Photoinduced Charge Accumulation and Prolonged Multielectron Storage for the Separation of Light and Dark Reaction

Article abstract of DOI:10.1021/jacs.0c03779

The utilization of solar energy is restricted by the intermittent nature of solar influx. We present novel noble-metal free complexes that can be photochemically charged in the presence of sacrificial electron donors and remain stable in its charged form for over 14 h. This allows the doubly reduced Cu(I) 4H-imidazolate complex to be stored after photochemical charging and used as a reagent in dark reactions, such as the reduction of methyl viologen or oxygen. Combined UV-vis/EPR spectroelectrochemistry indicates that a two-electron reduction is induced by introducing sacrificial electron donors that facilitate proton-coupled electron transfer. Repeated photochemical reduction and chemical oxidation reveals that the complex retained a charging capacity of 72% after four cycles. We demonstrate a chemical system that can decouple photochemical processes from the day-night cycle, which has been a barrier to realizing utilization of solar energy in photochemical processes on a global scale.

Full text of DOI:10.1021/jacs.0c03779

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Article
Photoinduced charge accumulation and prolonged multielectron
storage for the separation of light and dark reaction
Martin Schulz, Nina Hagmeyer, Frerk Wehmeyer, Grace Lowe, Marco Rosenkranz,
Bianca Seidler, Alexey Popov, Carsten Streb, Johannes G. Vos, and Benjamin Dietzek
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03779 • Publication Date (Web): 22 Aug 2020
Downloaded from pubs.acs.org on August 23, 2020
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Journal of the American Chemical Society
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Photoinduced charge accumulation and prolonged multielectron
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†
,‡
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†
#
‖
Martin Schulz *, Nina Hagmeyer , Frerk Wehmeyer , Grace Lowe , Marco Rosenkranz , Bianca
†
‖
#
§
†,‡,⫠
Seidler , Alexey Popov , Carsten Streb , Johannes G. Vos , Benjamin Dietzek
†
Institute of Physical Chemistry, Friedrich Schiller University Jena, Helmholtzweg 4, 07743 Jena, Germany.
Leibniz Institute of Photonic Technology (IPHT), Department Functional Interfaces, Albert-Einstein-Str. 9, 07745
‡
Jena, Germany.
#
Institute of Inorganic Chemistry I, Ulm University, Albert-Einstein-Allee 11, 89081 Ulm, Germany.
‖
Leibniz Institute for Solid State and Materials Research, Helmholtzstraße 20, 01069 Dresden, Germany.
§
SRC for Solar Energy Conversion, School of chemical Sciences, Dublin City University, Dublin, Ireland.
⫠
Centre for Energy and Environmental Chemistry (CEEC), Friedrich Schiller University Jena, Philosophenweg 7a,
0
7743 Jena, Germany.
ABSTRACT: The utilization of solar energy is restricted by the intermittent nature of solar influx. We present novel noble-
metal free complexes that can be photochemically charged in the presence of sacrificial electron donors and remain stable in
its charged form for over 14 hours. This allows the doubly reduced Cu(I) 4H-imidazolate complex to be stored after
photochemical charging and used as a reagent in dark reactions, such as the reduction of methyl viologen, or oxygen.
Combined UV-Vis/EPR spectroelectrochemistry indicates that a two-electron reduction is induced by introducing sacrificial
electron donors that facilitate proton-coupled electron transfer. Repeated photochemical reduction and chemical oxidation
reveals that the complex retained a charging capacity of 72% after four cycles. We demonstrate a chemical system that can
decouple photochemical processes from the day-night cycle, which has been a barrier to realizing utilization of solar energy
in photochemical processes on a global scale.
tuning of band edge positions and band gap energies in
INTRODUCTION
Harvesting solar energy by using photochemical
reactions to produce fuels and chemicals could drastically
order to harvest visible light, rather than UV light remains a
6,7
challenge.
Metal complexes conceptually offer the possibility to
achieve efficient harvesting of visible light together with
sufficient redox potentials for subsequent discharging
1
2
reduce global CO production. However, solar driven
photochemistry is currently limited to daylight hours which
makes it inherently unsuitable for large scale continuous
industrial processes. This problem has already been solved
by nature in photosynthesis; NADPH is formed by the
reduction of NADP during a light-dependent reaction and
stored for use in light-independent reactions as a
9–12
reactions by synthetic design.
