Synthesis of novel ruthenium dyes with thiophene or thienothiophene substituted terpyridyl ligands and their characterization

Article abstract of DOI:10.1080/15421406.2013.808147

We synthesized new materials for dye-sensitized solar cell (DSSC) which is incorporating highly conjugated π-electron-donating groups, such as thiophene or thieno[3,2-b]thiophene. The synthesized dyes showed similar molar extinction coefficient compared t

Full text of DOI:10.1080/15421406.2013.808147

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Synthesis of Novel Ruthenium Dyes
with Thiophene or Thienothiophene
Substituted Terpyridyl Ligands and Their
Characterization
Jaeun Ahn a , Ki Chang Lee b , Dajung Kim c , Chongchan Lee d ,
Sunae Lee c , Dae Won Cho c , Sukhun Kyung d & Chan Im a b c
a Department of Future Energy , Konkuk University , Gwangjin-gu ,
Seoul , Korea
b Department of Chemistry , Konkuk University , Gwangjin-gu ,
Seoul , Korea
c Konkuk University-Fraunhofer ISE Next Generation Solar Cell
Research Center , Gwangjin-gu , Seoul , Korea
d Dongjin Semichem Co., LTD., Yanggam-myun , Hwasung-si ,
Gyeonggi-do , Korea
Published online: 13 Sep 2013.
To cite this article: Jaeun Ahn , Ki Chang Lee , Dajung Kim , Chongchan Lee , Sunae Lee , Dae
Won Cho , Sukhun Kyung & Chan Im (2013) Synthesis of Novel Ruthenium Dyes with Thiophene or
Thienothiophene Substituted Terpyridyl Ligands and Their Characterization, Molecular Crystals and
Liquid Crystals, 581:1, 45-51
To link to this article: http://dx.doi.org/10.1080/15421406.2013.808147
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Mol. Cryst. Liq. Cryst., Vol. 581: pp. 45–51, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.808147
Synthesis of Novel Ruthenium Dyes with Thiophene
or Thienothiophene Substituted Terpyridyl Ligands
and Their Characterization
JAEUN AHN,1 KI CHANG LEE,2 DAJUNG KIM,3
CHONGCHAN LEE,4 SUNAE LEE,3 DAE WON CHO,3
SUKHUN KYUNG,4 AND CHAN IM1,2,3,∗
1Department of Future Energy, Konkuk University, Gwangjin-gu, Seoul, Korea
2Department of Chemistry, Konkuk University, Gwangjin-gu, Seoul, Korea
3Konkuk University-Fraunhofer ISE Next Generation Solar Cell Research
Center, Gwangjin-gu, Seoul, Korea
4Dongjin Semichem Co., LTD., Yanggam-myun, Hwasung-si, Gyeonggi-do,
Korea
We synthesized new materials for dye-sensitized solar cell (DSSC) which is incorpo-
rating highly conjugated π-electron-donating groups, such as thiophene or thieno[3,2-
b]thiophene. The synthesized dyes showed similar molar extinction coefficient compared
1
to that of N749. All compounds are analyzed by H- and 13C-NMR, and FT-IR spec-
troscopic measurements. The physical properties of dyes are characterized by UV-vis
absorption spectroscopy, cyclic voltamogram and DFT calculation. The power conver-
sion efficiencies (ηeff) of two synthesized dyes were 4.0 and 3.4%, respectively, under
AM1.5G light source. The reference cell (N749) was evaluated the ηeff of 6.0% at the
same conditions.
Keywords DFT calculation; dye-sensitized solar cell; Ru-complex; solar conversion
efficiency; thionothiophene; thiophene
Introduction
Ruthenium complexes have received great deal of attraction in the field of dye-based dye-
sensitized solar cells (DSSCs) because of the photon to current conversion efficiency over
10% [1]. The synthesis of new dyes is very important for increasing the energy conversion
efficiency of the solar cell. This is required that dyes have large absorption coefficient in
long wavelength up to near IR. Therefore, many researchers have tried to synthesis new
Ruthenium dyes. Black dye (N749) absorbs longer wavelength light than other Ruthenium
dyes such as N3 and N719. In this study, we synthesized new ruthenium dyes to expand
the π-conjugation, which have thiophene or thienothiophene moiety based on N749 dye.
The detail synthesis methods as shown in Scheme 1 for new dyes were investigated and
∗Address correspondence to Prof. Chan Im, Konkuk University, Seoul, Korea. E-mail: chanim@
konkuk.ac.kr
[423]/45

46/[424]
J. Ahn et al.
the photovoltaic performances for new dyes were also characterized compare to those of
reference N749 DSSC.
Experimental
1-[2-(4-Methoxycarbonylpyridin-2-yl)-2-oxo-ethyl]–pyridinium-iodide (1), Krohnke Re-
agent [2]: Methyl 2-acetylisonicotinate (0.24 g, 1.34 mmol) was added to a warmed a
solution of I2 (0.35 g, 1.38 mmol, 1.03 eq) in anhydrous pyridine (10 ml). The reaction
mixture was refluxed for 3 hr under Ar condition. After cooling to room temperature, a
precipitate was collected by filtration and wash with chloroform and diethyl ether to give 3
as powder (yield: 34%). 1H NMR (400 MHz, DMSO-d6), δppm: 9.10 (1H, d, J = 4.8 Hz),
9.09 (2H, d, J = 5.6 Hz), 9.00 (1H, t, J = 8 Hz), 8.36 (1 H, s), 8.30 (2H, t, J = 6.8 Hz), 8.28
(1H, dd, J = 1.6 Hz, 4.8 Hz), 6.52 (2H, s), 3.95 (3H, s); 13C NMR (100 MHz, DMSO-d6),
δ ppm: 191.20, 164.70, 152.07, 151.49, 146.92, 146.84, 139.21, 128.26, 127.88, 120.77,
67.71, 53.69.
Methyl-2-(3-Thiophen-2-yl-acryloyl)-isonicotinate (2): Piperidine (0.6 mL, 6.19
mmol) and acetic acid (0.35 mL, 6.19 mmol) were dissolved in methanol (10 mL) and
stirred for 5 min at room temperature. At room temperature, methyl-2-acetylisonicotinate
(1 g, 5.58 mmol) were added to solution and stirred for further 5 min. 2-
Thiophenecarboxaldehyde (0.62 g (0.52 mL), 5.58 mmol) were added to solution and
reflux for 5 h. When yellow precipitate was found on the solution, cool to room tempera-
ture. Precipiate was washed with small amount of methanol and no further purification is
needed. 1H NMR (400 MHz, DMSO-d6), δ ppm: 8.90 (1H, dd, J = 0.8 Hz, 4.8 Hz), 8.69
(1H, dd, J = 0.8 Hz, 1.6 Hz), 8.13 (1H, s), 8.09 (2H, m), 7.47 (2H, dd, J = 5.2 Hz, 10.8 Hz),
7.12 (1H, t, J = 3.6 Hz), 4.0 (3H, s); 13C NMR (100 MHz, CDCl3), δ ppm: 188.28, 165.16,
155.15, 149.75, 140.77, 138.69, 137.74, 132. 52, 129.50, 128.37, 125.88, 122.13, 119.47,
52.93.
Methyl-2-(3-thieno[3,2-b]thiophen-2-yl-acryloyl)-isonicotinate (2ꢀ): Piperidine (4.69
mL, 47.99 mmol, 1.1 eq) and acetic acid (2.73 mL, 47.99 mmol, 1.1eq) were added to a
stirred solution of thieno[3,2-b]thiophene-2-carbaldehyde (7.29 g, 43.33 mmol) and methyl-
2-acetylisonicotinate (8.6 g, 47.99 mmol, 1.1 eq) in methanol (50 mL). The mixture was re-
fluxed for 5 h and turned red with precipitation of orange crystals. After cooling to room tem-
perature, the crystals was filtered and washed with methanol. The crude product was chro-
matographed on silica gel, eluting with hexane: Ethyl acetate (5 : 1) and then it was recrystal-
lized with methanol to give 4 as orange crystals (yield: 58%). 1H NMR (400 MHz, DMSO-
d6), δ ppm: 9.03 (1H, d, J = 5.2 Hz), 8.44 (1H, s), 8.18 (1H, d, J = 16 Hz), 8.13 (1H, d,
J = 5.2 Hz), 8.05 (1H, d, J = 35.6 Hz), 7.93 (1H, d, J = 5.6 Hz), 7.53 (1H, d, J = 5.2 Hz),
3.95 (1H, s); 13C NMR (100 MHz, DMSO-d6), δ ppm: 187.48, 165.07, 154.72, 151.10,
142.27, 141.83, 140.10, 139.14, 138.91, 133.18, 127.61, 126.56, 121.35, 120.94, 118.71,
53.52.
4,4ꢀꢀ-Dimethoxycarbonyl-4ꢀ-thieno[3,2-b]thiophenyl-2,2ꢀ:6ꢀ2ꢀꢀ-terpyridine (3): En-
one reagent (2) (0.188 g, 0.69 mmol) and Krohnke’s reagent (1) (0.327 g, 0.85 mmol)
were added to a solution of excess ammonium acetate (2 g, 26 mmol) in methanol (3 mL)
and the resulting solution was heated to reflux for 4 h. After cooling to room temperature, a
precipitate was collected by filtration and triturated successively with methanol (2 × 2 mL)
and then with chloroform (2 × 2 mL). The chloroform extract was freed of solvent, and
then the crude was washed with methanol and then it was recrystallized with chloroform to
give 5 as a brown powder (yield: 27%). 1H NMR (400 MHz, DMSO-d6), δ ppm: 9.16 (2H,
s), 8.90 (2H, d, J = 4 Hz), 8.75 (2H, s), 8.00 (1H, s), 7.94 (2H, d, J = 4 Hz), 7.49 (1H, d, J

