Terpyridine- and 2,6-dipyrazinylpyridine-coordinated ruthenium(II) complexes: Synthesis, characterization and application in TiO2-based dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2010.06.001

Two new ruthenium(II) complexes bearing terpyridine- or 2,6-dipyrazinylpyridine ligands have been prepared and characterized by one- and two-dimensional NMR techniques, ESI mass spectrometry, as well as by UV-vis, emission, FTIR, Raman, and cyclic voltammetry studies. The structure of the terpyridine-coordinated complex resembles that of black dye (tris(thiocyanato) (2,2′:6′,2″-terpyridyl-4,4′,4″-tricarboxylato) ruthenium(II) tris(tetra butylammonium) salt), while the spectral data of the 2,6-dipyrazinylpyridine-coordinated complex are consistent with an unanticipated, unsymmetrical binuclear structure. The ruthenium(II)/(III) oxidation potential of the terpyridine-coordinated dye was measured at +0.87 V (vs. Ag/AgCl), about 200 mV higher than the oxidation potential of black dye. According to a series of desorption experiments, both new dyes were found to adsorb on TiO₂ to a greater extent than black dye. The photo-electrochemical properties of both dyes were investigated and compared to that of black dye; while the 2,6-dipyrazinylpyridine-coordinated complex was found to be a very poor sensitizer, the cells obtained from the terpyridine-coordinated dye show power-conversion efficiencies which are more than half of that attained by black dye. Finally, preliminary electron dynamics studies of the cells constructed with the terpyridine dye were carried out and the results are compared to that of black dye cells.

Full text of DOI:10.1016/j.jphotochem.2010.06.001

Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Terpyridine- and 2,6-dipyrazinylpyridine-coordinated ruthenium(II) complexes:
Synthesis, characterization and application in TiO -based dye-sensitized
2
solar cells
a
a
a
a
Georgios C. Vougioukalakis , Thomas Stergiopoulos , Georgios Kantonis , Athanassios G. Kontos ,
a
b
b
a,∗
Kyriakos Papadopoulos , Arta Stublla , Pierre G. Potvin , Polycarpos Falaras
Institute of Physical Chemistry, NCSR “Demokritos”, Agia Paraskevi Attikis, 15310 Athens, Greece
a
b
Department of Chemistry, York University, 4700 Keele Street, Toronto, ON, M3J 1P3, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new ruthenium(II) complexes bearing terpyridine- or 2,6-dipyrazinylpyridine ligands have been
prepared and characterized by one- and two-dimensional NMR techniques, ESI mass spectrometry,
as well as by UV–vis, emission, FTIR, Raman, and cyclic voltammetry studies. The structure of the
Received 16 March 2010
Received in revised form 11 May 2010
Accepted 1 June 2010
ꢀ
ꢀ
ꢀꢀ
terpyridine-coordinated complex resembles that of black dye (tris(thiocyanato)(2,2 :6 ,2 -terpyridyl-
Available online 8 June 2010
ꢀ
ꢀꢀ
4
,4 ,4 -tricarboxylato)ruthenium(II) tris(tetra butylammonium) salt), while the spectral data of the
,6-dipyrazinylpyridine-coordinated complex are consistent with an unanticipated, unsymmetrical bin-
2
Keywords:
uclear structure. The ruthenium(II)/(III) oxidation potential of the terpyridine-coordinated dye was
measured at +0.87 V (vs. Ag/AgCl), about 200 mV higher than the oxidation potential of black dye. Accord-
ing to a series of desorption experiments, both new dyes were found to adsorb on TiO2 to a greater extent
than black dye. The photo-electrochemical properties of both dyes were investigated and compared to
that of black dye; while the 2,6-dipyrazinylpyridine-coordinated complex was found to be a very poor
sensitizer, the cells obtained from the terpyridine-coordinated dye show power-conversion efficiencies
which are more than half of that attained by black dye. Finally, preliminary electron dynamics studies of
the cells constructed with the terpyridine dye were carried out and the results are compared to that of
black dye cells.
Dye-sensitized solar cells
Dyes
Ruthenium
TiO2
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
nanoparticulate film (7–20 ␮m) of a mesoporous semiconductor
oxide modified with a monolayer of sensitizer molecules, which
are chemically grafted via functional groups such as –COOH and
The development of renewable, non-polluting power resources
has become an extremely important need in view of fossil fuels
depletion, increasing world energy consumption, and concerns
over global environment preservation and climate change [1,2].
One of the most interesting and promising strategies seeking to
tackle this problem is to mimic nature in harvesting and collect-
ing sunlight, thus attaining clean and affordable solar electricity
–PO(OH) , and a Pt counter electrode, both deposited on conductive
2
glass substrates. An appropriate medium containing the redox cou-
−
−
ple (usually I /I3 ) is placed between the two electrodes to transfer
the charges.
The photo-electrochemical cycle in DSSCs begins when a dye
molecule undergoes transition into its excited state via light
absorption. The photoexcited dye molecule subsequently injects
an electron into the conduction band of the semiconductor. Ideally,
the photoinjected electron percolates through the nanoparticle
network, gets collected by the photoanode electrode, and then trav-
els through the external circuit performing electrical work. The
electron eventually reaches the counter electrode and reduces the
electron mediator species, generating the reducing agent that con-
comitantly reduces the ground state oxidized dye, completing the
circuit. The device maximum voltage (Vmax) that can be generated
equals the difference between the semiconductor Fermi level and
[
3,4]. Indeed, solid-state photovoltaic cells, based on inorganic
semiconductors, have proven to achieve this goal; however, their
high production cost imposes restrictions on their mass utilization
[
5,6]. Dye-sensitized solar cells (DSSCs) constitute an appeal-
ing alternative to the classical solid-state cells [7–15]. DSSCs are
based on nanocrystalline metal oxide semiconductors, sensitized
by molecular dyes (sensitizers) that promote photoinduced charge
separation [16,17]. The system is basically comprised of two fac-
ing electrodes: a transparent photoanode, consisting of a thin
−
−
the electron mediator species’ (i.e. I /I3 ) redox potential. Overall,
the device results in the conversion of photons to electrons, while
no chemical species are consumed nor transformed.
∗ Corresponding author. Tel.: +30 210 6503644; fax: +30 210 6511766.
E-mail address: papi@chem.demokritos.gr (P. Falaras).
1
010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.06.001

G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
23
Fig. 2. Molecular structures of the new terpyridine- and 2,6-dipyrazinylpyridine-
coordinated ruthenium(II) dyes (2) and (3).
Fig. 1. Molecular structures of ruthenium(II) dyes N3, N719, and black dye (1).
