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Towards the device: Anchoring the catalysts onto electrodes
multi-redox sites. The fabrication of photoelectrochemical devices is in fact
the ﬁnal step of the optimization of homogeneous water splitting screening.
In particular, the design of such an electrodic interface should promote
electron transfer, as well as provide high surface area, wettability, proton
translocation and robustness.
Moreover, the support material should: (i) be a good electron conductor;
(ii) be stable under the highly oxidizing catalytic conditions; (iii) display
optimized electrical contact between the catalyst and the electrode, in order
to have an eﬃcient electron transport.
One of the most signiﬁcant attempts to heterogenize a molecular WOC
regarded the immobilization of a phosphonate-modiﬁed blue dimer onto
ITO or nanocrystalline TiO2 and ZrO2.75 The catalyst retains its water
oxidation ability on the electrodes surface, as well as its proton-coupled
electron transfer (PCET) properties.
More recently, Llobet and coworkers reported the anodic electropolymerization of N-substituted pyrroles as a convenient method of
anchoring a redox-active dinuclear ruthenium catalyst onto conducting
solid supports, like vitreous carbon sponges (VCS) and ﬂuorine-doped tin
oxide (FTO).76 In the presence of Ce(IV) as the sacriﬁcial oxidant, turnover
numbers up to 76 have been achieved. A major improvement of the system
is accomplished by the copolymerization with a robust non active redox
species, able to further separate the catalytically active species on the solid
support, obtaining up to 250 catalytic cycles.
Oxygenic polyoxometalates can be also supported onto electrodes. A
proof-of-principle of such an electrode was recently reported, by using a
conductive bed of multi-wall carbon nanotubes (MWCNTs) as the support
material for the Ru4SiW10 OEC (Fig. 8).77
The anchoring of the anionic POM onto the surface of the tubes was
exploited by electrostatic interactions, functionalizing the nanotubes with
positively charged, dendrimeric ammonium moieties. Several spectroscopic
and microscopic techniques revealed that the structure of the OEC was
maintained in the hybrid material, and that its deposition on the tubes
occurred mainly as single molecules. The Ru4SiW10@MWCNTs material
was then drop cast onto ITO. Upon application of an external bias to the
resulting electrode, the catalytic wave due to water oxidation occurs at low
overpotentials of ca. 0.35 V in phosphate buﬀer at pH 7. Bare ITO alone
and ITO doped with multi-wall carbon nanotubes display instead very high
overpotentials, conﬁrming the role of the POM-based OEC in the catalytic
activity of the electrode. Signiﬁcantly, the conducting properties of the
nanotubes are essential to achieve eﬃcient catalysis, since their substitution
with amorphous carbon leads to a signiﬁcant abatement of the electrode
performance, in terms of catalytic current due to oxygen production.
Conclusions and Outlook: the artiﬁcial leaf
Conversion of solar light into chemical fuels translates into the ultimate
design of an ‘‘artiﬁcial leaf’’ device. This can be accomplished by a tailored
assembly of suitable chemical modules and their organization within a
photoelectrochemical cell (PEC). The simplest set up foresees the
Photochemistry, 2012, 40, 274–294 | 287
Fig. 8 Schematic representation of the complete electrochemical cell for water splitting where
Ru4SiW10 is anchored onto dendron-functionalized, positively charged, multi-wall carbon
Fig. 9 Photosensitizer (P) - oxygen evolving catalyst (OEC) dyad supported onto a nanostructured material, deposited onto an electrode. The resulting photoanode is then assembled
with a cathode and a membrane, yielding a device for artiﬁcial photosynthesis (i.e. an artiﬁcial
integration of an oxygen evolving photoanode with a hydrogen evolving
cathode. The concept is based on the tailored fabrication of composite
electrodes, replicating the natural architecture in two split half reactions.
The photoanode, in particular, can be assembled by organizing a photosensitizer and an oxygen evolving catalyst onto a nanostructured surface of
a semiconductor electrode (Fig. 9).
288 | Photochemistry, 2012, 40, 274–294
With this set-up, light absorption induces electron injection from the
photosensitizer to the conduction band of the semiconductor; while these
high energy electrons are transferred to the cathode for proton reduction to
hydrogen, the holes in the photosensitizer are ﬁlled by electron transfer
from the catalyst that evolves to its active form capable of water
Some interesting examples of photoanodes have been recently reported.
The majority of these works use colloidal or amorphous catalysts, deposited
onto semiconductor surfaces,47–49,78 or onto sensitized TiO2 electrodes (as
already described in Paragraph 4.1).38
Assemblies involving molecular catalysts are instead much more rare. In
2010, Sun and coworkers reported a photoelectrochemical device, able to
eﬃciently split water into O2 and H2, by immobilizing a molecular ruthenium catalyst in a Naﬁon membrane and depositing the material onto a
nanostructured TiO2 anode, sensitized with a ruthenium polypyridine dye.79
Visible light-driven water splitting was successfully achieved upon both
illumination and application of a small bias of À 0.325 V vs. Ag/AgCl to the
Recently, Dismukes, Spiccia and coworkers reported a tetranuclear
manganese [Mn4O4L6] catalyst (L=diarylphosphinate), which partially
mimics the OEC of PSII, encapsulated within a Naﬁon polymer matrix
through simple ion exchange.80 Combining the resulting material with TiO2
nanoparticles (sensitized with a [Ru(bpy)3]-like dye and supported onto
FTO electrodes), lead to the assembly of a composite photoanode, able to
produce a photocurrent of 31 mA Á cmÀ1 under white light irradiation in an
aqueous electrolyte at pH 6.5. However, a recent in-depth analysis on this
composite material revealed that the catalytically active species responsible
for water oxidation comes from the decomposition of the initial tetranuclear
manganese cluster in the Naﬁon membrane, to form Mn(II)-compounds,
which are then electro-oxidized, yielding dispersed nanoparticles of a disordered Mn(III/IV)-oxide phase.81
Another interesting example was reported by Brudvig, Crabtree and
coworkers.82 Both the dye (a zinc porphyrin) and the O2-evolving catalyst
(an iridium cyclopentadienyl complex) were covalently co-grafted onto the
surface of nanostructured TiO2, yielding a photoanode. Applying an
external bias 0.3 V and irradiating with visible light, a photocurrent density
of 30 mA Á cmÀ2 was obtained.
Photosensitizer-catalyst dyads supported onto electrodes can be investigated also by laser ﬂash photolysis. For example, Ru4SiW10 was supported
onto nanostructured titanium oxide, sensitized with a ruthenium polypyridine dye, covalently bound to the oxide surface by phosphonate ester
bonds.57 The polyanionic catalyst is anchored onto such organized surface
by exploiting electrostatic interactions with the cationic sensitizer. When the
catalyst-loaded samples are examined by laser ﬂash photolysis, a clear
acceleration of the bleach recovery is observed, increasing with the surface
concentration of the catalyst. At high catalyst loading, the fast component
of the recovery (ns timescale) becomes almost indistinguishable from the
laser pulse, while a long-lived (ms timescale) component of the bleach still
remains. The residual slow component can be very likely assigned to
Photochemistry, 2012, 40, 274–294 | 289
electron–hole recombination for oxidized sensitizer molecules that do not
have a catalyst in close proximity. Those that are in an ion-pair situation
with a catalyst, on the other hand, undergo very fast (sub-ns timescale) hole
Another chromophore-catalyst assembly supported onto an electrode
was reported by Meyer and coworkers.83 A ruthenium polypyridyl dye was
covalently linked both to a single site ruthenium OEC and to the TiO2
electrode. Initial transient laser and photocurrent measurements on the
nanosecond timescale reveal that excitation of the photosensitizer leads to a
rapid electron injection to the TiO2 followed by an intramolecular electron
transfer from the ruthenium center of the catalyst to the ruthenium center of
the dye, in a sub-ns timescale. However, injection from the chromophore
attached to TiO2 remains ineﬃcient (o 10 %), so this assembly needs further improvement to provide a key material for photoanode fabrication.
Hence, despite these recent developments, the current state of the art is
still far from meeting the requirements for a widespread production of such
devices. In particular, some speciﬁc points need to be addressed: (i) the
catalyst has to operate at low overpotentials and be highly active in terms of
turnover frequencies and long term stability; possibly, it has to be based on
earth abundant and cheap materials, in order to guarantee the possibility of
large scale production at a reasonable cost; (ii) photosensitizers with
extended absorption in the visible spectrum has to be considered, in order to
achieve an optimal matching with solar emission, and to maximize light
harvesting and utilization; moreover, the redox potential for the chromophore following injection must be suﬃciently positive to drive the highest
potential step in the water oxidation cycle; (iii) speciﬁc interactions between
the photosensitizer and the catalyst should be exploited to increase the
electron transfer rates from the catalyst to the oxidized form of the photosensitizer (hole scavenging), avoiding its self-bleaching; (iv) the use of
nanostructured material for the support of the photosensitizer-catalyst dyad
would guarantee eﬃcient electron transfer to the electrodes, thus achieving
fast catalysis; moreover the high surface area and the robustness of the
materials are key features that needs to be optimized; (v) the optimal
assembly should maximize injection by directing the lowest MLCT excited
state toward the semiconductor surface; (vi) the rate of water oxidation
must exceed 1 sÀ1 the approximate rate of excitation of individual chromophores in the TiO2 ﬁlm.
In conclusion, success in this area will involve an iterative design, evaluation, and redesign of the assemblies in order to optimize all the involved
processes and to ﬁnally produce an eﬃcient solar fuel device.
We thank the teams of Prof. M. Prato (University of Trieste), Prof.