Synthesis of Novel Ruthenium Dyes
[425]/47
= 8 Hz), 7.33 (1H, d, J = 4 Hz); 13C NMR (100 MHz, DMSO-d6), δ ppm: 165.97, 157.14,
155.83, 150.08, 144.28, 143.34, 140.36, 140.28, 139.69, 129.01, 123.24, 120.96, 119.88,
118.43, 117.66, 52.94.
Ruthenium(4,4ꢀꢀ-dimethoxycarbonyl-4ꢀ-thieno[3,2-b]thiophenyl-2,2ꢀ:6ꢀ,2ꢀꢀ-terpyrid-
ine)trichloride (4): Ethyl alcohol (80 mL) and ruthenium trichloride (0.60 g) were
reacted under argon. After the mixture stirred for 40 min, a solution of the ligand
4,4ꢀꢀ-dimethoxycarbonyl-4ꢀ-thieno[3,2-b]thiophenyl-2,2ꢀ:6ꢀ,2ꢀꢀterpyridine(5), 1.10 g, in
80 mL of dichloromethane, was then added. The reaction mixture was refluxed for 3 h.
The solution was concentrated to 10 mL, and the reaction mixture was cooled to room
temperature. The precipitated complex was collected on a sintered glass crucible and was
washed with ethanol to remove unreacted ruthenium trichloride. The product was air-dried
(yield: 75%). 1H NMR (400 MHz, DMSO-d6), δ ppm: 9.16 (2H, s), 8.90 (2H, d, J = 4 Hz),
8.75 (2H, s), 8.00 (1H, s), 7.94 (2H, d, J = 4 Hz), 7.49 (1H, d, J = 8 Hz), 7.33 (1H, d, J =
4 Hz).
Ruthenium (4,4ꢀꢀ-Dimethoxycarbonyl-4ꢀ-thiophenyl-2,2ꢀ:6ꢀ2ꢀꢀ-terpyridine)trithiocy-
anate (5): Ru(DMTT)Cl3 complex (1.47 g, 2.31 mmol) was dissolved in DMF (150 mL)
and [H4N]NCS (5.6 g, 7.39 mmol) was dissolved in H2O (29mL) and both were stirred
for 20 min at room temperature. Two solutions were mixed together in dark under argon
atmosphere by refluxing at 130◦C. Triethylamine (56 mL) and H2O (28 mL) were added,
and solution was refluxed for a further 24 hr to hydrolyze the ester groups on the terpyridine
ligand. The solvent volume was reduced on a rotary evaporator to about 10 mL, and then the
solution was added to H2O (200 mL). The resulting precipitate was filtered and dried. The
isolated solid was recrystallized from methanol - diethyl ether. After which it was further
1
purified on a 3 × 100 cm Sephadex LH-20 column, using methanol as eluent. H NMR
(400 MHz, DMSO-d6), δ ppm: 9.254 (6H, m), 8.353 (1H, s), 8.242 (2H, dd, J = 6 Hz,
13.2 Hz), 7.841 (1H, s), 7.341 (1H, s).
Ruthenium(4,4ꢀꢀ-dimethoxy-4ꢀ-thieno[3,2-b]thiophenyl-2,2ꢀ:6ꢀ,2ꢀꢀterpyridine)-trith-
iocyanate (5ꢀ): The compond (5ꢀ) was synthesized in dark under an argon atmo-
sphere by refluxing at 130◦C, a solution of [NH4]NCS (5.6 g, in 29 mL of H2O) and
Ru(trimethoxycarbonylterpy)Cl3 complex (1.47 g, in 150 mL of DMF) for 4 h [3]. Then,
56 mL of triethylamine and 28 mL of H2O were added, and the solution was refluxed for a
further 24 h to hydrolyze the ester groups on the terpyridine ligand. The solvent volume
was reduced on a rotary evaporator to about 10 mL, and then the solution was added to
70 mL of H2O. The resulting precipitate was filtered and dried. The isolated solid was
recrystallized from methanol-diethyl ether, after which it was further purified on a 100 cm
(φ = 3 cm) Sephadex LH-20 column, using methanol as eluent to give 7 as a brown
powder (yield: 5%). 1H NMR (400 MHz, DMSO-d6), δppm: 9.04 (2H, 4 Hz, d), 8.86 (4H,
m) 8.76 (1H, s), 8.11 (2H, 2 Hz, d), 7.84 (1H, 4 Hz, s), 7.341(1H, 4 Hz, d).
All Ru-compexes are triply purified using Sephadex LH-20 column to separate isomers
and impurity.
The detailed preparation of DSSC has been described earlier [4]. The current–voltage
(J–V) curves were measured using a source-measure unit under white-light irradiation from
a 1000 W Xe lamp (Thermo Oriel Instruments Co.). The incident light intensity and the
active cell area were 100 mW cm−2 and 0.25 cm2, respectively. The J–V curves were
used to calculate the short-circuit current (Jsc), open-circuit voltage (Voc) and fill factor
(FF) of the DSSC. The incident photon to current efficiency (IPCE) was measured as a
function of wavelength from 300 to 900 nm using the IPCE measurement (DEX7, PV
Measurements Co.). The absorption spectra were measured using an UV-vis absorption
spectrometer (Sinco, Neosys-2000).

