Photoinduced Electron Transfer in an Anthraquinone-[Ru(bpy)3]2+-Oligotriarylamine-[Ru(bpy)3]2+-Anthraquinone Pentad

Article abstract of DOI:10.1021/acs.inorgchem.5b02757

A molecular pentad comprised of a central multielectron donor and two flanking photosensitizer-acceptor moieties was prepared in order to explore the possibility of accumulating two positive charges at the central donor, using visible light as an energy input. Photoinduced charge accumulation in purely molecular systems without sacrificial reagents is challenging, because of the multitude of energy-wasting reaction pathways that are accessible after excitation with two photons. As expected, the main photoproduct in our pentad is a simple electron-hole pair, and it is tricky to identify the desired two-electron oxidation product on top of the stronger signal resulting from one-electron oxidation.

Full text of DOI:10.1021/acs.inorgchem.5b02757

Article
pubs.acs.org/IC
Photoinduced Electron Transfer in an Anthraquinone−[Ru(bpy)3]2+−
Oligotriarylamine−[Ru(bpy)3]2+−Anthraquinone Pentad
Annabell G. Bonn,†,§ Oleksandr Yushchenko,‡ Eric Vauthey,*,‡ and Oliver S. Wenger*,†
†Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
‡Department of Physical Chemistry, University of Geneva, 30 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
S
* Supporting Information
ABSTRACT: A molecular pentad comprised of a central multielectron
donor and two flanking photosensitizer-acceptor moieties was prepared
in order to explore the possibility of accumulating two positive charges
at the central donor, using visible light as an energy input. Photoinduced
charge accumulation in purely molecular systems without sacrificial
reagents is challenging, because of the multitude of energy-wasting
reaction pathways that are accessible after excitation with two photons.
As expected, the main photoproduct in our pentad is a simple electron−
hole pair, and it is tricky to identify the desired two-electron oxidation
product on top of the stronger signal resulting from one-electron
oxidation.
detail in the Supporting Information. The final compound was
characterized by nuclear magnetic resonance (NMR) spectros-
copy, high-resolution electron spin ionization (ESI) mass
spectrometry, and by elemental analysis.
INTRODUCTION
■
Electron−hole pairs have been observed after the photo-
excitation of many different dyads,1 triads,2 tetrads,3 and
pentads.4 However, light-driven accumulation of multiple
electrons or holes on a given molecular unit is much less
straightforward, at least when aiming at charge accumulation
without using sacrificial reagents.5 At present, only a handful of
prior studies have addressed this subject successfully.6
Charge accumulation is necessary for artificial photosyn-
thesis, because fuel-forming reactions such as H2 production or
CO2 reduction require multiple redox equivalents.5b,7 There-
fore, it is desirable to gain knowledge on how light-driven
charge accumulation can be performed efficiently, ideally
without sacrificial donors or acceptors. This was the purpose
of the research presented in this paper.
The molecular pentad in Scheme 1 is comprised of a central
oligotriarylamine (OTA) donor that is easily oxidized up to
three times. The OTA multielectron donor is connected to two
[Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) photosensitizers, each of
which bears an anthraquinone (AQ) acceptor. In principle, AQ
can accept two electrons; however, here, we aimed to explore
whether photoexcitation of both [Ru(bpy)3]2+ units could lead
to a charge-separated state in which OTA has been oxidized to
its dicationic state (OTA2+) while both AQ units have been
reduced to their monoanionic form (AQ−). To the best of our
knowledge, such photodriven hole accumulation in a purely
molecular system has never been achieved without sacrificial
electron acceptors.
Electrochemistry. Cyclic voltammetry was used to
determine the reduction potentials of the individual compo-
nents of the pentad. Measurements were performed in dry
CH3CN at 20 °C and provided the results shown in Table 1.
