Silane: A new linker for chromophores in dye-sensitised solar cells

Article abstract of DOI:10.1016/j.poly.2012.07.078

A series of ruthenium(II) polypyridyl complexes, with novel silane functionalisation, [Ru(bipy)₂(bipy-sil)](PF₆)₂ (3), [Ru(bipy-sil)₂Cl₂] (6), and [Ru(bipy-sil) ₂(NCS)₂] (7) have been synthesised and tested as chromophores (dyes) in TiO₂ and WO₃ based dye-sensitised solar cells (DSSCs). The performance of the respective DSSCs were compared to analogous dyes with ionic carboxylate ([Ru(bipy)₂(dcbipy)](PF ₆)₂ (1), [Ru(dcbipy)₂Cl₂] (4), [Ru(dcbipy)₂(NCS)₂] (5)) or phosphonate ([Ru(bipy) ₂(dpipy)](PF₆)₂ (2)) linking groups. The covalent silane-metal oxide linkage offers much needed improvement to the operating conditions, and lifetime of DSSCs, in terms of pH range and choice of solvent. UV-Vis spectroscopy of the deep-red solutions showed that the bis-bipy-sil complexes absorbed more visible light than the tris-bipy complex, as indicated by the presence of two absorption bands and higher ε values. The UV-Vis spectrum of (3) contained a single broad absorption at 400-600 nm with: λ_max = 457 nm; ε = 10 520 ± 440 L mol ^-1 cm^-1, whereas two intense broad absorption bands were observed for novel bis-bipy-sil complexes (6): 340-370 nm (λ _max(1) = 365 nm, ε₍₁₎ = 12 716 ± 180 L mol^-1 cm^-1); and 440-540 nm (λ_max(2) = 485 nm, ε₍₂₎ = 11 070 ± 150 L mol^-1 cm ^-1), and (7): 340-400 nm (λ_max = 371 nm, ε₍₁₎ = 20 690 ± 485 L mol^-1 cm^-1), and 460-530 nm (λ_max = 500 nm and ε₍₂₎ = 20 750 ± 487 L mol^-1 cm^-1). The bands in (7) being significantly more defined. A 10-fold improvement in the efficiency of the bipy-sil TiO₂-based DSSCs was observed from (3) to (6) to (7). This performance was lower than that of the commercial N3 dye, [Ru(dcbipy) ₂(NCS)₂] (5), but the current of (7) on WO₃, was comparable to that of the carboxylate system (4). There is considerable potential for further improvement by modification of the silyl linker, reducing the long non-conjugated propyl chain between the amide group and the silatrane (bipy-sil), to a short, conjugated link. During an extensive synthetic study, the most promising strategy was identified as direct linkage, the formation of a direct Si-C bond, using butyllithium with 4,4′-dibromo-2,2′- bipyridine and either trimethylsilane or 1-ethoxysilatrane, provided that the product can be captured and stabilised prior to binding to a metal oxide coated DSSC substrate.

Full text of DOI:10.1016/j.poly.2012.07.078

Polyhedron 52 (2013) 719–732
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Silane: A new linker for chromophores in dye-sensitised solar cells
a
b
b
Katherine Szpakolski a, Kay Latham a, , Colin Rix , Rozina A. Rani , Kourosh Kalantar-zadeh
⇑
a School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, VIC 3001, Australia
b School of Electrical Communications Engineering, RMIT University, GPO Box 2476V, Melbourne, VIC 3001, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 4 August 2012
A series of ruthenium(II) polypyridyl complexes, with novel silane functionalisation, [Ru(bipy)2(bipy-sil)]
(PF6)2 (3), [Ru(bipy-sil)2Cl2] (6), and [Ru(bipy-sil)2(NCS)2] (7) have been synthesised and tested as chro-
mophores (dyes) in TiO2 and WO3 based dye-sensitised solar cells (DSSCs). The performance of the
respective DSSCs were compared to analogous dyes with ionic carboxylate ([Ru(bipy)2(dcbipy)](PF6)2
(1), [Ru(dcbipy)2Cl2] (4), [Ru(dcbipy)2(NCS)2] (5)) or phosphonate ([Ru(bipy)2(dpipy)](PF6)2 (2)) linking
groups. The covalent silane–metal oxide linkage offers much needed improvement to the operating
conditions, and lifetime of DSSCs, in terms of pH range and choice of solvent. UV–Vis spectroscopy of
the deep-red solutions showed that the bis-bipy-sil complexes absorbed more visible light than the
This paper is dedicated to the life and works
of Professor Alfred Werner (1866–1919) on
the 100th anniversary of his award of Nobel
Prize in Chemistry, and in recognition of his
major contributions to the field of
Coordination Chemistry.
tris-bipy complex, as indicated by the presence of two absorption bands and higher
UV–Vis spectrum of (3) contained a single broad absorption at 400–600 nm with: kmax = 457 nm;
= 10520 440 L molꢀ1 cmꢀ1, whereas two intense broad absorption bands were observed for novel
bis-bipy-sil complexes (6): 340–370 nm (kmax(1) = 365 nm,
(1) = 12716 180 L molꢀ1 cmꢀ1); and 440–
e values. The
Keywords:
Ruthenium
Polypyridyl complexes
Silane linkage
DSSCs
e
e
540 nm (kmax(2) = 485 nm, e =
(2) = 11070 150 L molꢀ1 cmꢀ1), and (7): 340–400 nm (kmax = 371 nm, e(1)
20690 485 L molꢀ1 cmꢀ1), and 460–530 nm (kmax = 500 nm and
bands in (7) being significantly more defined.
e
(2) = 20750 487 L molꢀ1 cmꢀ1). The
TiO2
WO3
A 10-fold improvement in the efficiency of the bipy-sil TiO2-based DSSCs was observed from (3) to (6)
to (7). This performance was lower than that of the commercial N3 dye, [Ru(dcbipy)2(NCS)2] (5), but the
current of (7) on WO3, was comparable to that of the carboxylate system (4). There is considerable poten-
tial for further improvement by modification of the silyl linker, reducing the long non-conjugated propyl
chain between the amide group and the silatrane (bipy-sil), to a short, conjugated link. During an exten-
sive synthetic study, the most promising strategy was identified as direct linkage, the formation of a
direct Si–C bond, using butyllithium with 4,40-dibromo-2,20-bipyridine and either trimethylsilane or
1-ethoxysilatrane, provided that the product can be captured and stabilised prior to binding to a metal
oxide coated DSSC substrate.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
in the visible and ultra-violet region (Ru(II) complexes coordinated
to N-heterocycles have been explored extensively due to their pho-
Dye-sensitised solar cells (DSSCs) – so called ‘Gratzel’ cells
(Fig. 1), have the potential to make a significant contribution to so-
lar energy capture as they perform the photo-voltaic conversion of
solar radiation to electricity. However, despite considerable
research effort, their commercial implementation has been
hampered by high cost and low conversion efficiency. Proof-of-
principle cells have been developed in the laboratory, but are
generally based on expensive metals, particularly ruthenium, and
only have photo-voltaic conversions of the order of 10% [1,2].
Such cells are based on a redox-active chromophore, which can
be an organic ‘dye’ or a transition metal co-ordination compound
having a broad absorption range across the solar spectrum, usually
tochemical and photophysical behaviour) [3,4]. Following elec-
tronic excitation, the chromophore then needs to possess a
sufficiently long-lived excited state (i.e. a fluorescent or phospho-
rescent state) which allows the excited electron to be passed to
an external circuit, usually via a semi-conducting substrate such
as titanium dioxide, or a variety of other metal oxides, incorporat-
ing metals such as Sn [5], Zn [6], W [7], and Nb [8]. This step is a
major stumbling block in the efficient energy capture and conver-
sion process, since although relatively efficient chromophores are
available, it is the rate of electron transfer to the substrate that
limits the photo-current. This step is not only dependent on the
life-time of the excited electronic state, but also relies critically
on efficient electron transfer from the molecular orbitals centred
on the chromophore, to those in the conduction band of the sub-
strate. It is therefore dependent on the proximity of the exchanging
⇑
Corresponding author. Fax: +61 3 9925 3747.
E-mail address: kay.latham@rmit.edu.au (K. Latham).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.078