48/[426]
J. Ahn et al.
Scheme 1. Synthesis route for novel ruthenium dyes.
Results and Discussion
Figure 1 shows the absorption spectra of 5, 5ꢀ and N749, which are normalized at 600 nm.
The absorption bands around 600 nm of dyes studied here is dominated by metal-to-ligand
charge-transfer transitions (3MLCT). The 3MLCT band of 5 and 5ꢀ at around 600 nm were
slightly broader than that of N749. This is attributed to the extent of π-conjugation. At
600 nm, the extinction coefficients (ε600nm) of 5, 5ꢀ were 7600 and 7000 M−1 cm−1, which
are smaller than that (8400 M−1 cm−1) of N749. The absorption bands at around 410 nm can
be assigned to 1MLCT band. The ε410nm of 5, 5ꢀ and N749 are 9800, 10400 and 11400 M−1
cm−1, respectively. The ε values are reduced ∼15% in whole ranges and the absorption
maxima for 3MLCT band of 5 and 5ꢀ at 600 nm observed no shift to longer wavelength.
Figure 2 shows the energy diagram for HOMO (oxidation potential vs. NHE) and
LUMO of 5 and 5ꢀ. The HOMO levels are determined based on the measurement of cyclic
voltammetry. The LUMO energies of dyes are calculated from the oxidation potentials and
the E0–0 determined from the intersection of absorption and emission spectra. The HOMO
and LUMO levels of 5 and 5ꢀ are lied lower compare to that of N749. The LUMO levels
of 5 and 5ꢀ are higher than conduction band of TiO2. Therefore, the electron injection from
dyes to TiO2 can be occurred through the exothermic process.

