Strongly Reducing, Visible-Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals

Article abstract of DOI:10.1002/chem.201702926

Photoredox catalysis is a versatile approach for the construction of challenging covalent bonds under mild reaction conditions, commonly using photoredox catalysts (PCs) derived from precious metals. As such, there is need to develop organic analogues as sustainable replacements. Although several organic PCs have been introduced, there remains a lack of strongly reducing, visible-light organic PCs. Herein, we establish the critical photophysical and electrochemical characteristics of both a dihydrophenazine and a phenoxazine system that enables their success as strongly reducing, visible-light PCs for trifluoromethylation reactions and dual photoredox/nickel-catalyzed C?N and C?S cross-coupling reactions, both of which have been historically exclusive to precious metal PCs.

Full text of DOI:10.1002/chem.201702926

A Journal of
Accepted Article
Title: Strongly Reducing Visible Light Organic Photoredox Catalysts as
Sustainable Alternatives to Precious Metals
Authors: Ya Du, Ryan Pearson, Chern Hooi-Lim, Steven Sartor,
Matthew Ryan, Haishen Yang, Niels Damrauer, and Garret
Miyake
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201702926
Link to VoR: http://dx.doi.org/10.1002/chem.201702926
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COMMUNICATION
Strongly Reducing Visible Light Organic Photoredox Catalysts as
Sustainable Alternatives to Precious Metals
Ya Du[a]†, Ryan M. Pearson[a]†, Chern-Hooi Lim[a]†, Steven M. Sartor , Matthew D. Ryan , Haishen
[a]
[a]
Yang[a,b], Niels H. Damrauer[a,c], and Garret M. Miyake[a,c]
*
Abstract: Photoredox catalysis is a versatile approach for the
However, iridium and ruthenium are precious metals and
construction of challenging covalent bonds under mild reaction
conditions, commonly using photoredox catalysts (PCs) derived from
precious metals. As such, there is need to develop organic
analogues as sustainable replacements. Although several organic
PCs have been introduced, there remains a lack of strongly reducing
visible light organic PCs. Herein, we establish the critical
amongst the rarest elements on earth, escalating their costs and
presenting concerns related to sustainability and scalability,
driving the need to realize new PCs incorporating non-precious
metals[10] or to develop entirely organic replacements.[11] Several
organic molecules have proven successful as visible light PCs
[
12]
for small molecule and polymeric transformations.
The
majority of these organic PCs, such as Eosin Y,[ rhodamine
dyes,[14] acridinium salts,[15] perylene diimides,[16] and
carbazolyls[17] are excited state oxidants and operate through a
reductive quenching cycle. Although a few strongly reducing
organic PCs exist,[ many do not absorb visible light. PCs that
operate using mild visible light and do not require sacrificial
reductants are desired to minimize side reactions.
13]
photophysical and electrochemical characteristics of both
dihydrophenazine and a phenoxazine system that enables them
success as strongly reducing visible light PCs for
a
trifluoromethylations and dual photoredox/nickel catalyzed C-N and
C-S cross-couplings, reactions which have been historically
exclusive to precious metal PCs.
18]
Our interest in organic PCs[19] initiated with the
Visible light photoredox catalysis has gained prominence
orchestrating challenging chemical transformations under mild
development of organocatalyzed atom transfer radical
polymerization (O-ATRP).[20] ATRP has historically been
[
1]
reaction conditions. A large majority of this work has employed
precious metal polypyridyl iridium and ruthenium photoredox
catalysts (PCs). The rapid establishment of these metal
complexes as practical PCs leveraged their well-studied
photophysical and photoredox properties, that in turn have
enabled their incorporation in a range of applications including
photovoltaics,[2] light emitting diodes,[3] imaging and sensing in
biological systems,[4] therapeutics,[5] and redox active
antibiotics.[
mediated by transition metal catalysts, most commonly copper
or ruthenium complexes, which can contaminate the polymer
[21]
product and restrict applications.
A primary challenge in
developing a photoredox mediated O-ATRP is presented by the
strong reducing power that is required of the PC to activate a
dormant alkyl halide.[22] In general, PCs that possess such
strong excited state reducing powers are rare, and this is
particularly true for organic systems (vide supra).
6]
To address this challenge, we have introduced visible light
In regards to photoredox catalysis, these metal complexes
exhibit essential characteristics, including strong absorption of
visible light via spin-allowed metal-to-ligand charge transfer
organic
dihydrophenazines,
PCs
including
perylene,[19a]
N,N-diaryl
[19b, 23]
[24]
and N-aryl phenoxazines as organic
[25]
PCs to mediate O-ATRP via an oxidative quenching pathway.
Dihydrophenazine and phenoxazine contain electron rich
chromophore motifs that form rather stable radical cations upon
oxidation and enable them to be strong excited state
(
MLCT), efficient conversion to long-lived triplet MLCT excited
states ( MLCT), and redox reversibility.[7a, 8] Furthermore,
3
[7]
ligand and metal modifications tailor the redox properties of the
ground and excited states.[9] For example, fac-Ir(ppy)
phenylpyridinato-C ,N]iridium(III), 1, ppy = 2-phenylpyridine) is
(tris[2-
3
reductants.[26] However,
a detailed comprehension of the
2
characteristics of these molecules in regards to catalysis or their
ability to catalyze other transformations has not been
established. Herein, through investigation of their photophysical
and electrochemical properties, we report the critical
amongst the strongest reducing PCs, while [Ru(bpy)
tris(2,2’-bipyridine)ruthenium(II), 2, bpy 2,2′-bipyridine)
3
]2+
(
=
possesses redox properties that enable it to function either as a
3
reductant or as an oxidant from the MLCT state.
characteristics
of
N,N-5,10-di(2-naphthalene)-5,10-
dihydrophenazine (3) and 3,7-(4-biphenyl)-1-naphthalene-10-
phenoxazine (4) that enable them to serve as successful PCs.
We further establish 3 and 4 as PCs through their employment
in atom transfer radical additions or substitutions with CF I to
3
alkenes and heterocycles as well as dual photoredox/nickel
catalyzed C-N and C-S cross-couplings.
[a]
[b]
[c]
Dr. Y. Du, R. M. Pearson, Dr. C.-H. Lim, S. M. Sartor, M. D. Ryan,
Dr. H. Yang, Prof. N. H. Damrauer, Prof. G. M. Miyake
Department of Chemistry and Biochemistry
University of Colorado at Boulder
215 UCB, Boulder, Colorado 80309-0215, USA
E-mail: garret.miyake@colorado.edu
Dr. H. Yang
Shanghai Key Laboratory of Materials Protection and Advanced
Materials in Electric Power, College of Environmental and Chemical
Engineering
Shanghai University of Electric Power
Shanghai 200090, P. R. China
Prof. N. H. Damrauer, Prof. G. M. Miyake
Materials Science and Engineering Program
University of Colorado at Boulder
596 UCB, Boulder, Colorado 80309-0596, USA
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - A European Journal
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COMMUNICATION
The photophysical properties of
Precious Metals
Organic Photoredox Catalysts
3
and
4
were investigated and
A
C
B
4
compared to that of transition metal
complexes 1 and 2 (Figure 1). As
photoexcitation is the first step in
photoredox catalysis, PCs should be
strong light absorbers. In N,N-
dimethylacetamide (DMA), the molar
absorptivities (ε) for transition metal
complexes 1 and 2 are 13,100 and
1
2
3
-
2.50
E0*
-2.00
-
1.50
1.00
3MLCT*
3CT*
3
CT*
-
Etriplet
-1
-1
3MLCT*
1
2,500 M cm at their maximum
peak wavelengths of absorption
max,abs) of 377 nm and 454 nm,
-0.50
0
0
.00
.50
(
λ
1.00
.50
E0ox
1
respectively
Dihydrophenazine
molar extinction (εmax,abs = 5,950 M
(Figure
has
1A).
1 [h]
2 [i]
3
4
3
a
lower
0
[a]
-
E *
E0ox[b]
Etriplet, (V)[c]
λmax,abs(nm)[d]
-1.73
0.77
2.50
377
-0.81
1.29
2.10
454
-1.69 (-2.12)
0.21 (0.06)
1.90 (2.18)
343 (314)
5950
-1.80 (-1.70)
0.65 (0.42)
2.45 (2.11)
388 (351)
26600
1
-1
cm ;
λ
max,abs
=
343 nm) in
comparison to
1
and 2, while
phenoxazine 4 is an excellent light
-
1
-1 [d]
εmax,abs (M cm )
13100
494
12500
615
absorber and superior to the other
λ
max,em (nm)[e]
654 (569)
4.3 ꢀ 0.5
2.0 ꢀ 0.7
506 (588)
480 ꢀ 50
90 ꢀ 10
-
1
-1
three PCs (εmax,abs = 26,600 M cm ;
max,abs = 388 nm). In a similar fashion
to 1, although the λmax,abs values are
400 nm, the absorption profiles of
τtriplet (µs)[f]
Φtriplet (%)[g]
1.9
1.1
λ
~100
~100
<
organic PCs 3 and 4 extend into the Figure 1: Photophysical and electrochemical properties of precious metal and organic photoredox catalysts. (A)
UV-vis absorption spectra of PCs 1-4 in N,N-dimethylacetamide (DMA). (B) Structures of precious metal and
organic PCs. (C) Values enclosed in parentheses are from density functional theory (DFT) calculations. All
visible region and enable them to
function as visible light PCs.
experimental values for PCs 3 and 4 were measured in DMA at room temperature. [a] Triplet excited state
1
is known to be one of the reduction potential, E
*, in units of V vs. SCE. [b] Ground state oxidation potential [c] Triplet energy (Etriplet),
0
strongest excited state transition estimated from the fluorescence wavelength of the charge transfer singlet state. [d] Maximum absorption
wavelength (λmax,abs); molar absorptivity (εmax,abs) at λmax,abs. [e] Emission maximum wavelength (λmax,em). [f] Triplet
metal PC reductants available, with
3
excited state lifetime (τtriplet). [g] Quantum yield (Φtriplet) of charge transfer triplet excited state ( CT*), and metal-
an excited state reduction potential
3
ligand charge transfer triplet state ( MLCT*). [h] λmax,abs and εmax,abs were measured in this work in DMA. All other
E *) of E (Ir(IV)/ Ir(III)*) = -1.73 V vs. values were obtained from ref. [8a,9b], and were measured in acetonitrile, except for the λmax,em, which was
0
0
3
(
SCE (Figure 1C). Excitingly, organic measured in alcoholic solvent at 77K. [i] λmax,abs and εmax,abs were measured in this work in DMA solvent for
Ru(bpy) ](PF . All other values were obtained from ref. [8b,9a], and were measured in acetonitrile. See
Supporting Information for details.
[
3
6 2
)
PCs 3 and 4 are equally reducing
0
0 2
•+ 3
with E * = E ( PC / PC*) values of -
.69 and -1.80 V vs. SCE, respectively. Although 2 is not as
reducing in the excited state [E * = E (Ru(III)/ Ru(II)*) = -0.81 V
vs. SCE], the Ru(III) generated after participating in
photoreduction event is strongly oxidizing, with an oxidation
potential [E ox = E (Ru(III)/Ru(II))] of 1.29 V vs. SCE. Notably, 1
and the organic PC 4 have similar E ox values [E (Ir(IV)/Ir(III)) =
.77 and E ( PC / PC) = 0.65 V vs. SCE, respectively], while 3
forms a rather stable radical cation [E ox = E ( PC / PC) = 0.21
V vs. SCE]. In regards to triplet energy (Etriplet), both 1 and 4
have energetic triplets with Etriplet of 2.50 and 2.45 V, respectively.
Meanwhile, Etriplet of 2 and 3 are lower, with respective values of
The efficient access of a long-lived excited species by the
PC enables sufficient time for bimolecular electron transfer with
the desired substrate(s). In the case of transition metal
complexes 1 and 2, ultrafast intersystem crossing produces the
1
0
0
3
a
3
0
0
MLCT and is useful by way of extending excited state lifetimes
(τ). Photoexcitation quantitatively leads to a long-lived 3MLCT
which survives for 1.9 and 1.1 µs (in acetonitrile) for 1 and 2,
respectively (Figure 1C). Using nanosecond transient-absorption
0
0
0
2
•+ 1
0
0
0 2
•+ 1
(
TA) spectroscopy performed in DMA at room temperature, we
identified long-lived excited states for the organic PCs 3 and 4.
The τ of 3 was determined to be 4.3 ± 0.5 µs (Figure 2C)
whereas for 4, it is a remarkable two orders of magnitude longer
than that of 1, 2 or 3, with τ = 480 ± 50 µs (Figure 2D). By use of
a triplet-triplet energy transfer method, we have determined the
triplet quantum yield (Φtriplet) of 3 and 4 in DMA at ambient
temperature: 3’s Φtriplet is relatively low at 2.0 ± 0.7% while 4 has
an impressively high Φtriplet of 90% ± 10%.
Another critical characteristic for successful PCs is radical
stability following single electron transfer events. Transition
metal complexes 1 and 2 exhibit reversible waves in cyclic
voltammetry (CV), a property that indicates stability of the redox-
2
.10 and 1.90 V.
Upon photoexcitation, transition metal complexes 1 and 2
form a MLCT excited state, which is suggested to facilitate
electron transfer mechanisms in photocatalytic cycles.[8b, 9a]
Recently, we reported that the lowest energy excited state of
is also CT in nature.[
23]
Specifically,
dihydrophenazine
intramolecular CT occurs from the electron rich phenazine core
donor) to one of the 2-naphthyl N-substituents (acceptor). Here
we show that phenoxazine PC similarly undergoes
3
(
4
photoinduced intramolecular CT, as evidenced by a significant
solvatochromic effect in the emission (Figure 2A). Additionally,
the broad and featureless emission peaks are characteristic of
emission from a CT state (Figure 2B).[27]
[7a, 8]
altered catalyst.
Similarly, the CVs corresponding to the
PC / PC couple of 3 and 4 are reversible (Figure 2E and F). In
particular, the difference between the anodic and cathodic peak
2
●+ 1
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Chemistry - A European Journal
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COMMUNICATION
potential (ΔE
5
p
) for 3 is 67 mV (compared to theoretical value of
moderate to excellent yields (42% to 98%). For alkenes, the
presence of HCOOK base affords the substitution product while
the absence of HCOOK favors the addition product. The
9 mV),[28] while the ratio of the peak anodic current (ipa) to the
peak cathodic current (ipc) is 0.97 (compared to theoretical value
of 1) (Figure 2E). Redox reversibility of 3 is in part attributed to
reduction of CF
3
CF
2
I
was also accomplished, generating
2
●+
0
●
the stability of 3’s PC , as indicated by low E ox value of 0.21 V
vs. SCE; this value is even lower than the redox couple
producing ferrocenium (E0ox = ~ 0.4 V vs. SCE). Likewise, the
CF
3
CF
2
for substitution onto indoles and alkenes. The
trifluoromethylation of 3-methylindole was achieved with similar
yield using natural sunlight. The substitution reaction between
10-undecene-1-ol and CF I could be performed using lower
3
[29]
CV of 4 reveals ΔE
p
= 68 mV and ipa/ipc = 1.28.[30] Additionally, a
catalyst loading (0.25 mol %, 69 % yield) or on a
larger 10 mmol scale (1.74 g product, 73% yield)
while maintaining good yields.
Dual catalytic approaches integrating
photoredox catalysis using iridium PCs and
nickel-catalyzed cross-coupling reactions have
[
33]
C-S,[34] C-N[35] and
enabled access to C-O,
various C-C[
36]
reactions. Incorporating the
photoredox cycle introduces redox or energy
transfer[37] mechanisms with the nickel
complexes to complete otherwise demanding
catalytic cycles. Cross-coupling reactions have
traditionally been catalyzed by palladium
complexes at elevated temperatures to construct
such critical bonds.[38] Thus, to entirely remove
precious metals out of cross-couplings through
dual catalytic reactions, we sought to determine
if organic PCs 3 and 4 could also enable such
challenging reactions.
Previously,
a
dual photoredox/nickel
catalytic approach employing 0.02 mol % of
polypyridyl
Ir[dF(CF) ppy]
iridium
3
(dtbbpy)PF [dF(CF )ppy =
PC
2-
3
2
6
(
2,4-difluorophenyl)-5-(trifluoromethyl)pyridine;
4,4′-ditertbutyl-2,2′-bipyridine] in
•glyme could efficiently
Figure 2. Photophysical properties of organic PCs. (A) Photograph showing solvatochromic shifts dtbbpy
=
in the emission of 4 when irradiated with 365 nm light in solvents of increasing polarity; from left to
right: 1-hexene, benzene, dioxane, ethyl acetate, pyridine, and DMF. (B) Normalized emission
spectra of 4 in solvents of varying polarity. Transient absorption spectra of 3 (C) and 4 (D) in DMA
conjunction with NiBr
catalyze C-N bond formation under mild reaction
at room temperature. Cyclic voltammetry (CV) experiments with various scan rates for (E) 3 and (F) conditions.[
2
35a]
At similar reaction conditions,
4
. See Supporting Information for details.
albeit using a higher catalyst loading (0.4 mol %),
PC 3 or PC 4 in combination with NiBr •glyme
2
linear relationship between ipa and the square root of the scan
rate (ν1/2) reveals that the CV of 3 and 4 are diffusion limited
successfully catalyzed C-N coupling reactions at good to
excellent yields (68% to 96%, Figure 3B). The scope of amines
included both primary (aniline, furfurylamine, and propylamine)
and secondary amines (pyrrolidine and morpholine) and were
effectively coupled with electron rich, electron poor, and
heterocyclic aryl bromides. For secondary amines, both PC 3
and 4 catalyzed C-N bond formations, although PC 3 generally
gave slightly higher yields. Whilst PC 3 was unsuccessful in
effecting C-N cross-coupling involving primary amines, PC 4
proved to be effective to couple primary amines in high yields.
(
Figure 2E and F, insets); this supports the idea that electron
transfer between the PC and the electrode (for either 3 or 4) is
fast and likely facilitated by small structural reorganization
between PC and PC .
[
24]
1
2
●+
With the confirmation that
3 and 4 possess key
photophysical and electrochemical characteristics critical for
photoredox catalysis, we set out to establish their broader
catalytic ability and potential to replace precious metal PCs
through performing challenging chemical transformations,
particularly ones that have been previously directed by
polypyridyl iridium and ruthenium PCs such as 1 and 2.
In
photoredox/nickel
Ir[dF(CF) ppy] (dtbbpy)PF
coupled products under mild conditions. At analogous reaction
conditions, phenoxazine PC 4 achieved C-S cross-couplings at
good to excellent yields (64% to 98%, Figure 3C) but proved
efficient at a much lower PC loading of 0.2 mol %. Aryl thiol
regards
to
catalysis
and NiCl
C-S
cross-coupling,
the
mol
dual
%
with
2
3
2
6
2
•glyme produced C-S
[34]
First, we investigated if the strongly reducing
3
dihydrophenazine 3 could directly reduce CF I (peak reduction
potential (E
generating CF
) of -1.52 V vs. SCE on glassy carbon),[31] thereby
p
●
3
for the trifluoromethylation of unsaturated
substrates (Figure 3A).[32] Using white LED irradiation of 3 (1 to
mol %) in the presence of 1.5 eq. of potassium formate
HCOOK), CF was successfully installed onto five-membered
heteroarenes (indoles, pyrroles), arenes, and alkenes at
(
thiophenol), alkyl thiol (4-Methoxybenzyl mercaptan, 1-
5
(
octanethiol and cyclohexanethiol) and cysteine (N-(tert-
Butoxycarbonyl)-L-cysteine methyl ester) successfully coupled
with a variety of aryl bromides. It is important to note that aryl
3
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COMMUNICATION
bromide coupling partners were successfully incorporated with
C-N coupling suffered a 30% drop in yield. This lower yield was
attributed to limited light penetration owing to the opaque
solution mixture compounded by the lower molar absorptivity of
PC 3.
organic PC 4, which were shown to be inactive when using
3 2 6
ppy] (dtbbpy)PF
.[34] PC 3 was unsuccessful in C-S
Ir[dF(CF)
coupling reactions, presumably due to its stable radical cation
(
E0ox = 0.21 V vs. SCE) being unable to generate a thiol radical
In
sum,
photophysical
and
electrochemical
involved in the coupling reaction.[34]
characterizations on dihydrophenazine and phenoxazine PCs 3
and 4 reveal that these molecules possess the key attributes
vital to successful photoredox catalysis, including redox
reversibility and strong visible light absorption to efficiently
access a highly reducing triplet state through formation of CT
excited state. The triplet excited state of these organic PCs are
long-lived and accessed in 90% ± 10% quantum yield by PC 4.
Highlighting that 4 is an organic analogue of
These photoredox/nickel C-N and C-S cross-coupling
reactions could be driven by natural sunlight to obtain similarly
high yield. Furthermore, both the C-N and C-S couplings could
be performed on a larger 10 mmol scale reaction for C-N (1.22g,
53% yield) and C-S (2.92g, 98% yield) couplings. In these
scaled reactions, C-S coupling maintained the high yield while
A
the iridium complex 1, both PCs have almost
identical E *, E0ox and
0
Etriplet values. The
potential for replacement of polypyridyl iridium
and ruthenium complexes by organic PCs 3
and 4 is demonstrated by the ability of these
organic
trifluoromethylations
analogues
to
and
catalyze
dual
photoredox/nickel C-N and C-S cross-coupling
reactions using visible light, including natural
sunlight.
†Y. Du, R.M.Pearson and C.-H. Lim
contributed equally to this work.
B
Acknowledgements
This work was supported by the University of
Colorado Boulder and the Advanced Research
Projects Agency-Energy (DE-AR0000683).
Research reported in this publication was
supported by the National Institute of General
Medical Sciences of the National Institutes of
Health under Award Number R35GM119702.
The content is solely the responsibility of the
authors and does not necessarily represent the
official views of the National Institutes of Health.
Acknowledgement is made to the donors of
The American Chemical Society Petroleum
Research Fund for partial support of this
research (56501-DNI7). S.M.S and M.D.R.
acknowledge support from a DOE Graduate
Assistance in Areas of National Need
Fellowship. We gratefully acknowledge the use
of XSEDE supercomputing resources (NSF
ACI-1053575).
C
Figure 3. Photoredox catalyzed transformations using organic PCs
trifluoromethylations using PC 3 on alkenes, five-membered heteroarenes, arenes, and cross-addition
3 and 4. (A) Radical
Keywords: photocatalysis • photochemistry •
on alkenes. (B) Dual organic photoredox and nickel catalyzed C-N cross-coupling reaction scope. (C) organocatalysis • radicals • sustainable
Dual organic photoredox and nickel catalyzed C-S cross-coupling scope. Data reported as isolated
yields. Values in parentheses are the ratio of Z : E : β-hydride elimination product. aReaction was also
chemistry catalysis
conducted using sunlight for 1 week (67% yield for trifluoromethylation, 83% yield for C-N coupling,
9
4% yield for C-S coupling). bCF
3
CF
2
I was used instead of CF
3
c
I. Reaction time 6 hours. dReaction
[
1]
a) J. W. Tucker, C. R. J. Stephenson, J. Org.
was also conducted on a larger 10 mmol scale (73% yield for trifluoromethylation, 53% yield for C-N
coupling, 98% yield for C-S coupling). eReaction was also conducted at reduced catalyst loading of
Chem. 2012, 77, 1617-1622; b) C. K. Prier, D. A. Rankic,
0
.25 mol %, instead of standard 1.0 mol % (69% yield for trifluoromethylation, 53% yield for C-N D. W. C. MacMillan, Chem. Rev. 2013, 113, 5322-5363;
f
g
coupling, 98% yield for C-S coupling after 24 hours). Performed without HCOOK. Reaction performed
h
with 10 mol % pyrrolidine as the ligand and reduced nickel loading to 1.0 mol %. Reaction catalyzed
i
by PC 4. Reaction catalyzed by PC 3.
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Chemistry - A European Journal
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COMMUNICATION
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Chemistry - A European Journal
10.1002/chem.201702926
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COMMUNICATION
Precious Metals
Organic Photoredox Catalysts
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
E
0
* = -1.73
-0.81
-1.69
-1.80
V vs SCE
The new iridium. The photophysical and electrochemical properties of a strongly
reducing phenazine and phenoxazine enable their application as visible light
photoredox catalysts to catalyze reactions that were previously restricted to
precious metals. These organic visible light photoredox catalysts not only present
sustainable alternatives to precious metals, but can possess enhanced properties
to their metal counterparts for catalysis.
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