These approaches include
dinuclear rhodium complexes where the electrons are
13–15
stored on the metal centres,
as well as examples where
11,16–24
the ligands act as the storage medium.
Typically,
ruthenium polypyridyl complexes are photoexcited and
sequentially quenched by sacrificial donors to yield the
2
,3
photoredox equivalent.
To realise solar driven
photochemistry on a large scale, we also need to decouple
the reactions from the fluctuations in solar energy supply.
Mimicking nature, chemical systems must be developed,
which generate reactive photoredox equivalents that can be
stored and regenerated on demand. Artificial systems for
the storage of redox equivalents have used
polyoxometallates in an electrochemical as well as
photoelectrochemical scheme . The latter applied
tungsten oxide photoanode to harvest the visible light.
Additionally, solid state material based approaches have
been proposed for light harvesting and the storage of redox
19,25,26
multi-electron
reduced
species.
Donor-
27,28
29
30
photosensitizer-acceptor triads,
pentads or heptads
31
16
dendrimers, donor-acceptor-donor triads or sensitizer-
32
acceptor dyads were designed to study reversible
multielectron charge accumulation without sacrificial
electron donors. Typically, the photoredox equivalent
4
storage times are observed in the nanosecond-microsecond
5
a
27–32
range,
however, charge storage of at least 75 min was
30
obtained with a donor-photosensitizer-acceptor heptad.
But little attention has been paid to the consumption of
stored photoredox equivalents in dark reactions.
electron reduction has been achieved by potential inversion
through the ring opening of a strained dibenzo[1,2]dithiin.
The obtained two-electron reduced species was directly
23,33
Two-
6
,7
equivalents on the basis of metal oxides as well as carbon
6
–8
nitrides. Despite high charge storage capacities, these
solid materials mostly absorb in the UV region. The precise
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used in
a
photocatalytic transformation.23 These
this photochemical process, DMT is oxidized to DMT by the
excited complex and two protons are released followed by
•+
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contributions clearly demonstrate the feasibility of light-
induced multi-electron storage by molecular systems,
however, prolonged storage times still need to be
demonstrated to realistically address the challenges
originating from the day/night cycle, as well as the dark
consumption of stored photoredox equivalents.
4
2,43
dimerization to DMT
2
.
This means, that one proton is
released per transferred electron, available for charge
compensation. The reduced and protonated complex 1
(2 ), as proposed in Fig. 2, represents a Cu(I) 1H-
red
red
imidazolate complex. The proposed structure is in accord
with the spectroelectrochemistry data obtained in this
work (vide infra and SI) as well as with the report on the free
Herein we report the first example of noble-metal free
transition metal complexes for the accumulation, storage
and release of multiple photoredox equivalents. The Cu(I)
2
0
4
H-imidazole ligand from Beckert and coworkers . The
proposed -1 net charge of the proton-stabilised two-
electron reduced species is most probably compensated by
4
H-imidazolate complexes presented here are capable of
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storing photoelectrons for at least 14 hours prior to dark
consumption (Fig. 1). We present a molecular approach for
solar energy storage where the photosensitiser and the
charge storage entity are on the same (small) molecule. This
approach removes the need for intermolecular charge
transfer between a separate sensitizer and charge storage
entity.
+
+
2
DMTH or DMT H .
The photochemical reduction of 1 and 2 occurs slowly but
with high yields. After irradiation for ca. 8 hours in the
presence of DMT, the absorbance decreased to 0.6% (1) and
0.4% (2) of its original value, representing a photoreduction
yield better than 99 %. However, the quantum yield of the
photoreduction of 1 and 2 was estimated to be 0.08 % and
0.10 %, respectively (see SI for details). After irradiation the
red
red
solution of the photoreduced complexes 1 and 2 were
kept in the dark for ca. 14 hours during which only a
marginal increase of the absorption band was observed (e.g.
