Coupled sensitizer-catalyst dyads: Electron-transfer reactions in a perylen-polyoxometalate conjugate

Article abstract of DOI:10.1002/chem.200801880

A new strategy is introduced that permits covalent attachment of an organic chromophore to a polyoxometalate (POM) cluster. Two examples are reported that differ according to the nature of the anchoring group and the flexibility of the linker. Both POMs a

Full text of DOI:10.1002/chem.200801880

DOI: 10.1002/chem.200801880
Coupled Sensitizer–Catalyst Dyads: Electron-Transfer Reactions in a
Perylene–Polyoxometalate Conjugate
Fabrice Odobel,*[a] Marjorie Sꢀverac,[a] Yann Pellegrin,[a] Errol Blart,[a] Cꢀline Fosse,[b]
Caroline Cannizzo,[c] Cꢀdric R. Mayer,*[d] Kristopher J. Elliott,[e] and
Anthony Harriman*[e]
Abstract: A new strategy is introduced
that permits covalent attachment of an
organic chromophore to a polyoxo-
metalate (POM) cluster. Two examples
are reported that differ according to
the nature of the anchoring group and
the flexibility of the linker. Both POMs
are functionalized with perylene mono-
imide units, which function as photon
collectors and form a relatively long-
lived charge-transfer state under illumi-
nation. They are reduced to a stable p-
radical anion by electrolysis or to a
protonated dianion under photolysis in
the presence of aqueous triethanol-
AHCTUNGTERGaNNUN mine. The presence of the POM
fluorescence quenching, again due to
intramolecular electron transfer. In
most cases, it is difficult to resolve the
electron-transfer products because of
relatively fast reverse charge shift that
occurs within a closed conformer. Al-
though the POM can store multiple
electrons, it has not proved possible to
use these systems as molecular-scale
capacitors because of efficient electron
transfer from the one-electron-reduced
POM to the excited singlet state of the
perylene monoimide.
opens up an intramolecular electron-
transfer route by which the charge-
transfer state reduces the POM. The
rate of this process depends on the mo-
lecular conformation and appears to in-
volve through-space interactions. Prior
reduction of the POM leads to efficient
Keywords: dyes/pigments · electron
transfer · fluorescence · photochem-
istry · polyoxometalates
Introduction
fuel under visible-light illumination. Although much funda-
mental research has been conducted over the past four de-
cades or so, many severe barriers remain, and we are still
unable to engineer a viable photosystem for this purpose.
One of the critical steps relates to the need to charge selec-
The ever-increasing demands for renewable energy supplies
have led to increased effort being devoted to the design of
artificial photosynthetic systems able to generate a chemical
[a] Dr. F. Odobel, Dr. M. Sꢀverac, Dr. Y. Pellegrin, Dr. E. Blart
University of Nantes
[d] Dr. C. R. Mayer
Institut Lavoisier de Versailles, UMR-CNRS 8180
Universitꢀ de Versailles St-Quentin
Bꢂtiment Lavoisier, 45 avenue des Etats-Unis
78035 Versailles cedex (France)
Fax : (+33)1-39-25-43-81
CEISAM, Chimie Et Interdisciplinaritꢀ, Synthꢁse, Analyse, Modꢀlisa-
tion
Facultꢀ des Sciences et des Techniques
2, rue de la Houssiniꢁre, BP 92208, 44322 Nantes Cedex 3 (France)
Fax : (+33)2-51-12-54-02
E-mail: cmayer@chimie.uvsq.fr.
E-mail: Fabrice.Odobel@univ-nantes.fr
[e] K. J. Elliott, Prof. Dr. A. Harriman
[b] Dr. C. Fosse
Molecular Photonics Laboratory, School of Chemistry
University of Newcastle, Newcastle upon Tyne, NE17RU (UK)
Fax : (+44)191-222-8660
Ecole Nationale Supꢀrieure de Chimie Paris
Service de Spectromꢀtrie de Masse
Laboratoire de Chimie Inorganique et Bioinorganique
24 Rue Lhomond, 75231 Paris Cedex 05 (France)
E-mail: anthony.harriman@newcastle.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801880.
[c] Dr. C. Cannizzo
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
Laboratoire de Chimie Inorganique et Bioinorganique, EPFL-BCH
1015 Lausanne (Switzerland.)
3130
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3130 – 3138

FULL PAPER
tive catalysts with redox equivalents generated during the
photochemical event. Bearing in mind that fuel production
is a multi-electron process,[1] while light-induced redox reac-
tions invariably proceed by one-electron steps,[2] this is a
major problem. The most successful artificial systems use
colloidal redox catalysts to drive the multi-electron chemis-
try,[3–5] with the catalyst fulfilling the vital role of storing the
charges, at least in a temporary sense. Such systems have
proved to be invaluable in optimizing catalyst performance
in sacrificial systems (i.e., in which the fuel is produced from
a disposable organic substrate), but have evident limitations
under real-world conditions. Consequently, one of the main
requirements for sustained production of a useful chemical
fuel relates to the identification of alternative catalysts and
new routes by which to charge them with the appropriate
number of redox equivalents.[6]
Herein we introduce a soluble polyoxometalate (POM)
cluster as a redox catalyst,[7] functionalized with a perylene-
based photon collector. The intention was to explore the
electron-transfer chemistry of the resultant conjugate fol-
lowing illumination into the perylene-based unit. In princi-
ple, several dyes can be attached to the surface of a given
POM, and thereby offer the possibility to accumulate suc-
cessive charges on the catalyst, but first it is necessary to
fully characterize the initial steps. The perylene monoimide
dye used here is a close relative of compounds known to
form a charge-transfer state under illumination in a polar
solvent.[8,9] This unit is attached to the POM through a short
covalent tether intended to minimize the distance over
which secondary electron-transfer reactions must occur.
Direct attachment of the dye to the surface is a nontrivial
task[10] and we have used two different types of tether.
