Photophysical properties of new cyclometalated ruthenium complexes and their use in dye sensitized solar cells

Article abstract of DOI:10.1039/c2dt30482a

A new class of cyclometalated ruthenium complexes, Ru(C^N^N′) (N^N′^N′′)·Cl where N^N′^N′′ = 4,4′,4′′-tricarboxy-2,2′:6′,2′′- terpyridine and C^N^N′ = substituted 6-phenyl-2,2′-bipyridine, for Dye Sensitized Solar Cells (DSSCs) is proposed. We have investigated the effect of different substituents (R = COOH, thiophen-2-yl, F and OCH₃) on the ancillary C^N^N′ ligand on the photophysical properties and performance of the six different cyclometalated ruthenium complexes in DSSCs. Using an ionic liquid based electrolyte, efficiencies up to η = 3.06% have been attained under 1 sun irradiation. Moreover, the T66 based DSSC exhibited a good stability under 1000 W m² light soaking at 60 °C for 24 days, retaining 92.8% of its initial efficiency.

Full text of DOI:10.1039/c2dt30482a

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PAPER
Photophysical properties of new cyclometalated ruthenium complexes and
their use in dye sensitized solar cells†
Hawraa Kisserwan, Amanda Kamar, Tharallah Shoker and Tarek H. Ghaddar*
Received 28th February 2012, Accepted 22nd June 2012
DOI: 10.1039/c2dt30482a
A new class of cyclometalated ruthenium complexes, Ru(C^N^N′)(N^N′^N′′)·Cl where N^N′^N′′ =
4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine and C^N^N′ = substituted 6-phenyl-2,2′-bipyridine, for Dye
Sensitized Solar Cells (DSSCs) is proposed. We have investigated the effect of different substituents
(R = COOH, thiophen-2-yl, F and OCH3) on the ancillary C^N^N′ ligand on the photophysical properties
and performance of the six different cyclometalated ruthenium complexes in DSSCs. Using an ionic
liquid based electrolyte, efficiencies up to η = 3.06% have been attained under 1 sun irradiation.
Moreover, the T66 based DSSC exhibited a good stability under 1000 W m2 light soaking at 60 °C for
24 days, retaining 92.8% of its initial efficiency.
with low to mid molar absorption coefficients in conjunction
Introduction
with thick titania films (>10 μm). Recently, a new class of cyclo-
metalated ruthenium complexes that lack the SCN− ligand and
The dye sensitized solar cell (DSSC) is a type of solar cell
device that attracted the attention of many scientists since the
pioneer work of Grätzel and O’Regan in 1991.1 A DSSC is
based on the sensitization of titania, a wide band-gap semi-
conductor, with either organic or inorganic dyes which results in
the fabrication of an efficient and low cost solar cell. Recently,
DSSC’s efficiencies have reached 12.3% under AM 1.5 sunlight
irradiation, using a zinc–porphyrin complex as a sensitizer and a
cobalt-based electrolyte system.2 However, two main challenges
that are still facing the DSSC field are the long term stability of
the DSSC device under normal operating conditions, and the
DSSC’s efficiency being at the low side (lower than 15%).
Many research groups have been trying to tackle those chal-
lenges by optimizing the different components of the DSSC,
such as the titanium dioxide film, redox electrolyte, counter elec-
trode and most importantly the sensitizer dye structure.3 We are
one of these research groups who have been working on engin-
eering new ruthenium based dyes that address the DSSC’s chal-
lenges. Designing ruthenium based dyes that lack the
thiocyanate ligand (SCN−), which is considered from long-term
chemical stability tests as the weakest part in most ruthenium-
based dyes,4,5 is of great interest. In addition, red-shifting the
absorption band of the sensitizer and increasing the molar
absorption extinction coefficient in the visible and near-IR
region may have positive effects on the DSSCs’ efficiencies. As
such, thinner titania films can be used in a DSSC that harvest
most of the visible light with diminished electron recombination
processes than the conventional DSSCs that incorporate dyes
have better light harvesting properties than N719 has been intro-
duced by different research groups.4–12 The high interest in this
class of ruthenium based dyes is due to their encouraging long
term stability measurements and extended absorption in the
visible region (down to 800 nm).10
In the present study, we report the synthesis, electronic,
optical, and sensitizing properties of new cyclometalated dyes,
T65 through T71, Scheme 1. These ruthenium complexes bear a
(tctpy = 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine) moiety as the
anchoring ligand to titanium dioxide and a C^N^N′ moiety as
the ancillary ligand with different substituents on the three aro-
matic rings. The selection of the substituents in these dyes was
based on the following reasons: (a) the introduction of a thienyl
group on the middle pyridyl ring would increase the absorption
extinction coefficient of the dye in the visible and near-IR
region, as shown by many research groups,8,10,13,14 due to the
increase in conjugation, and the large radial extension in
bonding because of the presence of the electron rich sulfur
hetero-atom. (b) The carboxylic acid group on the terminal
pyridyl ligand may act as an additional anchoring group to the
TiO2 film, in addition to a neutralization effect of the positive
charge present on the ruthenium center. (c) The meta-fluorine
and the para-methoxy substituents on the phenyl ring, with
respect to the ruthenium center, were selected in order to tune
the redox potential of the ruthenium complexes.
Results and discussion
Department of Chemistry, American University of Beirut, Beirut
11-0236, Lebanon. E-mail: tarek.ghaddar@aub.edu.lb
†Electronic supplementary information (ESI) available: H-NMR spectra
of L65–L71 and T65–T71 are shown. See DOI: 10.1039/c2dt30482a
The successful synthesis of the six different C(H)^N^N′ ligands
(L65 through L71) was accomplished by multi-step reactions
following Krönke15 methodology, Scheme 1. The complexes
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Scheme 1 Synthesis of L65 through L71 and T65 through T71. Reaction conditions: (a) Piperidinium acetate, methanol, reflux 2 h for R =
COOCH3; NaOH, methanol–water 1 : 1, RT, 2 h for R = H (b) NH4OAc, ethanol, reflux 4 h; (c) (4,4′,4′′-trimethoxycarbonyl-2,2′:6′,2′′-terpyridine)-
RuCl3, 5 : 2 : 1 ethanol–N-methylmorpholine–H2O, reflux 48 h, acidification then tetra-butyl ammonium hydroxide (TBAOH). (d) KMnO4, NaOH,
water, reflux 18 h, acidification then TBAOH.
