Electronic optimization of heteroleptic Ru(II) bipyridine complexes by remote substituents: Synthesis, characterization, and application to dye-sensitized solar cells

Article abstract of DOI:10.1021/ic101909e

We prepared a series of new heteroleptic ruthenium(II) complexes, Ru(NCS)2LL0 (3a-3e), where L is 4,40-di(hydroxycarbonyl)-2,20-bipyridine and L0 is 4,40-di(p-Xphenyl)- 2,20-pyridine (X = CN (a), F (b), H (c), OMe (d), and NMe2 (e)), in an attempt to explore the structure-activity relationships in their photophysical and electrochemical behavior and in their performance in dye-sensitized solar cells (DSSCs). When substituent X is changed from electron-donating NMe2 to electron-withdrawing CN, the absorption and emission maxima reveal systematic bathochromic shifts. The redox potentials of these dyes are also significantly influenced by X. The electronic properties of the dyes were theoretically analyzed using density functional theory calculations; the results show good correlations with the experimental results. The solar-cell performance of DSSCs based on dye-grafted nanocrystalline TiO2 using 3a-3e and standard N3 (bis[(4,40-carboxy-2,20-bipyridine)(thiocyanato)]ruthenium(II)) were compared, revealing substantial dependences on the dye structures, particularly on the remote substituent X. The 3d-based device showed the best performance: η = 8.30%, JSC = 16.0 mA 3 cm^-2, VOC = 717 mV, and ff = 0.72. These values are better than N3-based device. 2011 American Chemical Society. 2011 American Chemical Society.

Full text of DOI:10.1021/ic101909e

ARTICLE
pubs.acs.org/IC
Electronic Optimization of Heteroleptic Ru(II) Bipyridine Complexes
by Remote Substituents: Synthesis, Characterization, and Application
to Dye-Sensitized Solar Cells
Won-Sik Han,† Jung-Kyu Han,† Hyun-Young Kim,† Mi Jin Choi,‡ Yong-Soo Kang,‡ Chyongjin Pac,*,† and
Sang Ook Kang*,†
†Department of Advanced Material Chemistry, Korea University, Sejong, Chungnam 339-700, South Korea
‡WCU Department of Energy Engineering and Center for Next Generation Dye-sensitized Solar Cells, Hanyang University, Seoul
133-791, South Korea
S Supporting Information
b
ABSTRACT: We prepared a series of new heteroleptic
ruthenium(II) complexes, Ru(NCS)2LL0 (3aꢀ3e), where L is
4,40-di(hydroxycarbonyl)-2,20-bipyridine and L0 is 4,40-di(p-X-
phenyl)-2,20-pyridine (X = CN (a), F (b), H (c), OMe (d), and
NMe2 (e)), in an attempt to explore the structureꢀactivity
relationships in their photophysical and electrochemical beha-
vior and in their performance in dye-sensitized solar cells
(DSSCs). When substituent X is changed from electron-donat-
ing NMe2 to electron-withdrawing CN, the absorption and
emission maxima reveal systematic bathochromic shifts. The redox potentials of these dyes are also significantly influenced by X. The
electronic properties of the dyes were theoretically analyzed using density functional theory calculations; the results show good
correlations with the experimental results. The solar-cell performance of DSSCs based on dye-grafted nanocrystalline TiO2 using
3aꢀ3e and standard N3 (bis[(4,40-carboxy-2,20-bipyridine)(thiocyanato)]ruthenium(II)) were compared, revealing substantial
dependences on the dye structures, particularly on the remote substituent X. The 3d-based device showed the best performance:
η = 8.30%, JSC = 16.0 mA cmꢀ2, VOC = 717 mV, and ff = 0.72. These values are better than N3-based device.
3
’ INTRODUCTION
based on heteroleptic complexes Ru(NCS)2LL0. The aim is
mainly to (1) broaden the absorption range and increase the molar
The development of systems for converting solar energy into
chemical fuels or electricity is among the most demanding
challenges that chemists face today. In light harvesting, the first
step in such conversion, the choice of a photosensitizer capable of
efficient visible-light absorption is crucial. Therefore, many
efforts have focused on the design and synthesis of various dyes,
including metal complexes1ꢀ3 and organic dyes.4,5 Among them,
Ru(II) polypyridine complexes1 have been intensively pursued as
a potential candidate for efficient sensitizers because of their
favorable photophysical properties, such as intense charge-trans-
fer (CT) absorption in the visible range and moderately intense
emission with fairly long lifetimes in fluid solutions at ambient
temperature. They also exhibit reversible redox behavior with tunable
potentials.6 Consequently, a variety of Ru(II) complexes have been
extensively applied to photoinduced hydrogen evolution,7 CO2
reduction,8 and dye-sensitized solar cells (DSSCs).9 For the devel-
opment of DSSCs, Ru(NCS)2L2 (L = 4,40-di(hydroxycarbonyl)-
2,20-bipyridine, called N3 dye) is regarded as a key molecule;
it displays intense visible-light absorption and rapid electron injection
from the excited states into the conduction band of TiO2.1e,9e
Thus, one of the basic strategies underlying the molecular
design of efficient sensitizers has been the modification of N3 dye
10
absorption coefficients by extending the conjugation length of L0
and (2) tune the highest occupied molecular orbital/lowest un-
occupied molecular orbital (HOMO/LUMO) levels by introducing
other σ-donor ligands and L0.11 Attempts have been made using
polypyridine ligands (L0) conjugated with a π system such as
thiophene,12 thienothiophene,13 alkoxybenzene moieties,14 or
ethenyl-linked phenyl groups.15 Alternative approaches are based
on the tuning of the light absorption to lower energies by introdu-
cing a ligand with a low-lying π* molecular orbital16 and/or
by destabilization of the metal t2g orbital with a strong donor
ligand.17
In this work, we focused on the structureꢀproperty relation-
ships of Ru(NCS)2LL0 by using 4,40-di(p-X-phenyl)-2,20-bipyr-
idine as L0. Significant π-conjugation of the phenyl rings should
certainly enlarge both the absorption range and the CT cross
section of relevant Ru(II) complexes. Furthermore, the remote
substituent (X) might provide indirect electronic perturbations
to relevant Ru(II) complexes through the phenyl rings, with a
Received: September 17, 2010
Published: March 21, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic101909e Inorg. Chem. 2011, 50, 3271–3280
|

Inorganic Chemistry
ARTICLE
potential for fine-tuning the HOMO/LUMO levels. Therefore,
we have prepared N3-dye-related heteroleptic complexes, Ru-
(NCS)2LL0 (L0 = 4,40-di(p-X-phenyl)-2,20-pyridine), in which
substituent X is strong electron-withdrawing CN (3a), weakly
electron-withdrawing F (3b) and H (3c), moderately electron-
donating OMe (3d), or strong electron-donating NMe2 (3e).
