Tetranuclear polypyridyl complexes of RuII and FeII: Synthesis, electrochemical, photophysical and photochemical behaviour

Article abstract of DOI:10.1002/ejic.200500106

Three heterotetranuclear complexes [{Ru^II(bpy) ₂(LL_n)}₃Fe^II]⁸⁺ {bpy = 2,2′-bipyridine, n = 2, 4, 6; denoted [{Ru(LL_n)} ₃Fe]⁸⁺}, in which one iron centre is complexed by three Ru^II-tris-bipyridine-like moieties containing covalently bridging bis-bipyridine LL_n ligands, have been synthesised and characterised. The stability and the electrochemical, photophysical and photochemical properties of these complexes have been investigated in CH₃CN. The cyclic voltammograms of all complexes exhibit two successive reversible oxidation processes in the positive region, corresponding to the Fe ^II/Fe^III and Ru^II/Ru^III redox couples. These systems are clearly separated (ΔE_1/2 about 300 mV), which indicates the absence of an electronic connection between the two subunits. In the negative region, three successive reversible four-electron systems are observed, corresponding to tho ligand-based reduction processes. The two oxidized forms of the complexes [{Ru^II(LL_n)} ₃Fe^III]⁹⁺ and [{Ru^III(LL _n)}₃Fe^III]¹²⁺, which are obtained by two successive exhaustive electrolyses, are very stable. The [{Ru(LL _n)}₃Fe]⁸⁺ complexes are luminescent, which shows that the covalent linkage between the Ru^II-tris-bipyridine and Fe^II-tris-bipyridine subunits leads to an only partial quenching of the Ru^II* excited states by energy transfer to the Fe ^II centre. The luminescence lifetime and quantum yield are found to be independent of the complexes' concentration, thus indicating that the energy-transfer process is only due to an intramolecular electron-exchange mechanism. Quantitative photoinduced oxidation of the tetranuclear complexes has been performed by continuous photolysis experiments in the presence of a large excess of a diazonium salt, which plays the role of a sacrificial oxidant. Two successive oxidation processes (Fe^II → Fe^III and Ru^II → Ru^III) are observed. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500106

FULL PAPER
Tetranuclear Polypyridyl Complexes of RuII and FeII: Synthesis,
Electrochemical, Photophysical and Photochemical Behaviour
Jean Lombard,[a] Sophie Romain,[a] Stéphane Dumas,[a] Jérôme Chauvin,[a]
Marie-Noëlle Collomb,[a] Denis Daveloose,[b] Alain Deronzier,*[a] and
Jean-Claude Leprêtre*[a]
Keywords: Bridging ligands / Electrochemistry / Iron / Photochemistry / Ruthenium
Three heterotetranuclear complexes [{RuII(bpy)2(LLn)}3FeII]8+
{bpy = 2,2Ј-bipyridine, n = 2, 4, 6; denoted [{Ru(LLn)}3Fe]8+},
in which one iron centre is complexed by three RuII-tris-bi-
pyridine-like moieties containing covalently bridging bis-bi-
pyridine LLn ligands, have been synthesised and character-
ised. The stability and the electrochemical, photophysical
and photochemical properties of these complexes have been
investigated in CH3CN. The cyclic voltammograms of all
tained by two successive exhaustive electrolyses, are very
stable. The [{Ru(LLn)}3Fe]8+ complexes are luminescent,
which shows that the covalent linkage between the RuII-tris-
bipyridine and FeII-tris-bipyridine subunits leads to an only
partial quenching of the RuII* excited states by energy trans-
fer to the FeII centre. The luminescence lifetime and quan-
tum yield are found to be independent of the complexes’ con-
centration, thus indicating that the energy-transfer process is
complexes exhibit two successive reversible oxidation pro- only due to an intramolecular electron-exchange mechanism.
cesses in the positive region, corresponding to the FeII/FeIII
and RuII/RuIII redox couples. These systems are clearly sepa-
rated (ΔE1/2 about 300 mV), which indicates the absence of
an electronic connection between the two subunits. In the
negative region, three successive reversible four-electron
systems are observed, corresponding to the ligand-based re-
duction processes. The two oxidized forms of the complexes
Quantitative photoinduced oxidation of the tetranuclear
complexes has been performed by continuous photolysis ex-
periments in the presence of a large excess of a diazonium
salt, which plays the role of a sacrificial oxidant. Two success-
ive oxidation processes (FeII Ǟ FeIII and RuII Ǟ RuIII) are ob-
served.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
[{RuII(LLn)}3FeIII]9+ and [{RuIII(LLn)}3FeIII]12+, which are ob- Germany, 2005)
molecular electron transfer can occur from the MnII site to
Introduction
the photogenerated RuIII species in the presence of an exter-
nal electron acceptor like viologen in acetonitrile.[8] How-
ever, the rate constant for this process is fairly slow and the
efficiency of the process depends on the distance between
the two metallic centres. In addition, the system is compli-
cated, since the MnII/MnIII redox couple is only poorly re-
versible as the MnIII species reacts with residual water to
form binuclear MnIII/MnIV oxo complexes.[9–11] Moreover,
in most cases the MnII sites quench the excited state of the
ruthenium(ii) complex, presumably by an energy-transfer
process.[8,12,13] With a view to studying this kind of intra-
molecular electron transfer, we have recently reported the
photoredox behaviour of a series of heterobinuclear met-
In recent years, important synthetic efforts have been
made to mimic the function of the donor site of the pho-
tosystem II (PSII) by the development of superstructured
heterometallic complexes.[1–3] The main strategy to elabo-
rate models of PSII involves the covalent coupling of a pho-
toactive [RuII(bpy)3]2+ (bpy = 2,2Ј-bipyridine) moiety,
which plays the role of the P680 chlorophyll photosensitizer,
to some monomanganese(ii), binuclear (ii,iii) or (iii,iii) and
trinuclear (ii,ii,ii) complexes that model the catalytic centre
that oxidizes water into oxygen.[4–7] It has been reported for
the mononuclear manganese(ii) complexes that an intra-
[a] Laboratoire dЈElectrochimie Organique et de Photochimie allic complexes of RuII and FeII, namely [RuII(bpy)2(LLn)-
FeII(bpy)2]4+ (denoted [Ru(LLn)Fe]4+), based on the use of
Rédox, Université Joseph Fourier,
UMR CNRS 5630, Institut de Chimie Moléculaire de Greno-
bridging bis-bipyridine ligands LLn (Scheme 1).[14] This sys-
ble, FR CNRS 2607
tem is simpler than the RuII/MnII one, since FeII/FeIII is a
B. P. 53, 38041 Grenoble Cedex 9, France
Fax: +33-4-7651-4267
E-mail: jean-claude.lepretre@ujf-grenoble.fr
perfectly reversible redox couple, with the oxidation of the
[FeII(bpy)3]2+-like complex occurring at a potential close to
alain.deronzier@ujf-grenoble.fr
that of [MnII(bpy)3]2+. In this previous study, we demon-
[b] Centre de Recherches du Service de Santé des Armées “Emile
Pardé”, Laboratoire de Biospectrométrie,
24 Avenue des Maquis du Grésivaudan, BP 87, 38702 La Tron-
strated that the 3MLCT excited state of the RuII-tris-bipyri-
che, Cedex, France
3320
dine centre is strongly quenched by the FeII-tris-bipyridine
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200500106
Eur. J. Inorg. Chem. 2005, 3320–3330

Tetranuclear Polypyridyl Complexes of RuII and FeII
FULL PAPER
unit through an energy-transfer process that is mainly inter- series of new heterobimetallic complexes of RuII and FeII:
molecular. Nevertheless, this strong energy transfer can be [{RuII(bpy)2(LLn)}3FeII]8+ {n = 2, 4, 6; denoted [{Ru-
easily short-circuited in the presence of an external irrevers- (LLn)}3Fe]8+} in which one iron centre is complexed by
ible electron acceptor like an aryldiazonium salt (ArN2+) by three RuII-tris-bipyridine moieties (Scheme 1). A previous
an electron transfer that leads finally to the photoinduced publication reported some photophysical data of this kind
formation of [RuII(LLn)FeIII 5+
It has been shown that the electron transfer between the situ.[15] Under these conditions, the presence of some un-
ground state of the RuIII and FeII species essentially occurs complexed [RuII(bpy)2(LLn)]2+ {denoted [RuII(LLn)]2+
]
species with high efficiency. of complex, but these compounds were only prepared in
}
by an intermolecular process. In order to gain more infor- species in the medium prevented the drawing of clear con-
mation about these photoprocesses, we have synthesised a clusions about the photophysical properties of the
[{Ru(LLn)}3Fe]8+ complexes. In the present study, we report
the synthesis and characterisation of the three heterotetran-
uclear complexes [{Ru(LLn)}3Fe]8+, their main redox and
photophysical properties and the comparison of the latter
properties with those of the heterobinuclear complexes
[Ru(LLn)Fe]4+. Moreover, we have investigated the possibil-
ity to photoinduce the oxidation of the tetranuclear com-
plexes in the presence of ArN2+. The expected multi-step
oxidation process is summarised in Scheme 2.