Voltammograms and further experimental details are given in
the Supporting Information. OTA is oxidized reversibly to its
monocationic and dicationic forms at −0.01 and 0.23 V vs Fc+/
Fc, which is consistent with prior studies.6b,8 The formation of
OTA3+ at 0.65 V is quasi-reversible, as reported earlier.6b,8 The
redox potentials of the AQ and the [Ru(bpy)3]2+ components
of the pentad are similar to those found for neat anthraquinone
or [Ru(bpy)3]2+ under identical conditions.9
Based on these potentials, excitation of the pentad at one of
its two photosensitizer units should lead to a charge-separated
state comprised of OTA+ and AQ−, similar to what was
observed in structurally related triads.2l,10 The resulting AQ−−
RuII−OTA+−RuII−AQ state is expected to be ca. 1.26 eV above
the initial (ground) state. Excitation of the second photo-
sensitizer unit can then potentially lead to reduction of the
second AQ unit and oxidation of OTA+ to OTA2+. The
resulting AQ−−RuII−OTA2+−RuII−AQ− state is expected to be
ca. 2.74 eV above the ground state, which is much lower in
energy than a hypothetical AQ2−−RuII−OTA2+−RuII−AQ state
in which two electrons are located on the same AQ unit. In the
following optical spectroscopic studies, we searched for the
AQ−−RuII−OTA2+−RuII−AQ− photoproduct.
RESULTS AND DISCUSSION
■
Synthesis. The key ligand for the AQ−RuII−OTA−RuII−
AQ pentad from Scheme 1 was synthesized in 18 steps using
standard C−C and N−C coupling reactions, as described in
Received: November 27, 2015
© XXXX American Chemical Society
A
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Scheme 1. Chemical Structure of the AQ−RuII−OTA−RuII−AQ Pentad
a
Table 1. Redox Potentials (V vs. Fc+/Fc) in CH3CN
b
pentad
[Ru(bpy)3]2+
0.90
AQ
Ru(III/II)
OTA+/0
OTA2+/+
OTA3+/2+
bpy0/−
0.91
−0.01
0.23
0.65
−1.67
−1.92
−2.20
−1.25
−1.72
−1.92
−2.17
bpy0/−
bpy0/−
AQ0/−
−1.23
−1.77
c
AQ2−/−
−2.0
a
Measured in the presence of 0.1 M TBAPF6 as a supporting
b
electrolyte using scan rates of 0.1 V/s. From ref 9a for neat 9,10-
anthraquinone in DMF; potential converted from SCE to Fc+/Fc,
c
following ref 11. Irreversible.
UV−vis Spectroscopy and Chemical Oxidation. The
black trace in Figure 1a is the absorption spectrum of the
pentad in neat CH3CN at 20 °C. Absorption bands with
maxima at 455, 324, and 250 nm are attributable to
[Ru(bpy)3]2+, AQ, and OTA components, respectively; an
additional band at 290 nm is due to bpy-localized π−π*
transitions. The red trace was measured in the presence of 2000
equiv of N,N′-diphenylthiourea, which is able to form tight
hydrogen-bonds to quinone radical anions.12 We anticipated
that this might facilitate AQ reduction, potentially leading to
longer-lived charge-separated states and increased chances of
observing the desired AQ−−RuII−OTA2+−RuII−AQ− photo-
Figure 1. (a) UV-vis absorption spectra obtained from 10−5
M
solutions of the pentad in CH3CN (black trace represents data in neat
solvent; red trace represents data in the presence of 2000 equiv of
N,N′-diphenylthiourea. (b) UV-vis difference spectra measured after
addition of increasing amounts of Cu(ClO4)2 to a 10−5 M solution of
the pentad in CH3CN; the spectrum prior to adding any Cu(II)
oxidant served as a baseline. Blue trace represents data measured after
the addition of 1.0 equiv of oxidant, spectrum attributed to OTA+;
green trace represents data measured after the addition of 2.5 equiv of
oxidant, spectrum attributed to OTA2+.
product.2l,8b,13 Bronsted acids or the strong hydrogen-bond
1130 nm (attributable to OTA2+) on top of a band at 1315 nm
(caused by OTA+).