With this series of Ru(II) complexes, we have found that by
varying X, we can fine-tune the lowest metal-to-ligand charge
transfer (MLCT) band position and the electrochemical redox
potentials associated with the HOMO/LUMO levels of the
ruthenium polypyridyl complexes. In this study, we report the
preparation of Ru(NCS)2LL0 complexes, their photophysical
and electrochemical behavior, and their possible application to
DSSCs. A DSSC using Ru(NCS)2LL0 with X = OMe (3d) shows
the best performance among the devices, with a higher power
conversion efficiency than N3 dye under identical conditions of
cell fabrication and measurement.
Recrystallization from water/ethanol produced small off-white crystals
in 90% yield (0.45 g). mp 130 °C; H NMR (300.1 MHz; CDCl3);
δ 8.61 (d, 2H), 8.48 (2H, d), 7.50 (2H, dd); HRMS (FAB) calcd for
C10H6Br2N2: 311.8898. Found: 311.8911 [M]þ; Anal. Calcd for
C10H6Br2N2: C, 38.25; H, 1.93; Found: C, 38.23; H, 1.94.
1
General Procedure for Suzuki Coupling of 4,40-Dibromo-
2,20-bipyridine. An oven-dried Schlenk flask was evacuated and
backfilled with nitrogen. The reaction flask was charged with corre-
sponding boronic acid (10 mmol), 4,40-dibromo-2,20-bipyridine
(4 mmol, 1.26 g), toluene (80 mL), and aqueous sodium carbonate
solution (2 M, 35 mL). After the biphasic mixture was bubbled for
15 min by nitrogen stream, Pd(PPh3)4 (5 mol % based on 4,40-dibromo-
2,20-bipyridine, 0.23 g) was added, and the mixture was refluxed for 14 h.
After the mixture cooled to room temperature, the aqueous layer was
separated and extracted with CH2Cl2 (3 ꢁ 50 mL). The combined
organic layers were washed with brine, dried over anhydrous magnesium
sulfate, filtered, and concentrated in vacuo. The crude material was
purified by flash chromatography on silica gel.
4,40-Bis(p-cyanophenyl)-2,20-bipyridine (2a). The coupling
of 4,40-dibromo-2,20-bipyridine (4 mmol, 1.26 g) with 4-cyanophenyl-
boronic acid (10 mmol, 1.47 g) was effected using the general procedure
with 5 mol % Pd(PPh3)4 (0.23 g). Flash chromatography using a 15:1
dichloromethane/methanol eluent gave the title compound as an off-
’ EXPERIMENTAL SECTION
General Procedures. Unless stated otherwise, all reagents were
purchased from Aldrich and used without further purification. Dry
acetonitrile was distilled over calcium hydride powder under argon.
Toluene was distilled over sodium and benzophenone ketyl before use.
N,N-dimethylformamide (DMF) was distilled from calcium hydride and
stored over 4 Å molecular sieves. 4,40-Dinitro-2,20-bipyridine N,N0-
dioxide was synthesized according to the literature.18 NMR spectra were
recorded on a Varian Mercury 300 spectrometer operating at 300.1 MHz
for 1H and 75.4 MHz for 13C. The chemical shift of the chloroform or
DMSO-d6 signal was used as a reference (δ = 7.26 or 2.49 ppm for 1H
and δ = 77.0 or 39.5 ppm for 13C, respectively). Elemental analyses
(Carlo Erba Instruments CHNS-O EA 1108 analyzer) and high-resolu-
tion tandem mass spectrometry (Jeol LTD JMS-HX 110/110A) were
performed at the Ochang Branches of the Korean Basic Science
Institute. Absorption and photoluminescence spectra were recorded
on a Shimadzu UV-3101PC UVꢀvis-NIR scanning spectrophotometer
and a Varian Cary Eclipse fluorescence spectrophotometer, respectively.
Cyclic voltammetry experiments were conducted on a BAS 100 electro-
chemical analyzer equipped with a three-electrode cell system (a glassy
carbon working electrode, a platinum wire counter-electrode, and a Ag/
AgNO3 reference electrode). Freshly distilled, degassed acetonitrile was
used as the solvent, and 0.1 M tetrabutylammonium tetrafluoroborate
was used as the supporting electrolyte. Potentials were initially refer-
enced to an internal ferrocene (Fc) standard.
1
white powder in 78% yield (1.12 g). H NMR (300.1 MHz; CDCl3);
δ 8.78 (t, 2H), 8.73 (s, 2H), 7.86 (dd, 4H), 7.80 (dd, 4H), 7.55 (dd, 2H);
HRMS (FAB) calcd for C24H14N4: 358.1218. Found: 359.1221 [M þ H]þ;
Anal. Calcd for C24H14N4: C, 80.43; H, 3.94; N, 15.63; Found: C, 80.35;
H, 3.92; N, 15.66.
4,40-Bis(p-fluorophenyl)-2,20-bipyridine (2b). The coupling
of 4,40-dibromo-2,20-bipyridine (4 mmol, 1.26 g) with 4-fluorophenyl-
boronic acid (10 mmol, 1.40 g) was effected using the general procedure
with 5 mol % Pd(PPh3)4 (0.23 g). Flash chromatography using a 15:1
dichloromethane/methanol eluent gave the title compound as an off-
1
white powder in 88% yield (1.21 g). H NMR (300.1 MHz; CDCl3);
δ 8.71 (d, 2H), 8.65 (s, 2H), 7.73 (dd, 4H), 7.49 (dd, 2H), 7.17 (dd,
4H); 13C NMR δ 164.5, 156.7, 149.9, 148.6, 134.5, 129.2, 122.3, 119.3,
116.4; HRMS (FAB) calcd for C22H14F2N2: 344.1125. Found:
345.1132 [M þ H]þ; Anal. Calcd for C22H14F2N2: C, 76.73; H, 4.10;
N, 8.13; Found: C, 76.71; H, 4.09; N, 8.14.
4,40-Diphenyl-2,20-bipyridine (2c). The coupling of 4,40-dibro-
mo-2,20-bipyridine (4 mmol, 1.26 g) with phenylboronic acid (10 mmol,
1.21 g) was effected using the general procedure with 5 mol % Pd
(PPh3)4 (0.23 g). Flash chromatography using a 10:1 dichloromethane/
methanol eluent gave the title compound as an off-white powder in 93%
1
yield (1.15 g). H NMR (300.1 MHz; CDCl3); δ 8.73 (d, 2H), 8.71
(s, 2H), 7.77 (dd, 4H), 7.54 (dd, 2H), 7.49 (dd, 4H), 7.43 (dd, 2H); 13C
NMR δ 156.9, 149.9, 149.6, 138.5, 129.3, 127.4, 121.9, 119.5; HRMS
(FAB) calcd for C22H16N2: 308.1313. Found: 308.1319 [M]þ; Anal.
