Heteroleptic ruthenium complexes containing uncommon 5,5′- disubstituted-2,2′-bipyridine chromophores for dye-sensitized solar cells

Article abstract of DOI:10.1039/c0dt01043j

Four new heteroleptic ruthenium sensitizers [Ru(4,4′-carboxylic acid-2,2′-bipyridine)(L)(NCS)₂] (L = 5,5′-bis(4- octylthiophen-2-yl)-2,2′-bipyridine (1), 5,5′-bis(N,N-diphenyl-4- aminophenyl)-2,2′-bipyridine (2), 5,5′-bis(5-(N,N-diphenyl-4- ami

Full text of DOI:10.1039/c0dt01043j

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www.rsc.org/dalton | Dalton Transactions
Heteroleptic ruthenium complexes containing uncommon 5,5¢-disubstituted-
2,2¢-bipyridine chromophores for dye-sensitized solar cells†
Feng-Rong Dai,a Wen-Jun Wu,b Qi-Wei Wang,a He Tian*b and Wai-Yeung Wong*a
Received 18th August 2010, Accepted 28th September 2010
DOI: 10.1039/c0dt01043j
Four new heteroleptic ruthenium sensitizers [Ru(4,4¢-carboxylic acid-2,2¢-bipyridine)(L)(NCS)2]
(L = 5,5¢-bis(4-octylthiophen-2-yl)-2,2¢-bipyridine (1), 5,5¢-bis(N,N-diphenyl-4-aminophenyl)-
2,2¢-bipyridine (2), 5,5¢-bis(5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2¢-bipyridine (3) and
5,5¢-bis(4-octyl-5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2¢-bipyridine (4)) were synthesized,
characterized by physicochemical and computational methods, and utilized as photosensitizers in
nanocrystalline dye-sensitized solar cells (DSSCs). The lmax of the metal-to-ligand charge transfer
(MLCT) absorption of these four ruthenium dyes (527 nm for 1, 535 nm for 2, 585 nm for 3 and 553 nm
for 4) can be tuned by various structural modifications of the ancillary ligand and it was shown that
increasing the conjugation length of such ligand reduces the energy as well as the molar absorption
coefficient of the MLCT band. The maximum incident photon to current conversion efficiency (IPCE)
of 41.4% at 550 nm, 38.6% at 480 nm, 39.4% at 470 nm and 31.1% at 480 nm for 1-, 2-, 3- and
4-sensitized solar cells were obtained. Respectable power conversion efficiencies of 3.00%, 2.51%, 2.00%
and 2.03% were realized, respectively, when the sensitizers 1, 2, 3 and 4 were used in DSSCs under the
standard air mass (AM) 1.5 sunlight illumination (versus 5.9% for standard N719).
engineer the structures of ruthenium complexes to broaden the
absorption range, increase the molar absorption coefficient and
improve the long-term stability. Since 4,4¢-dicarboxylic acid-2,2¢-
bipyridine (dcbpy) has been shown to be the most widely used an-
Introduction
There is a growing research interest in the design and synthesis
of functional materials for applications in molecular devices,
memory media and photovoltaic energy conversion.1–4 Much
attention has been focused on developing photovoltaic device
technology and searching for photovoltaic materials and related
device architectures.3–6 Since the first report of a dye-sensitized
solar cell (DSSC) by Gra¨tzel and O’Regan in 1991,7 DSSCs have
attracted considerable research interest due to their low cost, high
efficiency, flexible use of materials, and easy fabrication.8–14 The
best solar-to-electricity conversion efficiency of higher than 10%
was achieved by using ruthenium(II) polypyridyl sensitizers, such
as N3,15 N719,16 N907,17 and black dye18 under standard global
air mass (AM) 1.5 sunlight. It has been demonstrated that the
broad range and high molar absorption coefficient of the metal-
to-ligand charge transfer (MLCT) absorption in the visible to near-
IR region, as well as the appropriate localization of the frontier
orbitals of the ruthenium(II) polypyridyl sensitizers play important
roles in achieving higher photovoltaic performance for the energy
conversion. The scope and diversity of studies on these ruthenium-
based dyes in the realm of practical photovoltaic technology have
continued to expand. Attempts have been made to molecularly
choring ligand in such ruthenium(II) polypyridyl sensitizers,19 sev-
eral heteroleptic ruthenium(II)-polypyridyl sensitizers have been
recently reported using highly conjugated ancillary bipyridine
ligands, which were modified with light-harvesting chromophores
such as thiophene,20 furan,21 carbazole22 and thienothiophene
moieties,23 in place of one of the dcbpy anchoring ligands. To date,
the majority of work has proliferated exclusively to the use of 4,4¢-
disubstituted bipyridine derivatives as the ancillary ligands, while
research work of the corresponding 5,5¢-disubstituted bipyridine
congeners was not well elucidated and remains to be largely
studied.8,24 To advance the development of novel ruthenium-
based sensitizers to fill this gap, we have launched an initiative
to develop new synthetic routes towards the preparation of a new
class of 5,5¢-bifunctionalized bipyridine ligands that are capable
of affording another family of ruthenium polypyridyl sensitizer
dyes for DSSC applications. In fact, the starting precursor 5,5¢-
dibromo-2,2¢-bipyridine25,26 itself is synthetically more accessible
and hence less costly with a higher synthetic yield as compared
to the 4,4¢-dibromo-2,2¢-bipyridine.27,28 While most of the 4,4¢-
disubstituted-2,2¢-bipyridine ancillary ligands were prepared by
incorporating the conjugated chromophores and 4,4¢-dibromo-
2,2¢-bipyridine via appropriate coupling reactions for the aryl–aryl
bond formation, we can do the same to offer some interesting 5,5¢-
disubstituted-2,2¢-bipyridine counterparts in the present study.
