Synthesis and photophysical properties of trans-dithiocyanato bis(4,4′-dicarboxylic acid-2,2′-bipyridine) ruthenium(II) charge transfer sensitizer

Article abstract of DOI:10.1016/S0020-1693(99)00359-X

The ruthenium complexes trans-[Ru(dcbpyH₂)₂(Cl)₂] (1), trans-[Ru(dcbpyH₂)₂(NCS)₂] (2), were synthesized and characterized by UV-Vis absorption, emission, IR, and NMR spectroscopy. The absorption and emission maxima of the trans complexes are red-shifted when compared to the cis analogs. The low energy MLCT maximum of complex 2 shows at 582 nm and exhibits a luminescence consisting of a single band with a maximum at 870 nm, in DMF solution at 298 K. The red shift of the MLCT absorption of the trans-chloro complex 1 (662 nm) is even more pronounced than that of the trans-complex 2 and shows the onset of weak and broad emission signals above 900 nm. The proton NMR spectra of the trans-isomers show only three peaks corresponding to the two dcbpy ligands in which all the pyridine rings are equivalent. The ¹³C NMR spectrum of the trans-isomer is characterized by a relatively simple pattern of resonances from the four equivalent pyridine rings. The enhanced red response of the trans -complex 2 renders it an attractive candidate as a panchromatic charge transfer sensitizer in mesoporous oxide solar cells.

Full text of DOI:10.1016/S0020-1693(99)00359-X

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Inorganica Chimica Acta 296 (1999) 250–253
Synthesis and photophysical properties of trans-dithiocyanato
bis(4,4%-dicarboxylic acid-2,2%-bipyridine) ruthenium(II) charge
transfer sensitizer
S.M. Zakeeruddin, Md.K. Nazeeruddin *, R. Humphry-Baker, M. Gra¨tzel
Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland
Received 26 July 1999
Abstract
The ruthenium complexes trans-[Ru(dcbpyH2)2(Cl)2] (1), trans-[Ru(dcbpyH2)2(NCS)2] (2), were synthesized and characterized
by UV–Vis absorption, emission, IR, and NMR spectroscopy. The absorption and emission maxima of the trans complexes are
red-shifted when compared to the cis analogs. The low energy MLCT maximum of complex 2 shows at 582 nm and exhibits a
luminescence consisting of a single band with a maximum at 870 nm, in DMF solution at 298 K. The red shift of the MLCT
absorption of the trans-chloro complex 1 (662 nm) is even more pronounced than that of the trans-complex 2 and shows the onset
of weak and broad emission signals above 900 nm. The proton NMR spectra of the trans-isomers show only three peaks
corresponding to the two dcbpy ligands in which all the pyridine rings are equivalent. The 13C NMR spectrum of the trans-isomer
is characterized by a relatively simple pattern of resonances from the four equivalent pyridine rings. The enhanced red response
of the trans-complex 2 renders it an attractive candidate as a panchromatic charge transfer sensitizer in mesoporous oxide solar
cells. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Ruthenium(II) complexes; Photophysical properties; Optical properties; Charge transfer sensitizer; Dithiocyanato complexes
1. Introduction
geneous charge transfer sensitizers known to date and is
widely used in the nanocrystalline TiO2 solar cell [4].
The present study reports on the synthesis and charac-
terization of the trans isomer of this complex, which
was found to exhibit very interesting optical properties.
Photophysical and photochemical properties of
ruthenium polypyridyl complexes have been studied
extensively for many years [1,2] in part because of their
possible applications in energy conversion processes
[3,4]. The choice of ruthenium metal is attractive for its
octahedral coordination geometry, where one can intro-
duce specific ligands in a controlled manner to tune the
photophysical, photochemical and the electrochemical
properties. The majority of the ruthenium complexes,
reported to date containing two bipyridyl and two
monodentate ligands have a cis-configuration [5], al-
though the synthesis and crystal structure of a few
trans-complexes have been described [6]. A case of
particular interest is that of the dithiocyanato complex
Ru(II)(4,4%-dicarboxy-2,2%-bipyridine)2(NCS)2]. The cis-
form of this complex is one of the most efficient hetero-
2. Experimental
UV–Vis and fluorescence spectra were recorded in a
1 cm path length quartz cell on a Cary 5 spectrophoto-
meter and Spex Fluorolog 112 Spectrofluorometer, re-
spectively. The measured emission and excitation
spectra were routinely corrected for the wavelength
dependent features. The emission lifetimes were mea-
sured by exciting the sample with a pulse from an active
modelocked Nd YAG laser, using the frequency dou-
bled line at 532 nm. The emission decay was followed
on a Tektronix DSA 7912 Digitising Signal Analyser,
having used a Hamamatsu R928 photomultiplier to
convert the light signal to a voltage signal. Proton and
* Corresponding author. Tel.: +41-21-693 6124; fax: +41-21-693
4111.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00359-X