Calcd for C22H16N2: C, 85.69; H, 5.23; N, 9.08; Found: C, 85.67; H,
5.21; N, 9.09.
Synthesis of 4,40-Dibromo-2,20-bipyridine N,N0-dioxide. A
suspension of 4,40-dinitro-2,20-bipyridine N,N0-dioxide18 (13.6 mmol,
3.8 g) in glacial acetic acid (60 mL) and acetyl bromide (40 mL)
was stirred for 4 h at 100 °C. The resulting yellow-brown solution was
cooled to 0 °C, poured onto ice (125 g), and then neutralized with a
concentrated sodium hydroxide solution. An off-white solid was filtered
off, washed with water, and air-dried to give 4,40-dibromo-2,20-bipyr-
idine N,N0-dioxide in 80% yield (2.8 g). 1H NMR (300.1 MHz; DMSO-
d6 þ CF3COOH); δ 8.29 (d, 2H), 8.03 (d, 2H), 7.78 (dd, 2H); Anal.
Calcd for C10H6Br2N2O2 (345.97): C, 34.72; H, 1.75; N, 8.10; Found:
C, 34.32; H, 1.92; N, 8.20.
4,40-Bis(p-methoxyphenyl)-2,20-bipyridine (2d). The cou-
pling of 4,40-dibromo-2,20-bipyridine (4 mmol, 1.26 g) with 3-methox-
yphenylboronic acid (10 mmol, 1.52 g) was effected using the general
procedure with 5 mol % Pd(PPh3)4 (0.23 g). Flash chromatography
using a 15:1 dichloromethane/methanol eluent gave the title compound
1
as an off-white powder in 91% yield (1.34 g). H NMR (300.1 MHz;
Synthesis of 4,40-Dibromo-2,20-bipyridine. A suspension of
4,40-dibromo-2,20-bipyridine N,N0-dioxide (2.2 mmol, 0.58 g) and PBr3
(46 mmol, 12.5 g) in anhydrous acetonitrile (100 mL) was refluxed for
4 h. The resulting yellow-brown solution was poured into ice water
(200 g). After the solution was adjusted to pH 11 with concentrated
sodium hydroxide solution, a yellow-white suspension formed and was
extracted exhaustively with CHCl3. The combined organic layers were
dried over Na2CO3 and then evaporated under reduced pressure.
CDCl3); δ 8.69 (d, 2H), 8.65 (s, 2H), 7.73 (dd, 4H), 7.50 (dd, 2H), 7.00
(dd, 4H), 3.85 (s, 6H); 13C NMR δ 160.7, 156.8, 149.8, 149.0, 130.7,
128.6, 121.3, 118.8, 114.6, 55.6; HRMS (FAB) calcd for C24H20N2O2:
368.1525. Found: 368.1531 [M]þ; Anal. Calcd for C24H20N2O2: C,
78.24; H, 5.47; N, 7.60; Found: C, 78.20; H, 5.45; N, 7.62.
4,40-Bis(p-dimethylaminophenyl)-2,20-bipyridine (2e). The
coupling of 4,40-dibromo-2,20-bipyridine (4 mmol, 1.26 g) with
4-(dimethylamino)phenylboronic acid (10 mmol, 1.65 g) was effected
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Inorganic Chemistry
ARTICLE
using the general procedure with 5 mol % Pd(PPh3)4 (0.23 g). Flash
chromatography using a 15:1 dichloromethane/methanol eluent gave
the title compound as an off-white powder in 37% yield (0.58 g). H
basis set.19 All geometries were fully optimized in the ground states
(closed-shell singlet So). All calculations were performed with the
Gaussian 03W software package.20
1
NMR (300.1 MHz; CDCl3); δ 8.62 (s, 2H), 8.51 (d, 2H), 7.71 (dd, 4H),
7.52 (dd, 2H), 7.48 (dd, 2H), 6.80 (d, 4H), 3.03 (s, 12H); HRMS (FAB)
calcd for C26H26N4: 394.2157. Found: 395.2168 [M þ H]þ; Anal.
Calcd for C26H26N4: C, 79.16; H, 6.64; N, 14.20; Found: C, 79.13; H,
6.68; N, 14.22.
Emission Quantum Yield Calculations. Fluorescence emission
quantum yield values (ΦPL) of the ruthenium complexes were calcu-
lated employing the comparative method of William,21 which involves
the use of well characterized standards with known ΦPL values. For this
purpose, the UVꢀvis absorbance and corrected emission spectra of five
different concentrations (1 μM ∼ 5 μM) of reference standards
(Rhodamine B; Aldrich, ΦPL = 1.00) and ruthenium complexes were
recorded. Rhodamine B has been used in mixture of ethanol and 0.01%
HCl and ruthenium complexes have been used in DMF solution. Record
the UVꢀvis absorbance and fluorescence spectrum of sample and then
integrated fluorescence intensities of the ruthenium complexes were
plotted vs absorbance values. The gradients of the plots are proportional
to the quantity of the quantum yield. Absolute values are calculated using
the standard samples which have a fixed and known fluorescence
quantum yield value, according to the following equation:
General Procedure for Conventional One-Pot Synthesis of
Ru(NCS)2LL0. To a solution of dichloro(p-cymene)ruthenium(II)
dimer (0.11 g, 0.17 mmol) and L0 (2aꢀ2e) (0.34 mmol) in DMF
(50 mL) heated at 80 °C for 4 h under N2 in the dark, and then 2,20-
bipyridine-4,40-dicarboxylic acid (L) (0.08 g, 0.34 mmol) was added.
The reaction mixture was stirred at 140 °C for 4 h. After an excess of
NH4NCS (1.05 g, 13.7 mmol) was added to the resulting dark solution,
the mixture was stirred for another 4 h at 140 °C and then cooled to
room temperature. After the removal of DMF under vacuum followed by
the addition of water, the suspended solution was filtered on a sintered
glass crucible by suction filtration to give a solid. After washing with
water and with diethyl ether, the crude complex was dissolved in basic
methanol (tetrabutylammonium hydroxide) and purified on a Sephadex
LH-20 column using methanol as the eluent. The collected main band
was concentrated and slowly titrated with an acidic methanol solution
(HNO3) to pH 3.1. It should be noted that this titration should be done
very slowly. The precipitate was collected on a sintered glass crucible by
suction filtration and dried under air.
!
