Synthesis and characterization of dichloro(2,2′-bipyridyl-4,4′-dicarboxylate)bis(triphenylphosphine) ruthenium(II) for efficient photosensitization of titanium oxide

Article abstract of DOI:10.1039/a800779i

Ru(PPh₃)₂(dcbipy)Cl₂ (PPh₃ = triphenylphosphine, dcbipy = 2,2′-bipyridyl-4,4′-dicarboxylate) was prepared to use as a TiO₂ sensitizer in wet regenerative photoelectrochemical cells. The complex has been structurally characterized by IR, Raman, ES-MS and NMR spectroscopies. The broad bands in the visible spectrum as well as the reversibility of the redox couple Ru^III-Ru^III, established by cyclic voltammetry, make this complex potentially beneficial for the photosensitization process.

Full text of DOI:10.1039/a800779i

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Synthesis and characterization of dichloro(2,2º-bipyridyl-4,4º-
dicarboxylate)bis(triphenylphosphine)ruthenium(II) for efficient
photosensitization of titanium oxide
Polycarpos Falaras,*,a Antonis P. Xagasa and Anne Hugot-Le Go†b
L e t t e r
a Institute of Physical Chemistry, NCSR ““DemokritosÏÏ, 15310 Aghia Paraskevi Attikis, Greece
b Physique des L iquides et Electrochimie (CNRS UPR 15), Universite Pierre et Marie Curie,
75252 Paris cedex 05, France
Ru(PPh ) (dcbipy)Cl (PPh \ triphenylphosphine, dcbipy \ 2,2@-bipyridyl-4,4@-dicarboxylate) was prepared to
3 2
2
3
use as a TiO sensitizer in wet regenerative photoelectrochemical cells. The complex has been structurally
2
characterized by IR, Raman, ES-MS and NMR spectroscopies. The broad bands in the visible spectrum as well
as the reversibility of the redox couple RuIIÈRuIII, established by cyclic voltammetry, make this complex
potentially beneÐcial for the photosensitization process.
The photosensitization of wide bandgap semiconductors
(mainly TiO and SnO ) with organic dyes or transition metal
formulation
of
the
complex.
Anal.
calcd
for
C
H
N O P Cl Ru: C, 61.27; H, 4.04; N, 2.98. Found: C,
2
2
48 38
2
4 2
2
complexes is being intensively investigated for direct solar to
electrical energy conversion.1h3 Although a large number of
complexes have been synthesised and applied as photo-
sensitizers,2,4,5 the challenge for an efficient redox sensitizer
remains. The best efficiencies so far have been achieved by
using ruthenium(II) complexes that contain the 2,2@-bipyridyl-
4,4@-dicarboxylate ligand (henceforth called dcbipy) on high
61.69; H, 4.45; N, 2.38. IR8 (KBr) cm~1: 3058(m) [m(CwH)],
1724(ms) [m(CxO)], 1481(ms) [m d(CwC)], 1434(s)
[n d(CwC)], 1259(m) [m ], 1221(m) [m (COO)], 1159(w) [c
11
sym
d(CwH) in-plane], 1093(m) [q X-sensitive], 1025 [b d(CwH)
in-plane or
m ], 745(s) [(CwH) out-of-plane], 658(ms)
15
[dcbipy], 619(ms) [d (CwC) in-plane]. The Raman9 spectrum
contains the bands: 3056(w), 1722(vw), 1478(s), 1436(w),
1257(m), 1162(w), 1099(m), 1029(ms), 658(m), 616(w) and addi-
tionally 1615(s) [m ], 1538(s) [m ], 1315(m) [m (COO)],
surface area, nanocrystalline, thin TiO Ðlms.1,2 By keeping
2
the dcbipy ligand and introducing new chromophore groups
5
6
sym
around the RuII centre, one can alter the light-harvesting char-
acteristics and therefore the photoelectrochemical properties.
Ruthenium triphenylphosphine complexes have already been
employed in a great variety of chemical and catalytic reac-
tions.5,6 In this contribution, the preparation and physical
characterisation of a new RuII complex, containing both the
997(ms) [ring-breathing], 427(m) [m(RuwP)].
Results and Discussion
The ES-MS spectrum (positive mode) gave the following
fragments (all the m/z values are based on 102Ru, which
is the most abundant isotope) : [M]` (941), [M [ Cl]`
(906), [M [ PPh ]` (679), [M [ Cl [ PPh ]` (644),
[M [ Cl [ dcbipy]` (662). All these peaks have the appropri-
ate isotopic pattern. The system was also run in the negative
mode but no peaks were observed.
PPh and the dcbipy ligand, are described.
3
3
3
Experimental
All the solvents was of analytical grade and were dried and
degassed before use. RuCl (PPh ) was prepared in accord-
The 1H NMR spectrum (Bruker, 250 MHz) of the complex
exhibits a very complex pattern of resonances in the d region
2
3 3
ance with the previously described procedure.7 RuCl É 3H O
of 6.7È7.5, indicative of the presence of the (PPh ) RuII
3
2
3 2
and dcbipy were commercially available and were used
moiety.10 Moreover, there is a doublet at d 8.9 (2 H), attrib-
without further puriÐcation. PPh was recrystallized with
uted to the H6 and a singlet at d 8.4 (2 H), attributed to the
H3 of the dcbipy ligand. More structural evidence came from
the 31P NMR spectrum (Varian, 300 MHz, with 85% H PO
3
95% aqueous ethanol.
3
4
as external reference), which consists of a singlet at 22.1 ppm.
Synthesis
This fact indicates that both PPh are stereochemically equiv-
alent. This leads us, considering also the steric bulk of the two
3
RuCl (PPh ) (0.191 g, 0.2 mmol) was dissolved in 50 ml of
2
3 3
warm deaerated CH Cl and 0.049 g (0.2 mmol) of the dcbipy
PPh , to assume a trans conÐguration for the PPh (Fig. 1).
2
2
3
3
ligand was added. The reaction mixture was heated to reÑux
under Ar for 5 h. Then the solution was allowed to cool in this
inert atmosphere, its volume was reduced to about the one-
half on a rotary evaporator and was left to stand overnight.
The dark brown microcrystalline solid that precipitated was
Important structural evidence came from infrared and
Raman spectroscopic analysis. The most striking feature of
the IR spectrum is the strong sharp band at 1724 cm~1 due
to the CxO stretching vibration of the carboxylic group. This
band also appears in the Raman spectrum, but with very low
intensity. There is also a peak at 1221 cm~1, attributed to the
CwO stretch. The vibration observed at 3058 cm~1 is
ascribed to the CwH stretching vibration. Comparison with
the IR spectra of RuCl (PPh ) showed that additional bands
Ðltered, washed thoroughly with a 1 : 1 Et OÈpentane mixture
2
and dried in vacuum over CaCl and P O (mp 232È234 ¡C).
2
2
5
The compound is stable in the solid state and in solution. It is
soluble in ethanol, acetone and DMF, moderately
soluble in dichloromethane and chloroform and insoluble
in water and ether. Elemental analysis agreed well with the
2
3 3
exist in the Ðngerprint region, obviously due to the dcbipy
ligand (symmetrical CwC and CwN stretches as well as
New J. Chem., 1998, Pages 557È558
557

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vs. SCE. By considering the literature data,5,15 we attribute
this wave to the ruthenium(II)Èruthenium(III) couple:
RuII(PPh ) (dcbipy)Cl ¡ RuIII(PPh ) (dcbipy)Cl ] e~ (1)
3 2
2
3 2
2
For this couple, the anodic-cathodic peak current ratio (i /i )
pa pc
is equal to 1.0, indicating stability of the oxidized complex.
The anodic peak current i is proportional to the square root
p
of the scan rate (v") and the peak potential values E and E
pa
pc
are independent of the scan rate. The peak-to-peak separation
(*E \ E [ E ) as well as the di†erence o E [ E o are
p
pa
pc
p
p@2
both about 60 mV, as expected for a completely reversible
one-electron transfer process.
Fig. 1 Structure of the Ru(PPh ) (dcbipy)Cl complex
3 2
2
TiO nanocrystalline thin Ðlms, prepared by a sol-gel tech-
2
nique, were readily red-colored, after simple immersion in a
10~4 M solution of the complex in acetone or ethanol. It is
now widely accepted that dcbipy serves as an interlocking
group between the molecular “antennaÏ (the complex) and the
semiconductorÏs surface through an ester-like linkage between
the wCOOH group and the surface hydroxyl groups of
in-plane CwH wags).11 On the other hand, in the Raman
spectrum, the coupled symmetrical carbonyl stretch appears
at 1315 cm~1. The presence of PPh is conÐrmed by the
3
existence of some characteristic ring vibration modes (the
assignment of the characteristic vibration modes of PPh
are based upon the classic work of Whi†en on
3
TiO .3,16 The characterization of these sensitized cells by in
2
situ resonance Raman spectroscopy,17 under real photo-
monohalogenobenzenes).12 The results are in very good agree-
current conditions, conÐrmed the immutability of the dye
Raman spectra and revealed a reversible dependence of the
intensities of the bands on the polarization potential. The
observed photocurrents are satisfactorily high and very stable
while the photocurrent action spectrum matches well the
absorption spectrum of the complex. A detailed exploration of
the photoelectrochemical properties is now in progress.
ment with the data reported for (PPh ) RuIICl and other
3 3
related compounds.13 Compared to the free phosphine, we
note small shifts of the bands, due to complexation. Moreover,
there is a band at 427 cm~1 probably due to a RuIIwN
stretch.
The electronic spectrum (Perkin-Elmer Lambda 16) of the
complex (Fig. 2) displays two broad maxima in the visible
region at 521 and 413 nm, with molar extinction coefficients
3400 and 5800 M~1 cm~1, respectively, and a sharp increase
in the near UV region. No spectral changes were observed,
even after continuous illumination with visible light for several
days. It is now well-established that the two intense low
energy-bands are assigned to allowed metal-to-ligand charge-
Acknowledgements
We gratefully acknowledge Dr. A. Koutsodimou, Dr. S.
Coinis, Dr. F. Zanikos, Dr. G. Pistolis, X. Teas and E. Lyris
for their generous assistance.
transfer transitions of the type d (Ru) ] p*(dcbipy), which are
characteristic of N-heterocyclic complexes of ruthenium(II).
p
References
Similar assignments have been proposed for analogous com-
plexes.10,14 In contrast, no emission was detected in the solid
state and in ethanolic solutions (Perkin-Elmer LS50B and
Jasco FP-777). This may be due to a very short-lived excited
state and it is not unexpected for the family of RuÈdcbipy
complexes.2
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3 2
2
in DMF solution by cyclic voltammetry. The voltammogram,
taken in the region of interest (inset in Fig. 2), shows an oxida-
tion peak whose half-wave potential E is centered at ]0.64 V
8 Italics denote bands from the dcbipy ligand.
"
9 The Raman spectrum was taken using many di†erent excitation
wavelengths. The best results were attained by using the green
argon line at 514.5 nm. Since the absorption maximum of the
complex lies at 521 nm, we are very near resonance Raman condi-
tions.
10 R. Prasad, Polyhedron, 1995, 14, 2151.
11 T. J. Meyer, G. J. Meyer, B. W. Pfennig, J. R. Schoonover, C. J.
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Acta., 1994, 216, 191.
14 H. E. Toma, P. Santos and A. Bolanos, J. Chem. Res. (S), 1988,
124.
E / V vs. SCE
15 M. Haga, Md. M. Ali, S. Koseki, K. Fujimoto, A. Yoshimura, K.
Nozaki, T. Ohno, K. Nakajima and D. J. Stufkens, Inorg. Chem.,
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16 P. Falaras, Solar Energy Mater. Solar Cells, in press.
17 A. Hugot-Le Go†, S. Joiret, P. Falaras and A. P. Xagas, Electro-
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Wavelength / nm
Fig. 2 UV/VIS absorption spectrum of a 10~3 M solution of the
Ru(PPh ) (dcbipy)Cl complex in DMF. Inset: cyclic voltammogram
3 2
2
of the same solution (degassed), containing 0.2 M [(n-C H ) N]ClO .
4
9 4
4
Received in Montpellier, France, 30th December 1997;
Paper 8/00779I
Scan rate: 50 mV s~1, working electrode: Pt wire, counter electrode:
planar Pt electrode, reference: saturated calomel electrode (SCE)
558
New J. Chem., 1998, Pages 557È558