Charge Accumulation and Multi-Electron Photoredox Chemistry with a Sensitizer–Catalyst–Sensitizer Triad

Article abstract of DOI:10.1002/chem.201804037

Photoinduced electron transfer in donor–sensitizer–acceptor compounds usually leads to simple electron–hole pairs, and photoredox catalysis typically relies on single-electron transfer (SET) events. This work reports on a molecular triad able to accumulate two electrons on a central dibenzo[1,2]dithiin moiety flanked by two peripheral Ru^II photosensitizers. Under continuous illumination, the doubly reduced form of the dibenzo[1,2]dithiin undergoes thiolate–disulfide exchange with an aliphatic disulfide substrate, thereby acting as a two-electron catalyst after two initial SET events with triethylamine at the Ru^II sensitizers. The use of a relatively simple triad for coupling two separate SET processes to a subsequent two-electron reduction is an important conceptual advance from photoinduced SET and light-driven charge accumulation towards multi-electron photoredox catalysis. This is relevant for artificial photosynthesis and light-driven multi-electron chemistry in general.

Full text of DOI:10.1002/chem.201804037

A Journal of
Accepted Article
Title: Charge Accumulation and Multi-Electron Photoredox Chemistry
with a Sensitizer-Catalyst-Sensitizer Triad
Authors: Julia Nomrowski, Xingwei Guo, and Oliver S. Wenger
This manuscript has been accepted after peer review and appears as an
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using the Digital Object Identifier (DOI) given below. The VoR will be
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To be cited as: Chem. Eur. J. 10.1002/chem.201804037
Link to VoR: http://dx.doi.org/10.1002/chem.201804037
Supported by

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Charge Accumulation and Multi-Electron Photoredox Chemistry
with a Sensitizer-Catalyst-Sensitizer Triad
Julia Nomrowski,[a] Xingwei Guo,[a] and Oliver S. Wenger*[a]
Abstract: Photoinduced electron transfer in donor-sensitizer-
acceptor compounds usually leads to simple electron-hole pairs and
photoredox catalysis typically relies on single electron transfer (SET)
events. We report on a molecular triad able to accumulate two
electrons on a central dibenzo[1,2]dithiin moiety flanked by two
peripheral Ru(II) photosensitizers. Under continuous illumination, the
doubly reduced form of the dibenzo[1,2]dithiin undergoes thiolate-
disulfide exchange with an aliphatic disulfide substrate, thereby
acting as a two-electron catalyst after two initial SET events with
triethylamine at the Ru(II) sensitizers. The use of a relatively simple
triad for coupling two separate SET processes to a subsequent two-
electron reduction is an important conceptual advance from
photoinduced SET and light-driven charge accumulation towards
multi-electron photoredox catalysis. This is relevant for artificial
photosynthesis and light-driven multi-electron chemistry in general.
systems, it seemed logical to incorporate a catalyst flanked by
two peripheral photosensitizers in a molecular triad.
Investigations of long-range electron transfer often rely on multi-
component systems comprised of a donor (D), a photosensitizer
(PS), and an acceptor (A) to result in molecular dyads or triads
(Figure 1a).[1] Excitation of the sensitizer then commonly leads to
the formation of an electron-hole pair, and many studies focused
on obtaining such photoproducts with optimal efficiency and on
making them as long-lived as possible. Much of this research
was inspired by the primary electron transfer events occurring in
photosystem II, but it is yet a long way to artificial photosynthesis.
One fundamental problem is that simple electron-hole pairs are
not directly useable for fuel-forming reactions requiring multiple
redox equivalents.[2] Consequently, there is considerable interest
in photoinduced multi-electron transfer and the light-driven
accumulation of electrons and holes. In recent years, an
increasing number of molecular systems following the concept
illustrated by Figure 1b have been reported, in which excitation
of two photosensitizers leads to electron accumulation on a
central acceptor unit.[3] Most of these studies relied on sacrificial
donors, but there are now a handful of fully integrated (all-
covalent) systems capable of completely reversible
photoinduced multi-electron transfer.[4]
Using the accumulated redox equivalents to catalyze chemical
multi-electron conversions represents the next conceptually
important advance that needs to be taken on the way to artificial
photosynthesis (Figure 1c),[5] and for some nanoparticle or
quantum dot-based systems this has already been achieved.[6]
Building on the prior charge accumulation studies with molecular
Figure 1. (a) Donor-photosensitizer-acceptor triad for SET; (b) donor-
photosensitizer-acceptor ensemble for light-induced multi-electron transfer and
charge accumulation; (c) sensitizer-catalyst-sensitizer triad for multi-electron
photocatalysis; (d) specific system investigated in this work.
[a]
Dr. J. Nomrowski, Dr. X. Guo, Prof. Dr. O. S. Wenger
Department of Chemistry, University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: oliver.wenger@unibas.ch
Supporting information for this article is given via a link at the end of
the document.
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We chose a dibenzo[1,2]dithiin unit as a central acceptor and
catalyst (Figure 1d, red part), because this moiety is known to
facilitate the uptake of multiple electrons through redox potential
inversion (SI page S7),[7] and because its reduced (dithiolate)
form is able to undergo thiolate-disulfide exchange with aliphatic
disulfides. This combined electron-accumulating and catalyst
entity is connected on both sides to bpy (2,2’-bipyridine) ligands
that chelate [Ru(tBubpy)2]2+ fragments (tBubpy = 4,4’-di(tert-
butyl)-2,2’-bipyridine). The aim was to irradiate the resulting triad
in presence of excess electron donor (Et3N in Figure 1d), and to
investigate whether this would permit the light-driven two-
electron reduction of an aliphatic disulfide to its dithiolate form.
As a model substrate we chose trans-4,5-dihydroxy-1,2-dithiane
(green in Figure 1d, DTT(ox)) which can be reduced to 1,4-
dimercaptobutane-2,3-diol (blue in Figure 1d, DTT(red)). Whilst
this particular conversion is not of specific interest for solar
energy conversion its successful completion would represent an
important proof-of-concept, particularly given the relative
simplicity of the triad compared to previously reported donor-
sensitizer-acceptor compounds investigated in the context of
The synthesis and characterization of the triad is described in
the Supporting Information (SI, pages S2-S5). Its UV-Vis
absorption spectrum in CH3CN shows the expected MLCT and
-* absorptions of the Ru(II) sensitizers at 450 / 290 nm (Figure
2a) and a band at 330 nm caused by the dibenzo[1,2]dithiin.
Upon irradiation of the triad in de-aerated CH3CN (87 M) at 455
nm (LED, 3.5 W) in presence of 0.5 M Et3N, the spectral
changes in Figure 2b become observable. These changes signal
the
light-induced
reduction
of
the
sensitizer
and
dibenzo[1,2]dithiin units as discussed in the following. The
increased optical densities at 385 and 510 nm as well as the
bleaches at 290 / 450 nm are diagnostic for the reduced
photosensitizer, as confirmed by the spectro-electrochemical
data obtained for a [Ru(tBubpy)2(bpy)]2+ reference complex
(Figure 2c, see SI page S3 for molecular structure). However,
the difference spectra in Figure 2b exhibit an additional bleach at
330 nm that can be attributed unambiguously to the
dibenzo[1,2]dithiin reduction product based on the spectro-
electrochemical data of the isolated dibenzo[1,2]dithiin
compound in Figure 2d. Importantly, the spectral change at 330
nm reflects the two-electron reduction, which is more facile than
single reduction in this particular acceptor (SI page S7).[4g, 7]
One-electron reduction is known to lead to a disulfide radical
anion which dissociates only upon secondary reduction, thereby
releasing considerable ring tension and causing the potential
inversion in dibenzo[1,2]dithiin. Thus, based on the data in
Figure 2 we conclude that the two-electron reduced form of the
dibenzo[1,2]dithiin unit is obtained along with one-electron
reduced photosensitizer upon continuous irradiation with blue
light over several minutes (see SI page S8 for a mechanistic
discussion).
8]
reversible multi-electron transfer and charge accumulation.[4,
The triad in Figure 1d is comprised of only three components for
the coupling of two separate photoinduced SET events to an
overall two-electron reduction of an organic substrate,
complementing prior work on sensitizer-catalyst systems geared
at the photochemical H2 production or the light-driven reduction
of CO2.[9]
This electron accumulation sets the stage for multi-electron
catalysis. Aliphatic disulfides have one-electron reduction
potentials varying from -2.72 V vs. SCE for di-tert-butyl disulfide
to -1.77 V vs. SCE for 1,2-dithiolane,[10] but the one-electron
reduction potential of the [Ru(tBubpy)2(bpy)]2+ unit is only -1.36 V
vs. SCE (SI page S7). Consequently, SET from the one-electron
reduced photosensitizers to the DDT(ox) substrate should be a
disfavored reaction pathway. Moreover, DDT(ox) has a cyclic
structure that is expected to render disulfide bond cleavage
upon SET reduction entropically less favored compared to
acyclic disulfides. The combination of all these factors is
expected to favor thiolate-disulfide exchange as a reaction
pathway leading to the formation of DDT(red), and this is only
possible after initial accumulation of two electrons on the
dibenzo[1,2]dithiin. Under the photochemical conditions with
sacrificial Et3N this two-electron reduction is expected to lead to
a chemical equilibrium between dithiolate and dithiol forms of the
reduced dibenzo[1,2]dithiin, and only the former (present as a
minority species) can engage in thiolate-disulfide exchange with
the DTT(ox) substrate (see below and SI page S8). Nevertheless,
our system should be useable to test the concept illustrated in
Figure 1c.
Figure 2. (a) UV-Vis absorption spectrum of the triad from Figure 1d (87 M)
in CH3CN; (b) UV-Vis spectral changes observed in the course of irradiation of
the triad at 455 nm in presence of 0.5 M Et3N; (c) spectral changes associated
with the electrochemical one-electron reduction of [Ru(tBubpy)2(bpy)]2+ at -1.8
V vs. SCE in CH3CN; (d) spectral changes associated with two-electron
reduction of the disulfide acceptor to its dithiolate form at -2.0 V vs. SCE in
CH2Cl2, performed with the free (uncoordinated) key ligand of the triad.
However, it should be kept in mind that the oxidation of Et3N
liberates iminium ions (Et2N+=CHCH3) which can be reduced to
-alkyl amine radicals under the conditions used here, and
subsequent hydrogen atom transfer (HAT) with thiols (in
particular DTT(red)) leads back to Et3N (see SI page S9 for more
detailed discussion (eqs. S3, S4)).[11] This light-induced reverse
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reaction is expected to limit the overall attainable yield for
conversion of DTT(ox) to DTT(red) considerably. Moreover, the
chemical equilibrium between an aromatic (more electron-
deficient) dithiol (the reduced triad) and an aliphatic (more
electron-rich) dithiol (DTT(red)) is not in favor of DTT(red) formation
(SI page S9 (eq. S1)).
charge accumulation is feasible using the integrated sensitizer-
catalyst approach. This new strategy could provide access to
photochemical conversions that have so far been unattainable
with the classical SET approach.
Table 1. Turnover numbers (TONs) for the light-driven reduction of (22
mM) DTT(ox) to DTT(red) performed in de-aerated CH3CN at 22 °C under
various conditions.
entry
catalyst
Et3N
h
TON
42
0
1
2
3
4
triad
0.1 M
0.1 M
none
0.1 M
455 nm
none
triad
triad
455 nm
455 nm
0
[Ru(tBubpy)2(bpy)]2+
4
Nevertheless, photoirradiation experiments performed with 20
M triad, 22 mM DDT(ox) and 0.1 M Et3N in de-aerated CH3CN at
22 °C demonstrate that a proof-of principle is possible (Table 1,
entry 1). Following illumination with a 455-nm LED (3.5 W) over
20 hours, roughly 4% of DDT(ox) are converted to DTT(red)
according to 1H-NMR experiments (SI pages S10-S12), and this
corresponds to a turnover number (TON) of 42 (last column of
Table 1) for the triad catalyst. Control experiments performed in
absence of light or without Et3N lead to no product at all (Table 1,
Figure 3. Proposed mechanism for the photochemical conversion of DTT(ox) to
DTT(red): Electron accumulation on dibenzo[1,2]dithiin followed by dithiolate-
disulfide exchange. The dotted grey arrows mark
a counter-productive
entries
2 & 3). On the other hand, when using the
reaction. See SI pages S8-S9 (eqs. S1-S4) for further information.
[Ru(tBubpy)2(bpy)]2+ reference complex (40 M) instead of the
triad, some very modest conversion of DTT(ox) to DTT(red) is
observed (entry 4). The yield in this case is 0.7% and the TON is
4. The reaction under these conditions must be due to SET and
subsequent cleavage of the disulfide bond. After initial electron
uptake, the disulfide bond elongates and a loose radical anion is
formed with the SOMO localized at the disulfide bond, lowering
its bond dissociation energy.38-41 Cleavage of the S-S bond leads
to a thiyl radical that can then be reduced in a second step. The
use of Et3N favors that pathway to a certain extent, because the
Et3N+ oxidation product is known to react onwards as a
proton/electron donor.[12]
The key finding is that the TON for the triad is 10 times higher
than for the [Ru(tBubpy)2(bpy)]2+ reference complex, and this is
attributed to the mechanism illustrated by Figure 3. Light-driven
charge accumulation on the dibenzo[1,2]dithiin unit leads to an
aromatic dithiolate that subsequently undergoes dithiolate-
disulfide exchange with the DTT(ox) substrate. Electron
accumulation is favored by redox potential inversion and the use
of excess Et3N, but overall conversion is modest due to reverse
reactions involving the DTT(red) reduction product (see above and
SI page S9 (eqs. S3, S4); grey dotted arrows in Figure 3).
Nevertheless, the large observable difference in TONs between
the triad and the reference complex strongly suggests that the
mechanism in Figure 3 operates more efficiently than SET
pathways with our model substrate.
Acknowledgements
Funding from the Swiss National Science Foundation through
grant number 200021_178760 is gratefully acknowledged.
Keywords: electron transfer • donor-acceptor systems •
photochemistry • photocatalysis • UV/Vis spectroscopy
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Entry for the Table of Contents
COMMUNICATION
Photoinduced multi-electron transfer is
made applicable to catalysis of an
organic transformation, illustrating the
usefulness of covalent sensitizer-
catalyst constructs for initial charge
accumulation followed by organic
multi-electron photoredox chemistry.
Julia Nomrowski, Xingwei Guo, Oliver S.
Wenger*
Page No. – Page No.
Charge Accumulation and Multi-
Electron Photoredox Chemistry with
a Sensitizer-Catalyst-Sensitizer Triad
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