Photoredox vs. energy transfer in a Ru(II)-Fe(II) supramolecular complex built with an heteroditopic bipyridine-terpyridine ligand

Article abstract of DOI:10.1039/b710640h

A trinuclear [{Ru^II(bpy)₂(bpy-terpy)} ₂Fe^II]⁶⁺ complex (I) in which a Fe ^II-bis-terpyridine-like centre is covalently linked to two Ru ^II-tris-bipyridine-like moieties by a bridging bipyridine-terpyridine ligand has been synthesised and characterised. Its electrochemical, photophysical and photochemical properties have been investigated in CH ₃CN and compared with those of mononuclear model complexes. The cyclic voltammetry of (I) exhibits, in the positive region, two successive reversible oxidation processes, corresponding to the Fe^III/Fe ^II and Ru^III/Ru^II redox couples. These systems are clearly separated (ΔE_1/2 = 160 mV), demonstrating the lack of an electronic connection between the two subunits. The two oxidized forms of the complex, [{Ru^II(bpy)₂(bpy-terpy)}₂Fe ^III]⁷⁺ and [{Ru^III(bpy)₂(terpy-bpy)} ₂Fe^III]⁹⁺, obtained after two successive exhaustive electrolyses, are stable. (I) is poorly luminescent, indicating that the covalent linkage of the Ru^II-tris-bipyridine to the Fe ^II-bis-terpyridine subunit leads to a strong quenching of the Ru ^II* excited state by energy transfer to the Fe^II centre. Luminescence lifetime experiments show that the process occurs within 6 ns. The nature of the energy transfer process is discussed and an intramolecular energy exchange is proposed as a preferable deactivation pathway. Nevertheless this energy transfer can be efficiently quenched by an electron transfer process in the presence of a large excess of the 4-bromophenyl diazonium cation, playing the role of a sacrificial oxidant. Finally complete photoinduced oxidation of (I) has been performed by continuous photolysis experiments in the presence of a large excess of this sacrificial oxidant. The comparison with a mixture of the corresponding mononuclear model complexes has been made. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b710640h

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Photoredox vs. energy transfer in a Ru(II)–Fe(II) supramolecular complex
built with an heteroditopic bipyridine-terpyridine ligand†
Jean Lombard, Jean-Claude Lepreˆtre,‡ Je´roˆme Chauvin, Marie-Noe¨lle Collomb* and Alain Deronzier*
Received 12th July 2007, Accepted 6th November 2007
First published as an Advance Article on the web 23rd November 2007
DOI: 10.1039/b710640h
A trinuclear [{RuII(bpy)2(bpy-terpy)}2FeII]6+ complex (I) in which a FeII-bis-terpyridine-like centre is
covalently linked to two RuII-tris-bipyridine-like moieties by a bridging bipyridine-terpyridine ligand has
been synthesised and characterised. Its electrochemical, photophysical and photochemical properties
have been investigated in CH3CN and compared with those of mononuclear model complexes. The
cyclic voltammetry of (I) exhibits, in the positive region, two successive reversible oxidation processes,
corresponding to the FeIII/FeII and RuIII/RuII redox couples. These systems are clearly separated
(DE1/2 = 160 mV), demonstrating the lack of an electronic connection between the two subunits. The two
oxidized forms of the complex, [{RuII(bpy)2(bpy-terpy)}2FeIII]7+ and [{RuIII(bpy)2(terpy-bpy)}2FeIII]9+,
obtained after two successive exhaustive electrolyses, are stable. (I) is poorly luminescent, indicating
that the covalent linkage of the RuII-tris-bipyridine to the FeII-bis-terpyridine subunit leads to a
strong quenching of the RuII* excited state by energy transfer to the FeII centre. Luminescence lifetime
experiments show that the process occurs within 6 ns. The nature of the energy transfer process is
discussed and an intramolecular energy exchange is proposed as a preferable deactivation pathway.
Nevertheless this energy transfer can be efficiently quenched by an electron transfer process in the
presence of a large excess of the 4-bromophenyl diazonium cation, playing the role of a sacrificial
oxidant. Finally complete photoinduced oxidation of (I) has been performed by continuous
photolysis experiments in the presence of a large excess of this sacrificial oxidant. The comparison
with a mixture of the corresponding mononuclear model complexes has been made.
be engaged in an energy and/or electron transfer reaction with
Introduction
the neighbouring metallic site. This latter site can be used to
The development of systems mimicking at the molecular scale
transfer energy excitation like in light harvesting antenna systems
functions performed by nature such as solar energy conversion and
or can act as an electron donor or acceptor to photoinduce
storage, and/or photocatalysis has increased considerably during
a charge separation process or to store oxidative or reductive
the last decade.1 Among these systems a peculiar interesting design
equivalents. In that more specific case the second metallic site needs
to have perfectly reversible redox properties and FeII polypyridinyl
complexes appear then as good candidates to store a hole (i.e.
oxidative equivalent) on the metallic site.