These differ in terms of their internal flexibility and, most
notably, in their ability to keep the reactants apart. The
POM was selected for its facile multi-electron reduction,[11]
and we employed an organic solvent to prevent rapid dis-
charge of electrons accumulated on the catalyst.
et al. and Meldal et al. independently discovered that cop-
per(I) catalyzes this reaction very efficiently with exclusive
formation of the 1,3-triazole.[15] Recently, during the course
of this work, Malacria et al. reported the monofunctionaliza-
tion of Keggin-type polyoxometalates with floppy organotin
groups and their post-functionalization by the Huisgen reac-
tion.[16,17] Synthetic strategies allowing the bis-functionaliza-
tion of POMs are limited,[18] although they could be useful if
one envisions attaching several organic units to a single
POM or preparing copolymers based on a POM mono-
mer.[19] To this end, the new polyoxometalates 1 and 2 were
first prepared in good yield by treating K10[a2-
P2W17O61]·20H2O with an excess of 4-azidomethylphenyltri-
methoxysilane (3) and 2-azido ethylphosphonic acid (4), re-
spectively (Scheme 1). The tetrabutylammonium salts of
POMs 1 and 2 were isolated as air-stable white powders in
good yield (about 75–80%), and they are soluble in CH3CN
and DMF. In POMs 1 and 2, the characteristic IR signature
of the two azido groups could be observed at 2097 cmÀ1.
Scheme 1. Outline of the route used for preparation of the key POM syn-
thons 1 and 2. a) CH3CN/H2O, HCl, RT, 75%; b) CH3CN, HCl, reflux,
80%.
Results and Discussion
Synthesis: Usually, the association of a molecular unit to a
POM is attained by way of electrostatic interactions.[12,13]
However, we prefer to use a covalent linkage to increase
the stability of the conjugate, especially in polar solvents.
Furthermore, the former supramolecular approach is re-
stricted to utilization of positively charged adsorbates. To
the best of our knowledge, only two reports describe cova-
lent attachment of a sensitizer to a POM. The first example
concerns functionalization of a lacunary decatungstosilicate
with an organosilyl fulleropyrrolidine by Bonchio and co-
workers.[14a] In the second example, Peng and co-workers[14b]
connected hexamolybdate clusters to a conjugated polymer.
We initially developed a mild and efficient synthetic ap-
proach to covalently attach two functional groups to a
Dawson polyoxometalate. To this end, Huisgen 1,3-dipolar
cycloaddition was particularly appealing, since Sharpless
The Huisgen reaction was then tested with POMs 1 and 2
with three aromatic alkynes bearing substituents with elec-
tronic properties varying from electron-donating dihexyl-
AHCTUNGTREGaNNNU mino (5) through neutral, unsubstituted (6) to electron-
withdrawing cyano (7) groups. The classical conditions
(CuSO4·6H2O with ascorbic acid in DMF at 508C for 64 h)
afforded the expected bis-coupled products 8–13 in high
yield. In the resulting POMs 8–13, the intense IR band of
the azido group has disappeared completely.
To extend this approach to grafting light-harvesting anten-
na to the above POMs 1 and 2, we prepared novel perylene
monoimide (PMI) 16 bearing a terminal alkyne group and
the corresponding benchmark reference 17 (Scheme 2).
Compound 16 results from the Sonogashira reaction be-
tween the trimethylsilylacetylene and the known 2,5-di-tert-
butylphenoxyperylene-1,6-bis(4-tert-butyl-phenoxy)-9-
Chem. Eur. J. 2009, 15, 3130 – 3138
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3131

F. Odobel, C. R. Mayer, A. Harriman et al.
action of PMI 16 with POMs 1 and 2 under the above condi-
tions gave respectively POMs 18 and 19 in good yield. The
two new POMs 1 and 2 are valuable precursors in which the
organic arm is connected either by semirigid (1) or floppy
(2) spacers, and thus allow construction of triads with differ-
ent conformational freedoms and electronic couplings. Be-
sides, these azido-substituted polyoxometalates 1 and 2
make it possible to functionalize a Dawson-type polyoxo-
metalate with a wide diversity of substituents, since terminal
alkynes could be attached by copper-catalyzed Huisgen
reaction.[17]
Model compound PER: The
absorption spectrum of POM-
free reference compound PER
(17 in the synthetic schemes) is
given in Figure 1 and shows an
intense set of transitions with a
maximum at 525 nm in DMF.
The entire absorption envelope
can be resolved into a series of
four bands of equal half-width
and with a vibrational spacing
Scheme 2. Synthetic route to 16 and 17. Reagents and conditions: a) Trimethylsilylacetylene, [PdCl2ACTHNUTRGEN(NUG PPh3)2],
CuI, toluene, Et3N, 50 8C, overnight, 87%; b) Potassium carbonate, CH2Cl2, methanol, RT, 1 h, 100%; c) DMF,
CuSO4·5H2O, ascorbic acid, 2-azidoethyl diethyl phosphonate, 508C, 16 h, 100%.
of 1300 cmÀ1. The correspond-
ing fluorescence spectrum
bromo-3,4-dicarboximide (14)[20,21] followed by cleavage of
the trimethylsilyl group by potassium carbonate. Again, the
copper-catalyzed Huisgen reaction between PMI 16 and 2-
azidoethyl diethyl phosphonate gives 17 in quantitative
yield.
We then adapted the click-chemistry procedure to prepare
triads 18 and 19 with perylene monoimide sensitizer 16. Re-
Figure 1. Absorption and fluorescence spectra of PER recorded in a
range of organic solvents of differing polarity. The excitation wavelength
for the emission spectra was 490 nm.