2. Experimental
The efficiency of a DSSC largely depends on the charge gen-
eration step and, therefore, the sensitizer utilized. Some of the
requirements that an efficient sensitizer has to fulfill include
2.1. Materials and methods
[
7–14]: (i) a broad and strong absorption, preferably extending
Unless otherwise stated, all reagents were purchased from
Aldrich. Solvents were purchased from Fluka or Panreac and
used without further purification. Trihydrated ruthenium(III)
trichloride was purchased from Riedel-de Haën. Acetonitrile,
from the visible to the near-infrared; (ii) a firm, irreversible adsorp-
tion to the semiconductor’s surface and a strong electronic coupling
between its excited state and the semiconductor conduction band;
(
iii) chemical stability in the ground as well as in the excited and
oxidized states; (iv) a reduction potential sufficiently higher (by
150–200 mV [18]) than the semiconductor conduction band edge
used in the electrochemical studies, was degassed by
a
freeze–pump–thaw cycle and stirred overnight over P O ,
5
2
∼
at room temperature, under an argon atmosphere. It was
then distilled under argon and degassed again via another
freeze–pump–thaw cycle, before being stored over activated 4 Å
molecular sieves. All synthetic reactions were performed under
an argon atmosphere in degassed (15 min of argon bubbling)
in order to bring about an effective electron injection; and (v) an
oxidation potential sufficiently lower (by ∼200–300 mV [18]) than
the redox potential of the electron mediator species, so that it can
be regenerated rapidly.
ꢀ
ꢀ
ꢀꢀ
ꢀ
ꢀꢀ
A variety of organic dyes and transition-metal complexes
has been successfully employed as sensitizers in DSSCs thus far
solvents. Black dye (tris(thiocyanato)(2,2 :6 ,2 -terpyridyl-4,4 ,4 -
tricarboxylato)ruthenium(II) tris(tetrabutylammonium) salt) was
purchased from Solaronix.
[
19–24]; in terms of photovoltaic performance and long-term sta-
bility, Ru(II) polypyridyl complexes comprise the most successful
family of DSSCs sensitizers [25–29]. The first high-performance
Ru(II) sensitizer was reported in 1993 by Grätzel and co-workers
2
.1.1. Analytical measurements
1H, 13C, HSQC, and COSY Nuclear Magnetic Resonance (NMR)
(
N3 and N719, Fig. 1) [30,31]. In 1997, the same group published the
spectra were obtained with a Bruker Avance 500 MHz spectrom-
eter in (CD ) SO or CD OD. Chemical shifts are reported in ppm
downfield from Me Si, byusingthe residual solventpeak asinternal
standard. Data for NMR spectra are reported as follows: chemical
shift (ı ppm), multiplicity, coupling constant (Hz), and integra-
synthesis and evaluation of a terpyridyl analogue, i.e. “black dye” (1,
Fig. 1) [32,33]. The reported overall power-conversion efficiencies
of N3, N719, and black dye are 10.0%, 11.2%, and 10.4%, respectively.
Since then, a series of modifications of these early Ru(II) complexes
have, among others, led to sensitizers with amphiphilic prop-
erties and/or extended conjugation, achieving power-conversion
efficiencies up to 11.9% [34,35].
In this kind of ruthenium sensitizers, the stereoelectronic char-
acteristics of the polypyridine ligand along with the number of
functional binding groups are of high importance for efficient
sensitization. As part of our ongoing work in the field of DSSCs
3
2
3
4
−5
tion. Electronic spectra (UV–vis) in solution (1: 5.01 × 10 M; 2:
−5
−5
5
.62 × 10 M; and 3: 3.44 × 10 M in DMF) were recorded with
a Hitachi 3010 spectrophotometer. Diffuse reflectance and trans-
mittance UV–vis spectra of the dye-sensitized TiO2 films were
obtained on the same instrument using an integrating sphere with
−5
6
0 mm diameter. Emission spectra (2: 9.04 × 10 M in DMF) were
recorded on a JASCO FP 777 spectrophotometer. Infrared spectra
(FTIR) in the powder form were obtained using a FTIR Nicolet 6700
spectrometer. Data for FTIR spectra are reported as follows: fre-
[
36–40], we herein report the synthesis, characterization, and TiO2-
sensitizing ability evaluation of two new Ru(II) dye complexes,
coordinated with terpyridine (2) and 2,6-dipyrazinylpyridine (3)
ligands (Fig. 2). Both ligands encompass one peripheral carboxylic
anchoring group; furthermore, the 2,6-dipyrazinylpyridine moiety
is more electron-withdrawing than the terpyridine one, due to the
additional nitrogen atoms contained in the framework of the for-
mer. Comparison of sensitizers such as (2) and (3) with black dye (1)
provides more insight into the sensitizers optimum structure and
geometry, paving the way for a more detailed structure–function
correlation in these systems.
−1
quency (cm ) and strength (s = strong; m = medium; w = weak).
Despite our repeated attempts, we were unable to get reliable Ele-
mental Analysis data for any of the newly synthesized complexes,
possibly due to insufficient combustion. Micro-Raman spectra were
measured in the backscattering configuration using a Renishaw
inVia spectrometer with an Ar ion laser (ꢀ = 514.5 nm) and a near-
infrared (NIR) diode laser (ꢀ = 785 nm), as excitation sources. In all
+
2
cases, very low laser power density (0.05 mW/␮m ) was applied
to avoid dye degradation and thermal shifts of the modes. Cyclic

2
4
G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
voltammetry measurements were performed in 10−3 M solutions
of the corresponding complexes in acetonitrile containing 0.1 M
tetrabutylammonium tetrafluoroborate (TBATFB) as supporting
electrolyte, at a sweep rate of 100 mV/s, using the Autolab PGSTAT-
sensitized films were incorporated as the active photoelectrode
(PE) in a lamellar solar cell configuration of the following structure:
conductive glass/TiO2 + dye/electrolyte/Pt/TEC15. TEC15 conduc-
tive glasses (Pilkington), platinized by sputtering (about 10 nm
thick), were used as counter electrodes (CE). Non-sealed DSSCs
were fabricated by placing a drop of an ionic salt-based liquid
electrolyte (PMII, Dyesol Ltd.) onto the photoelectrode and sand-
wiching the CE on top of the PE.
3
0 potentiostat (Ecochemie). A three-electrode one-compartment
electrochemical cell was used, with Metrohm dot (6.0302.000)
and planar (6.0305.000) platinum working and counter electrodes,
respectively, as well as a Metrohm Ag/AgCl 6.0726.100 reference
electrode (LiCl saturated in ethanol). The experiments were per-
formed in deoxygenated solutions purified by Ar gas (99.9%) for
2.1.4. Photo-electrochemical measurements
3
0 min prior to use. During the experiments Ar was passing over
Current–voltage (I–V) measurements were performed by illumi-
nating the DSSCs using solar-simulated light (1 sun, 1000 W/m ),
2
the solution surface. Electrospray Ionization Mass spectra (ESI/MS)
were recorded using an AQA Navigator, Finnigan Mass Spectrom-
eter. Accurate mass measurements were obtained on solutions in
CH OH-CH Cl with an AB/Sciex QStar mass spectrometer with an
ESI source in either positive or negative ion mode and Time of Flight
detection.
from a 300 W Xe lamp, in combination with AM 1.5G and UV
optical filters (Oriel). The active area of the DSSCs was set at
2
0.152 cm , using a black metallic mask in front of the cell (while
3
2
2
2
the aperture area was larger, 0.360 cm ). I–V characteristics were
obtained using linear sweep voltammetry (scan speed: 50 mV/s) on
an Autolab PGSTAT-30 potentiostat. Electron diffusion coefficients
and lifetimes were determined by intensity-modulated pho-
tocurrent spectroscopy (IMPS) and intensity-modulated voltage
spectroscopy (IMVS), respectively. A red light (625 nm) emitting
diode was used as the light source for both AC and DC illumination
controlled by the FRA module. The LED intensity, calibrated by a Si
photodiode (Thorlabs), was varied using neutral optical filters.