ꢀ
ꢁ
GradX
ηX2
η2ST
ΦX ¼ ΦST
GradST
where the subscripts ST and X denote standard and ruthenium
respectively, Φ is the fluorescence quantum yield, Grad the gradient
from the plot of integrated fluorescence intensity versus absorbance, and
η the refractive index of the solvent.21
3a (X = CN): yield 72%; 1H NMR (300.1 MHz; DMSO-
d6 þ Bu4NOH); δ 9.43 (d, 1H), 9.37 (s, 1H), 9.33 (d, 1H), 9.17
(s, 1H), 9.14 (s, 1H), 8.98 (s, 1H), 8.49 (d, 1H), 8.44 (d, 1H), 8.42
(d, 2H), 8.29ꢀ8.32 (m, 1H), 8.19 (d, 2H), 8.12 (d, 2H), 8.04 (d, 2H),
8.02 (s, 1H), 7.59ꢀ7.65 (m, 2H); other data were signals of Bu4NOH;
Anal. Calcd for C38H22N8O4RuS2: C, 55.67; H, 2.70; N, 13.67; Found:
C, C, 55.78; H, 2.75; N, 13.59.
DSSC Fabrication. DSSCs were fabricated as follows. Transparent
conductive glass plates coated with an F-doped SnO2 (FTO, purchased
from Pilkington. Co. Ltd., 8 Ω/γ) were used to prepare both the photo-
and counter-electrodes. A Ti(IV) bis(ethyl acetoacetato)-diisopropox-
ide solution (2% w/w in 1-butanol) was spin-coated onto FTO
substrates, which were then heated stepwise to 450 °C and maintained
at this temperature for 20 min. Commercialized TiO2 paste (Ti-
Nanoxide T, Solaronix) was cast onto the heat-treated FTO substrates
by the doctor-blade technique and then sintered at 450 °C for 30 min.
The substrates with thick mesoporous TiO2 layers (ca. 13ꢀ18 μm) were
dipped into a 1:1 tert-butanol/acetonitrile solution of 3aꢀ3e or N3
(0.3 mM) and kept overnight. Pt-layered counter-electrodes were
prepared by spin-coating H2PtCl6 solution (0.05 M in isopropanol)
onto FTO glass and then sintered at 400 °C for 30 min. The dye-
adsorbed TiO2 electrodes and Pt counter-electrodes were assembled
into a sealed sandwich-type cell by heating at 80 °C using a hot-melt
ionomer film (Surlyn) as a spacer between the electrodes. The electro-
lyte was composed of 0.6 M 3-hexyl-1,2-dimethyl imidazolium iodide,
0.05 M iodine, 0.05 M LiI, and 0.5 M 4-tert butylpyridine in acetonitrile.
A drop of the electrolyte solution was placed in a hole drilled in the
counter-electrode and driven into the cell via vacuum backfilling. Finally,
the hole was sealed using additional Surlyn and a cover glass (0.1 mm
thick).
3b (X = F): yield 76%; 1H NMR (300.1 MHz; DMSO-
d6 þ Bu4NOH); δ 9.42 (d, 1H), 9.23 (d, 1H), 9.21 (s, 1H), 9.04
(s, 1H), 8.92 (s, 1H), 8.32 (d, 2H), 8.22 (d, 1H), 8.00 (t, 1H), 7.92
(d, 2H), 7.85ꢀ7.89 (m, 1H), 7.63 (t, 1H), 7.46 (t, 1H), 7.40 (t, 1H),
7.23 (d, 2H), 7.10 (d, 2H); other data were signals of Bu4NOH; Anal.
Calcd for C36H22F2N6O4RuS2: C, 53.66; H, 2.75; N, 10.43; Found: C,
53.41; H, 2.43; N, 10.52.
3c (X = H): yield 83%; 1H NMR (300.1 MHz; DMSO-
d6 þ Bu4NOH); δ 9.42 (d, 1H), 9.31 (d, 1H), 9.28 (s, 1H), 9.15
(s, 1H), 9.12 (s, 1H), 8.99 (s, 1H), 8.37 (d, 1H), 8.29 (d, 1H), 8.23
(d, 2H), 7.93 (d, 2H), 7.84 (d, 1H), 7.69 (t, 2H), 7.60ꢀ7.63 (m, 2H),
7.54ꢀ7.57 (m, 4H), 7.51 (t, 1H); other data were signals of Bu4NOH;
Anal. Calcd for C36H24N6O4RuS2: C, 56.17; H, 3.14; N, 10.92; Found:
C, 56.41; H, 3.19; N, 10.98.
3d (X = OMe): yield 75%; 1H NMR (300.1 MHz; DMSO-
d6 þ Bu4NOH); δ 9.39 (d, 1H), 9.19 (d, 1H), 9.16 (s, 1H), 9.11
(s, 1H), 9.00 (s, 1H), 8.95 (s, 1H), 8.28 (d, 1H), 8.25 (d, 1H), 8.18
(d, 2H), 7.88 (d, 2H), 7.84 (d, 1H), 7.59 (d, 1H), 7.47 (d, 1H), 7.40
(d, 1H), 7.19 (d, 2H), 7.05 (d, 2H), 3.87 (s, 3H), 3.78 (s, 3H); other data
were signals of Bu4NOH; Anal. Calcd for C38H28N6O6RuS2 (830.06):
C, 55.00; H, 3.40; N, 10.13; Found: C, 55.11; H, 3.48; N, 10.11.
3e (X = NMe2): yield 65%; 1H NMR (300.1 MHz; DMSO-
d6 þ Bu4NOH); δ 9.35 (d, 1H), 9.17 (d, 1H), 9.14 (s, 1H), 9.08
(s, 1H), 8.98 (s, 1H), 8.90 (d, 1H), 8.17 (d, 1H), 8.08 (d, 2H), 7.75
(d, 2H), 7.70 (d, 1H), 7.48 (d, 1H), 7.40 (d, 1H), 7.34 (d, 1H), 7.11
(d, 2H), 7.02 (d, 2H), 3.01 (s, 6H), 2.99 (s, 6H); other data were signals
of Bu4NOH; Anal. Calcd for C40H34N8O4RuS2: C, 56.13; H, 4.00; N,
13.09; Found: C, 56.28; H, 4.08; N, 13.02.
Solar Cell Efficiency. The photoelectrochemical performance
characteristics (short-circuit current Jsc (mA cmꢀ2), open-circuit vol-
3
tage Voc (V), fill factor ff, and overall energy conversion efficiency η)
were measured under illumination with a 1000 W xenon lamp (Oriel,
91193) using a Keithley Model 2400. The light intensity, which was
confirmed to be homogeneous over an 8 ꢁ 8 in2 area, was calibrated with
a Si solar cell (Fraunhofer Institute for Solar Energy Systems, Mono-
SiþKG filter, Certificate No. C-ISE269) for 1 sun light intensity (AM
1.5G, 100 mW cmꢀ2) and was double-checked with an NREL-cali-
3
brated Si solar cell (PV Measurements, Inc.). Accidental increases in
temperature inside the cell during measurement were prevented by
using a cooler with a propeller. Each measurement was repeated at least
three times to confirm reproducibility.
