Stepwise Photoinduced Electron Transfer in a Tetrathiafulvalene-Phenothiazine-Ruthenium Triad

Article abstract of DOI:10.1002/ejic.201900453

A molecular triad comprising a [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) photosensitizer, a primary phenothiazine (PTZ) donor and a secondary (extended) tetrathiafulvalene (exTTF) donor was synthesized and explored by UV/Vis transient absorption spectroscopy. Initial photoinduced electron transfer from PTZ to the ³MLCT-excited [Ru(bpy)₃]²⁺ occurs within less than 60 ps, and subsequently PTZ is regenerated by electron transfer from exTTF with a time constant of 300 ps. The resulting photoproduct comprising exTTF^·+ and [Ru(bpy)₃]⁺ has a lifetime of 6100 ps in de-aerated CH₃CN at room temperature. Additional one- and two-pulse laser flash photolysis studies of the triad were performed in the presence of excess methyl viologen (MV²⁺), to explore the possibility of light-driven charge accumulation on exTTF. MV²⁺ clearly oxidized [Ru(bpy)₃]⁺ and thereby re-instated ground-state [Ru(bpy)₃]²⁺ in triads in which exTTF had been oxidized to exTTF^·+, but further excitation of the solution containing the exTTF^·+-PTZ-[Ru(bpy)₃]²⁺ photoproduct did not provide evidence for exTTF²⁺. Nevertheless, it seems that the design principle of a covalent donor-donor-sensitizer triad (as opposed to simpler donor-sensitizer dyads) is beneficial for light-driven accumulation of oxidation equivalents. These investigations are relevant in the greater context of multi-electron photoredox chemistry and artificial photosynthesis.

Full text of DOI:10.1002/ejic.201900453

DOI: 10.1002/ejic.201900453
Full Paper
Charge Accumulation
Stepwise Photoinduced Electron Transfer in a
Tetrathiafulvalene-Phenothiazine-Ruthenium Triad
Michael Skaisgirski,[a] Christopher B. Larsen,[a] Christoph Kerzig,[a] and Oliver S. Wenger*[a]
Abstract: A molecular triad comprising a [Ru(bpy)3]2+ (bpy = (MV2+), to explore the possibility of light-driven charge accumu-
2,2′-bipyridine) photosensitizer, a primary phenothiazine (PTZ) lation on exTTF. MV2+ clearly oxidized [Ru(bpy)3]+ and thereby
donor and a secondary (extended) tetrathiafulvalene (exTTF) re-instated ground-state [Ru(bpy)3]2+ in triads in which exTTF
donor was synthesized and explored by UV/Vis transient ab- had been oxidized to exTTF·+, but further excitation of the solu-
sorption spectroscopy. Initial photoinduced electron transfer tion containing the exTTF·+-PTZ-[Ru(bpy)3]2+ photoproduct did
3
from PTZ to the MLCT-excited [Ru(bpy)3]2+ occurs within less not provide evidence for exTTF2+. Nevertheless, it seems that
than 60 ps, and subsequently PTZ is regenerated by electron the design principle of a covalent donor-donor-sensitizer triad
transfer from exTTF with a time constant of 300 ps. The result- (as opposed to simpler donor-sensitizer dyads) is beneficial for
ing photoproduct comprising exTTF·+ and [Ru(bpy)3]+ has a life- light-driven accumulation of oxidation equivalents. These inves-
time of 6100 ps in de-aerated CH3CN at room temperature. Ad- tigations are relevant in the greater context of multi-electron
ditional one- and two-pulse laser flash photolysis studies of the photoredox chemistry and artificial photosynthesis.