Synthesis of Novel Ruthenium Dyes
[427]/49
Figure 1. The normalized absorption spectra of 5, 5ꢀ and N749 in ethanol.
Figure 3(a) shows the J–V curves of the 5, 5ꢀ and N749 DSSCs performed under
same conditions (TiO2 thickness, dipping time, concentration of electrolyte, and etc). The
reference N749 DSSC showed a high short-circuit current density (Jsc) compared with
those of 5 and 5ꢀ DSSCs, as listed in Table 1. When compared to that of N749, the power
conversion efficiencies (ηeff) of 5 and 5ꢀ decreased by about 55 and 65%, respectively.
Although the fill factor (FF) value was not much changed the lower efficiency was due to
a drop in the Jsc and the open-circuit voltage (Voc) values. The small Jsc values of 5 and
5ꢀ could be attributed to that the extinction coefficients of 5 and 5ꢀ dyes were about 15%
Figure 2. Schematic diagram for HOMO and LUMO energy levels of 5, 5ꢀ, N719 and N749 dyes.

50/[428]
J. Ahn et al.
Figure 3. (a) Current-density–voltage (J–V) curves of the DSSC prepared with 5, 5ꢀ and N749 dyes.
(b) Incident photon to current conversion efficiency (IPCE) action spectra of DSSC prepared with 5ꢀ
and N749 dyes.
smaller than that of N749. The adsorption ability of 5, and 5ꢀ dyes onto TiO2 electrode may
be low, because of bulky thiophene moieties instead of hydrogen of N749.
As shown in Fig. 4, the electron of HOMO orbital is localized at NCS ligands and metal
ion. On the other hand, the electron of LUMO orbital is delocalized terphyridine ligand
including thiophene moieties. The electron in L+1 orbital is more populated at carboxyl
groups of terphyridine ligand, which is similar with the LUMO orbital of N749. The
L+1 orbital is proper for the electron injection into TiO2, because the electron is mostly
populated at the anchoring carboxyl group. The electron of LUMO orbital extended to
thiophene moieties. This indicates the π-conjugation is increased, but the electron density
is decreases at the anchoring group. We suggest this is also one reason for low efficiency
of 5 and 5ꢀ DSSCs.
Generally, the Voc values have close relation to recombination between photoinjected
electrons and the oxidized redox species in the electrolyte. In order to explain about lower
the Voc of 5 and 5ꢀ DSSCs, we suggest that the high rate of recombination results a
decreasing of the Voc values. Further experimentation, therefore, is necessary to prove our
expectation about the shorter electron lifetime of 5 and 5ꢀ compared to that of N749.
In conclusion, two thiophene-based N749 dyes have been synthesized to investigate
the π-conjugation effect on efficiency of DSSCs. Under same condition, the photovoltaic
performance for new dyes was lower than N749 DSSC. In order to optimize the cell
performance, we are carrying out further studies such as the increasing concentration of the
adsorbed dyes, the control of the size of TiO2 particle and the increasing of dipping time.
Table 1. Photovoltaic performances of DSSCs
Dyes
Voc (V)
Jsc (mA/cm2)
FF (%)
ηeff (%)
5
0.49
0.54
0.59
12.1
14.2
19.7
57.4
51.7
53.0
3.4
4.0
6.2
5ꢀ
N749

Synthesis of Novel Ruthenium Dyes
[429]/51
Figure 4. The orbital diagrams of 5ꢀ calculated by density-functional theory (DFT) at the B3LYP/
6-31G(d) level, and applied LANL2DZ methods for Ru ion (Gaussian 03 program).
Acknowledgment
This work has been supported by a Grant-in Aid for the Seoul R&BD program (WR090671)
from Seoul metropolitan government.
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