Theoretical Calculations. DFT calculations were carried out
using B3LYP* (Becke’s three-parameter exchange functional (B3) and
the LeeꢀYangꢀParr correlation functional (LYP)) and the LANL2DZ
IPCE. Incident photon-to-current conversion efficiency (IPCE)
was measured as a function of wavelength from 360 to 800 nm (PV
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Inorganic Chemistry
ARTICLE
Scheme 1. Synthesis Route of 3aꢀ3e and N3 Dye
(2c), OMe (2d), and NMe2 (2e)) synthesized by Pd-catalyzed
Suzuki coupling reaction were reacted with [Ru(p-cymene)Cl2]2
in DMF under nitrogen to give Ru(p-X-phenyl)2Cl3. The Ru-
(NCS)2LL0 complexes were obtained by the reaction of Ru(p-X-
phenyl)2Cl3 with 4,40-di(hy3droxycarbonyl)-2,20-bipyridine (L)
followed by treatment with a large excess of ammonium thio-
cyanate. All the products were isolated in moderate yields
(65ꢀ83%) by Sephadex column followed by acid precipita-
tion. The structures were unambiguously identified by their
spectroscopic properties and elemental analyses (Experimental
Section).
Spectroscopic and Electrochemical Properties. The UVꢀ
vis absorption spectra of 3aꢀ3e and N3 in DMF are shown in
Figure 1, and the optical data are summarized in Table 1. The
absorption spectra of 3aꢀ3e consist of three bands with a
spectral feature similar to that of N3 caused by ligand-based
charge transfer (LBCT) and/or MLCT transitions. It should be
noted that 3aꢀ3e exhibit higher extinction coefficients than N3
over the entire investigated region, which is attributed to the
conjugation effect of the p-X-phenyl substituents.
The high-energy bands in the UV region between 298 and
307 nm are assigned as being dominated by the L-centered πꢀπ*
transition overlapping the L0 πꢀπ* transition. The second
absorption band (abbreviated as band I), with maxima between
375 and 428 nm, should be assigned to LBCT plus MLCT, as
reported.6a,22 The low-energy absorption band located between
527 and 542 nm, abbreviated as band II, should be the result of
MLCT arising from the participation of the NCS moieties,
because such a low-energy MLCT transition does not appear
with related Ru(II) complexes without an NCS ligand. The
absorption maxima of bands I and II depend substantially on the
electron-withdrawing/donating nature of X; a systematic bath-
ochromic shift occurs as X changes from the strong electron-
donating NMe2 group (3e) to the strong electron-withdrawing
CN group (3a). It is of interest to note that 3a shows a
panchromatic-like absorption feature in the visible region, with
the maxima of bands I and II longer by 49 and 17 nm respectively
than in N3. Moreover, the molar extinction coefficients for bands
Figure 1. Electronic absorption spectra of 3aꢀ3e and N3 measured
in DMF.
Measurements, Inc.) using a standard tungsten-halogen lamp as mono-
chromatic light and a broadband bias light for approximating 1 sun light
intensity.
Laser Flash Photolysis. Nanosecond transient absorption mea-
surements were conducted by a laser flash photolysis technique for
completed DSSCs irradiated with 504 nm pulses generated by hydrogen
Raman shifting of third-harmonic generation (THG, 355 nm) from a
Q-switched Nd:YAG laser (Continuum, Surelite II, pulse width of 4.5 ns
fwhm). A 150 W xenon arc lamp (Newport, 70525) was focused into the
sample solution as the probe light for the measurements. Temporal
profiles were measured with a monochromator (DongWoo Optron,
Monora 500i) equipped with a photomultiplier (Zolix Instruments Co.,
CR 131) and a digital oscilloscope (Tektronix, TDS-784D). Reported
signals are averages of 1000 events. The decay profiles for the oxidized
dyes are monitored at 820 nm.
’ RESULTS AND DISCUSSION
Synthesis. Scheme 1 outlines the synthesis of Ru(NCS)2LL0.
Di(p-X-phenyl)-2,20-bipyridines (L0; X = CN (2a), F (2b), H
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Table 1. Photophysical Properties of 3aꢀ3e and N3
λmax (nm)a/ε [ ꢁ 104 Mꢀ1cmꢀ1
]
Ru complex (X)
LC
band I
band II
λPL (nm)b
HOMO (eV)c
LUMO (eV)c
ΦPLd ( ꢁ 10ꢀ4
)
τPLe (ns)
3a (CN)
3b (F)
306/3.27
307/4.21
307/3.95
306/4.45
298/4.05
309/3.51
428/1.06
390/1.46
389/1.36
388/1.56
375/2.69
379/1.06
542/1.04
533/1.24
533/1.25
530/1.32
527/1.47
525/1.07
800
762
773
759
744
764
ꢀ4.82
ꢀ4.60
ꢀ4.54
ꢀ4.46
ꢀ4.30
ꢀ4.95
ꢀ3.18
ꢀ2.97
ꢀ2.91
ꢀ2.83
ꢀ2.69
ꢀ3.24
2.0
4.2
3.3
5.4
5.7
4.0f
33.0
39.0
40.0
48.0
66.0
60.0g
3c (H)
3d (OMe)
3e (NMe2)
N3
a Measured in DMF. b Emission maxima observed by excitation at 530 nm. c HOMO and LUMO levels were determined using the following equations:
E
HOMO (eV) = ꢀ(Eox ꢀ EFc|Fcþ þ 4.8 eV), ELUMO (eV) = ꢀ(EHOMO ꢀ Eg; Eg = absorption threshold of the Ru complexes). d Emission quantum yields
for air-purged DMF solution. e Emission lifetimes for air-purged DMF solution. f Ref 1e. g Ref 9e.
