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Towards the device: Anchoring the catalysts onto electrodes

Towards the device: Anchoring the catalysts onto electrodes

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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.

6



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

nanotubes.



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

leaf).



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

oxidization.

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

device.

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

scavenging.

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.

Acknowledgements

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



Fondazione Cariparo (Nano-Mode, Progetti di Eccellenza 2010) are

gratefully acknowledged.

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Any colour you like. Excited state and

ground state proton transfer in flavonols

and applications

Stefano Protti*a and Alberto Mezzettib, c

DOI: 10.1039/9781849734882-00295



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

flavonols.



1



Introduction



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

a



PhotoGreen Lab, Department of Chemistry, University of Pavia, V.Le Taramelli 12, 27100

Pavia, Italy. E-mail: prottistefano@gmail.com

Laboratoire de Photocatalyse et Biohydroge`ne, SB2SM, CNRS URA 2096, CEA-Saclay,

DSV/iBiTecS, 91191 Gif-sur-Yvette cedex, France.

c

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.

b



Photochemistry, 2012, 40, 295–322 | 295



c



The Royal Society of Chemistry 2012



(a)



(b)



O

A



B



B



O

A



C



C

OH

O



Fig. 1



OH



OH

HO



O



OH



O



HO



OH



OH

1



OH



2



O



OH



O



OH



OH

OH

HO



HO



O



O



OMe



OH

OH

OH



3



OH



4



OH



O



O



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

processes

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

irradiation.

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



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