is based on heterobimetallic superstructured complexes where the
coordination sites are bridged by innocent groups, preventing
alteration of the individual properties of the different metallic
sites. In those designs, the different active sites are maintained in a
close proximity without electronic connection between them. Most
examples contain a [Ru(bpy)3]2+ like chromophore connected by
an alkyl chain to some other bipyridyl derivatives, acting as chro-
mophores such as Ru,2 Os,3 Re,4 Fe,5 or electron donor or acceptor
Mn,6 Co,7 Ni,7c Rh,7b,8 or Pt,9 for instance. The [Ru(bpy)3]2+
subunit confines to these superstructured complexes interesting
photoactivable properties since they absorb a significant part of
the visible spectrum and have adequate long-lived excited states to
In that context we have already investigated two series of
heteronuclear complexes of ruthenium and iron, issued from
the linkage of the [Ru(bpy)3]2+ and [Fe(bpy)3]2+ subunits by
covalently bridging bis-bipyridine Ln ligands denoted [RuII(bpy)2-
LnFeII(bpy)2]4+(IV) and [{RuII(bpy)2Ln}3FeII]8+ (V) (Scheme 1).5c,d
We have demonstrated that for these two series, a partial energy
3
transfer from the MLCT excited state of the RuII-tris-bipyridyl
centres to the FeII-tris-bipyridyl unit occurs. This energy transfer
operates only by an intramolecular process for the tetranuclear
series, while for the binuclear ones an intermolecular process is
also observed. The intramolecular quenching was evidenced by
steady state luminescence experiments but was too rapid to be
analyzed with a nanosecond luminescence lifetime experimental
setup. Nevertheless, the energy transfer process has been easily
short-circuited in the presence of an external irreversible electron
acceptor by an electron transfer reaction, leading with a reasonably
high quantum yield, to the photoinduced oxidation of the iron(II)
subunit of the complexes, followed by that of the ruthenium(II)
ones. Up to 4 oxidative equivalents could then be stored on
De´partement de Chimie Mole´culaire, UMR-5250, ICMG FR-2607,
CNRS Universite´ Joseph Fourier, BP-53, 38041, Grenoble cedex 9,
France. E-mail: marie-noe¨lle.collomb@ujf-grenoble.fr, alain.deronzier@
ujf-grenoble.fr
† Electronic supplementary information (ESI) available: Cyclic voltam-
mogram of (III) in CH3CN + 0.05 M Bu4NPF6 (Fig. S1). See DOI:
10.1039/b710640h
‡ Present adress:LEPMI/ELSA/ENSEEG, Domaine Universitaire, 1130
rue de la piscine, BP-75, 38042 Saint Martin dꢀHe`res, France
658 | Dalton Trans., 2008, 658–666
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each complex of the tetranuclear series. In order to gain more
information about these photoprocesses, we have prepared a
new trinuclear complex of Ru(II) and Fe(II), [{RuII(bpy)2(bpy-
terpy)}2FeII]6+ (I) based on the complexation of Fe2+ with the free
terpy unit of the [Ru(bpy)2(bpy-terpy)]2+ complex (II) we have
recently synthesized (Scheme 1).10 The bpy-terpy ligand presents
dissymmetrical coordination sites in which a bidentate bipyridine
(bpy) is bridged to a tridentate terpyridine (terpy). Example of
[Fe(terpy)2]2+ like unit linked to [Ru(bpy)3]2+ like subunits are
rare.11,12 The tridentate ligand gives to the system a more planar
structure than the series already studied5c,d which may influence the
intramolecular energy transfer process. In this study we present the
preparation and electrochemical properties of (I). Photophysics
are investigated in CH3CN with a luminescence lifetime decay set-
up that permit analysis of the sub-nanosecond decay. The (IV) and
(V) series are also reinvestigated and their data compared to those
of the trinuclear complex. Finally the possibility to photoinduce
electron transfer using an aryl diazonium cation as a sacrificial ox-
Results and discussion
Electrochemistry
We have first reinvestigated the electrochemical and spectroscopic
(UV-visible) properties of the mononuclear [Fe(tterpy)2]2+ (III) and
[Ru(bpy)2(bpy-terpy)]2+ (II) complexes in CH3CN and the stability
of their corresponding oxidized forms. Redox potentials and UV-
visible data of (I)–(III) are reported in Tables 1 and 2, respectively.
The cyclic voltammetry (CV) of (III) exhibits in the positive
potential region the reversible FeII/FeIII oxidation process at
E1/2 = 0.76 V, as previously described,13 and in the negative
one, four successive one-electron reversible waves corresponding
respectively to the first and the second one-electron reduction of
the two terpyridine ligands (Fig. S1†). Four successive reversible
one-electron reduction processes are also observed for (II), the
three first ones corresponding to the reduction of the bipyridine
ligands, the most negative one is due to the first reduction
of the metal-free terpyridine part while the reversible metal-
based oxidation process (RuII/RuIII) appears at E1/2 = 0.92 V.10
The respective oxidized forms of (III) and (II), [FeIII(tterpy)2]3+
and [RuIII(bpy)2(bpy-terpy)]3+, can be generated by exhaustive
electrolyses (at E = 0.84 V for (III) and 1.20 V for (II)). These
+
idant (ArN2 ) is presented and compared with the results obtained
from a simple mixture of both mononuclear parents complexes i.e.
[Ru(bpy)3]2+ and [Fe(tterpy)2]2+ (III) (tterpy = 4ꢀ-(4-methylphenyl)-
2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine) (Scheme 1) in the concentration ratio 1/10
+
with ArN2 .
Scheme 1 Chemical structure of the polypyridyl complexes (I)–(V).
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Table 1 Redox potentials of (I)–(III) in deoxygenated CH3CN + 0.1 M Bu4NClO4 at a scan rate of 100 mV s−1. E1/2(V) (DEp/mV) vs. Ag/Ag+ (AgNO3
0.01 M in CH3CN + 0.1 M Bu4NClO4)
Reduction processes
Complexes
FeII/FeIII
RuII/RuIII
1
2
3
4
[Fe(tterpy)2]2+ (III)a
0.76 (60)
—
0.76 (60)
—
0.92 (60)
0.92 (60)
−1.54 (60)
−1.67 (60)
−1.54 (60)
−1.65 (60)
−1.87 (70)
−1.66 (60)
−2.30 (90)
−2.13 (70)
−1.85 (60)
−2.50b
−2.17 (110)d
[Ru(bpy)2(bpy-terpy)]2+ (II)c
−2.37 (80)
[{Ru(bpy)2(bpy-terpy)}2Fe]6+ (I)
a The electrochemical properties of this complex have been studied in CH3CN + 0.05 M Bu4NPF6 owing to the low solubility of [FeIII(tterpy)2]3+ in the
.
b This value cannot be accurately determined due to the proximity of the solvent reduction. c Reference 10. d This value cannot be
−
presence of ClO4
accurately measured since the wave is strongly distorted by adsorption of the reduction product on the electrode surface.
