Photoelectric conversion at a [Ru(bpy)3]2+-based metallic triad anchored on ITO surface

Article abstract of DOI:10.1039/c4dt01884b

A tri-metallic triad based on a [Ru(bpy)₃]²⁺ moiety connected to Fe(ii) and Co(iii) bisterpyridine has been grafted on an ITO electrode by a stepwise procedure. Under visible light, in the presence of a sacrificial electron donor, the system produces electric current. The photo-current magnitude is compared to the one generated from a Co(iii)-Ru(ii) dyad and shows an increase of 40%.

Full text of DOI:10.1039/c4dt01884b

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Photoelectric conversion at a [Ru(bpy)3]2+-based
metallic triad anchored on ITO surface†
Cite this: Dalton Trans., 2014, 43,
12156
Rajaa Farran, Damien Jouvenot,* Frédérique Loiseau,* Jérôme Chauvin* and
Alain Deronzier*
Received 23rd June 2014,
Accepted 30th June 2014
DOI: 10.1039/c4dt01884b
www.rsc.org/dalton
A tri-metallic triad based on a [Ru(bpy)3]2+ moiety connected to
Fe(II) and Co(III) bisterpyridine has been grafted on an ITO electrode
by a stepwise procedure. Under visible light, in the presence of a
sacrificial electron donor, the system produces electric current.
The photo-current magnitude is compared to the one generated
from a Co(III)–Ru(II) dyad and shows an increase of 40%.
There is a significant challenge in mimicking Nature’s extra-
ordinary ability to efficiently transport charges across long dis-
tances under solar light. Since the first example of an efficient
synthetic system and its incorporation into a lipidic mem-
brane,1 intense research has been carried out to design mole-
cular triads (D–P–A) displaying this property by covalently
attaching an electron donor (D) and acceptor (A) to a photo-
sensitizer (P). Among the different examples of triads, those
containing Ru(II) polypyridinic complexes have been frequently
studied and proved their efficiency in solution.2 For instance
in a Mn2II/[Ru(bpy)3]2+/naphthalenediimide triad (bpy = 2,2′
bipyridine) under irradiation, an average charge separation
state lifetime of 600 μs was obtained at room temperature.3
Another example built around a rod-like [Ru(dqp)2]2+ (dqp =
2,6 di(quinoline-8yl)pyridine) covalently linked to benzo-
quinone and phenothiazine shows a charge separation state yield
≥95%.4 To utilize charge flow under irradiation in molecular
devices some photoactive D–P–A have been grafted onto a
conducting surface5–7 and P–A dyad connected between two
electrodes.8 Surprisingly, none of these works are based on a
[Ru(bpy)3]2+ photosensitizer. Moreover, in these systems the
donor, the acceptor or both entities are preferentially organic
fragments although transition metal complexes can offer oxi-
dized or reduced systems with better stability and with mul-
tiple redox or spin states suitable for applications in molecular
electronic devices or catalysis. This work describes the easy
Fig. 1 Structure of the photoactive ditopic metallo-ligand L1.
construction of the first molecular triad built with
a
[Ru(bpy)3]2+ photosensitive unit on an ITO surface, A and D
being two different metallic complexes. To obtain a well-
organized triad, we followed a stepwise approach based on
terpyridine coordination assembly. This layer-by-layer
methodology makes possible the formation of heteroleptic
complexes of labile systems such as first row transition metal
complexes.9–13 Our strategy to construct a linear triad on a
surface and overcome the formation of statistical isomers of
[Ru(bpy)3]2+, has been to anchor two terpyridine sites on the
same, symmetrically-substituted bipyridine (Fig. 1).
This ditopic metallo-ligand (see ESI† for synthetic pro-
cedure and characterization) is the photo-active spacer
between the donor and the acceptor moieties. We chose to
link the two coordination sites by an ether bond for ease of
synthesis, to ensure a weak electronic coupling and to obtain a
relative rigidity. Substitution on the 5,5′ position of the 2,2′-
bipyridine increases the distance between the donor and the
acceptor site14 in comparison to the more common 4,4′ modi-
fication that we previously proposed.15 On the other hand, this
substitution conserves well the photophysical properties of the
[Ru(bpy)3]2+ subunit.16 (see ESI† for details).
Université Grenoble Alpes, DCM, CNRS UMR-5250, BP-53, 38041 Grenoble Cedex 9,
France. E-mail: jerome.chauvin@ujf-grenoble.fr; Fax: (+33) 476514267;
Tel: (+33) 476514267
The assembly of the triad on the electrode has been carried
out stepwise (Scheme 1), taking advantage of the strong self-
assembling interaction between phosphonates with ITO and
†Electronic supplementary information (ESI) available: Experimental part, syn-
thesis, grafting procedures, photophysics, cyclic voltammogram of L1, L2, and
S3. See DOI: 10.1039/c4dt01884b
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Table 1 Oxidation potentials E1/2 (ΔEp, mV) of S1, S2 and S3 in V vs.
Ag/AgNO3 10−2 M, scan rate: 20 mV s−1 in CH3CN + 0.1 M TBAP
ECoIII/II/V
EFeIII/II/V
ERuIII/II/V
S1a
S1b
S2
0.75 (20)
−0.08 (55)
−0.08 (40)
−0.09 (40)
0.97 (10)
0.98 (44)
S3
0.75 (30)
Scheme 1 Stepwise assembly of the trimetallic triad on ITO (S3). For
clarity, charges have been omitted (ttpy
terpyridine).
=
4’-ptolyl-2,2’:6’,2’’-
between terpyridine ligands with first row transition metals. In
a first step, the surface is grafted with a phosphonate-substi-
tuted terpyridine. This modified surface is then dipped in an
ethanolic solution of Co(BF4)2 at room temperature. After
thorough rinsing, the surface is again dipped in a solution
containing the photosensitizing moiety (L1) that self-assem-
bles on the pending metallic center, building the acceptor-
photosensitizer part of the triad (see further in the text). The
free terpyridine exposed to the outer solution can then be
involved in a coordination process with a metallic center,
namely Fe2+. After rinsing, the system is capped with a final
Fig. 2 Cyclic voltammogram of the modified electrodes in a CH3CN +
0.1 M TBAP scan rate = 20 mV s−1. Right: plot of current intensity under
oxidation peak vs. scan rate.
4′-ptolyl-2,2′:6′,2″-terpyridine producing
a
bis-terpyridine
complex acting as the electron donor unit (see ESI† for con-
struction details). For comparison purposes, we also prepared
mono and dinuclear system grafted on ITO (Scheme 2). The
construction of the dyad (S2) was carried out using a mono-
functionalized analog of complex L1 denoted L2 (see ESI† for
synthetic procedure and characterization).
All systems were characterized by cyclic voltammetry in
acetonitrile + 0.1 M tetrabutylammonium perchlorate (TBAP).
The oxidation potentials are collected in Table 1. The ether
linkage between units allows each complex to behave indepen-
dently. For all systems the potential of the redox process
assigned to a given metallic center remains unchanged
throughout the series. For S3 (Fig. 2), the first reversible peak,
around −0.1 V, is characteristic of the CoIII/CoII couple. Then,
at higher potentials, a reversible monoelectronic peak at
0.75 V assesses the presence of the [Fe(tpy)2]2+ fragment while
the oxidation of RuII into RuIII is observed slightly below
1 V. The linear relationship between the intensity of the peaks
and the scan rate clearly indicates that the processes observed
occur on a grafted surface (Fig. 2). The same cyclic voltammo-
gram was obtained after a period of two weeks, showing the
stability of the assembly.
For S3 the surface coverage value Γ17 obtained by inte-
gration under the cobalt oxidation peak (1.1 × 10−11 mol cm−2
)
is almost similar to the value obtained under the iron peak
(0.8 × 10−11 mol cm−2). It shows that the stepwise construction
is efficient, leading mainly to a trimetallic triad with a limited
amount of the simple RuII/CoII dyad immobilized on surface.
For S2 and S1a,b the surface coverage values are higher
(between 4 and 3 × 10−11 mol cm−2). The difference is attribu-
ted to the larger structure of ligand L1 which does not allow
the full coverage of the pending cobalt-terpyridine initially
modified surface.
Scheme 2 Structure of the mono and dinuclear systems. Charges have
been omitted for clarity.
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Fig. 3 Photocurrent density of S3 (full line), S2 (dashed line) and S1a,b
(dotted line) vs. time. Irradiation periods are highlighted in yellow.
■
Fig. 4 Photoaction spectrum ( ) of S3 along with the UV/vis spectra of
triad’s building block: [Ru(bpy)3]2+ (full line), [Fe(ttpy)2]2+ (dashed line)
and [Co(ttpy)2]3+ (dotted line).
Upon visible light irradiation, the modified electrodes S2
and S3 to which we apply a bias (0.12 V) are able to generate
current in the presence of triethanolamine (TEOA) (1 M in
CH3CN) as sacrificial electron donor. The value of the bias has
been selected to keep the cobalt ion in its oxidation state III,
thus making it a good electron acceptor towards the excited
state of [Ru(bpy)3]2+.18 Fig. 3 shows the typical response as the
excitation light is turned on and off.
the photocurrent results only from absorption of light by the
[Ru(bpy)3]2+ subunit of the assembly. The mechanism is based
on the assumption that intramolecular electron transfer as
well as electron injection to the electrode are faster than inter-
molecular electron transfer. After absorption of light, En.T.
between Ru(II)* and Fe(II) is efficiently short-circuited by an
electron transfer process toward the Co(III) subunit as proven
by the resulting photocurrent generation (step a). Further
experiments would be necessary to elucidate the sequential
pathway of electron transfer between step b and b′ (Scheme 3).
However, the photocurrent intensity of S3 is higher than S2
which allows us to presume that the electron transfer between
Ru(III) and Fe(II) reduces the efficiency of the electron recombi-
nation between Ru(III) and Co(II).
In conclusion, we designed a bis-terpyridine ditopic ligand
bearing a [Ru(bpy)3]2+ moiety and used it in a stepwise con-
struction with Co(III) and Fe(II) centers. In such a triad the
energy transfer between Ru(II)* and Fe(II) is advantageously
short-circuited by an electron transfer process resulting in an
efficient photon to electron conversion. To our knowledge, this
is the first example of the use of [Ru(bpy)3]2+ in a grafted
photoinduced charge-separation system. The stepwise con-
struction proved to be an efficient way to assemble a tri-
metallic D–P–A triad on ITO. This strategy can easily be trans-
posed to other metallic centers notably those exhibiting redox
catalytic properties to photoinduce chemical reactions.
For S3 an anodic photocurrent magnitude of 14 μA cm−2 is
quickly reached. The irradiation process was repeated over 2 h
providing reproducible photocurrent response proving the
mechanical and photophysical stability of the film (see ESI†).
It should be noted that the photolysis in the absence of TEOA
or the irradiation of S1a,b in presence of TEOA causes only
negligible photocurrent. The photocurrent originating from
irradiation of S3 is 40% higher than S2 (10 μA cm−2). Despite
the energy transfer (En.T.) process between Ru(II)* and Fe(II)
subunits which reduce the excited state lifetime of Ru(II)*,19 S3
is still able to generate a large amount of current. In S3, the
En.T. is short-circuited by an electron transfer process from
Ru(II)* to the Co(III) subunit. Moreover, the Fe(II) subunit reacts
efficiently with the photogenerated Ru(III) species to compete
with the back electron transfer (BET) reaction between Co(II)
and Ru(III) species. The incident photon-to-electron conversion
efficiency (IPCE)20 at 455 nm is estimated to 0.031% for S3 and
0.013% for S2.
The mechanism we propose for electron transfer process
at the triad is depicted in Scheme 3. As illustrated in Fig. 4,
Acknowledgements
The authors acknowledge support from ICMG FR 2607 and
LabEx ARCANE (ANR-11-LABX-0003-01).
Notes and references
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Scheme 3 Proposed mechanism of the electron transfer across the
triad S3.
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This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 12156–12159 | 12159