triad were performed in the presence of excess methyl viologen
Introduction
(> 10 ns) charge accumulation in the absence of sacrificial rea-
gents,[6] sometimes exploiting proton-coupled electron transfer
(PCET),[7] or the concept of redox potential inversion.[8]
The photoinduced transfer of single electrons in donor–ac-
ceptor compounds has been thoroughly investigated for sev-
eral decades, but light-driven multi-electron transfer processes
are still very poorly understood. For small-molecule activation
and artificial photosynthesis multi-electron transfer is essential,
and therefore it is highly desirable to understand the basic prin-
ciples of this reaction type.[1] Fully integrated molecular systems
comprising covalently connected donors, sensitizers, and ac-
ceptors are ideally suited for mechanistic studies with time-re-
solved laser spectroscopy. In most cases explored to date, light-
induced charge accumulation relies on sacrificial reagents that
decompose after electron transfer,[2] but this does not permit
sustainable light-to-chemical energy conversion. However, in
absence of sacrificial reagents all photoinduced electron trans-
fer steps are reversible, and this commonly leads to a multitude
of undesired reverse electron transfers which are counter-
productive, making charge accumulation very difficult.[1,3] Two
pioneering studies reported on light-driven charge accumula-
tion in molecular systems without sacrificial reagents more than
20 years ago,[4] but the field had then been dormant until 2010
when TiO2 nanoparticles were used to facilitate accumulation
of oxidative equivalents on covalently attached donors.[5] More
recently, we and others reported on several fully integrated do-
nor-sensitizer–acceptor compounds that permitted long-lived
Primary electron–hole separation after excitation with a first
photon is usually facile, but the processes occurring after the
absorption of a second photon are often complicated, as illus-
trated by Scheme 1. In simple donor-sensitizer (D–S) com-
pounds (left part), primary photoinduced electron transfer
yields D+–S–, and the latter can donate its additional electron
to free acceptors (A) in a bimolecular reaction. When the result-
ing D+–S intermediate is excited with a second photon (line ii),
then the fastest reaction is usually oxidative quenching of the
excited sensitizer, because D+ is a strong acceptor. This step is
unproductive for charge accumulation because it occurs in the
wrong direction. In the present study we aimed to explore
whether this fundamental problem can be overcome in donor-
donor-sensitizer (D2–D1–S) compounds.
When D2 is the stronger donor than D1, then initial excitation
of S will lead to D2+–D1–S– (right part of Scheme 1, line i) in a
sequence of electron transfer steps that will be studied in detail
below. After electron transfer from S– to a first acceptor mole-
cule (A) and subsequent secondary excitation of S (line ii), elec-
tron transfer from D1 to S* could now outcompete the more
+
exergonic electron transfer from S* to D2 because of the
shorter distance associated with the desirable reductive
quenching of S* by D1 (line iii). After bimolecular electron trans-
fer between S– and a second acceptor molecule (A) (line iv) and
+
+
[a] Department of Chemistry, University of Basel,
St. Johanns-Ring 19, 4056 Basel, Switzerland
intramolecular charge-shift between D2 and D1 (line v), hole
accumulation on D2 will have occurred (line vi).
E-mail: oliver.wenger@unibas.ch
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900453.
Successful realization of this concept requires a primary do-
nor (D1) that can reductively quench the excited sensitizer, and
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Scheme 2. Molecular structures of the triad under study (a) and two reference
compounds (b, c).
been incorporated into a variety of supramolecular constructs
relevant in the greater context of molecular electronics.[14]
Scheme 1. Possible reaction pathways after primary charge-separation and
excitation with a second photon in simple donor-sensitizer (D–S) dyads (left)
and in donor-donor-sensitizer triads (right).
Results and Discussion
The exTTF-PTZ-Ru(bpy)32+ triad in Scheme 2a was synthesized
from commercial building blocks as described in detail in the
Supporting Information (SI, pages S1–S7). The final product was
characterized by NMR spectroscopy, high-resolution mass spec-
trometry, and combustion analysis (SI, page S7). The multi-step
ligand synthesis (Scheme S1) involved the preparation and iso-
lation of compounds ref-exTTF (Scheme 2b) and ref-PTZ
(Scheme 2c), which served as convenient reference substances
for electrochemical and optical spectroscopic investigations.
Cyclic voltammetry of the three compounds from Scheme 2
a stronger secondary donor (D2) that can be oxidized twice.