2.1.2. Crystal structure determination
Crystals of ethyl 2,6-dipyrazin-2-ylisonicotinate were obtained
from CHCl3 by slow evaporation, and analyzed by Dr. A. Lough
University of Toronto). X-ray diffraction data were collected on
(
a Bruker-Nonius Kappa-CCD diffractometer using monochromated
Mo K␣ radiation and were measured using a combination of f
scans and w scans with k offsets, to fill the Ewald sphere. The data
were processed using the Denzo-SMN package. Absorption correc-
tions were carried out using SORTAV. The structure was solved and
refined using SHELXTL V6.1 for full-matrix least-squares refine-
ment that was based on F . All H atoms were included in calculated
positions and allowed to refine in riding-motion approximation
with Uiso tied to the carrier atom. Crystallographic data: formula
C16H13N5O ; Fw 307.31; T 150(2) K; triclinic space group P1 (#2);
2
2
.2. Synthesis
ꢀ
ꢀ
ꢀꢀ
ꢀ
.2.1. Synthesis of Ethyl 2,2 :6 ,2 -terpyridine-4 -carboxylate
-Acetylpyridine (290 mg, 2.4 mmol, 2 equiv.) and ethyl glyox-
2
2
alate (0.238 mL, 1.2 mmol, 1 equiv.) were dissolved in CH OH (8 mL)
3
by stirring for 5 min, followed by addition of 15% KOH (7.2 mL) and
conc. NH OH (0.8 mL). The mixture was allowed to stand at ambi-
4
¯
2
ent temperature for 3 days. The emulsion formed was filtered off
and washed with CHCl3 (4 mL) and cold 1:1 CH OH:H O (4 mL).
lattice constants: a 101.013(2) Å, b 9.6459(5) Å, c 19.6025(9) Å; ˛
◦
◦
◦
3
3
2
1
1
3
2
01.013(2) , ˇ 91.151(3) , ꢁ 100.872(3) , V 696.97(5) Å ; Z 2; D
calc
The crude product was suspended in CH OH:H O (80:20) and the
3
−1
◦
3
2
.464 Mg/m ; ꢂ 0.102 mm ; ꢀ(Mo K␣) 0.71073 Å; 2ꢃmax 27.49 ;
117 independent reflections measured (Rint 0.0333), of which
336 were considered to be observed with I > 2ꢄ(I); max. resid-
◦
mixture was stirred and sonicated at 35 C until a clear solution
was obtained. This was then acidified to pH 2, by addition of 1 M
HCl, to give a suspension that was collected by filtration and care-
fully washed with a small amount of slightly acidified water and a
small amount of cold water. After drying under vacuum, the crude
−3
ual electron density 0.300 and −0.295 e/Å ; 209 parameters (the
position of the H atoms were calculated at idealized positions); R1
0.0495; wR2 0.1458. Crystallographic data have been deposited at
+
+
K /NH4 salt was dissolved in EtOH in the presence of a catalytic
amount of H SO and heated to reflux for 2.5–3 days. The pure prod-
the Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, U.K., and copies can be obtained on request, free
of charge, by quoting the publication citation and the deposition
number 7757703, or via www.ccdc.cam.ac.uk/data request/cif.
2
4
uct was isolated as white flakes after extraction with CH Cl (yield
2
2
1
5
2
4%). H NMR (CDCl , 400 MHz): ı 1.48 (t, 3H), 4.50 (q, 2H), 7.38 (dd,
3
13
1
H), 7.89 (dd, 2H), 8.67 (d, 2H), 8.76 (d, 2H), 9.00 (s, 2H). C{ H}
NMR (CDCl , 100 MHz): ı 14.3, 61.8, 120.5, 121.2, 124.1, 136.9,
3
2.1.3. Device assembly
140.1, 149.3, 155.4, 156.4, 165.3. Anal. Found for C18H15N O : C,
3 2
Titania films were prepared using the doctor-blade technique
70.81; H, 5.05; N, 13.43. Calcd: C, 70.8; H, 4.95; N, 13.76. EI-MS: m/z
305 (43%), 276 (3%), 260 (6%), 233 (100%). FTIR vmax (cm ) 1725
(C O str).
−
1
by depositing three different layers of TiO2 on a 6 mm transpar-
ent conductive glass (fluorine-doped tin oxide, FTO) provided with
an antireflective coating (Activ Clear, Pilkington). The first layer
was prepared from a paste of nanocrystalline powder (Degussa
P25) dispersed in distilled water, following a standard exper-
ꢀ
ꢀ
ꢀꢀ
ꢀ
2.2.2. Synthesis of [Ru(ethyl 2,2 :6 ,2 -terpyridine-4 -
carboxylate)Cl ] (4)
3
◦
imental procedure [30]. The films were then heated at 120 C
A two-necked, 50 mL round-bottomed flask, equipped with a
for 30 min. A second layer was deposited on top of the former
films by doctor-blading a paste of large scattering particles (Ti-
Nanoxide 300, Solaronix), mixed with Degussa P25 in order to
improve the films’ adherence. Afterwards, the films were ther-
stirring bar and a reflux condenser, was charged with RuCl ·3H O
3
2
(130.7 mg, 0.5 mmol, 1 equiv.) and degassed EtOH (14 mL) at room
ꢀ
ꢀ
ꢀꢀ
temperature under argon. The ligand ethyl 2,2 :6 ,2 -terpyridine-
ꢀ
4 -carboxylate (152.7 mg, 0.5 mmol, 1 equiv.) was dissolved in
◦
mally treated at 550 C for 1 h. Finally, a third, very thin layer
degassed CH Cl2 (14 mL) and added to this solution. The reaction
2
was added through a simple immersion of the as-annealed films
mixture was refluxed in the dark under argon, for three and a half
hours, before let to cool down to ambient temperature. This deep
red solution was left overnight at room temperature under argon,
in the dark, then filtered on a G4 sintered glass crucible, and washed
in an aqueous solution of TiCl . The films were then thoroughly
4
◦
cleaned with water and ethanol and re-annealed at 550 C. The
final thickness of the resultant films (22 ␮m) was measured by
a profilometer. The thermally treated titania films were sensi-
tized by overnight immersion in 0.3 mM ethanolic dye solutions.
In order to evaluate their photovoltaic performance, the dye-
thoroughly with EtOH (5 × 5 mL) and Et O (5 × 5 mL). The remain-
2
ing dark brown powder was dried overnight under high vacuum
−
1
(210.0 mg, 82%). FTIR vmax (cm ) 1720.2 (s), 1600.6 (m), 1552.4

G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
25
(
1
(
(
3
w), 1463.7 (w), 1419.4 (s), 1365.4 (m), 1346.1 (w), 1257.4 (s),
245.8 (s), 1162.9 (w), 1118.5 (m), 1012.5 (m), 912.2 (w), 865.9
w), 794.5 (s), 765.6 (s), 725.1 (s), 651.8 (w), 433.9 (m). ESI/MS
1H), 8.74 (s, 1H), 3.27 (q, J = 7.2 Hz, 24H), 1.35 (t, J = 7.2 Hz, 36H).