0
.6% to 2.1% for 1; cf. Fig. 2c and Fig. S36). The slight
increase of the absorption is probably due to residual trace
amounts of air dissolved in the Teflon stirring bar or to an
electron back-transfer from the photoreduced complex to
the photooxidized DMT. Control experiments with 1 and 2
carried out in the dark and in the presence of DMT did not
show spectral changes over a period of ca. 20 hours (Fig.
S20 and S39). Likewise, irradiation of 1 or 2 in the absence
of DMT did not furnish any changes of the UV-Vis absorption
spectrum (Fig. S21 and S38).
Figure 1. Presentation of the general concept of this study and
of the investigated Cu(I) 4H-imidazolate complexes 1 and 2.
The complexes 1 and 2 are photoreduced under irradiation
with visible light and in the presence of a sacrificial electron
donor. The photoreduced complexes are stable for at least 14
hours and can subsequently reduce an electron acceptor in a
dark reaction.
RESULTS AND DISCUSSION
Cu(I) 4H-imidazolate complexes have earlier been
3
4,35
introduced
and are a relatively new class of Cu(I)
3
6
coordination compounds. Cu(I) 4H-imidazolates combine
the necessary prerequisites for solar light harvesting and
intermolecular photoredox reactions, such as broad and
intense visible absorptions as well as the ability to store two
3
4,35,37
electrons.
The corresponding Ru(II) 4H-imidazolates
are also able to store electrons but possess excited state
38–
lifetimes that are too short for intermolecular reactions.
41
In this study we have investigated the complexes 1 and 2
(Fig. 1) towards their photocharging and discharging
behaviour. Complex 1 was newly synthesized and shows
improved solubility behaviour, while complex 2 was
Figure 2. Presentation of the photoreduction of the Cu(I) 4H-
imidazolate complex 1 (6 ∙ 10 mol/L) to the two-electron
photoreduced species
chromophore of 1 is drawn with bold lines. Two-electron
reduction leads to formation of the 1H-imidazolate ligand of
1
presence of DMT (2 ∙ 10 mol/L). Due to the high initial
absorbance of complex 1, the spectrum prior to irradiation was
measured in a 1 mm cuvette and multiplied with a factor of ten
to match with the spectra taken during irradiation in a 10 mm
3
4,35,37
–4
previously reported by us.
To study the photocharging
red
of Cu(I) 4H-imidazolates, the complexes 1 and 2 were
1
(proposed structure) The
photoreduced by irradiation with white LED-light
2
(
20 mW/cm ) in the presence of the sacrificial electron
4
2,43
red
donor N,N-dimethylamino-p-toluidine (DMT)
in
. a – UV-vis spectra obtained during irradiation of 1 in the
–
1
acetonitrile (see SI for experimental details and the
photoreduction of 2). As exemplarily shown for complex 1
in Figure 2, irradiation in the presence of excess DMT leads
to a dramatic decrease of the visible absorption bands
cuvette. The insets show photographs of a solution of 1 and
(
photobleach) due to the formation of the two-electron
red
1
. b - Zoom-in into the low absorbance region of graph a,
red
red
red
red
reduced species 1 (2 ) (1 (2 ) denote the proposed,
reached during irradiation. c – UV-vis spectra of the reaction
mono-protonated two-electron reduced species). During
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solution in the dark (after irradiation) showing the stability of
ceased after about 30 minutes, yielding ca. 90 % of the
initial absorbance (Fig. S22, Fig. S37). The reoxidation of 1
with molecular oxygen from air was used to exemplarily
study the stability during repeated cycles of
photoreduction/reoxidation (Fig. 4). As described above, 1
red
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1
in the dark.
Importantly, however, the photoreduced complex shows a
remarkable long-term stability. This long-term stability
allows for the harvest of photoredox equivalents following
the light reaction and makes them available for dark
reactions.
–
5
(
6 ∙ 10 mol/L) was photoreduced under irradiation for 8
hours with white LED-light in the presence of a 1000-fold
excess of DMT to accumulate photoredox equivalents.
