Light-Driven Electron Accumulation in a Molecular Pentad

Article abstract of DOI:10.1002/anie.201604030

Accumulation and temporary storage of redox equivalents with visible light as an energy input is of pivotal importance for artificial photosynthesis because key reactions, such as CO₂reduction or water oxidation, require the transfer of multiple redox equivalents. We report on the first purely molecular system, in which a long-lived charge-separated state (τ≈870 ns) with two electrons accumulated on a suitable acceptor unit can be observed after excitation with visible light. Importantly, no sacrificial reagents were employed.

Full text of DOI:10.1002/anie.201604030

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International Edition: DOI: 10.1002/anie.201604030
German Edition: DOI: 10.1002/ange.201604030
Solar-Energy Conversion Very Important Paper
Light-Driven Electron Accumulation in a Molecular Pentad
Margherita Orazietti+, Martin Kuss-Petermann+, Peter Hamm,* and Oliver S. Wenger*
Abstract: Accumulation and temporary storage of redox
equivalents with visible light as an energy input is of pivotal
importance for artificial photosynthesis because key reactions,
such as CO2 reduction or water oxidation, require the transfer
of multiple redox equivalents. We report on the first purely
molecular system, in which a long-lived charge-separated state
(t ꢀ 870 ns) with two electrons accumulated on a suitable
acceptor unit can be observed after excitation with visible light.
Importantly, no sacrificial reagents were employed.
M
olecular CO2 reduction and water oxidation catalysts are
commonly explored with sacrificial electron donors or accept-
ors to enable the underlying multi-electron redox chemistry,[1]
however, in view of sustainable solar-energy conversion, the
light-driven accumulation and temporary storage of redox
equivalents without the use of sacrificial reagents is highly
desirable.[2] Photoinduced electron transfer in molecular
systems has been investigated for several decades, but the
vast majority of prior studies have reported exclusively on the
transfer of single electrons.[3] Studies of photodriven charge
accumulation commonly made use of sacrificial reagents,[4]
with only a handful exceptions to that statement.[5] Nonethe-
less, until now it has not been possible to obtain long-lived
charge-separated states (t > 5 ns), in which two electrons are
accumulated on a single molecular acceptor.
Scheme 1. Molecular structures of pentad I and triad II.
The UV/Vis transient absorption spectra recorded in dry
CH3CN after pulsed excitation at l = 532 nm (SI page S10)
are compatible with the formation of TAA+ and reduced
AQ,[6] but it is difficult to distinguish between AQÀ and AQ2À
on the basis of these data. Infrared spectroscopy is signifi-
cantly better suited for this purpose.
With that goal in mind, we designed the pentad I shown in
Scheme 1. We anticipated that two electrons can be accumu-
lated on the central anthraquinone (AQ) unit after excitation
The transient IR difference spectra in Figure 1a were
measured after 0.5 ns following excitation of 1 mm solutions
of I and II in dry, de-aerated CD3CN under an inert
atmosphere. Excitation occurred at l = 415 nm selectively
2+
of the two Ru(bpy)3 (bpy = 2,2’-bipyridine) photosensitiz-
ers; the two terminal triarylamine (TAA) moieties were
expected to act as reversible (and not sacrificial) one-electron
donors. The molecular triad II served as a reference com-
pound, in which AQ can only be reduced by one electron after
photoexcitation. Synthesis procedures and product character-
ization data are in the Supporting Information (SI). Both the
UV/Vis spectra (SI page S8) and the cyclic voltammograms
(SI page S9) are indicative of weak electronic coupling
between the individual molecular components of I and II.
1
into the MLCT (MLCT= metal-to-ligand charge transfer)
manifold of the RuII photosensitizers (SI page S8) with pulses
of approximately 100 fs duration. Most of the spectral
changes observed in the n˜ = 1300–1700 cmÀ1 range are similar
for I and II, for example a prominent new absorption at
1575 cmÀ1 and a bleach at 1510 cmÀ1, both caused by
oxidation of TAA to TAA+.[7] Bleaches related to the
depletion of AQ are detected at 1322, 1600, and 1680 cmÀ1
along with new bands at around 1450 and approximately
1485 cmÀ1. The band at around 1485 cmÀ1 can clearly be
assigned to AQÀ, as evidenced by the comparison to the IR
spectro-electrochemical data shown in Figure 1b, while the
band at around 1450 cmÀ1 also might have contributions from
TAA+.[7] The most important observation in Figure 1a is
a band at 1366 cmÀ1, which does only appear for pentad I but
not for triad II. The IR spectro-electrochemical data in
Figure 1b, obtained from 5 mm 9,10-anthraquinone in water-
free CD3CN, demonstrate that this band is caused by AQ2À.
At potentials up to À1.45 V versus Fc+/Fc (Fc = [h-
(C5H5)2Fe], bands caused by AQÀ form at 1404 and
1492 cmÀ1. However, at more negative potentials these
absorptions disappear at the expense of a new band at
[*] M. Orazietti,[+] Prof. Dr. P. Hamm
Department of Chemistry
University of Zurich
Winterthurerstrasse 190, 8057 Zurich (Switzerland)
E-mail: peter.hamm@chem.uzh.ch
Dr. M. Kuss-Petermann,[+] Prof. Dr. O. S. Wenger
Department of Chemistry
University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: oliver.wenger@unibas.ch
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201604030.
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Figure 2. Temporal evolution of some of the key signals observed for
pentad I in Figure 1. The inset shows the normalized early rise of the
bands originating from AQÀ and AQ2À
.
Figure 1. a) Transient IR difference spectra measured after excitation of
1 mm solutions of pentad I and triad II in dry, de-aerated CD3CN at
415 nm with laser pulses of approximately 100 fs duration. The
excitation pulse energy was 2 mJ, the spectra were recorded 0.5 ns after
excitation. b) IR difference spectra measured after application of differ-
ent potentials to a 5 mm solution of 9,10-anthraquinone in CD3CN.
The IR spectrum measured prior to application of any potential served
as a baseline. Potentials are reported versus Fc+/Fc.
The electron-transfer sequence observed above is in line
with the energy level diagram in Scheme 2, which was
established on the basis of redox potentials determined
from cyclic voltammetry (SI page S11) and UV/Vis data.
Absorption of a single photon leads to the TAA+-RuII-AQÀ-
RuII-TAA (CSS1) state at around 1.53 eV (t = 40 ps), the
majority photo-product seen also in triad II (TAA+-RuII-
AQÀ). Absorption of two photons within the same 100 fs
pulse populates the TAA-*RuII-AQ-*RuII-TAA state at
1366 cmÀ1, which thus can be attributed to AQ2À [8]
We
.
conclude that after photoexcitation of pentad I, a charge-
separated state with two TAA+ and one AQ2À unit is formed.
By comparing the transient IR spectra with a weighted
superposition of spectro-electrochemical difference spectra of
AQÀ and AQ2À we estimate that around 15% of the excited
pentads end up in a doubly reduced state (CSS2), while 85%
end up in a singly reduced state (CSS1; SI page S12). In
contrast in the triad II, an ordinary TAA+/ AQÀ couple forms,
because there is only a single TAA donor (it is however not
clear why the second AQÀ band observed in the spectro-
electrochemical difference spectrum at 1404 cmÀ1 shows up in
the transient IR spectra only for the triad II, and not for the
pentad I).
The TAA+ signal at 1575 cmÀ1 observed after excitation of
pentad I rises with an instrumentally limited time constant of
about 10 ps (blue trace in Figure 2), indicating the formation
of the charge-separated states TAA+-RuI-AQ-RuII-TAA and
TAA+-RuI-AQ-RuI-TAA+. The reduced photosensitizer
(RuI) contributes a little bit to the absorption at 1575 cmÀ1,
hence the observation of a partial decay (ca. 15%) of the
initial signal with a time constant of 40 ps when the electron
transfer from RuI to AQ occurs.[6a] Reduction of AQ
manifests in a bleach of the signal at 1675 cmÀ1 with t =
40 ps and a rise of the signal at 1485 cmÀ1 (AQÀ) with the
same time constant (Figure 2). The AQ2À-related signal at
1366 cmÀ1 rises somewhat slower with t = 65 ps (inset of
Figure 2).
Scheme 2. Energy-level diagram established on the basis of electro-
chemical and UV/Vis data (Supporting Information page S11). Energy
estimates are accurate to approximately Æ0.1 eV.
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4.24 eV. Reductive MLCT excited-state quenching of both
photosensitizers leading to the TAA+-RuI-AQ-RuI-TAA+
state at 3.90 eV cannot be temporally resolved on our setup.
Subsequently, two-electron transfer steps from the two RuI
centers to AQ occur, going via the TAA+-RuII-AQÀ-RuI-
TAA+ state that is estimated to be at 3.48 eV to finally the
TAA+-RuII-AQ2À-RuII-TAA+ state (CSS2) at 3.56 eV. Due to
the multitude of overlapping processes, we cannot disentangle
them all, nevertheless it is reasonable to assume that the last
electron-accumulating step occurring with a 65 ps time
constant is rate-determining, because it has significantly
smaller driving-force than the preceding electron-transfer
events. Based on the redox potentials (SI page S11) that
electron-accumulating step would even be slightly endergonic
(DGET0 = 0.08 eV), but it should be kept in mind that the
energy estimates in Scheme 2 are associated with uncertain-
ties of Æ 0.1 eV.