T65 through T71 were synthesized in a one-pot reaction involv-
ing the respective ligand (L65 through L71), Ru(trimethoxy-
carbonylterpy)Cl3 and N-methylmorpholine in an ethanol–water
solvent mixture. In this one-pot reaction the N-methylmorpholine
plays the role of a reductant and base, where the ruthenium
center gets reduces to RuII during the reaction and the ester
groups on the N^N′^N′′ get hydrolyzed to the carboxylate form.
Purification of the different complexes was accomplished using
preparative C-18 column with methanol–water solvent mixture
as the eluent. The six different cyclometalated complexes (T65
the visible region two intense absorption peaks around
410–422 nm (ε = 1.35–1.90 × 104 M−1 cm−1) and at
520–554 nm (ε = 1.35–1.86 × 104 M−1 cm−1) with shoulders up
to 682 nm such in the case of T66, Fig. 1A. Fig. 1B shows the
light harvesting efficiencies (LHE) of the different dyes when
anchored on 4 μm thick TiO2 films. A close look at the absorp-
tion data of the different complexes shows an interesting effect
of substituting the C^N^N′ at the 4 position with a carboxylic
acid group, such as in the case of the different complexes’ pairs
T65–T66 and T70–T71. As can be seen from Table 1, the
absorption extinction coefficients of the complexes with a car-
boxylic acid moiety at the 4 position (T66 and T71) are higher
than their analogs (T65 and T70, respectively) that lack the car-
boxylic acid substituent by Δε = 2.0–4.6 × 103 M−1 cm−1. In
order to gain insight into the electronic structure of the different
dyes and to correctly assign the origin of the electronic tran-
sitions in the visible region, we performed DFT and TD-DFT
calculations using the B3LYP/6-31G* exchange correlation func-
tional with the Los Alamos effective core potential LanL2DZ16
as implemented in the Gaussian03 program package.17 The geo-
metries of the complexes (T65 through T71) were optimized in
water using the C-PCM algorithm.18 As an example, the frontier
1
through T71) were characterized by H-NMR, APPI-MS, differ-
ential pulse voltammetry, steady state and lifetime emission and
UV/Vis spectroscopy.
The 1H-NMR spectra (Experimental section and ESI†)
confirmed the cyclometallation by the appearance of a doublet
signal between 5.6 to 6.1 ppm for the different ruthenium com-
plexes, which is a characteristic signal for the ortho hydrogen
1
atom to the ruthenium center. In addition, the H-NMR of the
cyclometalated confirmed the sufficient purity of the above com-
plexes to be used as dyes in a DSSC.
The UV/Vis data for T65 through T71 are depicted in Table 1.
The UV/Vis absorption spectra of complexes in ethanol show in
10644 | Dalton Trans., 2012, 41, 10643–10651
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Table 1 Measured and calculated spectroscopic data for T65 through T71
λabs, nm
λem, nm
TD-DFT excitation
energies (nm)
Oscillator
strengthb
(ε, 104 M−1 cm−1 a
)
(τem, ns)a
Assignment
T65
T66
T67
418 (1.44), 524 (1.50),
570 (sh), 664 (sh)
810 (8.0)
801 (14.5)
799 (13.6)
532.5
501.1
524.7
502.6
535.0
520.2
526.5
0.1054
0.0547
0.1044
0.2137
0.0839
0.1406
0.1010
H − 1 to L + 1 (94%)
H − 2 to L + 1 (53%), H to L + 1(21%), H − 1 to L (9%),
H to L + 2 (6%),
H − 1 to L + 2 (88%), H-1 to L + 1 (7%)
418 (1.90), 530 (1.86),
574 (sh), 682 (sh)
H − 2 to L + 1 (34%), H to L + 2 (32%) H − 1 to L (15%),
H − 2 to L + 2 (11%)
H − 1 to L + 1 (90%), H to L + 3 (5%)
422 (1.39), 534 (1.45),
664 (sh)
H − 1 to L (41%), H to L + 1 (26%) H − 2 to L + 1 (11%),
H to L + 4 (7%)
H − 1 to L + 1 (94%)
T68
T70
410 (1.35), 518 (1.35),
556 (sh), 650 (sh)
791 (1.1)
803 (8.7)
500.6
521.2
0.0790
0.1005
H − 2 to L + 1 (48%), H to L + 1 (25%) H − 1 to L (14%)
H − 1 to L + 1 (94%)
410 (1.46), 520 (1.35),
568 (sh), 654 (sh)
491.3
514.7
494.6
0.1506
0.0991
0.1859
H to L + 1 (+5%), H − 1 to L (28%) H − 2 to L + 2 (11%),
H − 1 to L + 3 (6%) H to L + 4 (5%)
H − 1 to L + 2 (93%)
T71
410 (1.77), 522 (1.60),
570 (sh), 660 (sh)
812 (9.3)
H to L + 2 (40%), H − 2 to L + 1 (32%) H − 1 to L (19%)
a Measured in basic aerated methanol and λex = 510 nm. b Only bands with oscillator strength f ≥ 0.05 are listed.
orbitals of T65 and T66 are shown in Scheme 2, and the
TD-DFT excitation transitions in the visible regions for the six
complexes are shown in Table 1. The HOMO, HOMO − 1 and
HOMO − 2 of all of the complexes have a ruthenium t2g charac-
ter in addition to some contributions from the N^N′^N′′ and
C^N^N′ ligands. The LUMO and LUMO + 1 of the complexes
that lack the carboxylic acid substituent at the 4 position of the
C^N^N′ ligand (T65, T67, T68 and T70) are π* orbitals loca-
lized on the N^N′^N′′ ligand. However, the LUMO + 1 for the
complexes with the carboxylic acid substituent at the 4 position
of the C^N^N′ ligand (T66 and T71) is a π* orbital localized on
the C^N^N′ ligand, and the LUMO and LUMO + 2 are π* orbi-
tals localized on the N^N′^N′′ ligand. The computed ground-
state vertical excitation energies with oscillator strength (f)
greater than 0.05 are shown in Table 1. As can be seen, the pre-
dicted vertical excitations to the red of 500 nm are mainly
MLCT transitions with small contributions from intra-ligand
LLCT transitions. However, a close look at these transitions
reveals an interesting finding, where T66 and T71 lack a HOMO
to LUMO + 1 transition and instead a major contribution from
the HOMO to LUMO + 2 is found. Moreover, the calculated
oscillator strength for the transitions involving the HOMO to
LUMO + 2 contributions in T66 and T71 are higher than that of
T65 and T70, respectively. This finding might explain the above
mentioned experimental results that a carboxylic acid substituent
at the 4 position of the C^N^N′ ligand causes an increase in the
molar extinction coefficient of the absorption bands in the
visible region.