2+
Methyl viologen (MV ) was chosen as a prototype
These photoredox equivalents could then be reacted with
electron acceptor because of the characteristic absorption
red
molecular oxygen under reoxidation of 1
to 1. The
•+ 44
bands of its one-electron reduced form (MV ). In the dark,
photoreduction/reoxidation cycle was repeated four times
Fig. 4 and Fig. S25-S32) and the progress was followed by
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sub-stoichiometric amounts of MV in acetonitrile were
2+
(
added to the solution of the photoreduced complex 1 and 2
UV-Vis absorption spectroscopy. The charge accumulated
and subsequently transferred was calculated from the
absorption data, assuming two electrons per photoreduced
(addition of 0.3 and 0.5 equivalents respectively). The
•+
characteristic absorption bands of MV
appeared
immediately at 398, 608, 664 and 735 nm together with the
absorption band of the neutral complex at ca. 500 nm (Fig.
red
complex 1 . Thereby, a charge accumulation yield of 83%
was obtained in the first and second cycle, which
3
; Fig. S36), demonstrating the transfer of photoredox
–7
red
corresponds to a charge of 57 mAs (2.9 ∙ 10 mol of 1 ). In
2
+
equivalents from the photoreduced complex to MV in a
dark reaction. Estimation of the molar ratio of the
reoxidized complex and the formed MV
the third and fourth cycle the charge accumulation
–7
red
efficiency was 72%, 49 mAs (2.6 ∙ 10 mol of 1 ). Under
strict inert conditions a photoreduction efficiency of more
than 99% can be achieved (vide supra). The observed drop
of the photoreduction yield and the somewhat slower
kinetics in the third and fourth cycle (Fig. 4) are attributed
to the formation of water as the product of oxygen reduction
and complex degradation as a result of the increasing water
concentration.
•+
gave
•+
•+
1
/MV = 0.6 ±0.1 and 2/MV = 0.8 ±0.1, which supports
the proposed re-oxidation, within the experimental error.
2+
About 20 min after the addition of MV , 6% of the initial
absorbance was recovered and 74% was recovered after 8
days (Fig. S33). However, a more elaborate experimental
setup will be necessary to study the kinetics of this
reoxidation process. This is at present under investigation.
As a control experiment, the neutral 4H-imidazolate
ligand was subjected to the same photoreduction
conditions as complex 1. Irradiation of the ligand in the
presence of DMT resulted in a decrease of the ligand
absorption band to 51% of the initial absorbance over 89
hours (Fig. S23). The aerobic reoxidation and hence the
reoccurrence of the initial absorption band ceased at 78%
of the initial absorbance. This observation indicates that the
photoreduction of the ligand is limited. The incomplete
aerobic reoxidation of the photoreduced ligand suggests
that decomposition reactions may play a significant role.
2
+
Control experiments in the absence of 1 or 2 with MV and
DMT in acetonitrile did not furnish any detectable amounts
•+
of MV neither in the dark nor under irradiation for 20
hours with white LED-light.
Figure 4. Presentation of the photoreduction/reoxidation
–
5
cycles of 1 (6 ∙ 10 mol/L) in the presence of a 1000-fold
excess of DMT: light-driven charge accumulation (8 hours) and
transfer of photoredox equivalents to oxygen (air) in a dark
process (1 hour). The theoretical maximum charge
accumulation in this experiment is at 68 mAs (horizontal black
line).
Figure 3. Presentation of the visible absorption upon addition
2+
red
of 0.3 equivalents of MV to 1 . The absorption spectrum
shows the appearance of the absorption bands of 1 together
•
+[44]
with the absorption bands of MV
at 398 nm, 608 nm,
red
2+
664 nm and 735 nm, as a result of 1 oxidation by MV . The
To obtain further insights into the light-driven electron
transfer processes, nanosecond time-resolved transient
absorption quenching studies with varying DMT
concentrations were conducted between 20 and 500 ns
photographs show the recolouration of the solution.
The photoreduced complexes 1red and 2red are also
readily reoxidised by molecular oxygen. Exposure of the
(
pumped at 530 nm) as detailed below. A detailed study of
red
red
solution of 1
or 2
to air immediately led to a re-
the excited state processes in the absence of a quencher was
colouration due to the formation of 1 or 2, respectively and
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earlier reported34 for 2 and ns time-resolved transient
these are one-electron transfer reactions, although the
1
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6
absorption data of 1 can be found in the supporting
information (Fig. 5 and Fig. S6-S7). The time-dependent
spectral profiles of 1 and 2 (Fig. 5) are similar and the
spectral profile of 2, obtained in acetonitrile in this study,
larger than ideal separations indicate that the electron
transfer reactions are not perfectly reversible. However, the
•–
voltammetry also shows that 1 is stable on a voltammetric
timescale.
resembles the one obtained earlier34 in CH
2 2
Cl . On a
For the photocharging reaction we have suggested a
nanosecond timescale and in the absence of a quencher,
both complexes 1 and 2 decay monoexponentially to the
ground state with lifetimes of 18 and 119 ns, obtained by a
global fit, respectively. Typically, in the presence of a
quencher, dynamic electron transfer quenching of excited
state molecules leads to diminished excited state lifetimes,
due to this additional decay pathway. Here a quenching
proton-coupled process, enabled by the release of protons
45,46
from the sacrificial donor.
Proton-coupled electron
transfer (PCET) is known to greatly affect redox potentials.
To investigate the effect of the protons released by the
sacrificial electron donor on the redox behaviour (vide
supra), ammonium was added as a proton source to the
supporting electrolyte during voltammetry of 1. A very
weak acid was used due to the known instability Cu(I) 4H-
0
1
2
3
4
5
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2
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0
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2
3
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7
8
9
0
–5
study was conducted, by taking ns-TA data of 1 (3 ∙ 10
–5
mol/L) and 2 (2 ∙ 10 mol/L) in the presence of 2, 25, 100,
35
imidazolates in the presence of strong acids. In the
and 1000 equivalents of DMT. Surprisingly, the addition of
DMT did neither influence the observed excited state
lifetimes nor the time-dependent spectral profile. To
exclude that this observation is due to a too low quencher
concentration, the hypothetical quenching rates and
lifetime changes were calculated using the diffusion rate
4 6
presence of NH PF the two one-electron redox waves
merge and the reduction peak shifts positive to a half peak
+
potential (Ep/2) −0.78 V vs. Fc/Fc (Fig. 6g and Fig. S10-S12).
The resulting redox couple has a much larger peak-to-peak
separation of 1.06 V and the peak height ratio for the
corresponding reduction and oxidation processes (ipa/ipc
was 0.4 (Fig. S10). Voltammograms recorded at varying
concentrations of ammonium showed correlation
)
1
0
–1 –1
constant in acetonitrile (2 ∙ 10 M s ) as a limiting value
see SI for details). The obtained quenching rates show that
(
a
an intermolecular encounter between the Cu(I) complex
and DMT is not limited by diffusion and should at least for 2
lead to a detectable decrease of the excited state lifetime
between Ep/2 of the reduction of 1 and the negative decadic
+
logarithm of the ammonium concentration, p[NH
4
] (Fig.
S11). This concentration dependence indicates a PCET-like
(
the lifetime of 1 is at the lower technical limit of the TA
process. However, a linear fit of the data gives a slope of 142
setup). At the current stage, further conclusions on the
quenching mechanism cannot be drawn and further
experiments are necessary to obtain the full picture of the
electron transfer, which might take place on even shorter
time scales. It is interesting to note that the spectral profiles
obtained in the ns time-resolved experiments at any DMT
concentration, did not show signs of the one electron
+
mV/ p[NH
4
], which is much larger than 30 mV expected for
a protonation coupled to two electron transfer steps in
46,47
+
strong acids.
4
With the weak acid NH in aprotic media,
the PCET-like behaviour is likely due to hydrogen-bonding
and association of the ammonium to the reduced complex.
Similar behaviour was reported for quinones in the
presence of weakly associating or hydrogen bonding
reduced
species
as
obtained
by
UV-Vis-
46,47
species.
Also the small ipa/ipc value can indicate a
spectroelectrochemistry experiments (cf. Fig. 6). Thus, the
one-electron reduced species did not accumulate with
appreciable amounts on the investigated time scale.
chemical step proceeding electron transfer, which could be
•–
the association of the ammonium to 1 after reduction. The
•–
•–/ 2–
merging of both waves 1/1 and 1
1 indicates a much
stronger stabilisation of the second reduction process over
2–
the first, possibly by protonation of 1 . The large peak-to-
peak separation and large positive shift of the reoxidation
support this interpretation.
UVVis-spectroelectrochemistry was carried out to further
study the reduction of 1 and the formation of a two-electron
reduced species. In the absence of protons, two distinct
absorption profiles were recorded at the potentials of the
•–
two reduction waves reflecting one- (1 ) and two-electron
2–
(
1 ) reduced species, respectively (Fig. 6b-c and Fig. S14
Figure 5. Presentation of the time-dependent spectral profile of
and S16). The absorption profile observed
1
(a) and 2 (b) obtained during a nanosecond time-resolved
spectroelectrochemically in the presence of the proton
source matches with the spectrum observed after the
second reduction in the absence of protons (cf. Fig. 6d, Fig.
S19 and SI, page S12ff, for further discussions), as well as
with the absorption spectrum obtained via photochemical
reduction in the presence of DMT.
transient absorption experiment in acetonitrile and in the
absence of a quencher.
The ground state redox behaviour of 1 was investigated
by cyclic voltammetry (voltammetry of 2 was reported
3
5
earlier ). Voltammetry of 1 was performed in acetonitrile
containing 0.1 tetrabutylammonium
(
M
The formation of a two-electron reduced, diamagnetic
hexafluorophosphate) in the absence of the sacrificial
electron donor DMT. The data showed two redox waves at
species
is
further
supported
by
EPR-
spectroelectrochemistry data: a broad unstructured EPR
signal was observed upon potential sweep to the first
reduction wave in the absence of protons, which is in line
with the formation of an organic radical (Fig. S15 and S18).
•–
•–
2–
+
–
1.43 V (1/1 ) and –1.97 V (1 /1 ) vs. Fc/Fc with peak-
to-peak separations of 79 and 92 mV, respectively (cf. Fig.
f and Fig. S8). The peak-to-peak separations indicate that
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and 0.0 V, respectively. Although these potentials are
The signal is unstructured possibly due to an unresolved
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hyperfine coupling with the four nitrogen atoms. At the
potential of the second reduction wave, this signal
decreases and reappears during the re-oxidation (Fig. S17).
In contrast, when conducting the same EPR-
spectroelectrochemical experiment in the presence of
theoretically sufficient to oxidise DMT the nature of the
intermediated species under photochemical conditions and
the respective reduction potentials are unknown. Also, the
DMT cation is less basic than ammonium and although it
•−
could electrostatically associate to 1 the protons released
by oxidative dimerization of DMT could play more of a role
in the photocharging process making it a PCET. However,
several mechanisms for the overall two-electron reduction
must be taken into consideration, yet the major pathway
remains elusive at the current stage. One possible pathway
is a PCET-like process, which progresses via a transient
intermediate with a reduction potential more positive than
the oxidation potential of DMT. This would result in a rapid
second electron transfer and the observed two-electron
photocharging. But also, a second photoreduction through
photoexcitation of the intermediate radical species is
possible as well as disproportionation of the intermediate
radical species. The observed two-electron photoreduction
likely depends on several parameters, such as light
intensity, pH or donor concentration and is subject of
ongoing work. Future mechanistic studies will provide a
more detailed insight into this. Crucially, the voltammetry
with ammonium begins to explain the differences between
the voltammetry of 1 and its photocharging behaviour, as
well as proving the importance of protons in the
photocharging mechanism.
4 6
protons (NH PF ), no EPR signal could be detected, which
indicates the formation of the two-electron reduction
product. Despite the absence of an EPR signal the fast
consumption of an intermediate one-electron reduced
species cannot be ruled out. The spectroelectrochemical
data highlight the role of the sacrificial electron donor in the
photocharging mechanism: the DMT acts both as an
electron donor and proton source.
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CONCLUSIONS
In this contribution the efficient harvesting and storing of
light energy in the form of photoredox equivalents is
described. Both, light harvesting and storage are achieved
on one molecular entity only, namely a noble-metal free
Cu(I) 4H-imidazolate complex. Spectroelectrochemistry
suggests a proton-coupled electron transfer mechanism
that drives the overall two-electron photoreduction. With
prolonged photocharge storage observed for 14 hours,
timescales are realised, that are relevant to cope with the
day/night cycle. The stored photoredox equivalents were
efficiently transferred to the electron acceptors methyl
viologen and oxygen in a dark reaction. This demonstrates
the effective decoupling of the light reaction from the dark
reaction and avails discrete control of both reactions. We
are convinced that many of the photocharge accumulating
systems known today will show prolonged storage times, as
these systems are often ground state species. The question
then is, if they are stable (or inert) enough against
decomposition or side reactions. In this work, four cycles of
photoreduction and charge transfer in the dark were
carried out and underscore the reversibility of the system.
The molecular approach of merging photosensitiser and
charge accumulation on one entity not only allows for
tailoring its properties with chemical methods, but also
enables the direct conversion and storage of solar energy
without charge transfer losses. The presented principle of
decoupling the photocharging of a multi-electron species
from dark charge transfer is anticipated to impact on the
development of solar energy conversion and storage
schemes as well as photo(redox)catalysis.
Figure 6. Presentation of the spectral changes during
–
4
(
spectro)electrochemical reduction of 1 (4 ∙ 10 mol/L; a-d).
For the electrochemical reduction shown in f and g, the
–
3
concentration of 1 was 2 ∙ 10 mol/L. See SI for further details.
•–
a - EPR signal of 1 obtained at the first reduction wave (cf. f:
•–
1
/1 ). b - absorption difference profile at the first reduction
•–
•–
wave (cf. f: 1/1 ) and c - at the second reduction wave (cf. f: 1
2
–
/1 ) in the absence of protons. d - absorption difference
red
profile at the merged reduction wave (cf. g: 1/1 ), in the
presence of protons (2.5 eq. NH
+
4
). e - inverted absorption
spectrum of 1.
Applying these (spectro)electrochemical insights to
address the two-electron photocharging behaviour it is
clear that at least the first reduction step has to occur at a
potential, which is more positive, then that of DMT (the
4
2
+
DMT oxidation potential is +0.32 V vs. Fc/Fc ). The ground
state reduction potentials neither in the presence nor in the
absence of a proton source are sufficient to oxidise DMT. It
is thus plausible to suggest that the first reduction proceeds
via a redox-active excited state probably stabilised by
protonation or hydrogen bonding. An estimation of the
excited state redox potentials from the ground state
potentials in absence of a proton source and with an excited
state energy of about 2 eV, approximated from the onset of
the visible absorption band’s red edge, gives about +0.5 V
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Supporting Information. Detailed synthetic methods and
structural characterization data, electrochemistry, UV-
Vis/EPR-spectroelectrochemistry, and ns-transient absorption
data as well as irradiation methods and data.
This material is available free of charge via the Internet at
http://pubs.acs.org
O. G.; Rajh, T.; Mujica, V.; Groy, T. L.; Gust, D.; Moore, T. A.;
Moore, A. L. A Bioinspired Redox Relay That Mimics Radical
Interactions of the Tyr–His Pairs of Photosystem II. Nat. Chem.
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AUTHOR INFORMATION
Corresponding Author
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*
martin.schulz.1@uni-jena.de.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors gratefully acknowledge the help of Peter Mons
with the implementation of the spectroscopic investigations
and Mathias Micheel for supporting the quantum yield
determination. Rainer Beckert is gratefully acknowledged for
the provision of the 4H-imidazole ligands. Funded by the
Deutsche Forschungsgemeinschaft (DFG, German Research
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SYNOPSIS TOC
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Presentation of the general concept of this study and of the investigated Cu(I) 4H-imidazolate complexes 1
and 2. The complexes 1 and 2 are photoreduced under irradiation with visible light and in the presence of a
sacrificial electron donor. The photoreduced complexes are stable for at least 14 hours and can subsequently
reduce an electron acceptor in a dark reaction.
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ACS Paragon Plus Environment

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Journal of the American Chemical Society
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ACS Paragon Plus Environment

Journal of the American Chemical Society
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40x177mm (300 x 300 DPI)
ACS Paragon Plus Environment

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Journal of the American Chemical Society
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ACS Paragon Plus Environment