Table 2. Electrochemical Properties of 3aꢀ3e and N3
comparable with that of the carboxylic substituent for one-
electron oxidation of the Ru(II) center.
b
At negative potentials versus Fc|Fcþ, two peaks that can be
attributed to ligand-based reductions were observed.23 As shown
in Table 2, electron-withdrawing ligands clearly result in more
positive shifts of redox potential. In the reduction of Ru(II)
polypyridine complexes, an electron is added to the LUMO,
usually the π* orbital of a polypyridine ligand.24 Therefore, each
of the bpy ligands L and L0 can accept an electron in each
electrochemically accessible unoccupied molecular orbital. As
expected, 3aꢀ3e all exhibited two clear cathodic peaks, an
irreversible reduction wave Epc1 spanning from ꢀ1.84 (3a) to
ꢀ2.19 V (3e) and a reversible one Epc2 spanning from ꢀ2.15 to
ꢀ2.55 V, which might be attributable to the one-electron
reduction of L and L0, respectively. The irreversibility of the first
wave should arise from eq 1, that is, reductive H2 evolution from
the carboxylic acid groups located at the 4,40-positions in the 2,20-
bipyridine ring.25
Ered
E1/2/Va (RuIII/II) V vs Fc|Fcþ Epc1 Epc2 Epa1 Epa2
3a (CN)
3b (F)
0.39
0.36
0.34
0.31
0.26
ꢀ1.84 ꢀ2.15 ꢀ2.04 ꢀ1.74
ꢀ2.01 ꢀ2.44 ꢀ2.30 ꢀ1.93
ꢀ2.00 ꢀ2.47 ꢀ2.20 ꢀ1.93
ꢀ2.07 ꢀ2.49 ꢀ2.26 ꢀ1.99
ꢀ2.19 ꢀ2.55 ꢀ2.33 ꢀ2.08
3c (H)
3d (OMe)
3e (NMe2)
a The Ag/AgNO3 reference electrode was calibrated with a ferrocene/
ferrocinium (Fc|Fcþ) redox couple. The electrochemical experiments
were carried out in 0.1 M [nBu4N]BF4/MeCN solution. b Epc1 and Epc2
denote cathodic peaks at negative scan, whereas Epa1 and Epa2 represent
anodic peaks at reverse scan.
I and II depend substantially on the substituent X, as revealed by
the decrease from the exceptionally high values of 3e to the values
of 3a comparable with those of N3. All these observations clearly
demonstrate that the electronic transitions of Ru(NCS)2LL0
have been tuned by the choice of remote substituent X.
1
R ꢀ CO2H þ eꢀ f R ꢀ CO2ꢀ þ H2
ð1Þ
2
Table 1 also shows the emission parameters (maxima, quan-
tum yields, and lifetimes) of 3aꢀ3e in DMF solution at 298 K,
which again reveal significant dependences on the substituent X.
The emission maximum moved by 56 nm, from 744 nm for 3e to
800 nm for 3a; the latter was longer by 36 nm than that of N3.
The emission quantum yields showed a ∼2.8-fold difference
between the highest value for 3e and the lowest one for 3a. The
lifetimes increased systematically from 33 ns for 3a to 66 ns
Computational Studies. For further insights into the electro-
nic structures of 3aꢀ3e with regard to the HOMO and LUMO
levels, we performed DFT calculations using the B3LYP/6-31G*
exchange correlation functional with the Los Alamos effective
core potential LanL2DZ19 as implemented in the Gaussian03
package.20
Table 3 and Figure 2 summarize the results. Notably, both the
HOMO and LUMO levels increase systematically as X changes
from the strong electron-withdrawing CN group to the strong
electron-donating Me2N group, indicating that the remote
substituent X can tune the electronics of Ru(NCS)2LL0. For
comparison, DFT calculations were also performed for N3 (L =
L0), yielding deeper HOMO and LUMO levels than those of
3aꢀ3e. This seems reasonable because the strong electron-
withdrawing CO2H groups are directly attached to the two bpy
rings. The calculated HOMO/LUMO levels correlate well with
the experimentally estimated values listed in Table 1. Good
correlations were also found between the first reduction poten-
tials and the LUMO levels and between the oxidation potentials
and the HOMO levels. Figure 2 shows the energy levels and
orbital population densities of the HOMO-1, HOMO, LUMO,
and LUMOþ1 for the heteroleptic Ru(II) complexes, demon-
strating the exclusive population of HOMO and HOMO-1 on
the Ruꢀ(NCS)2 part and the localized amplitude of LUMO on
3
for 3e. All these results again indicate that the MLCT-state
behavior of Ru(NCS)2LL0 depends substantially on the remote
substituent X.
Cyclic voltammetric measurements for acetonitrile solutions
of 3aꢀ3e were performed under identical conditions, typically
yielding pseudoreversible oxidation and reduction waves, as
shown in Figure S2 (Supporting Information) for a typical trace
of 3d. The oxidation and reduction potentials obtained from the
voltammograms are presented in Table 2. Whereas the oxidation
potential of 3e was significantly more positive by 0.05ꢀ0.13 V
than the others, 3aꢀ3d show small variations in the oxidation
waves. It is worth noting that the oxidation potential of 3a was
only slightly negative (by 0.01 V) compared with the value
reported for N3,9d even though the latter has the strong electron-
withdrawing carboxylic groups directly bound the bipyridine
ligand. This suggests that the p-cyanophenyl group has an effect
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Table 3. Calculated Energies for 3aꢀ3e
(eV)
N3
3a (RuꢀCN)
3b (RuꢀF)
3c (RuꢀH)
3d (RuꢀOMe)
3e (RuꢀNMe2)
LUMOþ3
LUMOþ2
LUMOþ1
LUMO
ꢀ2.367
ꢀ2.667
ꢀ3.184
ꢀ3.238
ꢀ4.952
ꢀ5.089
ꢀ5.333
ꢀ5.442
1.714
ꢀ2.585
ꢀ2.612
ꢀ3.048
ꢀ3.184
ꢀ4.816
ꢀ5.007
ꢀ5.089
ꢀ5.197
1.632
ꢀ2.095
ꢀ2.422
ꢀ2.667
ꢀ2.966
ꢀ4.599
ꢀ4.789
ꢀ4.843
ꢀ4.980
1.633
ꢀ2.041
ꢀ2.367
ꢀ2.585
ꢀ2.912
ꢀ4.544
ꢀ4.735
ꢀ4.781
ꢀ4.898
1.632
ꢀ1.960
ꢀ2.286
ꢀ2.422
ꢀ2.830
ꢀ4.463
ꢀ4.646
ꢀ4.708
ꢀ4.816
1.633
ꢀ1.823
ꢀ2.150
ꢀ2.231
ꢀ2.694
ꢀ4.300
ꢀ4.490
ꢀ4.544
ꢀ4.684
1.606
HOMO
HOMO-1
HOMO-2
HOMO-3
Eg
Figure 2. (Top) Theoretically calculated energy levels of 3aꢀ3e. HOMOꢀLUMO gaps are reported in eV. (Bottom) Isodensity plots for HOMOꢀ1,
HOMO, LUMO, and LUMO þ1 (from bottom to top).
L, with one exception (3a). These features are very similar to
those reported for N3,26 indicating that 3aꢀ3e basically maintain
the electronic structure of N3.
