CHN analyzer. Mass
spectrometry was performed with a JMS-700 double focusing
mass spectrometer (JEOL, Tokyo, Japan). UV-vis absorption
spectra were recorded with a UV-vis spectrophotometer (Hewlett
Packard 8453). The square-wave voltammetry was performed on
a CH Instruments electrochemical analyzer in DMF solutions
(10ꢂ3 M) with a platinum plate as the working electrode, a plat-
inum wire auxiliary electrode, and a nonaqueous Ag/AgNO3
reference electrode. The supporting electrolyte was tetrabuty-
lammonium tetrafluoroborate (0.1 M) and ferrocene was selected
as the internal standard. The solutions were bubbled with N2 for
10 min before measurements. The scan rate for cyclic voltam-
metry (CV) was 100 mV sꢂ1. The square-wave voltammograms
were swept with a potential step increment of 10 mV and
a frequency of 25 Hz. Electrochemical impedance spectra (EIS)
of the DSCs were obtained using a potentiostat/galvanostat
equipped with FRA2 modules under a constant light illumina-
tion of 100 mW cmꢂ2 and the frequency range used was from 10
mHz to 65 kHz. The applied bias voltage and ac amplitude were
set at the open-circuit voltage (Voc) of the DSCs and 10 mV,
respectively.
Fig. 10 Nyquist plots of electrochemical impedance spectra of JF-2-,
JF-3- and JF-4-sensitized solar cells.
3-(10H-Phenothiazin-10-yl)thiophene
To a two-necked flask containing 3-bromothiophene (1092.4 mg,
6.7 mmol), phenothiazine (1335.1 mg, 6.7 mmol), Pd(dba)2 (80.5
mg, 0.14 mmol) and sodium tert-butoxide (1105.3 mg, 11.5
mmol) were added a solution of tri-tert-butylphosphine in
toluene (0.5 M, 0.25 mL) and 35 mL of anhydrous toluene under
an argon atmosphere. The mixture was refluxed for 90 min. The
hot reaction mixture was filtered to remove insoluble solids. The
filtrate was concentrated under reduced pressure to give a yellow
powder. The crude product was purified by chromatography
using CH2Cl2/hexane (2 : 8) as the eluent to afford 1248.1 mg
(0.78 mmol, yield 66%) of 3-(10H-phenothiazin-10-yl)thiophene
as pure white solid. 1H NMR (400 MHz, DMSO-d6, d): 7.88 (d, J
¼ 8.0 Hz, 1H), 7.75 (s, 1H), 7.21 (d, J ¼ 8.0 Hz, 1H), 7.09 (d, J ¼
7.2 Hz, 2H), 6.99 (d, J ¼ 6.8 Hz, 2H), 6.88 (d, J ¼ 6.8 Hz, 2H),
6.32 (d, J ¼ 7.2 Hz, 2H); EIMS (m/z): 281.1 [M+].
Fig. 11 Bode plots of electrochemical impedance spectra of JF-2-, JF-3-
and JF-4-sensitized solar cells.
data are consistent with the trend for the photovoltaic
performance.
3. Conclusions
In conclusion, we report on the design and synthesis of two new
ruthenium dyes, JF-3 and JF-4, containing ancillary ligands with
electron-donating groups. The dye JF-3, which contains an
electron-donating phenothiazine group, exhibits superior device
performance. A comparison of the phenothiazine and N,N-
diphenylamino groups utilized in molecular engineering for
ruthenium dyes demonstrates that the phenothiazine group can
efficiently broaden band II and increase its molar extinction
coefficient in the UV-vis absorption spectrum, reduce device
resistances, increase electron lifetime, and therefore enhance
power-conversion efficiency. This finding of up and down
adjustment provides an alternative route for improving the light-
harvesting capability of ruthenium dyes as well as DSC perfor-
mance via a multifunctionalized design of the ancillary ligand.