13
1
C{ H} NMR (CD OD, 125 MHz): 164.73, 162.61, 162.47, 152.41,
3
152.24, 150.42, 150.22, 149.95, 148.95, 147.62, 141.62, 141.16,
+
+
−1
+ mode): m/z 495.0 ([M–Cl+H O] , 40%), 477.0 ([M–Cl] , 21%),
140.93, 119.39, 118.94, 118.46, 118.37, 45.40, 6.59. FTIR vmax (cm
)
2
+
05.1 ([L+H] , 30%). Exact mass calculated for C18H17N O Cl Ru
2098.2 (s), 1716.4 (w), 1571.7 (s), 1467.6 (w), 1446.4 (w), 1371.2 (s),
1240.0 (w), 1178.3 (w), 1157.1 (w), 1116.6 (w), 1054.9 (w), 1016.3
3
3
2
+
[
M–Cl+H O] 494.9684, observed 494.9665.
2
(
w), 904.5 (w), 848.5 (w), 788.8 (m), 775.3 (m), 688.5 (w), 648.0 (s),
ꢀ
ꢀ
ꢀꢀ
2
4
.2.3. Synthesis of {(CH CH ) NH}[Ru(2,2 :6 ,2 -terpyridine-
455.1 (w).
3
2 3
ꢀ
-carboxylic acid)(NCS) ] (2)
3
A two-necked, 50 mL round-bottomed flask, equipped with a
stirring bar and a reflux condenser, was charged with 4 (154.5 mg,
.3 mmol, 1 equiv.) and degassed DMF (9.3 mL) at room temper-
3. Results and discussion
0
3.1. Synthesis, NMR, and MS studies of ruthenium dyes
ature under argon. NH NCS (730.8 mg, 9.6 mmol, 32 equiv.) was
4
ꢀ
ꢀ
ꢀꢀ
ꢀ
dissolved in degassed H O (3.6 mL) and added to this solution. The
The synthesis of ethyl 2,2 :6 ,2 -terpyridine-4 -carboxylate has
been reported in the literature [41–47] but it and the completely
new ethyl 2,6-dipyrazin-2-ylisonicotinate were prepared by a new
method, both in 54% yields, from commercial materials, and both
were fully characterized. Fig. 3 reports the crystal structure for the
latter ligand. As is typical for tridentates, exemplified by the 4-p-
tolyl analogue [48], the pyrazine rings are rotated out of the metal-
binding conformation to minimize dipole–dipole repulsions. The
ethyl ester group is twisted out of coplanarity with the pyridine ring
2
◦
reaction mixture was heated at 135 C in the dark under argon
for 45 min. Next, degassed H O (5.4 mL) and Et N (10.8 mL) were
2
3
◦
added, and the solution was refluxed (135 C) for 20 more hours to
hydrolyze the ester functionality. The reaction mixture was cooled
down to room temperature and the solvent volume was reduced to
about 0.5 mL on a rotary evaporator. H O (15 mL) was then added,
2
causing a dark brown powder to precipitate. This was washed
thoroughly (on a G4 sintered glass crucible) with H O (6 × 5 mL)
2
◦
and Et O (6 × 5 mL), redissolved in the minimum amount of DMF,
by 15.5 , although this may simply be to facilitate the packing of
2
precipitated with MeOH, and washed on a G4 sintered glass cru-
the ethyl groups in the voids between stacks of tridentate moieties.
Complex 2 was prepared via the two-step synthetic route
presented in Scheme 1, in 57% total isolated yield, following a mod-
ification of previously reported procedures [32,33]. All reactions
were monitored by removing small aliquots, which were subse-
cible with Et O (3 × 5 mL), to afford complex 2 as a black powder
2
1
that was dried overnight under high vacuum (137.5 mg, 70%).
H
NMR (CD OD, 500 MHz – due to peak overlap only the major iso-
3
mer’s data are given): ı 8.99 (d, J = 4.9 Hz, 2H), 8.76 (s, 2H), 8.48
1
(
(
1
1
d, J = 7.9 Hz, 2H), 8.03 (t, J = 7.6 Hz, 2H), 7.71 (t, J = 6.5 Hz, 2H), 3.21
quently analyzed via UV–vis and H NMR spectroscopy. Complex 2
q, J = 7.3 Hz, 6H), 1.30 (t, J = 7.3 Hz, 9H). 13C{ H} NMR (CD OD,
1
was characterized by 1H, C, COSY, and HSQC NMR experiments,
UV–vis and FTIR spectroscopy (vide infra), as well as ESI-TOF and
ESI mass spectrometry. Also, it has to be noted that, despite our
repeated attempts, we were unable to isolate and purify the car-
boxylic analogue of complex 4 via the reaction of RuCl ·3H O with
13
3
25 MHz): 167.33, 162.72, 160.21, 154.57, 138.01, 137.58, 134.32,
−
1
29.45, 128.41, 123.89, 121.64, 48.11, 9.36. FTIR vmax (cm ) 2104.0
(
1
(
(
2
s), 1693.2 (m), 1596.8 (m), 1471.4 (w), 1421.3 (m), 1336.4 (m),
276.7 (m), 1234.2 (m), 1155.2 (w), 1041.4 (m), 1014.4 (w), 892.9
w), 881.3 (w), 790.7 (m), 763.7 (s), 727.0 (m), 651.8 (w), 528.4
3
2
ꢀ ꢀ ꢀꢀ ꢀ
2,2 :6 ,2 -terpyridine-4 -carboxylic acid.
−
−
m). ESI/MS (− mode): m/z 552.9 ([M] , 100%), 493.9 ([M–H–NCS] ,
Both 1D and 2D NMR data of 2, i.e. chemical shifts, integra-
tions, peak multiplicity, and 2D homo- and hetero-nuclear coupling
experiments, suggested the existence of two (detectable) isomers
in a 76/24 ratio. Thus, in the aromatic region of the H NMR spec-
tra of 2 (Supporting Information), two sets of resonances were
−
−
6%). Exact mass calculated for C19
H
11
N O S Ru [M] 552.9154,
6
2 3
observed 552.9160. ESI/MS (− mode): m/z 553.3 ([M] , 100%).
1
2.2.4. Synthesis of 4-Ethyl 2,6-dipyrazin-2-ylisonicotinate
Using 2-acetylpyrazine (295 mg, 2.4 mmol) instead of 2-
acetylpyridine, this was prepared in exactly the same fashion as
ꢀ
ꢀ
ꢀꢀ
ꢀ
1
was ethyl 2,2 :6 ,2 -terpyridine-4 -carboxylate (yield 54%). H NMR
(
2
6
CDCl , 400 MHz): ı 1.50 (t, 3H), 4.53 (q, 2H), 8.70 (d, 2H), 8.72 (d,
3
13
1
H), 9.02 (s, 2H) 9.85 (s, 2H). C{ H} NMR (CDCl , 100 MHz): 14.3,
3
2.1, 121.4, 140.6, 143.4, 143.8, 145.0, 150.1, 155.0, 164.7. Anal.
Found for C16H13N5O : C, 62.55; H, 4.25; N, 22.64. Calcd: C, 62.53;
2
H, 4.26; N, 22.79. EI-MS: m/z 307.19 (97%), 279.17 (8%), 262.15 (7%),
−1
2
35.15 (100%). FTIR vmax (cm ) 1720 (C O str).