Moreover, the one-electron oxidized form of the secondary
two-electron donor (D2+) must be thermodynamically able to
reduce the one-electron oxidized form of the primary donor
(D1+). Ideally, all relevant intermediates and photoproducts (S*,
S–, D1+, D2+, D22+, A–) should have diagnostic spectral signatures
that can easily be detected by transient absorption spectro-
scopy, and mutual spectral overlaps should be minimal. These
combined factors represent a very stringent set of selection
criteria for the individual components of suitable D2–D1–S tri-
ads. Based on previously published electrochemical and spec- (Figure S1) was performed in de-aerated CH3CN (triad) or DMF
troscopic data, we identified the combination of a pheno- (reference substances) and provided the redox potentials in
thiazine (PTZ) primary donor (D1) with an extended tetrathia- Table 1, which are all in line with previously reported potentials
fulvalene (exTTF) unit as a secondary two-electron donor (D2), for related molecular components.[16] Based on the known
2+
2+
and a Ru(bpy)3 sensitizer (S) as a promising molecular design redox potentials for Ru(bpy)3 and an energy of 2.12 eV for its
(Scheme 2a).
photoactive 3MLCT excited state, the reduction potential of the
excited sensitizer unit (S*) is ca. 0.8 V vs. SCE. Thus, primary
There have been several prior studies of photoinduced elec-
tron and energy transfer in which either TTF-RuII dyads,[9] or
PTZ-RuII compounds have been explored,[10] but never in a
combined triad system such as that in Scheme 2a. To the best
of our knowledge, there are no prior published reports that
tested the concept outlined in Scheme 1. Furthermore, we are
unaware of previous papers reporting the light-driven charge
accumulation on exTTF, and it seems that merely one-electron
oxidation of exTTF has been achievable in photoinduced man-
ner until now.[11] However, pulse radiolysis studies provided evi-
dence for disproportionation of exTTF·+ into exTTF2+ and
electron transfer from PTZ to 3MLCT-excited Ru(bpy)3 is
2+
0
slightly exergonic (ΔGET ≈ - 0.05 eV), and charge-shift from
exTTF to PTZ·+ has a reaction free energy of ca. –0.7 eV based
on the potentials in Table 1. Furthermore, even the more chal-
lenging electron transfer from exTTF·+ to PTZ·+ (an anticipated
necessary process in the course of possible charge accumula-
tion; Scheme 1, right, line v) is exergonic by ca. 0.3 eV, hence
the thermodynamic requirements for hole accumulation are ful-
filled.
In the UV/Vis absorption spectrum of the exTTF-PTZ-
exTTF.[12] The redox chemistry of so-called π-extended versions Ru(bpy)32+ triad (Figure S2), the MLCT absorption bands of the
of TTF has received considerable attention,[13] and exTTF has sensitizer unit appear as a low-energy shoulder to more intense
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Table 1. Redox potentials (E0 in V vs. SCE) of the individual components of the triad, the two reference compounds, and the photosensitizer.
ref-exTTF[b]
ref-PTZ[b]
Ru(bpy)3
2+[a]
2+[c]
Redox couple
exTTF-PTZ-Ru(bpy)3
exTTF·+/0
exTTF2+/·+
PTZ·+/0
0.42
0.05
0.68
1.29
0.43
0.10
0.75
3+/2+
Ru(bpy)3
1.29
–1.33
2+/+
Ru(bpy)3
[a] In CH3CN. [b] In DMF. [c] From ref.[15]
exTTF-localized π–π* absorption bands. Nevertheless, selective transfer products (dotted vertical arrows). Reduction of
2+
2+
+
excitation of the Ru(bpy)3 unit is readily possible, especially Ru(bpy)3 to Ru(bpy)3 at –1.4 V vs. SCE caused an increase in
into the low-energy tail at 532 nm.
extinction at 510 nm (Figure 1b, Figure S3), in line with prior
reports – hence the observable bands at that wavelength in
Figure 1a are assigned to the one-electron reduced sensi-
tizer.[17] In the transient absorption spectra in Figure 1a, a pro-
nounced decrease of change in absorbance at wavelengths
shorter than 520 nm (accompanied by decreased signal-to-
noise ratio) can be observed, which does not follow the differ-
ence spectrum in Figure 1b. This is due to the high optical
density of the sample used for the picosecond transient absorp-
tion measurements, precluding detection at wavelengths below
500 nm.