Results and Discussion
Synthesis and Stability of the Complexes
Eliott and co-workers[15] have reported that the complex-
ation of some [RuII(bpy)2(L-L)]2+ complexes (L-L = coval-
ently linked bipyridines) with Fe2+ in situ in methanol/water
(1:1) forms the tetranuclear complexes [{RuII(bpy)2-
(L-L)}3FeII]8+. However, in this medium, it has been shown
that such compounds undergo some dissociation (associa-
tion constants between 1013 and 1015). As a result, the pres-
ence of uncomplexed [RuII(bpy)2(L-L)]2+ species leads to
the observance of a double exponential for the emission
Scheme 1. Chemical structure of the ligands LLn and of the com-
plexes [Ru(LLn)]2+, [Ru(LLn)Fe]4+ and [{Ru(LLn)}3Fe]8+
.
decay. The fast component of the decay corresponds to
emission from ruthenium “ligands” complexed to iron, and
the long one to the uncomplexed [RuII(bpy)2(L-L)]2+
centres. Moreover, isolation of tetranuclear complexes in a
pure form by this group was unsuccessful. In this work, we
have synthesised, isolated and purified three heterotetranu-
clear complexes [{Ru(LLn)}3Fe]8+ (n = 2, 4 and 6) by the
treatment of 0.4 equivalents of Fe2+, as its perchlorate salt,
with one equivalent of the corresponding [Ru(bpy)2(LLn)]2+
complexes in ethanol/water (12:1). This synthetic method
allows us to obtain the expected compounds with fairly
good yields; their purity was checked by TLC and elemental
analysis. In order to determine their photophysical proper-
ties in CH3CN solution properly, we also verified their sta-
bility in this solvent by UV/Vis spectroscopy. The visible
absorption spectra of [{Ru(LLn)}3Fe]8+ in the concentra-
tion range 10–4–10–7 m exhibit the expected regular bands
at 356, 396 (shoulder), 430 (shoulder), 454 and 530 nm
(shoulder) corresponding to the superimposition of the ab-
sorbance of the RuII- and FeII-tris-bipyridine subunits
(Table 1).[15] For the three complexes [{Ru(LLn)}3Fe]8+
,
there is no significant variation of the ratio of the ab-
sorbance values at 356, 454 and 530 nm with concentration
(in the range 10–4 and 10–7 m), showing that no dissociation
of the complexes occurs. The linearity of the absorbance vs.
Scheme 2. Schematic presentation of the photooxidation mecha-
nism of [{Ru(LLn)}3Fe]8+ in the presence of an external electron
acceptor (ArN2+).
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A. Deronzier, J.-C. Leprêtre et al.
Table 1. UV/Vis data for [RuII(LLn)]2+ and [{RuII(LLn)}3FeII]8+ in deoxygenated CH3CN + 0.1 m Bu4NClO4 at 25 °C.
FULL PAPER
Complexes
λabs (ε)
Ref.
[FeII(dmbpy)3]2+
356 nm (7100 m–1 cm–1), 395[a] (4000)
this work
494[a] (7400), 528 (8000)
[FeIII(dmbpy)3]3+
[RuII(LL2)]2+
417 (2200)
this work
354[a] (5500), 396[a] (5100)
[14]
430[a] (10300), 454 (12000)
354[a] (5500), 396[a] (5200)
[RuII(LL4)]2+
[14]
[14]
430[a] (10700), 454 (12300)
354[a] (5700), 396[a] (5400)
[RuII(LL6)]2+
430[a] (11000), 454 (12700), 356[a] (25000)
396[a] (20500), 430[a] (35000)
455 (40900), 530[a] (9300)
[{RuII(LL2)}3FeII]8+
[{RuII(LL4)}3FeII]8+
[{RuII(LL6)}3FeII]8+
this work
this work
this work
356[a] (26000), 396[a] (21400), 430[a] (36600)
454 (42500), 530[a] (11800)
356[a] (27300), 396[a] (22200), 430[a] (36600)
454 (42300), 530[a] (12700)
[a] Shoulder.
addition of Fe2+ allows the determination of the association
constant as being greater than 1022. This constant value is
higher than those estimated in a methanol/water mixture
(1013–1015),[15] presumably due to the more dissociative
properties of this medium compared to the acetonitrile one.
This high stability was also confirmed by following the ab-
sorbance at 530 nm of a [Ru(LLn)]2+ acetonitrile solution
after progressive addition of Fe(ClO4)2·8H2O (Figure 1).
The absorbances at 356 and 530 nm increase linearly during
the addition of Fe2+ until a maximum is reached for the
expected ratio of one iron for every three [Ru(LLn)]2+ units.
Addition of an excess of Fe2+ does not lead to any spectro-
scopic changes. The absence of free [Ru(LLn)]2+ is also con-
firmed by luminescence measurements in CH3CN, since, for
all synthesised complexes, the emission decay curves are
clean single exponentials (see below).
Electrochemistry
In CH3CN containing 0.1 m Bu4NClO4 the electrochemi-
cal behaviour of the three [{Ru(LLn)}3Fe]8+ complexes is
similar (see Table 2). The length of the aliphatic bridges has
only a slight effect on the E1/2 values of the oxidative and
reductive processes. In the negative region, although the
successive one-electron reductions of the bipyridine ligands
in the [Fe(dmbpy)3]2+ (dmbpy = 4,4Ј-dimethyl-2,2Ј-bipyri-
dine) and [Ru(LLn)]2+ complexes are clearly separated (by
115–140 mV, see Table 2), the reduction pattern of the
[{Ru(LLn)}3Fe]8+ complexes shows only three successive re-
duction waves {see Figure 2 for [{Ru(LL4)}3Fe]8+}, as ob-
served previously for the binuclear [Ru(LLn)Fe]4+ com-
plexes.[14] Moreover, only the first reduction wave is clearly
seen. A strong adsorption phenomenon coupled to the sec-
ond reduction process leads to an important distortion of
the third one. It should be mentioned that each reversible
redox system corresponds to the overall exchange of four
electrons corresponding to the one-electron reduction of
one bipyridine unit per metallic centre. The exchange of
Figure 1. A) Evolution of the visible spectrum of a solution of
[Ru(LL4)]2+ (0.105 mm) in CH3CN + 0.1 m Bu4NClO4 during the
addition of Fe(ClO4)2·8H2O: a) initial solution, b) after addition
of 0.055 equiv. of Fe2+, (c) 0.11, (d) 0.167, (e) 0.22, (f) 0.28, (g) 0.33
and 0.5. B) Evolution of the absorbance at 530 nm (l = 1 cm).
3322
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 3320–3330

Tetranuclear Polypyridyl Complexes of RuII and FeII
FULL PAPER
Table 2. Electrochemical data for [RuII(bpy)2(LLn)]2+ and [{RuII(LLn)}3FeII]8+ in deoxygenated CH3CN + 0.1 m Bu4NClO4 using a plati-
num electrode at a scan rate of 100 mV s–1.[a]
Reduction processes
Complexes
FeII/FeIII
RuII/RuIII
1
2
3
Ref.
[FeII(dmbpy)3]2+
0.585 (60)
–
–
–
0.607 (60)
0.588 (60)
0.588 (60)
–
–1.775 (60)
–1.665 (50)
–1.665 (50)
–1.665 (50)
–1.687 (60)
–1.647 (70)
–1.658 (60)
–1.970 (70)
–1.855 (50)
–1,862 (50)
–1.865 (50)
–1.827 (80)
–1.806 (50)
–1.818 (70)
–2.195 (70)
–2.105 (50)
–2.115 (50)
this work
[14]
[RuII(LL2)]2+
0.912 (60)
0.905 (60)
0.905 (60)
0.907 (60)
0.905 (60)
0.902 (60)
[RuII(LL4)]2+
[14]
[14]
[RuII(LL6)]2+
–2.125 (50)
[{RuII(LL2)}3FeII]8+
[{RuII(LL4)}3FeII]8+
[{RuII(LL6)}3FeII]8+
this work
this work
this work
[b]
[b]
[b]
[a] E1/2 (V) (ΔEp in mV) vs. Ag/Ag+ (0.01 m AgNO3 in CH3CN + 0.1 m Bu4NClO4). [b] This value cannot be accurately measured since
the waves are strongly distorted by adsorption phenomena (see text).
four electrons is confirmed by the fact that the height of the
first reduction wave at a rotating disk electrode is equal to
the sum of the heights of the two oxidation waves (see be-
low and Figure 2).
In the positive region, the cyclic voltammograms of the
three [{Ru(LLn)}3Fe]8+ complexes exhibit two well-sepa-
rated reversible redox systems (Figure 2). The potentials of
these systems are closed to those of the mononuclear com-
plexes [Fe(dmbpy)3]2+ and [Ru(LLn)]2+, thus allowing us to
assign the first oxidative process to the FeII/FeIII redox cou-
ple and the more anodic one to that of the RuII/RuIII system
(Table 2). Moreover, rotating disk electrode experiments
confirmed that the complexes are obtained in pure form
and that no dissociation occurs in CH3CN, since the height
of the RuII/RuIII wave is three times that of FeII/FeIII, in
accordance with the 3:1 Ru/Fe stoichiometry in the tetranu-
clear complexes [Equations (1) and (2)].
[{RuII(bpy)2(LLn)}3FeII]8+ Ǟ [{RuII(bpy)2(LLn)}3FeIII 9+
]
+ e– (1)
[{RuII(bpy)2(LLn)}3FeIII 9+ Ǟ [{RuIII(bpy)2(LLn)}3FeIII 12+
]
]
+ 3e–
(2)
As previously observed for the [Ru(LLn)Fe]4+ com-
plexes,[14] the E1/2 values of the FeII/FeIII and RuII/RuIII re-
dox systems decrease slightly upon increasing the number
of methylenes in the aliphatic bridge (Table 2). This E1/2
variation is due to a slightly higher electron-donor effect of
the LL6 ligand compared to LL4, and of the latter to LL2.
Otherwise, the RuII/RuIII redox system involves the si-
Figure 2. (−) Cyclic voltammograms of a 0.32 mm solution of
[{Ru(LL4)}3Fe]8+ in CH3CN + 0.1 m Bu4NClO4 at a platinum elec-
trode; scan rate: 100 mVs–1. (---) Voltammograms at a platinum
rotating disk electrode at ω = 600 rmin–1; scan rate: 10 mVs–1.
multaneous exchange of three electrons. In order to esti- (10 mV) in the order LL2 Ͼ LL4 Ͼ LL6. Since the two
mate the magnitude of the electronic connexion between oxidized forms of the complexes, [{RuII(LLn)}3FeIII 9+
and
the three ruthenium centres, computational fitting of the [{RuIII(LLn)}3FeIII 12+
are expected to be produced during
]
]
cyclic voltammetry curves of [{Ru(LLn)}3Fe]8+ was per- the photoinduced electron-transfer processes (Scheme 2),
formed, which led to the determination of the formal poten- the stabilities of these species were evaluated by exhaustive
tials Ef1, Ef2 and Ef3 for the three ruthenium oxidation steps electrolyses. Taking into account the large potential differ-
(Table 3) (see Figure 3 for [{Ru(LL4)}3Fe]8+ and Experi- ence between the FeII/FeIII and RuII/RuIII redox systems
mental Section).[16,17] Since the difference between the ex- (ΔE1/2 = 296 mV for LL2, 315 for LL4 and 317 for LL6),
treme Ef values (ΔEf3–1 = Ef3 – Ef1) is close to the theoreti- the mixed-valent [{RuII(LLn)}3FeIII 9+
species are perfectly
]
cal value (57 mV) for the three tetranuclear complexes,[16] stable. Indeed, two successive exhaustive electrolyses carried
the absence of an electronic connexion within the studied out at 0.80 and 1.10 V consumed one and three electrons,
compounds is confirmed. One can see, as observed for the respectively, per [{Ru(LLn)}3Fe]8+, and allowed the bulk
FeII/FeIII system, that these Ef values are in accordance with build-up of [{RuII(LLn)}3FeIII]9+ and [{RuIII(LLn)}3FeIII]12+
.
the electron-donor character of the bridging ligand; the This is illustrated by the evolution of the absorption
E1/2 value of the RuII/RuIII couple decreases slightly spectra of the solutions after these sequential electrolyses
Eur. J. Inorg. Chem. 2005, 3320–3330
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A. Deronzier, J.-C. Leprêtre et al.
FULL PAPER
Table 3. Oxidation potentials of [{Ru(LLn)}3Fe]8+ determined by fitting of the experimental cyclic voltammograms.
E1/2 [mV] vs. Ag/Ag+ 10 mm
FeII/FeIII
E1/2
RuII/RuIII
RuII/RuIII
RuII/RuIII
RuII/RuIII
E1/2
moy
Complexes
Ef1
Ef2
Ef3
(ΔEf3–1
)
[{RuII(LL2)}3FeII]8+
[{RuII(LL4)}3FeII]8+
[{RuII(LL6)}3FeII]8+
614
584
572
882
860
852
1
2
1
915
906
897
2
2
4
934
931
818 10
3
2
910 (58)
899 (71)
889 (66)
(see Figure 4 for LL4). The typical initial bands of the FeII- Energy Transfer
tris-bipyridine unit (λ = 356 and 530 nm) disappear after
one electron per complex has been passed (Table 1). After
The emission wavelength (λemis), luminescence lifetime
(τ), and luminescence quantum yield (φL) of the [{Ru-
(LLn)}3Fe]8+ complexes are reported in Table 4, along with
those obtained for the corresponding [Ru(LLn)]2+ and
[Ru(LLn)Fe]4+ parent complexes. On the nanosecond times-
cale, the luminescence decay of all [{Ru(LLn)}3Fe]8+ com-
plexes appears as a single exponential with a luminescence
lifetime slightly lower than those of the monometallic
[Ru(LLn)]2+ complexes. Further experiments were per-
formed on a subnanosecond timescale using a picosecond
Ti:Sapphire laser and single photon counting detection. No
additive short component in the decay was observed. Nev-
ertheless, if the luminescence is detected, it is largely
quenched. Indeed, compared to the [Ru(LLn)]2+ complexes,
the luminescence quantum yield is lowered from a factor of
6 for [{Ru(LL6)}3Fe]8+ to almost 10 for [{Ru(LL4)}3Fe]8+
(Table 4). These data indicate that, as observed for [Ru(LLn)-
Fe]4+, the FeII-tris-bipyridine unit quenches the 3MLCT ex-
cited state in the [{Ru(LLn)}3Fe]8+ complexes.
exchange of three other electrons, the bands of the RuII-
tris-bipyridine units (λ = 356 and 454 nm) disappear and
are replaced by those of the RuIII ones (428 and 644 nm).
Both oxidized forms are stable for several hours. Finally, we
found that exhaustive reduction of all these oxidized species
leads to the nearly quantitative recovery of the starting ma-
terial, thus demonstrating the high reversibility of the pro-
cess.
The luminescence lifetime and quantum yield of the
[{Ru(LLn)}3Fe]8+ complexes were found to be independent
of the concentration of the samples (concentration range
used from 10–6 to 2×10–5 m), which means that, in contrast
to what we demonstrated for the binuclear [Ru(LLn)Fe]4+
complexes, the intermolecular quenching is not efficient in
such tetranuclear systems. As a result, the partial inhibition
of the luminescence of the RuII centre is only due to an
intramolecular interaction with the central FeII-tris-bipyri-
dine unit.
Figure 3. Cyclic voltammograms of a 0.14 mm solution of [{Ru-
(LL4)}3Fe]8+ in CH3CN + 0.1 m Bu4NClO4 at a platinum electrode;
scan rate 200 mVs–1: (---) experimental curve, (−) fitted curve.
Electron transfer (ET) between RuII* and FeII centres
has to be ruled out for this quenching pathway. Indeed,
considering the potential values of the RuII*/RuI [in the
range 0.34–0.38 V for [{Ru(LLn)}3Fe]8+ vs. Ag/Ag+ in
CH3CN with 0.1 m Bu4NClO4][18] and FeII/FeIII (in the
range 0.58–0.60 V) redox couples, the electron-transfer
quenching process is strongly endergonic (Ն0.2 V). Like-
wise, the potential value of the RuII*/RuIII redox couple
(between –1.12 and –1.14 V) compared to that of the first
reduction potential of the iron unit (between –1.647 and
–1.687) leads to a strongly endergonic process (Ն0.52 V).
Thus, a pure electronic energy transfer (EET) can be con-
sidered as the main quenching process.[14,19]
The quenching rate constant (kq) was determined from
Ref
S
Equation (3), where φL
and φL are the luminescence
Figure 4. Absorption spectra of a 0.32 mm solution of [{Ru-
(LL4)}3Fe]8+ in CH3CN + 0.1 m Bu4NClO4: (a) initial solution, (b)
after exhaustive electrolysis at 0.80 V, (c) after exhaustive electroly-
sis at 1.10 V (l = 1 mm).
quantum yields of the [Ru(LLn)]2+ and [{Ru(LLn)}3Fe]8+
complexes, respectively, and τRef the luminescence lifetime
of [Ru(LLn)]2+
.
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Tetranuclear Polypyridyl Complexes of RuII and FeII
FULL PAPER
Table 4. Photophysical data determined for deoxygenated solutions of the ruthenium complexes in CH3CN + 0.1 m Bu4NClO4 at 25 °C.
[a]
Complexes
λemis [nm]
τ [μs]
φL
kq
Ref.
[14,29]
[RuII(bpy)3]3+
603
611
612
612
612
612
612
612
612
612
–
1.06
1.11
1.12
0.062
0.060
0.054
0.059
0.022[c]
0.019[c]
0.023[c]
0.008
0.005
0.009
0.06
9.0×109 m–1 s–1[b]
[14]
[RuII(LL2)]2+
1.4×1010
1.8×1010
1.8×1010
m
m
m
–1 s–1[b]
–1 s–1[b]
–1 s–1[b]
[14]
[RuII(LL4)]2+
[14]
[RuII(LL6)]2+
1.13
[14]
[RuII(LL2)FeII]4+
[RuII(LL4)FeII]4+
[RuII(LL6)FeII]4+
[{RuII(LL2)}3FeII]8+
[{RuII(LL4)}3FeII]8+
[{RuII(LL6)}3FeII]8+
[{RuII(LL2)]3ZnII}8+
1.10[c]
1.03[c]
1.00[c]
1.07
3.5×108 m–1 s–1
2.2×109 m–1 s–1
3.3×109 m–1 s–1
5.9×106 s–1
8.8×106 s–1
4.9×106 s–1
–
[14]
[14]
this work
this work
this work
this work
1.03
1.04
1.07
[a] Quenching rate constant of the electronic energy transfer (EET) between RuII* and FeII. [b] Values determined in the presence of
[Fe(bpy)3]2+. [c] Values determined for a 0.04 mm solution of binuclear complexes.
centres is less than R0, energy transfer is faster than radia-
tive and non-radiative relaxation of the donor. Neverthe-
(3)
less, the fact that τ does not exhibit a systematic dependence
on the Ru–Fe distance in the [{Ru(LLn)}3Fe]8+ series sug-
The same order of magnitude of kq (5×106 Յ kq
Յ
gests that the Förster mechanism should be considered as
minor. Thus, the deactivation mainly proceeds by the Dex-
ter mechanism. Therefore, since it involves the contact be-
tween the acceptor and the donor, the folding up of the
molecule is required. Taking into account that the length of
the alkyl chain does not lead to an important variation in
τ, and that the deactivation process is mainly intramolecu-
lar, the experimental luminescence data can be understood
if one considers that the tetranuclear complexes coexist in
at least two forms in a thermodynamic equilibrium after
irradiation in the MLCT band of the [Ru(LLn)]2+ subunit
(Scheme 3). At the ground state level, the more stable geom-
etry of the complexes is presumably the one which presents
the four cationic metallic centres sufficiently distant to
minimize the electrostatic repulsion. Quickly after the laser
pulse irradiation, at the excited state level, the complexes
undergo a sub-nanosecond (less than 30 ps) rearrangement,
which cannot be detected by our setup, into two forms A
and B (Scheme 3). In the A form, the relatively important
RuII/FeII distance does not allow an efficient energy transfer
(via a Dexter mechanism) and the luminescence of the ru-
thenium centre is detected with almost no perturbation due
to the presence of the iron centre. It is well-known that,
after irradiation in the MLCT band of the [Ru(LLn)]2+ moi-
eties, an electron from the HOMO, which is localised
around the metallic centre, is injected into the LUMO,
which is delocalised over the bipyridine ligand, to form the
transient bpy·– radical anion.[22,23] In the B form, the two
linked cationic metallic centres fold up in a way that con-
fines the bpy·– radical. Because two metallic centres are
close, an efficient intramolecular EET is now feasible and
contributes to the extinction of the luminescence of the
complexes. From the luminescence quantum yield measure-
ments, it appears that the B form is the more stable one at
room temperature since φL decreases drastically from a fac-
tor of between 6 and 10 compared to that of [Ru(LLn)]2+
(Table 4). Moreover, if the A form and the B form are in
rapid equilibrium, the excited state lifetime of the tetranu-
clear samples would be shorter than the corresponding
9×106 s–1; Table 4) was obtained for all three compounds,
which shows that the EET process is moderately efficient
and clearly not connected to the length to the aliphatic
chain of the bridging ligand. Eliott and co-workers[15] have
obtained a similar magnitude of kq (8×106 Յ kq
Յ
197×106 s–1) in aqueous methanolic solution, but, in con-
trast to our results, these authors found a dependence of
the kq value with the distance beween RuII and FeII: the
more important the RuII–FeII distance, the weaker the kq
value. One can suggest that the nature of the solvent might
have an influence on the deactivation process. This influ-
ence of solvation conditions is confirmed by the lower τ
values obtained in aqueous methanolic medium for the pre-
cursor ruthenium complexes {τ
= 356 ns for [Ru-
(LL2)]2+}.[15]
EET processes occur according to two mechanisms −
Dexter (or electron exchange interaction)[20] and Förster (or
coulombic interaction)[21] − both of which involve the spec-
tral overlap of the luminescence of the donor with the ab-
sorption spectra of the acceptor. The Dexter mechanism
requires contact between the donor and the acceptor,
whereas the Förster one proceeds over a larger distance. For
the Förster process, a critical transfer distance, R0, can be
calculated according to Equation (4)
(4)
where φD is the donor emission quantum yield {i.e.
[Ru(LLn)]2+}, n the solvent refractive index and the integral
term represents the spectral overlap of the normalised do-
nor emission with the acceptor absorption. For the three
[{Ru(LLn)}3Fe]8+ complexes, an R0 value of 20 1 Å has
been calculated, whereas the distance between the ruthe-
nium and iron centres in theses structures is in the range
from 10 to 12 Å according to space-filling molecular mod-
els.[15] When the estimated distance between both metallic
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A. Deronzier, J.-C. Leprêtre et al.
FULL PAPER
[Ru(LLn)]2+ as the relaxation channel for the B form is can only be due to a higher probability of a dynamic
much faster than for the A form.[24] This effect is weak for quenching between Fe and Ru by folding up of the molecule
[{Ru(LL2)}3Fe]8+ {τ = 1.07 μs compared to 1.11 μs for in the tetranuclear structures than in the binuclear ones.
[Ru(LL2)]2+} and larger for complexes having a longer ali-
phatic bridge such as n = 4 and n = 6 (Table 4).
Photoinduced Electron Transfer
It has been established previously[14,26,27] that addition of
an irreversible electron acceptor like 4-bromophenyldiazon-
–
ium tetrafluoroborate (ArN2+BF4 ) to a solution of the
[RuII(bpy)3]2+ complex in CH3CN + 0.1 m Bu4NClO4 al-
lows the efficient production of [RuIII(bpy)3]3+ under con-
tinuous irradiation (quantum yield 0.34). This permanent
build up of [RuIII(bpy)3]3+ arises because of the following
electron transfer quenching reaction [Equations (5) and (6)];
the back-electron transfer reaction between [RuIII(bpy)3]3+
·
and ArN2 radical is avoided by the rapid evolution of
·
ArN2 into ArH and N2.[25,26]
[RuII(bpy)3]2+* + ArN2 Ǟ [RuIII(bpy)3]3+ + ArN2
(5)
(6)
+
·
CH3CN
·
ArN2
Ǟ ArH + N2
Scheme 3. Illustration of the photophysical behaviour of [{Ru-
(LLn)}3Fe]8+ in CH3CN + 0.1 m Bu4NClO4 after irradiation at
337 nm.
In the binuclear [Ru(LLn)Fe]4+ complexes, it has been
demonstrated that ArN2 can also react preferentially with
+
the excited state of the [Ru(LLn)]2+ subunit to advan-
tageously short-circuit the energy transfer.[14] For tetranu-
clear complexes, a similarly efficient photogeneration of the
corresponding RuIII species, i.e. [RuIIIRuII2(bpy)6(LLn)3-
FeII]9+ is expected (Scheme 2). Indeed, the quenching of the
RuII* centre in the [{Ru(LLn)}3Fe]8+ complexes by EET is
incomplete and the luminescence lifetime of the RuII* ex-
cited state is sufficiently long-lived (1 μs) to allow a bimol-
ecular reaction using an external electron acceptor. In ad-
dition, the lifetime of the excited state of the FeII moiety is
Although this phenomenon involves the proximity of cat-
ionic species, and therefore electrostatic repulsion, such an
EET mechanism has already been suspected for other sys-
tems containing a [Ru(bpy)3]2+*-like donor with cationic
iron[14,15] or manganese[12,25] complexes as acceptor. Never-
theless, the luminescence quantum yield of the [{Ru-
(LLn)}3Fe]8+ complexes is much lower than that of
[Ru(LLn)Fe]4+. To understand the reason for this difference,
and to investigate the possibility of a self-quenching phe-
nomenon of the RuII-tris-bipyridine units in the tetranu-
clear structure, we synthesised a similar tetranuclear com-
plex in which Fe2+ is replaced by Zn2+, namely [{RuII-
(bpy)2(LL2)}3ZnII]8+ {denoted [{Ru(LL2)}3Zn]8+}. The
ZnII-tris-bipyridine centre has no absorption in the visible
region and, as expected, the absorption spectrum of the
too short (Ͻ1 ns) to undergo an efficient bimolecular elec-
+ [19]
tron transfer with ArN2
.
In order to photoinduce the oxidation of the [{Ru-
+
(LLn)}3Fe]8+ complexes, ArN2 was added to a solution of
the complexes in deoxygenated CH3CN + 0.1 m Bu4NClO4.
Since the photogenerated RuIII subunit plays the role of oxi-
dant towards FeII (see electrochemical part), irradiation
[{Ru(LL2)}3Zn]8+ complex is close to that of [Ru(LL2)]2+
.
should lead, in a first step, to [{RuII(LLn)}3FeIII 9+
]
That excludes any possibility of an energy transfer between
the RuII-tris-bipyridine units and the ZnII-tris-bipyridine
one after light irradiation. Moreover, since the Zn2+ site is
not able to be oxidized, a quenching of the RuII* lumines-
cence by an electron transfer is thermodynamically pro-
hibited. The quantum yield and luminescence lifetime of
[{Ru(LL2)}3Zn]8+ were determined to be φL = 0.06 and τ =
1.07 μs, respectively. These values are similar to those ob-
tained for [Ru(LL2)]2+ (φL = 0.06, τ = 1.11 μs), only the
lifetime is weakly decreased as for [{Ru(LL2)}3-
(Scheme 2). In a second step, the formation of the final
+
[{RuIII(LLn)}3FeIII 12+
species is expected as ArN2 is
]
added in excess. The rate constant of oxidant quenching
(kET) of the excited state of the tetranuclear complexes by
+
ArN2 was determined from the Stern–Volmer plot, see
Equation (7)
+
τ/τAr = 1 + kET·τ·[ArN2
]
(7)
Fe]8+, indicating that the possible self-quenching of the sim- where τ and τAr are the luminescence lifetime of the com-
+
ilar [Ru(LL2)]2+ centre has to be considered as a minor pro- plexes without and with a variable concentration of ArN2
,
cess. Thus, the drastically lowered φL value for the respectively, and kET is the rate constant of the electron
[{Ru(LLn)}3Fe]8+ complexes compared to [Ru(LLn)Fe]4+ transfer reaction according to Equation (8).
3326
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Tetranuclear Polypyridyl Complexes of RuII and FeII
FULL PAPER
kET
Photooxidation by Continuous Irradiation
[RuII*RuII2(bpy)6(LLn)3FeII]8+
+
ArN2
Ǟ
[RuIIIRuII
-
2
+
·
(bpy)6(LLn)3FeII]9+ + ArN2
(8)
Successive photogeneration of the oxidized [{RuII-
(LLn)}3FeIII 9+ [{RuIII(LLn)}3FeIII 12+
and species
]
]
(Scheme 2) was followed by UV/Vis absorption spec-
troscopy. Solutions containing [{Ru(LLn)}3Fe]8+ (0.03–
The Stern–Volmer plots are linear for the three com-
plexes (Figure 5). This indicates that the electron transfer is
a simple bimolecular reaction, i.e. only one RuII centre re-
acts each time with ArN2+. The kET values obtained for the
tetranuclear [{Ru(LLn)}3Fe]8+ compounds (Ͼ109 m–1 s–1)
depend only slightly on the Ru–Fe distance in the com-
plexes, and are around 10 times greater than the kET values
determined for the binuclear [Ru(LLn)Fe]4+ complexes
(Table 5). The markedly faster electronic quenching process
in the case of tetranuclear complexes is presumably due to
the lower efficiency of the competitive energy transfer by
the Fe2+ centre (Table 4). Nevertheless, it is difficult to com-
pare the energy-transfer constants of these two kinds of
complexes since, for the tetranuclear complexes, the energy
transfer occurs via an intramolecular process while for the
binuclear ones it is intermolecular.
+
0.04 mm) in the presence of an excess of ArN2 (15 mm) in
CH3CN + 0.1 m Bu4NClO4 were irradiated with a mercury
lamp (250 W). Two successive changes of the absorption
spectrum were observed during the irradiation. In the first
step, the band of the FeII unit at 530 nm decreases regularly
until it disappears totally, in accordance with the formation
of [{RuII(LLn)}3FeIII 9+
]
via the transient photogenerated
RuIII species (Figure 6). In a second step, irradiation in-
duces the quantitative formation of [{RuIII(LLn)}3FeIII 12+
,
]
as illustrated by the decrease of the MLCT band of the RuII
unit at around 455 nm and the appearance of two new ones
at 428 and 644 nm typical of the RuIII species.
Figure 6. Spectral change of
a
mixture of [{Ru(LL6)}3Fe]8+
(0.032 mm) and ArN2+ (15 mm), under visible irradiation: (a) initial
solution, (b) 3 s, (c) 12 s, (d) 24 s, (e) 120 s, (f) 220 s, (g) 700 s, (l =
1 cm).
Figure 5. Stern–Volmer plots for the [{Ru(LLn)}3Fe]8+ complexes
From a kinetic point of view, one can consider that, dur-
ing the first step, there is a steady-state concentration of the
RuII and RuIII species since the sacrificial oxidant is present
in large excess. Thus, the overall kinetic law for the oxi-
dation of the FeII-tris-bipyridine unit can, as we previously
proposed,[14] be approximated to a pseudo-first-order equa-
tion [Equation (9)], where [RuII] and [FeII] represent the
concentration of the two subunits of the [{Ru(LLn)}3Fe]8+
complexes in solution.
+
in deoxygenated CH3CN (0.01 mm) with ArN2 (between 0 and
0.6 mm): [{Ru(LL2)}3Fe]8+ (᭿), [{Ru(LL4)}3Fe]8+ (᭹) and
[{Ru(LL6)}3Fe]8+ (᭡).
Table 5. Electron transfer rate constant, kET, for [Ru(LLn)Fe]4+ and
[{Ru(LLn)}3Fe]8+ in the presence of a variable concentration of
+
ArN2 at room temperature in CH3CN + 0.1 m Bu4NClO4.
Complexes
kET [m–1 s–1]
[RuII(LL2)FeII]4+
[RuII(LL4)FeII]4+
[RuII(LL6)FeII]4+
[{RuII(LL2)}3FeII]8+
[{RuII(LL4)}3FeII]8+
[{RuII(LL6)}3FeII]8+
3.7×108
5.3×108
6.4×108
3.6×109
3.2×109
4.0×109
v = k[RuII][FeII][ArN2+] = KFe[FeII] = –d[FeII]/dt
(9)
In a similar fashion, during the second step involving the
formation of [{RuIII(LLn)}3FeIII 12+
, a pseudo-first-order
]
equation is also obtained, see Equation (10).
Eur. J. Inorg. Chem. 2005, 3320–3330
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A. Deronzier, J.-C. Leprêtre et al.
FULL PAPER
+
Table 6. Apparent oxidation rate constants for [{Ru(LLn)}3Fe]8+ during irradiation in the presence of a large excess of ArN2
.
Complexes
KFe [s–1]
KRu [s–1]
[{RuII(LL2)}3FeII]8+
[{RuII(LL4)}3FeII]8+
[{RuII(LL6)}3FeII]8+
5.4×10–2 0.8×10–2
8.4×10–2 0.6×10–2
11.7×10–2 1×10–2
4.99×10–3 4×10–5
6.63×10–3 6×10–5
12.24×10–3 3×10–5
v = kЈ[RuII][FeIII][ArN2+] = KRu[RuII] = –d[RuII]/dt
(10)
photogeneration of the RuIII moieties. This process requires
+
contact between two cationic species {i.e. ArN2 and
[{Ru(LLn)}3Fe]8+} and involves an electrostatic repulsion
phenomenon which should be important for the complex
exhibiting the smaller Ru–Fe distance − [{Ru(LL2)}3Fe]8+
− whereas it is less pronounced for [{Ru(LL6)}3Fe]8+. The
These two oxidation steps were followed by the evolution
of the absorbance at 530 and 454 nm, respectively, with the
irradiation time. In all cases the absorbance variation was
fitted by a monoexponential curve, as illustrated in Fig-
ure 7, in accordance with Equations (9) and (10), thus al-
lowing the determination of KFe and KRu for the three het-
erotetranuclear complexes. These values are given in
Table 6. It appears that both the FeII and RuII oxidation
rate depend on the nature of the bridging ligand, and that
the efficiency of the photoassisted process increases when
the length of the alkyl chain increases, oxidation of
[{Ru(LL6)}3Fe]8+ being twice as fast as oxidation of
second process involves the oxidation of FeII into FeIII
.
Since the electron transfer operates according to an intra-
molecular process, it requires the folding-up of the complex
in order to allow the formation of FeIII through the photo-
generated RuIII moieties. Taking into account that the flexi-
bility of the aliphatic bridging ligand increases with the
number of methylene units, the contact between RuIII and
FeII should be favoured in the case of [{Ru(LL6)}3Fe]8+
,
whereas for [{Ru(LL2)}3Fe]8+ the folding-up is more diffi-
cult and therefore the photooxidation rate is lowered.
Finally, the quantum yield of formation of [{RuII-
[{Ru(LL2)}3Fe]8+
.
(LLn)}3FeIII 9+
(φF) was determined in CH3CN (see Experi-
]
mental Section) and was found to be equal to 0.22 for the
most efficient system, i.e. [{Ru(LL6)}3Fe]8+
.
Conclusions
In this study we have demonstrated that for the tetranu-
clear [{RuII(LLn)}3FeII]8+ complexes a partial energy trans-
fer from the 3MLCT excited state of the RuII-tris-bipyridine
centres to the FeII-tris-bipyridine unit occurs by a pure in-
tramolecular process, in contrast to what was observed for
the corresponding binuclear complexes. This is a conse-
quence of the structure of the tetranuclear complexes, in
which the presence of the three RuII subunits prevents the
close contact of the FeII centre with the RuII subunits of
another tetranuclear complex and therefore an intermo-
lecular energy transfer. The rate constant of the process is
not clearly connected to the length of the aliphatic chain of
the bridging ligand. This energy-transfer process can be eas-
ily short-circuited in the presence of an external irreversible
electron acceptor by an electron-transfer process, leading,
with a high quantum yield, to the photoinduced oxidation
Figure 7. Evolution of the absorption intensity at 530 nm (A) and
454 nm (B) with irradiation time for the [{Ru(LL6)}3Fe]8+ com-
plexes (0.032 mm) in CH3CN + 0.1 m Bu4NClO4, in the presence
of ArN2+ (15 mm) (l = 1 cm). Bold lines correspond to the fit using
a monoexponetial function. The fit parameters KFe and KRu are
given in Table 6.
of the iron(ii) subunit of the tetranuclear complexes (φF
0.22) followed by that of the ruthenium(ii) subunits.
=
An extension of this work is currently underway with
similar tetranuclear polypyridyl complexes of RuII and
MnII. An efficient photoinduced oxidation phenomenon is
also expected for this kind of complex since the energy
Two processes can control the overall efficiency of the transfer between the two different metallic subunits is
oxidation steps and explain the influence of the aliphatic known to be less efficient than for the parent FeII com-
chain length on the photooxidation rate. The first one is the pounds.
3328
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 3320–3330

Tetranuclear Polypyridyl Complexes of RuII and FeII
FULL PAPER
Experimental Section
(11)
General: Acetonitrile (Rathburn, HPLC grade) was used as re-
ceived and stored under argon in a glovebox. Tetra-n-butylammo-
nium perchlorate (Bu4NClO4, Fluka) was used as received. 1H
NMR spectra were recorded on a Bruker AC 250 spectrometer.
The electrospray ionisation mass spectrometry (ESI-MS) experi-
ments were performed on a triple quadrupole mass spectrometer
Quattro II (micromass, Altrincham, UK). The ESI source was
heated to 80 °C. The sampling cone voltage was set within the
range 6–17 V according to the complex studied. Complexes in solu-
tion (1 mgmL–1 in CH3CN) were injected using a syringe pump at
a flow rate of 10 μLmin–1. The electrospray probe (capillary) volt-
age was optimised in the range 2–5 kV for positive ion electrospray.