720
K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
to a range of metal oxides (e.g. TiO2, WO3) using silyl groups is
explored within this paper.
light (hv)
The synthesis of two novel bis-bipyridyl ruthenium(II) com-
plexes which contain a silatrane-propyl amide link on the (4,40-
dicarboxylic acid)-2,20bipyridine ligand (bipy-sil) are reported,
and their photophysical properties, and efficiency as dyes in TiO2
and WO3 – based DSSCs, tested and compared against carboxylic
acid and phosphonate functionalised bipy analogues (dcbipy and
dpbipy respectively) (Fig. 3). In addition, an extensive study, which
concentrated on strategies to synthesise pyridine-type ligands
with short distances between the silatrane linker and the pyridine
ring, was undertaken.
Ru3+
Cu2+
Reductive regeneration
Reductive regeneration
Cu+/Ru2+
Oxidative release of e–
Oxidative release of e–
N
N
where A = anchoring group
e–
e–
A
A
2. Materials and methods
Unless otherwise stated, all chemicals were of at least analytical
reagent grade and used without further purification. Ammonium
hexafluorophosphate (95+%), 2,2-bipyridine (bipy, 99+%) n-
butyllithium in hexane (1.567 M), and triethylphosphite (98%)
from Aldrich Chemicals. Triisopropyl phosphite (90+%) from Alfa
Aesar. Triethanolamine (P98%), triethylamine (P99%), and tri-
phenylphosphine, (P98%) from BDH Chemicals. 4,40-Dibromo-
2,20-bipyridine and 4,40-dicarboxylic acid-2,20-bipyridine (dcbipy)
from Carbosynth and Dyesol, respectively. Hexamethyldisilazane
(bis(trimethylsilyl)amine, 98%) and thionyl chloride (P98%) from
Merck. (3-aminopropyl)triethoxysilane (P98%), nicotinamide
(98%), and tetraethylorthosilicate (99%) from Sigma Chemicals,
and ruthenium(III) chloride hydrate (99.9%) and tetrakis(triphenyl-
phosphine)palladium(0) (99.9+%) from Strem Chemicals.
TiO2 Semiconductor surface
Fig. 1. Schematic of dye complex (Cu+/Cu2+ and Ru2+/Ru3+ systems), anchored to a
semiconductor substrate in DSSC (regenerative cell), absorbing light and
a
transferring the excited electron to the semiconductor substrate.
centres, and the presence of an electron-conducting pathway be-
tween them. Hence, the nature of the attachment of the chromo-
phore to the surface of the metal oxide is especially important.
The use of highly polar carboxylate (COOꢀ) and phosphonate
(P(O)2(OH)ꢀ or PO32ꢀ) ionic linker groups, to anchor the appropri-
ate metal-based chromophore to a metal-oxide surface, has been
explored extensively in previous studies [4,9–24], but the surface
binding of these groups to metal oxides is not stable over the full
pH range, and they are not compatible with some organic solvents
[15,25], To overcome these deficiencies, a covalent linker was
deemed necessary. The purpose of the current report is to explore
a new linker, a silyl ester, which may, by skilful design, improve the
efficiency of electron transfer between the chromophore and me-
tal-oxide semiconductor substrate.
One of the many reasons why silyl coupling has not been ex-
plored is the rather tedious synthetic procedure required to pre-
pare appropriate precursor ligands [26,27]. An in-situ synthetic
method has been reported [26], whereby the first step involved
modification of titania with organoalkoxysilane materials (3-chlo-
ropropyl-methoxysilane), but this investigation focused on photo-
catalytic activity for the degradation of phenols. It is, however, still
representative of a silyl linkage to a metal oxide. A silyl linkage has
also been prepared by in-situ pyrazole silylation on the ruthenium
complex, [Ru(py-pzH)3]2+ (where py-pzH = 3-(20-pyridyl)pyrazole),
Fig. 2 [28]. Although these linkers have not been exploited to any
degree in DSSCs, Brennan et al. [29] have investigated silicon–oxy-
gen based linkages with SnO2 semiconducting surfaces, and found
the silyl-ester bond a promising alternative linker to carboxylates
and phosphonates. To this end, the potential for anchoring of ‘dyes’
2.1. 1-(30-Aminopropylsilatrane)
1-(30-Aminopropylsilatrane) was synthesised following the pro-
cedure of Semenov [30]. A mixture of 3-aminopropyl(trieth-
oxy)silane (8.84 g, 0.04 mol) and triethanolamine (5.69 g,
0.04 mol) was heated (ꢁ70 °C) in a flask fitted with a Dean–Stark
apparatus, and the ethanol produced was removed as it formed.
The product was recrystallised twice, from toluene and then chlo-
roform, to give a pale yellow powder in 62% yield. IR (KBr):
v
= 3411 (b), 2954 (m), 2900 (m), 1660(m), 1576 (m), 1494 (b),
1124 (m), 1096 (m), 946 (s), 917 (s), 778 (m) cmꢀ1. (where
b = broad, m = medium and s = strong); 1H NMR (CDCl3,
300 MHz): d 0.4 (t, 2H, J = 5 Hz, –CH2–Si–), 1.53 (m, 2H, –CH2–),
2.60 (t, 6H, J = 4.5 Hz, –CH2–N), 2.79 (t, 2H, J = 3.5 Hz, CH2–NH2),
3.75 (t, 6H, J = 2.5 Hz, CH2–O–) ppm.
2.2. 1-Ethoxysilatrane
1-Ethoxysilatrane was prepared using a similar procedure to that
described for 1-(30-aminopropylsilatrane), however a large excess
of benzene was added to act as an azeotropiser. A mixture of
tetraethylorthosilicate (8.33 g, 0.04 mol), triethanolamine (5.69 g,
0.04 mol) and benzene (20 mL) were heated (ꢁ70 °C) in a flask fit-
ted with a Dean–Stark apparatus; the benzene-ethanol azeotrope
produced was removed as formed. The product was recrystallised
twice from toluene and then chloroform, to give a yellow solid in
73% yield. 1H NMR (CDCl3-d6, 300 MHz): d 1.10 (t, 3H, J = 6 Hz,
CH3–), 2.6 (t, 6H, J = 4 Hz, CH2–N), 3.9 (m, 2H, CH2–O), 3.9 (m,
6H, O–CH2) ppm.
2.3. 1-Chlorosilatrane
1-Chlorosilatrane was prepared following the procedure of
Kazakova et al. [31] 1-ethoxysilatrane (220 mg, 1 mmol) was
reacted with excess thionyl chloride (0.4 mL, 5 mmol) under a
Fig. 2. Possible linkage of silyl to TiO2, where R = (a) a palladium 2-aminothiazole
complex [20], and (b) [Ru(py-pzH)3]2+ (where py-pzH = 3-(20-pyridyl)pyrazole) [22].

K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
721
O
C
O
C
(a)
(b)
(c)
HdO
Hc Hc
OHd
Hb
Hb
N
N
Ha
Hl
Ha
Hl
Hm Hm
Hk
Hk
N
N
Hj
OHe
Hj
OHe
HdO
P
O
Hc Hc
O
P
OHd
Hb
Hb
N
N
Ha
Ha
(d)
Hg Hg
He
He
N
O
O
H
NHd
C
Si
N
Si
O
O
O
Hc
C
Hf
Hc
Hf
O
N
O
O
h
Hi
H
Hb
Hb
h
Hi
H
N
N
Ha
Ha
Fig. 3. Location of protons on (a) dcbipy, (b) bipy, (c) dpbipy, and (d) bipy-sil for 1H NMR.
nitrogen atmosphere for 4 h. The excess thionyl chloride was re-
moved under reduced pressure, and the product recrystallised
from DCM and recovered as a white powder in 80% yield.
4H, –CH2–), 2.66 (t, 12H, J = 5 Hz, –CH2–N), 2.84 (t, 4H, J = 4 Hz,
CH2–NH2), 3.75 (t, 12H, J = 4 Hz, CH2–O–), 7.8 (m, 2H, Ar–H), 8.05
(s, 2H, Ar–H), 8.6 (m, 2H, Ar–H) ppm.
2.5. 4,40-Disilatrane-2,20-bipyridine
2.4. 2,20-Bipyridine-4,40-dicarboxylic acid bis[(3-silatranylpropyl)
amide] (bipy-sil)
2.5.1. Attempt (1) – Grignard method
In a 250 mL two-neck round bottom flask1 fitted with a reflux
condenser, a rubber septum and a stir bar, a mixture of 1-ethoxysi-
latrane (1.33 g, 6 mmol), anhydrous THF (10 mL), freshly washed
magnesium turnings (121.5 mg, 5 mmol) and a small crystal of
iodine2 was heated to 70 °C under a nitrogen atmosphere. Then, a
concentrated THF solution containing 4,40-dibromo-2,20-bipyridine
(314 mg, 1 mmol) was added dropwise via syringe over half an hour.
The mixture was allowed to reflux for an additional 3 h. The solvent
was removed under reduced pressure, and hexane (30 mL) added to
the crude product. The resulting mixture was filtered, and an off-
white precipitate formed within a day. Spectroscopic examination
(NMR, IR) of the white precipitate indicated that starting material
was isolated.
2,20-Bipyridine-4,40-dicarboxylic
acid
bis[(3-silatranylpropyl)
amide] (bipy-sil) was synthesised following the procedure of Bren-
nan et al. [29] 2,20-bipyridine-4,40-dicarboxylic acid (371 mg,
1.52 mmol) and thionyl chloride (4 mL, 55 mmol) were refluxed
under nitrogen for 3 h, and any remaining unreacted thionyl chlo-
ride was removed under vacuum. To this, a solution containing tri-
ethylamine (450 lL, 3.23 mmol) and dichloromethane (5 mL,
78 mmol) was added. This mixture was then transferred to a solu-
tion of toluene (15 mL) containing 3-aminopropylsilatrane
(712 mg, 3.06 mmol) and heated at 60 °C for 1 h, and then stirred
at ambient temperature for 12 h. The solvent was removed under
vacuum and the dark yellow product purified by column chroma-
tography on silica gel (9 CHCl3: 1 CH3OH) to give a white powder in
10% yield. IR (KBr):
(s), 1463 (s), 1280 (s), 1119 (s), 1094 (s), 757 (s) cmꢀ1
(DMSO-d6, 300 MHz): d 0.35 (t, 4H, J = 6 Hz, –CH2–Si–), 1.50 (m,
v
= 3456 (b), 2937 (m), 2891 (m), 1659 (s), 1540
1H NMR
1
;
Glassware was flame dried before use.
The same method was used with dibromoethane (375 mg, 2 mmol).
2