Picosecond transient absorption studies were performed on
solutions of the triad in de-aerated CH3CN at room temperature,
using an excitation wavelength of 532 nm and a pulse duration
of ca. 30 ps. Measurements with a streak camera (see Experi-
mental Section for details) permitted simultaneous recording of
temporally and spectrally resolved data. Time-integrating be-
tween 0.5 and 3.5 ns following excitation, a fundamentally dif-
ferent ΔOD spectrum was recorded than between 1 and 6 ns
(black solid vs. gray dotted traces in Figure 1a). Both spectra
exhibit absorption bands at ca. 510 nm and 660 nm, but the
early spectrum features an additional band near 700 nm not
present in the 1–6 ns spectrum. Spectro-electrochemical experi-
ments with [Ru(bpy)3](PF6)2 (Figure 1b), ref-PTZ (Figure 1c), as
well as a nanosecond flash-photolysis experiment that pro-
duced ref-exTTF·+ (Figure 1d) were useful to assign the individ-
ual transient absorption bands in Figure 1a to different electron
The band at 660 nm is attributed to the one-electron oxid-
ized TTF donor based on the data in Figure 1d, in line with
prior studies.[12] To obtain the difference spectrum in Figure 1d,
a solution containing 30 μ
M
[Ru(bpy)3](PF6)2 and 2 m
M
ref-
exTTF in de-aerated DMF was excited at 532 nm with laser
pulses of ca. 10 ns duration (see SI page S10). Bimolecular elec-
2+
tron transfer between ref-exTTF and photoexcited Ru(bpy)3
produced a difference spectrum with overlapping contributions
from ref-exTTF·+ and Ru(bpy)3+ (Figure S4a). Subtraction of the
contribution of the latter (obtained via spectro-electrochemis-
try; Figure S3/S4b) yielded the difference spectrum shown in
Figure 1d, which is very similar to previously reported spectra
for one-electron oxidized exTTF.[12]
Lastly, the band observable near 700 nm in the 0.5–3.5 ns
spectrum of Figure 1a can be attributed to PTZ·+ based on the
spectro-electrochemical data in Figure 1c (see Figure S5 and
SI page S10), and this difference spectrum is compatible with
previously published PTZ·+ spectra.[10d,18] At longer detection
times (1–6 ns spectrum, gray dotted trace in Figure 1a), a shift
in absorption band maximum closer to 660 nm suggests that
PTZ·+ is a very short-lived intermediate whilst exTTF·+ is the
longer-lived photoproduct.
This interpretation is corroborated by the transients shown
in Figure 2. The black trace in (a) is the temporal evolution of
the absorption signal integrated between 580 and 700 nm after
excitation at 532 nm with laser pulses of ca. 30 ps duration.
This corresponds to the spectral range in which both PTZ·+ and
exTTF·+ absorb (Figure 1a/c/d). This signal exhibits an initial
rapid increase that is instrumentally limited, followed by a rise
with a time constant of ca. 300 ps. The initial rapid rise parallels
the instrumentally limited evolution of the 3MLCT absorption
features of [Ru(bpy)3]2+ in a reference sample (gray trace in Fig-
ure 2b),[19] and a time-resolved emission experiment with the
triad shows that the 3MLCT-excited state decays with instru-
mentally limited kinetics (Figure S6). Consequently, we attribute
Figure 1. (a) Transient absorption spectra of the triad in de-aerated CH3CN
after excitation at 532 nm (see Experimental Section for details). Detection
occurred by time-integration between 0.5–3.5 ns (black solid trace) and be-
tween 1–6 ns (gray dotted trace) following excitation with pulses of ca. 30 ps
duration. (b) UV/Vis difference spectrum obtained as a result of reduction of
2+
+
Ru(bpy)3 to Ru(bpy)3 in dry, de-oxygenated CH3CN (with 0.1
M TBAPF6) at
a potential of –1.4 V vs. SCE (see Figure S3). (c) UV/Vis difference spectrum
obtained as a result of conversion of the PTZ moiety of the triad to PTZ·+ in
dry CH2Cl2, using SbCl5 as chemical oxidant (see Figure S5). (d) Difference
spectrum resulting from photochemical oxidation of ref-exTTF to ref-
exTTF·+ in DMF (see Figure S4).
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the initial rapid rise in the black trace of Figure 2a to primary
3
2+
electron transfer from PTZ to MLCT-excited Ru(bpy)3 occur-
ring with a time constant shorter than 60 ps (ET1 in Scheme 3).
In related systems, similarly rapid initial electron transfers were
observed.[20] The slower rise (300 ps) can be attributed to sec-
ondary electron transfer from exTTF to PTZ·+ (ET2 in Scheme 3),
leading to the final exTTF·+-PTZ-Ru(bpy)3 photoproduct.
+
Based on the redox potentials from Table 1, that photoproduct
stores 1.76 eV of energy. Given the relatively large reorganiza-
tion energy associated with oxidation of TTF to TTF·+ and the
comparatively low driving-forces for ET1 and ET2,[21] these proc-
esses are quite rapid.
Scheme 3. Energy level diagram illustrating photoinduced charge-shift and
thermal reverse charge-shift reactions in the triad from Scheme 2a. Energy
levels were estimated based on the electrochemical data in Table 1.
In the following, we attempted to obtain evidence for charge
accumulation on the exTTF unit of our triad by using methyl
+
viologen (MV2+) as an acceptor that can oxidize Ru(bpy)3 bi-
molecularly following the initial rapid intramolecular electron
transfer sequence observed above. Toward this end, the triad
(2.5 × 10–5 MV2+
M) was excited at 532 nm in presence of 50 mM
in de-aerated CH3CN using pulses of ca. 10 ns duration. The
resulting transient absorption spectrum (Figure 3a, solid trace)
is dominated by spectral changes caused by the reduction of
MV2+ to MV·+. This is evident from the dashed trace in Figure 3a,
which is the difference spectrum obtained from electrochemical
reduction of MV2+ to MV·+ (see Figure S7). To make transient
absorption changes occurring from the triad more evident, the
two spectra in Figure 3a were scaled to an identical ΔOD value
at the MV·+ absorption maximum at 394 nm, and then the
dashed trace was subtracted from the solid trace. The resulting
difference spectrum (Figure 3b) exhibits a prominent absorp-
tion band centered around 650 nm, and based on the reference
spectrum for exTTF·+ in Figure 3c (same spectrum as in Fig-
ure 1d and S4), this can be assigned to the one-electron oxid-
ized terminal donor of the triad. Thus, the reaction sequence
up to line (ii) in the right part of Scheme 1 has indeed occurred.
Given the rapid kinetics of intramolecular photoinduced
Figure 2. Temporal evolution of the emission-corrected transient absorption
signal of exTTF-PTZ-Ru(bpy)3 in CH3CN integrated between 580 and
2+
700 nm (black trace) and [Ru(bpy)3]2+ between 550 and 680 nm (gray trace)
following excitation at 532 nm with laser pulses of ca. 30 ps duration. (b)
Transient absorption of the solution from (a) in a longer time window. Laser
excitation pulse occurs at t = 0.5 ns on the 5 ns time window and at t = 2 ns
on the 20 ns time window.
electron transfer (300 ps, see above) and assuming diffusion-
+
After ca. 1.5 ns following the excitation pulse the transient limited bimolecular electron transfer between Ru(bpy)3 and
absorption signal in Figure 2a begins to decay with a time con- 50 m
M M /
MV2+ (1.9 × 1010 –1 s–1 in CH3CN at 25 °C), E0 (MV2+
stant of 6100 ps, reflecting the return of the molecular triad to MV·+ = –0.69 V vs. SCE),[23] secondary excitation of the regener-
+
its initial state via reverse thermal charge shift from Ru(bpy)3
ated photosensitizer, Ru(bpy)32+, within the duration of the
to exTTF·+. The longest observable time window on our ps tran- same 10-ns laser pulse might be possible. This could then in-
sient absorption setup is 20 ns, and on that timescale the signal duce the sequence of reactions (iii)–(vi) in Scheme 1, leading
does not return completely to baseline (Figure 2b). On our ns ultimately to exTTF2+ (D22+ in Scheme 1). Indeed, the spectrum
flash photolysis setup, we observed a residual signal even after in Figure 3b exhibits a weak increase of absorbance at 490 nm,
more than 500 ns (not shown), and we tentatively attribute coincident with the wavelength at which the difference spec-
this to a photodegradation product. Furthermore, we cannot trum resulting from chemical oxidation of exTTF to exTTF2+
exclude the population of a (long-lived) exTTT-based triplet ex- with SbCl5 has a maximum (Figure 3d). At shorter wavelengths,
3
2+
cited state via energy transfer from MLCT-excited Ru(bpy)3
.
there are bleaches in the spectrum of Figure 3b resembling
Computational studies suggest that the lowest triplet excited those in Figure 3d, but they merely reflect disappearance of
states of TTF and PTZ lie higher in energy than the states con- neutral exTTF (Figure S2) and do not permit to distinguish be-
sidered here.[22]
tween exTTF·+ and exTTF2+
.