3-(N,N-Diphenylamino)thiophene
The compound 3-(N,N-diphenylamino)thiophene was synthe-
sized following a similar procedure to that described for the
3-(10H-phenothiazin-10-yl)thiophene, except that N,N-diphe-
nylamine was used in place of phenothiazine. The crude product
was purified by chromatography using CH2Cl2/hexane (2 : 8) as
the eluent to afford 3-(N,N-diphenylamino)thiophene as a pure
white solid (yield 70%). 1H NMR (400 MHz, DMSO-d6, d): 7.51
(d, J ¼ 8.0 Hz, 1H), 7.27 (t, J ¼ 7.6 Hz, 4H), 6.99 (m, 6H), 6.84 (d,
J ¼ 5.6 Hz, 2H); EIMS (m/z): 251.2 [M+].
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2-Bromo-3-(10H-phenothiazin-10-yl)thiophene
2-bromo-3-(10H-phenothiazin-10-yl)thiophene.
The
crude
product was purified by chromatography using hexane as the
NBS (979.3 mg, 5.5 mmol) in 20 mL of DMF was added drop-
wise under darkness to a solution of 3-(10H-phenothiazin-10-yl)
thiophene (1407.9 mg, 5.0 mmol) in DMF at 0 ꢁC. The reaction
mixture was stirred at room temperature for another 10 h before
being poured into water. The organic layer was separated, and
the aqueous layer was extracted with methylene chloride. The
organic layers were collected, dried over anhydrous MgSO4 and
the removal of the solvent gave the crude product. The crude
product was purified by chromatography using CH2Cl2/hexane
(2 : 8) as the eluent to afford 1447.5 mg (4.02 mmol, yield 80%) of
2-bromo-3-(10H-phenothiazin-10-yl)thiophene as pure white
eluent to afford 3-(N,N-diphenylamino)-2-octylthiophene as
1
colorless liquid (yield 62%). H NMR (400 MHz, CD2Cl2, d):
7.25 (t, J ¼ 8.0 Hz, 4H), 7.18 (d, J ¼ 5.6 Hz, 1H), 7.03 (d, J ¼ 8.0
Hz, 4H), 6.96 (t, J ¼ 7.2 Hz, 2H), 6.82 (d, J ¼ 5.6 Hz, 1H), 2.49 (t,
J ¼ 8.0 Hz, 2H), 1.89 (m, 2H), 1.34 (m, 10H), 0.95 (t, J ¼ 6.4 Hz,
3H); EIMS (m/z): 363.3 [M+].
4-(10H-Phenothiazin-10-yl)-5-octylthiophene-2-carbaldehyde
3-(10H-Phenothiazin-10-yl)-2-octylthiophene (1966.5 mg, 5.0
mmol) was dissolved in 30 mL THF in a well-dried flask under
the protection of a N2 flow. The solution was cooled in a liquid
nitrogen/acetone cooling bath, and n-butyllithium (3.5 mL, 5.6
mmol, 1.6 M in hexane) was then added dropwise. The cooling
bath was removed, and the solution was allowed to warm to
room temperature and N-formylpiperidine (572.3 mg, 5.1 mmol)
was then added in one portion. After 6 h, the solution was poured
into 200 mL of cool water. The organic layer was separated, and
the aqueous layer was extracted with ether. The organic layers
were collected, dried over anhydrous MgSO4 and the removal of
the solvent gave the crude product. The crude product was
purified by chromatography using CH2Cl2/hexane (1 : 1) as the
eluent to afford 1432.5 mg (3.4 mmol, yield 68%) of 4-(10H-
phenothiazin-10-yl)-5-octylthiophene-2-carbaldehyde as light
1
solid. H NMR (400 MHz, DMSO-d6, d): 7.98 (d, J ¼ 5.2 Hz,
1H), 7.28 (d, J ¼ 5.2 Hz, 1H), 7.07 (d, J ¼ 8.4 Hz, 2H), 6.99 (t,
J ¼ 7.6 Hz, 2H), 6.88 (t, J ¼ 7.6 Hz, 2H), 6.17 (d, J ¼ 8.4 Hz, 2H);
EIMS (m/z): 359.0 [M+].