In this paper, four new heteroleptic ruthenium(II)-polypyridyl
sensitizers, [Ru(dcppy)(L)(NCS)2] (L = 5,5¢-bis(4-octylthiophen-
2-yl)-2,2¢-bipyridine (1), 5,5¢-bis(N,N-diphenyl-4-aminophenyl)-
2,2¢-bipyridine (2), 5,5¢-bis(5-(N,N-diphenyl-4-aminophenyl)-
thiophen-2-yl)-2,2¢-bipyridine (3) and 5,5¢-bis(4-octyl-5-(N,N-
aInstitute of Molecular Functional Materials‡, Department of Chemistry
and Centre for Luminescence Materials, Hong Kong Baptist University,
Waterloo Road, Kowloon Tong, Hong Kong, P.R. China. E-mail: rwywong@
hkbu.edu.hk; Fax: +852 34117348
bKey Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science and Technology, Meilong Road 130,
Shanghai, 200237, P.R. China. E-mail: tianhe@ecust.edu.cn; Fax: +86 21-
64252288
† CCDC reference numbers 789721–789723. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt01043j
‡ Areas of Excellence Scheme, University Grants Committee (Hong
Kong).
diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2¢-bipyridine
(4))
2314 | Dalton Trans., 2011, 40, 2314–2323
This journal is The Royal Society of Chemistry 2011
©

Table 1 Crystallographic data for L1–L3
based on 5,5¢-disubstituted bipyridine ancillary ligands were
synthesized using a one-pot synthetic procedure. This is the
first time an ancillary ligand incorporating 5,5¢-disubstituted
bipyridine unit for ruthenium dyes is presented. Alkylthiophene
substituents on the bipyridine moiety are expected to increase
the p-conjugation length of the ancillary ligand and influence
the photophysical properties of the corresponding ruthenium
complexes. The triphenylamine is electron donating in nature
and can extend the p-electron delocalization and also facilitate
charge transfer.29–32 The photophysics and localization of the
frontier orbitals of these ruthenium complexes were studied,
and the performance of the DSSCs using these complexes as
photosensitizers was investigated.
Compound
L1
L2
L3
Formula
C34H44N2S2
544.83
C46H34N4
642.77
C54H38N4S2
807.00
Formula weight
Crystal system
Space group
Triclinic
Monoclinic Triclinic
¯
¯
P1
P1
C2/c
14.9729(15) 10.7447(10)
25.621(3) 11.8346(11)
11.2567(11) 17.2692(16)
˚
a/A
5.5379(4)
14.5782(11)
19.5515(14)
107.1120(10)
92.4750(10)
91.9740(10)
1505.32(19)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
90
95.596(2)
97.058(2)
108.418(2)
2045.9(3)
2
128.422(2)
90
3383.1(6)
4
3
˚
V/A
Z
T/K
173(2)
0.202
1.202
9521
173(2)
0.074
1.262
10474
4042
173(2)
0.175
1.310
12904
m(Mo-Ka)/mm-1
D(calcd)/g cm-3
Total no. of reflections
Unique reflections
[R(int)]
Results and discussion
6869 (0.0185)
9318 (0.0233)
Syntheses and characterization
(0.0299)
2535
Observed reflections
[I > 2s(I)]
5758
6845
The synthetic route for the preparation of the 5,5¢-disubstituted-
2,2¢-bipyridine ligands L1–L4 is illustrated in Scheme 1. 5,5¢-Bis(4-
octylthiophen-2-yl)-2,2¢-bipyridine (L1) and 5,5¢-bis(5-(N,N-
R1 (Fo)a
0.0495
0.1431
1.006
0.576, -0.781
0.0585
0.1609
1.031
0.341,
-0.222
0.0445
0.1148
1.015
0.408, -0.312
wR2 (Fo)b
GOF
Dr/e A
-3
diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2¢-bipyridine
(L3)
˚
were synthesized from 5,5¢-dibromo-2,2¢-bipyridine and
tributyl(4-octylthiophen-2-yl)stannane or N,N-diphenyl-4-(5-
(tributylstannyl)thiophen-2-yl)benzenamine via the Stille coupling
protocol. The Suzuki coupling reaction of 5,5¢-dibromo-2,2¢-
bipyridine with N,N-diphenyl-4-aminophenylboronic acid yielded
5,5¢-bis(N,N-diphenyl-4-aminophenyl)-2,2¢-bipyridine (L2). 5,5¢-
Bis(4-octyl-5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,
2¢-bipyridine (L4) was obtained through Suzuki coupling reaction
between N,N-diphenyl-4-aminophenylboronic acid and 5,5¢-
bis(4-octyl-5-bromo-thiophen-2-yl)-2,2¢-bipyridine, which was
prepared by bromination of L1 with NBS in CHCl3.
The heteroleptic ruthenium sensitizers 1–4 were obtained in a
typical one pot synthesis from the sequential reaction of ruthenium
dimer [RuCl2(p-cymene)]2 with each of the newly prepared 5,5¢-
disubstituted-2,2¢-bipyridine ligands, followed by the reaction of
the resulting mononuclear complex with 4,4¢-dicarboxylic acid-
2,2¢-bipyridine (dcbpy). The chloro complexes then reacted with
an excess of ammonium thiocyanate ligand to afford the target
heteroleptic ruthenium complexes 1–4. The products were purified
by chromatography on silica gel. The addition of long octyl chains
on the thiophene rings in 4 relative 3 can notably improve the
solubility of the dyes in organic solvents as well as to prevent the
complexes from water-induced desorption of the dye molecules
from the TiO2 surface.12
ꢀ
ꢀ
ꢀ
a R1
=
(|Fo|
-
|Fc|)/ |Fo|. b wR2
=
{
[w(Fo
-
Fc2)2]/
2
ꢀ
1/2
[w(Fo2)2]}
.