S.M. Zakeeruddin et al. / Inorganica Chimica Acta 296 (1999) 250–253
251
13C NMR spectra were measured on a Bruker 200 MHz
2.2. Synthesis of trans-[RuII(4,4%-dicarboxylic
acid-2,2%-bipyridine)2(NCS)2] (2)
spectrometer. The reported chemical shifts are against
TMS. Infrared spectra were obtained with a Perkin–
Elmer Paragon 1000 FTIR spectrophotometer at a
resolution of 5 cm−1 with the samples dispersed in
compressed KBr pellets. The cis-[Ru(II)(4,4%-dicar-
boxylic acid-2,2%-bipyridine)2(Cl)2] and cis-[RuII(4,4%-di-
carboxylic acid-2,2%-bipyridine)2(NCS)2] complexes were
prepared by a literature procedure [4]. All other chemi-
cals and solvents were reagent grade and used without
further purification.
This dithiocyanato complex was synthesized by heat-
ing under magnetic stirring a solution of trans-
[RuII(4,4%-dicarboxylic acid-2,2%-bipyridine)2(Cl)2] (50
mg) and ammonium thiocyanate (90 mg) in DMF (10
ml) for 3 h at 100°C in the dark under an argon
atmosphere. The solvent DMF was evaporated com-
pletely on a rotary evaporator under vacuum. The
resulting solid product was collected on a sintered glass
crucible, washed with methanol and followed by diethyl
2.1. Synthesis of trans-[RuII(4,4%-dicarboxylic
acid-2,2%-bipyridine)2(Cl)2]·2H2O (1)
ether. Yield 15%. H NMR (D2O/NaOD) l ppm: 9.48
1
(4H, d), 8.83 (4H, s), 8.03 (4H, d). 13C NMR (D2O/
NaOD) l ppm: 172.46 (COOH), 160.43 (C-6), 157.46
(C-2), 155.27 (NCS), 146.20 (C-4), 124.55 (C-3) and
122.40 (C-5). UV–Vis: u max. 582, 528, 402 and 314
nm.
The cis-[Ru(II)(4,4%-dicarboxylic acid-2,2%-bipyridine)2-
(Cl)2] complex was dissolved in ethanol (1×10−3 M)
and filtered in order to remove insoluble by-products.
The filtered solution was photolyzed with a 250-W
sunlamp to induce cis–trans isomerization. The trans
isomer precipates from the solution. After 3 h of irradi-
ation precipitation appeared complete. The same pre-
cipitate was also obtained by leaving an ethanolic
3. Results and discussion
Scheme 1 shows the details of the strategy adopted
for the synthesis of trans-complexes. The trans-dichloro
complex 1 was obtained by photolyzing the cis-dichloro
complex that gave the precipitate in ethanol solution.
Heating the complex 1 in the presence of ammonium
thiocyanate resulted in the formation of the trans-
dithiocyanate complex 2. The UV–Vis absorption spec-
trum of complex 2 in DMF shows three intense
metal-to-ligand charge transfer (MLCT) bands at 402,
528 and 582 nm, with a molar extinction coefficient of
solution
of
cis-[RuII(4,4%-dicarboxylic
acid-2,2%-
bipyridine)2(Cl)2] complex under room light for several
days. The precipitate was filtered off, washed with
ethanol followed by diethyl ether and dried under vac-
uum. Yield 90%. Anal. Calc. for RuC24H16N4Cl2O8: C,
41.38; H, 2.87; N, 8.05; Cl, 10.20. Found: C, 41.52; H,
1
2.92; N, 8.17; Cl, 10.23%. H NMR (D2O/NaOD) l
ppm: 9.66 (4H, d), 8.87 (4H, s), 8.11 (4H, d). UV–Vis:
u max. 662, 584, 424 and 314 nm.
Scheme 1.

252
S.M. Zakeeruddin et al. / Inorganica Chimica Acta 296 (1999) 250–253
from the two anionic NCS ligands to ruthenium center.
The red shift of the MLCT absorption of the trans-
chloro complex 1 (662 nm) is even more pronounced
than that of the trans-complex 2. This is expected as the
Cl− ligand is a stronger donor than NCS−.
When excited at the lowest energy MLCT absorption
band, the trans- and cis-NCS isomers in DMF solution
at 298 K exhibit a luminescence consisting of a single
band with a maximum at 870 and 780 nm, respectively
(Fig. 2). Excitation of these complexes at different
wavelengths within the manifold of the MLCT bands
gave the same emission maxima. This phenomenon
shows that exciting the complex between 380 and 650
nm leads to population of the same luminescent state.
The excited state lifetimes of trans- and cis-isomer,
measured at 298 K under aerobic conditions, are 12
and 24 (95) ns, respectively. We could not observe any
detectable emission up to 900 nm for complex 1, in
DMF solution at room temperature. However, complex
1 showed at room temperature onset of a weak and
broad emission signals above 900 nm that did not allow
for lifetime measurements. Hence, the emission from
the trans-complex 1 is expected to be in the 900 to 1100
nm region, beyond the detector response of our
equipment.
Fig. 1. UV–Vis absorption spectra of cis- and trans-dithiocyanato-
bis(2,2%-bipyridine-4,4%-dicarboxylic acid) ruthenium(II) complexes,
1.5×10−5 M in DMF solution, 1-cm cell.
The trans configurations of the complexes 1 and 2
also have been confirmed by the proton and 13C NMR
spectra. The cis-isomer in a D2O+NaOD solution
shows six peaks in the aromatic region corresponding
to two different pyridine ring protons (Fig. 3(a)). In the
trans-isomer, two dcbpy ligands are equivalent in which
all the pyridine rings are trans to each other and show
only three peaks (Fig. 3(b)). For the complex 2, no
evidence was found for the substitution of thiocyanate
ligand or the isomerization to cis during several days
under room light in DMF solution.
The carbon 13 NMR spectroscopy proved to be the
most valuable tool for determining the geometric
configuration of the ruthenium complex and the linkage
Fig. 2. Emission spectra of cis- and trans-dithiocyanatobis(2,2%-
bipyridine-4,4%-dicarboxylic acid) ruthenium(II) complexes measured
at 298 K under aerobic conditions in DMF solution.
12500 M−1 cm−1 for the lowest energy band. The
band at 314 nm is assigned to the intra ligand p–p*
transition of dcbpy ligand. There is a striking difference
in the visible absorption spectra of the trans- and
cis-isomer (Fig. 1). The lowest energy MLCT maximum
shifts from 538 to 582 nm upon cis-to-trans isomeriza-
tion. This is a significantly larger red shift than for any
of the Ru(II) bis-bipyridyl complex reported to date in
the literature [7].
The bathochromic shift in the trans-complex 2 com-
pared to its cis-analog is attributed to a destabilization
of the ruthenium t2g orbital due to electron donation
Fig. 3. Proton NMR spectra of (a) cis- and (b) trans-dithiocyanato-
bis(2,2%-bipyridine-4,4%-dicarboxylic acid) ruthenium(II) complexes
measured in D2O+NaOD solution.

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253
properties of the NCS ligand. Since, the NCS ligand is
an ambidentate ligand it can coordinate to the metal
through N or S end. The proton decoupled carbon 13
NMR spectra of cis- and trans-isomers show different
carbon resonances that were identified by comparing
with known bipyridine complexes [8]. Carbon 13 NMR
spectrum of the cis-isomer shows 12 resonance peaks
consisting of six sets due to two different dcbpy ligands.
The single resonance peak at 132.84 ppm we assign to
the carbon of N-coordinated NCS. The two downfield
resonances at 172.59 and 172.24 are assigned to the
carboxylate carbons.
nanocrystalline TiO2 films derivatized with this
sensitizer.
Acknowledgements
It is a pleasure to acknowledge Dr F.P. Rotzinger for
his enlightening discussions. This work was supported
by grants from the Swiss Federal Institute for Energy
(OFEN) and the Institut fu¨r Angewandte Photochemie
(INAP), Germany.
The 13C NMR spectrum of the trans-isomer is char-
acterized by a relatively simple pattern of resonances
from the four equivalent pyridine rings. The pattern
consists of six single resonance peaks coming from the
pyridine carbons and one resonance peak at 155.27
ppm that was assigned to the trans NCS ligands. There
is a noteworthy difference between the cis NCS carbon
resonance (132 ppm) and the trans NCS (155.27 ppm),
which could be due to the trans NCS carbons being
more deshielded than the cis analog. In an axial posi-
tion, both the thiocyanate ligands have less p back
bonding from the ruthenium center compared to cis
NCS. Our observation is consistent with a known
ruthenium complex that contains trans NCS ligands [9].
A perfect trans-symmetry with two NCS ligands
trans to each other is expected to give rise to a single
w(NC) stretch [10]. The IR spectrum of trans-complex 2
shows an intense single band at 2106 w(NC) and 782
w(CS) cm−1 due to N-coordinated NCS ligand. The
band at 172192 cm−1 is due to the carboxyl groups.
The enhanced red response of the trans-complex 2
renders it an attractive candidate for substituting the cis
analog as a panchromatic charge transfer sensitizer in
mesoporous oxide solar cells. Initial experiments
confirm efficient photoinduced electron injection into
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