2-Bromo-3-(N,N-diphenylamino)thiophene
The compound 2-bromo-3-(N,N-diphenylamino)thiophene was
synthesized following a similar procedure to that described for 2-
bromo-3-(10H-phenothiazin-10-yl)thiophene, except that 3-(N,
N-diphenylamino)thiophene was used in place of 3-(10H-phe-
nothiazin-10-yl)thiophene. The crude product was purified by
chromatography using CH2Cl2/hexane (2 : 8) as the eluent to
afford 2-bromo-3-(N,N-diphenylamino)thiophene as pure white
solid (yield 87%). 1H NMR (400 MHz, DMSO-d6, d): 7.70 (d, J ¼
5.6 Hz, 1H), 7.28 (t, J ¼ 7.6 Hz, 4H), 6.99 (t, J ¼ 7.2 Hz, 2H), 6.90
(m, 5H); EIMS (m/z): 329.1 [M+].
1
yellow liquid. H NMR (400 MHz, DMSO-d6, d): 9.96 (s, 1H),
8.31 (d, J ¼ 2.8 Hz, 1H), 7.12 (d, J ¼ 7.6 Hz, 2H), 7.01 (t, J ¼ 7.6
Hz, 2H), 6.91 (t, J ¼ 7.2 Hz, 2H), 6.31 (d, J ¼ 8.0 Hz, 2H), 2.75 (t,
J ¼ 7.6 Hz, 2H), 1.52 (m, 2H), 1.33 (m, 10H), 0.98 (t, J ¼ 8.0 Hz,
3H); EIMS (m/z): 421.3 [M+].
3-(10H-Phenothiazin-10-yl)-2-octylthiophene
4-(N,N-Diphenylamino)-5-octylthiophene-2-carbaldehyde
2-Bromo-3-(10H-phenothiazin-10-yl)thiophene (1081.2 mg, 3.0
mmol) was dissolved in 30 mL THF in a well-dried flask under
the protection of a N2 flow. The solution was cooled in a liquid
nitrogen/acetone cooling bath, and n-butyllithium (2.1 mL, 3.36
mmol, 1.6 M in hexane) was then added dropwise. The cooling
bath was removed, and the solution was allowed to warm to
room temperature and 1-bromooctane (579.1 mg, 3.0 mmol) was
then added in one portion. After 6 h, the solution was poured
into 200 mL of cool water. The organic layer was separated, and
the aqueous layer was extracted with ether. The organic layers
were collected, dried over anhydrous MgSO4 and removal of the
solvent gave the crude product. The crude product was purified
by chromatography using hexane as the eluent to afford 787.6
mg (2.0 mmol, yield 67%) of 3-(10H-phenothiazin-10-yl)-2-
octylthiophene as a colorless liquid. 1H NMR (400 MHz,
DMSO-d6, d): 7.66 (d, J ¼ 5.2 Hz, 1H), 7.16 (d, J ¼ 5.2 Hz, 1H),
7.04 (d, J ¼ 7.2 Hz, 2H), 6.93 (d, J ¼ 7.2 Hz, 2H), 6.83 (t, J ¼ 7.2
Hz, 2H), 6.22 (d, J ¼ 8.0 Hz, 2H), 2.82 (t, J ¼ 8.2 Hz, 2H), 1.68
(m, 2H), 1.34 (m, 10H), 0.95 (t, J ¼ 7.6 Hz, 3H); EIMS (m/z):
393.3 [M+].
The compound 4-(N,N-diphenylamino)-5-octylthiophene-2-car-
baldehyde was synthesized following a similar procedure to that
described for 4-(10H-phenothiazin-10-yl)-5-octylthiophene-2-
carbaldehyde, except that 3-(N,N-diphenylamino)-2-octylth-
iophene was used in place of 3-(10H-phenothiazin-10-yl)-2-
octylthiophene. The crude product was purified by chromato-
graphy using CH2Cl2/hexane (1 : 1) as the eluent to afford 4-(N,
N-diphenylamino)-5-octylthiophene-2-carbaldehyde as light
yellow liquid (yield 64%). 1H NMR (400 MHz, CD2Cl2, d): 9.74
(s, 1H), 7.55 (s, 1H), 7.25 (t, J ¼ 6.4 Hz, 4H), 7.01 (m, 6H), 2.52
(t, J ¼ 7.6 Hz, 2H), 1.52 (m, 2H), 1.25 (m, 10H), 0.88 (t, J ¼ 6.4
Hz, 3H); EIMS (m/z): 391.3 [M+].
2-(4-(10H-Phenothiazin-10-yl)-5-octylthiophen-2-yl)-1H-
imidazo[4,5-f][1,10]phenanthroline (potip)
A mixture of 1,10-phenanthroline-5,6-dione (210.5 mg, 1.0
mmol), 4-(10H-phenothiazin-10-yl)-5-octylthiophene-2-carbal-
dehyde (422.3 mg, 1.0 mmol), ammonium acetate (1550.0 mg,
20.1 mmol) and glacial acetic acid (30 mL) was refluxed for 2 h.
After the reaction, the mixture was poured into 200 mL of cool
water and the resulting precipitate was isolated by filtration. The
crude products were washed with water and purified by chro-
matography using CH2Cl2/hexane/MeOH (5 : 5 : 1) as the eluent
to afford potip (355.3 mg, 0.581 mmol, 58%) as yellow solid. 1H
NMR (400 MHz, DMSO-d6, d): 9.02 (d, J ¼ 4.0 Hz, 2H), 8.80
3-(N,N-Diphenylamino)-2-octylthiophene
The compound 3-(N,N-diphenylamino)-2-octylthiophene was
synthesized following a similar procedure to that described for
3-(10H-phenothiazin-10-yl)-2-octylthiophene, except that 2-
bromo-3-(N,N-diphenylamino)thiophene was used in place of
136 | J. Mater. Chem., 2012, 22, 130–139
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(d, J ¼ 7.6 Hz, 2H), 7.95 (s, 1H), 7.81 (s, 2H), 7.11 (d, J ¼ 8.0 Hz,
2H), 7.04 (d, J ¼ 6.4 Hz, 2H), 6.91 (t, J ¼ 7.6 Hz, 2H), 6.47 (d,
J ¼ 8.0 Hz, 2H), 2.76 (t, J ¼ 7.2 Hz, 2H), 1.56 (m, 2H), 1.17 (m,
10H), 0.85 (m, 3H); FABMS (m/z): 612.2 [M + H]+.
Molecular modeling
The geometrical properties of the dyes, JF-3 and JF-4, were
studied with density functional theory (DFT) and time-depen-
dent density functional theory (TDDFT) calculations using the
Gaussian 03 (G03) program package,44 employing the DFT
method with Becke’s three-parameter hybrid function45 and Lee–
Yang–Parr’s gradient corrected correlation function46 (B3LYP).
The LanL2DZ effective core potential47 was used for the ruthe-
nium atom and the split-valence 6-31G** basis set48 was applied
for hydrogen, sulfur, carbon, oxygen and nitrogen atoms. The
ground-state geometries of the dye molecules were optimized in
the gas phase. Molecular orbitals were visualized using ‘Gauss
View 3.09’. TDDFT calculations for JF-3 and JF-4 were per-
formed using the conductor-like polarizable continuum model
method (C-PCM)49–51 with dimethylformamide (DMF) as the
solvent.52 The empirical solvent data, molecular radius and
2-(4-(N,N-Diphenylamino)-5-octylthiophene-2-yl)-1H-imidazo
[4,5-f][1,10]phenanthroline (dpotip)
The compound dpotip was synthesized following a similar
procedure to that described for potip, except that 4-(N,N-diphe-
nylamino)-5-octylthiophene-2-carbaldehyde was used in place
of 4-(10H-phenothiazin-10-yl)-5-octylthiophene-2-carbaldehyde.
The crude products were washed with water and purified by
chromatography using CH2Cl2/hexane/MeOH (5 : 5 : 1) as the
eluent to afford dpotip (yield 57%) as brown solid. 1H NMR (400
MHz, DMSO-d6, d): 8.99 (d, J ¼ 2.4 Hz, 2H), 8.78 (d, J ¼ 8.4 Hz,
2H), 7.78 (dd, J ¼ 6.0, 8.4 Hz, 2H), 7.65 (s, 1H), 7.30 (d, J ¼ 5.8 Hz,
4H), 6.99 (m, 6H), 2.78 (t, J ¼ 7.6 Hz, 2H), 1.61 (m, 2H), 1.24 (m,
10H), 0.85 (t, J ¼ 6.4 Hz, 3H); FABMS (m/z): 582.3 [M + H]+.
ꢀ
dielectric constant, employed in C-PCM are 2.647 A and 36.71,
respectively.
Preparation of TiO2 electrode
[Ru(dcbpy)(potip)(NCS)2] (JF-3)
Preparation of the TiO2 precursor and electrode fabrication were
carried out based on previous reports.53 The TiO2 film, serving as
the photoanode, was prepared by the general sol–gel method.
The precursor solution was prepared according to the following
procedure: to 430 mL of a vigorously stirred 0.1 M nitric acid
solution, 72 mL of Ti(C3H7O)4 was slowly added dropwise to
form a mixture. After hydrolysis, the mixture was heated at 85 ꢁC
in a water bath and stirred vigorously for 8 h in order to achieve
peptization. When the mixture was cooled to room temperature,
the resulting colloid was ꢁfiltered, and then heated in an autoclave
at a temperature of 240 C for 12 h to permit the growth of the
TiO2 particles. After cooling the colloid to room temperature, it
was ultrasonically vibrated for 10 min. The TiO2 colloid was
concentrated to 13 wt%, and 30 wt% (with respect to TiO2
weight) of poly(ethylene glycol) (PEG, Mw ¼ 20 000 and
200 000) was added to prevent the film from cracking during
drying. To fabricate the TiO2 electrode, TTIP (titanium(IV) iso-
propoxide) was vigorously mixed with ME (2-methoxylethanol)
(in the weight ratio of 1 : 3) to form a metallorganic solution. The
metallorganic solution was then spin-coated onto clean con-
ducting fluorine-doped tin oxide (FTO) glasses with a sheet
resistivity of 13 U per square, followed by annealing at 500 ꢁC for
30 min to form a compact, thin TiO2 layer. A TiO2 paste was
applied thrice to the top of this compact film using a glass rod to
produce the appropriate thickness. For the first coating (paste 1),
the TiO2 colloid mixed with PEG having a molecular weight of
200 000 was used. The second coating used a TiO2 paste (paste 2)
containing a TiO2 colloid and PEG with a molecular weight of
20 000. Paste 2 mixed with the light scattering particles of TiO2
(300 nm, 30 wt% in total TiO2) was used for the third (final)
coating to reduce light loss by back scattering.
[RuCl2(p-cymene)]2 (306.4 mg, 0.5 mmol) and potip (582.5 mg,
1.0 mmol) were added to dry DMF (20 mL). The reaction
ꢁ
mixture was heated at 80 C under N2 for 4 h and then dcbpy
(4,40-dicarboxylic acid-2,20-bipyridine; 244.3 mg, 1.0 mmol) was
added. The reaction mixture was refluxed at 160 ꢁC for another 4
h in the dark. Excess NH4NCS was added to the reaction mixture
and heated at 130 ꢁC for 5 h. After the reaction, the solvent was
removed by a rotary evaporator. The product was collected and
washed with water and diethyl ether. The crude product was
dissolved in methanol and then passed through a column using
methanol as the eluent. The main band was collected and
concentrated, 375.3 mg (0.35 mmol, 35%) of black solid was
obtained. 1H NMR (400 MHz, DMSO-d6, d): 9.52 (m, 2H), 9.05
(m, 2H), 8.87 (s, 1H), 8.74 (s, 1H), 8.32 (m, 2H), 8.03 (s, 1H), 7.85
(d, J ¼ 4.8 Hz, 1H), 7.68 (d, J ¼ 5.6 Hz, 1H), 7.62 (d, J ¼ 6.4 Hz,
1H), 7.47 (d, J ¼ 5.6 Hz, 1H), 7.13 (d, J ¼ 8.0 Hz, 2H), 7.04 (t, J
¼ 7.6 Hz, 2H), 6.92 (t, J ¼ 7.6 Hz, 2H), 6.47 (d, J ¼ 8.0 Hz, 2H),
2.75 (t, J ¼ 6.8 Hz, 2H), 1.59 (m, 2H), 1.23 (m, 10H), 0.86 (t, J ¼
6.8 Hz, 3H); FABMS (m/z): 1073.2 [M+]. Anal. Calcd for
C51H41N9O4RuS4: C 57.07, H 3.85, N 11.75, S 11.95; found: C
56.67, H 3.74, N 11.49, S 11.68%.
[Ru(dcbpy)(dpotip)(NCS)2] (JF-4)
The photosensitizer JF-4 was synthesized following a similar
procedure to that described for JF-3, except that dpotip was used
in place of potip. The crude product was dissolved in methanol
and then passed through a column using methanol as the eluent.
The main band was collected and concentrated, and a black solid
1
was obtained as the product (yield 32%). H NMR (400 MHz,
DMSO-d6, d): 9.50 (m, 2H), 9.12 (s, 1H), 9.02 (s, 1H), 8.93 (s,
1H), 8.66 (s, 1H), 8.32 (d, J ¼ 4.8 Hz, 2H), 7.81 (d, J ¼ 4.0 Hz,
1H), 7.71 (d, J ¼ 4.8 Hz, 1H), 7.65 (d, J ¼ 4.8 Hz, 1H), 7.56 (d,
J ¼ 5.2 Hz, 1H), 7.44 (d, J ¼ 5.2 Hz, 1H), 7.27 (m, 4H), 6.99 (m,
6H), 2.77 (t, J ¼ 6.8 Hz, 2H), 1.59 (m, 2H), 1.28 (m, 10H), 0.85 (t,
J ¼ 6.8 Hz, 3H); FABMS (m/z): 1043.2 [M+]. Anal. Calcd for
C51H43N9O4RuS3: C 58.72, H 4.15, N 12.08, S 9.22; found: C
58.39, H 4.46, N 11.79, S 9.13%.
Device fabrication of dye-sensitized solar cells
The TiO2 film electrode with a 0.4 ꢀ 0.4 cm2 geometric area was
immersed overnight in acetonitrile/tert-butanol mixtures
(volume ratio 1 : 1) containing 2 ꢀ 10ꢂ4 M dye sensitizers. A
platinized FTO was used as a counter-electrode and had an
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J. Mater. Chem., 2012, 22, 130–139 | 137

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active area of 0.16 cm2, produced by adhering a polyester tape
with a thickness of 60 mm. The dye-sensitized photoanode was
rinsed with acetonitrile and air-dried. After filling the spacer with
electrolyte, the photoanode was placed on top of the counter-
electrode and was then tightly clipped to form a cell. The elec-
trolyte was composed of 0.6 M butylmethylimidazolim (BMII),
0.1 M LiI, 0.5 M 4-tert-butylpyridine, 0.03 M I2, and 0.5 M
guanidinium thiocyanate (GuSCN) dissolved in acetonitrile. The
photovoltaic characterizations of the solar cells with a mask (0.5
ꢀ 0.5 cm2) were carried out using a 150 W Peccell solar simulator
(PEC-L11). Light intensity attenuated by a neutral density filter
(Optosigma, 078–0360) at the measuring (cell) position was
calibrated to be 100 mW cmꢂ2 according to the reading from
a radiant power meter (Oriel, 70260) connected by means of
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