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K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
2.5.2. Attempt (2) – palladium method
rosilatrane (214 mg, 1 mmol), and hexamethyldisilizane3 (2 mL)
were refluxed for 5 h. A brown powder was isolated, which on
spectroscopic examination (NMR) was identified as starting material
was isolated.
Anhydrous toluene (33 mL) containing 4,40-dibromo-2,20-bipyr-
idine (500 mg, 1.6 mmol), 1-ethoxysilatrane (1.5 g, 7 mmol), tri-
ethylamine (1.0 mL, 7 mmol), triphenylphosphine (2.1 g, 8 mmol)
and PdðPPh3 ) Þ4 (0.37 g, 0.32 mmol) were refluxed for 18 h under
nitrogen. The solution was filtered and distilled. A milky-white
liquid product was isolated. Spectroscopic examination (NMR) of
the milky-white liquid indicated that starting material was
isolated.
2.8. 4,40-Di(trimethylsilyl)-2,20-bipyridine
4,40-Di(trimethylsilyl)-2,20-bipyridine was synthesised following
the procedure of Anderson et al. [33], with some modification. In
a 100 mL two-neck round bottom flask fitted with two rubber
septums and a stir bar under a nitrogen atmosphere, a solution
of n-butyllithium in hexane (0.2 mL, 1.567 M, 0.32 mmol) was
2.5.3. Attempt (3) – butyllithium method
An attempt was made to synthesise 4,40-disilatrane-2,20-bipyri-
dine following the procedure of Reidmiller et al. [32], with some
modification. In a 100 mL two-neck round bottom flask1 fitted with
two rubber septums and a stir bar under a nitrogen atmosphere, a
solution of 4,40-dibromo-2,20-bipyridine (100 mg, 0.32 mmol) in
anhydrous diethyl ether (5 mL), a solution of n-butyllithium in
hexane (0.2 mL, 1.567 M, 0.32 mmol) was added dropwise whilst
cooling to –78 °C. The resulting solution was stirred and main-
tained at –78 °C for 1 h. A pre-cooled (–78 °C) solution of 1-ethox-
ysilatrane (140 mg, 0.64 mmol) in anhydrous diethyl ether (5 mL)
was added to the former solution slowly via cannula. The resulting
solution was stirred and maintained at –78 °C for 1 h, after which
the solution was allowed to warm to room temperature. The sol-
vent was evaporated and pentane (5 mL) added to the residue.
The solution was filtered and the product isolated by distillation.
A yellow liquid product was isolated. Spectroscopic examination
(NMR) of the yellow liquid indicated the presence of aromatic
and silatrane peaks, although not in correct ratios. The product
decomposed rapidly, preventing further analysis by other methods.
added dropwise to
a
solution of 4,40-dibromo-2,20-bipyridine
(100 mg, 0.32 mmol) in anhydrous diethyl ether (5 mL), whilst
cooling to –78 °C. The resulting solution was stirred and main-
tained at –78 °C for 1 h. A pre-cooled (–78 °C) solution of TMS-Cl
(70 mg, 0.64 mmol) in anhydrous diethyl ether (15 mL) was added
to the former solution slowly via cannula. The resulting solution
was stirred and maintained at –78 °C for 1 h, after which the solu-
tion was allowed to warm to room temperature. The solution was
filtered and the product isolated by distillation. A yellow-brown
liquid product was isolated. Spectroscopic examination (NMR) of
the liquid indicated that 4,40-di(trimethylsilyl)-2,20-bipyridine
was isolated in 12% yield. 1H NMR (DMSO-d6, 300 MHz): 0.4
(s, 9H, CH3–Si), 7.2 (d, 1H, J = 3.5 Hz, Ar–H), 7.6 (d, 1H, J = 3 Hz,
Ar–H), 8.5 (m, 2H, Ar–H), 8.7(s, 2H, Ar–H) ppm.
2.9. 4,40-Dicarboxysilylestersilatrane-2,20-bipyridine – attempted ester
linkage
2.5.4. Attempt (3a) – butyllithium and 1-chlorosilatrane
4,40-Dicarboxysilylestersilatrane-2,20-bipyridine – attempted ester
linkage following the procedure of Chen et al. [36], with some mod-
ification. In a two neck round bottom flask fitted with a Dean–Stark
dcbipy (100 mg, 0.4 mmol), 1-ethoxysilatrane (90 mg, 0.4 mmol)
and chlorobenzene (5 mL) were refluxed for 3 h under nitrogen.
The ethanol and chlorobenzene were removed by azeotropic distil-
lation. The pale yellow precipitate that formed was recrystallised
from a 1:1 DCM/hexane solution. However, spectroscopic exami-
nation (NMR, IR) indicated that only starting material was isolated.
An attempt was made to synthesise 4,40disilatrane-2,20-bipyri-
dine following the procedure of Anderson et al. [33], with some
modification. In a 100 mL two-neck round bottom flask, fitted with
two rubber septums and a stir bar, and under a nitrogen atmo-
sphere, a solution of n-butyllithium in hexane (0.2 mL, 1.567 M,
0.32 mmol) was added dropwise to a solution of 4,40-dibromo-
2,20-bipyridine (100 mg, 0.32 mmol) in anhydrous diethyl ether
(5 mL), whilst cooling to –78 °C. The resulting solution was stirred
and maintained at –78 °C for 1 h. A pre-cooled (–78 °C) solution of
1-chlorosilatrane (133 mg, 0.64 mmol) in anhydrous diethyl ether
(15 mL) was added to the former solution slowly via cannula.
The resulting solution was stirred and maintained at –78 °C for
1 h, after which the solution was allowed to warm to room temper-
ature. The solution was filtered and the product isolated by distil-
lation. A yellow-brown liquid product was isolated. Spectroscopic
examination (NMR) of the liquid indicated that only the starting
materials had been recovered.
2.10. 2,20-Bipyridine-4,40-(diethyl phosphonate) (depbipy)
2,20-Bipyridine-4,40-(diethyl phosphonate) (depbipy) was synthes-
ised following the procedure of Kanaizuka [37]. The product was
purified by column chromatography on silica gel (99 CH2Cl2/1 CH3-
OH), to give a white powder in 76% yield. IR (KBr):
2149 (m), 1968 (m), 1637 (s), 1435 (s), 1398 (s), 1365 (s), 1227
(m), 1043 (m), 567 (m) cmꢀ1 1H NMR (CDCl3, 300 MHz): d 1.37
v = 3455 (b),
;
(t, 12H, J = 6 Hz, –CH3), 4.20 (m, 8H, –CH2–), 7.28 (s, 2H, Ar–H),
7.73 (dd, 2H, J = 4 Hz, 4.5 Hz, Ar–H), 8.82 (m, 2H, Ar–H) ppm.
2.6. N-(Trimethylsilyl)pyridine-3-carboxamide
N-(Trimethylsilyl)pyridine-3-carboxamide was prepared follow-
ing the procedure of Gilg et al. [34,35] A mixture of model com-
pound, nicotinamide (250 mg, 2 mmol), chlorotrimethylsilane
(224 mg, 2 mmol), and hexamethyldisilizane (2 mL) were refluxed
for 5 h. A pale yellow powder was isolated in 65% yield. 1H NMR
(DMSO-d6, 300 MHz): ꢀ0.1 (s, 3H, J = 6 Hz, CH3–Si), 0.1 (s, 3H,
CH3–Si), 0.3 (s, 3H, CH3–Si), 7.23 (m, 1H, Ar–H), 8 (m, 1H, Ar–H),
8.6 (dd, 1H, J = 5 Hz, 4.5 Hz, Ar–H), 8.9 (d, 1H, J = 3 Hz, Ar–H) ppm.
2.11. Synthesis of complexes of Ru(II) with bipy, and ligands based on
bipy
2.11.1. cis-[Ru(bipy)2Cl2]
cis-[Ru(bipy)2Cl2] was synthesised following the procedure of
Sprintschnik et al. [38] A dark red-purple microcrystalline product
was isolated in 65–70% yield. IR(KBr):
(s), 1445 (s), 1441 (s), 1315 (s), 1268 (s), 1159 (s), 1019 (s), 769 (s)
cmꢀ1 1H NMR (D2O/DMSO-d6, 300 MHz): d 7.47 (d, 4H, J = 4 Hz,
v = 3460 (b), 1602 (s), 1465
;
2.7. N-(1-Silatranyl)nicotinamide – attempted synthesis
Ar–H), 7.99 (m, 8H, Ar–H), 8.40 (d, 4H, J = 3.5 Hz, Ar–H) ppm.
An attempt was made to synthesise N-(1-silatranyl)nicotin-
amide following the procedure of Gilg et al. [34,35] with some
modification. A mixture of nicotinamide (125 mg, 2 mmol), 1-chlo-
3
The same method was used with toluene, benzene and chlorobenzene with
similar results.

K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
723
2.11.2. [Ru(bipy)2(dcbipy)](PF6)2 (1)
can occur with –COOHd to form –COOD and therefore the Hd pro-
ton would not be observed.
Two synthetic methods were investigated for the synthesis of
[Ru(bipy)2(dcbipy)](PF6)2. The first synthesis followed the proce-
dure of Browne et al. [39]. A red microcrystalline product was
isolated in 58% yield. The second, and higher yielding [Ru(bipy)2(-
dcbipy)](PF6)2 synthesis, followed the procedure of Terpetschnig
et al. [40]. A red microcrystalline product was isolated in 71% yield.
2.11.6. cis-[Ru(dcbipy)2(NCS)2] ‘N3 dye’ (5)
cis-[Ru(dcbipy)2(NCS)2] ‘N3 dye’ (5) was synthesised following
the procedure of Nazeeruddin et al. [23]. The product was isolated
as a dark red-microcrystalline powder 60% yield. IR(KBr):
(b), 2929 (b), 2119 (s), 2002 (s), 1722 (s), 1637 (m), 1553 (s), 1407
(s), 1379 (s), 1308 (s), 1324 (s), 1025 (s), 771 (s) cmꢀ1 1H NMR
v = 3449
IR(KBr):
v
= 3463 (b), 2936 (m), 1739 (s), 1608 (m), 1470 (s), 1449
(s), 1236 (m), 850 (s), 764 (s), 559 (s) cmꢀ1
.
1H NMR (D2O/ DMSO-
;
d6, 300 MHz): d 7.20 (q, J = 5 Hz, Ar–H), 7.55 (dd, J = 2 Hz, 5 Hz, Ar–
H), 7.60 (d, 4H, J = 5 Hz, Ar–Hm), 7.65 (d, 2H, J = 5 Hz, Ar–Hb), 7.82
(d, 2H, J = 7 Hz, Ar–Ha), 7.90 (m, 4H, Ar–Hk, l), 8.37 (d, 4H, Ar–Hj),
8.82 (s, 2H, Ar–Hc), 10.6 (s, 2H, –OHd) ppm (see Fig. 3a and b for
proton assignments).
(D2O, 300 MHz): d 7.97 (d, 4H, J = 5 Hz, Ar–H), 8.82 (s, 4H, Ar–H),
8.91 (d, 4H, J = 5 Hz, Ar–H) ppm (see Fig. 3a for proton
assignments).
2.11.7. cis-[Ru(bipy-sil)2Cl2] (6)
2.11.3. [Ru(bipy)2(dpbipy)](PF6)2 (2)
cis-[Ru(bipy-sil)2Cl2] (6) was prepared using a similar procedure
as cis-[Ru(dcbipy)2Cl2] (4), except bipy-sil (100 mg, 0.1 mmol) was
used in place of dcbipy. A dark red microcrystalline product was
[Ru(bipy)2(dpbipy)](PF6)2 (2) was prepared using the Terpetsch-
nig et al. [40] procedure for [Ru(bipy)2(dcbipy)](PF6)2, except dep-
bipy (190 mg, 0.6 mmol) was used in place of dcbipy, and NaHCO3
was not added. The diethyl ester was hydrolysed by adjusting
the pH to 4 with HCl (4 M). A red microcrystalline product was iso-
isolated in 40% yield. IR(KBr):
v = 3423 (b), 2939 (m), 2892 (m),
1651 (s), 1551 (m), 1463 (s), 1281 (s), 1088 (s), 1022 (s), 916 (s),
759 (s) cmꢀ1
.
1H NMR (DMSO-d6, 300 MHz): d 0.40 (m, 4H, –
CH2g–Si), 1.54 (m, 4H, –CH2ꢀf), 2.60 (m, 12H, –CH2 –N), 2.81 (t,
4H, J = 6 Hz, –CH2e–NH), 3.70 (dt, 12H, J = 6 Hz, –CH2h–O), 7.75
(m, 4H, Ar–Hb), 8.76 (m, 4H, Ar–Ha), 8.78 (s, 2H, Ar–Hc) ppm (see
Fig. 3d for proton assignments). Elemental analysis calcd. (%) for
i
lated in 40% yield. IR(KBr):
v
= 3484 (b), 2993 (m), 1891 (s), 1589
(s), 1541 (s), 1485 (s), 1439 (s), 1191 (s), 1121 (s), 1027 (s), 723
(s), 698 (s) 538 (s) cmꢀ1
.
1H NMR (D2O/ DMSO-d6, 300 MHz): d
7.45–7.52 (m, Ar–Hj,l,m), 7.53–7.60 (m, Ar–Hj,l,m), 7.65–7.75 (m,
Ar–Hj,l,m), 7.76 (dd, 2H, J = 2 Hz, 6 Hz, Ar–Hb), 8.54 (s, 2H, Ar–Hc),
8.79 (dd, 2H, J = 1.5 Hz, 9 Hz, Ar–Ha), 8.85 (t, J = 6 Hz, Ar–Hk)
ppm. 31P NMR (D2O/DMSO-d6, 121.45 MHz): d ꢀ10 (s, 2P, –P–)
ppm (see Fig. 3b and c for proton assignments).
C60H88N12O16Si4RuCl2 (Mr = 1517.740): C 47.48, H 5.84, N 11.07,
Cl 4.67. Found (%): C 47.88, H 5.36, N 11.32, Cl 4.20.
2.11.8. cis-[Ru(bipy-sil)2(NCS)2] (7)
cis-[Ru(bipy-sil)2(NCS)2] (7) was prepared from [Ru(bipy-sil)2Cl2]
(6). [Ru(bipy-sil)2Cl2] (60 mg, 0.04 mmol) was dissolved in DMF
(5 mL) under subdued light. In a separate beaker sodium thiocya-
nate (32 mg, 0.40 mmol) was dissolved in water (1 mL), and then
added to the DMF solution. The resulting mixture was refluxed
and stirred for 6 h under N2. The solution was cooled slowly over-
night, and the solvent removed under vacuum. The powder was
dissolved in water and the suspension filtered through a sintered
glass crucible. The solution was cooled to 4 °C for 18 h. The solu-
tion was allowed to warm to room temperature and within several
hours a microcrystalline solid formed. This was filtered and subse-
quently washed with water, acetone and anhydrous diethyl ether.
A dark red microcrystalline product was isolated in 35% yield.
2.11.4. [Ru(bipy)2(bipy-sil)](PF6)2 (3)
[Ru(bipy)2(bipy-sil)](PF6)2 (3) was synthesised following the pro-
cedure of Brennan et al. [29]. [Ru(bipy)2Cl2] (32 mg, 0.06 mmol)
and bipy-sil (48 mg, 0.07 mmol) dissolved in DMF (3 mL) were
heated to 90 °C under N2 in subdued light for 72 h. Most of the sol-
vent was then removed under vacuum and water (2 mL) added.
The resulting solution was filtered, and to the filtrate was added
a saturated NH4PF6 solution (0.4 mL). The solution was then ex-
tracted with ethyl acetate and the solvent recovered by distillation.
A red powder was isolated in 42% yield. IR(KBr):
v
= 3367 (b), 2961
(m), 2896 (m), 1663 (s), 1551 (s), 1462 (s), 1277 (s), 1066 (m), 918
(s), 802 (s) cmꢀ1. 1H NMR (CDCl3, 300 MHz): d 0.30 (m, 4H, –CH2
–
g
f
i
Si), 1.35 (m, 4H, –CH2ꢀ ), 2.45 (t, 12H, J = 6 Hz, –CH2 –N), 2.75 (m,
4H, –CH2e–NH), 3.29–3.70 (m, 12H, –CH2h–O), 7.27 (d, 4H,
J = 3 Hz, Ar–Hm), 7.41 (d, 4H, J = 2 Hz, Ar–Hl), 7.45 (d, 2H,
J = 2.5 Hz, Ar–Hb), 8.15 (d, 2H, J = 2 Hz, Ar–Ha), 8.20 (t, 4H,
J = 2 Hz, Ar–Hk), 8.77 (m, 4H, Ar–Hj), 9.05 (s, 2H, Ar–Hc) ppm (see
Fig. 3b and d for proton assignments). Elemental analysis calcd.
(%) for C50H60N10O8Si2RuP2F12 (Mr = 1376.249): C 43.64, H 4.39, N
10.18, P 4.50. Found (%): C 43.21, H 4.73, N 9.64, P 4.11.
IR(KBr):
v
= 3419 (b), 3084 (b), 2945 (b), 2070 (s), 2000 (s),
1950 (s), 1664 (s), 1555 (s), 1470 (s), 1407 (s), 1104 (b) cmꢀ1
:
1H
NMR (DMSO-d6, 300 MHz): d 0.39 (m, 4H, –CH2–Si), 1.55 (m, 4H,
–CH2–), 2.62 (m, 12H, –CH2–N), 2.80 (t, 4H, J = 5.5 Hz, –CH2–NH),
3.68 (m, 12H, –CH2–O), 7.78 (m, 4H, Ar–H), 8.76 (m, 4H, Ar–H),
8.82 (s, 2H, Ar–H) ppm (see Fig. 3d for proton assignments). Ele-
mental
analysis
calcd.
(%)
for
C62H88N14O16S2Si4Ru
(Mr = 1563.001): C 47.64, H 5.67, N 12.55, S 4.10. Found (%): C
47.72, H 5.34, N 12.25, S 4.45.
2.11.5. cis-[Ru(dcbipy)2Cl2] (4)
cis-[Ru(dcbipy)2Cl2] (4) was synthesised following the procedure
of Liska et al. [21]. RuCl3ꢂxH2O (60 mg, 0.229 mmol) and dcbipy
(113 mg, 0.463 mmol) were refluxed in DMF under N2 for 8 h.
The resulting solution was cooled slowly overnight, and then fil-
tered. The majority of the DMF solvent was evaporated off to leave
approximately 1 mL of solution. Acetone was then added dropwise
until a dark-red precipitate formed. The precipitate was collected
by filtration and dried under vacuum to give a 55% yield. IR(KBr):
2.12. Preparation of semiconductors (TiO2 and WO3)
The TiO2 substrates were purchased from Dyesol: they consist
of a glass substrate with a TiO2 coated area of 0.8 cm2. The WO3
substrates were prepared by the School of Electrical and Computer
Engineering at RMIT University [7]. The dimensions of the glass
substrates were 190 mm ꢃ 140 mm ꢃ 20 mm, and had a WO3
coated area of 0.25 cm2. The glass platelets of TiO2/WO3 substrates
were annealed, prior to coating with ‘dye’, by placing them in a
tube furnace open to the air. The substrates were then heated
slowly (2 °C/min) from room temperature to 450 °C, and held at
450 °C for an hour and then cooled at a rate of 2 °C/min.
v
= 3467 (b), 3115 (m), 2475 (b), 1989 (m), 1725 (s), 1660 (m),
1463 (s), 1368 (s), 1309 (s), 1142 (s), 1070 (s), 1014 (s), 767 (s),
685 (s) cmꢀ1 1H NMR (D2O, 300 MHz): d 7.93 (d, 4H, J = 4 Hz,
.
Ar–Hb), 8.84 (s, 4H, Ar–Hc), 8.91 (d, 4H, J = 5 Hz, Ar–Ha) ppm (see
Fig. 3a for proton assignments). Note: In D2O, proton exchange

724
K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
2.13. Preparation of test batch solar cells (TiO2 and WO3)
rate of 60 nm/min, and using a 1 cm path-length quartz UV cell.
50–100 mg of sample was dissolved in 25 mL of an appropriate
solvent (water, ethanol, DMF, or acetonitrile). The molar extinction
Dyes containing ruthenium complexes prepared in-house and
possessing carboxylic or phosphonic linkages were coated on to
the annealed semiconductor substrate by immersion in a 0.3 mM
(acetonitrile or ethanol) solution of the dye, for 24 h. Dyes contain-
ing a silyl linkage were adhered to the semiconductor by immers-
ing the oxide substrate in a 0.3 mM DMF solution of the dye and
heating (ꢁ70 °C) for 1 h, in subdued light. In both cases a thin col-
oured film formed on the semiconductor, which was subsequently
washed with ethanol and air dried. Two holes were then drilled
into a second FTO-glass plate (FTO – fluorine doped tin oxide),
the counter electrode; this was done to allow the injection of the
liquid electrolyte. A heat-activated gasket (SurlynÒ-30), with a film
coefficient of the molecule (
e
) can be calculated from the spectra
using the Beer–Lambert Law:
A ¼ ꢃ c ꢃ l
e
ð1Þ
(A = absorbance, c = concentration, l = path length).
3. Results and discussion
3.1. Preparation and characterisation of complexes
A series of ruthenium(II) coordination complexes containing
either bis- or tris-bipy ligands were investigated (Table 1). Whilst
the tris-chelated ruthenium complexes are known to be less effi-
cient in terms of solar energy capture, not all of the ligands with
anchoring groups could be incorporated into bis-chelated ruthe-
nium complexes. For example, the dpbipy analogue could not be
successfully prepared in a bis-form. Therefore, the tris complexes
were used to compare the efficiency of various anchoring ligands
and to make a reasonable comparison between existing ruthenium
dyes and those prepared in this study.
The bis-bipy ruthenium dye complexes were synthesised by
refluxing ruthenium(III) chloride with various 2,20-bipyridine
derivatives in DMF, under N2, to form the ruthenium(II) cis-
dichloro species (4,6) [21]. These were then refluxed with excess
NaNCS to form the cis-dithiocyanato species (5,7) (Scheme 1).
The N-coordinated thiocyanato group enabling fine-tuning of the
physico-chemical properties of such ruthenium species, to opti-
mise their function in a DSSC. The tris-bipy ruthenium complexes
(1–3), whereby one of the bipy ligands contained a linker and the
others were unsubstituted, were synthesised by refluxing
[Ru(bipy)2Cl2] with various 2,20-bipyridine derivatives in DMF un-
der N2. Each complex was precipitated from solution by addition
of excess NH4PF6 (Scheme 2).
The reaction scheme for the preparation of the hydrolysis-
resistant bipy-sil ligand is illustrated in Scheme 3. The tris-bipy
ruthenium complex prepared by Brennan et al. [29], bis(2,20-bipyr-
idine)-4,4-dicarboxyl-(3-silatranylpropylamide)ruthenium(II)
dihexafluorophosphate, [Ru(bipy)2(bipy-sil)](PF6)2, was also pre-
pared here, and numbered (3), together with two new bis-bipy-sil
ruthenium complexes: [Ru(bipy-sil)2Cl2] (6), and [Ru(bipy-sil)2
(NCS)2] (7). [Ru(bipy-sil)2Cl2] (6) was prepared by coordination of
the bipy-sil ligand to ruthenium(III) trichloride, utilising methods
published previously, but replacing dcbipy with bipy-sil (see Sec-
tion 2). [Ru(bipy-sil)2(NCS)2] (7), was prepared by reacting [Ru(bi-
py-sil)2Cl2] (6) with excess sodium thiocyanate. All ligands and
complexes were characterised using FT-IR, 1H, 13C and, where pos-
sible, 31P NMR, UV–Vis spectroscopic analysis (see Section 2 for
summary FTIR and NMR data; and Supplementary data for ex-
tended interpretation).
thickness of 30 lm was then placed around the dye-coated surface.
The second FTO-glass plate was then placed on top and heated to
approximately 100 °C. The liquid electrolyte, Iꢀ=I3ꢀ was then in-
jected into the cell through the pre-drilled holes, approximately
1 drop in each hole. The holes were then sealed with a heat acti-
vated gasket. See Fig. 4 for solar cell schematic.
2.14. Solar cell testing unit
All cells containing the metal complex ‘dye’ were tested on a
custom-made solar cell testing station [7]. The station comprised
an ABET Technologies LS-120 solar light source fitted with an AM
1.5 filter, where the illumination power density was calibrated to
90 mW/cm2, according to the protocol described by Zheng et al.
[7]. Voltage and current measurements were collected using a
Keithly 2602 sourcemeter.
2.15. Instrumental measurements
FT-IR absorption spectra were obtained on a Perkin Elmer Spec-
trum 100 spectrometer using KBr discs. The discs were prepared by
grinding the sample (1–2 mg) and IR grade KBr (100 mg), into a
homogeneous powder using a mortar and pestle. The powder
was then placed in a die press (Specac 13 mm) and compacted un-
der vacuum for approximately 5 min using 8 tonne of pressure. The
spectra were collected using the following conditions: scan range
4000–400 cmꢀ1; number of scans 8; single beam; resolution:
4 cmꢀ1. Selected elemental analysis (C, H, N, S, P, halogens) was
performed by the Campbell Microanalytical Laboratory at the Uni-
versity of Otago, New Zealand. NMR Spectra were collected using a
Bruker AVANCE 300 MHz NMR spectrometer, at a frequency of
300 MHz for 1H, 75.45 MHz for 13C and 121.45 MHz for 31P. Each
sample was locked and shimmed for spectral optimisation. Sam-
ples were prepared using deuterated solvents: typically 20 mg of
sample was dissolved in the appropriate solvent (either D2O, CDCl3
or DMSO-d6 (CD3)2SO) and placed in a 5 mm diameter glass NMR
tube. UV–Vis spectra were then recorded on a Varian Cary 50 spec-
trophotometer, over the wavelength range 200–800 nm, at a scan
Two holes for
the electrolyte
Table 1
Observed wavelength maxima (kmax) and molar extinction coefficient (
complexes (1)–(7).
e) data for
FTO-glass
Dyea
Number
kmax (nm)
e )
(L molꢀ1 cmꢀ1
[Ru(bipy)2(dcbipy)](PF6)2
[Ru(bipy)2(dpbipy)](PF6)2
[Ru(bipy)2(bipy-sil)](PF6)2
[Ru(dcbipy)2Cl2]
[Ru(dcbipy)2(NCS)2] N3
[Ru(bipy-sil)2Cl2]
(1)
(2)
(3)
(4)
(5)
(6)
(7)
460
450
457
467
492
485
500
14350
17600
10520
20790
29630
11070
20750
Gasket
FTO-glass with
TiO2 coating
[Ru(bipy-sil)2(NCS)2]
Dye (metal complex)
a
The dcbipy, dpbipy and bipy-sil ligands in complexes (1)–(7) are shown in
Scheme 1.
Fig. 4. Schematic of DSSC.

K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
725
R
R
R
R
R
R
R
R
N
N
NaNCS
reflux
DMF
+
RuCl3
N
N
N
N
DMF
reflux
N
Ru2+
N
Ru2+
NCS
(reduction)
N
Cl
N
NCS
Cl
R
R
N
where R =
O
O
Si
O
O
O
O
C
C
P
OH
OH
OH
HN
phosphonate
"dpbipy"
silatrane-amide
"bipy-sil"
carboxylate
"dcbipy"
Scheme 1. Overall reaction sequence for the formation of bis-chelated Ru(II) complexes (4–7).
N
DMF
+
RuCl3
N
N
reflux
N
Ru2+
N
(reduction)
N
Cl
Cl
R
R
DMF
reflux, then
NH4PF6
N
N
N
N
N
N
–
2 PF6
Ru2+
N
N
R
R
Scheme 2. Overall reaction sequence for the formation of tris-chelated Ru(II) complexes (1–3).
3.2. UV–Vis spectroscopy
solid-state, all three complexes were orange-red in colour and this
colour persisted in solution.
UV–Vis absorption studies of tris-bipy complexes (Fig. 5):
[Ru(bipy)2(dcbipy)](PF6)2 (1), [Ru(bipy)2(dpbipy)](PF6)2 (2), and
[Ru(bipy)2(bipy-sil)](PF6)2 (3) [29] were carried out in DMF solution
at concentrations of 2, 3 and 4 mM. The molar extinction coeffi-
cients (Table 1) are averages of the three concentrations. In the
In the spectrum of [Ru(bipy)2(dcbipy)](PF6)2 (1), one distinct
band in the visible region between 400 and 600 nm was observed
with a kmax at 460 nm. The mean molar extinction coefficient of (1)
was calculated to be 14350 320 L molꢀ1 cmꢀ1 using the Beer–
Lambert relationship (see Table 1 for a summary of kmax and
e

726
K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
COOH
OH
HOOC
O
Si
O
O
N
N
N
HO
OH
thionyl chloride
reflux, N2
Heat
Dean-Stark
H2N
COCl
ClOC
OH
N
+
O
H2N
Si
O
N
N
O
toluene
triethylamine
DCM
N
O
O
H
Si
NH
N
Si
O
O
O
O
C
C
N
O
O
N
N
Scheme 3. Reaction scheme for synthesis of bipy-sil [23].
complexes, the bands are believed to arise from metal-to-ligand
charge transfer (MLCT) transitions. The UV–Vis absorption studies
of bis-bipy complexes, [Ru(dcbipy)2Cl2] (4), and [Ru(dcbipy)2(NCS)2]
(5), were carried out in acetonitrile solution. In the solid state, both
(4) and (5) are dark red in colour, and this dark colour persisted in
solution for complex (5), but (4) formed a red-pink solution. The
UV–Vis spectrum of complex (4) exhibited one broad absorption
band from 410 to 510 nm, with a pronounced shoulder from 420
to 440 nm, not unlike the absorbance profiles of (1) and (2). The
absorption band of (4) had a kmax at 467 nm, with a corresponding
mean molar extinction coefficient of 20790 510 L molꢀ1 cmꢀ1
.
The UV–Vis spectrum of complex (5), the favoured commercial
‘N3’ dye, [Ru(dcbipy)2(NCS)2], displayed two intense broad absorp-
tion bands in the visible region: band (1) from 425 to 550 nm
(kmax(1) = 492 nm,
from 610 to 750 nm (kmax(2) = 685 nm,
e
(1) = 29630 870 L molꢀ1 cmꢀ1); and band (2)
Fig. 5. UV–Vis spectra of complexes (1), (2) and (3) (3 mM, 300–800 nm).
e
(2) = 23380 690 L molꢀ1
cmꢀ1) as seen in Fig. 6, which is characteristic of bis-bipy ruthe-
nium complexes and has been reported previously for N3 [16].
UV–Vis absorption studies of the two new bis-bipy-sil com-
plexes (6) and (7) were carried out in DMF solution. In the solid
state both (6) and (7) are dark red in colour and this colour per-
sisted in solution. Like the commercial N3 dye, two broad bands
were observed in the spectrum of (6) [Ru(bipy-sil)2Cl2], one in
the UV region, band (1) from 340 to 370 nm (kmax(1) = 365 nm,
data). The UV–Vis spectrum of the dpbipy complex [Ru(bipy)2
(dpbipy)](PF6)2 (2) is similar to that of the dcbipy complex (1), with
a broad band observed between 400 and 500 nm (kmax = 450 nm;
shoulder k = 425 nm), and a second small band at 545 nm. The
structure of the absorption at 450 nm is characteristic of tris-bipy
ruthenium complexes and arises from metal-to-ligand charge
transfer (MLCT), from the metal t2 orbital to the excited t1 orbital
[41,42]. The mean molar extinction coefficient of (2) was slightly
higher than (1) at 17600 640 L molꢀ1 cmꢀ1. The UV–Vis spec-
trum of the Brennan [23] bipy-sil complex, [Ru(bipy)2(bipy-
sil)](PF6)2 (3), is similar to both (1) and (2), with a kmax at
457 nm, but with a lower absorbance. The mean molar extinction
e
(1) = 12716 180 L molꢀ1 cmꢀ1); and the second, band (2) in the
visible from 440 to 540 nm (kmax(2) = 485 nm, e(2) = 11070 150 L
molꢀ1 cmꢀ1).
The UV–Vis spectrum of complex, [Ru(bipy-sil)2(NCS)2] (7)
(Fig. 7), is similar to that observed in complex (6). But the bands
in (7) are better defined and contain a band in the UV-region rang-
ing from 340 to 400 nm (band (1) with kmax at 371 nm and molar
extinction coefficient of 20690 485 L molꢀ1 cmꢀ1, and the second
visible band range from 460 to 530 nm (band (2) with a kmax at
coefficient of (3) was calculated to be 10520 440 L molꢀ1 cmꢀ1
.
As stated previously, tris-bipy ruthenium complexes have lim-
ited absorbance in the visible region, which has been observed
here in the UV–Vis spectra of complexes (1), (2) and (3). In all three

K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
727
observed in their absorption data (refer to Table 1). But they re-
quire testing in a simulated solar cell to rate their efficiency in con-
ditions (b) and (c). As mentioned earlier (Section 1), the surface
binding of the carboxylic acid and phosphonic acid analogues is
not stable across a wide pH range, however the silyl ester bond
is, and this is one of its advantages [28]. The tris-bipyridine based
ruthenium complexes were found to desorb from TiO2 in aqueous
solution at pH ꢁ4 for the carboxylate, and pH ꢁ7 for the phospho-
nate [24,28,29]. The stability of the silyl linkage allows it to func-
tion over a wider pH range and in the presence of significant
amounts of organic solvents [43]. The silatrane is able to bind to
the metal oxide surface after mild heating, upon which the trieth-
anolamine group is lost and a silyl ester bond is formed at the sur-
face, according to the reaction shown in Fig. 8.
All seven complexes were deposited successfully onto nano-
structured TiO2 and/or WO3 films supported on an FTO-glass (Fluo-
rine doped Tin Oxide) wafer. This was achieved by immersing an
annealed, TiO2 or WO3 coated, FTO-glass substrate into a 0.3 mM
solution of the dye species. Complexes (1) [Ru(bipy)2(dcbipy)]
(PF6)2, (2) [Ru(bipy)2(dpbipy)](PF6)2, (4) [Ru(dcbipy)2Cl2] and (5)
[Ru(dcbipy)2(NCS)2] were dissolved in acetonitrile or ethanol, and
the substrate immersed in the solution for 24 h. Whilst complexes
(3) [Ru(bipy)2(bipy-sil)](PF6)2, (6) [Ru(bipy-sil)2Cl2] and (7) [Ru
(bipy-sil)2(NCS)2] were dissolved in DMF, and then heated gently
at approximately 70 °C for 1 h, under subdued light, in a glass con-
tainer fitted with a condenser. The coated-substrates were then
washed with ethanol, to remove excess dye, and air dried.
Fig. 6. UV–Vis spectrum of complex (5) ‘N3 dye’ (3 mM, 300–800 nm).
A thin film of the dye solution coated the substrate, as evi-
denced by a colour change. Attempts to quantify the degree of
binding of the dye to the coated substrates by FT-IR or Raman spec-
troscopy were not successful, as there were no observable differ-
ences in the spectra of the pure semiconductor layer and that of
the dye deposited on the semiconductor, despite an obvious
change in the colour of the substrate. This is most likely due to
the very thin film of the dye i.e. too low a concentration of the
dye to be detected. As a result, the most effective way to determine
whether the dye had adhered to the semiconductor was by visual
comparison. Photographs of the DSSCs produced using complexes
(1) to (7) are shown in Fig. 9.
The performance of the DSSCs was tested using a custom-made
solar cell testing station, with a simulated solar light source. The
recorded current and voltage for each of the solar cells is listed
in Table 2. The voltage is the energy carried by the charge, and in
DSSCs this corresponds to the difference between the Fermi level
(which is the energy of the electrons in a semiconductor under illu-
mination), compared to the Nernst potential of the electrolyte re-
dox couple [4]. The macroscopic voltage and current are the sum
total of all the individual molecular photo-excitation events. It is
found that the array generates a voltage which can be modelled
as a series of cells linked in parallel. Hence, the numerical value
of the voltage is determined by the characteristics of the dye/semi-
conductor combination, and its capacity (total Joule/Coulomb) by
the number of such molecular units in the array [4,44–46].
The current is directly linked to the efficiency of electron trans-
fer between the semiconductor and the dye, and the reductive
Fig. 7. UV–Vis spectrum of complex (7) (3 mM, 300–800 nm).
500 nm and molar extinction coefficient of 20750 487 L molꢀ1
-
cmꢀ1. Hence, the absorbance and extinction coefficient values are
significantly greater than those observed for (6), and the Brennan
bipy-sil complex (3) [29].
As expected, the bis-bipy ruthenium complexes absorbed more
visible light than the tris-bipy ruthenium complexes, as indicated
by the presence of two absorption bands (Figs. 6 and 7) and the
higher e values (Table 1). The absorption bands have been assigned
to metal-to-ligand charge transfer (MLCT) transitions, whilst the
narrow intense bands at ꢁ300 nm observed in the UV region are
attributed to ligand p–
p⁄ transitions.
3.3. Solar cells
Clearly, the most efficient dye complexes will be those that: (a)
absorb the most light; (b) are able to transfer the excited electron
to the semi-conductor substrate; and (c) are readily reduced to the
starting complex, to continue the photo-excitation redox cycle (see
Fig. 1). All seven complexes (1)–(7) satisfy condition (a), which was
O
O
Si
+ N(CH2CH2OH)3
Si
O
Ti
O
O
O
OH OH OH
Ti Ti Ti
O
Ti
O
Ti
heat
N
Fig. 8. Silyl linkage: ‘‘chemisorption’’ to TiO2.

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K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
Fig. 9. Photos of DSSCs produced using complexes (1)–(7).
Table 2
Current, voltage and efficiency of DSSCs prepared on TiO2/FTO (1)–(7), and WO3/FTO (7).
Dye
Number
Short circuit current density, Isc (mA)
Open circuit voltage, Voc (mV)
Fill factor (ff)a
Efficiency (%)b
[Ru(bipy)2(dcbipy)](PF6)2
[Ru(bipy)2(dpbipy)](PF6)2
[Ru(bipy)2(bipy-sil)](PF6)2
[Ru(dcbipy)2Cl2]
[Ru(dcbipy)2(NCS)2] N3
[Ru(bipy-sil)2Cl2]
(1)
(2)
(3)
(4)
(5)
(6)
(7) TiO2
(7) WO3
0.184
0.002
0.110
1.000
1.352
0.163
0.360
0.985
390
140
160
590
650
350
480
160
0.19
0.24
0.23
0.25
0.26
0.25
0.27
0.23
0.015
0.00007
0.0045
0.164
0.254
0.016
0.052
0.040
[Ru(bipy-sil)2(NCS)2]
[Ru(bipy-sil)2(NCS)2]
a
The fill factor (ff) was calculated using Eq. (2) [2]
Impp ꢃ Vmpp
ff ¼
ð2Þ
ISC ꢃ VOC
Impp is the current measured at the power point (maximum), the power point is the maximum product obtained from the cell voltage and
current. Vmpp is the voltage measured at the power point (maximum). ISC is the photocurrent density. VOC is the voltage measured at short
circuit.
b
The efficiency was calculated using the ‘power conversion efficiency’ Eq. (3) [2]
ISCVOCff
gglobal
¼
ð3Þ
IS
global is the Overall conversion efficiency. ff is the ‘‘fill factor’’, which is the ratio of the maximum power output of the cell to the potential
power output of the cell. IS is the intensity of the incident light (90 mW/cm2 from calibration).
g
regeneration of the metal centre. The current can be modelled as a
series sum of the number of electrons collected from each molec-
ular event; it is this factor which is critical in the efficiency of pho-
ton-to-electrical conversion, and will be dependent on the extent
of coating, the tightness of dye binding, and the facilitation of eꢀ
transfer from the dye to the semiconductor [4,13,44–46].
The tris-bipy complexes (1) to (3) formed light yellow-orange
layers on the semiconducting surface, the light colour of the dye
on the surface implies that only a limited amount of dye has been
adsorbed on the oxide (two anchoring groups compared to four in
the bis-bipy derivatives). This in turn limits the amount of light
which can be absorbed, and therefore a high current or voltage
was not expected. The main purpose of investigating the tris-bipy
ruthenium complexes was to compare them with the bis-bipy com-
plexes and note any effects of the anchoring ligands. In particular,
the Brennan complex [29], which is the only previous report of a
tris-bipy-sil complex binding to a semiconductor substrate (in their
case SnO2). To the best of our knowledge, dye complexes contain-
ing silyl linkers have not been tested previously in TiO2 or WO3
based DSSCs.
Of this group, the anchoring ligand which gave the best current,
voltage and overall efficiency was dcbipy (1) whilst dpbipy (2) gave
surprisingly low results considering that the molar extinction coef-
ficient of complex (2), [Ru(bipy)(dpbipy)](PF6)2, was the highest of

K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
729
the three tris-bipy complexes (17603 L molꢀ1 cmꢀ1), indicating
Ru
strong light absorption in solution. The bipy-sil complex (3) [29]
gave about 60% of the current, and 40% of the voltage observed
for the dcbipy complex (1). The efficiency of this cell is therefore
quite low; approximately 30% that of the dcbipy complex cell. This
is likely due to inefficient electron-transfer through the long satu-
rated propyl chain of the silyl linker.
N
N
Considerably more promising results were attained for the bis-
bipy systems. Complex (4), [Ru(dcbipy)2Cl2], formed a dark orange-
red layer on the semiconductor surface, and the voltage (590 mV)
and current (1 mA) of the cell were good. These results improved
for the commercially-favoured dithiocyanato analogue, (5,N3)
[Ru(dcbipy)2(NCS)2], which gave a dark orange-red layer on the
semiconductor surface, and a measured current and voltage of
1.352 mA and 650 mV, respectively. Complex (6), [Ru(bipy-sil)2Cl2],
formed a dark red layer on the semiconductor surface, but despite
the appearance, the current (0.163 mA) and voltage (350 mV) were
quite low, with the measured fill-factor of 25%, then the overall
efficiency of the dye is 0.016%. This is however, on comparison
with (3) [29], a 4-fold increase in efficiency.
Complex (7), [Ru(bipy-sil)2(NCS)2], gave a significant improve-
ment in the current, voltage and overall efficiency when compared
to (6) and (3). Complex (7) also formed a dark-red film over the
TiO2. Whilst (6) and (3) contain the same silyl ligand, complex
(7) also contains the thiocyanate anion which is known to improve
efficiency and extend wavelength absorption (see Fig. 7). The
measured current and voltage were 0.360 mA and 480 mV, respec-
tively, and the overall efficiency 0.052%. Thus, for (7), the voltage
has increased by 130 mV, and the current, and hence the overall
efficiency, has increased 4-fold compared to that of (6), and in-
creased 10-fold to that of (3) [29]. Therefore, by changing the coun-
ter-anion from Clꢀ to NCSꢀ the efficiency of the bipy-sil based cell
can be improved considerably.
C
O
O
C
HN
NH
poor e– transfer
efficiencies
Si
Si
O
O
O
O
O
O
TiO2 Semiconductor surface
Fig. 10. The long, non-conjugated propyl chain linker of bipy-sil to TiO2.
favourable strategies were: (a) direct addition (Si–C) of a silatrane
group to the bipyridine ligand, (b) amide linkage (Si–N(H)–C(O)–
C), or (c) ester linkage (using a carboxylate group – Si–O–C(O)–C),
as indicated in Fig. 11.
Strategy (a) direct linkage was investigated initially, in an at-
tempt to produce the target ligand: 4,40-disilatrane-2,20-bipyridine.
Three different organic schemes ((1) Grignard reagent, (2) Pd-cat-
alyst, and (3) butyllithium), using a silatrane form of tetraethylor-
thosilicate (TEOS) (in place of 1-(3-aminopropyl)triethoxysilane
used in bipy-sil synthesis) were trialled.
Amongst the solar energy community, there is also interest in
using the intrinsic semiconductor, WO3, as a substrate in DSSCs,
as WO3 can absorb light directly. When complex (7) was adsorbed
onto
a WO3 semiconductor substrate, the current measured
(0.985 mA) was significantly higher than that observed with the
same complex adsorbed on TiO2 (0.360 mA), and comparable to
that of the bis-dcbipy complex (4) However, the voltage measure-
ment was significantly lower at 160 mV and is attributed to a com-
bination of low dye coverage, which may be due to poor adhesion
of the dye to the substrate, and the difference between the Fermi
level of WO3, when compared to the Nernst potential.
In summary, the measured current and voltage, on TiO2 sub-
strates, of the two new bis-bipy-sil complexes (6) and (7) which
contain silyl linkages, are lower than those of bis-dcbipy analogues
(4) and (5), but significant improvements to the overall efficiency
have been achieved by replacing the chloride counter ion with
thiocyanate, and there is considerable potential for further
improvement through modification of the silyl linker.
3.4.1. (a) Direct linkage: (1) Grignard reagent method
The predicted reaction sequence, using 4,40-dibromo-2,20-bipyr-
idine (bipy-Br2) as the starting material, is illustrated in Fig. 12.
Two methods were investigated to activate the magnesium
turnings, dibromoethane and iodide. Dibromoethane was found
to be an ineffective activating agent for the bipyridine. This result
M
=
Direct bond
O
3.4. Synthesis of modified silyl linkers
N
N
The presence of saturated bonds in the bridge between the
anchoring group and the dye is known to lower the efficiency of
electronic coupling between the dye and the semiconductor sub-
strate [47]. Thus, the non-conjugated propyl chain between the
amide group and the silatrane (bipy-sil) in (6) and (7) is likely to
reduce electron transfer significantly (Fig. 10). To improve this
transfer/efficiency, the linker either needs to incorporate conjuga-
tion, e.g. replace the propyl chain with a phenyl group, or the dis-
tance between the dye and substrate needs to be considerably
shorter e.g. by removal of the amide-propyl group (Fig. 10). The
former strategy was considered, but the cost of the starting
material proved prohibitive, and thus not a viable option. More
C
O
O
H
N
This type of silyl linkage
ensures tight binding
Si
Si
C
O
O
O
O
O
O
TiO2 semiconductor surface
Fig. 11. Short silyl linkage options: direct addition (Si–C); amide linkage (Si–N(H)–
C(O)–C); and ester linkage (Si–O–(O)C–C).

730
K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
Br
Br
BrMg
MgBr
Magnesium turnings
THF, (I2 or C2H4Br2)
reflux, N2
N
N
N
N
O
O
Si
O
O
N
N
N
?
O
O
O
O
Si
O
Si
O
N
N
Fig. 12. Grignard reaction sequence – target ligand 4,40-disilatrane-2,20-bipyridine.
was not unexpected as a similar method, whereby 2-bromopyri-
dine was reacted with magnesium powder and dibromoethane un-
der sonication, has been reported in the literature [48], and was
also found to be unsuccessful. Iodine, a more aggressive activator,
appeared to be more effective in activating the magnesium – oxi-
dation was observed at the magnesium surface. However, reaction
of the subsequent ‘Grignard reagent’ with 1-ethoxysilatrane was
ultimately unsuccessful, and NMR and mass spectroscopy con-
firmed that only the starting materials bipy-Br2 and 1-ethoxysila-
trane, were isolated.
It is possible that the Grignard reagent did not form: this is dif-
ficult to ascertain, as isolation of the reagent is often not possible
owing to its instability, and thus immediate use is preferential.
Also, whilst Grignard reagents are able to form quite readily with
phenyl rings, pyridine rings generally prove to be more trouble-
some. Nitrogen is more electronegative than carbon, and therefore
3.4.3. (a) Direct linkage: (3) Butyllithium method
The third synthetic method for direct linkage involved the use
of butyllithium, which out of all the methods attempted in forming
a direct silicon–carbon bond, was the most successful. Riedmiller
et al. have reported the successful reaction of n-butyllithium and
TEOS at ꢀ78 °C under N2 to form 3-triethoxysilylpyridine [32]. In
general, this method has been found to be successful for the
formation of ortho- and meta-substituted bromo-pyridine systems,
but para-substituted bromo-pyridine systems were not investi-
gated. The Riedmiller et al. [32] method was adapted here using
4,40-dibromo-2,20-bipyridine and 1-ethoxysilatrane instead of
3-bromopyridine and tetraethoxysilane. NMR data of the product
indicated that peaks corresponding to the silatrane group and aro-
matic groups were present, although not in the expected ratios.
Unfortunately, the product decomposed rapidly, and hence further
analysis could not be undertaken. This route was therefore not
considered to be viable, as the compound was unstable under
ambient conditions.
the carbon
p-orbitals may be more attracted to the nitrogen atom
thus deforming the orbitals, and consequently producing
p-defi-
cient pyridine rings [49]. In addition to this, the lone pair of elec-
trons on the nitrogen are basic and not delocalised over the ring.
Subsequently, a Pd-catalysed reaction was investigated as an
alternative.
In summary, the formation of a direct Si–C bond from a Br–C
bond in a para-substituted bromopyridine ring was not found to
be possible by any of the three organic routes, using 1-ethoxysila-
trane. This may be due to the pyridyl-nitrogen atoms inhibiting the
formation of a direct bond, the insufficient reactivity of the 1-
ethoxysilatrane group to enable this formation, and/or the bulky
nature of the latter. Attempts to react bipy-Br2 with 1-chlorosilatra-
ne were also unsuccessful. In a final attempt to form a direct Si–C
bond on bipy, a more reactive and less-bulky group (trimethylsi-
lane, TMS) was investigated. 4,40-di(trimethylsilyl)-2,20-bipyridine
was produced successfully by reaction of TMS-Cl with 4,40-dibro-
mo-2,20-bipyridine, as confirmed by 1H NMR data (see Section 2).
Unfortunately, the material proved to be extremely unstable,
decomposing rapidly in the presence of air, and could not be tested
in a DSSC.
3.4.2. (a) Direct linkage: (2) Pd-catalysed method
The second route employed a Palladium-catalyst, Pd(PPh3)4. The
use of Pd-catalysts in organic synthesis has been studied exten-
sively, in particular for the formation of C–C bonds. Indeed, in this
paper, and in the literature [37], a Pd-catalyst was used success-
fully to form a C–P bond, in the synthesis of 4,4-diphosphonic
acid-2,20-biypridine (dpbipy). However, the use of a Pd-catalyst
with bipy may lead to deactivation of the Pd-catalyst, as bipy can
compete with the triphenylphosphine ligand for coordination to
the metal centre. To limit this from occurring, a large excess of tri-
phenylphosphine was added to the reaction mixture [50–51]. The
initial oxidative-addition of the Pd-catalyst is analogous to the for-
mation of the Grignard reagent, where the carbon–halogen cova-
lent bond is cleaved to form a new complex. However, this
method was also unsuccessful. NMR data indicating that only the
bipyridine starting material was isolated.
3.4.4. (b) Amide linkage: (1) TMS-Cl and 1-chlorosilatrane method
To determine whether a silicon-amide bond (Si–NH–C(O)), or a
direct silicon bond (Si–C) to the pyridine ring would be possible,
reaction of trimethylsilyl chloride (TMS-Cl) with an amide-
pyridine or bromo-pyridine, followed by reaction with 1-chlorosi-
latrane (a more reactive group than ethoxy), was investigated.
N-(trimethylsilyl)pyridine-3-carboxamide has been prepared

K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
731
OH
O
O
O
Benzene
Si
O
Si
O
3 ethanol
benzene
N
O
N
O
O
HO
OH
Cl
HOOC
COOH
or
N
N
N
N
?
O
O
O
O
O
Si
Si
O
O
O
O
O
C
C
N
N
Fig. 13. Carboxy-sil reaction sequence – target ligand 4,40-dicarboxysilatrane-2,20-bipyridine.[46].
previously by Gilg et al. [34,35], and this was repeated successfully
in this work (as confirmed by 1H NMR data, see Section 2). How-
ever, reaction of nicotinamide with 1-chlorosilatrane (target
ligand; N-(1-silatranyl)nicotinamide) proved to be more difficult,
with no reaction taking place. This route was therefore not
explored with substituted bipy derivatives. As with the Grignard
(direct Si–C) route, a possible reason for the lack of reaction may
be due to the bulkier nature of the silatrane group when compared
to that of trimethylsilyl. There is also the possibility that the
hexamethyldisilizane may compete with the chlorosilatrane for
bonding to the ring, and it is may be possible that both hexame-
thyldisilizane and TMS-Cl contribute to the formation of an
amide-silicon bond. Synthetic methods in which hexamethyldisil-
izane was replaced by e.g. benzene, toluene, chlorobenzene were
also investigated, but unfortunately, were also unsuccessful.
in DSSCs, was tested and compared to previously favoured carbox-
ylate (dcbipy) and phosphonate (dpbipy) anchors. The bipy-sil lin-
ker has been trialled on SnO2 [29], but has not been explored
previously with TiO2 or WO3 DSSC systems.
The linkers were examined as both tris-bipy systems:
[Ru(bipy)2(dcbipy)](PF6)2 (1), [Ru(bipy)2(dpbipy)](PF6)2 (2), and (3)
[Ru(bipy)2(bipy-sil)](PF6)2, and bis-bipy systems with either cis-di-
chloro: [Ru(dcbipy)2Cl2] (4) and [Ru(bipy-sil)2Cl2] (6), or cis-dithio-
cyanato ligands: [Ru(dcbipy)2(NCS)2] (5) and [Ru(bipy-sil)2(NCS)2]
(7). From UV–Vis spectroscopy solution measurements, all seven
complexes were found to have the desired properties for solar en-
ergy capture. Whilst the commercial carboxylate linker, dcbipy,
was found to outperform the phosphonate linker, dpbipy, and the
silyl linker, for both the tris-bipy and bis-bipy systems, in terms
of the current, voltage and efficiency of their respective DSSC cells,
it must be said that this is the first test of a silyl linker in TiO2 and
WO3 DSSCs, and that the overall performance of the silyl linker in
TiO2 based cells has been significantly improved over the course of
this study. A 10-fold increase in efficiency was attained in moving
from the Brennan complex, [Ru(bipy)2(bipy-sil)](PF6)2 (3) [29], to
[Ru(bipy-sil)2Cl2] (6) to [Ru(bipy-sil)2(NCS)2] (7). Also, despite the
disappointing voltage of (7) on WO3, the current was comparable
to that of the dcbipy-TiO2 system (4), [Ru(dcbipy)2Cl2]. Further
experiments to optimise the binding of (7) to WO3 are likely to ex-
tend the performance well beyond that of the currently-favoured
N3 dye (5), [Ru(dcbipy)2(NCS)2], on TiO2.
In this work, despite significant effort, strategies to synthesise
4,40-disilyl-functionalised bipyridyl ligands, with a short distance
(more efficient e-transfer pathway) between the Si link and the
bipy framework, by formation of (a) direct Si–C bonds, (b) amide
Si–NH–C(O) links, or (c) ester Si–O–C(O) bonds to the silatrane,
proved unsuccessful, or the products were insufficiently stable to
allow their incorporation into Ru(II) complexes, and subsequent
DSSC fabrication. It is however, believed that the silyl linkage will
be an important component of more robust dyes used to sensitise
metal oxide surfaces (TiO2, WO3, ZnO, SnO2, etc.) and it is therefore
recommended that further studies be pursued in this challenging
synthetic area. The most promising strategy in this work appears
to be direct linkage (a), the formation of a direct Si–C bond, using
butyllithium with 4,40-dibromo-2,20-bipyridine and either trimeth-
ylsilane or 1-ethoxysilatrane, provided that the product can be
3.4.5. (c) Ester linkage: (i) Carboxy-sil method
As strategies (a) – direct linkage, and (b) – amide linkage, were
unsuccessful, the formation of a carboxy-sil bond (Si–O–C(O))
was considered. This type of bonding has been observed previously
with ferrocenyl carboxylic acid and 1-ethoxysilatrane [52], where,
a carboxylic acid and a silatrane were reacted together in an azeo-
tropic solvent to form 1-ferrocenecarboxysilatrane. This method
was successful for Chen et al. [52], with isolation of 1-ferrocene-
carboxysilatrane in good yields and crystals of suitable quality
for single-crystal XRD analysis. Ferrocene is able to undergo similar
reactions to those observed with aromatic compounds.
Owing to the success of Chen et al. [52] a similar method, using
the reaction scheme illustrated in Fig. 13, was explored. However,
as observed with the other routes, a mixture of starting materials
was isolated. The NMR spectrum indicated the presence of aro-
matic and methylene protons, however mass spectroscopy sug-
gested that no new oxygen-silicon bond had formed. This result
is not surprising, as this bond is known to be quite unstable,
decomposing rapidly [43]. This point is further emphasised by
the limited amount of research in this field [53].
4. Conclusion
The efficiency, and efficacy of a new silyl linker, bipy-sil, for
binding ruthenium(II) bipyridine chromophores to metal oxides

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K. Szpakolski et al. / Polyhedron 52 (2013) 719–732
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