(0.18ꢀ0.19 eV) between HOMO and HOMOþ1. These
results should be correlated with the systematic blue-shift of
band I from 3a to 3e, assuming the HOMO-to-LUMOþ1
transition contributes considerably to band I. For 3a, a small
but non-negligible population of LUMO density exists on L0,
in sharp contrast to the complete absence in the cases of
3bꢀ3e. These DFT calculations clearly showed that the
HOMO/LUMO levels can be systematically tuned by chan-
ging X from the strong electron-donating NMe2 group to the
However, LUMOþ1 was largely populated on L0, except for
3e, which contains the strong electron-donating NMe2 group.
It is of interest to note that the energy gap between LUMO
and LUMOþ1 increased as X changed from the strong
electron-withdrawing CN group to the strong electron-donat-
ing NMe2 group, in contrast to the almost constant differences
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Figure 3. IPCE plot versus excitation wavelengths for DSSCs using
3aꢀ3e and N3.
strong electron-withdrawing CN group while maintaining the
basic electronic structure of N3.
Application of 3aꢀ3e to Dye-Sensitized Solar Cells. As
discussed above, the present N3-related dyes (3aꢀ3e) reveal
uniquely tunable electronic properties associated with different
X. Therefore, the application of 3aꢀ3e to DSSCs can be
expected to provide clues to exploring the dye-related structure
ꢀactivity relationships controlling the performance of DSSCs in
solar cells. The primary requirement for DSSCs is that relevant
dyes must have excited-state potentials capable of injecting an
electron into the conduction band of TiO2. The excited-state
oxidation potentials of these dyes (<ꢀ1.55 V vs Fc|Fcþ) are
sufficiently more negative than the conduction-band level of
TiO2 (ECB ≈ 0.13 V vs Fc|Fcþ). Another essential requirement
is that the dyes must have HOMO levels that are more positive
than the redox potential of the I3ꢀ/Iꢀ couple (ꢀ0.06 V vs Fc|
Fcþ) for efficient reduction of the oxidized dyes to occur after
electron injection. The HOMO levels of 3aꢀ3e should be
positive enough for the electron transfer (ET) to proceed
efficiently, as shown by the oxidation potentials (0.26ꢀ0.39 V
vs Fc|Fcþ) listed in Table 2. In addition, the relatively high molar
extinction coefficients of 3aꢀ3e over an expanded wavelength
region might be beneficial for harvesting visible light. Therefore,
we investigated the performance of the DSSCs in relation to the
electronic properties of 3aꢀ3e.
Figure 4. Current densityꢀvoltage curves for DSSCs using 3aꢀ3e
measured (a) in the dark and (b) under 1 sun simulated sunlight
illumination (AM 1.5G, 100 mW cmꢀ2).
3
Table 4. Photovoltaic Properties of DSSCs Using 3aꢀ3e and
N3 as a Sensitizer under 1 Sun Conditions (AM 1.5G,
100 mW cmꢀ2
)
3
dye (X)
Voc (mV)
Jsc (mA cmꢀ2
)
ff (%)
η (%)
3
3a (CN)
3b (F)
668
724
706
717
688
742
10.0
14.3
15.2
16.0
8.41
14.6
73.7
73.1
71.7
72.4
73.3
71.9
4.90
7.59
7.72
8.30
4.24
7.80
3c (H)
3d (OMe)
3e (NMe2)
N3
The photovoltaic performance of DSSCs using 3aꢀ3e were
studied under 1 sun (AM 1.5G, 100 mW cmꢀ2) and compared
3
with those of a standard DSSC using N3. Experimental condi-
tions for cell fabrication and measurement of solar cell character-
istics were carefully kept as unchanged as possible so that the cell
characteristics would reflect the functions of the dyes.
the cells taken in the absence of the electrolyte (Figure S1 of the
Supporting Information).
Part a of Figure 4 shows the currentꢀvoltage characteristics of
the devices in the dark, which reveal sufficient rectification effects
with significantly different behavior depending on the device.
The turn-on voltages associated with hole transfer from the dye-
grafted TiO2 electrode to the electrolyte increase in the order 3e
< 3a < 3b < 3c < 3d, similar to the IPCE behavior. Part b of
Figure 4 shows the photovoltaic performances of the devices,
again indicating remarkable dependences on the dyes, with a
trend that parallels those of the IPCE and dark-current behavior.
Table 4 summarizes the characteristic parameters abstracted
from part b of Figure 4, including the open-circuit voltages
(Voc), short-circuit currents (Jsc), fill factors (ff), and power
conversion efficiencies (η). The ff values are almost constant
(71.7ꢀ73.7%), indicating that the device configurations are
Figure 3 shows the monochromatic IPCE values for devices
using 3aꢀ3e and N3; broad curves cover almost the entire visible
region from 400 to 720 nm for all of the devices. Substantial
variations in IPCE were observed, depending on the devices. The
efficiencies increase with increasing electron-donating power of
X, reaching a maximum for the 3d-based device, but with a large
drop for 3e, that is, 3e < 3a < 3b < N3 ≈ 3c < 3d. This suggests
that the remote substituent X should control the DSSC’s
performance to a considerable degree. Alternatively, these differ-
ences might simply arise from different light-harvesting efficien-
cies in the dyes adsorbed on the TiO2 surface. However, this is
clearly not the case, as demonstrated by the absorption spectra of
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Chart 1. Kinetic Processes in DSSCs
Table 5. Kinetics for Electron Transport and Recombination
Dynamics for 3aꢀ3d and N3
τ1 (μs)
τ2 (μs)
amplitude A1/A2
RuꢀCN (3a)
RuꢀF (3b)
RuꢀH (3c)
RuꢀOMe (3d)
N3
0.36
0.40
0.41
0.48
0.44
80
79/21
80/20
85/15
87/13
80/20
177
209
211
117
more enhanced compared with the other devices, as indicated by
the dark current behavior in part a of Figure 4. The relatively
shallow HOMO level of 3e should provide a weak driving force
for ET from Iꢀ to 3eþ•, enhancing participation in CR. Further-
more, a well-known fact that amines interact with I2 to form CT
complexes should be taken into consideration. Such interactions
might lead to an increase in local concentration of the oxidized
electrolyte in contact with the dyes, thus enhancing CR.
Figure 5. Temporal decay profile for the 3d-based device monitored at
820 nm.