σ-donating C^N^N′ ligand among these three complexes, while
in the case of T67 the σ-donation of the C^N^N′ ligand is the
highest. It is well known that strong σ-donating ligands in
general raise the 3MC states of bis-tridentate ruthenium com-
plexes which results in an increase in the energy gap between
3
3
the MC and MLCT excited states.19–22 As a consequence, the
efficiency of the thermally activated crossing processes from the
3MLCT state to the 3MC state decreases, which in turns prolongs
the emission lifetime, such as in the case of T67. However,
when comparing the different complexes’ pairs T65–T66 and
T70–T71, an introduction of the electron-withdrawing carboxyl
group at the 4 position of the C^N^N′ results in a stabilization of
the N^N′^N′′ LUMO π* ligand orbitals more than the HOMO
π(t2g) metal orbital (Scheme 2). Therefore, the energy gap
3
3
between the MC and MLCT excited states increase and thus
longer emission lifetimes are seen such as in the case of T66 and
T71 when compared to T65 and T70, respectively.23,24
The electrochemical properties of the different ruthenium
complexes (T65 through T71) were evaluated by differential
pulse voltammetry in CH3CN with 0.1 TBAPF6 at a scan rate =
100 mV s−1. In order to overcome solubility difficulties of the
complexes in CH3CN and to imitate the working environment in
a DSSC, a TiO2 film stained with the respective ruthenium
complex was used as the working electrode.25 The RuII/III redox
potentials of the ruthenium complexes adsorbed on TiO2 films
were measured to be between 0.87 and 0.96 V vs. NHE depend-
ing on the substituents present on the C^N^N′ ligand. As
expected, T68 had the highest oxidation potential due to the
presence of the fluorine substituent, whereas, T67 had the lowest
oxidation potential due to the electron donating effect of the
methoxy group on the ruthenium center. It is worth to mention
here that from previous work on T66 the redox potential was
evaluated to be E1/2 = 0.65 V vs. NHE in CH3CN solution,8
All of the complexes showed weak emission around 800 nm
with emission lifetimes ranging between 1.1 and 15.3 ns,
however, among the different complexes T68 had the shortest
emission lifetime τem = 1.1 ns. By comparing the closely related
complexes (T65, T67 and T68), T68 owns the weakest
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Fig. 1 (A) Absorption spectra of T65 through T71 in ethanol. (B)
Light harvesting efficiency (LHE) for T65 through T71 when anchored
on 4 μm TiO2 films.
Scheme 2 Frontier Molecular Orbitals of T65 through T71 and the
isodensity plots for the HOMO − 1, HOMO, LUMO, LUMO + 1 and
LUMO + 2 of T65 and T66.
however, when anchored on TiO2 (present work) the redox
potential shifted to E1/2 = 0.89 V vs. NHE. Berlinguette et al.
reported high dependence of the RuII/III redox potentials on the
medium when conducting cyclic voltammetry measurements on
similar cyclometalated ruthenium complexes in different solvent
media.10 Moreover, the extent of protonation of the carboxylic
acid groups has a profound effect on the RuII/III redox potential
of cyclometalated ruthenium complexes as it was shown in a
different study by Berlinguette et al. who reported a 0.3 V shift
in the RuII/III redox potential in DMF towards less positive
values for a tris-bidentate cyclometalated ruthenium complex
(Bu4N)3[Ru(dcbpy)(dcbpyH)(C^N)]PF6 when compared to the
similarity between the effect of protonation of the carboxylate
groups with H+ and their coordination to Ti+4 sites in the TiO2
film. Moreover, the E0–0 values of the different dyes were esti-
mated to be in the range E0–0 = 1.7–1.8 eV from the intersection
of the corresponding normalized absorption and emission
spectra. The approximated oxidation potentials (Eox*) of the six
different complexes in their excited states fall between ∼−0.8
and −0.9 V vs. NHE, which are more negative than the conduc-
tion band level of nanocrystalline TiO2 (−0.5 V vs. NHE).
Therefore, upon photo excitation of these complexes fast elec-
tron injection into TiO2 is expected.
fully protonated form [Ru(dcbpyH2)2(C^N)]PF6 (dcbpyH2
=
4,4′-dicarboxy-2,2′-bipyridine and C^N = 2-(2,4-difluorophenyl)-
pyridine).26 Therefore, we rationalize the different measured E1/2
values for T66 in solution and when anchored on TiO2 to a
The photocurrent vs. voltage (IV) response of cells made with
the different cyclometalated complexes (T65 through T71) and
N719 as a reference are shown in Fig. 2. The non-volatile
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Fig. 3 Impedance spectra of T65 to T68 and N719 DSSCs as Nyquist
plots measured at an applied potential of −0.6 V under dark conditions.
(Inset) the used electrochemical circuit model.
respectively. In order to understand the reasons behind the four
different (T65 to T68), yet similar, dyes’ performance in the
studied DSSCs, we measured the incident photon to electron
conversion efficiency (IPCE) and the electrochemical impedance
spectra (EIS) of these cells.
Fig. 2 (A) Photocurrent–voltage (IV) curves of T65 through T71.
Measured under 100 mW cm−2 simulated AM1.5 spectrum with an
active area = 0.126 cm2 and 0.6 M DMPII, 0.05 M LiI, 0.5 M TBP,
0.1 M GuSCN and 0.03 M I2 in MPN as the electrolyte. (B) Dark
currents of the corresponding cells.