Table 2 UV/Vis data for (I)–(III) in deoxygenated CH3CN solution at 298 K
abs
max
Complexes
k
/nm (e/M−1cm−1
)
[Fe(tterpy)2]2+ (III)
285 (68 200), 310a (60 400), 320 (62 800), 364a (8100), 494a (7600), 525a (1100), 567 (25 900), 625a (2700)
257 (57 800), 288 (96 700), 326a (21 900), 354a (6400), 430a (12 000), 455 (14 300)
[Ru(bpy)2(bpy-terpy)]2+ (II)b
[{Ru(bpy)2(bpy-terpy)}2Fe]6+ (I)
257 (99 600), 286 (215 600), 318a (85 000), 356 (24800), 430a (26 500), 455 (32 000), 568 (26 000)
a Shoulder. b Reference 10.
species are obtained with high yield (∼97%) as determined by
rotating disk electrode (RDE) experiments, and are characterized
by their UV-visible spectrum. The formation of [FeIII(tterpy)2]3+ is
evidenced by the emergence of an intense band at 393 nm (sh, e ∼
26 300 M−1cm−1) and a less intense large one centred at 670 nm (e ∼
2600 M−1cm−1) at the expense of the initial FeII → terpy metal to
ligand charge transfer (MLCT) band at 567 nm of (III) (Fig. 1A)
(Table 2). In the case of (II), the formation of [RuIII(bpy)2(bpy-
terpy)]3+ leads to the replacement of the initial RuII → bpy MLCT
band at 455 nm by two new ones at 430 (e ∼ 3000 M−1cm−1) and
660 nm (e ∼ 600 M−1cm−1), typical of RuIII-tris-bipyridine species
(Fig. 1B).5c,d,14
−1.85 V) is due to the reduction of the second bipyridine of the
two Ru(II) units (eqn (3)). Comparison of the wave heights by
RDE experiments is in accordance with the number of electrons
exchanged for each process (1/3/2) (Fig. 2B).
[{RuII(bpy)2(bpy-terpy)}2FeII]6+ + e−
ꢀ [{RuII(bpy)2}2(bpy-terpy)(bpy-terpy•−) FeII]5+
(1)
(2)
(3)
[{RuII(bpy)2}2(bpy-terpy)(bpy-terpy•−)FeII]5+ + 3e−
ꢀ [{RuII(bpy)(bpy•−)(bpy-terpy•−)}2 FeII]2+
[{RuII(bpy)(bpy•−)(bpy-terpy•−)}2 FeII]2+ + 2e−
ꢀ [{RuII(bpy•−)2(bpy-terpy•−)}2 FeII]0
The CV of (I) is actually the superposition of the electrochemical
behaviour of both subunits. In the positive potential region, two
The stability of the two oxidized forms of (I) i.e. [{RuII(bpy)2-
(bpy-terpy)}2FeIII]7+ and [{RuIII(bpy)2(bpy-terpy)}2FeIII]9+ has
been evaluated by exhaustive electrolyses. Two successive ex-
haustive electrolyses carried out at 0.84 and 1.10 V consumed
one and two electrons respectively per initial complex, and
allowed the build up of [{RuII(bpy)2(bpy-terpy)}2FeIII]7+ and
[{RuIII(bpy)2(bpy-terpy)}2FeIII]9+. RDE experiments show that
these oxidized forms are obtained with the respective yields of
95 and 90% (Fig. 2B). Taking into account the large difference of
potential between the FeIII/FeII and RuIII/RuII redox processes, the
mixed valent complex [{RuII(bpy)2(bpy-terpy)}2FeIII]7+ is perfectly
stable. These two oxidised forms of (I) are identified by their visible
absorption spectrum (Fig. 3). Absorption spectrum of (I) closely
results from the overlap of the two parents subunits (Table 2).
The two bands at 455 and 568 nm are attributed respectively
to the RuII → bpy and FeII → terpy MLCT. The bands in the
UV part of the spectrum are due to the p → p* transitions of
the ligands. After one electron per complex has been passed at
0.84 V, the initial band of the FeII–bis-terpyridine unit at 568 nm
disappears on at the appearance of a shoulder around 390 nm
and a low intensity band centered at 670 nm typical of the FeIII
species (Fig. 3). The slight loss of intensity of the RuII visible band
(kmax = 454 nm) is due to the lower absorbance of the FeIII–bis-
terpyridine subunit compared to FeII in this wavelength region.
well-separated reversible redox systems are observed at E1/2
=
0.76 and 0.92 V (Fig. 2A). The potentials of these systems are
similar to those of the mononuclear parents, allowing assignment
of the first oxidative process to the FeII/FeIII redox couple and the
more positive one to the RuII/RuIII system. These systems are well
separated (DE1/2 = 160 mV) which demonstrates the absence of
an electronic connection between the metallic centres. Moreover,
RDE experiments confirm that the trinuclear complex is obtained
in pure form and that no dissociation occurs in CH3CN, since the
height of the RuII/RuIII wave is two times higher that of FeII/FeIII,
in accordance with the 2 : 1 Ru/Fe stoichiometry (Fig. 2B).
In the negative region, the CV of (I) exhibits three successive
ligand centred reversible reduction waves involving respectively
one, three, and two electrons, the last one being strongly distorted
by some electroprecipitation–redissolution phenomena (Fig. 2A).
These processes can also be identified by comparison with
the behaviour of the mononuclear parent complexes. The first
reduction process located at E1/2 = −1.54 V is attributed to the
first reduction of one terpyridine ligand of the Fe(II) subunit
(eqn (1)). The second system (E1/2 = −1.66 V) is associated to
the first reduction of the second terpyridine ligand (exchange of
one electron) and the reduction of one bipyridine per Ru(II) centre
(exchange of two electrons) (eqn (2)). Finally, the third one (E1/2
=
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Fig. 2 (A) Cyclic voltammograms of a 0.19 mM solution of (I) in
CH3CN + 0.1 M Bu4NClO4 at a platinum electrode (0.5 cm of diameter);
scan rate: 100 mV s−1. (B) Voltammograms at a platinum electrode (0.2 cm
of diameter); rotation rate x = 600 rot. min−1, scan rate: 10 mV s−1
;
(a) initial solution, (b) after exhaustive electrolysis at 0.84 V, (c) after
exhaustive electrolysis at 1.10 V.
Fig. 1 (A) Absorption spectra of a 0.46 mM solution of (III) in CH3CN +
0.05 M Bu4NPF6 (a) initial solution, (b) after exhaustive electrolysis at
0.84 V (one electron consumed). (B) Absorption spectra of a 0.5 mM
solution of (II) in CH3CN + 0.1 M Bu4NClO4 (a) initial solution, (b) after
exhaustive electrolysis at 1.20 V (one electron consumed); l = 1 mm.
After exchange of two additional electrons at 1.10 V, the band of
the RuII–tris-bipyridine units at 455 nm disappears and is replaced
by those of the RuIII ones (shoulder around 430 nm and a slight
increase of the band around 660 nm).
Photophysics
Fig. 3 Absorption spectra of a 0.16 mM solution of (I) in CH3CN +
0.1 M Bu4NClO4 (a) initial solution, (b) after exhaustive electrolysis at
0.84 V, (c) after exhaustive electrolysis at 1.10 V, l = 2 mm.
Data relative to the emission of (I) and (II) are reported in Table 3.
The emission spectra of (I) and (II) are typical of the MLCT
3
excited state of [Ru(bpy)3]2+ like complexes, nevertheless emission
intensity of (I) is drastically lowered vs. (II). It represents only
1.3% of the emission of the iron free complex (II) based on
the ratio of their quantum yield. This strong inhibition of the
luminescence of the RuII centres by the FeII core in (I) results from
an energy transfer process (En. T.), as already demonstrated for
some relevant polypyridinyl mixed iron–ruthenium systems,5,15,16
and is illustrated by the large overlap between the emission
spectrum of the RuII* subunit and the absorption of the FeII one
(Fig. 4).
The quenching by electron transfer can be ruled out since it
would be endothermic by 0.39 V. This value is evaluated from
the difference of the potential of the FeIII/FeII redox couple
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emi
max
Table 3 Luminescence maxima (k ) and emission quantum yield (φem
)
both linear and give respectively a quenching rate constant of:
kq1 = 11.5 109 M−1 s−1 and kq2 = 4.6 109 M−1 s−1.
of (I) and (II) in deoxygenated CH3CN + 0.1 M Bu4NClO4 solution at
298 K
ꢀ
= 1 + s kq1[Fe(tterpy) ]2+
(5)
(6)
φ0
φ
0
2
emi
max
Complexes
k
/nm
φem
ꢀ
= 1 + s kq2[Fe(tterpy) ]2+
s0
s
0
2
[Ru(bpy)2(bpy-terpy)]2+ (II)a
609
611
0.054
0.0007
[{Ru(bpy)2(bpy-terpy)}2Fe]6+ (I)
φ0, φ and s0, s are respectively the luminescence quantum yield and
lifetime of [Ru(bpy)3]2+ without and with a variable concentration
of (III) (s0 = 0.870 ls18).
a Reference 10.
The difference between kq1 and kq2 could be due to the fact that
the quenching process can not be considered as a pure dynamic
quenching. An additive radiative mechanism of En. T. where the
luminescence of [Ru(bpy)3]2+* is simply reabsorbed by (III) also
occurs since kq1 is higher than kq2.
Luminescence lifetime of (I) is on the contrary described by
a double exponential, reflecting contributions from unquenched
and quenched ruthenium centres. The shorter component of the
decay (6 ns) which is attributed to the quenching process represents
around 92% of the total intensity decay (Table 4, Fig. 5). The longer
component of the decay is close to that of (II) (s0 = 0.997 ls10) and
then assigned to the unquenched RuII* centre. A similar situation
was previously observed for a relevant RuII/FeII trinuclear complex
linked with a tyrosine-tolyl-terpyridine modified linker. This
Fig. 4 UV-visible absorption spectra of (I) (black line) and (III) (dotted
line), emission spectrum of (I) (dashed line) in deoxygenated CH3CN at
room temperature.
(0.76 V) and that of the RuII*/RuI one (0.37 V) calculated at
room temperature using the Rehm–Weller equation (eqn (4)):
Eoox* = Eoox + E
(4)
where Eoox is the formal redox potential for the RuII/I couple and E
is the energy gap between the ground and excited state, estimated
from the emission spectra.17
We have firstly investigated the quenching that occurs in a
mixture of [Ru(bpy)3]2+ and (III). The En. T. quenching may arise
from different processes that can, in part be identified. A dynamic
quenching involves a collisional encounter between the donor in
its excited state (i.e. [Ru(bpy)3]2+*) and the acceptor (i.e. (III)), then
lifetime of the emission of the donor (s0) and quantum yield (φ0)
both decreased vs. concentration of quencher. A static or radiative
quenching reduces the luminescence quantum yield of the donor
but does not decrease the measured lifetime of emission. A static
quenching also induces in most cases a change in the electronic
absorption spectrum compared to the sum of the spectra of those
of the donor and acceptor alone. The Stern–Volmer plots of φ0/φ
and s0/s vs. concentration of (III) following eqn (5) and (6) are
Fig. 5 Evolution of the normalized absorbance at 568 nm under visible
irradiation in CH3CN + 0.1 M Bu4NClO4 for: (a) complex (I) (0.051 mM),
(b) a mixture of [Ru(bpy)3]2+ (0.010 mM) and (III) (0.053 mM) in the ratio
+
+
1/5 + ArN2 (15 mM), (c) complex (I) (0.052 mM) + ArN2 (15 mM).
Table 4 Lifetime of the quenched (s1) and unquenched (s2) ruthenium centres determined in deoxygenated CH3CN + 0.1 M Bu4NClO4 solution at
298 K. kq is the intramolecular quenching constant determined from eqn (7). φem the emission quantum yield of the complexes
Complexes
s1/ns (%)a
s2/ns (%)a
kq/s−1
φem
φem/φ0 (%)b
[{Ru(bpy)2L2}3Fe]8+,c (V)
[{Ru(bpy)2L4}3Fe]8+,c (V)
[{Ru(bpy)2L6}3Fe]8+,c(V)
[Ru(bpy)2L2Fe(bpy)2]4+ (IV)
[{Ru(bpy)2(bpy-terpy)}2Fe]6+ (I)
30 (71)
33 (65)
47 (91)
11 (81)
6 (92)
670 (29)
816 (35)
923 (9)
640 (19)
785 (8)
3.2 × 107
2.9 × 107
2.0 × 107
8.9 × 107
1.6 × 108
0.008d
0.005d
0.009d
0.022e
0.0007
13
9.3
15
37
1.3
a These percentages correspond to the fractional intensities of the decay components. φem/φ0 represents the ratio of the emission of the mentioned
complexes vs. the emission of the corresponding iron free parent complexes. c The structure of the complex is depicted in Scheme 1. d Reference 5d.
e Reference 5c.
b
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abs
max
is higher too (k /nm (e M−1cm−1) = 568 (26 000) and 516
behaviour was attributed to a partial decomplexation of the iron
ion in this trinuclear complex at low concentration in CH3CN. To
investigate if decomplexation occurs in our complex a large excess
of Fe(ClO4)2·8H2O (up to 60 molar equivalents) was added to
a solution of (I) (0.01 mM in CH3CN). Fe2+ does not quench
the luminescence of [Ru(bpy)3]2+* by energy or electron transfer,
and the changes in the luminescence properties of the solution
would then only be due to the complexation of iron on the
terpyridine moieties if some iron free complex (II) is present in
solution. The UV-visible spectrum as the luminescence quantum
yield of the solution remains unchanged after addition of Fe2+.
This result indicates that there is no net decoordination of the
Fe2+ ion for a 10−5 M concentration of (I) in CH3CN. Moreover
continuous irradiation of a solution of (I) in CH3CN with a Xe
lamp does not lead to any change in the UV-visible spectrum
(Fig. 5), indicating that direct photogeneration (or formation via
the energy transfer process from RuII*) of excited states located
on the Fe subunit does not lead either to any decomplexation.
The bi-exponential behaviour of the luminescence decay could
be attributed to the relative kinetic of the lability of iron ion in
(I) vs. the luminescence lifetime of the complex. The exchange
facilities of iron ions between bipyridine and terpyridine ligands
has early on been shown to take place in water.19,20 Even if CH3CN
has a weaker coordinating character compared to water, the bi-
exponential decay of a solution of (I) in CH3CN could nevertheless
be a consequence of the reversible lability of Fe2+ in the complex
and would suggest that the exchange kinetic takes more than the
6 ns of the luminescence decay of (I).
(8600) for (I) and (IV) respectively). These results can also be
understood, considering that the terpyridine ligand substituted
with the [Ru(bpy)3]2+ like center is a strongly chelating ligand for
the iron ion compared to the bipyridine one. In a solution of (I),
the unquenched [Ru(bpy)3]2+ centre represent only 8% of the total
luminescence intensity decay while it is approximately 20–35% for
complexes (IV) and (V).
In the series (V), kq is only weakly dependant on the length of
the methylene bridge. The En. T. process is presumably attributed
to an electron exchange mechanism in such mixed metal systems.5
This process involves the interchange of electron between RuII*
and FeII and therefore overlap of the orbitals of the two subunits.
Since in our structures the linker is an innocent electron-saturated
and flexible bridge, En. T. would proceed mainly after folding up
of the molecule resulting in the close contact between the RuII*
and the FeII centre and the length of the bridge is therefore not a
crucial parameter.
[Ru(bpy)2L2Fe(bpy)2]4+ (IV) has a more efficient quenching rate
constant compared to the (V) series. It suggests that the motion
of the bridging ligand in (V) is less facilitated. Although the ratio
(Ru/Fe) number of centre is 3 times higher in (V) compared to
(IV), φ is 3–4 times smaller in (V). It may suggest an efficient
additive self quenching of the surrounding ruthenium subunit in
em
the (V) series to explain this large difference.
Photoinduced oxidation
Luminescence lifetime and quantum yield of (I) are constant
vs. the concentration of the solution over the range used 0.002–
0.015 mM, meaning that the quenching of the luminescence is only
due to an intramolecular process. The intramolecular quenching
constant kq (Table 4) was determined from eqn (7) :
To induce an electron transfer between the RuII and the
FeII subunits, an external irreversible electron acceptor is re-
quired. We have previously demonstrated that for the binu-
clear (IV) series, the 4-bromophenyl diazonium tetrafluoroborate
+
(ArN2 , BF4−) reacts preferentially with the excited state of
[RuII(bpy)3]2+ and leads to [RuIII(bpy)3]3+ which plays the role
of an oxidant toward [FeII(bpy)3]2+ subunit.5c As a consequence,
continuous irradiation leads to the quantitative formation of
[RuII(bpy)2LnFeIII(bpy)2]5+, followed by the formation of the
1
1
kq =
−
(7)
s1 s2
where s1 and s2 represent respectively the short and the long
component of the bi-exponential decay of the excited state (I*).
We have moreover reinvestigated some of our previous sys-
tems where a [Fe(bpy)3]2+ core is covalently linked to one
and to three [Ru(bpy)3]2+ by a bis-bipyridine bridging ligand,
[RuII(bpy)2L2FeII(bpy)2]4+ (IV) and the [{RuII(bpy)2Ln}3FeII]8+
(n = 2, 4, 6) (V) series (Scheme 1).5c,d In all these systems the
luminescence decays appear also as a bi-exponential function,
the short component, in our previous publications, escaped our
previous experimental set-up. So the intramolecular rate constants
have been recalculated and are presented in Table 4.
fully oxidized [RuIII(bpy)2LnFeIII(bpy)2]6+ species since ArN2 is
+
added in large excess. The same behaviour was observed for the
tetranuclear (V) complexes,5d and also here for the trinuclear one
(I). We found that the unquenched part of the emission of (I)
is efficiently quenched by the added diazonium salt. The rate
constant (kET) of this oxidative quenching (eqn (8)) was determined
from the Stern–Volmer plots of φ0/φ and s0/s vs. the concentration
+
of ArN2 .
∗
[{RuII (bpy)2(bpy-terpy)}FeII{(bpy-terpy)(bpy)2RuII}]6+
k
ET
+ ArN+ −−→ [{RuIII(bpy)2(bpy-terpy)}FeII{(bpy-
The comparison of the photophysical data for the three kinds
of ruthenium–iron structures show the following points:
2
terpy)(bpy)2RuII}]7+ + ArN2•
(8)
Iron terpyridyl core is a markedly more efficient quencher
compared to its bipyridyl equivalent. For instance, (I) has a φem
about 30 times smaller than (IV), although the quenching constant
is twice. Moreover it should be noted that the ratio (Ru/Fe)
number of centre in (I) is twice than in (IV). The main origin
of the difference results in the fact that the spectral overlap of
the RuII* emission with the iron subunit is markedly higher in
the case of the terpyridine core since absorption wavelength is
shifted to the higher wavelength and molar absorption coefficient
The kET is close to 4 × 109 M−1s−1. It should be recalled that the
quenching rate by ArN2+ in the regular [Ru(bpy)3]2+ has been found
to be equal to 2 × 109 M−1s−1.21 The high value illustrates that the
photogeneration of [RuIII(bpy)3]3+ like species in (I) is also efficient.
Even if the energy transfer by the Fe2+ center in (I) is fast and
rather efficient, it is possible to advantageously short-circuit it by
+
a bimolecular electron transfer reaction with ArN2 , whereas the
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•
photogenerated reduced form (ArN2 ) (eqn (8)) evolves irreversibly
almost disappears totally, on behalf of the progressive emergence
of a shoulder around 390 nm and a band centred at 670 nm typical
of the FeIII species. The [{RuII(bpy)2(bpy-terpy)}2FeIII]7+ formation
is reached after about 9 min of irradiation. These results confirm
that the photoinduced oxidation of (I) is efficient as predicted by
the less anodic potential values of FeII/FeIII compared to those
of RuII/RuIII subunits (Table 1). In a second step (Fig. 6B), a
prolonged irradiation induces the formation of [{RuIII(bpy)2(bpy-
terpy)}2FeIII]9+ illustrated by the decrease of the MLCT band of
RuII unit at 455 nm and the appearance of a shoulder at 430 nm
and the slight increase of the band centered at 660 nm due to
the RuIII species. The electronic absorption spectra obtained after
these two steps are identical to those obtained after exhaustive
electrolyses conducted on a solution of (I) in CH3CN + 0.1 M
Bu4NClO4 (Fig. 3), indicating that the oxidized species are almost
quantitatively produced.
mainly to the corresponding hydrocarbon ArH derivative, as
previously demonstrated.22
Continuous photolysis of a solution of (I) (0.052 mM) and
+
ArN2 (15 mM) in CH3CN + 0.1 M Bu4NClO4 irradiated with
a Xenon lamp, leads to the successive photogenerations of the
oxidized [{RuII(bpy)2(bpy-terpy)}2FeIII]7+ and [{RuIII(bpy)2(bpy-
terpy)}2FeIII]9+. During the irradiation two successive changes
of the absorption spectra are observed (Fig. 6). In a first step
(Fig. 6A), in accordance with the formation of [{RuII(bpy)2(bpy-
terpy)}2FeIII]7+ through the transient photogenerated RuIII species,
the band of the FeII unit at 568 nm decreases regularly until it
The kinetic of the overall photoinduced oxidation of FeII into
FeIII in (I) is represented on Fig. 5 and compared with the evolution
of a simple mixture of complexes [Ru(bpy)3]2+ and (III) (in the
+
concentration ration 1/10) with ArN2 . Taking into account that
RuII is regenerated along this first step, the concentration of
FeII should follow a monoexponential decrease with an overall
constant (k) which depends on the initial RuII concentration.5d
The decreases of the normalised absorbance at 568 nm are almost
superimposable and can be fit by a monoexponential function,
demonstrating, as expected, that ArN2+ can efficiently quench the
RuII* state and that the electron transfer from the photogenerated
RuIII species to FeII ones is very efficient in both cases. Although
in the unconnected system the RuII concentration is weaker than
that in the connected one, we obtained similar k values, i.e. k =
2.9 × 10−3 s−1 for (I) and k = 2.3 × 10−3 s−1 for the mixture of (II)
and (III). As demonstrated for the tetranuclear series (V),5d which
presents close kinetic constants, the comparison of the k values
suggests that in the connected system, contrary to the unconnected
one in which the process is obviously intermolecular, the electron
transfer process should be also intramolecular.
Conclusion
In this study we have demonstrated that in the trinuclear
[{RuII(bpy)2(bpy-terpy)}2FeII]6+ complex, the energy transfer from
the 3MLCT excited state of the RuII–tris-bipyridine centres to the
FeII–bis-terpyridine unit is strongly efficient. It occurs by a pure
intramolecular process in contrast to what was observed for the
binuclear [RuII(bpy)2LnFeII(bpy)2]4+ (IV) complexes where Ln is a
bis-bipyridine bridging ligand. This is, in part, a consequence of
the larger overlap of the emission spectrum of the [RuII(bpy)3]2+
subunit with the absorption of the [FeII(tterpy)2]2+ one compared
to the [FeII(bpy)3]2+core. It is also a consequence of the structure
of the bridging ligand which is more linear and promotes the
electronic energy transfer. Although this energy transfer process is
dramatically more efficient than in (IV), it can also be easily short-
circuited in the presence of an external irreversible electron accep-
tor by an electron transfer process, leading to the close quantitative
oxidation of the iron(II) subunit, the process occurring with a
similar rate constant. The reversible redox characteristics of theses
metallic centres suggest that this superstructured complex is an
ideal candidate for an energy storage device. Extension of this work
is currently underway using an heteroditopic ligand containing one
Fig. 6 Visible absorption change of a mixture of (I) (0.052 mM)
and ArN2 (15 mM) under visible irradiation. (A) Photogeneration of
+
[{RuII(bpy)2(bpy-terpy)}2FeIII
]
7+, (a) initial, (b) after 1 min 30 s, (c) after
2 min 30 s, (d) 3 min 10 s, (e) 3 min 50 s, (f) 4 min 30 s, (g) 5 min 10 s,
(h) 6 min 30 s, (i) 7 min 30 s, (j) 8 min 50 s. (B) Photogeneration of
[{RuIII(bpy)2(bpy-terpy)}2FeIII
]
9+, (j) 8 min 50 s, (k) 10 min 20 s, (l) 13 min
20 s, (m) 17 min 40 s, (n) 23 min 20 s, (o) 26 min 40 s, (p) 33 min 20 s,
(q) 47 min, (r) 68 min 40 s, l = 1 cm.
664 | Dalton Trans., 2008, 658–666
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2,2ꢀ-bipyridine and two 2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridine moieties to afford the
self-construction of coordination polymeric structures, giving a
well organized alternated and photoactivable ruthenium(II) and
iron(II) system.
the CH2Cl2 was removed under reduced pressure. The red–brown
product was reprecipitated in CH3CN/diethyl ether, washed with
diethyl ether and dried under vacuum. Yield: 36 mg (31%). The
1
purity of the complex was confirmed by elemental analysis, H
NMR, electrochemistry and by the observation of a single spot by
TLC (eluent H2O/CH3CN (10 : 90) containing KPF6 (10 mM)).
Elem. anal. Calcd for C108H86N18Ru2FeP6F36·4H2O (2835.79): C,
45.74; H, 3.34; N, 8.89. Found: C, 45.70; H, 3.30; N, 8.83. ESI-
MS: m/z (%) 1237.0 (68) [M − 2PF6]2+, 776.4 (29) [M − 3PF6]3+,
546.2 (7) [M − 4PF6]4+, 407.8 (100) [M − 5PF6]5+, 315.7 (39) [M −
6PF6]6+.
Experimental
General
Materials
Acetonitrile (CH3CN, Rathburn, HPLC grade), tetra-n-
butylammonium perchlorate (Bu4NClO4, Fluka) were used as
received and stored under argon in a dry-glovebox (Jaram).
4-Bromophenyl diazonium tetrafluoroborate p-BrC6H4N2(BF4)
(ArN2(BF4)) was synthesised as described previously.21 Elemental
analyses were performed by the Service Central d’Analyse du
CNRS at Vernaison (France). 1H NMR spectra was recorded
on a Bruker AC 250 spectrometer. The electrospray ionization
mass spectrometry (ESI-MS) experiments 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 solution (1 mg mL−1 in
CH3CN) were injected using a syringe pump at a flow rate
of 10 lL min−1. The electrospray probe (capillary) voltage was
optimized in the range 2–5 kV for positive ion electrospray.
Absorbance and emission
UV-visible spectra were obtained using a Cary 1 absorption
spectrophotometer on 1 cm path length quartz cells. For emission
experiments, samples were prepared under Ar in a glove-box
and contained in a 1 cm path length quartz cuvette. Samples
were maintained under anaerobic conditions with a Teflon cap.