Figure 3. Dependence of the AQ bleach at 1322 cmÀ1 and the AQ2À
absorption band at 1366 cmÀ1 on the excitation pulse energy. The data
have been averaged over various time points ranging from 1 to 100 ns
to enhance the signal-to-noise ratio. The inset shows the change in
The TAA+-RuII-AQÀ-RuII-TAA (CSS1) and TAA+-RuII-
AQ2À-RuII-TAA+ (CSS2) states then decay with time con-
stants of 980 and 870 ns, respectively. While the lifetime of
CSS1 is not surprising,[6,9] the relatively long lifetime of CSS2
is remarkable in view of the multitude of decay channels
which in principle are open to a state which is energetically
approximately 3.56 eV above the ground state. However, we
note that relaxation of CSS2 to CSS1 is associated with
optical density (OD) at 1366 cmÀ1 compared to that at 1322 cmÀ1
which reports on the bleaching of AQ and as such is used as an
,
internal standard (which is free of the uncertainty originating from, for
example, the alignment of the spatial overlap between pump- and
probe pulse). The data are fitted to a quadratic function up approx-
imately 0.03 mJ, and to a linear function beyond.
0
DGET ꢀ À2.0 eV, hence it is possible that this reaction is
decelerated by an inverted driving-force effect,[10] similar to
what we observed for the charge-recombination from CSS1 in
donor-bridge-acceptor molecules which are structurally
closely related to triad II.[6b,9]
the largest pump pulse energies tested (Ep = 2 mJ). At this
stage, we do not know what the photo-physical processes are
that give rise to both observations. Double-pulse excitation
experiments with high time-resolution, along the lines of
Ref. [2a], might elucidate these processes.
Since population of CSS2 requires absorption of two
photons, a quadratic dependence of the intensity of the signal
at 1366 cmÀ1 on excitation power is expected. This is indeed
the case, as the data in Figure 3 show. At an excitation energy
of 0.01 mJ, a bleach of the AQ band at 1322 cmÀ1 is already
detectable whereas at 1366 cmÀ1 no trace of AQ2À is
recognizable yet, indicating the exclusive formation of AQÀ
(CSS1) at the lowest pulse energy (Ep). At Ep > 0.01 mJ, the
AQ2À signal at 1366 cmÀ1 (CSS2) becomes observable, and for
Ep > 0.04 mJ the bleach at 1322 cmÀ1 and the signal at
1366 cmÀ1 increase in parallel in essentially a linear fashion.
The inset of Figure 3 highlights this behavior by showing the
expected quadratic power dependence seen up to approx-
imately 0.03 mJ. By comparing the amplitude of the transient
IR response with the spectro-electrochemical difference
spectrum (see SI Figure S4), we estimate that the fractions
of excited pentads, averaged over the excited volume,
amounts to 2% at Ep = 0.03 mJ. As an independent estimate
of the excitation probability, we can also consider the
extinction coefficient of the photosensitizer and the peak
irradiance of the pump pulse, revealing an excitation prob-
ability of 3% at Ep = 0.03 mJ (SI page S12). The second
estimate is somewhat higher, as it refers to the peak, in
contrast to the averaged, excitation probability. In any case,
the changeover to a linear power dependence (see Figure 3,
inset) at such low excitation probabilities is highly surprising.
Likely, the early changeover from a quadratic into a linear
regime has the same origin as the low maximal excitation
probability of CSS2 (15% relative to CSS1) observed even for
In summary, we have achieved long-lived (t = 870 ns)
electron accumulation in a purely molecular system without
sacrificial reagents, using visible light as the only energy input.
This observation is of key importance in the context of solar-
energy conversion, because the generation of so-called solar
fuels (e.g., H2, HCOOH, or CH3OH) from small inert
molecules, such as H2O and CO2, invariably relies on multi-
electron redox reactions. Consequently, it is important to
elucidate the basic principles of photodriven accumulation
and temporary storage of redox equivalents without relying
on sacrificial reagents. Our study reports on an important
proof-of-concept in this regard.
Acknowledgements
This research was funded by the Swiss National Science
Foundation through grant number 200021-146231/1 to O.S.W.
and through grant number CRSII2_160801/1 to P.H., as well
as by the URRP LightChEC of the University of Zꢀrich to
P.H.
Keywords: donor–acceptor systems · electron transfer ·
energy conversion · photochemistry ·
time-resolved spectroscopy
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Received: April 26, 2016
Published online: && &&, &&&&
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Solar-Energy Conversion
M. Orazietti, M. Kuss-Petermann,
P. Hamm,*
O. S. Wenger*
&&&& — &&&&
Light-Driven Electron Accumulation in
a Molecular Pentad
No sacrifice at all: Long-lived (t=870 ns)
electron accumulation has been achieved
in a molecular pentad without the use of
sacrificial reagents. This is an important
proof-of-concept for solar-energy conver-
sion and multi-electron photoredox
chemistry.
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