Fig. 3 shows the typical EIS Nyquist plots measured under
dark conditions at a forward bias of −0.6 V. The equivalent
circuit used to fit the experimental data is shown in Fig. 3 as an
inset. Rs is the series resistance due to the transport resistance of
the FTO and the electrolyte. Cμ and Rct are the chemical capaci-
tance and the charge recombination resistance at the TiO2/
electrolyte interface, respectively. Whereas, CPt and RPt are
the interface capacitance and charge-transport resistance at the
platinum/electrolyte interface, respectively. The larger semicircle
at mid frequencies represents the interfacial charge recombina-
tion resistance (Rct) at the dyed TiO2/electrolyte interface. From
the fitted Rct and Cμ values, we calculated similar values for the
T65, T66 and T68 cells (Rct = 41, 39 and 41 Ω cm−2 and Cμ =
7.9, 7.8 and 7.3 × 10−4 F cm−2 for T65, T66 and T68, respect-
ively), which suggests that the electron recombination rates are
very similar in these three cells. However, in the case of T67, a
smaller Rct value (Rct = 34 Ω cm−2 for T67) and a higher Cμ
value (Cμ = 8.2 × 10−4 F cm−2) than T65 were calculated.
Therefore, we speculate that the reason behind the smaller Voc
value measured in the T67 cell is mainly due to an acceleration
in the electron recombination processes (Rct: T68–T65–T66 >
T67). Such a finding was not of a surprise to us, since we always
hypothesized in previous studies that the presence of an electron-
rich sp3-hybridized hetero-atom, such as oxygen in the case of
T67, accelerates the electron recombination processes at the
TiO2/dye/electrolyte interface most probably due to iodine
binding to such hetero-atoms.27,28
Table 2 Redox potentials and DSSCs’ performance of T65 through
T71 and N719 dyes
Eredoxa (V)
Jsc (mA cm−2
)
Voc (mV)
FF
ηb (%)
T65
T66
T67
T68
T70
T71
N719
0.90
0.89
0.87
0.96
0.93
0.94
1.12
8.8
9.3
8.9
8.8
7.8
4.6
12.1
535
535
528
555
525
539
607
0.61
0.61
0.63
0.63
0.64
0.61
0.67
2.88
3.05
2.95
3.06
2.62
1.50
4.93
a Redox potentials of the dyes on TiO2 were measured in CH3CN with
0.1 M TBAPF6 at a scan rate of 100 mV s−1 vs. NHE. b Measured under
100 mW cm−2 simulated AM1.5 spectrum with an active area =
0.126 cm2. Electrolyte: 0.6 M DMPII, 0.05 M LiI, 0.5 M TBP, 0.1 M
GuSCN and 0.03 M I2 in MPN.
iodide/triiodide electrolyte used (see Table 2) was one which
gives a lower voltage but promotes high quantum efficiency of
electron injection in low volatility solvent mainly designed for
N719. As can be seen from the data shown in Table 2, DSSCs
made with T65 to T68 showed similar efficiencies, η = 2.88,
3.05, 2.95 and 3.06%, respectively. Whereas, T70 and T71
showed lower DSSCs’ efficiencies at η = 2.62 and 1.50%,
Comparing the similar pair of dyes (T70 and T71), T70
showed better performance as a DSSC dye (η = 2.62 and 1.50%
for T70 and T71, respectively). We speculate that the extra
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Fig. 4 Relative IPCE spectra of T65 through T71 with (0.6 M DMPII,
0.05 M LiI, 0.5 M TBP, 0.1 M GuSCN and 0.03 M I2 in MPN) as the
electrolyte.
Fig. 5 Evolution of the photocurrent–voltage (IV) curves of T66
during visible-light soaking (100 mW cm−2 simulated AM1.5) at 60 °C.
Electrolyte: 0.5 M N-methylbenzimidazole, 0.6 M DMPII, 0.05 M LiI
and 0.05 M I2 in MPN.
carboxylic acid moieties on the C^N^N′ ligand in these two dyes
alter the dyes’ adsorption profiles on the TiO2 film which might
influence the electron injection and electron recombination rates
to and from the TiO2 film, respectively. This former effect could
be profound especially in the T71 case where the position of the
two carboxylic acid moieties at the 4 and 4′ position of the
C^N^N′ ligand act as strong anchoring motifs to the TiO2 film,
thus rendering the efficiency of electron injection from the
LUMO which resides on the N^N′^N′′ ligand. Such a phenom-
enon would explain the lower photocurrents seen for T70 and
T71 (Jsc = 7.8 and 4.6 for T70 and T71, respectively) and the
lower IPCE’s when compared to T65, Fig. 4. In addition, the
differences between the IPCE spectra (Fig. 4) and the LHE
spectra (Fig. 1B) for T70 and T71 also suggest that these two
dyes have lower electron injection yields than T65 through T68.
Finally, the lower performance of the different cyclometalated
complexes when compared to N719 is mainly attributed to the
positive charge that is intrinsic of these complexes. Such a
charge is expected to accelerate charge recombination processes
at the TiO2/electrolyte interface as evident in the measured lower
Rct values of these complexes than that of N719 (Rct = 92 Ω
cm−2 for N719), Fig. 3.
η = 4.01% to 3.44% on days 1 and 24, respectively (Jsc = 9.7
and 8.2 mA cm−2 and Voc = 640 and 650 mV on days 1 and 24,
respectively). Such a result, suggests that the use of cyclometa-
lated ruthenium complexes that lack labile mono-dentate ligands
is a good strategy for engineering dye complexes that perform
well under prolonged operational environment.