determined by single crystal X-ray diffraction. Crystallographic
data and structure refinement parameters of L1–L3 are summa-
rized in Table 1, and ORTEP drawings of L1, L2 and L3 are
1
depicted in Fig. 1–3, respectively. The H NMR spectra of 1–4
exhibit complicated multiple peaks, which indicate that the dcbpy
and 5,5¢-disubstituted-2,2¢-bipyridine ligands are magnetically
non-equivalent. The carboxylic acid protons of the ruthenium
complexes were observed at 9.59 and 9.24 ppm for 1, 9.60 and
9.26 ppm for 2, 9.72 and 8.58 ppm for 3, and 9.56 and 9.42 ppm
for 4, respectively, suggesting that the bipyridine ligand contains
carboxylic acid groups. FT-IR spectra of 1–4 in KBr pellet
show the characteristic intense signal band of the NCS group at
around 2100 cm-1, which can be attributed to the N-coordinated
isothiocyanate group, thus confirming the successful coordination
of the NCS moiety to the ruthenium(II) center.
Electronic absorption spectra
The electronic absorption and photoluminescence spectra of the
free ligands L1–L4 in DMSO : EtOH = 2 : 3 (v/v) solutions are
shown in Fig. 4. They show one or two intense absorption bands in
the UV-vis region, which can be assigned to the p → p* transitions.
All of the new organic ligands and their ruthenium complexes
1
were characterized by UV-vis, IR, and H and 13C NMR spec-
troscopy. The solid-state structures of L1, L2 and L3 were also
Fig. 1 ORTEP drawing of L1 with the atom-labeling scheme, showing 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity.
This journal is The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 2314–2323 | 2315
©

Scheme 1 Synthetic routes to L1–L4 and the ruthenium-based sensitizers 1–4.
The order of the absorption maximum for these metal-free ligands
also offer an additional source of visible light for excitation of the
MLCT transition of the corresponding ruthenium complexes.
Fig. 5 shows the UV-vis spectra of the ruthenium complexes 1–4
in DMSO : EtOH = 2 : 3 (v/v) solutions at 293 K, in addition to
that of the benchmark compound N719 used as a reference. The
electronic absorption spectral data are summarized in Table 2. The
ruthenium complexes show three or four characteristic absorption
bands caused by the ligand-centered (LC) electronic transitions
is L1 (358 nm) < L2 (383 nm) < L4 (399 nm) < L3 (423 nm), which
is partly a manifestation of the increasing conjugated length of the
ligands. Comparing L3 with L4, the addition of a large alkyl chain
on the thiophene ring would result in a less coplanar configuration
for the aromatic rings, making it less conjugated for L4 than L3.
The emission maximum of L1–L4 is centered at 419, 521, 568
and 565 nm, respectively. The emitted light from the ligand may
2316 | Dalton Trans., 2011, 40, 2314–2323
This journal is
The Royal Society of Chemistry 2011
©

Fig. 2 ORTEP drawing of L2 with the atom-labeling scheme, showing 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity.
Fig. 3 ORTEP drawing of L3 with the atom-labeling scheme, showing 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity.
Table 2 Absorption and electrochemical data of 1–4 and N719
The low-energy broad absorption bands centred at 527–585 nm
are characteristic of the MLCT transitions from the occupied 4d
orbitals of ruthenium to the lowest unoccupied p* orbitals of the
bipyridine ligands in the corresponding ruthenium complexes 1–
4. The lmax of 1–4 in the low-energy region is chemically tunable
by structural design and spans a wide absorption range from 527
to 585 nm (cf. 541 nm for N719). The absorption wavelength
order follows 1 (527 nm) < 2 (535 nm) < 4 (553 nm) < 3 (585
nm), which is in line with the increasing conjugation length of
the 5,5¢-disubstituted ancillary bipyridines L1–L4 (vide supra). In
contrast, the molar absorption coefficient for the MLCT band
of these ruthenium dyes displays the order 1 (9000 M-1 cm-1) > 2
(6400 M-1 cm-1) > 4 (5900 M-1 cm-1) > 3 (3200 M-1 cm-1), which are
smaller than that of N719 (18600 M-1 cm-1). These results indicate
that increasing the conjugation length of the ancillary ligand can
lower the MLCT energy, but also decrease the absorption intensity
of the MLCT transition.
HOMO/ LUMO/
l
max/nm (e/104 M-1 cm-1)a
eVb
eVc
Eg/eVd
1
2
3
310 (3.85), 383 (5.55), 527 (0.90) -5.33
305 (5.94), 431 (4.25), 535 (0.64) -5.66
303 (2.34), 353 (2.71), 476 (3.44), -5.46
585 (0.32)
-3.55
-3.89
-3.66
1.78
1.77
1.80
4
310 (4.11), 343 (2.84), 446 (3.42), -5.64
-3.85
1.79
553 (0.59)
N719 317 (5.94), 400 (1.75), 541 (1.86) -5.45e
-3.85e
1.60e
a Measured in DMSO : EtOH = 2 : 3 (v/v). b Calculated from Eox - EFc/Fc+
+
4.8. c Calculated from EHOMO + Eg. d Optical bandgaps were determined
from onset of absorption in DMSO : EtOH = 2 : 3 (v/v). e Data from ref.
34.
and/or MLCT transitions. The strong absorption bands centered
at 303–353 nm are dominated by LC absorptions due to p → p*
transitions of the bipyridine ligands. The absorption bands with
intermediate energy at 383–476 nm can be ascribed to predom-
inant p → p* LC transitions, mixed with some character from
d → p* MLCT transitions. These absorption bands result partly
from p → p* LC transitions because the single 5,5¢-disubstituted
ancillary bipyridines show almost the same absorption pattern.