2.2.5. Synthesis of [Ru(ethyl 2,6-dipyrazin-2-ylisonicotinate)Cl3]
(
5)
This complex was prepared and purified as described for com-
−1
plex 4 (86% yield). FTIR vmax (cm ) 1722.1 (s), 1583.3 (m), 1562.1
w), 1467.6 (w), 1375.0 (s), 1346.1 (w), 1243.9 (s), 1168.7 (w),
118.5 (m), 1091.5 (w), 1020.2 (m), 910.2 (s), 850.5 (w), 771.4
m), 688.5 (m), 651.8 (w), 615.2 (w). ESI/MS (+ mode): m/z 497.0
(
1
(
(
3
+
+
+
[M–Cl+H O] , 7%), 479.0 ([M–Cl] , 4%), 442.0 ([M–OEt–Cl+H O] ,
2
2
+
+
%), 330.1 ([L+Na] , 7%), 308.1 ([L+H] , 100%). Exact mass calculated
+
for C16H15N5O C Ru [M–Cl+H O] 496.9589, observed 496.9606.
3
12
2
Fig. 3. ORTEP-type drawing of the crystal structure of ethyl 2,6-dipyrazin-2-
2
2
.2.6. Synthesis of {(CH CH ) NH} [Ru(NCS) (2,6-dipyrazin-
ylisonicotinate, with 50% probability ellipsoids and hydrogen atoms omitted for
3
2
3
4
3
◦
clarity. Selected bond lengths (all ± 0.002 Å) and angles (all ± 0.1 ): C3-N1 1.341 Å,
-ylisonicotinic acid)Ru(NCS)5] (3)
C12-N1 1.340 Å, C2-C3 1.397 Å, C12-C13 1.400 Å, C1-C2 1.386 Å, C1-C13 1.392 Å,
C1-C14 1.500 Å, C14-O1 1.208 Å, C14-O2 1.330 Å, C3-C4 1.483 Å, C12-C11 1.487 Å,
This complex was prepared and purified as described for com-
1
plex 2 (88% yield). H NMR (CD OD, 500 MHz): ı 10.78 (s, 1H), 10.57
◦ ◦ ◦ ◦
N1-C3-C4 116.4 , N1-C12-C11 115.5 , C3-C4-N2 116.9 , C12-C11-N5 118.0 , N1-C3-
3
◦
◦
(
s, 1H), 9.40 (s, 1H), 9.26 (s, 1H), 9.13 (s, 1H), 9.07 (s, 1H), 8.77 (s,
C4-N2 179.8 , N1-C12-C11-N5 −172.3 .

2
6
G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
Scheme 2. Synthesis of the 2,6-dipyrazinylpyridine-coordinated ruthenium com-
plex 3.
species was attempted. Complex 3, although obtained somewhat
1
13
impure, was characterized by H and C NMR experiments, UV–vis
and FTIR spectroscopy, as well as ESI-TOF mass spectrometry.
In the 1H NMR spectra of 3 (Supporting Information), the inte-
grated ratio between the aromatic (dipyrazinylpyridine) and the
Scheme 1. Synthesis of the terpyridine-coordinated ruthenium dye 2.
aliphatic (Et NH) resonances shows the presence of four Et NH
3
3
counter ions per 2,6-dipyrazin-2-ylisonicotinic acid ligand, sug-
gesting a total charge of −4 for this ruthenium complex. Moreover,
eight instead of the expected four resonances in the aromatic region
suggest that the two pyrazine rings of the dipyrazinylpyridine
ligand are not magnetically equivalent, and, therefore, the com-
plex is unsymmetrical. We speculate that this “desymmetrization”
occurs through the attachment of a second metal center on one of
the peripheral pyrazine nitrogen atoms of the ligand (complex 3).
This behavior has been in situ observed in the past with a similar
dipyrazinylpyridine ruthenium(II) complex [48–52]; that complex
showed the ability to bind Lewis acids, for instance forming Ru–Fe
binuclear clusters that bear structural resemblance to the herein
proposed binuclear structure.
observed. In particular, between ı 9.05 and 8.70 ppm, where peaks
do not overlap significantly, two doublets at ı 8.99 and 8.95 ppm,
for the major and minor isomers, respectively, and two singlets
at ı 8.80 and 8.76 ppm, for the minor and major isomers, respec-
tively, were recorded. The two low-field doublets correspond to the
two protons ortho to the nitrogen atoms of the terpyridine ligand,
whereas the singlets arise from the protons of the central pyridine
moiety. Both the major and the minor isomers show five signals in
the aromatic region, sometimes overlapping with each other, sug-
gesting that the peripheral pyridine rings of the terpyridine ligand
are magnetically equivalent; i.e. that both complexes (major and
minor isomers) are symmetrical. Also, the integration ratios of the
Finally, although it has not been possible to observe the
molecular tetraanion neither via ESI-TOF/MS nor via ESI/MS (Sup-
porting Information), some of the detected fragments could very
well originate from complex 3. For example, the monoanion
at m/z = 772.7612 in the negative-mode ESI-TOF spectrum could
terpyridine ligand resonances and of the aliphatic peaks of Et NH
3
(
at ı 3.21 and 1.30 ppm), suggest that each Et NH cation corre-
3
sponds to one ruthenium center. By taking these spectral data into
consideration, we conclude that complex 2 is most probably iso-
lated in the form of two isomers due to the ambidentate nature
of its NCS ligands; that is we attribute the two different sets of
NMR resonances to two linkage isomers of 2 (N-coordinated vs.
S-coordinated NCS – also refer Section 3.3).
−
be [M–3NCS] (requiring 772.7605). Other fragments suggest
2−
the presence of trinuclear ([L Ru (SCN) ]
with L representing
the dipyrazinylisonicotinic acid ligand: m/z = 663.8375, requiring
63.8332) and mononuclear (m/z = 555.9) analogues of 3, but these
2
3
8
6
The molecular ion of 2 was detected by negative-mode ESI-
TOF/MS at m/z 552.9160 (2 requires 552.9154); a fragment
may have been formed in situ (as possible sampling artifacts) during
the ESI-TOF experiments.
−
corresponding to [M–H–NCS] was also found at m/z 493.9.
Indisputable structural assignment of complex 3 would arise
from the crystallographic analysis of a single-crystal; however,
it has thus far been impossible to obtain single-crystals suitable
for X-ray crystallographic analysis from any of the newly synthe-
sized complexes. To conclude, although the structure of complex
We also targeted the 2,6-dipyrazinylpyridine-substituted ana-
logue of complex 2, to study the effect of this more electron-
withdrawing ligand on the sensitizing properties of the resulting
ruthenium(II) complex. The preparation of complex 5 (Scheme 2),
that is, the precursor to the dipyrazinylpyridine analogue of 2,
was straightforward, following the reaction conditions utilized
for parent complex 4 (Scheme 1). However, in the next step
of the synthetic procedure, binuclear complex 3 (Scheme 2)
was isolated instead of the anticipated dipyrazinylpyridine ana-
logue of 2 (vide infra), along with some very minor unidentified
impurities that we were unable to completely remove. Appar-
ently, a part of the 2,6-dipyrazinylpyridine-substituted trichloride
complex 5 falls apart under these conditions, affording free
3
has not been proven beyond any doubt, the structure depicted
in Scheme 2 is consistent with all existing spectroscopic data
described above. Despite its accidental formation, the unusual
structure of bimetallic 3 complex presents high interest from both
structural and electrochemical point of view and warrants further
investigation, however that extends beyond the objectives of the
present study.
2
,6-dipyrazin-2-ylisonicotinic acid and the corresponding de-
3.2. UV–vis and emission studies
coordinated ruthenium species; in fact, we were indeed able to
isolate free 2,6-dipyrazin-2-ylisonicotinic acid from the reaction
mixture, but no investigation of the fate of the resultant ruthenium
UV–vis absorption spectra of the dyes in solution (DMF) as well
as upon TiO2 films, from transmittance and diffuse reflectance

G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
27
Fig. 5. Resonant Raman spectra of dyes 2 and 3 in the powder form and after
sensitization of thick TiO2 films. A Raman spectrum of black dye (1) is shown for
comparison. The modes discussed for dye 2 are marked with a star.
2
080 cm 1 [ꢅ(C N) stretching] and 700 cm [ꢅ(C S) stretching]
− −1
are suggestive of S-coordination. Another methodology that has
been utilized to distinguish between N- and S-coordination modes
of NCS is based on the differences in the intensity of the ꢅ(C N)
stretching band [58,59]. The FTIR spectra of 2 were recorded in the
Fig. 4. Absorption spectra of dyes 1, 2, and 3 in DMF solution (a), and corresponding
spectra of the dyes chemisorbed onto TiO2 films (b). The emission spectrum of dye
2
(a) is also given (arbitrary units).
−
1
4
000–400 cm region (Supporting Information). The strong and
−
1
relatively broad absorption observed at 2104 cm is attributed to
experiments, respectively, are shown in Fig. 4. The absorption spec-
tra of dye 2 are dominated by MLCT transitions with absorption
maxima at 588, 535 (as a shoulder), and 390 nm. High-energy
narrow bands, due to ␲–␲* intra ligand transitions, are shown
at 327 and 280 nm. The molar extinction coefficient at 588 nm is
the characteristic ꢅ(C N) stretching vibration of both the N- and
the S-coordinated NCS ligands (both linkage isomers of complex 2
−1
–
vide supra). Moreover, the two bands at 791 and 764 cm [ꢅ(C S)
stretching] can be assigned to the all-N- and all-S-coordinated
isomers of ruthenium(II) complex 2, respectively, in close agree-
ment with the corresponding linkage isomers of black dye (1,
Fig. 1), where the analogous stretching bands are observed at 788
− −1
1
7
640 M cm . Black dye (1), the UV–vis spectrum of which is
shown for comparison, presents MLCT transitions with absorption
maxima at 627, 543, and 425 nm (Table S1 in Supporting Informa-
tion). Dye 3, on the other hand, has a completely different spectrum
with a broad MLCT band at 503 nm, and ␲–␲* transitions at 293 and
−1
−1
and 757 cm , respectively [59]. Finally, the band at 1693 cm is
assigned to the C O stretching band of the protonated carboxyl
group in 2 (the precursor ester moiety in complex 4 is observed at
2
65 nm, which are similar to those of tris(bipyrazine)ruthenium(II)
−1
1
720 cm – Supporting Information). Dye 3 shows similar char-
complexes [48,50]. A very broad absorption band at the NIR region
is also observed, possibly due to the Ru(NCS)5 unit. Upon TiO2
sensitization, the wavelengths of the absorption maxima are only
slightly affected. The absorption band of dye 2 keeps its shape, while
for dye 3 a clear broadening is observed. This effect could be a sign
of possible dye 3 agglomeration on the TiO2 film. Finally, as can be
seen in Fig. 4, dye 2 shows an approximate 40 nm blue shift of ꢀmax
in comparison with black dye (1) [33].
acteristics with dyes 1 and 2, presenting a broad C N stretching
−1
−1
mode at 2098 cm , and the C S modes at 789 and 775 cm , due to
both N- and S-coordinated NCS ligands. The carboxyl C O stretch-
−1
ing mode in 3 is observed at 1716 cm (the precursor ester group in
−1
5
resonates at 1722 cm – Supporting Information). The rich FTIR
spectra in the fingerprint region (1000–1800 cm ) for both dyes
−1
(
(
2 and 3) is due to the various atomic vibrations of the terpyridine-
dye 2) and 2,6-dipyrazinylpyridine-ligands (dye 3). It should be
The emission properties of the dyes were studied in DMF. A
typical spectrum of complex 2 (presenting a maximum at 720 nm)
superimposed on the corresponding absorbance curve is given in
Fig. 4(a). From the intersection between the UV–vis and emission
spectra [53–55], the excitation transition energy E0-0 is determined
to be 1.80 eV (690 nm). Using the absorption and emission data of
black dye, the corresponding absorption threshold was estimated
at about 1.49 eV.
also noted that the infrared spectra of dye 2 exert pronounced
similarities with that of dye 1, as expected [33]. Complementary
vibrational analysis, carried out by off-resonance Raman (under
NIR) and Resonance Raman (under visible excitation) scattering,
is discussed in the following section.
3.4. Raman and Resonance Raman studies
Off-resonance Raman peaks of dye 2 under NIR excitation
3
.3. Infrared studies
(Fig. S1, Supporting Information) could be observed neither for
the dye in powder form, nor when it is adsorbed on the TiO2
surface, due to the strong photoluminescence band recorded
in the emission spectra (Fig. 4(a)). Resonant Raman spectra of
dye 2 are shown in Fig. 5, both in powder form as well as when
anchored on the film. The spectra are very similar to that of black
dye (1), which we also recorded and included in the same figure
for comparison purposes. The C N frequency of the NCS ligand is
The most direct evidence regarding the coordination mode
of ambidentate ligands arises from the corresponding infrared
spectra. In the case of NCS ligands, the presence of absorp-
−1
−1
tion bands at about 2100 cm [ꢅ(C N) stretching] and 780 cm
ꢅ(C S) stretching] is commonly considered as a confirmation
of N-coordination [56,57]. On the contrary, absorptions at about
[

2
8
G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
shown at 2120 cm 1, with a shoulder at 2068 cm−1, indicating both
−
Table 1
HOMO–LUMO energies of Ru(II) complexes and absorption threshold values.
N- and S-type coordination to ruthenium(II), in perfect agreement
with the IR analysis. The high-frequency pyridine C C and C
modes do not change significantly, shifted by less than 4 cm , as
expected from their rigid character. However, some remarkable
differences are predominant in the range of 1100–1400 cm
Sensitizer
HOMOa (V vs.
Ag/AgCl)
E0-0c nm (eV)
LUMOd (V vs.
Ag/AgCl)b
N
b
−
1
Dye (2)e
Black dye (1)
0.87
0.66
690(1.80)
835 (1.49)
−0.93
−0.83
f
−
1
,
a
where vibrations of single C–C, C–H, and C–N modes are expected.
Electrochemical data.
Under these conditions, the Fc/Fc redox potential value was determined at
b
+
The most evident differences between dye 2 and black dye (1)
−
1
+0.60 V vs. Ag/AgCl and was used as the standard reference.
are the modes emerging in the spectra at 1162 and 1340 cm ; the
latter may be assigned to the combined in-plane C–H and C–C vibra-
tions in the pyridine rings, as also observed in non-carboxylated
pyridine-containing ligands such as in Ru(terpy) [PF ] , and oth-
c
Derived from the spectra in Fig. 5 and Ref. [32].
Calculated.
In CH3CN.