Conclusions
In conclusion, six different cyclometalated ruthenium com-
plexes, T65 through T71, have been synthesized and the effect
of different substituents (R = COOH, thiophen-2-yl, F and
OCH3) on the ancillary C^N^N′ ligand has been investigated on
the photophysical properties and performance of these com-
plexes in DSSCs. In the case of the T65 through T68 dyes,
where the C^N^N′ is a 6′-phenyl-4′-thiophen-2-yl-[2,2′]bi-
pyridinyl with different substituents on either the phenyl ring or
the 2′-pyridyl groups, similar solar cells’ efficiencies were attained
(η = 2.88, 3.05, 2.95 and 3.06%, respectively). However, in the
case of the two dyes T70 and T71 where the ancillary C^N^N′
ligand is
a 6-phenyl-2,2′-bipyridine-4-carboxylic acid and
In order to test the robustness of such cyclometalated com-
plexes under light soaking, we performed a preliminary long-
term stability test on a T66 DSSC incorporating a low volatility
electrolyte (0.5 M N-methylbenzimidazole, 0.6 M DMPII,
0.05 M LiI and 0.05 M I2 in MPN). Fig. 5 shows the IV curves
of a T66 cell taken during a 24 days period while illuminated
with a 100 mW cm2 light intensity using a white LED source at
60 °C. Interestingly, the T66 DSSC’s efficiency dropped only by
7.2% on day 24 (η = 2.63 and 2.44% on days 1 and 24, respect-
ively), mainly due to a drop in the short-circuit current (Jsc = 7.0
and 6.3 mA cm−2 on days 1 and 24, respectively) while the
photo-voltage increased by 20 mV (Voc = 589 and 609 mV on
days 1 and 24, respectively). N719 based cells studied under
the same conditions showed a 14.3% drop in the efficiency from
6-phenyl-2,2′-bipyridine-4,4′-dicarboxylic acid, respectively,
lower efficiencies were measured (η = 2.62 and 1.50% for T70
and T71, respectively). The lower efficiencies of these two dyes
could be attributed to less efficient electron injection rates than
the T65 to T68 dyes. Moreover, T66 based DSSC exhibited a
good stability under 1000 W m2 light soaking at 60 °C for
24 days, retaining 92.8% of its initial efficiency. Finally, the
lower efficiencies of the six studied cyclometalated complexes is
attributed to their intrinsic positive charge that might accelerate
the electron recombination processes in the DSSC and/or to
inefficient dye regeneration by the electrolyte when compared to
N719. We are currently engineering new cyclometalated com-
plexes that resemble T66 but with long alkyl-chains, which we
expect to show retardation in the electron recombination
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processes. In addition, we will be investigating these, to be made
complexes, with different electrolyte systems that may cause
faster dye regeneration than the iodide/triiodide one.
using the doctor blading method and then heated to 480 °C for
30 min. This afforded an 8 μm thick TiO2 film, on top of which
a 4 μm TiO2 scattering layer (400 nm TiO2 particles) was depos-
ited and reheated at 480 °C for 30 min. A TiCl4 post-treatment
was applied to the films following reported procedures in the lit-
erature.32 These films were then reheated at 480 °C for 30 min.
Dyeing was accomplished using 0.3 mM solutions in ethyl
alcohol for all of the studied dyes. The counter electrodes were
fabricated by applying a 2–3 μl cm−2 of 5 mM H2PtCl6 in
2-propanol to the FTO glass, followed by heating in an oven at
400 °C for 20 min. Cells assemblies were formed by sealing the
counter electrodes to the TiO2 electrode with a 60 μm Surlyn
(Dupont) spacer at ∼100 °C for 3 min. The corresponding elec-
trolyte was introduced through two small holes, previously
drilled through the counter electrode, which were then sealed
with Surlyn. The electrolyte used was as follows: methoxy-
propionitrile (MPN), 0.6 M 1,2-dimethyl-3-propylimidazolium
iodide (DMPII), 0.1 M guanadinium thiocyanate (GuSCN), 0.05
M LiI, 0.5 M tert-butyl pyridine (TBP), and 0.03 M I2.
Experimental
Materials and instrumentation
All organic chemicals were purchased from Sigma-Aldrich and
used as supplied. C-18 resin was purchased from YMC (Japan).
The N719 dye was purchased from Solaronix (Switzerland).
FTO glass “Tec15” was purchased from Pilkington (USA). TiO2
colloids were purchased from Dyesol (Australia). The different
di-aryl propenones,8 the different 1-acetylaryl pyridinium
iodides,29
(4,4′,4′′-trimethoxycarbonyl-2,2′:6′,2′′-terpyridine)-
RuCl3,30 and methyl 2-acetyl isonicotinate31 were prepared
according to reported procedures in the literature. The NMR
spectra (1H and 13C) were measured on a Bruker AM 300 MHz
spectrometer. UV-Vis spectra were recorded on a Jasco V-570
UV/Vis/NIR. Steady state and lifetime emission spectra were
measured on a JobinYvon Horiba Fluorolog-3 spectrofluoro-
meter. The electrochemical setup consisted of a three-electrode
cell, with a stained TiO2 film on FTO with the respective dye as
the working electrode, a Pt wire ∼1 mm diameter as the counter
electrode, and Ag/Ag+ (10 mM AgNO3) as the reference elec-
trode. All solution phase electrochemical measurements were
done in 0.1 M tetra-butyl ammonium hexafluorophosphate
(TBAPF6) in acetonitrile, and Fc/Fc+ standard (0.63 vs. NHE in
acetonitrile) was used as a reference. Electrochemical impedance
spectra of the DSSCs were performed with a CH Instruments
760B (USA). The obtained impedance spectra were fitted with
the Z-view software (v2.8b, Scribner Associates Inc.), and the
reported fitted parameters are within a 2% error. The spectra
were performed at a forward bias voltage (−0.6 V) in the fre-
quency range 0.1 Hz–105 Hz with oscillation potential ampli-
tudes of 10 mV at RT under dark conditions. The photo-anode
was connected to the working electrode. The Pt based cathode
was connected to the auxiliary electrode and the reference elec-
trode. IPCE spectra were recorded using a Newport 74 000
Cornerstone™ monochromator and a solar simulator illuminated
by a Xenon arc lamp (Oriel) through an AM1.5 simulation filter
(ScienceTech). Photocurrent vs. Voltage characteristics were
measured with a Keithley 2400 sourcemeter. The irradiated area
of the cell was 0.126 cm2.
Preparation of L65, L66, L67, L68, L70a and L71a
The di-aryl propenone (10 mmol) and the corresponding 1-acetyl-
aryl pyridinium iodide (10 mmol) were added to a solution of
excess ammonium acetate (4 g) in methanol (20 ml). The result-
ing solution was then refluxed for 4 h. The precipitate formed
upon cooling was collected by filtration, washed with cold
methanol and air dried. This afforded the different ligands in
quantitative amounts (Yield: 70–84%).