In addition, these absorption bands may be caused by one spin
allowed d → p* MLCT transitions.33
Although the molar absorption coefficients for the MLCT
bands of 1–4 are lower than those of the state-of-the-
art dyes N719 (cis-di(thiocyanato)bis(2,2¢-bipyridine-4,4¢-
dicarboxylate)ruthenium(II)
bis(tetrabutylammonium)
salt
(18600 M-1 cm-1 at 541 nm) and CYC-B3 (cis-di(thiocyanato)-
4,4¢-di(octylthienyl)-2,2¢-bipyridine-4,4¢-carboxylic
acid-2,2¢-
bipyridine ruthenium(II))20e (15700 M-1 cm-1 at 544 nm in DMF),
the light-harvesting ability of 1–4 in the intermediate energy
region appears higher than those of the dyes N719 and CYC-B3.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2314–2323 | 2317
©

be established from the electrochemical and absorption data. As
shown in Fig. 6, the oxidation of all complexes is irreversible
since the oxidation potential of the thiocyanate ligand is close
to that for Ru(II), as observed for other related ruthenium dyes.19,35
The Ru(III/II) oxidation potential of 1, 2, 3 and 4 were found
to be 0.90, 1.23, 1.03, and 1.21 (V vs. SCE), respectively, which
are higher than the I-/I3 electrolyte (0.5 V), ensuring effective
-
sensitizer regeneration process.36 The Ru(III/II) oxidation potential
order 1 < 4 can be rationalized from the increased degree of
conjugation in the presence of electron-releasing triphenylamino.
The corresponding order 3 < 4 is caused by the increased electron
richness of the bipyridyl ligand in 4 due to the presence of an
electron-releasing octyl substituent, rendering the Ru(II) center less
susceptible to oxidation in 4. The LUMO levels of these sensitizers
are all much higher than the lower bound of the conduction band
of TiO2 (-4.4 eV), indicating that the efficiency of charge injection
from the excited sensitizer molecule to the TiO2 conduction band
is viable.36 The energy level diagrams of 1–4, N719, TiO2, and the
-
I-/I3 redox couple are depicted in Fig. 7.
Fig. 4 (a) Normalized absorption and (b) photoluminescence spectra of
L1–L4 in DMSO : EtOH = 2 : 3 (v/v) at 293 K.
Fig. 6 Oxidative cyclic voltammetry of the photosensitizers adsorbed on
the electrode coated with a thin layer of TiO2.
Computational studies
Fig. 5 UV-vis absorption spectra of 1–4 and N719 in DMSO : EtOH =
2 : 3 (v/v).
The geometry optimization, electronic structure and localization
of the frontier orbitals of complexes 1–4 were carried out by the
density functional theory (DFT) calculations based on the Becke’s
three parameters employing Lee-Yang-Parr exchange functional
(B3LYP) with 6-31G* basis sets. As shown from the orbital profiles
in Fig. 8, the components of the frontier orbitals of 1–4 are similar
to each other. The highest occupied molecular orbitals, HOMO
and HOMO-1, of the four ruthenium dyes are mainly located at
the ruthenium metal and the NCS ligands, in which the ruthenium
metal character is about 11–19% and the NCS ligand character is
about 74–86%. Within the NCS ligands, the amplitude is located
on the sulfur atom. The lowest unoccupied molecular orbitals
(LUMO) of 1–4 have amplitudes mostly on the anchoring ligands
Electrochemistry
To evaluate the possibility of electron transfer from the excited
dye molecule to the conductive band (Ecb) of TiO2, oxidative
cyclic voltammetry was performed in the acetonitrile solvent
using 0.1 M tetrabutylammonium hexafluorophosphate as the
supporting electrolyte, TiO2 films stained with sensitizer as the
working electrode, Pt as the counter electrode and saturated
calomel electrode (SCE) as the reference electrode. Under these
conditions, the E1/2 of ferrocene was 0.37 V vs. SCE.
The band structures of 1–4, which should match the energy
level of the semiconductor anode and the redox electrolyte, can
2318 | Dalton Trans., 2011, 40, 2314–2323
This journal is
The Royal Society of Chemistry 2011
©

-
Fig. 7 Energy level diagrams of 1–4, N719, TiO2, and I-/I3
.
Fig. 9 Photocurrent action spectra of the TiO2 electrode sensitized by
1–4.
Fig. 8 Contour plots of selected frontier orbitals of 1–4.
(dcbpy) that can facilitate electron injection from the excited metal
complex to TiO2, while the LUMO+1 is mainly located on the
ancillary bipyridine ligands. It was revealed that the HOMO–
LUMO excitation transferred the electrons from the NCS ligands
to the anchoring ligands (dcbpy), and the photoinduced electron
injection from the dye to TiO2 can be efficiently mediated by the
HOMO–LUMO transition.
Fig. 10 Photocurrent-voltage curves for DSSCs based on photosensitiz-
ers 1–4.
Performance of dye-sensitized solar cells
The photocurrent action spectra of DSSCs with sensitizers 1–4 are
presented in Fig. 9. Considering the incident photon to current
conversion efficiency (IPCE) of these ruthenium dyes, the IPCE
curves cover almost the entire visible spectrum with a maximum
of IPCE values of 41.4% at 550 nm, 38.6% at 480 nm, 39.4% at
470 nm and 31.1% at 480 nm for 1-, 2-, 3- and 4-sensitized solar
cells, respectively.
The short-circuit photocurrent density (Jsc), open-circuit pho-
tovoltage (Voc), and fill factor (ff ) of the DSSCs were measured
under standard AM 1.5 sunlight illumination. The photocurrent-
voltage curves for DSSCs based on sensitizers 1–4 are shown in
Fig. 10 and the detailed device performance data are listed in
Table 3. The Jsc of the solar cells with 1, 2, 3 and 4 are 6.56, 5.60,
4.93 and 4.49 mA cm-2, respectively. These values are lower than
that of the optimized solar cells with N719 dye (13.81 mA cm-2),
and the difference can be attributed to the notion that MLCT
molar extinction coefficients of 1–4 are lower than that of N719
in solution (vide supra). The power conversion efficiency (h) of 1-,
2-, 3- and 4-sensitized solar cells are moderate at 3.00%, 2.51%,
2.00% and 2.03%, respectively (vs. 5.9% for N719 under the same
device-fabrication process and measuring parameters). The lower
h is largely due to the lower Jsc (or molar extinction coefficient) of
the sensitizers as compared to N719 that is associated with their
weaker light-harvesting capacity and bulky molecular structures,
although the ff of the present devices are all slightly higher than
that for N719. The values of the devices are consistent with the
MLCT band absorption coefficient of the photosensitizers.
Table 3 Photovoltaic performance of DSSCs with different 1-, 2-, 3- and
4- and N719-sensitizers under the AM 1.5 sunlight
Jsc/mA cm-2
V
oc/mV
ff
h (%)
1
6.56
5.60
4.93
4.49
13.81
649
592
584
633
657
0.70
0.76
0.69
0.71
0.65
3.00
2.51
2.00
2.03
5.9
2
3
4
N719
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2314–2323 | 2319
©

Concluding remarks
X-Ray crystallography†
A series of heteroleptic ruthenium complexes containing un-
common 5,5¢-disubstituted-2,2¢-bipyridine ancillary ligands were
synthesized and fully characterized by spectroscopic and DFT
studies. The different substituent on bipyridine acts as a good
functional chromophore to tune the overall properties of the
complex. The relationship between the nature of the ancillary
ligand and the DSSC performance of the dye was investigated. As
demonstrated by the absorption spectral studies, the increase in
conjugation length of the ancillary ligand results in a lower MLCT
energy and a diminished intensity of the transition. Photosensitizer
1 gave the highest conversion efficiency of 3.00%, and among 1–4,
the performance results correlate well with the molar absorption
coefficient of the MLCT band of the dyes and their different
light-harvesting ability. These findings suggest that bipyridine
chromophores substituted at the synthetically more accessible 5,5¢-
positions opens an alternative good class of ancillary ligands over
the traditional 4,4¢-disubstituted ones for use in DSSC research
based on the photosensitizing metal polypyridyl core. Work is still
underway for device optimization and it is possible to gain higher
efficiencies with a thicker TiO2 film.
X-Ray diffraction data were collected at 173 K using graphite-
˚
monochromated Mo-Ka radiation (l = 0.71073 A) on a Bruker
Axs SMART 1000 CCD diffractometer. The collected frames
were processed with the software SAINT+43 and an absorption
correction (SADABS)44 was applied to the collected reflections.
The structure was solved by the Direct method (SHELXTL)45
in conjunction with standard difference Fourier techniques and
subsequently refined by full-matrix least-squares analyses on F2.
Hydrogen atoms were generated in their idealized positions and
all non-hydrogen atoms were refined anisotropically.
Fabrication of photovoltaic devices
A screen-printed double layer of TiO2 particles was used as
photoelectrodes. A 10 mm thick film of 13 nm-sized TiO2 particles
(Ti-Nanoxide T/SP) was first printed on the FTO conducting
glass and further coated by a 4 mm thick second layer of 400 nm
light-scattering anatase particles (Ti-Nanoxide 300). Sintering was
carried out at 450 ◦C for 30 min. Before immersion in the dye
solution, these films were immersed into a 40 mM aqueous TiCl4
solution at 70 ◦C for 30 min and washed with water and ethanol.
◦
Then the films were heated again at 450 C for 30 min followed
◦
by cooling to 80 C and dipping into a 3 ¥ 10-4 M solution of
Experimental
dyes in DMSO : EtOH = 2 : 3 (v/v) for 12 h at room temperature.
Chenodeoxycholic acid (CDCA) as a coadsorbent was adsorbed
on the TiO2 films in a 3 ¥ 10-4 M CDCA in acetonitrile solution
for 0.5 h or 1 h before dipping into the solution of dyes. To prepare
the counter electrode, the Pt catalyst was deposited on the cleaned
FTO glass by coating the FTO with a drop of H2PtCl6 solution
(0.02 M in 2-propanol solution) with the heat treatment at 400 ◦C
for 15 min. A hole (0.8 mm diameter) was drilled on the counter
electrode by a Drill-press. The perforated sheet was cleaned by
ultrasound in an ethanol bath for 10 min. About the assemblage
of DSSCs, the dye-covered TiO2 electrode and Pt-counter electrode
were assembled into a sandwich type cell and sealed with a hot-
melt gasket of 25 mm thickness made of the ionomer Surlyn 1702
(Dupont). The size of the TiO2 electrodes used was 0.25 cm2 (i.e.,
5 mm ¥ 5 mm). A drop of the electrolyte was put on the hole in the
back of the counter electrode. It was introduced into the cell via
vacuum backfilling. The hole in the counter electrode was sealed
by a film of Surlyn 1702 and a cover glass (0.1 mm thickness) using
a hot iron bar.
Materials and reagents
All manipulations were performed under dry nitrogen at-
mosphere by using Schlenk techniques. Solvents were dried
by standard methods and distilled prior to use. Di-m-chloro-
6
bis[(h -p-cymene)chlororuthenium(II)] ([RuCl2(p-cymene)]2) was
purchased from Strem Chemical Inc. (USA). N719 dye
was purchased from Solaronix SA Co. (Switzerland). 4,4¢-
Dicarboxylic acid-2,2¢-bipyridine (dcbpy) was prepared according
to the reported procedure in the literature.37 5,5¢-Dibromo-
2,2¢-bipyridine,25 tributyl(4-octylthiophen-2-yl)stannane,38 N,N-
diphenyl-4-aminophenylboronic acid39 and N,N-diphenyl-4-(5-
(tributylstannyl)thiophen-2-yl)benzenamine40 were prepared ac-
cording to the published method, and other chemicals are
commercially available and used as received.
Physical measurements
Infrared spectra were recorded as KBr pellets using a Perkin-
Elmer Paragon 1000 PC or Nicolet Magna 550 Series II FTIR
spectrometer. MALDI-TOF mass spectra were measured on an
Autoflex Bruker MALDI-TOF MS instrument. NMR spectra
were measured in CDCl3 or DMSO-d6 on a Bruker AM 400 MHz
FT-NMR spectrometer, and chemical shifts were quoted relative
DSSC measurements
Photovoltaic measurements employed an AM 1.5 solar simulator
equipped with a 1000 W xenon lamp (Model No. 91160, Oriel).
The power of the simulated light was calibrated to 100 mW cm-2
using a Newport Oriel PV reference cell system (Model 91150V).
J–V curves were obtained by applying an external bias to the
cell and measuring the generated photocurrent with a Keithley
model 2400 digital source meter. The voltage step and delay time
of photocurrent were 10 mV and 40 ms, respectively. Cell active
area was tested with a mask of 0.158 cm2. The photocurrent action
spectra were measured with an IPCE test system consisting of a
Model SR830 DSP lock-in amplifier and a Model SR540 optical
chopper (Stanford Research Corporation, USA), a 7IL/PX150
xenon lamp and power supply, and a 7ISW301 spectrometer.
to tetramethylsilane for H and 13C nuclei. UV-Vis spectra were
obtained on a HP-8453 diode array spectrophotometer.
1
DFT computational method
All calculations were performed using the Gaussian 03 program
package.41 The B3LYP (Becke-3-Lee-Yang-Par)42 DFT method
was chosen because of its high accuracy and it is not computa-
tionally demanding. The LAN2DZ basis set was applied for Ru
and S atoms, and 6-31G* basis set was employed for C, H, N and
O atoms.
2320 | Dalton Trans., 2011, 40, 2314–2323
This journal is
The Royal Society of Chemistry 2011
©

Impedance spectra were done using a CHI-660c electrochemical
station.
120.89 (Ar) ppm. MS (MALDI-TOF): m/z = 807.31 (M+H)+.
Anal. calc. for C54H38N4S2: C, 80.37; H, 4.75; N, 6.94. Found: C,
80.55; H, 4.86; N, 7.08.
Synthesis
5,5¢-Bis(4-octyl-5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-
yl)-2,2¢-bipyridine (L4). A solution of L1 (0.41 g, 0.75 mmol)
in chloroform (15 ml) was stirred and cooled to 0 ◦C. N-
Bromosuccinimide (NBS) (0.14 g, 0.75 mmol) was added in
small portions. After stirring overnight at room temperature, the
reaction mixture was poured into water. The organic phase was
separated and dried over anhydrous Na2SO4. After the solvent was
evaporated, the crude product was purified by recrystallization
from methanol to afford 5,5¢-bis(4-octyl-5-bromo-thiophen-2-yl)-
2,2¢-bipyridine in 87% yield (0.46 g, 0.65 mmol). 1H NMR
(400 MHz, CDCl3): d = 8.84 (s, 2H, Ar), 8.40 (d, J = 8.3 Hz, 2H,
Ar), 7.90 (d, J = 8.3 Hz, 2H, Ar), 7.13 (s, 2H, Ar), 2.61–2.57 (m, 4H,
alkyl), 1.63–1.60 (m, 4H, alkyl), 1.36–1.28 (m, 20H, alkyl), 0.90–
0.87 (m, 6H, alkyl) ppm. 13C NMR (100 MHz, CDCl3): d = 154.37,
145.80, 143.64, 139.56, 133.20, 129.83, 125.29, 121.00, 109.90 (Ar),
31.89, 29.74, 29.66, 29.58, 29.39, 29.26, 22.68, 14.14 (alkyl) ppm.
The procedure described for the synthesis of L2 was followed
to produce L4 from N,N-diphenyl-4-aminophenylboronic acid
(0.44 g, 1.50 mmol) and 5,5¢-bis(4-octyl-5-bromo-thiophen-2-yl)-
2,2¢-bipyridine (0.35 g, 0.50 mmol). The crude product was purified
by column chromatography on silica gel with dichloromethane–
ethyl acetate = 50 : 1 (v/v) as eluent to give 0.36 g of the product as
an orange solid (0.35 mmol, 70%). 1H NMR (400 MHz, CDCl3):
d = 8.90 (s, 2H, Ar), 8.38 (d, J = 8.3 Hz, 2H, Ar), 7.93 (d, J =
8.3 Hz, 2H, Ar), 7.32–7.24 (m, 14H, Ar), 7.15–7.13 (m, 8H, Ar),
7.09–7.07 (m, 4H, Ar), 7.05–7.01 (m, 4H, Ar), 2.69–2.64 (m, 4H,
alkyl), 1.66–1.63 (m, 4H, alkyl), 1.32–1.20 (m, 20H, alkyl), 0.90–
0.87 (m, 6H, alkyl) ppm. 13C NMR (100 MHz, CDCl3): d = 154.06,
147.50, 147.35, 145.87, 139.63, 138.96, 137.44, 132.99, 130.26,
129.85, 129.42, 127.86, 126.91, 124.80, 123.30, 122.95, 120.87
(Ar), 31.97, 31.10, 29.63, 29.49, 29.35, 28.99, 22.76, 14.24 (alkyl)
ppm. MS (MALDI-TOF): m/z = 1031.62 (M+H)+. Anal. calc. for
C70H70N4S2: C, 81.51; H, 6.84; N, 5.43. Found: C, 81.76; H, 6.80;
N, 5.28.
5,5¢-Bis(4-octylthiophen-2-yl)-2,2¢-bipyridine
(L1). 5,5¢-
Dibromo-2,2¢-bipyridine (0.32 g, 1.00 mmol), tributyl(4-
octylthiophen-2-yl)stannane (1.1 g, 2.25 mmol) and Pd(PPh3)4
(0.116 g, 0.10 mmol, 5 mol%) were dissolved in dry toluene (30
mL) and the mixture was refluxed under N2 for two days. After
evaporation of the solvent under reduced pressure, the resulting
solid was purified by column chromatography on silica gel using
CH2Cl2 as eluent to afford L1 as a yellow solid (0.41 g, 0.75 mmol,
75%). 1H NMR (400 MHz, CDCl3): d = 8.91 (s, 2H, Ar), 8.40 (d,
J = 8.3 Hz, 2H, Ar), 7.97 (d, J = 8.3 Hz, 2H, Ar), 7.26 (s, 2H,
Ar), 6.97 (s, 2H, Ar), 2.66–2.62 (m, 4H, alkyl), 1.68–1.64 (m, 4H,
alkyl), 1.36–1.28 (m, 20H, alkyl), 0.90–0.87 (m, 6H, alkyl) ppm.
13C NMR (100 MHz, CDCl3): d = 154.23, 146.10, 144.75, 139.97,
133.43, 130.44, 125.66, 120.86, 120.84 (Ar), 31.89, 30.56, 30.47,
29.44, 29.34, 29.27, 22.68, 14.13 (alkyl) ppm. MS (MALDI-TOF):
m/z = 545.29 (M+H)+. Anal. calc. for C34H44N2S2: C, 74.95; H,
8.14; N, 5.14. Found: C, 75.10; H, 8.20; N, 5.25.
5,5¢-Bis(N,N-diphenyl-4-aminophenyl)-2,2¢-bipyridine
(L2).
To a solution of 5,5¢-dibromo-2,2¢-bipyridine (0.33 g, 1.05 mmol),
N,N-diphenyl-4-aminophenylboronic acid (0.87 g, 3.00 mmol)
and Pd(PPh3)4 (0.116 g, 0.10 mmol, 5 mol%) in a mixture of
toluene (10 mL) and THF (20 mL) was added a solution of K2CO3
(2 M, 4 mL). The reaction mixture was stirred under reflux for
two days. After evaporation of the solvent under reduced pressure,
water (50 mL) and dichloromethane (50 mL) were added. The
organic layer was separated, washed with water and brine,
and dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure, and the crude product was purified by
column chromatography on silica gel with dichloromethane–ethyl
acetate = 10 : 1 (v/v) as the eluent to afford L1 as an orange solid
1
in 68% yield (0.44 g, 0.68 mmol). H NMR (400 MHz, CDCl3):
d = 8.91 (s, 2H, Ar), 8.46 (d, J = 8.2 Hz, 2H, Ar), 8.00 (d, J =
8.3 Hz, 2H, Ar), 7.55–7.53 (m, 4H, Ar), 7.27–7.31 (m, 8H, Ar),
7.19–7.14 (m, 12H, Ar), 7.09–7.05 (m, 4H, Ar) ppm. 13C NMR
(100 MHz, CDCl3): d = 154.09, 148.10, 147.40, 147.18, 135.82,
134.54, 130.90, 129.38, 127.66, 124.75, 123.51, 123.34, 120.89
(Ar) ppm. MS (MALDI-TOF): m/z = 643.17 (M+H)+. Anal.
calc. for C46H34N4: C, 85.95; H, 5.33; N, 8.72. Found: C, 86.10; H,
5.15; N, 8.98.
General procedure for the synthesis of the ruthenium sensitizers
In a typical one-pot synthesis, [RuCl2(p-cymene)]2 (138 mg, 0.23
mmol) was dissolved in dry DMF (25 mL) and the respective 5,5¢-
disubstituted-2,2¢-bipyridine ligand (L1, L2, L3 or L4, 0.45 mmol)
was added. The reaction mixture was stirred at 80 ◦C under argon
for 4 h and then dcbpy (110 mg, 0.45 mmol) was added. The
reaction mixture was refluxed at 160 ◦C for another 4 h in the
dark to avoid the photoinduced cis-to-trans isomerization. After
addition of excess NH4NCS (330 mg, 4.50 mmol), the reaction
mixture was stirred at 130 ◦C for 5 h. After the reaction, the
solvent was removed by using a rotary evaporator. Then, water was
added to the resulting mixture to remove any excess NH4NCS. The
water-insoluble product was collected, washed with distilled water,
followed by diethyl ether, and dried in air. Purification of the crude
product was achieved by column chromatography on silica gel
using chloroform–methanol (2 : 1, v/v) as the eluent. The main
band was collected and a few drops of 0.01 M HNO3(aq) was
added. One to four successive chromatographic separation was
necessary to ensure the desired purity.
5,5¢-Bis(5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2¢-
bipyridine (L3). The procedure described for the synthesis
of L1 was followed to produce L3 from N,N-diphenyl-4-(5-
(tributylstannyl)thiophen-2-yl)benzenamine (0.92 g, 1.50 mmol)
and 5,5¢-dibromo-2,2¢-bipyridine (0.21 g, 0.67 mmol). The crude
product was purified by column chromatography on silica gel
eluting with dichloromethane–ethyl acetate = 10 : 1 (v/v) to yield
1
0.40 g of the product as an orange solid (0.49 mmol, 73%). H
NMR (400 MHz, CDCl3): d = 8.94 (s, 2H, Ar), 8.43 (d, J = 8.3 Hz,
2H, Ar), 7.99 (d, J = 8.3 Hz, 2H, Ar), 7.52–7.49 (m, 4H, Ar),
7.41–7.40 (m, 2H, Ar), 7.30–7.24 (m, 10H, Ar), 7.15–7.13 (m, 8H,
Ar), 7.10–7.04 (m, 8H, Ar) ppm. 13C NMR (100 MHz, CDCl3):
d = 154.11, 147.66, 147.36, 145.90, 145.12, 138.51, 133.10, 130.23,
129.36, 127.75, 126.54, 125.29, 124.66, 123.41, 123.33, 123.28,
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2314–2323 | 2321
©

[Ru(dcbpy)(L1)(NCS)2] (1). Purple solid. 1H NMR (400 MHz,
DMSO-d6): d = 9.59 (s, 1H, Ar), 9.24 (s, 1H, Ar), 8.87 (s, 1H, Ar),
8.76 (d, J = 8.7 Hz, 2H, Ar), 8.68 (s, 1H, Ar), 8.61 (d, J = 8.5 Hz, 1H,
Ar), 8.46 (d, J = 8.0 Hz, 1H, Ar), 8.21–8.18 (m, 2H, Ar), 7.88 (s, 1H,
Ar), 7.65 (s, 1H, Ar), 7.52–7.42 (m, 5H, Ar), 7.23–7.20 (m, 1H, Ar),
1.66 (m, 2H, alkyl), 1.49 (m, 2H, alkyl), 1.31–1.21 (m, 24H, alkyl),
0.85–0.81 (m, 6H, alkyl) ppm. IR (KBr) (cm-1): 2103 s (NCS).
MS (MALDI-TOF): m/z = 1043.24 (M+2H2O+H)+, 948.29 (M–
NCS)+. Anal. calc. for C48H52N6O4S4Ru: C, 57.29; H, 5.21; N, 8.35.
Found: C, 57.40; H, 5.25; N, 8.56.
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[Ru(dcbpy)(L2)(NCS)2] (2). Black solid. 1H NMR (400 MHz,
DMSO-d6): d = 9.60 (s, 1H, Ar), 9.26 (s, 1H, Ar), 8.82–8.80 (m,
2H, Ar), 8.67–8.65 (m, 2H, Ar), 8.53 (d, J = 8.4 Hz, 1H, Ar),
8.16–8.11 (m, 2H, Ar), 7.94 (d, J = 8.6 Hz, 1H, Ar), 7.64–7.53 (m,
4H, Ar), 7.40–7.31 (m, 10H, Ar), 7.25–7.23 (m, 2H, Ar), 7.15–
7.10 (m, 10H, Ar), 7.04–7.02 (m, 4H, Ar), 6.86–6.84 (m, 2H,
Ar) ppm. IR (KBr) (cm-1): 2099 s (NCS). MS (MALDI-TOF):
m/z = 1141.28 (M+2H2O+H)+, 1046.35 (M–NCS)+. Anal. calc.
for C60H42N8O4S2Ru: C, 65.26; H, 3.83; N, 10.15. Found: C, 65.53;
H, 3.98; N, 10.24.
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[Ru(dcbpy)(L3)(NCS)2] (3). Black solid. 1H NMR (400 MHz,
DMSO-d6): d = 9.72 (s, 1H, Ar), 8.75 (d, J = 8.5 Hz, 1H, Ar), 8.57
(d, J = 8.4 Hz, 2H, Ar), 8.21 (d, J = 8.2 Hz, 1H, Ar), 8.00 (d, J =
3.7 Hz, 1H, Ar), 7.64 (s, 1H, Ar), 7.60 (d, J = 3.2 Hz, 1H, Ar), 7.55
(d, J = 3.7 Hz, 1H, Ar), 7.47 (d, J = 8.6 Hz, 2H, Ar), 7.38–7.26 (m,
12H, Ar), 7.10–6.96 (m, 20H, Ar), 6.87–6.85 (m, 3H, Ar) ppm. IR
(KBr) (cm-1): 2094 s (NCS). MS (MALDI-TOF): m/z = 1210.19
(M–NCS)+. Anal. calc. for C68H46N8O4S4Ru: C, 64.39; H, 3.66; N,
8.83. Found: C, 64.54; H, 3.98; N, 8.60.
[Ru(dcbpy)(L4)(NCS)2] (4). Black solid. 1H NMR (400 MHz,
DMSO-d6): d = 9.56 (s, 1H, Ar), 9.42 (s, 1H, Ar), 9.05 (s, 1H, Ar),
8.86 (s, 1H, Ar), 8.66 (s, 1H, Ar), 8.50 (s, 1H, Ar), 8.34 (s, 1H, Ar),
8.29 (s, 1H, Ar), 7.95 (s, 1H, Ar), 7.90 (s, 1H, Ar), 7.39 (d, J =
8.2 Hz, 2H, Ar), 7.35–7.30 (m, 10H, Ar), 7.22 (d, J = 8.4 Hz, 2H,
Ar), 7.12–7.00 (m, 18H, Ar), 6.96–6.94 (d, 2H, Ar), 1.66 (s, 4H,
alkyl), 1.44 (s, 4H, alkyl), 1.23–1.20 (m, 20H, alkyl), 0.82–0.81 (m,
6H, alkyl) ppm. IR (KBr) (cm-1): 2104 s (NCS). MS (MALDI-
TOF): m/z = 1529.42 (M+2H2O+H)+, 1434.41 (M–NCS)+. Anal.
calc. for C84H78N8O4S4Ru: C, 67.58; H, 5.27; N, 7.51. Found: C,
67.66; H, 5.40; N, 7.78.
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Acknowledgements
This work has been supported by the University Grants Commit-
tee Areas of Excellence Scheme (AoE/P-03/08) and the Hong
Kong Baptist University. We also thank the National Science
Foundation of China (NSFC) (project number: 21029001) for
financial support.
23 F. Gao, Y. Wang, J. Zhang, D. Shi, M. Wang, R. Humphry-Baker,
P. Wang, S. M. Zakeeruddin and M. Gra¨tzel, Chem. Commun., 2008,
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