In CH3CN.
d
e
f
2
6 2
ers [39,60,61]. Also, it is very interesting that low-frequency modes
at 922, 694 (in-plane pyridine deformation), and 535 (out-of-plane
pyridine deformation) soften to 890–900, 680, and 530 cm , which
stems from the missing carboxyl groups on the pyridine moieties
of dye 2.
3
.5. Electrochemical studies
−
1
The cyclic voltammogram (Fig. S3, Supporting Information) of
dye 2 in acetonitrile, containing 0.1 M tetrabutylammonium tetra-
boronfluoride (TBATBF), presents a reversible wave at +0.87 V (vs.
Ag/AgCl) attributed to the Ru3+/Ru redox couple as well as a
reversible ligand reduction centered at −1.35 V (vs. Ag/AgCl). Under
identical experimental conditions, the corresponding voltammo-
gram of black dye (1) shows a reversible wave at +0.66 V (vs.
Ag/AgCl) [32,33]. Using the excitation HOMO–LUMO transition
energy value E0-0 = 1.80 eV from Fig. 4, the LUMO energy level for
Upon dye 2 sensitization of the films, notable changes in the Res-
onant Raman spectra are observed. First off, the TiO anatase modes
2
2+
−1
at low frequencies (100–650 cm ) emerge, the strongest being
−
1
the 143 cm band. Furthermore, important changes in the relative
intensities of the major C O and C N peaks at high frequencies are
observed, as well as marked variations of the C–H and C–C modes
−1
in the 1250–1350 cm
region, in agreement with the findings
in other dyes [40] where such changes are attributed to surface-
enhanced Raman scattering effects. New modes are also detected
2
is estimated to about −0.93 V vs. Ag/AgCl (Table 1). The corre-
sponding LUMO level in the case of black dye was estimated at
−
due to the linkage of the dye to the semiconductor (via COO ), at
approximately −0.83 V, by considering the derived E = 1.49 eV
−1
0-0
1
C
373 cm (symmetric stretching mode of the carbonyl groups ꢅs
O (COO )), and at 1560 cm (possibly the corresponding anti-
value [32].
−
−1
The LUMO levels of both dyes 1 (black dye) and 2 lie well
symmetric mode) [62]. All the above differences, together with
above the TiO conduction band (−0.6 V vs. Ag/AgCl [8]) permitting
−
1
2
the absence of the protonated carboxyl group at about 1700 cm
,
electron injection from the excited dye molecules into the semicon-
ductor conduction band. Greater injection rates are expected for
dye 2, as the driving force (energetic difference between LUMO and
CB) is larger than in the case of dye 1. The corresponding HOMO lev-
verify chemisorption of the dye on the semiconducting surface,
most likely via bidentate chelation or bridging-type coordination
[
63].
The analysis of the off-resonance Raman spectrum of dye 3
Fig. S1, Supporting Information) shows several strong lines, due
−
−
els lie below the I /I3 redox potential (+0.3 V vs. Ag/AgCl) allowing
easy cation reduction for both dyes.
(
−
1
to C N related vibrations, at 1480 and 1516 cm , as well C-N
related modes at 1200 and 1282 cm−1, in accordance with results
on tris(bipyrazine)ruthenium(II) complexes and heteroleptic com-
plexes with pyridine and bipyrazine [64,65]. The strong intensity
of the above modes is justified by the increased number of such
bonds in the pyrazine rings, and the rich spectrum in these regions
due to the co-existence of one pyridine moiety. Peaks at 1150
The corresponding CV of binuclear complex
3 was also
recorded under similar experimental conditions. This complex
presents an irreversible ruthenium (II)-based oxidation wave at
+
1.35 V (vs. Ag/AgCl), together with two quasi irreversible ligand-
based reductions centered at −0.66 and −1.14 V (vs. Ag/AgCl,
Fig. S3 in Supporting Information). The bridging pyrazine ligand is a
well-known redox active center, being reduced at negative poten-
tials because of its strong ␲-acidity and usually accommodating
two reduction waves in bimetallic complexes [67]. Furthermore,
the existence of only one oxidation wave in bimetallic complex
−1
and 1340 cm
are also observed in this case, as for dye 2, and
these are attributed to combined C–H and C–C internal ring vibra-
tions.
The Resonant Raman spectrum of dye 3 (Fig. 5) was obtained
by exciting the dye at 514 nm, very close to its MLCT transi-
tion (503 nm). Similarities with the Resonance Raman spectra
in an earlier work [64,65], are pronounced both in the gen-
eral pattern of Raman lines as well as their frequencies. Marked
variations are observed in comparison to the spectrum obtained
under NIR excitation of the dye, mainly regarding the inten-
sity of several vibration bands. Thus, strong modes at 1034
3
is indicative of poor electronic communication between the
two ruthenium centers and/or low stability of the corresponding
oxidized ruthenium species [68]. In the case of complex 3, the deter-
mination of the energetic diagrams was not possible, due to both
relative electrochemical irreversibility and poor fitting between
the corresponding UV–vis and emission spectra (Fig. S2, Support-
ing Information).
−1
and 1460 cm , observed under NIR excitation, are nearly miss-
ing under visible excitation, while, on the other hand, intense
3.6. Quantitative determination of dyes’ adsorption
−1
modes at 1024 and 1540 cm
emerge. This is due to the
excitation into the MLCT which gives rise to resonance enhance-
ment of symmetric stretching vibrations [33,66], and selective
enhancement of the pyrazine modes relative to pyridine ones
As described above, the presence of the sensitizer on the pho-
toelectrode was qualitatively confirmed by applying Raman and
UV–vis spectroscopy on the dye-sensitized titania films. A next
step was then to quantify the chemisorption of the dye on the
titania films; to determine the amount of the dye adsorbed, des-
orption experiments (carried out in EtOH/H2O NaOH solutions)
were performed following standard procedures [69]. Using this
approach [70], the surface density of the adsorbed dye was esti-
[
64].
Resonant Raman spectra of TiO2 films sensitized with dye 3 do
not show important differences as to the corresponding spectra of
the dye in the powder form, besides the appearance of the TiO2
modes. Sharp features, like those observed with dye 2, were not
found in this case suggesting poor dye sensitization of the film.
mated to be 2.63 × 10 mol/cm2 for dye 2, more than 2.5 times
−
7

G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
29
Table 2
(
Voc) of 616 mV, and a filling factor (FF) of 0.65. On the other hand,
Performance parameters of DSSCs based on titania films sensitized by different dyes.
The surface density of adsorbed dyes is also given.
the corresponding efficiency for the dye 3-based cell was extremely
poor, of the order of only 0.02% (Supporting Information, Fig. S4).
Additional experiments using deoxycholic acid as co-adsorbent in
order to reduce agglomeration did not improve the efficiency of
dye 3, the poor value of which is obviously due to the molecu-
lar structure of the complex. Thus, it can be considered that the
LUMO level lies below the CB, in part because of the added nitro-
gens, in part because of the appended metal, both serving to shift
the redox potentials to more positive values. Comparative results
performed on identical DSSCs prepared using the black dye (con-
taining two more carboxylic acid moieties) [33] showed that the
efficiency of the cells obtained from dye 2 is about the half of that
attained by the black dye (Table 2). The obtained result is rather
satisfactory, especially if we bear in mind that black dye is among
the very few dyes that give overall energy conversion efficiency
over 10% [72]. The efficiency losses arise from the lower Voc (about
Sensitizer
Jsc (mA/cm2)
Voc (mV)
FF
ꢆ (%)
Dye (mol/cm2)
−7
−7
−8
Dye 2
Dye 3
Black dye (1)
6.19
0.27
10.23
616
214
717
0.65
0.34
0.63
2.48
0.02
4.64
2.63 × 10
1.09 × 10
4.65 × 10
higher than the loading of dye 3 (1.09 × 10−7 mol/cm , Table 2).