shows reasonably good mirror symmetry with the absorption
transition and has a maximum at 585 nm in DMF. The emis-
sion maximum shifts towards lower energy with increasing
dielectric constant of the solvent (Figure 1), but the absorp-
tion spectrum is less affected. This behavior confirms that a
marked increase in the molecular dipole moment occurs on
promotion to the first excited singlet state. Indeed, fitting
the experimental data to the Lippert–Mataga expression[22]
allows us to conclude that the dipole moment of the first ex-
cited singlet state exceeds that of the ground state by about
16 D. This value is reached on the basis that the molecule
resides in a cavity of radius 5 ꢄ. Furthermore, DFT calcula-
tions at the B3LYP/6-311+G* level give a ground-state
3132
www.chemeurj.org
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3130 – 3138

Coupled Sensitizer–Catalyst Dyads
FULL PAPER
dipole moment of 4.8 D; hence, the dipole moment of the S1
state must be about 21 D. This behavior is in general agree-
ment with earlier work with closely related compounds.[8,9]
The S1 state is highly fluorescent in solution.[23] The emis-
sion quantum yield FF recorded in dilute DMF is 0.58, while
the excited state lifetime tS is 5.4 ns. Decay curves were well
represented by single-exponential kinetics. Again, the de-
rived photophysical values are consistent with earlier re-
ports.[8,9] The spectral data can be used to determine the
energy level of the S1 state as 2.22 eV. Even in a nonpolar
solvent, the emission spectrum is relatively broad and fea-
tureless and there is no clear indication for initial formation
of a p,p* excited state. At 77 K, the fluorescence spectrum
recorded in a butyronitrile glass is blue-shifted by about
20 nm and shows some fine structure (see Supporting Infor-
mation). The individual bands resolved by a Gaussian analy-
sis remain fairly broad, while the Stokes shift is reduced to
900 cmÀ1. Thus, the S1 state has less charge-transfer charac-
ter under these conditions, but a significant change in geom-
etry on excitation still occurs.[24] In a butyronitrile glass con-
taining 15% iodoethane and maintained at 77 K, we were
unable to detect phosphorescence at l<800 nm. Likewise,
nanosecond laser flash photolysis of PER in deoxygenated
DMF did not detect a transient species that might be the
triplet excited state.[25] Under these conditions (l=532 nm,
Figure 2. Spectro-electrochemical investigation of the reduction of PER
in DMF containing background electrolyte and after thorough deoxyge-
nation. Electrolysis was carried out at À0.85 V versus NHE for incremen-
tal time periods.
To investigate the photochemical reduction of PER, a set
of experiments was performed following addition of trietha-
nolamine (TEOA). It is well known that oxidation of
TEOA, for which the one-electron oxidation potential is re-
ported to be 0.82 V versus NHE,[30] is followed by loss of a
proton and concomitant formation of a reducing radical.[31]
Thus, one molecule of TEOA provides two reducing equiva-
lents. A linear Stern–Volmer plot was obtained for the
quenching of the PER S1 state by TEOA in wet DMF, from
which the bimolecular rate constant was found to be (1.2Æ
0.2)ꢅ109 mÀ1 sÀ1. The thermodynamic driving force for this
reaction is estimated to be about 0.48 eV from the respec-
tive reduction potentials. Steady-state illumination of PER
in deoxygenated DMF containing TEOA and 5 vol% water
leads to rapid buildup of the PER p-radical anion
(Figure 3), which accumulates in solution but disappears on
aeration. Longer irradiation leads to formation of a second
species, which absorbs primarily at 490, 608, and 647 nm.
This species, which can be assigned to a protonated form of
the PER p dianion,[32] presumably arises because of the
presence of small amounts of water in the system. Indeed,
laser flash photolysis of PER in deoxygenated, wet DMF
containing TEOA (0.01m) confirmed formation of the p-
full width at half-maximum (FWHM)=4 ns),
a small
amount of sample decomposition led to formation of the p-
radical anion as a permanent species.[26] Aeration of the so-
lution reduced the lifetime of this species to about 2 ms. The
yield of the p-radical anion was very low and probably re-
flects reaction of the polar S1 state with reducing impurities
present in the solvent.
The cyclic voltammogram recorded for PER in DMF
shows that the compound readily undergoes both reductive
and oxidative electrochemical steps.[27] Indeed, the voltam-
mogram shows four one-electron waves, some of which are
quasi-reversible. On reductive scans, two one-electron waves
are seen and allow determination of the corresponding half-
wave potentials as À0.92 and À1.36 V versus a normal hy-
drogen electrode (NHE). The corresponding one-electron
waves seen on oxidative scans are poorly reversible and cor-
respond to peak potentials of 1.25 and 1.58 V versus NHE,
respectively. Poor reversibility is a common feature of the
oxidation of aryl hydrocarbons[28] and is often associated
with formation of a dimer radical cation.[29] Spectro-electro-
chemical studies in dry DMF were performed to follow the
course of the reductive scans, and it was found that electrol-
ysis at À0.85 V versus NHE results in clean conversion of
PER to the corresponding p-radical anion[26] (Figure 2). A
clear isosbestic point at 560 nm is preserved throughout
electrolysis. The p-radical anion has a narrow and intense
absorption band centered at 620 nm, with additional bands
prominent at 808 and 864 nm. There is no indication for dis-
proportionation of the p-radical anion under these condi-
tions, and this finding is consistent with the large spacing
(DE=0.44 eV) between successive reduction waves in the
cyclic voltammograms.
Figure 3. Absorption spectral changes recorded during steady-state irradi-
ation of PER in deoxygenated DMF containing water (5% v/v) and
TEAO (0.1m). The inset shows a Stern–Volmer plot for quenching of the
PER S1 state by TEOA under the same conditions.
Chem. Eur. J. 2009, 15, 3130 – 3138
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3133

F. Odobel, C. R. Mayer, A. Harriman et al.
radical anion. Under these conditions, this species decays
with second-order kinetics with a half-life of about 450 ms.
Using the molar absorption coefficient derived by spectro-
electrochemistry (e620 =56300mÀ1 cmÀ1), the corresponding
bimolecular rate constant becomes 1.9ꢅ109 mÀ1 sÀ1. This
decay process is presumed to involve concomitant protona-
tion and disproportionation.[33] Again, oxygen catalyzes the
return to the ground-state dye; in air-equilibrated DMF the
lifetime falls to about 2 ms.
Studies on PSI–POM: In PSI–PER, two PER units are at-
tached to a single POM residue by way of a silyl anchoring
group (compound 18 in the synthetic schemes). In DMF, the
absorption and fluorescence spectra remain unaffected by
the presence of the cluster; this indicates the absence of any
significant electronic interactions between these units. There
is a modest decrease in FF (i.e., 40%), while the emission
decay curves are nonexponential. Reasonable, but far from
perfect, analyses can be obtained by a biexponential fit with
lifetimes of 5.3 (67) and 1.0 ns (33%). Cyclic voltammetry
shows that the oxidative electrochemistry of the PER units
remains unchanged from that recorded for PER, but several
new quasireversible, one-electron reduction waves are ap-
parent (Figure 4). These peaks, which have half-wave poten-
Figure 5. Spectro-electrochemical investigation of the stepwise reduction
of PSI–POM 18 in deoxygenated DMF. The applied potential was initial-
ly À0.5 V versus NHE and finally À0.89 V versus NHE.
duced form of the POM cluster. According to the cyclic vol-
tammograms, reduction of the PER unit occurs after addi-
tion of two electrons to the POM.
The linkage used to attach the PER unit to the surface of
the POM is semirigid and favors the existence of different
conformations.[37] In a crude sense, we can assign the longer
lifetime found in the time-resolved emission records to a
conformation in which the PER and POM units are held far
apart. Consequently, the shorter lifetime can be assigned to
species having the two units in relatively close contact. This
type of analysis does not rule out the presence of conform-
ers in which the redox-active units are in direct contact and,
in fact, integrating the decay curves for PER and PSI–POM
suggests that about 17% of the ground-state mixture exists
in such a closed conformation that does not fluoresce. Thus,
the combined fluorescence data require the coexistence of a
minimum of three families of conformers that differ accord-
ing to their ability to undergo intramolecular electron-trans-
fer reactions. This situation was confirmed by a series of
laser flash photolysis studies with excitation into the PER
unit (l=532 nm, FHWM=20 ps). Thus, the transient ab-
sorption spectrum recorded 30 ps after excitation of PSI–
POM is identical to that recorded for PER in dry DMF
(Figure 6). This spectrum shows bleaching of the ground-
state absorption bands and strong absorption centered at
around 620 nm that is reminiscent of the PER p-radical
anion. There is no clear spectroscopic signature for the oxi-
dized component of the charge-transfer state at wavelengths
between 420 and 700 nm (see Supporting Information). The
transient signal returns to the ground state by way of nonex-
ponential kinetics. However, decay profiles recorded at 620
and 525 nm are remarkably similar, and there is no sign of
either the one-electron-reduced POM or the oxidized
donor. The latter species would be most apparent as delayed
return of the ground-state absorption, since it has no clear
spectral signature. The transient absorption signal observed
at longer wavelengths, at which the reduced POM is the sole
absorber, were too small to provide useful kinetic informa-
tion.
Figure 4. Cyclic voltammogram recorded for PSI–POM 18 in dry DMF
containing background electrolyte and after thorough deoxygenation.
tials of À0.59 and À0.84 V versus NHE, are due to reduc-
tion of the POM cluster.[34] The second reduction step over-
laps with the first reduction wave associated with the PER
unit. These data can be used to estimate the thermodynamic
driving force for electron transfer from the PER S1 state to
the nearby POM as 0.34 eV. We attribute the observed fluo-
rescence quenching process to intramolecular electron trans-
fer from the initially formed charge-transfer state to the
POM.
Spectro-electrochemical studies in deoxygenated DMF
showed that reductive electrolysis leads to buildup of the re-
duced POM cluster (Figure 5). This product displays weak
absorption over the 600–1100 nm range and has a clear blue
color.[35] The one-electron-reduced POM is stable over many
hours in the absence of oxygen.[36] Further electrolysis at
À0.89 V versus NHE results in the appearance of the PER
p-radical anion. Clearly, this species can coexist with the re-
We can conclude, therefore, that charge recombination
(DG0 =À1.84 eV) is faster than intramolecular electron
3134
www.chemeurj.org
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3130 – 3138

Coupled Sensitizer–Catalyst Dyads
FULL PAPER
1.2ꢅ109 mÀ1 sÀ1. Again, this quenching process is attributed
to reduction of the S1 state to form the corresponding PER-
based p-radical anion. Thus, steady-state illumination of
PSI–POM in deoxygenated wet DMF containing excess
TEOA leads to accumulation of the reduced POM (see Sup-
porting Information). On prolonged irradiation, there was
no clear indication for further reduction to give the PER p-
radical anion and, in particular, no bleaching of the PER ab-
sorption bands occurred. This is because the S1 state local-
ized on the PER unit undergoes an electron-transfer reac-
tion with the reduced POM in preference to electron ab-
straction from TEOA. This step was confirmed by fluores-
cence spectro-electrochemistry, which showed that prior re-
duction of the POM lowered FF for the PER unit from 0.35
to 0.09. The fluorescence lifetime is decreased to about 1 ns.
It is also notable that absorption spectra recorded for the re-
duced POM in dry and wet DMF are different. Presumably,
this is because of protonation of the reduced POM in the
presence of water.
Figure 6. Top: Transient differential absorption spectra recorded at differ-
ent time delays following pulsed laser excitation of PSI–POM in deoxy-
genated DMF. Bottom: Decay traces recorded at 620 (gray) and 520 nm
(black).
Nanosecond laser flash photolysis studies in the presence
of excess TEOA (0.1m) in deoxygenated wet DMF failed to
detect transient formation of the p-radical anion. Presuma-
bly, this is because the rate constant for intramolecular elec-
tron transfer to the appended POM is too high; we can esti-
mate the lower limit for this value as being >2ꢅ108 sÀ1.