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fraction of primary photoproduct present, the transient absorp-
tion spectrum recorded after the second (blue) pulse (Figure
S9a) is very similar to that obtained via single-pulse excitation
(Figure 3a). Notably, the ratio between 490 and 650 nm absorb-
ance changes is essentially identical in one- and two-pulse ex-
periments (Figure S9b). Given this finding and the above-men-
tioned power-dependent studies, it seems unlikely that the
weak feature at 490 nm arises from exTTF2+, and it is more
plausible that this feature is an artefact resulting from the im-
perfect correction of the spectra in Figure 3 and Figure S9.
Conclusions
Intramolecular photoinduced electron transfer in the triad from
Scheme 2a occurs in stepwise fashion, involving oxidation of
3
2+
PTZ by MLCT-excited Ru(bpy)3 on a sub-60-ps timescale, fol-
lowed by oxidation of exTTF by PTZ·+ within 300 ps. The result-
ing photoproduct, exTTF·+-PTZ-Ru(bpy)3+, has a lifetime of
6100 ps in de-aerated CH3CN at room temperature. In presence
Figure 3. (a) Solid trace: Transient absorption spectrum recorded after 532 nm
excitation (200 ns after excitation and time-integrated over 200 ns; laser pulse
energy, 80 mJ) of exTTF-PTZ-Ru(bpy)3 (2.5 × 10–5
M) in de-aerated CH3CN
2+
+
of excess methyl viologen Ru(bpy)3 is rapidly oxidized, and
(laser pulse duration ca. 10 ns) in presence of 50 m
M MV(PF6)2. Dashed trace:
2+
there is clear evidence for exTTF·+-PTZ-Ru(bpy)3 and MV·+.
Difference spectrum resulting from reduction of MV2+ to MV·+ (see Figure
S7); scaled to the ΔOD value of the solid trace at 394 nm. (b) Difference
spectrum generated by subtracting the dashed trace in (a) from the solid
trace in (a). (c) Difference spectrum resulting from photochemical oxidation
of ref-exTTF to ref-exTTF·+ in DMF (same spectrum as in Figure 1d). (d)
Difference spectrum resulting from chemical oxidation of the exTTF moiety
of the triad to exTTF2+ in DMF (see Figure S5).
Weak transient absorption at 490 nm suggests the formation of
2+
exTTF2+-PTZ-Ru(bpy)3 resulting from twofold excitation of a
given triad molecule within the same laser pulse and reaction
along the sequence outlined in the right part of Scheme 1
(causing one-electron reduction of two MV2+ acceptor species).
However, in a two-color two-pulse flash photolysis experiment
the intensity of the signal at 490 nm does not increase relative
to the absorbance at 660 nm caused by the exTTF·+ photoprod-
uct resulting from single excitation and one-electron oxidation
of the terminal donor, and excitation-power dependent experi-
ments did not provide any evidence for exTTF2+ either. Thus,
there is likely no multi-electron transfer and charge accumula-
tion in our triad.
Nevertheless, the strategy outlined in Scheme 1 appears
promising, but more photochemically robust donor-donor-sen-
sitizer triads will be desirable for further exploration of this con-
cept. The combination with acceptors exhibiting less significant
absorption changes upon reduction would be beneficial to
more clearly detect the individual oxidation products, and to
permit more straightforward discrimination between one- and
two-electron oxidized donor species. Ultimately, it might even
be desirable to extend this concept to fully integrated (all-cova-
lent) donor-donor-sensitizer–acceptor–acceptor compounds, in
which twofold oxidation of the terminal donor and twofold re-
duction of the terminal acceptor in the same molecular con-
struct might become possible. To date, this has not been ob-
served in molecular systems, and this would certainly represent
a conceptual milestone on the way to emulating natural photo-
synthesis in artificial systems.
Excitation power dependent measurements can often help
to distinguish between photoproducts resulting from one- and
two-photon excitation processes,[6a,24] and therefore we re-
corded transient absorption spectra at two different laser pulse
energies (15 and 80 mJ). However, shapes and relative signal
intensities of the resulting spectra were completely identical.