This difference can be attributed to the chemical structure of 3,
most probably being a binuclear complex (vide supra), that steri-
cally inhibits its sensitization efficiency by providing much fewer
dye monolayers on the TiO2 semiconductor in comparison with
dye 2. In any case, both complexes present reasonable dye amount
values in relation to other efficient dyes, recently reported in lit-
erature [71], implying that light-harvesting efficiency will not be
a limiting factor for attaining high photovoltaic efficiency. More-
over, it is interesting to observe that both novel dyes are adsorbed
on TiO2 to a greater degree than black dye 1 (in qualitative agree-
ment with the reflectance spectra of the dyes on TiO2 substrates,
Fig. 4). This result is closely related to the number of carboxylic
groups that are available for adsorption in every dye; dyes 2 and 3
contain only one carboxylic group, while black dye contains three
carboxylic moieties and is adsorbed through two of them [63]. A
factor that could account for better adsorption in the case of dyes
2
1
4
00 mV) and mainly due to the lower photocurrent (more than
2
mA/cm ) gained by the dye-2 based cell. Trying to explain the
above differences, preliminary studies on the electron dynamics
governing the cells constructed with the two different dyes were
conducted.
Initially, the electron lifetimes were determined by performing
IMVS experiments on the two cells (Fig. 7(a)). It was found that
2
and 3, in relation to black dye, is the great flexibility of the dyes
attached via one anchoring group on TiO , which would require less
2
space, allowing more dye molecules to adsorb on the titania surface.
In any case, especially concerning dye 3, probable agglomeration
on the TiO2 substrate (previously confirmed by optical measure-
ments) could also lead to higher loading. However, aggregation
usually implies either intermolecular quenching of the excited state
energy and/or reduced injection efficiency since additional layers
on top of titania particles cannot inject electrons directly into the
semiconductor.
3.7. Photovoltaic studies
The photovoltaic performance of the sensitized photoelectrodes
was examined following their incorporation into sandwich-type
liquid DSSCs. The I–V characteristics were then obtained under 1
sun (AM1.5) illumination (Fig. 6). The power-conversion efficiency
(
ꢆ) determined for the dye 2-based cells was as high as 2.48%, with a
2
short-circuit current (Jsc) of 6.19 mA/cm , an open-circuit potential
Fig. 7. (a) Electron lifetimes derived from the IMVS diagrams with varying photo-
voltage (with variation of the LED DC illumination power), and (b) electron diffusion
coefficients derived from the IMPS diagrams with varying Jsc.
Fig. 6. I–V characteristics of DSSCs using TiO2 electrodes sensitized by different dyes
under illumination of 1 sun (AM 1.5).

3
0
G.C. Vougioukalakis et al. / Journal of Photochemistry and Photobiology A: Chemistry 214 (2010) 22–32
the black dye-based DSSCs gave much higher electron lifetimes
12.7 ms at high illumination intensities) than the ones deter-
4. Conclusions
(
mined for the dye 2-based cell (only 1.56 ms at identical incident
powerdensity). These results fully confirm the differences observed
at Voc values, implying clearly less recombination in the case of
the black dye. In terms of the two dyes structures, the greater
dye coverage of dye 2 on TiO2 (in relation to black dye) implies
Two novel ruthenium(II) dyes, coordinated with terpyridine-
or 2,6-dipyrazinylpyridine-based ligands, have been synthesized
and characterized. The structure of the terpyridine-bearing com-
plex resembles that of black dye; on the other hand, the spectral
data of the 2,6-dipyrazinylpyridine-coordinated complex suggest
an unexpected binuclear structure in which one of the peripheral
pyrazine nitrogen atoms of the ligand is attached to a sec-
ond ruthenium(II) center. The Ru2+/Ru oxidation potential of
the terpyridine-coordinated dye in acetonitrile was measured at
+0.87 V (vs. Ag/AgCl); this is about 200 mV higher than the oxida-
tion potential of black dye. Both new dyes were found to adsorb
on TiO2 films to a higher extent than black dye. Furthermore, the
photo-electrochemical properties of both dyes were investigated;
interestingly, solar cells obtained from the terpyridine-coordinated
dye show power-conversion efficiencies which are more than half
of those attained by the black dye. The electron dynamics in cells
containing the new terpyridine dye were also studied and com-
pared to that of black dye.
−
higher protection from I3 ions, which scavenge the surface elec-
trons when closely surrounding the TiO particles. Thus, the higher
2
3+
recombination observed with dye 2 probably suggests increased
−
recombination with dye cations (and not I3 ). This back-reaction
is frequently considered important under open-circuit conditions
in a DSSC, decreasing Voc values [73]. Nevertheless, differences in
the conduction band edges of TiO2 upon adsorption of the two
dyes cannot be excluded as a possible reason to explain the Voc
differences, due to different sensitizer adsorption geometry induc-
ing dipolar fields of different magnitude and orientation at the
surface of the semiconductor particle [74]. In this context, we
have performed electrochemical impedance (EIS) measurements
(
Fig. S5, Supporting Information) in order to estimate the chemi-
cal capacitance of the semiconductor for the two cells [75]. It was
found that the at the same film capacitance, the dye 2-based cell
presents a higher voltage (about 250 mV) than the one correspond-
ing to black dye, implying that the adsorption of dye 2 on the
Acknowledgments
The authors acknowledge financial support from the General
Secretariat for Research and Technology (GSRT-Greece) through
PENED project 03ED 118/2005. T. Stergiopoulos thanks the Greek
State Scholarships Foundation (IKY) for a fellowship. Helpful assis-
tance by Dr. L. Leontiadis in obtaining the ESI-MS spectra is also
gratefully acknowledged.
semiconductor shifts the TiO conduction band towards more neg-
2
ative potentials [76]. However, this shift is not accompanied by an
enhancement of Voc due to the dramatic increase of the recombi-
nation rate (that finally diminishes the overall Voc values by about
1
00 mV).
On the other hand, significant differences were observed on the
photocurrent delivered by the two cells. The Jsc in a DSSC depends
on the light harvesting of the photoelectrode, the injection effi-
ciency of the electrons from the dye onto the semiconductor, and,
lastly, the collection efficiency of the electrons at the back contact.
The light-harvesting ability of dye 2 is significantly lower than that
of dye 1 above 600 nm (Fig. 4(b)), however this change cannot solely
account for the large Jsc differences. Differences in charge injection
efficiencies (concerning the driving force for electrons injection)
are difficult to be evaluated since the LUMO of dye 2 is higher
than that of dye 1 (Table 1), but coincidently the conduction band
edge of TiO2 is also higher upon adsorption of dye 2 in comparison
with dye 1. Thus, the only probability that remains to justify the
observed Jsc differences is the higher charge collection efficiency
in the case of black dye (1). This parameter is usually estimated
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.06.001.
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