In general, CR between the injected electron and the oxidized
dye (Dye•þ) may occur in competition with ET from Iꢀ to
Dye•þ, with considerable effect on DSSC performance, particu-
larly Voc and Jsc. The kinetic processes in DSSCs are described in
Chart 1. To obtain insights into the ET and CR processes, we
investigated the transient dynamics of completed DSSCs by
monitoring the decay of Dye•þ absorption at 820 nm. Figure 5
shows a typical temporal decay profile for the 3d-based device,
which can be fitted to a biexponential kinetics consisting of fast
(τ1) and slow (τ2) components reasonably well. Similar biexpo-
nential decays of the absorption at 820 nm were observed for the
other devices as well; the decay times (τ1 and τ2) and their
amplitudes (A1 and A2) are shown in Table 5.
It is well established that ET occurs in a time domain of
0.1ꢀ10 μs, whereas CR proceeds in a range of hundreds of
microseconds or even a few milliseconds.28 Therefore, the
observed fast (τ1) and slow (τ2) kinetics can be attributed to
ET and CR, respectively. As shown in Table 5, the τ1 values are
fairly constant (0.36ꢀ0.48 μs), indicating that the ET process
occurs at reaction rates very similar to that for the N3 device
independently of the dyes. This can be reasonably attributed to
the deep HOMO levels of all the dyes compared with the redox
potential of the electrolyte. In contrast, a substantial variation can
be observed for the slow τ2 component, from 80 μs for the 3a-
based device to 211 μs for the 3d-based device, indicating that the
CR process is slower in the order 3a > N3 > 3b > 3c > 3d. In line
with this, the A1/A2 ratio increases systematically from 79/21 for
the 3a-based device to 87/13 for the 3d-based device, a result
associated with the decrease in the CR contribution to Dye•þ
decay in the order 3a > N3 ≈ 3b > 3c > 3d. The substantial
differences in the CR kinetics between the 3a and 3d devices
similar and the device performance reflects the electronic nature
of the dyes. The Voc and Jsc values are clearly poor for the device
using 3a, with the strong electron-withdrawing CN group, and
still worse for the device using 3e, with the highly electron-
donating NMe2 group. In contrast, the device using 3d, with the
moderately electron-donating MeO group, has the highest Jsc.
Consequently, the η values systematically increase with increas-
ing electron-donating power of X to reach a maximum for the 3d-
based device but drop greatly for 3e, an observation in line with
the IPCE behavior. It should be noted that the 3d-based DSSC
gives the best conversion efficiency (η = 8.30%) of the cells
investigated, which is higher than that of the N3-based device
(η = 7.80%) measured under identical conditions.
The poor performances of the 3a- and 3e-based devices are
reminiscent of the reported behavior of DSSCs based on Ru-
(NCS)2LL0, with L0 = NO2- or NH2-substituted phenanthroline
ligand.27 For the NO2-substituted ligand, the poor performance
has been discussed in terms of the unusually deep LUMO level
coupled with the dominant LUMO population on L0 remote
from the TiO2 surface, which cause inefficient electron injection
from the excited-state dye into the TiO2 conduction band.
Although this would also work, at least in part, for the 3a device,
3a has a sufficiently high LUMO compared with ECB and a very
small population of LUMO density on L0. Therefore, other
factors should be considered, that is, enhanced charge recombi-
nation (CR) between the injected electrons and the oxidized dye
(below). For the NH2-substituted ligand, on the other hand, the
device behavior has been attributed to enhanced CR between the
injected electrons and the oxidized electrolyte. In the 3e device,
the charge recombination with the oxidized electrolyte should be
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ARTICLE
appear to arise from the different driving forces of the CR
process, as suggested by the difference of ∼0.35 eV estimated
from the HOMO levels of 3a and 3d in Tables 1 and 3.
Presumably, CR would be a long-distance type of electron
transfer for the electron diffusing inside TiO2, so the kinetics
should be sensitive to differences in the driving force. However,
ET should occur at short-range because Iꢀ diffuses in solution to
approach the oxidized dye closely; that is, a diffusion-controlled
type of process occurs. Therefore, the slow CR in the 3d device,
associated with the dye’s electronic nature, can be concluded to
play an essential role in the better device performance compared
with the others, whereas the poor behavior of the 3a device
should be dominated by the relatively fast CR.
’ ACKNOWLEDGMENT
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science, and Technol-
ogy (MEST) of Korea (2010-0018456) and by the Center for Next
Generation Dye-Sensitized Solar Cells (No. 2011-0001055).
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Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Gr€atzel, M.; Officer,
D. L. J. Phys. Chem. C 2007, 111, 11760.
’ CONCLUSIONS
We have investigated the photophysical and electrochemical
properties of a series of newly synthesized heteroleptic
ruthenium(II) complexes, Ru(NCS)2LL0 (3aꢀ3e), and their
behavior in DSSCs in an attempt to explore the structureꢀactiv-
ity relationships associated with the dyes. We demonstrated that
the remote substituent X substantially affects steady-state ab-
sorption and emission maxima and redox potentials of the dyes.
These properties, which are tuned by X, have been shown to
correlate well with the results of DFT calculations, which show
that 3aꢀ3d maintain the electronic structure of N3, with
exclusive orbital populations of HOMO on the Ru(NCS)2 part
and LUMO on L, accompanied by continuous variation in the
HOMO/LUMO levels depending on X. We compared the solar-
cell performance of DSSCs based on 3aꢀ3e and N3 and found
that the power-conversion efficiencies increase in the order
3e < 3a < 3b < 3c < N3 < 3d. Transient spectroscopic studies
of the completed devices revealed that charge recombination
between the photoinjected electrons and the oxidized dye is
slower in the order 3a > N3 > 3b > 3c > 3d, whereas the reaction
rates for electron transfer from the electrolyte to the oxidized dye
are fairly constant independent of the dyes. The close relation-
ship between the charge-recombination kinetics and the device
performance suggests that slow charge recombination is the main
contributor to the efficient solar cell behavior of the 3d-based
device, whereas the poor performances of the 3a device is likely
due to fast charge recombination. A particular case is the 3e
device for which the poor solar-cell behavior has been attributed
to the particular chemical nature of the Me2N substituent, which
enhances another charge recombination between the injected
electrons and the oxidized electrolyte as well as quenching of the
excited-state dye. The present observations imply that a meth-
odology based on fine electronic tuning of dyes provides a
technique for the molecular design of optimum dyes for applica-
tion to DSSCs as well as to artificial photosynthesis.
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’ ASSOCIATED CONTENT
S
Supporting Information. Absorption spectra of 3aꢀ3e
b
dye-adsorbed TiO2 films, and cyclic voltammogram of 3d. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
(11) Anderson, P. A.; Strouse, G. F.; Treadway, T. A.; Keene, F. R.;
Meyer, T. J. Inorg. Chem. 1994, 33, 3863.
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Corresponding Author
*Tel: þ82-41-860-1334, fax: þ82-41-867-5396, e-mail: jjpac@
korea.ac.kr (C.P.); sangok@korea.ac.kr (S.O.K.).
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