6-Phenyl-4-thiophen-2-yl-[2,2′]bipyridine,
L65. 1H-NMR
(CDCl3, 300 MHz), δ (ppm): 7.18 (dd, 1H, J1 = 5.1 Hz, J2 =
3.9 Hz), 7.35–7.37 (m, 1H), 7.44–7.54 (m, 4H), 7.7 (dd, 1H,
J1 = 3.6 Hz, J2 = 1.2 Hz), 7.86 (td, 1H, J1 = 6.0 Hz, J2
=
1.5 Hz), 7.94 (d, 1H, J = 1.8 Hz), 8.17 (m, 2H), 8.62 (d, 1H, J =
1.5 Hz), 8.67–8.72 (m, 1H), 8.74 (m, 1H). 13C-NMR (CDCl3,
75 MHz), δ (ppm): 115.6, 116.8, 121.5, 123.9, 125.6, 127.0,
127.1, 128.4, 128.7, 129.2, 136.9, 139.2, 141.9, 143.2, 149.1,
156.1, 156.4, 157.3. MS (APPI): m/z = 315.1 [M + H+]+ calcu-
lated for C20H15N2S, 315.4.
6′-Phenyl-4′-thiophen-2-yl-[2,2′]bipyridine-4-carboxylic acid,
L66. 1H-NMR (CDCl3, 300 MHz), δ (ppm): 4.02 (s, 3H), 7.19
(dd, 1H, J1 = 4.8 Hz, J2 = 3.9 Hz), 7.45–7.56 (m, 4H), 7.71 (dd,
1H, J1 = 3.6 Hz, J2 = 0.9 Hz), 7.89 (dd, 1H, J1 = 4.8 Hz, J2 =
1.5 Hz), 7.97 (d, 1H, J = 1.5 Hz), 8.20 (m, 2H), 8.63 (d, J =
1.8 Hz), 8.86 (dd, 1H, J1 = 5.1 Hz, J2 = 0.9 Hz), 9.14 (m, 1H).
13C-NMR (CDCl3, 75 MHz), δ (ppm): 52.8, 115.9, 117.2,
120.8, 122.9, 125.7, 127.1, 128.4, 128.8, 129.3, 138.4, 139.1,
141.7, 143.3, 149.8, 155.7, 157.3, 157.6, 165.9. MS (APPI): m/z
= 373.1 [M + H+]+ calculated for C22H17N2O2S, 373.4.
Computational methods
Calculations were carried out using Gaussian 03.17 Geometries
were optimized using the 6-31G* basis set with (B3LYP),
together with the Los Alamos effective core potential LanL2DZ16
in water (C-PCM algorithm).18 TD-DFT calculations were per-
formed using the C-PCM with water as the solvent. Fifty singlet
excited states were determined from the optimized structures.
6-(3-Methoxy-phenyl)-4-thiophen-2-yl-[2,2′]bipyridine,
L67. 1H-NMR (CDCl3, 300 MHz), δ (ppm): 3.94 (s, 3H), 7.02
(dd, 1H, J1 = 2.7 Hz, J2 = 0.9 Hz), 7.18 (dd, 1H, J1 = 5.1 Hz,
J2 = 3.6 Hz), 7.35–7.46 (m, 1H), 7.44 (m, 2H), 7.70 (m, 3H),
7.86 (td, 1H, J1 = 7.5 Hz, J2 = 1.5 Hz), 7.93 (d, 1H, J = 1.5 Hz),
8.62 (d, 1H, J = 1.5 Hz), 8.66 (d, 1H, J = 8.4 Hz), 8.72–8.74
(m, 1H). 13C-NMR (CDCl3, 75 MHz), δ (ppm): 55.4, 112.8,
114.5, 115.8, 116.9, 119.5, 121.5, 123.9, 125.6, 127.0, 128.4,
Solar cell fabrication
Dye sensitized solar cells were fabricated using standard pro-
cedures. The TiO2 films were made from colloidal solutions
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 10643–10651 | 10649

View Article Online
129.8, 136.9, 140.8, 141.8, 143.2, 149.1, 156.1, 156.4, 157.1,
160.0. MS (APPI): m/z = 345.0 [M + H+]+ calculated for
C21H17N2OS, 345.4.
mixture was then refluxed overnight. After that, the reaction was
left to cool to RT then filtered over Cellite to get rid of the
MnO2. The pH of the filtrate was adjusted to 3.5 by the addition
of 2 M HCl. The precipitate formed was filtered and washed
with water. Yield (0.68 g, 38%). 1H-NMR (DMSO-d6,
6-(4-Fluoro-phenyl)-4-thiophen-2-yl-[2,2′]bipyridine, L68. 1H-NMR
(CDCl3, 300 MHz), δ (ppm): 7.17–7.21 (m, 3H), 7.34–7.38 (m,
1H), 7.45 (d, 1H, J = 5.1), 7.71 (d, 1H, J = 3 Hz), 7.84–7.90 (m,
2H), 8.17 (m, 2H), 8.61–8.64 (m, 2H), 8.72 (d, 1H, J = 4.2 Hz).
13C-NMR (CDCl3, 75 MHz), δ (ppm): 115.6–115.8 (d), 116.4,
121.5, 124.0, 125.7, 127.1, 128.4, 128.8–128.9 (d), 135.4,
136.9, 141.7, 143.3, 149.1, 156.0, 156.3, 156.4, 162.1, 165.1.
MS (APPI): m/z = 333.0 [M + H+]+ calculated for C20H14FN2S,
333.4.
300 MHz) δ (ppm): 7.52–7.64 (m, 3H), 7.95 (dd, 1H, J1
=
4.8 Hz, J2 = 1.5 Hz), 8.25 (d, 2H, J = 7.2 Hz), 8.39 (s, 1H), 8.82
(s, 1H), 8.94–8.97 (m, 2H). 13C-NMR (DMSO-d6, 300 MHz)
δ (ppm): 118.3, 119.5, 119.8, 123.6, 126.8, 129.1, 129.8, 137.6,
139.6, 140.9, 150.7, 155.4, 155.5, 157.0, 166.0, 166.1. MS (APPI):
m/z = 319.2 [M − H+]− calculated for C18H11N2O4, 319.3.
Preparation of T65 through T71
4-Furan-2-yl-6-phenyl-[2,2′]bipyridine,
L70a. 1H-NMR
0.27 mmol of the corresponding ligand (L65 through L71) and
(4,4′,4′′-trimethoxycarbonyl-2,2′:6′,2′′-terpyridine)RuCl3 (0.16 g,
0.27 mmol) were dissolved in ethanol–H2O (5 : 1 v/v, 60 ml) fol-
lowed by the addition of N-methylmorpholine (20 ml). The
mixture was then refluxed under Ar for 48 h. The solvent was
then evaporated under reduced pressure. The obtained dark solid
was dissolved in water containing excess tetra-butyl ammonium
hydroxide and applied to a preparative C18-column. The com-
pound was purified by a gradient elution with water/methanol
100% : 0% to 60% : 40%. Concentrating the solvent that con-
tains the major band under reduced pressure and acidifying to
pH 3.0 with 0.1 M HCl resulted in a dark precipitate. The solid
was filtered to yield a dark violet crystalline solid, which was
dried under vacuum at 80 °C for 24 h. The mono-protonated
form of the different ruthenium complexes were obtained by
adding the corresponding mole equivalents of tetra-butyl
ammonium hydroxide (TBAOH) in methanol, stirred under N2
for 1 h and then dried under vacuum at 80 °C for 24 h.
(CDCl3, 300 MHz), δ (ppm): 6.56–6.58 (m, 1H), 7.06 (d, 1H,
J = 3.6 Hz), 7.32–7.36 (m, 1H), 7.47–7.59 (m, 4H), 7.85 (td,
1H, J1 = 7.5 Hz, J2 = 1.8 Hz), 8.05 (d, 1H, J = 1.5 Hz),
8.19–8.22 (dd, 2H, J1 = 7.2 Hz, J2 = 0.9 Hz), 8.65 (d, 1H, J =
1.5 Hz), 8.7 (d, 1H, J = 7.8 Hz), 8.73 (dd, 1H, J1 = 4.8 Hz, J2 =
0.9 Hz). 13C-NMR (CDCl3, 75 MHz), δ (ppm): 108.9, 112.2,
113.6, 114.4, 121.4, 123.9, 127.0, 128.7, 129.1, 136.9, 139.29,
139.32, 143.6, 149.0, 151.9, 156.19, 156.20, 157.2. MS (APPI):
m/z = 299.0 [M + H+]+ calculated for C20H15N2O, 299.3.
4′-Furan-2-yl-6′-phenyl-[2,2′]bipyridine-4-carboxylic
acid
methyl ester, L71a. 1H-NMR (DMSO-d6, 300 MHz) δ (ppm):
3.97 (s, 3H), 6.76 (dd, 1H, J1 = 3.3 Hz, J2 = 1.8 Hz), 7.51–7.63
(m, 4H), 7.94–7.97 (m, 2H), 8.26–8.32 (m, 3H), 8.63 (d, 1H,
J = 1.2 Hz), 8.95–8.98 (m, 2H). 13C-NMR (DMSO-d6,
300 MHz) δ (ppm): 52.9, 110.7, 112.7, 112.8, 114.7, 119.2,
123.1, 126.8, 128.9, 129.5, 138.2, 139.2, 145.1, 150.6, 150.6,
154.9, 156.2, 156.7, 165.2. MS (APPI): m/z = 357.3 [M + H+]+
calculated for C22H17N2O3, 357.4.
T65.2TBA. 1H-NMR (MeOD + 0.1% KOD, 300 MHz),
δ (ppm): 0.92 (t, 24H), 1.30 (m, 16H), 1.56 (m, 16H), 3.33 (m,
16H), 5.67 (dd, 1H, J1 = 7.5 Hz, J2 = 0.9 Hz), 6.52 (td, 1H, J1 =
6.0 Hz, J2 = 0.9 Hz), 6.77 (td, 1H, J1 = 6.0 Hz, J2 = 0.9 Hz),
7.14 (m, 1H), 7.36 (dd, 1H, J1 = 5.1 Hz, J2 = 3.9 Hz), 7.49–7.54
(m, 5H), 7.73 (dd, 1H, J1 = 5.1 Hz, J2 = 0.9 Hz), 7.92–7.95 (m,
2H), 8.12 (dd, 1H, J1 = 3.9 Hz, J2 = 0.9 Hz), 8.51 (d, 1H, J =
1.2 Hz), 8.73 (d, 1H, J = 8.1 Hz), 8.79 (d, 1H, J = 1.2 Hz), 8.96
(d, 2H, J = 0.6 Hz), 9.22 (s, 2H). MS (APPI): m/z = 1262.8
[M − Cl−]+ calculated for C70H94N7O6RuS, 1262.7.
T66.3TBA. 1H-NMR (MeOD + 0.1% KOD, 300 MHz),
δ (ppm): 0.96 (t, 36H), 1.42 (m, 24H), 1.64–1.69 (m, 24H), 3.33
(m, 24H), 5.64 (d, 1H, J = 7.2 Hz), 6.51 (t, 1H, J = 7.5 Hz),
6.77 (t, 1H, J = 7.2 Hz), 7.39 (dd, 1H, J1 = 4.8 Hz, J2 = 3.6 Hz),
7.52–7.62 (m, 6H), 7.74 (d, 1H, J = 4.5 Hz), 7.79 (d, 1H, J =
7.5 Hz), 8.14 (d, 1H, J = 2.7 Hz), 8.53 (s, 1H), 8.78 (s, 1H),
8.97 (s, 2H), 9.06 (s, 1H), 9.23 (s, 2H). MS (APPI): m/z =
1548.0 [M − Cl−]+ calculated for C87H129N8O8RuS, 1548.1.