̈
donor hexafluoroisopropanol lead to oxidation of OTA in
darkness and, therefore, were not considered further.
Nanosecond Transient Absorption. A deaerated 10−5
M
The UV-vis spectral changes occurring in the course of
chemical oxidation of OTA to OTA+ and OTA2+ by Cu(ClO4)2
(E0 = 0.57 V vs Fc+/Fc in CH3CN)11,14 are shown in Figure 1b.
The UV-vis spectrum measured prior to adding any oxidant
served as a baseline. The blue and green traces were measured
after addition of 1.0 and 2.5 equiv of Cu(ClO4)2 and are
attributed to OTA+ and OTA2+, respectively, which is
consistent with prior studies.6a,b,8b,c These chemical oxidation
experiments demonstrate that OTA+ and OTA2+ are best
distinguishable from each other in the near-infrared (NIR)
spectral range. Therefore, the nanosecond transient absorption
studies reported below focused on the detection of a band at
solution of the pentad containing 2000 equiv of N,N′-
diphenylthiourea was excited with laser pulses of ∼10 ns
duration at 532 nm, resulting in the transient absorption spectra
shown in Figure 2a. The black and blue traces were obtained
using pulse energies of 10 and 60 mJ, respectively. We estimate
that this corresponds to a flux of ca. ∼4 and ∼24 photons per
molecule and pulse, respectively, and therefore multiphoton
processes should be possible.
Both transient absorption spectra in Figure 2a exhibit
prominent bands at 1280 and 410 nm, which are compatible
with the predominant formation of OTA+. An additional band
at 540 nm in transient absorption can be attributed to
B
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Transient absorption decays measured after excitation with
pulse energies of 60 mJ are biexponential (see Table 2 and
Figure S2). In principle, such behavior is compatible with two
independently decaying photoproducts such as the AQ−−RuII−
OTA+−RuII−AQ majority state and the desired AQ−−RuII−
OTA2+−RuII−AQ− state. However, the relative contributions of
the two decay components are roughly 60% (τ = 290 ns) to
40% (τ = 100 ns) at all detection wavelengths (Figure S2 in the
Supporting Information), which would imply a large con-
tribution from the desired AQ−−RuII−OTA2+−RuII−AQ− state
to the overall decay. Therefore, it seems more likely that the
biexponential decay behavior at high pulse energies is an effect
that is not directly related to the charge-accumulated state. It is
conceivable that this is a thermal effect.
Pump−pump−probe experiments in which a first excitation
pulse at 532 nm was followed by a second excitation pulse at
480 nm (with a delay time of 10−20 ns) did not provide clearer
evidence for the formation of OTA2+ either (Figure S3 in the
Supporting Information).
Figure 2. (a) Transient absorption spectra measured after excitation at
532 nm with laser pulses of ∼10 ns duration. Both spectra were time-
integrated over a detection period of 200 ns immediately after
excitation. Black trace: excitation with 10 mJ/pulse; blue trace:
excitation with 60 mJ/pulse. (b) Red trace: Difference spectrum
obtained after subtraction of the black trace in panel (a) from the blue
trace in panel (a); prior to the subtraction, the two spectra from panel
(a) were scaled to identical intensities at 1285 nm (see text for further
details). Green trace represents the UV-vis difference spectrum
associated with the formation of OTA2+ (green trace from Figure 1b).
As noted above, the desired AQ−−RuII−OTA2+−RuII−AQ−
state is 2.74 eV above the ground state, and, consequently,
many decay channels to energetically lower states do potentially
exist. Therefore, one might expect this state to be rather short-
lived, and, for this reason, transient absorption studies with
higher temporal resolution were performed.
Femtosecond Transient Absorption. Excitation of the
pentad in deaerated CH3CN at 385 nm using a setup with an
∼150 fs instrument response function16 yielded the transient
absorption spectra in Figures 3a and 3b. Differences between
the spectra obtained at low (0.4 mJ/cm2, Figure 3a) and high
excitation densities (8.0 mJ/cm2, Figure 3b) are observed
between 470 nm and 540 nm. The time evolution of the
transient absorption at low excitation density could be well
reproduced by global target analysis, assuming three consec-
utive exponential steps A → B → C → D and yielded the
species-associated decay spectra (SADS)17 shown in Figure 3c.
Precise interpretation of these species/states is not
straightforward, but step A most probably corresponds to the
3MLCT state of the [Ru(bpy)3]2+ sensitizer, although some
contribution from the triplet excited state of AQ cannot be
excluded.18 On the other hand, species C is more compatible
with AQ−−RuII−OTA+−RuII−AQ than with the desired AQ−−
RuII−OTA2+−RuII−AQ− state. Unfortunately, measurements
in the NIR spectral range, which would permit distinguishing
more clearly between between OTA+ and OTA2+, cannot be
performed on the femtosecond setup. The spectrum of state B
resembles that of state C but has a smaller amplitude. This
suggests that state B might be associated with a reaction
intermediate (for example, a charge-separated state with the
electron or the hole on the sensitizer). In any case, these data
indicate that the quenching of the excited sensitizer (A → B)
occurs within a few picoseconds, whereas the AQ−−RuII−
OTA+−RuII−AQ state (C) is populated within <400 ps.
At high excitation density, two additional steps must be
added to properly account for the observed transient
absorption dynamics, with the SADS and time constants
shown in Figure 3d. The last two species, D and E, have similar
spectra and lifetimes as B and C in the low intensity
measurements, indicating that the AQ−−RuII−OTA2+−RuII−
AQ− state is still not populated. The SADS of species A, B, and
C in the high-intensity measurements are all very similar, except
for their amplitude, and resemble the SADS of state A
measured at low intensity. This indicates that these four spectra
AQ−,10a,b,15 and, hence, it is clear that the majority photo-
product is the AQ−−RuII−OTA+−RuII−AQ state. This is not
unexpected because only a fraction of all pentads that have
absorbed a first photon will also absorb a second photon.6a−c
The transient absorption signals at 410, 540, and 740 nm
(Supporting Information) decay with a lifetime of 160 ns,
irrespective of whether N,N′-diphenylthiourea is present (Table
2 and Figure S2). Importantly, the AQ−−RuII−OTA+−RuII−
Table 2. Lifetimes of Photoproducts in Deaerated CH3CN at
20 °C
τ0 [ns]
τ1 [ns]
neat, 10 mJ
160
160
100
2000 eq thiourea, 10 mJ
2000 eq thiourea, 60 mJ
290
AQ state is formed within 10 ns, hence the possibility for
absorption of a (second) photon from this state within the
same laser pulse is indeed given.
In our search of evidence for the AQ−−RuII−OTA2+−RuII−
AQ− state, we subtracted the black trace from the blue trace in
Figure 2a, in order to obtain a difference spectrum in which the
dominant contribution from the simple AQ−/OTA+ electron−
hole pair is eliminated. For this purpose, the two spectra in
Figure 2a were scaled to equal intensities at 1285 nm, which is
an isosbestic point appearing in the course of the formation of
OTA+ and OTA2+ (Figure 1b). The result is shown as a red
trace in Figure 2b, together with the UV-vis difference spectrum
obtained for OTA2+ in the chemical oxidation experiment
(green trace from Figure 1b). We note that there is indeed
some agreement between the two difference spectra in Figure
2b, particularly in the NIR spectral range (∼1100 nm) and near
400 nm where OTA2+ has characteristic absorption bands. This
observation suggests that a small fraction of the desired AQ−−
RuII−OTA2+−RuII−AQ− state is indeed formed.