Elemental analyses were performed by the Service Central d’Ana-
lyse du CNRS at Vernaison (France). UV/Vis spectra were ob-
tained using a Cary 1 and a Cary 100 absorption spectrophotome-
ters on 1-mm path-length quartz cells for electrochemical experi-
ments and 1-cm cells for irradiation experiments.
where the emission intensity, IL, was calculated from the spectrum
area ͐I(λ)dλ, and OD represents the optical density at the exci-
tation wavelength (450 nm). The superscripts “S” and “Ref” refer
to the sample and to the standard, respectively.
Continuous Irradiation: The irradiation experiments were per-
formed using a mercury lamp (Oriel 66901, 250 W) whose UV and
IR radiations were filtered with a large band-pass filter centred
around λ = 560 nm (irradiation range between 410 and 710 nm).
The solutions contained a mixture of [{Ru(LLn)}3Fe]8+ complex
+
(0.03–0.04 mm) and ArN2 (15 mm) in order to obtain an ab-
sorbance below 1.5 at 454 nm in CH3CN + 0.1 m Bu4NClO4. Un-
der these conditions the concentration of ArN2+ can be considered
as constant during the experiments.
The quantum yields of formation of [{RuII(LLn)}3FeIII 9+
] , φF, mea-
Electrochemistry: All electrochemical measurements were per-
formed under argon in a glovebox at room temperature. Cyclic vol-
tammetry and controlled potential electrolysis experiments were
performed using an EG&G PAR model 173 potentiostat/galvanos-
tat equipped with a PAR model universal programmer and a PAR
model 179 digital coulometer. A CHI440 Electrochemical Analyzer
(CH Instruments, Texas, USA) was used for the determination of
formal potentials. A standard three-electrode electrochemical cell
was used. Potentials were referenced to an Ag/10 mm AgNO3 refer-
ence electrode in CH3CN + 0.1 m Bu4NClO4. Potentials referred to
that system can be converted into the ferrocene/ferrocenium couple
by subtracting 87 mV and to SCE or NHE by adding 298 mV or
548 mV, respectively.[28] The working electrode was a platinum disk
polished with 2-μm diamond paste (Mecaprex Presi) with a dia-
meter of 5 mm for cyclic voltammetry [Epa: anodic peak potential;
Epc: cathodic peak potential; E1/2 = (Epa + Epc)/2; ΔEp = Epa – Epc]
and 2 mm for rotating disk electrode experiments (RDE). Exhaus-
tive electrolyses were carried out with a platinum plate. The auxil-
iary electrode was a Pt wire in CH3CN + 0.1 m Bu4NClO4. For
electrochemical experiments, electronic absorption spectra were re-
corded on a Hewlett–Packard 8452 A diode array spectrophotome-
ter. Initial and electrolyzed solutions were transferred to a conven-
tional 1-mm quartz cell in the glovebox.
sured after continuous irradiation at 436 nm, were performed with
a 250-W Hg Lamp (Oriel 66901). The desired mercury emission
line was isolated using a band-pass filter. The quantum yields were
determined by actinometry at the wavelength of the maximum ab-
sorption of [Fe(bpy)3]2+ (526 nm) by comparing the absorption of
the sample before and after irradiation and using the quantum
+
yield formation of [Ru(bpy)3]3+ in the system [Ru(bpy)3]2+/ArN2
as reference (φFRef = 0.34).[26] Samples were prepared with the same
absorbance (Abs = 2) at 454 nm and the variation of absorbance
during irradiation was checked to be less than 0.1. The quantum
yield of formation, φFS, of the sample was deduced from Equation
(12).
(12)
Synthesis of the Diazonium Salt: 4-bromophenyl diazonium tetra-
–
–
fluoroborate p-BrC6H4N2+·BF4 (ArN2+·BF4 ) was synthesised as
described previously.[26]
Synthesis of Ligands LLn and of [RuII(bpy)2(LLn)](PF6)2: The li-
gands 1,2-bis[4-(4Ј-methyl-2,2Ј-bipyridinyl)]ethane (LL2), 1,2-bis[4-
(4Ј-methyl-2,2Ј-bipyridinyl)]butane (LL4), 1,2-bis[4-(4Ј-methyl-2,2Ј-
bipyridinyl)]hexane (LL6) and the [RuII(bpy)2(LL2)](PF6)2,
[RuII(bpy)2(LL4)](PF6)2, [RuII(bpy)2(LL6)](PF6)2 complexes were
synthesised as described previously.[14]
Luminescence: The steady-state emission spectra were recorded on
a Photon Technology International (PTI) SE-900M spectrofluo-
rimeter. All the samples for luminescence and photooxidation ex-
periments were prepared in a glovebox in deoxygenated CH3CN +
0.1 m Bu4NClO4 and contained in a 1-cm quartz cell. The samples
were maintained under aerobic conditions with a Teflon cap. The
luminescence lifetime of the complexes was recorded after irradia-
tion at λ = 337 nm with a 4-ns pulsed laser (spectra physics 337-
201) and recorded at λ = 600 nm using a monochromator and a
Synthesis of [{RuII(bpy)2(LLn)}3FeII](PF6)8: An aqueous solution of
0.4 molar equivalents of Fe(ClO4)2·8H2O was added to an orange
solution of [RuII(bpy)2(LLn)](PF6)2 (100 mg) in ethanol (10 mL) at
60 °C, leading to the formation of the red-orange complex
[{RuII(bpy)2(LLn)}3FeII]8+. After stirring the solution for one hour
photomultiplicator tube (Hammamatsu R928) coupled with an ul- at 60 °C, the ethanol was removed under reduced pressure and the
tra-fast oscilloscope (Tektronix TDS 520A). For subnanosecond
decay experiments, sample excitation was performed at 400 nm,
which was obtained by the second harmonic of a Titanium: Sap-
phire laser (picosecond Tsunami laser at an 80-MHz repetition
rate). For decay acquisition, a GaAs microchannel plate photomul-
tiplier (Hamamatsu model R 3809 U-51) followed by a homemade
single-photon correlator were used. The ultimate time resolution
of the entire chain was close to 30 ps. The emission quantum yield,
φL, was determined at 25 °C in deoxygenated acetonitrile solutions
heterotetranuclear complex was extracted with CH2Cl2. The re-
sulting solution was washed three times with an aqueous KPF6
–
–
solution (0.1 m) in order to exchange the ClO4 anion with PF6 ,
and then twice with water. After drying over Na2SO4, CH2Cl2 was
removed under reduced pressure. The red-orange product obtained
corresponding to [{RuII(bpy)2(LLn)}3FeII](PF6)8 was reprecipitated
in CH3CN/diethyl ether, washed with diethyl ether, and dried under
vacuum. The purity of each complex was confirmed by elemental
analysis, 1H NMR spectroscopy and by the observation of a single
spot by TLC [eluent H2O/CH3CN (10:90) containing KPF6
(10 mm)].
ref
with a CH3CN solution of [RuII(bpy)3](PF6)2 (φL = 0.062)[29] ac-
cording to Equation (11)
Eur. J. Inorg. Chem. 2005, 3320–3330
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3329

A. Deronzier, J.-C. Leprêtre et al.
FULL PAPER
[{RuII(bpy)2(LL2)}3FeII](PF6)8 ([{Ru(LL2)}3Fe]8+): 51 mg (45%).
C132H114F48Fe1N24P8Ru3·6H2O (3663.3): calcd. C 43.28, H 3.46, N
9.18; found C 43.42, H 3.33, N 9.36. ESI-MS: m/z (%) = 3410.28
[3] C. A. Bignozzi, J. R. Schoonover, F. Scandola, Prog. Inorg.
Chem. 1997, 44, 1, special volume on molecular level artificial
photosynthetic materials (Eds.: G. J. Meyer, K. D. Karlin),
John Wiley & Sons, New York.
[4] L. Sun, L. Hammarström, B. Akermark, S. Styring, Chem. Soc.
Rev. 2001, 30, 36.
[M – PF6]+, 1632.70 (2) [M – 2PF6]2+, 1040.15 (6) [M – 3PF6]3+
,
743.87 (10) [M – 4PF6]4+, 566.10 (42) [M – 5PF6]5+
.
[{RuII(bpy)2(LL4)}3FeII](PF6)8 ([{Ru(LL4)}3Fe]8+): Yield: 46 mg
(41%). C138H126F48Fe1N24P8Ru3·6H2O (3747.5): calcd. C 44.34, H
3.71, N 9.04; found C 44.82, H 3.72, N 9.03. ESI-MS: m/z (%):
3494.44 [M – PF6]+, 1674.75 (1) [M – 2PF6]2+, 1068.18 (8) [M –
[5] L. Hammarström, Curr. Opin. Chem. Biol. 2003, 7, 666.
[6] D. Burdinski, K. Wieghardt, S. Steenken, J. Am. Chem. Soc.
1999, 30, 10781.
[7] D. Burdinski, B. Eberhardt, K. Wieghardt, Inorg. Chem. 2000,
39, 105.
[8] K. E. Berg, A. Tran, M. K. Raymond, M. Abrahamsson, J.
Wolny, S. Redon, M. Andersson, L. Sun, L. Hammarström, H.
Toftlund, B. Akermark, Eur. J. Inorg. Chem. 2001, 1019.
[9] M. M. Morrisson, D. T. Sawyer, Inorg. Chem. 1978, 17, 333.
[10] M. N. Collomb Dunand-Sautier, A. Deronzier, J. Electroanal.
Chem. 1997, 428, 65.
[11] M. N. Collomb Dunand-Sautier, A. Deronzier, I. Romero, J.
Electroanal. Chem. 1997, 436, 219.
[12] H. Berglund-Baudin, L. Sun, R. Davidov, M. Sundahl, S. Styr-
ing, B. Akermark, M. Almgren, L. Hammarström, J. Phys.
Chem. A 1998, 102, 2512.
[13] M. Abrahamsson, H. Baudin, A. Tran, C. Philouze, K. E.
Berg, M. K. Raymond-Johansson, L. Sun, B. Akermark, S.
Styring, L. Hammarström, Inorg. Chem. 2002, 41, 1534.
[14] F. Lafolet, J. Chauvin, M.-N. Collomb, A. Deronzier, H. Lagu-
itton-Pasquier, J.-C. Leprêtre, J.-C. Vial, B. Brasme, Phys.
Chem. Chem. Phys. 2003, 5, 2520.
3PF6]3+, 764.90 (17) [M – 4PF6]4+, 582.92 (53) [M – 5PF6]5+
.
[{RuII(bpy)2(LL6)}3FeII](PF6)8 ([{Ru(LL6)}3Fe]8+): Yield: 24 mg
(22%). C144H138F48Fe1N24P8·Ru36H2O (3831.65): calcd. C 45.14,
H 3.94, N 8.78; found C 45.63, H 3.91, N 8.96. ESI-MS: m/z (%):
3578.60 [M – PF6]+, 1726.80 (1) [M – 2PF6]2+, 1096.22 (8) [M –
3PF6]3+, 785.92 (22) [M – 4PF6]4+, 599.74 (59) [M – 5PF6]5+
.
Synthesis of Zn(CF3SO3)2: Pure CF3SO3H (1 mL, 11.2 mmol) was
added to a solution of CH3CN (10 mL) containing Zn powder
(7.13 g, 112 mmol). After stirring for one day, the excess of Zn was
removed by filtration. Addition of diethyl ether to the filtrate
yielded the formation of a white precipitate of Zn(CF3SO3)2, which
was filtered off, washed with diethyl ether and dried under vacuum
(yield: 1.02 g, 50%).
Synthesis of [{RuII(bpy)2(LL2)}3ZnII](PF6)8: Solid Zn(CF3SO3)2
(2.36 mg, 6.48 μmol) was added to
a
solution of [RuII-
[15] R. H. Schmehl, R. A. Auerbach, W. F. Wacholtz, C. M. Elliott,
R. A. Freitag, J. W. Merkert, Inorg. Chem. 1986, 25, 2440.
[16] A. E. Kaifer, M. Gomez-Kaifer, Supramolecular Electrochemis-
try, Chap. 8 “Electrochemical Considerations for Supramolec-
ular Systems”, Wiley-VCH Weinheim, Germany ,1999, p. 89.
[17] J. B. Flanagan, S. Margel, A. J. Bard, F. C. Anson, J. Am.
Chem. Soc. 1977, 100, 4248.
(bpy)2(LL2)](PF6)2 (20.8 mg, 19.4 μmol) in ethanol (80 mL), lead-
ing to the formation of the [{RuII(bpy)2(LL2)]3ZnII}8+ complex.
This solution was stirred for one hour at reflux. After cooling to
room temperature, an aqueous solution (20 mL) of KPF6 (15 mg,
64.8 μmol) was added and then the ethanol was removed under
reduced pressure. The heterotetranuclear complex was extracted
from the aqueous phase with CH2Cl2. The resulting CH2Cl2 solu-
tion was washed three times with an aqueous KPF6 solution
(0.1 m), and then twice with water. After drying over Na2SO4,
CH2Cl2 was removed under reduced pressure. The orange product
obtained corresponding to [{RuII(bpy)2(LL2)}3ZnII](PF6)8 was re-
precipitated from CH3CN/diethyl ether, washed with diethyl ether,
[18] The potentials of the RuII*/RuI and RuII*/RuIII redox couples
were estimated by subtracting 2.03 eV (λemis = 612 nm) from
the potential of RuII/RuI and RuII/RuIII of the [{Ru(LLn)}3-
Fe]8+ complexes.
[19] C. Creutz, M. Chou, T. Netzel, M. Okumura, N. Sutin, J. Am.
Chem. Soc. 1980, 102, 1309.
[20] L. D. Dexter, J. Chem. Phys. 1953, 21, 838.
[21] T. H. Förster, Discuss. Faraday Soc. 1959, 27, 7.
[22] A. Juris, V. Balzani, F. Barrigelletti, S. Campagna, P. Belser, A.
von Zelewsky, Coord. Chem. Rev. 1988, 84, 85–277.
[23] G. F. Strousse, J. R. Schoonover, R. Duesing, S. Boyde, W. E.
Jones, T. J. Meyer, Inorg. Chem. 1995, 34, 473.
and
dried
under
vacuum
(yield:
18.5 mg,
80%).
C132H114F48N24Ru3P8Zn1 (3564.8): calcd. C 44.47, H 3.22, N 9.43;
found C 44.49, H 3.29, N 9.60.
[24] J. N. Demas, Excited State Lifetime Measurements, Academic
Press, New York, 1983.
Acknowledgments
[25] C. Baffert, S. Dumas, J. Chauvin, J.-C. Leprêtre, M.-N. Col-
lomb, A. Deronzier, Phys. Chem. Chem. Phys. 2005, 7, 202.
[26] H. Cano-Yelo, A. Deronzier, J. Chem. Soc., Faraday Trans. 1
1984, 80, 3011.
We thank Dr. Jean-Claude Vial from the Laboratoire de Spectrom-
étrie Physique (CNRS UMR 5588, Université Joseph Fourier, Gre-
noble I) for allowing us to use his picosecond emission decay setup.
We also thank one referee for pointing out that the kinetics of two
excited states in rapid equilibrium were worked out thoroughly by
J. N. Demas.
[27] H. Laguitton-Pasquier, A. Martre, A. Deronzier, J. Phys.
Chem. B 2001, 105, 4801.
[28] V. V. Pavlishchuk, A. W. Addison, Inorg. Chim. Acta 2000, 298,
97.
[29] J. V. Caspar, T. J. Meyer, J. Am. Chem. Soc. 1983, 105, 5583.
Received: February 2, 2005
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[2] V. Balzani, A. Juris, M. Venturi, Chem. Rev. 1996, 96, 759.
Published Online: July 7, 2005
3330
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 3320–3330