The signal at 490 nm in Figure 3b is very weak, and in an
attempt to find stronger evidence for hole accumulation on the
exTTF unit of our triad, we performed additional two-color two-
pulse flash photolysis experiments,[25] relying on a setup devel-
oped recently.[26] Specifically, an initial 532 nm laser pulse was
followed by a secondary pulse exciting the same sample at
1
460 nm. At that wavelength near the MLCT absorption maxi-
mum of the sensitizer, absorption by exTTF, exTTF·+ and MV·+
and is minimal (Figure S2/S4/S7). Thus, both pulses predomi-
2+
nantly excited the Ru(bpy)3 sensitizer of the triad, and they
both had a duration of ca. 10 ns but occurred with a delay of
500 ns. That delay time is ideal based on a two-pulse experi-
ment monitoring kinetic transient absorption signals at the
long-wavelength absorption band of MV·+ at 610 nm (Figure
S8). We anticipated that this double excitation with a time delay
of 500 ns would lead to more efficient two-fold oxidation of
exTTF, assuming that the first pulse could induce the reaction
sequences (i)–(ii) of Scheme 1 (right-hand side) whilst the sec-
ond pulse could trigger reactions (iii)–(vi). Assuming the exclu-
sive formation of the exTTF·+-PTZ-Ru(bpy)32+/MV·+ charge-
separated state with the first laser pulse, about 21 % of all triad
molecules were converted to that state after primary excitation
at 532 nm (estimated with the well-known molar absorption
Experimental Section
1H-NMR spectra were recorded on a Bruker Avance III NMR spec-
trometer with an operation frequency of 250 or 400 MHz at 298 K.
Chemical shifts (δ) are given in ppm and were referenced on resid-
coefficient of MV·+ at 395 nm).[6c] However, despite the large ual solvent peaks.[27] Coupling constants are reported in Hz. ESI
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mass spectra were measured on a Bruker Esquire 3000 plus ion-
trap ESI-MS. High resolution ESI mass spectra were recorded by
Dr. Michael Pfeffer on a Bruker maXis 4G QTOF ESI spectrometer.
Elemental analysis was performed by Ms Sylvie Mittelheisser on a
Vario Micro Cube from Elementar.
UV/Vis absorption spectra were measured on a Varian Cary-5000
UV/Vis-NIR spectrometer. Cyclic voltammograms were recorded
with a Versastat3–200 potentiostat from Princeton Applied Re-
search. A three-electrode setup containing a glassy carbon working
electrode, a silver wire counter electrode and an SCE reference elec-
trode was used to measure the cyclic voltammograms. For spectro-
electrochemical measurements, the Cary-5000 UV/Vis-NIR spec-
trometer and the Versastat3–200 potentiostat were used in combi-
nation. Here, a platinum net was used as working electrode, a plati-
num wire served as counter electrode and an SCE as reference elec-
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(GBE02-A or GBE05-A, both from Thorlabs) to bring their diameters
to either ∼1.4 cm (blue laser; maximum laser intensity per area,
13 mJ/cm2) or ∼1.2 cm (green laser; maximum laser intensity per
area, 70 mJ/cm2). The beam expansion ensured completely homo-
geneous laser excitation in the whole detection volume (ca.
1.2 cm3).
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Emission-corrected picosecond transient absorption spectra were
measured using a TRASS instrument from Hamamatsu and a mode-
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Acknowledgments
Financial support by the Swiss National Science Foundation
through grants number 200021_178760 and 206021_157687
(to O. S. W.) and by the German National Academy of Sciences
(to C. K., postdoctoral fellowship LPDS 2017-11) is gratefully ac-
knowledged.
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Keywords: Electron transfer · Time-resolved spectroscopy ·
Donor-acceptor systems · Charge transfer · Photochemistry
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Received: April 23, 2019
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Full Paper
Charge Accumulation
Photoinduced charge-shift reactions
and the possibility of light-driven
charge accumulation were explored in
a molecular triad comprising two do-
nors and one photosensitizer.
M. Skaisgirski, C. B. Larsen,
C. Kerzig, O. S. Wenger* ................... 1–8
Stepwise Photoinduced Electron
Transfer in
a Tetrathiafulvalene-
Phenothiazine-Ruthenium Triad
DOI: 10.1002/ejic.201900453
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim