Accumulative charge separation inspired by photosynthesis

Article abstract of DOI:10.1021/ja104809x

Molecular systems that follow the functional principles of photosynthesis have attracted increasing attention as a method for the direct production of solar fuels. This could give a major carbon-neutral energy contribution to our future society. An outsta

Full text of DOI:10.1021/ja104809x

Published on Web 12/07/2010
Accumulative Charge Separation Inspired by Photosynthesis
Susanne Karlsson,† Julien Boixel,‡ Yann Pellegrin,‡ Errol Blart,‡ Hans-Christian Becker,†
Fabrice Odobel,*,‡ and Leif Hammarstro¨m*,†
Department of Photochemistry and Molecular Science, Uppsala UniVersity, Box 523, SE-751 20 Uppsala, Sweden,
and CEISAM, Chimie Et Interdisciplinarite´, Synthe`se, Analyse, Mode´lisation CNRS, UMR CNRS 6230, UFR des
Sciences et des Techniques 2, rue de la Houssinie`re - BP 92208, 44322 Nantes Cedex 3, France
Received June 2, 2010; E-mail: Fabrice.Odobel@univ-nantes.fr; Leif@fotomol.uu.se
Scheme 1. OTARu Dye-Anchored onto Nanocrystalline TiO2
Abstract: Molecular systems that follow the functional principles
of photosynthesis have attracted increasing attention as a method
for the direct production of solar fuels. This could give a major
carbon-neutral energy contribution to our future society. An
outstanding challenge in this research is to couple the light-
induced charge separation (which generates a single electron-hole
pair) to the multielectron processes of water oxidation and fuel
generation. New design considerations are needed to allow for
several cycles of photon absorption and charge separation of a
single artificial photosystem. Here we demonstrate a molecular
system with a regenerative photosensitizer that shows two
successive events of light-induced charge separation, leading to
high-yield accumulation of redox equivalents on single compo-
nents without sacrificial agents.
Photosynthetic reaction centers provide a blueprint for the
conversion of light to chemical energy by a chain of photoinduced
electron transfer events.1,2 Each photon absorption induces rapid
transfer of an electron from the central chlorophylls, and by
subsequent migration of the hole onto the donor side cofactors,
the chlorophylls are regenerated and ready to undergo another cycle
of charge separation. The resulting redox equivalents are ac-
cumulated on separate sites, storing parts of the combined energy
of the absorbed photons for use in subsequent multielectron catalytic
reactions. There are currently several strong efforts to develop
similar chemistry in human-made molecular systems by functional
mimics of this process.3-9 The target is the direct production of
solar fuels, such as molecular hydrogen or the reduction products
of carbon dioxide. An outstanding challenge in this research is to
couple the single-electron-photoinduced charge separation in a
donor-acceptor pair to multielectron catalytic reactions without
sacrificial agents other than the catalysis substrates. This calls for
the development of molecular systems that are capable of several
cycles of light absorption and charge separation before recombina-
tion occurs, leading to the accumulation of chemical redox
equivalents by analogy with the photosynthetic reaction centers.
Only a handful of molecular donor-acceptor systems with such
functionalities have been reported in the literature.5,10-13 The
majority of these systems accumulate only either electrons or holes
and rely on external sacrificial electron donors or acceptors for the
other half-reaction. This terminates the electron transfer chain
instead of providing both electrons and holes for the catalytic
reactions. Herein we report a donor-acceptor system (Scheme 1)
that gives a fully reversible two-electron charge-separated state in
high yield upon multiple photoexcitation. The system is based on
the regenerative use of a single photosensitizer, mimicking natural
photosystems, and no sacrificial reactions are required.
A general scheme for the electronic states involved in stepwise
excitation of a donor-photosensitizer-acceptor (DPA) system is
given in Figure 1. The yield of the single-electron charge-separated
state D+PA- (CSS1) is a result of kinetic competition between
the productive and unproductive reaction pathways, in which the
charge-separated state recombines and the system returns to the
ground state as indicated by dashed arrows. Further excitation of
the first charge-separated state (D+PA-) gives the D+P*A- state.
In addition to the productive pathway (1) leading to the second
charge-separated state, D2+PA2- (CSS2), a number of additional
decay pathways are available, some of which have no direct parallel
in the decay pathways from the singly excited DP*A state. For
example, in the D+P*A- state, the excited photosensitizer P* is
surrounded by both a reductant (A-) and an oxidant (D+), which
might result in reverse electron transfer from A- (2) or to D+ (3).
Additional unproductive reactions due to low-lying excited states
Figure 1. General reaction scheme for photoinduced electron transfer in a
donor-photosensitizer-acceptor (DPA) system upon single (blue) and
double (red) photoexcitation.
† Uppsala University.
‡ Chimie Et Interdisciplinarite´, Synthe`se, Analyse, Mode´lisation CNRS.
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J. AM. CHEM. SOC. 2010, 132, 17977–17979 17977

C O M M U N I C A T I O N S
or the paramagnetic nature of the A- and D+ intermediates may
also occur. The challenge these additional decay pathways present
are beautifully met in photosystem II, where intermediate acceptors
and donors between the central chlorophylls and the final donor
(the oxygen-evolving CaMn4 cluster) and acceptor (plastoquinone)
ensure that the electrons and holes are quickly removed from the
proximity of the photosensitizer. From this we learn that a viable
strategy for promoting the productive formation of the charge-
accumulated state D2+PA2- in high yield in a DPA system is to
make sure that process (1) in Figure 1 is very fast and kinetically
outcompetes the unproductive reactions.
To investigate this possibility to promote charge accumulation
upon multiple photoexcitation and electron transfer, we designed
a DPA system consisting of an oligo(triarylamine)ruthenium(II)
polypyridine dye, OTARu, anchored onto titanium dioxide nano-
particles via the carboxylate groups (see Scheme 1). The TiO2
nanoparticle acceptor was chosen because of the documented rapid
ultrafast electron injection from ruthenium(II) polypyridine dyes
and subsequent slow recombination in similar systems as well as
its ability to accept several electrons per particle.14-16 In cyclic
voltammetry of OTARu in solution (CH2Cl2, Bu4PF6 0.15 M), the
first two oxidation peaks at +0.33 and +0.58 V vs SCE were
assigned to the first and second oxidations of the OTA donor unit,
respectively;17 these are well below the RuIII/II oxidation of the
sensitizer at +0.93 V (Table S1 in the Supporting Information).
Thus, if RuIII is produced by photoinduced injection into the TiO2,
it is thermodynamically able to oxidize the OTA donor unit in two
steps upon multiple excitations.
The OTA+ and OTA2+ species are spectroscopically distinct, as
shown by chemical oxidation in solution and spectroelectrochemical
investigations on OTARu-sensitized TiO2 films (Figures S3 and
S5 in the Supporting Information). The electronic absorption of
the singly oxidized species OTA+ is characterized by positive
absorption above 435 nm with band maxima at 495 and 590 nm vs
the uncharged species. Below 435 nm, the OTA+ absorbs less than
the uncharged OTA species, resulting in net bleaching in the
difference absorption spectrum in this region. When the oligotri-
arylamine moiety is further oxidized from OTA+ to OTA2+, the
most significant changes are a shift of the bleached region to
400-500 nm with positive absorption above 500 nm and increased
intensity in the broad absorption band at 600 nm. From studies of
similar dicationic polytriarylamine structures, we can assume that
the holes are delocalized over the OTA2+ unit.16
Figure 2. Transient absorption spectra for OTARuTiO2 directly after
single-pulse (P1, black O) and double-pulse (P1 + P2, green O) excitation
(480 nm, 10 ns pulses separated by 1 µs). The double difference spectrum
(orange O) highlights the features of OTA2+ formed after the second pulse
(see the text). Difference spectra of OTA+ and OTA2+ (solid lines) obtained
from spectroelectrochemistry and linearly scaled for a best fit are shown
for comparison. Inset: Transient absorption at 620 nm after single-pulse
(black) and double-pulse (green) excitation.
well with the expected signature from the OTA+ monocation (blue
solid line in Figure 2) combined with the weak, structureless
absorption from electrons injected into the TiO2 conduction band
(∆ε800 ≈ 800 M-1 cm-1 for a 4 µm thick film).20 Approximately
30% of the dye molecules were excited and produced
OTA+RuIITiO2 after a single pulse, as estimated from the
-
magnitude of the generated OTA+ absorption relative to the initial
dye absorption (OD480 ) 0.1; Figure S4). Recombination of the
charge-separated state was found to be multiexponential (Figure 2
inset, black trace), which is typical for Ru dyes on TiO2,16 with
complete recombination on the millisecond time scale.
When a second laser pulse at 480 nm was applied 1 µs after the
first one, the regenerated Ru sensitizer was again excited and
initiated further charge separation. In comparison with the single-
excitation experiments, the transient spectrum immediately after
the second excitation pulse (green circles in Figure 2) showed
increased absorption at 600 nm along with bleaching at 400-500
nm, causing a red shift of the isosbestic point to 460 nm. The unique
features after the second excitation pulse are highlighted by the
double difference spectrum (orange circles in Figure 2), which was
obtained by subtracting 1.4 times the first pulse spectrum from
the double-pulse spectrum to remove contributions from
To investigate the formation of CSS1, the OTARu-sensitized
TiO2 film was excited using ∼120 fs laser pulses at 510 nm in a 1
M LiClO4 (propylene carbonate) electrolyte. Multiexponential
electron injection from the excited Ru moiety to the TiO2 was
observed, forming OTARuIIITiO2- within 5 ps (Figure S6).18 The
Ru sensitizer was then regenerated by hole transfer (HT) to the
OTA donor on a longer time scale; a biexponential fit yielded HT
rate constants (kHT) of (29 ps)-1 and (1.2 ns)-1 (see the Supporting
Information). This was shown by the growth of OTA+ features
concomitant with RuII ground-state recovery at 490 nm. The total
OTA+RuIITiO2 species.21 These features agree well with the
-
optical signature of OTA2+ observed upon electrolysis of the dye
on TiO2, which is also included in Figure 2. We conclude that in
the second excitation pulse, a fraction of the second charge-
separated state, OTA2+RuIITiO22-, was formed from the excited
state OTA+(RuII)*TiO2-. Hole transfer in the OTA+RuIIITiO22- state
after the second excitation was faster than the time resolution (∼15
ns) of the nanosecond experiments.22 The OTA2+RuIITiO22- product
was also observed in high-photon-flux single-pulse experiments,
where multiple excitations can occur within the same pulse (Figure
S10).
yield of charge separation was found to be very high (ΦCSS1
≈
100%), as estimated from the amplitude of the maximum OTA+
absorption (ε495 ) 5 × 103 M-1 cm-1; Figure S5) relative to the
initial ground-state bleach a few picoseconds after excitation
[∆ε495(RuIII - RuII) ≈ -1 × 104 M-1 cm-1].19 The high yield is in
agreement with the rapid kinetics of injection and hole migration,
which compete efficiently with intrinsic *RuII excited-state decay
The nanosecond transient absorption spectra shown in
Figure 2 were used to evaluate the yield of formation of the
OTA2+RuIITiO22- state. As shown above, 30% of the dye molecules
were excited and produced OTA+RuIITiO2- by a single pulse. The
two excitation pulses were equivalent in pulse intensity and duration,
and RuIIITiO2 recombination, respectively.
-
Upon nanosecond single-pulse excitation (480 nm, 10 ns pulse
duration) the obtained spectrum (shown in black in Figure 2) agreed
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C O M M U N I C A T I O N S
so the fraction of dye molecules that were excited by two photons
was in total ∼10% (0.302 ) 0.09) of the sample in the probe
volume. The fraction of OTA2+ formed after the second pulse was
∼10%, as calculated from the transient spectral amplitudes at 495
and 600 nm immediately after the second excitation using the
extinction coefficients of OTA+ and OTA2+ (Figure S5). This
minimum losses. The fundamental study of photoinduced ac-
cumulative electron transfer in model systems such as the one
described here aids our understanding of the specific challenges
that accompany accumulative charge separation, which is important
for the efficient use of photodriven multielectron processes, such
as production of solar fuels.
corresponds to a 100% quantum yield of formation (ΦCSS2
)
Acknowledgment. This work was supported by the Swedish
Research Council, the Swedish Energy Agency, the K & A
Wallenberg Foundation, COST D35, and the ANR “PhotoCumElec”
Programme.
of the second charge-separated state, OTA2+RuIITiO22-, from
OTA+(RuII)*TiO2-. This remarkably high yield is primarily ascribed
to the very fast electron injection into TiO2 [process (1) in Figure
1] that successfully competes with unproductive pathways. It should
also be noted that while the electronic coupling favors the injection
process, the electrons and holes are sufficiently decoupled in
OTA+RuIIITiO22- to prevent unproductive recombination from this
intermediate state.
Supporting Information Available: Synthesis of OTARu, experi-
mental details, and additional data. This material is available free of
charge via the Internet at http://pubs.acs.org.
The OTA2+ signature was shown to contribute to the spectral
features with an approximately constant fraction, at least up to 100
µs (Figure S11). Following the multiexponential decay at separate
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supported by the fact that the OTA2+RuIITiO2 state shows the
2-
same slow decay kinetics as the OTA+RuIITiO2- state, suggesting
that electron transfer between neighboring OTA and OTA2+ units
is very slow in spite of a moderate driving force (∆G0 ) -0.25
eV). It is therefore reasonable to assign the observed hole
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(18) TiO2 is used here to denote that stoichiometrically one electron per Ru
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excitation.
In conclusion, we have demonstrated here a system in which
the absorption of two photons leads to accumulation of two holes
at the donor site and two electrons at the acceptor site in nearly
100% yield by the regenerative use of a single photosensitizer.23
The doubly charge-separated state conserves much of the energy
of the absorbed photons, in contrast to systems that rely on sacrificial
donors or acceptors. The rapid electron injection is the key to
obtaining the doubly charge-separated state in high yield in
OTARuTiO2, avoiding the competing unproductive pathways (2)
and (3) as illustrated in Figure 1. This strategy could most likely
be incorporated in the design of other molecular systems for
photoinduced electron transfer to obtain charge accumulation with
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(21) The difference spectrum was obtained by linear subtraction of the spectrum
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A scaling factor was used to account for the singly charge-separated state
OTA+RuIITiO2 that was created from the remaining unreacted dyes in
-
the second excitation.
(22) In ultrafast double-pulse experiments, the fraction of doubly excited dyes
was too small to give a clearly distinguishable signal.
(23) It should be noted that this result is very different from that for
dye-sensitized TiO2 cells, where the holes are transferred to an external
electrolyte instead of being accumulated on a molecular unit by the
regenerative action of a sensitizer.
JA104809X
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