F. Scandola (University of Ferrara) and Prof. F. Paolucci (University of
Bologna) for their invaluable and constructive collaboration. Financial
supports from the University of Padova (Progetto di Ateneo CPDA104105/
10 and Progetto Strategico 2008 HELIOS prot. STPD08RCX), MIUR
(PRIN no. 20085M27SS, FIRB ‘‘Nanosolar’’ RBAP11C58Y) and
290 | Photochemistry, 2012, 40, 274–294
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294 | Photochemistry, 2012, 40, 274–294
Any colour you like. Excited state and
ground state proton transfer in flavonols
Stefano Protti*a and Alberto Mezzettib, c
The photoinduced and ground state proton transfer processes occurring in ﬂavonols
are responsible for their multi-wavelength emission. This peculiar behavior has
touched on a wide range of research areas, ranging from biology to chemistry of
materials leading, among others, to the development of ﬂuorescent probes for
physical and biophysical parameters, laser dyes, and wavelentgh shifting devices.
This account aims to be a brief introduction to the multi-faceted applications of
Flavonoids are polyphenolic compounds which usually occur in plants as
glycosides and play a key-role in the UV photoprotection1 of internal tissues
of leaves and stems. Furthermore, molecules belonging to this class have
been reported to play a key role to preserve plants under hostile conditions
(e.g. presence of toxic metals, frost and drought).2
The ﬂavonoid structure consists of 2 aromatic rings (namely A and B in
Fig. 1a) connected by a pyrane moiety (C). The presence of several phenolic
hydroxyl functions within the ring classiﬁes ﬂavonoids in diﬀerent families,
including ﬂavones, ﬂavanols, isoﬂavones and ﬂavonols, the latter characterized by a 3-hydroxypyran-4-one ring (Fig. 1b).
Flavonols are widespread (more than 200 ﬂavonols aglycones have been
identiﬁed) in leaves and outer parts of the plant, but only four of these,
quercetin (1, Fig. 2), kaempferol (2), myricetin (3) and isorhamnetin (4) are
commonly present as 3-glycosilated derivatives in fruits.1 Quercetin is the
most abundant ﬂavonol in the daily diet and it can be found in onions,
apples and, though in lower concentrations, in tea and wine.3 Multiple
biological activities of ﬂavonols have been reported in literature,4 including
vasodilatatory, antibacterial, hepatoprotective, antidiabetics, antifungal,
antiviral and even antitumor5 eﬀects. In particular, their antioxidant
activity6 (mainly related to their free radical scavenging activity)7,8 makes
them cardioprotective agents.9,10
Due to the presence of chelating groups such as a-hydroxy carbonyl and
catechol moieties, ﬂavonols form complexes with several metal cations11
and have found several applications in analytical chemistry.12 Furthermore,
ﬂavonol-metal complexes have important biological activities13 and their
PhotoGreen Lab, Department of Chemistry, University of Pavia, V.Le Taramelli 12, 27100
Pavia, Italy. E-mail: firstname.lastname@example.org
Laboratoire de Photocatalyse et Biohydroge`ne, SB2SM, CNRS URA 2096, CEA-Saclay,
DSV/iBiTecS, 91191 Gif-sur-Yvette cedex, France.
Laboratoire de Spectrochimie Infrarouge et Raman UMR CNRS 8516, Universite´ de Sciences
et Technologies de Lille, Bat. C5, Cite´ Scientiﬁque, 59655, Villeneuve d’Ascq, France.
Photochemistry, 2012, 40, 295–322 | 295
The Royal Society of Chemistry 2012
Fig. 2 Quercetin (1), Kaempferol (2), Myricetin (3) and Isorhamnetin (4).
use for technological applications has been proposed.14 Complexation of
ﬂavonols with metal cations has been also employed as model system to
investigate soil organic matter-metal interactions.15
2 3-Hydroxyﬂavone (3HF) as a model molecules for proton transfer
The synthetic derivative 3-hydroxyﬂavone (3HF, 5, Fig. 3) is the simplest
ﬂavonol. The strong interest for 5 is mostly due to its peculiar photophysical
and photochemical properties, and in particular, as discovered by Sengupta
and Kasha,16 to the proton transfer process that 3HF undergoes during
As described in Scheme 1, the absorption of a photon (hn1 B 340 nm) in
3HF leads to the excited state 1N, then Excited State Intramolecular Proton
Transfer (ESIPT),17 from the 3-hydroxy group to the neighbouring 4-carbonyl takes place, aﬀording the corresponding excited tautomer 1T (also
described by a benzopyrylium ion structure), the emission of which (hnT) at
510–540 nm, depends on the surrounding microenvironment. Emission
from 1T state results in a large Stokes shift (Dl=180 nm) that is the basis of
several technological applications of 3HF (see following sections). Emission
from 1N state (at higher energies, hnN=400–430 nm) has been also
observed. The system ﬁnally undergoes non-radiative back proton transfer
296 | Photochemistry, 2012, 40, 295–322