C
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Figure 3. Transient absorption spectra recorded at different delay times after excitation at 385 nm with femtosecond laser pulses at (a) low excitation
densities and (b) high excitation densities.16 (c, d) Species-associated decay spectra (SADS) obtained from global target analyses of the data in
panels (a) and (b), assuming a series of three consecutive exponential steps (panel (c)) or five consecutive exponential steps (panel (d)).
are due to the same species/state, most probably [Ru(bpy)3]2+
in the MLCT state, and that, at high excitation density, the
excitation powers in nanosecond transient absorption spectros-
copy, a weak signal that is compatible with the formation of
OTA2+ becomes detectable, suggesting that small amounts of
the desired AQ−−RuII−OTA2+−RuII−AQ− state are indeed
formed. However, it has not been possible to corroborate this
interpretation by exploring the excitation power dependence of
this weak signal, or by measuring its temporal evolution. We
conclude that there is some evidence for photodriven charge
accumulation, but the case is not clear-cut.
As noted in the Introduction, we are unaware of prior studies
that have reported on light-driven hole accumulation in purely
molecular systems without using sacrificial electron acceptors.
The reason why this is such a tricky task is the multitude of
unproductive (energy-wasting) reaction pathways that are
available after excitation with two photons. These pathways
include thermal charge-recombination, energy transfer pro-
cesses, and triplet−triplet annihilation events. Moreover, once
the AQ−−RuII−OTA+−RuII−AQ state is formed, excitation of
the second ruthenium photosensitizer could lead to reduction
of OTA+ to OTA, because OTA+ is a good electron acceptor.
Understanding the basic principles of photodriven charge
accumulation remains an important challenge, in view of
artificial photosynthesis.
3
decay of this state is not single exponential. Such dependence
on the excitation intensity is typical of exciton annihilation, and
has been observed with many multichromophoric systems and
aggregates.19 In the present case, triplet−triplet annihilation
3
between two [Ru(bpy)3]2+ units in the MLCT state is the
most probable origin of the nonexponential decay at higher
excitation density. Such process is strongly detrimental to
efficient accumulative electron transfer, but can be minimized
by using sufficiently long excitation pulses, so that the first
photoinduced charge separation process occurs before the
second sensitizer is excited. This is clearly not possible with the
100 fs pulses used here for the femtosecond measurements,
3
because quenching of the [Ru(bpy)3]2+ in the MLCT state
occurs within a few picoseconds (step A → step B in Figure
3a). In the nanosecond measurements, pulses longer by a factor
105 are used, and therefore the probability for a pentad to have
two excited [Ru(bpy)3]2+ units, none of which has yet
participated in photoinduced electron transfer, is very low,
even with 60 mJ pulses. In the latter case, even though the
energy per pulse is large, the irradiance is only 84 MW/cm2
(2.25 × 1014 photons/cm2/ps), compared to 80 GW/cm2 (1.55
× 1017 photons/cm2/ps) with the high-intensity femtosecond
measurements. This could explain why the double charge-
separated state, AQ−−RuII−OTA2+−RuII−AQ−, might be
observed in the nanosecond measurements but not in the
femtosecond studies.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02757.
SUMMARY AND CONCLUSIONS
■
Not surprisingly, photoexcitation of the pentad from Scheme 1
in CH3CN leads primarily to the population of a charge-
separated state in which one AQ is reduced and OTA is
oxidized. This state is formed rapidly (<1 ns) and it lives for
160 ns in deaerated CH3CN at 20 °C. When using high
Syntheses and product characterization data, as well as
additional electrochemical and optical spectroscopic data
(PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: eric.vauthey@unige.ch (E. Vauthey).
*E-mail: oliver.wenger@unibas.ch (O. S. Wenger).
Present Address
§Department of Chemistry, Ångstrom Laboratories, Uppsala
̈
University, Box 523, 751 20 Uppsala, Sweden.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Swiss National Science
Foundation, through Grant Nos. 200021_146231/1 and
200020_147098.
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