Preparation of 6-phenyl-2,2′-bipyridine-4-carboxylic acid (L70)
In a round bottom flask, L70a (1.8 g, 6.1 mmol) and potassium
permanganate (1.16 g, 7.3 mmol) were dissolved in 100 ml
water. The pH of the solution was adjusted to 10.0 with the
addition of NaOH. The mixture was then refluxed for 2 h. After
that, the reaction was left to cool then filtered over Cellite to get
rid of the MnO2. The pH of the filtrate was adjusted to 3.5 by
the addition of 2 M HCl. The precipitate formed was filtered and
1
washed with water. Yield (0.67 g, 40%). H-NMR (DMSO-d6,
300 MHz), δ (ppm): 7.49–7.60 (m, 4H), 8.04 (td, 1H, J1
=
7.2 Hz, J2 = 1.8 Hz), 8.28 (dd, 2H, J1 = 8.4 Hz, J2 = 1.5 Hz),
8.35 (d, 1H, J = 1.5 Hz), 8.62 (d, 1H, J = 8.1 Hz), 8.75 (m, 1H),
8.79 (d, 1H, J = 1.5 Hz). 13C-NMR (DMSO-d6, 75 MHz),
δ (ppm): 118.1, 119.2, 120.8, 124.8, 126.8, 129.0, 129.7, 137.6,
137.7, 140.8, 149.5, 154.4, 156.1, 156.7, 166.2. MS (APPI): m/z
= 275.0 [M − H+]− calculated for C17H11N2O2, 275.2.
T67.2TBA. 1H-NMR (MeOD + 0.1% KOD, 300 MHz),
δ (ppm): 0.91 (t, 24 H), 1.30 (m, 16 H), 1.56 (m, 16 H), 2.74 (s,
3H), 3.33 (m, 16H), 5.95 (d, 1H, J = 7.8 Hz), 6.70 (t, 1H, J =
7.8 Hz), 7.01 (m, 1H), 7.16 (d, 1H, J = 3.9 Hz), 7.25 (dd, 1H,
J1 = 5.1 Hz, J2 = 3.9 Hz), 7.36 (d, 4H, J = 1.2 Hz), 7.59 (d, 1H,
J = 7.5 Hz), 7.61 (dd, 1H, J1 = 5.1 Hz, J2 = 1.2 Hz), 7.73 (td,
1H, J1 = 8.4 Hz, J2 = 1.5 Hz), 7.99 (dd, 1H, J1 = 4.8 Hz, J2 =
Preparation 6-phenyl-2,2′-bipyridine-4,4′-dicarboxylic acid (L71)
In a round bottom flask, L71a (2.0 g, 5.6 mmol) and sodium
hydroxide (0.3 g, 7.5 mmol) were refluxed for 4 h in 100 ml of
water. Potassium permanganate (1.0 g, 6.3 mmol) was then
added and the pH of the solution was adjusted to 10.0. The
10650 | Dalton Trans., 2012, 41, 10643–10651
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0.9 Hz), 8.41 (d, 1H, J = 1.5 Hz), 8.55 (d, 1H, J = 8.1 Hz), 8.61
(d, 1H, J = 1.2 Hz), 8.79 (s, 2H), 9.06 (s, 2H). MS (APPI): m/z
= 1292.8 [M − Cl−]+ calculated for C71H96N7O7RuS, 1292.7.
T68.2TBA. 1H-NMR (MeOD + 0.1% KOD, 300 MHz),
δ (ppm): 0.99 (t, 24H), 1.35 (m, 16 H), 1.61 (m, 16H), 3.24 (m,
16H), 5.35 (dd, 1H, J1 = 8.7 Hz, J2 = 2.4 Hz), 6.45 (td, 1H, J1 =
8.7 Hz, J2 = 2.4 Hz), 7.14 (t, 1H, J = 6.3 Hz), 7.35 (dd, 1H, J1 =
4.8 Hz, J2 = 3.6 Hz), 7.51 (m, 5H), 7.73 (dd, 1H, J1 = 5.1 Hz,
J2 = 0.9 Hz), 7.93 (td, 1H, J1 = 8.1 Hz, J2 = 0.9 Hz), 7.99 (dd,
1H, J1 = 8.7 Hz, J2 = 5.4 Hz), 8.12 (dd, 1H, J1 = 3.6 Hz, J2 =
0.9 Hz), 8.48 (d, 1H, J = 1.2 Hz), 8.73 (d, 1H, J = 8.1 Hz), 8.78
(d, 1H, J = 1.2 Hz), 8.97 (s, 2H), 9.23 (s, 2H). MS (APPI): m/z
= 1280.7 [M − Cl−]+ calculated for C70H93FN7O6RuS, 1280.7.
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T70.3TBA. 1H-NMR (MeOD + 0.1% KOD, 300 MHz),
δ (ppm): 0.99 (m, 36H), 1.36 (m, 24H), 1.65 (m, 24H), 3.35 (m,
24H), 5.60 (d, 1H, J = 7.8 Hz), 6.45 (t, 1H, J = 7.6 Hz), 6.73 (t,
1H, J = 7.5 Hz), 7.11 (t, 1H, J = 7.5 Hz), 7.42 (d, 3H, J =
5.7 Hz), 7.48 (dd, 2H, J1 = 6.0 Hz, J2 = 1.8 Hz), 7.84–7.93 (m,
2H), 8.59 (d, 1H, J = 8.1 Hz), 8.71 (s, 1H), 8.91 (d, 3H, J =
7.2 Hz), 9.19 (s, 2H). MS (APPI): m/z = 1222.8 [M − H+
−
TBA − Cl−]− calculated for C67H90N7O8Ru, 1222.6.
T71.4TBA. 1H NMR (MeOD + 0.1% KOD, 300 MHz)
δ (ppm): 0.99 (m, 48H), 1.36 (m, 32H), 1.65 (m, 32H), 3.35 (m,
32H), 5.62 (dd, 1H, J1 = 7.5 Hz, J2 = 0.9 Hz), 6.46 (td, 1H, J1 =
8.7 Hz, J2 = 0.9 Hz), 6.73 (td, 1H, J1 = 7.2 Hz, J2 = 1.2 Hz),
7.42–7.49 (m, 6H), 7.84 (d, 1H, J = 7.5 Hz), 8.71 (d, 1H, J =
0.9 Hz), 8.92 (d, 2H, J = 0.9 Hz), 8.96 (dd, 2H, J1 = 3.9 Hz,
J2 = 1.2 Hz), 9.19 (s, 2H). MS (APPI): m/z = 1508.1 [M − H+ −
TBA − Cl−]− calculated for C84H125N8O10Ru, 1508.0.
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Acknowledgements
This work was supported by the University Research Board
(URB) at the American University of Beirut (AUB), the
Lebanese National Council for Scientific Research (LNCSR),
and the Munib and Angela Masri Institute of Energy and Natural
Resources.
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