Even with improved temporal resolution it was not possible
to obtain a good estimate for this rate constant, because the
rate of formation of the PER p-radical anion remains
modest (kꢀ7ꢅ108 sÀ1) even when the concentration of
TEOA is increased to 0.6m. Under these conditions, the life-
time of the PER p-radical anion is less than 5 ns.
transfer in this system. A possible explanation for this be-
havior is that electron transfer to the POM takes place
through folded conformers in which the reactants are in
close contact (Scheme 3). The rate constant for electron
Laser flash photolysis studies were also performed with a
sample in which the POM had been reduced. Here, the S1
state of the PER chromophore is seen immediately after ex-
citation (l=400 nm; FWHM=20 ps). This species decays
with first-order kinetics, with a lifetime of 0.8Æ0.2 ns, to re-
store the pre-pulse baseline. We attribute this quenching
process to intramolecular electron transfer from the reduced
POM to the PER S1 state to form the PER p-radical anion
(Scheme 4). This reaction is extremely favorable, with a
thermodynamic driving force of about 1.9 eV, and is likely
to fall within the Marcus inverted region.[38] Light-induced
electron transfer is followed by a thermal electron-transfer
step to recover the reduced POM; the driving force for this
second step is 0.34 eV. Under these conditions, it was not
possible to observe intermediate formation of the p-radical
anion, since the rate for the
Scheme 3. Light-induced electron transfer from the excited singlet state
of the PER-based chromophore to the POM in the different families of
conformations present in slow equilibrium (csr=charge-shift reaction).
transfer is difficult to resolve from the diffusional processes.
However, charge recombination is facilitated since the re-
quired geometry has already been attained. The modest
fraction of ground-state dyad that exists in a folded confor-
mation (i.e., ca. 17% as deduced from the fluorescence
studies) does not provide evidence for a relatively long lived
species in which the charge is transferred to the POM.
return charge-shift reaction ex-
ceeds that for the forward reac-
tion. Furthermore, we are
unable to distinguish between
through-bond and through-
Scheme 4. Light-induced elec-
tron transfer (et) from the re-
duced POM to the excited sin-
glet state of the PER-based
The residual fluorescence from the PER S1 state in PSI–
POM was quenched by TEOA in wet DMF, and a linear
Stern–Volmer plot was obtained. Under the assumption that
the longer-lived species will be more susceptible to bimolec-
ular quenching, the derived rate constant is estimated as
space processes on the basis of
these results alone and, as a
consequence, attention was
turned to the more flexible PP–
POM system.
chromophore
shift reaction).
(csr=charge-
Chem. Eur. J. 2009, 15, 3130 – 3138
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3135

F. Odobel, C. R. Mayer, A. Harriman et al.
Studies on PP–POM: The PP–POM system has two PER
units covalently attached to a POM residue by way of flexi-
ble linkages with phosphoryl anchoring groups (compound
19 in the synthetic schemes). Again, the presence of the
POM had no evident effect on either absorption or fluores-
cence spectra recorded in DMF. In this case, however, fluo-
rescence from the PER S1 state was heavily quenched, and
FF fell to 0.02. Time-resolved fluorescence decay curves
could be analyzed satisfactorily in terms of two exponential
components, with lifetimes of 5.5 (8) and 0.38 ns (92%). As
above, fluorescence quenching is attributed to intramolecu-
lar electron transfer from the PER S1 state to the POM, for
which the thermodynamic driving force is 0.87 eV. The
decay records do not eliminate the presence of very fast
components, and integration of the entire signal suggests
that such species make an important contribution. Indeed,
we estimate that some 75% of the dyad must exist in a non-
fluorescent form. These different lifetimes are assigned to
conformations that differ according to their propensity to
undergo light-induced electron transfer. They may differ in
terms of mutual separation distance[39] and/or orientation.[40]
The fact that PP–POM is more heavily quenched than PSI–
POM, in which the linker is more rigid, can be used to
argue that light-induced electron transfer requires orbital
contact between the redox-active units.
from the reduced POM to the PER S1 state, followed by a
charge-shift reaction to restore the reduced POM
(Scheme 4). In this case, the rather low fluorescence quan-
tum yield found for PP–POM prevented monitoring this
process by steady-state fluorescence spectroscopy. Because
of the extensive quenching of the S1 state by the POM, it
was difficult to conduct meaningful laser flash photolysis
studies in the presence of TEOA.
Laser excitation of PP–POM after prior reduction of the
POM in dry DMF generates the PER S1 state (Figure 7).
This species decays over about 100 ps with nonexponential
Figure 7. Transient differential absorption spectra recorded at different
time delays following pulsed laser excitation of PP–POM in deoxygenat-
ed DMF after prior reduction of the POM. Delay times are 0, 20, 40, 80,
120, 180, 250, and 500 ps.
Laser flash photolysis of PP–POM in dry DMF (l=
400 nm; FWHM=20 ps) confirmed formation of the PER
S1 state within the excitation pulse. The derived decay pro-
files were complex, but most of the initial signal (60–70% in
a typical experiment) decayed rapidly with a lifetime of
about 60 ps. We attribute this process to intramolecular elec-
tron transfer from S1 to the POM in a family of folded con-
formers (Scheme 3). This situation appears to be fully con-
sistent with the fluorescence results mentioned above. The
decay process is most notable at 620 nm, where the PER p-
radical anion is the dominant absorber. On longer time-
scales, the PER S1 state decays by nonexponential kinetics
to restore the pre-pulse baseline. Overall, this behavior is
consistent with the forward reaction being slower than the
reverse process. This can be well explained in terms of the
forward reaction being limited by the need for mass trans-
fer, while the reverse step occurs within the resultant confor-
mer.
kinetics. As the signal decays, a slight blueshift (5–10 nm)
and narrowing of the absorption band occur, possibly due to
the expected light-induced charge-shift reaction (Scheme 4).
However, there are no evident spectroscopic signatures by
which to monitor this process. Kinetic measurements indi-
cate that part of the signal grows in after the excitation
pulse, and this must be indicative of the charge-shift reac-
tion. On the basis of a global analysis of the spectroscopic
data, it can be argued that the PER S1 state has an average
lifetime of about 70 ps, while mono-reduced POM is re-
stored with a lifetime of about 160 ps in DMF at room tem-
perature (Figure 8).
Spectro-electrochemical reduction of PP–POM in dry
DMF shows stepwise buildup of the reduced POM, followed
by the appearance of the PER p-radical anion, as described
above for PSI–POM (see Supporting Information). Prior re-
duction of the POM does not prevent electron addition to
the PER unit. Photoreduction of PP–POM in wet DMF con-
taining excess TEOA (0.5m) leads to the slow accumulation
of the reduced POM (see Supporting Information). Again,
the absorption spectra are somewhat different to those ob-
tained by electrochemical reduction in dry DMF and most
likely correspond to a protonated species. Under photo-
chemical conditions, the PER p-radical anion does not accu-
mulate even on prolonged irradiation. This behavior is
easily explained in terms of intramolecular electron transfer
Figure 8. Typical decay traces recorded for the system described in the
legend to Figure 7. The PER p-radical anion absorbs more strongly than
the PER S1 state at 620 nm, while they show comparable absorption coef-
ficients at 640 nm. The solid line running through each data set corre-
sponds to the results of a global fit to the spectroscopic data in which the
lifetime of the S1 state is 70 ps and that of the charge-shifted state is
160 ps.
3136
www.chemeurj.org
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3130 – 3138

Coupled Sensitizer–Catalyst Dyads
FULL PAPER
Conclusion
Experimental Section
Experimental details for the preparation of the various compounds and
their characterization are collected in the Supporting Information. Ab-
sorption spectra were recorded with a Hitachi U3310 or Perkin–Elmer
Lambda 35 UV/Visible spectrophotometer, while fluorescence studies
were made with a Hitachi F-4500 fluorescence spectrophotometer in
DMF. Measurements were made on optically dilute solutions after deoxy-
genation by purging with N2. Fluorescence quantum yields were mea-
sured by comparison to standard fluorophores.[42] Corrected excitation
spectra were also recorded under optically dilute conditions. Fluores-
cence lifetimes were measured by time-correlated, single-photon count-
ing following excitation with an ultrashort laser diode emitting at 525 nm
by using a PTI Easy-Life spectrometer. After deconvolution of the in-
strumental response function, the temporal resolution of this setup was
about 50 ps. Steady-state irradiations were performed in DMF by using a
Fiber Lite PT 900 regulated illuminator with a 150 W quartz-halogen
bulb with <400 nm filtering and by using a sealed, N2 purged cuvette.
We have described a methodology for the covalent attach-
ment of organic chromophores to the surface of a redox-
active POM. The linking tether is either flexible, and there-
by facilitates close contact between chromophore and POM,
or semirigid. The resultant dyads undergo successive electro-
chemical reduction steps in which the POM stores two elec-
trons before the PER unit is reduced. Both the reduced
POM and the PER p-radical anion are susceptible to proto-
nation when water is added to a solution of the samples in
DMF. The PER p-radical anion can also be formed by pho-
tochemical reduction using TEOA as a sacrificial electron
donor. A major point of interest in this work is intramolecu-
lar electron transfer between the terminals, and several such
steps have been identified. Firstly, the excited singlet state
resident on the PER chromophore transfers an electron to
the POM. Electron transfer from the S1 state, which has
considerable charge-transfer character,[8,9] appears to take
place within a folded conformation that brings the reactants
into close proximity. Thus, the dynamics of the overall pro-
cess involve a combination of mass transfer and electron
transfer. In this respect, the flexible tether provides for
faster rates of electron transfer and more extensive fluores-
cence quenching. The charge-shifted products are not seen,
because the rate of the reverse process exceeds that of the
forward step. Likewise, except in one case, we were unable
to record the kinetics for intramolecular electron transfer
from the PER p-radical anion to the appended POM. The
problem here is related to the relatively long timescale asso-
ciated with generation of the PER p-radical anion. For PP–
POM 19, this reaction has a half-life of about 160 ps.
Finally, the S1 state of the PER chromophore enters into
electron-transfer reactions with the one-electron-reduced
form of the POM.[41] In DMF at ambient temperature, this is
a fairly fast reaction that takes place preferentially within
folded conformations. It is followed by a thermal charge-
shift reaction to restore the original system. In principle, the
PER S1 state could transfer an electron to the mono-re-
duced POM to form the doubly reduced species. Such reac-
tions would be facilitated by a high photon flux, as is easily
achieved with short laser pulses, and are an essential feature
of a molecular-scale capacitor. This study shows that the ad-
dition of a second electron to the POM is not observed
under our conditions with 18 and 19, although the thermo-
dynamics are favorable. This is ascribed to the fact that
second electron transfer does not compete with the intramo-
lecular charge shift from the excited-state sensitizer to the
mono-reduced POM. This aspect could be very useful to
take into consideration in the future for the design of new
photochemical devices to achieve photo-accumulative elec-
tron transfer, which is an important function for mimicking
artificial photosynthesis.
Cyclic voltammetry experiments were performed using an HCH Instru-
ments Electrochemical Analyzer and a three-electrode setup consisting
of a platinum working electrode, a platinum wire counterelectrode, and a
silver wire reference electrode. All studies were performed in deoxygen-
ated dichloromethane containing tetra-n-butylammonium tetrafluorobo-
rate (TBATFB, 0.1m) as background electrolyte and ferrocene as internal
standard. The solute concentrations were typically 0.5 mm. Reduction po-
tentials were reproducible to within Æ15 mV. Spectro-electrochemical
fluorescence experiments were performed by using a Specac Omnicell,
which was aligned 458 to the excitation source and illuminated at 495 nm.
Scattered and incident light was removed by a 515 nm filter placed
before the detector. For bulk electrolysis on a HCH instruments electro-
chemical analyzer, the working electrode potential was set at À0.61 V.
Emission spectra were monitored over time.
Acknowledgement
We thank the Agence Nationale pour la Recherche (ANR Blanc “Photo-
CumElec”) and the University of Newcastle for financial support of this
work.
[1] J. R. Darwent, P. Douglas, A. Harriman, G. Porter, M.-C. Richoux,
Coord. Chem. Rev. 1982, 44, 83–126.
[2] a) A. Krasna, Photochem. Photobiol. 1979, 29, 267–271; A. Harri-
man, J. Photochem. 1984, 25, 33–36.
[3] J.-M. Lehn, J.-P. Sauvage, Nouv. J. Chim. 1977, 1, 441–446.
[4] A. Moradour, E. Amouyal, P. Keller, H. Kagan, Nouv. J. Chim.
1978, 2, 547–551.
[5] A. Harriman, G. Porter, M.-C. Richoux, J. Chem. Soc. Faraday
Trans. 2 1981, 77, 1939–1948.
[6] a) B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jorgensen, J. H. Niel-
sen, S. Horch, I. Chorkendorff, J. K. N. Orskov, J. Am. Chem. Soc.
2005, 127, 5308–5309; b) P. Liu, J. A. Rodriguez, J. Am. Chem. Soc.
2005, 127, 14871–14878; c) I. Bhugun, D. Lexa, J.-M. Savꢀant, J.
Am. Chem. Soc. 1996, 118, 3982–3983; d) P. V. Bernhardt, L. A.
Jones, Inorg. Chem. 1999, 38, 5086–5090.
[7] a) E. Gkika, P. Kormally, S. Antonaraki, D. Dimoticali, E. Papacon-
stantinou, A. Hiskia, Int. J. Photoenergy 2004, 6, 227–231; b) A.
Hiskia, A. Mylonas, E. Papaconstantinou, Chem. Soc. Rev. 2001, 30,
62–69; c) K. Karnicka, M. Chojak, K. Miecznikowski, M. Skunik, B.
Baranowska, A. Kolary, A. Piranska, B. Palys, L. Adamczyk, P. J.
Kulesza Bioelectrochemistry 2005, 66, 79–87; d) E. Gkika, P. Korma-
li, S. Antonaraki, D. Dimoticali, E. Papaconstantinou, A. Hiskia,
Int. J. Photoenergy 2004, 6, 227–231; e) T. Kida, Langmuir 2008, 24,
7648–7650.
[8] a) S. E. Miller, Y. Zhao, R. Schaller, V. Mulloni, E. M. Just, R. C.
Johnson, M. R. Wasielewski, Chem. Phys. 2002, 275, 167–183; b) D.
Chem. Eur. J. 2009, 15, 3130 – 3138
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3137

F. Odobel, C. R. Mayer, A. Harriman et al.
Gosztola, M. P. Niemczyk, W. Svec, A. S. Lukas, M. R. Wasielewski,
J. Phys. Chem. A 2000, 104, 6545–6551.
[21] A. Morandeira, J. Fortage, T. Edvinsson, L. Le Pleux, E. Blart, G.
Boschloo, A. Hagfeldt, L. Hammarstrçm, F. Odobel, J. Phys. Chem.
C 2008, 112, 1721–1728.
[22] a) E. Lippert, Z. Naturforsch. 1955, 10, 541–546; b) N. Mataga, Y.
Kaitu, M. Koizumi, Bull. Chem. Soc. Jpn. 1955, 28, 690–695.
[23] M. Sliwa, C. Flors, I. Oesterling, J. Hotta, K. Mullen, F. C. de
Schryver, J. Hotkens, J. Phys. Condens. Matter 2007, 19, 445004.
[24] M. Goes, M. De Groot, M. Koeberg, J. W. Verhoeven, N. R. Lokan,
M. J. Shephard, M. N. Paddon-Row, J. Phys. Chem. A 2002, 106,
2129–2134.
[25] a) W. E. Ford, P. V. Kamat, J. Phys. Chem. 1987, 91, 6373–6380;
b) A. Prodi, C. Chiorboli, F. Scandola, E. Leugo, E. Alessio, R. Do-
brawa, F. Wurther, J. Am. Chem. Soc. 2005, 127, 1454–1462.
[26] M. Andrzejak, M. Sterzel, M. T. Pawlikowski, Spectrochim. Acta A
2005, 61, 2029–2032.
[27] J. Cremer, P. Bauerle, J. Mater. Chem. 2006, 16, 874–884.
[28] S. K. Lee, Y. Zu, A. Herrmann, Y. Geerts, K. Mullen, A. J. Bard, J.
Am. Chem. Soc. 1999, 121, 3513–3520.
[9] a) L. E. Sinks, B. Rybtchinski, M. Limura, B. A. Jones, A. J. Goshe,
X. B. Zuo, D. M. Tiede, X. Y. Li, M. R. Wasielewski, Chem. Mater.
2005, 17, 6295–6303; b) S. Q. Xiao, M. E. El-Khouly, Y. L. Li, Z. H.
Gan, L. Jiang, Y. Araki, O. Ito, D. Zhu, J. Phys. Chem. B 2005, 109,
3658–3667; c) A. Marcos Ramos, E. H. A. Beckers, T. Orfermans,
S. C. J. Meskers, R. A. J. Janssen, J. Phys. Chem. A 2004, 108, 8201–
8211; d) C. Flors, I. Oesterling, T. Schnitzler, E. Fron, G. Schweitzer,
M. Sliwa, A. Herrmann, M. van der Auweraer, F. C. De Schryver, K.
Mullen, J. Hofkens, J. Phys. Chem. C 2007, 111, 4861–4870.
[10] a) M. J. Calhorda, J. Organomet. Chem. 1994, 475, 149–155; b) S.
Triki, L. Ouaheb, J. Padiou, D. Grandjean, J. Chem. Soc. Chem.
Commun. 1989, 1068–1070; c) M. H. Alizadeh, M. Mirzaei, H.
Razavi, Mater. Res. Bull. 2008, 43, 546–555; d) T. Kida, K. Lemura,
H. Kai, M. Nagano, J. Am. Ceram. Soc. 2007, 90, 618–621; e) M. K.
Seery, N. Fay, T. McCormac, E. Dempsey, R. J. Forster, T. E. Keyes,
Phys. Chem. Chem. Phys. 2005, 7, 3426–3433.
[11] a) D. Ingersoll, P. J. Kulesza, L. R. Faulkner, J. Electrochem. Soc.
1994, 141, 140–147; b) L. H. Bi, T. McCormac, S. Beloshapkin, E.
Dempsey, Electroanalysis 2008, 19, 37–46; c) K. Karnicka, M.
Chojak, K. Miecznikowski, M. Shunik, B. Baranowska, A. Kolary,
A. Piranska, B. Palys, L. Adamczyk, P. J. Kulesza, Bioelectrochemis-
try 2005, 66, 79–87.
[29] J. L. Sessler, Y. Kubo, A. Harriman, J. Phys. Org. Chem. 1992, 114,
5714–5722.
[30] V. Anbazhagan, A. Kathiravan, M. Asha Jhonsi, R. Renganathan,
Z. Phys. Chem. (Muenchen Ger.) 2007, 221, 929–939.
[31] M. Kirch, J.-M. Lehn, J.-P. Sauvage, Helv. Chim. Acta 1979, 62,
1345–1384.
[12] a) T. E. Keyes, E. Gicquel, L. Guerin, R. J. Forster, V. Hultgren,
A. M. Bond, A. G. Wedd, Inorg. Chem. 2003, 42, 7897–7905; b) C.
Allain, S. Favette, L.-M. Chamoreau, J. Vaissermann, L. Ruhlmann,
B. Hasenknopf, Eur. J. Inorg. Chem. 2008, 3433–3441.
[13] a) T. Tachikawa, S. Tojo, M. Fujitsuka, T. Majima, Chem. Eur. J.
2006, 12, 3124; b) J. Xie, B. F. Abrahams, A. G. Wedd, Chem.
Commun. 2008, 576–577; c) A. Yokoyama, T. Kojima, K. Ohkubo,
S. Fukuzumi, Chem. Commun. 2007, 3997–3998; d) A. Tsuda, E.
Hirahara, Y.-S. Kim, H. Tanaka, T. Kawai, T. Aida, Angew. Chem.
2004, 116, 6487–6491; Angew. Chem. Int. Ed. 2004, 43, 6327–6331.
[14] a) M. Bonchio, M. Carraro, G. Scorrano, A. Bagno, Adv. Synth.
Catal. 2004, 346, 648–654; b) B. Xu, M. Lu, J. Kang, D. Wang, J.
Brown, Z. Peng, Chem. Mater. 2005, 17, 2841 and 2851.
[15] a) V. V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless,
Angew. Chem. 2002, 114, 2708–2711; Angew. Chem. Int. Ed. 2002,
41, 2596–2599; b) C. W. Tornøe, C. Christensen, M. Meldal, J. Org.
Chem. 2002, 67, 3057–3064.
[16] K. Micoine, B. Hasenknopf, S. Thorimbert, E. Lacote, M. Malacria,
Org. Lett. 2007, 9, 3981–3984.
[17] V. D. Bock, H. Hiemstra, J. H. van Maarseveen, Eur. J. Org. Chem.
2006, 51–68.
[18] a) J. Kang, B. Xu, Z. Peng, X. Zhu, Y. Wei, D. R. Powell, Angew.
Chem. 2005, 117, 7062–7065; Angew. Chem. Int. Ed. 2005, 44, 6902–
6905; b) Z. Peng, Angew. Chem. 2004, 116, 948–953; Angew. Chem.
Int. Ed. 2004, 43, 930–935.
[19] a) S. Binauld, F. Boisson, T. Hamaide, J.-P. Pascault, E. Drockenmul-
ler and E. Fleury, J. Polym. Sci. Polym. Chem. Ed. 2008, 46, 5506–
5517; b) D. J. V. C. van Steenis, O. R. P. David, G. P. F. van Strij-
donck, J. H. van Maarseveen, J. N. H. Reek, Chem. Commun. 2005,
4333–4335.
[32] K. Kano, K. Mori, T. Konse, B. Uno, T. Kubota, Anal. Sci. 1989, 5,
651–656.
[33] L. Michaelis, Chem. Rev. 1935, 35, 243–270.
[34] a) M. Fromon, C. Triener, R. Bury, M. Fournier, J. Chem. Soc. Fara-
day Trans. 1 1985, 81, 2513–2523; b) R. Wlodarczyk, M. Chojak, K.
Miecznikowski, A. Kolary, P. J. Kulesza, R. Marassi, J. Power Sour-
ces 2006, 159, 802–809; c) P. J. Kulesza, M. Skunik, B. Baranowska,
K. Miecznikowski, M. Chojak, K. Karnicka, E. Frackowiak, F.
Beguin, A. Kuhn, M.-H. Delville, B. Starobrzynska, A. Ernst, Elec-
trochim. Acta 2006, 51, 2373–2379.
[35] a) G. J. Zhang, H. H. Ke, T. He, D. B. Xiao, Z. H. Chen, W. S. Yang,
J. N. Yao, J. Mater. Res. 2004, 19, 496–500; b) Y. Shan, G. Yang, Y.
Sun, S. Pang, J. Gong, Z. Su, L. Qu, Electrochim. Acta 2007, 53,
569–574.
[36] T. Rꢆther, V. M. Hultgren, B. P. Timko, A. M. Bond, W. B. Jackson,
A. G. Weddi, J. Am. Chem. Soc. 2003, 125, 10133–10143.
[37] a) Z. Bajzer, F. G. Prendergast, Biophys. J. 1993, 65, 2313–2323;
b) A. T. Russo, L. Brand, J. Fluoresc. 2004, 14, 333–342; c) C. K.
Johnson, B. D. Slaughter, J. R. Unruh, E. S. Price, Rev. Fluoresc.
2006, 239–259; d) G. Blondeel, D. De Keeukeleire, A. Harriman,
L. R. Milgrom, Chem. Phys. Lett. 1985, 118, 77–82.
[38] R. A. Marcus, J. Chem. Phys. 1956, 24, 966–971.
[39] J. Zhou, L. V. Lukin, C. L. Braun, J. Phys. Chem. A 2008, 112, 7507–
7513.
[40] L. A. Curtiss, C. A. Naleway, J. R. Miller, Chem. Phys. 1993, 176,
387–405.
[41] J. C. Scaiano, C. Aliaga, M. N. Chretien, M. Frenette, K. S. Focanea-
nu, L. Mikelsons, Pure Appl. Chem. 2005, 77, 1009–1018.
[42] D. Magde, J. Brannol, T. Cremers, J. Olmsted, J. Phys. Chem. 1979,
83, 696–699.
[20] R. T. Hayes, M. R. Wasielewski, D. Gosztola, J. Am. Chem. Soc.
2000, 122, 5563–5567.
Received: September 12, 2008
Published online: February 5, 2009
3138
www.chemeurj.org
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 3130 – 3138