Emission spectra were recorded at room temperature on a Photon
Technology International (PTI) SE-900 M spectrofluorimeter. The
excitation wavelength was 450 nm. Spectra were recorded from 500
to 850 nm. Emission quantum yield φL were determined at 25 ◦C
in deoxygenated acetonitrile solutions with [Ru(bpy)3](PF6)2 used
ref.
as a standard (φL = 0.06218) and using methodology already
presented.22 Emission lifetime measurements were performed after
irradiation at k = 400 nm obtained by the second harmonic
of a Titanium : Sapphire laser (picosecond Tsunami laser spectra
physics 3950-M1BB + 3986–03 pulse picker doubler) at a 400 kHZ
repetition rate. Fluotime 200 from AMS technologies was used for
the decay acquisition. It consists of a GaAs microchannel plate
photomultiplier tube (Hamamatsu model R3809U-50) followed
by a time-correlated single photon counting system from Pico-
quant (PicoHarp300). The ultimate time resolution of the system
is close to 30 ps. Luminescence decays were analyzed with Fluofit
software available from Picoquant. The fractional intensity of the
bi-exponential decay is defined as:
Synthesis of [Ru(bpy)2(bpy-terpy)](PF6)2 (II)
The heterodipotic bpy-terpy ligand and its ruthenium complex (II)
were synthesized following a procedure already published.10
Synthesis of [Fe(tterpy)2](PF6)2 (III)
A solution of FeSO4·7H2O (393 mg, 1.41 mmol) in water (100 mL)
was added to a tolyl-terpyridine solution (192 mg, 0.59 mmol) in
ethanol (150 mL). The resulting purple solution was heated at
reflux for 2 h. After cooling to room temperature, addition of an
aqueous KPF6 solution (1 g in 100 mL of water) allowed the partial
precipitation of the complex. Ethanol was then removed under
vacuum to give more precipitate which was extracted by CH2Cl2.
The organic layer was washed twice with an aqueous KPF6
solution (0.1 M) and once with water. After drying over anhydrous
Na2SO4, CH2Cl2 was removed under reduced pressure. The purple
product obtained was reprecipitated in CH3CN/diethyl ether,
washed with diethyl ether and dried under vacuum. Yield: 122 mg
(42%). Elem. anal. Calcd for C44H34N6FeP2F12 (992.55): C, 53.24;
H, 3.45; N, 8.47. Found: C, 53.06; H, 3.45; N, 8.47.
Aisi
i Aisi
ꢁ
xi =
where Ai and si are the parameter of the bi-exponential fit (pre-
factor and luminescence lifetime).
Continuous irradiations
Photoinduced oxidations were performed using a Xe lamp (250
Watt) in which UV and IR wavelength were cut off with a
UV heatbuster filter (Melles griot 03MHG101). The samples
received around 4.5 mW at 450 nm. For these experiments, all the
solutions were prepared in a dry-glove box under Ar. The solutions
Synthesis of [{RuII(bpy)2(bpy-terpy)}2FeII](PF6)6 (I)
To a stirred solution of (II) (50 mg, 0.041 mmol) in EtOH (17 mL),
an aqueous solution of Fe(SO4)2·7H2O (27 mg; 0.097 mmol) was
added. The red–brown solution was heated at reflux for 1 h.
After cooling to room temperature, addition of an aqueous KPF6
solution (1 g in 100 mL of water) allowed the partial precipitation
of the complex. Ethanol was then removed under vacuum to give
more precipitate which was extracted by CH2Cl2. The organic
layer was washed three times with an aqueous KPF6 solution
(0.1 M) and once with water. After drying over anhydrous Na2SO4,
+
contained a mixture of (I) (0.052 mM) and ArN2 (15 mM), or
by a mixture of the mononuclear parent complexes [Ru(bpy)3]2+
+
(0.010 mM) and (III) (0.053 mM), with ArN2 (15 mM).
+
Under these conditions, the concentration of ArN2 can be
considered as constant during the irradiation experiments. Photo-
induced oxidation experiments were followed by UV-visible spec-
troscopy.
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Electrochemistry
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All electrochemical measurements were run under an argon atmo-
sphere in a dry-glovebox at room temperature. Cyclic voltammetry
(CV) and controlled potential electrolysis experiments were per-
formed using an EG&G PAR model 173 potentiostat/galvanostat
equipped with a PAR model universal programmer 175 and a
PAR model 179 digital coulometer. A standard three-electrode
electrochemical cell was used. Potentials were referenced to an
Ag/10 mM AgNO3 reference electrode in CH3CN + 0,1 M
Bu4NClO4. Potentials referenced to that system can be converted
to the ferrocene/ferricinium couple by subtracting 87 mV and to
SCE or NHE by adding 298 mV or 548 mV, respectively.23 The
working electrode was a 5 mm diameter platinum disk polished
with 2 lm diamond paste (Mecaprex Presi) for CV. The rotating
disk electrode (RDE) was a 2 mm diameter platinum disk with
a rotation rate x = 600 rot. min−1. The UV-visible spectra of
the initial and electrolyzed solutions were transferred to 1 mm
path length quartz cuvette cells and were recorded directly in the
glovebox with a Cary 50 using optical fibers.
Acknowledgements
The authors thank the ANR Jeunes Chercheurs 2005 program
(Polycomsa) for financial support, the technical support from the
chemistry platform “NanoBio campus” in Grenoble (France), on
which luminescence lifetime measurement have been performed
and the Centre de Recherches du service de sante´ des Arme´es
“Emile Parde´”, Laboratoire de Biospectrome´trie, la Tronche,
France, for ESI-MS experiments.
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666 | Dalton Trans., 2008, 658–666
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©