Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis

Article abstract of DOI:10.1021/jacs.8b12223

The hydrated electron is experiencing a renaissance as a superreductant in lab-scale reductions driven by light, both for the degradation of recalcitrant pollutants and for challenging chemical reactions. However, examples for its sustainable generation under mild conditions are scarce. By combining a water-soluble Ir catalyst with unique photochemical properties and an inexpensive diode laser as light source, we produce hydrated electrons through a two-photon mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of trifluoromethyl to difluoromethyl groups.

Full text of DOI:10.1021/jacs.8b12223

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Unexpected Hydrated Electron Source for Preparative Visible-Light
Driven Photoredox Catalysis
Christoph Kerzig,* Xingwei Guo, and Oliver S. Wenger*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
S
* Supporting Information
ABSTRACT: The hydrated electron is experiencing a
renaissance as a superreductant in lab-scale reductions driven
by light, both for the degradation of recalcitrant pollutants and
for challenging chemical reactions. However, examples for its
sustainable generation under mild conditions are scarce. By
combining a water-soluble Ir catalyst with unique photo-
chemical properties and an inexpensive diode laser as light
source, we produce hydrated electrons through a two-photon
mechanism previously thought to be unimportant for laboratory applications. Adding cheap sacrificial donors turns our new
hydrated electron source into a catalytic cycle operating in pure water over a wide pH range. Not only is that catalytic system
capable of detoxifying a chlorinated model compound with turnover numbers of up to 200, but it can also be employed for two
novel hydrated electron reactions, namely, the decomposition of quaternary ammonium compounds and the conversion of
trifluoromethyl to difluoromethyl groups.
pulsed lasers for illumination,10,27,28 so far only the latter
INTRODUCTION
■
pathway has been viable with nonpulsed light sources such as
Many important chemical reactions require very strong
reductants, but photocatalytic mechanisms driven by a single
those nowadays available in most chemistry laboratories.11−13
However, these recent studies rely on the exploitation of
anionic micelles11,12 or sophisticated Coulombic interactions13
to stabilize relevant reaction intermediates, rendering these
methods somewhat cumbersome.
visible photon can only provide limited reducing power. For
instance, the widely used Ir(ppy)3 complex is capable of
reducing aryl iodides and bromides,1 but aryl chlorides are out
of range and require multiphoton excitation processes when
using visible photons.2 In 2014, the interest in two-photon
approaches received a boost by the discovery of novel
mechanisms2,3 with far-reaching laboratory applications for
reductive transformations, in organic solvents4−9 and
water.10−13 Given that intense irradiation is required for two-
photon mechanisms to be efficient, the availability of cheap
light sources with high photon fluxes such as high-power LEDs
and cw lasers laid the grounds for the ongoing rapid
development14−25 in that research field.
•−
During the past decade, eaq has been employed for the
decomposition of persistent halogenated pollutants,29 for
nitrogen30 or carbon dioxide fixation,31 and for versatile
chemical syntheses.11−13 However, the lack of easy-to-handle
•−
visible-light driven eaq sources still hampers its broad usage.
This initiated our quest for a novel catalytic system operating
in pure water without relying on supramolecular interactions.
As we will show by addressing both detailed mechanistic
investigations and preparative photoredox catalysis (see
Supporting Information for details), a water-soluble analogue
of fac-Ir(ppy)3 (Irsppy, Figure 1c)32 has ideal properties to act
Concerning challenging reductions in aqueous solution, the
hydrated electron (eaq•−) is the most promising initiating
species, because with a standard potential as high as that of
alkali metals (−2.9 V vs. NHE)26 and a lifetime much longer
than excited singlet or doublet states can possess (∼1 μs under
conditions suitable for applications), that species provides the
perfect balance between thermodynamic and kinetic reactivity.
•−
as a photocatalyst for eaq production via the mechanism of
Figure 1b. In contrast to the long-standing opinion that pulsed
lasers (∼$20 000) with power densities of ∼100 MW cm−2 are
required for that mechanism to work,3,6,27 we will demonstrate
preparative-scale applications with an inexpensive (∼$1 000)
blue cw laser and optics for beam collimation (Figure 1d),
whose combination ensures a photon flux per area on the order
of 1 kW cm−2 (Supporting Information, section 1.2).
•−
Two basic pathways for the catalytic generation of eaq are
conceivable, which both consume two visible photons and a
sacrificial donor.3,27 Following the first light absorption event
by the catalyst, either the excited state is directly photoionized,
producing the one-electron oxidized form of the catalyst as
byproduct (Figure 1b),27 or a photoinduced electron transfer
converts the catalyst into its one-electron reduced form,
precursor (Figure 1a).3 Whereas the
•−
serving as eaq
Received: November 14, 2018
applicability of both mechanisms was demonstrated using
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.8b12223
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 1. (a) Catalytic cycle of aqueous radical anion photo-
ionization.3 (b) Catalytic generation of eaq through the visible-light
•−
driven ionization of the photocatalyst (PC) Irsppy (structure given in
(c)) with subsequent catalyst regeneration using a sacrificial donor
Dsac. (d) Photograph showing the experimental conditions required
•−
for the novel eaq production system; the key point is the beam
collimation of a blue continuous wave (cw) laser (447 nm, 1 W) to a
spot smaller than 1 mm2 (as visualized by the catalyst emission). For
further explanations, see text and Supporting Information (section
1.2).
RESULTS AND DISCUSSION
■
Mechanistic Investigations. Our trisulfonated Irsppy is a
perfect water-soluble stand-in for its famous parent compound
fac-Ir(ppy)3,33 as the photophysical properties presented in
Figure 2a demonstrate: its ground state absorbs below 500 nm
(cyan trace), the 3MLCT shows characteristic green
luminescence (light green trace) with a very high quantum
yield, and the emissive excited state decays with a lifetime of
∼1.6 μs, which is shown in the inset of Figure 2a by time-
resolved luminescence (510 nm), excited-state absorption (367
nm), and ground-state bleaching measurements (278 nm).
Beyond these steady-state and lifetime experiments, we
carefully corrected and calibrated the excited-state absorption
spectrum of Irsppy (orange trace in Figure 2a; see Supporting
Information for details), which forms the basis for quantitative
measurements.
Figure 2. Photochemical properties of Irsppy in its ground and
excited state (panel a); two-pulse experiments on the photoionization
mechanism (Figure 1b) of Irsppy (panel b); and unambiguous
•−
identification of eaq as product by monitoring its reaction with
chloroacetate ClAc− (panel c). All experiments carried out in Ar-
saturated aqueous solutions containing 50 μM Irsppy. (a) Calibrated
absorption and corrected luminescence spectra; wavelengths used for
excitation (laser flash photolysis, 430 and 532 nm; prep. photolysis,
447 nm) indicated as vertical lines. Inset: Kinetic traces upon
excitation with 430 nm laser pulses (5 mJ). (b) Kinetic data for
excited Irsppy (upper traces, left y-axis) and eaq•− (lower traces, right
y-axis) in a representative two-pulse experiment (pulse scheme above
traces) with the second laser (135 mJ) blocked (cyan) or unblocked
Thermodynamic considerations (Supporting Information,
section 3) predict the photoionization (i.e., the light-driven
release of eaq•−) of excited Irsppy (3Irsppy) to be more
favorable than that of excited ruthenium(tris)bipyridine34
3
[Ru(bpy)3]2+ by ∼0.6 eV. The ionization of [Ru(bpy)3]2+
requires highly energetic photons (λ < 400 nm),3,35,36 and
therefore we anticipated that the absorption of a second low-
energy photon by 3Irsppy might result in efficient eaq
•−
•−
(blue). (Inset) Excited Irsppy bleaching/eaq formation (blue) at
formation via an “all-visible” biphotonic ionization mechanism
(mechanism of Figure 1b). To test this, we performed two-
pulse laser flash photolysis23,28,37,38 using a pulse sequence as
illustrated at the top of Figure 2b. An initial 430 nm pulse was
followed by a 532 nm pulse (delayed by 450 ns). This allowed
the generation of 3Irsppy with the first pulse, while the second
different intensities of the second laser and reference reaction
(excitation of [Ru(bpy)3]2+, orange) used for relative actinometry,
both relative to the prepulse concentration of the respective signal
•−
precursor. (c) Experimental eaq decay (dots) in the presence of
variable amounts of ClAc− after excitation with the pulse scheme
shown above the traces. (Inset) Corresponding Stern−Volmer plot.
For further details, see text and Supporting Information.
3
pulse (532 nm) selectively excites that Irsppy photoproduct
B
DOI: 10.1021/jacs.8b12223
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Figure 3. Lab-scale reactions carried out with the new blue-light driven eaq•− source in Ar-saturated solution with the pertinent conditions given at
the respective equations. (a) Detoxification of ClAc−. (b) Monodefluorination of 4-(trifluoromethyl)benzoate. (c) Degradation of the
benzyltrimethylammonium cation. Quantitative NMR measurements (ClAc− and benzyltrimethylammonium cation, 1H NMR; 4-
(trifluoromethyl)benzoate, 19F NMR) are displayed before (upper trace) and after (lower trace) irradiation; the same color codes as in the
reaction equations have been used. For further details, see the text and Supporting Information, sections 1 and 7.
(vertical lines in Figure 2a; Supporting Information, pp S12−
With the insights gained in both the ionization mechanism
and the catalyst stability, the lab-scale application of our novel
eaq source to challenging reductions became a realistic
3
S15). The second pulse brings about Irsppy bleaching on a
•−
large scale (Figure 2b, blue trace), as is evidenced by two-pulse
experiments with luminescence detection (λ = 510 nm),
because these measurements are able to monitor Irsppy (the
objective. With an excited-state oxidation potential of −1.64 V
vs NHE, 3Irsppy is itself capable of reducing numerous
compounds. In addition to imine reduction,32 we observed the
direct reductive acetophenone activation with a rate constant
close to the diffusion limit and the preparative dehalogenation
of 4-chlorobenzoic acid (Supporting Information, p S9), i.e.,
3
proposed intermediate in our consecutive two-photon
mechanism) without interferences from other species (see
section 6 of the Supporting Information for details).
Monitoring additional transient absorptions under the same
experimental conditions revealed the appearance of a new
second-pulse induced species absorbing in the red and
possessing a 1.4 μs lifetime. Quenching experiments of the
absorptions of this species with chloroacetate (ClAc−, Figure
2c) yielded a rate constant (1.15 × 109 M−1 s−1) practically
identical with that previously observed for the reaction
•−
two reactions that hitherto required eaq to be initiated in
aqueous solution.10,12 On these grounds, we carefully
performed control experiments (see Supporting Information)
with all substrates of Figure 3 and confirmed that catalyst-
derived species do not contribute to the observed lab-scale
transformations.
26
•−
between eaq and ClAc−, which is a reliable reference
Applications of the Novel Hydrated Electron Source.
We first investigated the preparative dehalogenation of ClAc−
using a collimated 447 nm diode laser (Supporting
Information, section 7), which can excite both Irsppy and
3Irsppy (blue vertical line in Figure 2a), and employed
ascorbate (HAsc−) because that sacrificial donor is known to
reduce the one-electron oxidized catalyst,32 thereby completing
the catalytic photoionization mechanism of Figure 1b.
1H NMR studies revealed the successful catalytic decom-
position of 66% (Figure 3a, lower limit due to interferences
with other NMR signals) of that model compound for
recalcitrant and toxic chloro-organics,10,27,29 together with
the formation of acetic acid as main product (Supporting
Information, section 7.1). Because no species in our system
other than eaq•− is able to reduce nonactivated chloro-organics,
reaction.11,28,29 Control experiments established that Irsppy
3
itself does not react with chloroacetate (Supporting
Information, last paragraph of section 5). All these results,
together with additional two-pulse experiments with spectral
detection (Supporting Information, section 6), unambiguously
identify the new species as eaq•−. By changing the energy of the
second pulse with all other parameters unmodified, the
•−
intensity dependence of eaq formation was obtained. That
dependence (Figure 2b, inset) exhibits a linear low-intensity
3
regime indicating the Irsppy ionization to be monophotonic.
Relative actinometry with the excitation of [Ru(bpy)3]2+ as
reference28,35 (Supporting Information, section 6) gave a
quantum yield of 1.3% for the green-light ionization of 3Irsppy.
Compared to a two-photon mechanism with quasi-persistent
radical anions as intermediates (Figure 1a), the mechanism of
Figure 1b with rather short-lived excited states unavoidably
militates for many unproductive catalyst excitations. This
prompted us to study the stability of our catalyst. In
comparative stability assays using blue light (Supporting
Information, section 2), Irsppy turned out to be much more
photostable than the widely employed catalyst [Ru(bpy)3]2+.
•−
the reaction must occur via the eaq induced dissociative
electron transfer.29 That reasoning is further substantiated by
the electrochemical detection of chloride ions corresponding
to 76% conversion. Furthermore, we carried out chloroacetate
degradation experiments at different excitation power densities.
The photon flux per area was modified by either changing the
beam size (Supporting Information, section 1.2) or the total
C
DOI: 10.1021/jacs.8b12223
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
output of our light source (Supporting Information, section 7.1
and Figure S11). Both sets of experiments confirmed the
biphotonic character (mechanism of Figure 1b) of the catalytic
photoreaction.
treatment) causes accumulation of QACs in the environ-
ment.45 Although QACs have long been known to be
environmentally risky, the quest for efficient QAC elimination
procedures is still ongoing. Using the benzyltrimethylammo-
nium cation, which is the basic structure of many QACs, we
observed very efficient degradation with our photocatalytic
system (Figure 3c and Supporting Information). We regard
We next tested triethanolamine (TEOA) as sacrificial donor,
because that donor is not prone to fast oxidation in solution
facilitating the experimental procedure. Under standardized
conditions, we indeed observed efficient ClAc− dechlorination
with a TON as high as 203, whereas in experiments with
[Ru(bpy)3]2+ there is no ClAc− conversion at all, only rapid
catalyst decomposition (Supporting Information, section 7.1).
These experiments clearly demonstrate that the combination
of a highly stable photocatalyst with favorable thermodynamic
properties and a collimated cw laser allows production of the
•−
our novel lab-scale approach relying on eaq generated with
visible photons as a potentially very interesting alternative to
gamma-ray45−47 or UVC-photon48,49 induced degradation
methods for benzyltrialkylammonium compounds. Traditional
monophotonic ionizations of inorganic anions with wave-
lengths around 250 nm29,50 would not be applicable to the
conversion of both phenyl-containing substrates presented
herein, because they are efficient light blockers below 300 nm.
Compared to the only other system that is able to produce
•−
superreductant eaq for laboratory applications. Compared to
other photoredox studies relying on two-photon excitation, our
TON of 203 is encouraging. For reference, dehalogenation
reactions with Rhodamine 6G5 or perylene diimide (PDI)2
seemed to require catalyst loadings of at least 5%, limiting the
maximum achievable TON to 20.
•−
eaq with visible photons from a nonpulsed light source in
homogeneous aqueous solution,13 our new eaq source has
•−
despite operating through a different mechanismboth similar
performance for applications and setup costs (with our setup
being cheaper by ∼50%). Furthermore, both systems rely on a
tailor-made metal complex catalyst. The more attractive
reaction conditions of our system (pH range from 7 to 10 vs
strongly alkaline solutions13 even able to attack glass),
however, offer a decisive advantage. Under acidic conditions,
the hydrated electrons would be quenched by protons, yielding
less-reactive hydrogen atoms.
Subsequently, we turned to the activation of trifluoromethyl
arenes, which is currently of significant interest in pharma-
ceutical research.39 In contrast to the only photoredox strategy
available for that task,40 an eaq•− based approach for the direct
reduction would likely not be limited to activated trifluoro-
methyl arenes with strongly electron-withdrawing groups. Our
first try with the model compound 4-(trifluoromethyl)-
benzoate afforded a conversion of 66% but yielded only traces
of the desired monodefluorinated product. However, by simply
switching off the light once the maximum concentration of 4-
(difluoromethyl)benzoate is reached, quite promising results
(Figure 3b) were obtained given the well-known selectivity
problems of such reactions.36 Simultaneously, we upscaled the
reaction and extracted a crude product (30 mg) containing 4-
(trifluoromethyl)benzoate and 4-(difluoromethyl)benzoate in
a 6/1 ratio (Supporting Information, section 7.2), showing the
usefulness of our method for larger-scale applications. The
reaction yield is still low, but the proof-of-concept is now
made. Separation of that mixture (compare main plot of Figure
S12) was not carried out. However, the isolation of 4-
(difluoromethyl)benzoic acid might be feasible via column
chromatography41 or preparative high-performance liquid
chromatography (HPLC).42 Given that difluoromethyl
arenesif commercially available at allare usually much
more expensive than the corresponding trifluoromethyl
derivatives, both the further optimization of our catalytic
system and its application to other defluorination reactions
seem worthy of further investigation in future studies.
Diode lasers currently have practically the same acquisition
costs as comparable high-power LEDs suitable for immediate
laboratory use (equipped with housing, cooling unit, and
power supply). Moreover, the usage of diode lasers operating
in continuous wave (cw) mode is comparable to that of an
LED, because cw lasers do not suffer from the health and safety
hazards connected with pulsed lasers.
CONCLUSIONS
■
In summary, we have developed a new system for the catalytic
generation of the superreductant eaq under very attractive
•−
conditions and demonstrated its lab-scale application to
challenging reductions of relevance for pharmaceutical research
and for the degradation of environmentally problematic
detergent components. Operating in pure water while only
consuming blue photons from an inexpensive light source and
•−
an extremely cheap sacrificial donor, our novel eaq source
could further contribute to the ongoing rethinking of using
green solvents such as water.51,52 Given the current interest in
two-photon phenomena, we anticipate that our first photo-
redox applications with a nonpulsed light source through the
mechanism presented hereinthe consecutive absorption of
two photons with an excited state as intermediatehave
important implications for future directions in harnessing light
for challenging chemical reactions. Following the recent surge
of interest in replacing precious one-photon catalysts,53−55 we
think that our first example for the mechanism of Figure 1b,
which uses a precious Ir-based complex, could initiate the
search for alternative (cheaper) photocatalysts with properties
suitable for the consecutive two-photon absorption.
Prior spectroscopic investigations using pulse radiolysis gave
a rate constant for the direct reduction of trifluoromethyl-
43
benzene by eaq as high as 1.8 × 109 M−1 s−1. Hence, the
•−
•−
eaq induced activation (with subsequent defluorination)
should in principle be applicable to a very broad range of
trifluoromethyl arenes. Another attractive feature of our
method is that deuteration12,44 of intermediate radicals is
feasible when the reactions are carried out in heavy water,
which is illustrated by the 1:1:1 triplets of both the CH2 NMR
signal in Figure 3a and the CF2 NMR signal in Figure 3b (inset
of the figure).
Finally, we addressed the reduction of quaternary
ammonium compounds (QACs). QACs are large-scale
industrial products with many applications; however, their
stability (hampering the degradation during wastewater
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b12223.
D
DOI: 10.1021/jacs.8b12223
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Remediations with a Visible LED or Even Sunlight. Chem. - Eur. J.
2018, 24, 17557−17567.
(14) Haring, M.; Perez-Ruiz, R.; Jacobi von Wangelin, A.; Díaz, D.
D. Intragel Photoreduction of Aryl Halides by Green-to-Blue
Upconversion under Aerobic Conditions. Chem. Commun. 2015, 51,
16848−16851.
(15) Aguirre-Soto, A.; Kaastrup, K.; Kim, S.; Ugo-Beke, K.; Sikes, H.
D. Excitation of Metastable Intermediates in Organic Photoredox
Catalysis: Z-Scheme Approach Decreases Catalyst Inactivation. ACS
Catal. 2018, 8, 6394−6400.
General experimental details, additional results and
control experiments, further laser flash photolysis data,
and application-related details (PDF)
́
̈
AUTHOR INFORMATION
■
Corresponding Authors
*christoph.kerzig@unibas.ch
*oliver.wenger@unibas.ch
ORCID
́
(16) Lopez-Calixto, C. G.; Liras, M.; de la Pena O’Shea, V. A.;
̃
Xingwei Guo: 0000-0002-5461-0360
Oliver S. Wenger: 0000-0002-0739-0553
́
Perez-Ruiz, R. Synchronized Biphotonic Process Triggering C-C
Coupling Catalytic Reactions. Appl. Catal., B 2018, 237, 18−23.
(17) Slanina, T.; Oberschmid, T. Rhodamine 6G Radical: A Spectro
(Fluoro) Electrochemical and Transient Spectroscopic Study.
ChemCatChem 2018, 10, 4182−4190.
Notes
The authors declare no competing financial interest.
(18) Marchini, M.; Gualandi, A.; Mengozzi, L.; Franchi, P.; Lucarini,
M.; Cozzi, P. G.; Balzani, V.; Ceroni, P. Mechanistic Insights into
Two-Photon-Driven Photocatalysis in Organic Synthesis. Phys. Chem.
Chem. Phys. 2018, 20, 8071−8076.
(19) Kerzig, C.; Wenger, O. S. Sensitized Triplet−Triplet
Annihilation Upconversion in Water and Its Application to Photo-
chemical Transformations. Chem. Sci. 2018, 9, 6670−6678.
(20) Kobayashi, Y.; Mutoh, K.; Abe, J. Stepwise Two-Photon
Absorption Processes Utilizing Photochromic Reactions. J. Photochem.
Photobiol., C 2018, 34, 2−28.
(21) Seegerer, A.; Nitschke, P.; Gschwind, R. M. Combined In Situ
Illumination-NMR-UV/Vis Spectroscopy: A New Mechanistic Tool
in Photochemistry. Angew. Chem., Int. Ed. 2018, 57, 7493−7497.
(22) Christensen, J. A.; Phelan, B. T.; Chaudhuri, S.; Acharya, A.;
Batista, V. S.; Wasielewski, M. R. Phenothiazine Radical Cation
Excited States as Super-Oxidants for Energy-Demanding Reactions. J.
Am. Chem. Soc. 2018, 140, 5290−5299.
(23) Fujitsuka, M.; Majima, T. Reaction Dynamics of Excited
Radical Ions Revealed by Femtosecond Laser Flash Photolysis. J.
Photochem. Photobiol., C 2018, 35, 25−37.
(24) Hu, A.; Chen, Y.; Guo, J.-J.; Yu, N.; An, Q.; Zuo, Z. Cerium-
Catalyzed Formal Cycloaddition of Cycloalkanols with Alkenes
through Dual Photoexcitation. J. Am. Chem. Soc. 2018, 140,
13580−13585.
(25) Rauch, M. P.; Knowles, R. R. Applications and Prospects for
Triplet−Triplet Annihilation Photon Upconversion. Chimia 2018, 72,
501−507.
(26) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B.
Critical Review of Rate Constants for Reactions of Hydrated
Electrons, Hydrogen Atoms and Hydroxyl Radicals in Aqueous
Solution. J. Phys. Chem. Ref. Data 1988, 17, 513−886.
(27) Kohlmann, T.; Naumann, R.; Kerzig, C.; Goez, M. 3-
Aminoperylene and Ascorbate in Aqueous SDS, One Green Laser
Flash... and Action! Sustainably Detoxifying a Recalcitrant Chloro-
Organic. Photochem. Photobiol. Sci. 2017, 16, 1613−1622.
(28) Kerzig, C.; Goez, M. Combining Energy and Electron Transfer
in a Supramolecular Environment for the “Green” Generation and
Utilization of Hydrated Electrons through Photoredox Catalysis.
Chem. Sci. 2016, 7, 3862−3868.
ACKNOWLEDGMENTS
■
Financial support provided by the German National Academy
of Sciences to C.K. (postdoctoral fellowship LPDS 2017-11)
and by the Swiss National Science Foundation (Grant no.
200021_178760) to O.S.W. is gratefully acknowledged.
REFERENCES
■
(1) Nguyen, J. D.; D’Amato, E. M.; Narayanam, J. M. R.;
Stephenson, C. R. J. Engaging Unactivated Alkyl, Alkenyl and Aryl
Iodides in Visible-Light-Mediated Free Radical Reactions. Nat. Chem.
2012, 4, 854−859.
̈
(2) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; Konig, B. Reduction of Aryl
Halides by Consecutive Visible Light-Induced Electron Transfer
Processes. Science 2014, 346, 725−728.
(3) Goez, M.; Kerzig, C.; Naumann, R. An “All-Green” Catalytic
Cycle of Aqueous Photoionization. Angew. Chem., Int. Ed. 2014, 53,
9914−9916.
(4) Zeng, L.; Liu, T.; He, C.; Shi, D.; Zhang, F.; Duan, C. Organized
Aggregation Makes Insoluble Perylene Diimide Efficient for the
Reduction of Aryl Halides via Consecutive Visible Light-Induced
Electron-Transfer Processes. J. Am. Chem. Soc. 2016, 138, 3958−
3961.
̈
(5) Ghosh, I.; Konig, B. Chromoselective Photocatalysis: Controlled
Bond Activation through Light-Color Regulation of Redox Potentials.
Angew. Chem., Int. Ed. 2016, 55, 7676−7679.
̃
(6) Neumeier, M.; Sampedro, D.; Majek, M.; de la Pena O’Shea, V.
A.; Jacobi von Wangelin, A.; Perez-Ruiz, R. Dichromatic Photo-
catalytic Substitutions of Aryl Halides with a Small Organic Dye.
Chem. - Eur. J. 2018, 24, 105−108.
́
́
(7) Gong, H.-X.; Cao, Z.; Li, M.-H.; Liao, S.-H.; Lin, M.-J.
Photoexcited Perylene Diimide Radical Anions for the Reduction of
Aryl Halides: A Bay-Substituent Effect. Org. Chem. Front. 2018, 5,
2296−2302.
̈
(8) Meyer, A. U.; Slanina, T.; Heckel, A.; Konig, B. Lanthanide Ions
Coupled with Photoinduced Electron Transfer Generate Strong
Reduction Potentials from Visible Light. Chem. - Eur. J. 2017, 23,
7900−7904.
̈
(9) Haimerl, J.; Ghosh, I.; Konig, B.; Vogelsang, J.; Lupton, J. M.
Single-Molecule Photoredox Catalysis. Chem. Sci. 2019,
(29) Li, X.; Ma, J.; Liu, G.; Fang, J.; Yue, S.; Guan, Y.; Chen, L.; Liu,
X. Efficient Reductive Dechlorination of Monochloroacetic Acid by
Sulfite/UV Process. Environ. Sci. Technol. 2012, 46, 7342−7349.
(30) Zhu, D.; Zhang, L.; Ruther, R. E.; Hamers, R. J. Photo-
Illuminated Diamond as a Solid-State Source of Solvated Electrons in
Water for Nitrogen Reduction. Nat. Mater. 2013, 12, 836−841.
(31) Zhang, L.; Zhu, D.; Nathanson, G. M.; Hamers, R. J. Selective
Photoelectrochemical Reduction of Aqueous CO2 to CO by Solvated
Electrons. Angew. Chem., Int. Ed. 2014, 53, 9746−9750.
(32) Guo, X.; Okamoto, Y.; Schreier, M. R.; Ward, T. R.; Wenger,
O. S. Enantioselective Synthesis of Amines by Combining Photoredox
and Enzymatic Catalysis in a Cyclic Reaction Network. Chem. Sci.
2018, 9, 5052−5056.
DOI: 10.1039/C8SC03860K.
(10) Naumann, R.; Kerzig, C.; Goez, M. Laboratory-Scale
Photoredox Catalysis Using Hydrated Electrons Sustainably Gen-
erated with a Single Green Laser. Chem. Sci. 2017, 8, 7510−7520.
(11) Naumann, R.; Lehmann, F.; Goez, M. Generating Hydrated
Electrons for Chemical Syntheses by Using a Green Light-Emitting
Diode (LED). Angew. Chem., Int. Ed. 2018, 57, 1078−1081.
(12) Naumann, R.; Lehmann, F.; Goez, M. Micellized Tris-
(bipyridine)ruthenium Catalysts Affording Preparative Amounts of
Hydrated Electrons with a Green Light-Emitting Diode. Chem. - Eur.
J. 2018, 24, 13259−13269.
(13) Naumann, R.; Goez, M. First Micelle-Free Photoredox
Catalytic Access to Hydrated Electrons for Syntheses and
E
DOI: 10.1021/jacs.8b12223
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(33) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti,
F. Photochemistry and Photophysics of Coordination Compounds:
Iridium. Top. Curr. Chem. 2007, 281, 143−203.
(52) Lippincott, D. J.; Linstadt, R. T. H.; Maser, M. R.; Gallou, F.;
Lipshutz, B. H. Synthesis of Functionalized 1,3-Butadienes via Pd-
Catalyzed Cross-Couplings of Substituted Allenic Esters in Water at
Room Temperature. Org. Lett. 2018, 20, 4719−4722.
(34) Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.;
Balzani, V. Photochemistry and Photophysics of Coordination
Compounds: Ruthenium. Top. Curr. Chem. 2007, 280, 117−214.
(53) Wenger, O. S. Photoactive Complexes with Earth-Abundant
Metals. J. Am. Chem. Soc. 2018, 140, 13522−13533.
(35) Mu
̈
ller, P.; Brettel, K. [Ru(bpy)3]2+ as a Reference in Transient
Absorption Spectroscopy: Differential Absorption Coefficients for
Formation of the Long-Lived 3MLCT Excited State. Photochem.
Photobiol. Sci. 2012, 11, 632−636.
(36) Tarnovsky, A. N.; Gawelda, W.; Johnson, M.; Bressler, C.;
Chergui, M. Photexcitation of Aqueous Ruthenium(II)-tris-(2,2‘-
bipyridine) with High-Intensity Femtosecond Laser Pulses. J. Phys.
Chem. B 2006, 110, 26497−26505.
(54) Du, Y.; Pearson, R. M.; Lim, C.-H.; Sartor, S. M.; Ryan, M. D.;
Yang, H.; Damrauer, N. H.; Miyake, G. M. Strongly Reducing,
Visible-Light Organic Photoredox Catalysts as Sustainable Alter-
natives to Precious Metals. Chem. - Eur. J. 2017, 23, 10962−10968.
(55) Qiao, Y.; Schelter, E. J. Lanthanide Photocatalysis. Acc. Chem.
Res. 2018, 51, 2926−2936.
(37) Tran, T.-T.; Ha-Thi, M.-H.; Pino, T.; Quaranta, A.; Lefumeux,
C.; Leibl, W.; Aukauloo, A. Snapshots of Light Induced Accumulation
of Two Charges on Methylviologen Using a Sequential Nanosecond
Pump−Pump Photoexcitation. J. Phys. Chem. Lett. 2018, 9, 1086−
1091.
(38) Redmond, R. W.; Scaiano, J. C.; Johnston, L. J. Photochemistry
of Diphenylketyl Radicals in Polar Solvents. J. Am. Chem. Soc. 1990,
112, 398−402.
(39) Jaroschik, F. Picking One out of Three: Selective Single C−F
Activation in Trifluoromethyl Groups. Chem. - Eur. J. 2018, 24,
14572−14582.
̈
(40) Chen, K.; Berg, N.; Gschwind, R.; Konig, B. Selective Single
C(Sp3)−F Bond Cleavage in Trifluoromethylarenes: Merging Visible-
Light Catalysis with Lewis Acid Activation. J. Am. Chem. Soc. 2017,
139, 18444−18447.
(41) Ma, J.; Yi, W.; Lu, G.; Cai, C. Transition-Metal-Free C−H
Oxidative Activation: Persulfate-Promoted Selective Benzylic Mono-
and Difluorination. Org. Biomol. Chem. 2015, 13, 2890−2894.
(42) van der Born, D.; Sewing, C.; Herscheid, J. K. D. M.;
Windhorst, A. D.; Orru, R. V. A.; Vugts, D. J. A Universal Procedure
for the [18F]Trifluoromethylation of Aryl Iodides and Aryl Boronic
Acids with Highly Improved Specific Activity. Angew. Chem., Int. Ed.
2014, 53, 11046−11050.
(43) Anbar, M.; Hart, E. J. The Reactivity of Aromatic Compounds
toward Hydrated Electrons. J. Am. Chem. Soc. 1964, 86, 5633−5637.
(44) Loh, Y. Y.; Nagao, K.; Hoover, A. J.; Hesk, D.; Rivera, N. R.;
Colletti, S. L.; Davies, I. W.; MacMillan, D. W. C. Photoredox-
Catalyzed Deuteration and Tritiation of Pharmaceutical Compounds.
Science 2017, 358, 1182−1187.
(45) Tezel, U.; Pavlostathis, S. G. Quaternary Ammonium
Disinfectants: Microbial Adaptation, Degradation and Ecology. Curr.
Opin. Biotechnol. 2015, 33, 296−304.
(46) Bobrowski, K. Pulse Radiolysis of Aqueous Solutions of
Benzyltrialkylammonium Cations. Reactions with the Primary
Transients from Water Radiolysis. J. Phys. Chem. 1981, 85, 382−388.
(47) Xu, Z.; Zhang, X.; Huang, N.; Hu, H. Oxidation of
Benzalkonium Chloride by Gamma Irradiation: Kinetics and Decrease
in Toxicity. J. Radioanal. Nucl. Chem. 2017, 312, 631−637.
(48) Huang, N.; Wang, T.; Wang, W.-L.; Wu, Q.-Y.; Li, A.; Hu, H.-
Y. UV/Chlorine as an Advanced Oxidation Process for the
Degradation of Benzalkonium Chloride: Synergistic Effect, Trans-
formation Products and Toxicity Evaluation. Water Res. 2017, 114,
246−253.
(49) Lee, M.-Y.; Wang, W.-L.; Wu, Q.-Y.; Huang, N.; Xu, Z.-B.; Hu,
H.-Y. Degradation of Dodecyl Dimethyl Benzyl Ammonium Chloride
(DDBAC) as a Non-Oxidizing Biocide in Reverse Osmosis System
Using UV/Persulfate: Kinetics, Degradation Pathways, and Toxicity
Evaluation. Chem. Eng. J. 2018, 352, 283−292.
(50) Gu, Y.; Dong, W.; Luo, C.; Liu, T. Efficient Reductive
Decomposition of Perfluorooctanesulfonate in a High Photon Flux
UV/Sulfite System. Environ. Sci. Technol. 2016, 50, 10554−10561.
(51) Yu, H.; Ru, S.; Dai, G.; Zhai, Y.; Lin, H.; Han, S.; Wei, Y. An
Efficient Iron(III)-Catalyzed Aerobic Oxidation of Aldehydes in
Water for the Green Preparation of Carboxylic Acids. Angew. Chem.,
Int. Ed. 2017, 56, 3867−3871.
F
DOI: 10.1021/jacs.8b12223
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX