A Toolbox Approach to Construct Broadly Applicable Metal-Free Catalysts for Photoredox Chemistry: Deliberate Tuning of Redox Potentials and Importance of Halogens in Donor-Acceptor Cyanoarenes

Article abstract of DOI:10.1021/jacs.8b08933

The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor-acceptor photocatalysts, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor substituents as well as the acceptors' molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts (e.g., for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the catalysts' redox properties. Based on their preeminent electrochemical and photophysical characteristics, all novel, purely organic photoredox catalysts were evaluated in three challenging, mechanistically distinct classes of benchmark reactions (either requiring balanced, highly oxidizing or strongly reducing properties) to demonstrate their enormous potential as customizable photocatalysts, that outperform and complement prevailing typical best photocatalysts.

Full text of DOI:10.1021/jacs.8b08933

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Article
A Toolbox Approach to Construct Broadly Applicable Metal-Free
Catalysts for Photoredox Chemistry – Deliberate Tuning of Redox
Potentials and Importance of Halogens in Donor-Acceptor Cyanoarenes
Elisabeth Speckmeier, Tillmann Fischer, and Kirsten Zeitler
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Oct 2018
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A Toolbox Approach to Construct Broadly Applicable Metal-Free
Catalysts for Photoredox Chemistry – Deliberate Tuning of Redox
Potentials and Importance of Halogens in Donor-Acceptor Cyanoarenes
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Elisabeth Speckmeier,§ Tillmann Fischer§ and Kirsten Zeitler*
Institut für Organische Chemie, Universität Leipzig, Johannisallee 29, D-04103 Leipzig, Germany.
KEYWORDS: photoredox catalysis, organic dyes, redox potential, donor acceptor molecules, thermally activated delayed
fluorescence (TADF)
ABSTRACT: The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of
challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design
strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor-acceptor photocatalysts, using
1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor
substituents as well as the acceptor´s molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts
(e.g. for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox
potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the
catalysts´ redox properties. Based on their preeminent electrochemical and photophysical characteristics all novel, purely organic
photoredox catalysts were evaluated in three challenging, mechanistically distinct classes of benchmark reactions (either requiring
balanced, highly oxidizing or strongly reducing properties) to demonstrate their enormous potential as customizable photocatalysts,
that outperform and complement prevailing typical best photocatalysts.
INTRODUCTION
▀
Visible light photoredox catalysis undoubtedly has proven
to be a powerful tool for the activation of small molecules, and
hence has found widespread applications in a myriad of syn-
theses and functionalizations to enable formerly challenging or
even impossible chemical transformations1,2, , , by selective
3 4 5
addressing and activation of specific functional groups or
bonds. Upon absorption of visible light typical photocatalysts
(PCs), either being transition metal-based complexes, or or-
ganic dyes, respectively, may reach electronically excited
states that enable single electron transfer (SET) and/or energy
transfer (ET) to various substrates. Due to their excellent pho-
tophysical properties, which have extensively been studied,
ruthenium and iridium based photocatalysts6 are ubiquitously
applied as photoredox catalysts and their prominent utility and
impressive performance is well established.1c However, em-
ployment of these precious metal catalysts is associated with
elevated costs, and, also in striking contrast to a commitment to
sustainability due to their terrestrial scarcity and environ-
mental issues connected with their exploitation.7 Conse-
quently, broad interest in the development of more sustainable
replacement photocatalysts has emerged. Only recently, alter-
natives based on more earth-abundant first-row transition
metals,8 including Cu,9 Cr10 and Fe11 complexes, have been
revisited or newly explored.12,13
Figure 1. I + II. Compared common approaches for tailoring
photocatalysts. III. Conceptual donor & acceptor modifications
for accessing both strongly reductive and oxidative properties.
and other challenges connected with the use of transition metal
To address and circumvent the aforementioned limitations
complexes (e.g. removing of potential corresponding contami-
has hence recently attracted enormous interest in the field of
nations from products)14 the development of purely organic
photocatalysts as sustainable and cost-effective alternatives,
photoredox catalysis.1d,15 While structure-property relationships
and the effects of changing metal centers, ligands or counteri-
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ons are well-investigated and understood for metal-based both reduction and oxidation processes, referring to the great suc-
complexes (Figure 1/I),16 comparable investigations to elucidate
molecular design principles for organic dyes remain remarkably
underexplored.
cess and versatile applicability of Ru(bpy)32+ (also see Figure 5).
While ground state potentials are rather independent of each
other and a consequence of the electronic distribution, the sim-
plified mathematical equation for the energy of photoinduced
electron transfer25 directly correlates the excited state potentials
to the excited state energy E0,0 and the corresponding ground
state potential pursuant to the fact that the photocatalyst´s excited
state is both a better oxidant and reductant than the PC´s ground
state. In other words: within a specific oxidative (or reductive)
quenching cycle high ground state potentials lead to low corre-
sponding excited state potentials. As a consequence extremely
high ground state redox potentials, such as E1/2 (IrIII/IrII) = –2.19 V
vs SCE for fac-Ir(ppy)3 are barely usable, because the low corre-
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Despite a rich history and the great potential of organic dyes as
photocatalysts1d,15,
17
, a conceptually similar treatment as known
for transition metal-based complexes, considering the transfer of
electron density from electron-rich (metal) centers to electron-
deficient positions (ligand) to access charge-transfer excited states
(MLCT) and accordingly modify the complex´s redox properties,
has not been available for donor-acceptor based organic photo-
catalysts. Only very recently, structural core modifications in
strongly oxidizing acridinium based fluorophores14,18 were inves-
tigated to alleviate their high excited state oxidation potential
allowing for a more balanced performance in net redox neutral
transformations.14 Similarly, alterations in N-aryl substituents of
phenothiazines, dihydrophenazines and lately phenoxazines were
used to modify their strong reductive power. (Figure 1/II)19
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sponding excited state potential (E1/2 (IrIII*/IrII) = +0.31 V; E0,0
=
2.50 eV) curtails its accessibility.1c On the other hand, due to the
same interconnection of potentials and E0,0, very high excited state
potentials, such as E1/2 (*Mes-Acr-Me+/Mes-Acr-Me●) = +2.06 V
consequently relate to rather low ground state potentials
E1/2 (Mes-Acr-Me+/Mes-Acr-Me●) = –0.57 V, depending on the
respective E0,0 energy (E0,0 (Mes-Acr-Me+) = 2.63 eV). This may
also lead to limitations in applicability, especially within (highly
desirable) net redox neutral transformations, where often both
significant exited state as well as ground state oxidative, resp.
reductive power (or vice versa) are required to close the catalytic
cycle.14 To better visualize these correlations and their depend-
ency on the excited state energy E0,0, we have designed an ex-
tended Latimer-type diagram as shown in Figure 2.
As recent reports highlight the options to tailor certain organic
dye classes for specific catalytic purposes,18,19,20,21 providing
22
either strongly oxidizing or powerful reducing properties, we
accordingly questioned whether we could develop tuning guide-
lines to access broadly applicable, purely organic photocatalysts
reaching both high oxidation and reduction potentials from a
single, common scaffold. Prompted by the essential importance
of having spatially separated HOMO and LUMO in a non-
overlapping frontier molecular orbital (FMO) structure for the
construction of thermally activated delayed fluorescence
OLED emitters (TADF OLEDs),23,24 and the initial proof of
concept for their photocatalytic activity,22,23a, we reasoned the
intrinsic charge transfer processes in such TADF materials to be
a suitable starting point for our investigations (Figure 1/III).
Building on their favorable photophysical properties, herein,
we report a comprehensive study on the deliberate molecular
design and modifications of both electron donor and electron
acceptor moieties of cyanobenzene derived organic dyes to
broadly customize their electrochemical properties and provide
insight into their versatile photoredox catalytic activities. Our
initial findings for the first time demonstrate the importance of
subtle scaffold-based variations, especially with respect to halo-
gen substituents and their previously not considered, ambivalent
properties, and hence offer new possibilities to develop tailored
organic photocatalysts according to the structure-property rela-
tionships established within this study.
Figure 2. Visualization of ground & excited state (PC*) redox proper-
ties of a photocatalyst (PC) in correlation to the corresponding excited
state energy E0,0 in an extended Latimer-type diagram. Different shades
of grey correspond to increasing values of E0,0 (here: 2.7 eV). Processes
of oxidative power are marked in golden brown, processes of reductive
RESULTS AND DISCUSSION
▀
●
+
General “design” considerations. To identify an appro-
priate initial dye test system to evaluate the ability of altering
the photocatalysts´ electronics and hence tuning their redox
properties by modifying core and substituents to achieve dif-
ferences in their charge transfer (CT) characteristics, a number
of additional pre-requisites had to be met. For a broadly applica-
ble, competitive performance and to concurrently allow to uncov-
ering direct correlations between structural modifications of the
donor-acceptor units and the photocatalyst´s (PC) properties, we
aimed at visible light absorbing organic dyes exhibiting sufficient
stability. We hence sorted out e. g. acridinium, pyrylium and
xanthene derivatives at an early state of our investigations due to
their undesirable predisposition for nucleophilic attacks, pH-
dependency and ionic character. Apart from accessing longer
excited state lifetimes, we centered our efforts on derivatives that
could provide a wide redox window allowing for a broad range of
power in green. Oxidative quenching of PC* generates PC (left);
reductive quenching leads to PC●− (right). For a comparative visualized
overview on individual photocatalyst examples and their redox
properties using such diagrams, see Figure 5.
For visible light excitation (λ > 400 nm) the maximum theo-
retical energy of ≈3.0 eV emphasizes the importance for an
almost lossless transformation into excited state energies E0,0
to allow for photocatalysts reaching higher, but at the same
time still more balanced redox potentials.
Aiming at organic photocatalysts being competitive to current
(metal) organic derivatives, we questioned whether thermally
activated delayed fluorescence (TADF) emitters (a class of
OLED materials23b,c) would be ideally suited candidates for
structure-property related studies as their inherent characteris-
tics would match our two key requirements for the molecular
design of photocatalysts. With the known feature of TADF
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should be possible due to the well-known spatial separation of
materials of small singlet-triplet energy gaps ∆EST, which can
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significantly be further decreased by HOMO and LUMO
separation in these molecular structures, such donor-acceptor
type molecules, including cyanobenzenes, aryl triazines,
sulfones and others,23b,c should qualify for deliberate structure-
correlated design to obtain high oxidation and reduction poten-
tials. Using cyanobenzenes as relatively strong acceptor moie-
ties, we opted to
– combine them with rather weak carbazole and diphenyl-
amine dervatives as corresponding donors and additionally
– modify the aromatic acceptor core unit by targeted inter-
change of the cyano groups with alternative (electron-
withdrawing) substituents26 to thereby
– gain a profound understanding of such unprecedented
core modifications and their additional and interconnecting
effects on the redox properties of the corresponding catalyst
derivatives.
Being aware that these modifications could deteriorate the
chemical stability, we planned to then examine and check this
issue in corresponding test reactions in comparison to established
catalyst systems (see section “Application and evaluation”).
HOMOs, being localized at the donor moieties, and LUMOs
distributed over the cyanoarene acceptor core. Sterical hindrance
even intensifies this separation of electron-donating aryl amines
and the cyanobenzenes as electron acceptor (typical dihedral
angle of about 60 ° for 4CzIPN) allowing (in a first, simplified
model) for a nearly independent consideration of reduction and
oxidation power by simple, discrete changes at either donor or
acceptor part of the photocatalyst PC. The ground state reduction
potential E1/2 (PC/PC●−) is therefore mainly determined by the
HOMO of the donor, while the ground state oxidation potential
E1/2 (PC●+/PC) is determined by the LUMO of the acceptor (Fig-
ure 3). In addition, electron-donating effects (either by strength or
number) should increase the reducing capability of the donor-
acceptor system, and inversely LUMO lowering effects should
contribute to improved oxidative power.
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Selection of building blocks. Founded on Adachi´s most well-
known, highly efficient, isophthalonitrile-based green TADF
emitter
1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene
(4CzIPN)23a Zhang and co-workers recently published their ef-
forts to apply a series of positionally interchanged dicyanoben-
zenes23a as donor-acceptor fluorophore-derived photocatalysts.22
Interestingly, despite the variations of the two cyano groups´
substitution pattern at the benzene core and different numbers of
donor carbazoles (Cz), all investigated six carbazolyl dicyanoben-
zenes (plus one diphenylamine (DPA) derivative) only slightly
differed in their redox potentials, but rather showed variations in
their stability. Valuing the photocatalysts´ qualification in a de-
carboxylative photoredox/Ni-catalyzed cross-coupling revealed
only minor activity for most derivatives. Nevertheless, 4CzIPN
and 3DPAFIPN27 (titled as 4DPAIPN by Zhang)28 afforded the
products in good yields. Since then, especially 4CzIPN has suc-
cessfully been applied as catalyst in a number of different
photoredox reactions, 29 most probably due to its balanced redox
potential, which proved especially useful for reductive quenching
cycles ((E1/2 (PC/PC●−) = –1.24 V; E1/2 (PC*/PC●−) = +1.43 V vs
SCE; for a full data set, see Table 1 and Figure 5).
Figure 3. Correlation of redox potentials and influences of
donor/acceptor strength and modification.
Following these principles for our catalyst design to set up a
series of well-considered donor-acceptor variations of 4CzIPN
as a structural foundation, we chose three donor and five ac-
ceptor units (stemming from only three different synthetic
precursors) to allow for their correlation to the corresponding
redox potentials achieved by the particular donor-acceptor
combinations (Figure 4). Our selection of the donor units
(3,6-dimethoxy-9H-carbazole (D1, MeOCz), diphenylamine
(D2, DPA) and carbazole (D3, Cz)) was considered to allow
for good comparability of mesomeric, inductive and steric
effects as well as the corresponding oxidation potentials as
measure for their donor strength.30
As our intention was the design of powerful, yet easily ac-
cessible organic dyes, we focused on commercially available
cyanobenzenes cores as acceptor units,31 albeit allowing for
further acceptor variations by integration of different halogen
substituents from these precursors, as the virtue of the halo-
gens´ dichotomous character, i. e. having strongly inductive
electron withdrawing properties (σ-acceptor), but at the same
time mesomeric donating abilities (π-donor character, as well-
established in their Hammett σ-factors32), seemed to be ideally
suited to allow for additional fine-tuning the acceptor proper-
ties to alter the overall redox characteristics of the donor-
Guided by these initial results, we assumed that even higher
(ground state) potentials to both sides, oxidation and especially
reduction, could be accessible by deliberate structural modifi-
cations of these cyanoarene-based donor-acceptor molecules
using 4CzIPN as our starting point. We commenced our photo-
catalyst design by taking advantage of the “class-inherent”
options to both adjust the electronic properties of TADF mate-
rials and concomitantly benefit from the extremely small ∆EST
values (between S1 and T1: ∆EST < 0.1 eV; ∆EST (4CzIPN) =
0.08 eV23a). Provided by this energetically nearly lossless
intersystem crossing (ISC) and reversed intersystem crossing
(RISC) processes this would lead to a series of photocatalysts,
which are able to transform almost all the absorbed energy of
a visible light photon into the excited state, resulting in high
excited state energies E0,0, and consequently extended redox
windows in the excited state. This hence would then allow for
both challenging oxidative and reductive quenching cycle
processes. With respect to appropriate donor-acceptor combi-
nations and therefore the targeted tunability of the catalysts´
redox potentials, we hypothesized that rational catalyst design
,
acceptor dyes.33 34 While the application of fluorine represents
an important and mature tool in the design of pharmaceuticals
and agrochemicals,35 fluorine substituents have only been
scarcely used in the development of new TADF materials,
mainly to blue-shift their emission (higher E0,0) or increase the
emitters´ solubility.36 Despite the simplicity of their synthesis
from commercially available, inexpensive starting materials,
the deliberate core implementation of halogens into donor-
acceptor dyes to smoothly alter their redox properties (espe-
cially with respect to exploit their potential π-donating charac-
ter) has not yet been realized.37
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Figure 4. Overview on considered fluorophores, their donor and acceptor units (I) as well as the corresponding synthetic precursors (II).
Organic fluorophores are sorted by their ground state redox potentials (III). All potentials in Volt vs SCE. See supporting information for
further details. D1 : 3,6-dimethoxy-9H-carbazole (MeOCz); D2: diphenylamine (DPA); D3: carbazole (Cz).
In total, we studied eight different organic dyes, thereof
In order to systematically relate the structural alterations
three completely new
(3DPAClIPN,
3DPA2FBN,
starting from 4CzIPN, to the corresponding more oxidizing,
respectively more reducing ground state redox potentials of
our novel photocatalysts, we expected two “effect categories”
to be relevant: (1) donor-based effects, which would lead to
increased reducing power upon rising donor strength and/or
number as well as (2) acceptor-core related modifications
where both number and kind of withdrawing substituents (CN,
Cl, F) would matter to increase the oxidation potential. With
respect to the special nature of fluorine as σ-acceptor, but
π-donor substituent, an additional influence of the core substi-
tution on the reduction potential would be of special interest to
reach more exceptional potentials.
5MeOCzBN), two of which have only been mentioned in
TADF patent literature without any synthesis (3CzClIPN,
4MeOCzIPN),38 the structurally revised dye 3DPAFIPN
(formerly described as 4DPAIPN)22,28 as well as the known
fluorophores 4CzIPN23a and 5CzBN39 (see Figure 4 for an
overview on all catalyst structures). It is important to note that
apart from 4CzIPN (and 3DPAFIPN28) none of these organic
donor-acceptor molecules has previously been used in any
photocatalytic application.
Notably, the synthesis of compounds with more than two vicinal
diphenylamine groups (e.g. in a combination D2-A2) was not
possible following the simple one-step nucleophilic substitution
SNAr protocol as published by Adachi23a for 4CzIPN and further
used by Zhang22 for the (misleadingly) reported preparation of
alleged 4DPAIPN.28,22 The largely increased steric demand
caused by the ortho-hydrogen atoms of the diphenylamine moie-
ties prevents the synthesis of such D2-substituted acceptors ex-
ceeding the number of three such entities. Nevertheless, this in-
herent structural restriction caused by the DPA (D2) substituent to
catalyst structures without neighboring DPA residues enables
the utilization of a simple synthetic protocol to favorably
access the corresponding halogen substituted candidates
(D-A1Cl, D-A1F, D-A3).
Scheme 1 illustrates a series of such stepwise modifications
and their particular impact on the redox properties of the cor-
responding dye.
Discussion and analysis of structure – redox property rela-
tionships. A comprehensive survey and analysis of all eight
donor-acceptor-type catalysts regarding their ground state oxida-
tion and reduction potential reveals the major principles for the
alteration of the corresponding redox potentials. This conse-
quently allows for a predictability of redox properties within our
conceptual design approach (see Figure 4). According to a simpli-
fied view of the aforementioned HOMO/LUMO separation, an
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tronic effects to redox potentials focus on oxidation poten-
accepted electron (i. e. reduction of the catalyst ↔ oxidative power)
would predominately be localized within the cyanoarene moiety.
tials33,34). Additional to the simple electron donating power of the
respective donors (HOMO part of the dye) the consecutive delo-
calization of the “deficient electron” (i. e. “hole” or positive
charge) within the molecule needs to be considered. Due to the
missing availability of certain structural patterns a direct compari-
son to D2-substituted dyes mainly fails, while catalysts featuring
carbazole donors D1 and D3 are well compared according to
their structural similarity. The remarkable strongest ground state
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7
The acceptor core´s strong electron withdrawing ability can
hence be rationally adjusted by the attached donors (according
to their strength) allowing for a significant contribution to
further tune the oxidation potential E1/2 (PC●+/PC). The ex-
perimentally determined ground state oxidation potentials (via
CV measurements) of our accordingly diversified donor-
acceptor molecules are in full agreement with this prediction.
reducing potential E1/2 (PC/PC●−) = -1.92
V vs SCE of
8
9
3DPA2FBN originates from a difluorocyano benzene core in
combination with the diphenylamine as strong donor. The rather
weak acceptor core may be instrumental to an enhanced delocal-
ization of the “hole” by improved interaction with the π-
conjugated donor moieties. Additionally, the twofold fluorine
substitution may beneficially contribute to further increase the
electron density according to fluorine´s well established electron
donating ability (+M). This special fluorine effect becomes more
apparent in the direct comparison of the two halogen substituted
Scheme 1. Stepwise alterations of ground state redox
potentials for donor-acceptor dyes starting from 4CzIPN.a
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isophthalonitrile derivatives (3DPAFIPN vs 3DPAClIPN) where
fluorine clearly increases the reduction power (cf. Hammett fac-
tors32).
Table 1. Excited and ground state redox potentials as well
as excited state Energy E0,0 of photocatalysts examined.
E1/2
E1/2
E1/2
E1/2
E0,0
−
−
Photocat (PC●+/PC*) (PC*/PC● ) (PC●+/PC) (PC/PC●
)
[eV]
[V]
[V]
[V]
[V]
4CzIPN
3DPAFIPN
5CzBN
-1.18
-1.38
-1.42
-1.60
-0.93
-1.34
-1.50
-1.79
+1.43
+1.09
+1.31
+0.92
+1.56
+1.24
+1.27
+1.15
+1.49
+1.30
+1.41
+1.24
+1.79
+1.31
+1.11
+1.02
-1.24
-1.59
-1.52
-1.92
-1.16
-1.41
-1.34
-1.66
2.67
2.68
2.83
2.84
2.72
2.65
2.61
2.81
3DPA2FBNa
3CzClIPN
3DPAClIPN
4MeOCzIPNb
5MeOCzBNa
a Eox = E1/2 (PC●+/PC); Ered = E1/2 (PC/PC●−). All values are given
in V vs SCE.
All potentials in Volt vs SCE. Measurements were performed in
a
MeCN unless otherwise noted. Measured in DCM. Measured in
b
As expected the combination of the strongest acceptors ClIPN
with the unsubstituted carbazole Cz (D3), offering the weakest
+M effect (as illustrated by the highest oxidation potential of
the free donors (E1/2 (ox) = +1.25 V vs SCE)), assemble to
isophthalonitrile type 3CzClIPN, reaching the highest ground
oxidation potential E1/2 (PC●+/PC) = +1.79 V vs SCE within our
series. All investigated catalysts follow a ranking primarily
dominated by the strength of the donors’ +M effect (cf. increas-
ing donor oxidation potential relates to decreasing donor´s +M-
effect). Within this classification the ranking then follows the
acceptor strength shown in Figure 4/I which of course is intrin-
sically interconnected with the number of donors contributing to
lower the oxidation potential upon their increasing count. Inter-
estingly, interchanging chlorine and fluorine as core substituents
at the central benzene ring (3DPAClIPN and 3DPAFIPN) does
not display a difference in the observed oxidation potential.
Possibly, electron withdrawing inductive effects are counterbal-
anced by size-dependent mesomeric effects.32,34
MeCN/DCM 5:1 v/v.
Considering the structural similar catalysts containing D1
(MeOCz) and D3 (Cz) donors the reduction potentials then
follow a logical ranking referring to the overall donation power of
the system. MeOCz as better electron donor, according to its
stronger, 5-methoxy-group induced +M-effect, outranges the
corresponding Cz substituted derivatives by reductive power.
Apart from this, stronger acceptors (as judged by the electron-
withdrawing cyano and chlorine substituents of the arene core)
display their inferior ability to contribute to the delocalization
of the “hole” and hence determine the overall ranking of the
ground state reduction potentials. Notably, within the series of
examined organic dyes seemingly small structural changes
allow to access largely different redox potentials (see Table 1).
Our deliberate structural modifications of the parent cyano-
arene scaffold led to a considerable bandwidth of ∆(∆E1/2
(PC/PC●−)) = 0.76 V and ∆(∆E1/2 (PC*/PC●−) = 0.64 V) of the
reductive quenching side and ∆(∆E1/2 (PC●+/PC)) = 0.77 V
and ∆(∆E1/2 (PC●+/PC*)) = 0.86 V for oxidative quenching.
For our targeted tuning of the reduction potential of these donor-
acceptor scaffolds general considerations turned out to be substan-
tially more complicated (notably, most studies correlating elec-
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1
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60
Figure 5. Overview of common visible light photocatalysts and all investigated photocatalysts in this study, displaying their redox
potentials vs SCE40 plus E0,0 energies in extended Latimer-type diagrams.
probe and highlight their stability and specific reactivity in
Remarkably, the excited state energies E0,0 also clearly
both common oxidative and reductive transformations. While
all eight photocatalysts should be capable of catalyzing a
alterations significantly higher E0,0 values up to 2.84 V could
exceed the range as observed for 4CzIPN. With our structural
broad range of reactions due to their large, yet balanced redox
windows (cf. Figure 5), we focused for their evaluation on
challenging reactions either requiring strongly oxidative or
be observed (3DPA2FBN – as triggered by the fluorine
substituents36). These high E0,0 values offer, besides the high
ground state redox potentials E1/2 (PC/PC●−) and
E1/2 (PC●+/PC) as function of the donor-acceptor properties,
highly reductive power, as well as such in need of counterbal-
anced potentials (e.g. for net redox neutral transformations).
additionally strong excited state redox potentials
E1/2 (PC●+/PC*) and E1/2 (PC*/PC●−). Such an equal distribu-
tion of redox potentials so far was only available for transition
metal based40 complexes such as for Ir(ppy)2(dtbpy)+,
To evaluate and compare the photocatalysts´ performance
according to their customized redox properties, we therefore
selected four mechanistically distinct reactions.
2+
Ir(dF-CF3-ppy)2(dtbpy)+ and Ru(bpy)3 (cf. Figure 5), albeit
(1) Decarboxylative, Giese-type conjugate addition.
with considerably lower absolute values. Comparison of the
redox properties of the current, most common photocatalysts
with our newly designed organic dyes (Figure 5), clearly re-
veals the advantage of our donor-acceptor photocatalysts both
with respect to their remarkable redox properties, but addition-
ally regarding to the balanced distribution of reductive and
oxidative power. The predictable tunability combined with
these extended, balanced redox windows provides a broad
range of novel photocatalysts of versatile usability to outper-
form and largely complement current best-practice photocata-
lysts for both oxidative and reductive applications.
The photocatalytic, decarboxylative conjugate addition of
N-protected amino acids to electron-poor olefins was initially
investigated by MacMillan and co-workers using
Ir(dF(CF3)ppy)2(dtbpy)+ as best-performing catalyst.41 This
transformation does not only strongly rely on the (exited state)
oxidative power of the photocatalyst to mediate the
decarboxylation process (cf. e. g. Boc-Pro-OCs, E1/2 (red) =
+0.95 V vs SCE) – notably, a dramatic decrease of product
yields was observed if employing less oxidizing Ir-complexes
– but also subsequently requires sufficient reducing potential
to then yield the corresponding enolate by SET reduction of
the in situ generated α-acyl radical (cf. E1/2 (red) = –0.60 V vs
SCE).41 Hence, this C-C cross coupling reaction was already
successfully applied as test reaction for acridinium based
Application and evaluation of photocatalytic performance.
Having established promising structure-property relationships
for our donor-acceptor dyes, we aimed to challenge the newly
designed family of photocatalysts in suitable test reactions to
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Journal of the American Chemical Society
photocatalysts by DiRocco and Nicewicz14 and turned out to
their excited CT state, and hence by such an internal
quenching may prevent their engagement in productive
catalytic cycles. 42
1
2
3
be highly suitable to carve out the catalysts´ distinct
differences. We investigated the performance of our
photocatalysts in comparison to the formerly best acridinium
catalyst as identified by DiRocco.14
(2) Oxidative photocatalytic bromination of arenes.
4
5
6
7
8
9
Considering the favorable oxidative properties of our catalyst
3CzClIPN, we were interested to further explore even more
challenging oxidative transformations in order to assess its
operating scope. We hence additionally examined the
photocatalytic bromination of anisole, originally developed by
Fukuzumi and co-workers.43 Formerly this reaction, which
requires the initial, rather demanding oxidation of anisole to its
arene radical cation (E1/2 (MeOAr/MeOAr●+) = +1.79 V vs
SCE) could only be conducted using powerful oxidative
photocatalysts, such as Fukuzumi´s catalyst Mes-Acr-Me+ (for
redox data, see Figure 5). Gratifyingly, 3CzClIPN could
equally perform this challenging reaction following the
reference conditions (Table 3).
Table 2: Photocatalytic decarboxylative conjugate addition
of Cbz-proline to diethyl maleate.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
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60
entrya
photocatalyst
yieldb of 3
1
2
3
4
5
6
7
8
9
80%c
16%
37%
20%
77%c
12%
3%
4CzIPN
3DPAFIPN
Table 3: Photocatalytic bromination of anisole.
5CzBN
3DPA2FBN
3CzClIPN
3DPAClIPN
4MeOCzIPN
5MeOCzBN
Mes-4MeOAcr-2MeOPh+
reaction
entry
catalyst
yieldc of 5
timeb
16 h
16 h
1%
73%c,d
Mes-Acr-Me+
3CzClIPN
1
2
87%
89%
a irradiation with blue LEDs. byield determined by GC-FID using
mesitylene as internal standard.
As the excited state oxidation potential of 3CzClIPN
(E1/2 (PC*/PC●−) = +1.56 V vs SCE) might be slightly too low
to operate the reaction, as mainly referred, following a
reductive quenching cycle, an oxidative quenching cycle
accessing the better fitting catalyst´s ground state oxidation
potential (E1/2(PC●+/PC) = +1.79 V vs SCE) by initial
reduction of molecular oxygen could be considered (Scheme
3). This alternative mechanistic option again clearly points to
the intrinsic advantages of photocatalysts with well-balanced
redox potentials.
a conditions: Cbz-Pro-H (1) (0.20 mmol), diethyl maleate 2
(0.22 mmol), photocatalyst (2.0 mol %), K2HPO4 (0.22 mmol),
MeCN (4 mL), irradiation with blue LEDs at rt. b yield
determined by GC-FID using mesitylene as internal standard.
d
c yield of isolated product. according to ref. 14: yield 86% as
calculated via HPLC assay.
With excited state potentials E1/2 (PC*/PC●−) from +1.56 V
(3CzClIPN) to +0.92 V (3DPA2FBN) and corresponding
ground state reduction potentials in the range of E1/2 (PC/PC●−)
from –1.92 V (3DPA2FBN) to –1.16 V (3CzClIPN) vs SCE
(reductive quenching cycle) most of our catalysts should
engage in a SET oxidation of the carboxylate formed by
deprotonation of Cbz-proline (1), radical addition to
diethylmaleate (2) and the subsequent reduction to close the
net redox neutral catalytic cycle. Our best oxidizing candidates
3CzClIPN (77%) and 4CzIPN (80%) were monitored to
afford even higher yields of addition product 3 than DiRocco´s
best candidate Mes-4MeOAcr-Ph+.14
In good agreement with the results of MacMillan41 other
candidates with less oxidation power were also able to
perform the reaction, albeit with considerably lower yields
(12-37%, entries 2-4,6). Despite their fitting excited state
oxidation potentials (Figure 5 and Table 1), the two MeOCz-
substituted dye derivatives only could barely generate
product 3 (entries 7 and 8). We asssume that these both
catalysts (4MeOCzIPN and 5MeOCzBN) perform a fast
intramolecular, unproductive (back) electron transfer from
Scheme 2. Comparison of both possible quenching cycles for
the mechanism of oxidative arene bromination.
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Page 8 of 14
(3) Photocatalytic, dual carbonyl reduction reactions for catalysts, except for eosin Y (Table 4, entry 10) used in a
1
2
3
4
5
6
7
8
sequential C-O bond activation based C-C bond formation.
control reaction. To account for a more detailed analysis of
the reductive properties of our catalysts, we therefore
additionally monitored the irradiation time to reach full
conversion of starting material 6 to acetophenone 7 and
further recorded the yield of pinacol 8 (determined by
isolation) in the second, more demanding reduction. Here we
could observe significant difference for the time to reach full
conversion of compound 6.
To assess the reductive performance of our catalyst series we
selected the reductive C-O bond cleavage reaction of a lignin
model derivative (6) as a benchmark reaction. While this
transformation developed by Stephenson and co-workers44 had
already been accordingly used for the photocatalytic evalua-
tion of a 4CzIPN-derived carbazolic porous organic frame-
work,45 we considered this reaction to be an especially favor-
able, yet challenging test system for additional reasons.
Based on the similarity of conditions of Stephenson44 to such
as published by Rueping for a reductive pinacol-type coupling
of acetophenone derivatives,46 we hypothesized that the
generated product p-methoxy acetophenone (7) would hold the
option for a second, even more challenging reduction process
to be possible, subject to the stability and efficiency of the
catalysts employed. This would allow for a unprecedented
sequential combination of two different, consecutive
reductions by a single photocatalyst (Scheme 3).
Table 4. Sequential C–O-bond cleavage with subsequent
reductive pinacol coupling.
9
10
11
12
13
14
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17
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Scheme 3. A sequential, double photocatalytic reduction
strategy for the transformation of lignin derivatives
(cycle I) to pinacols (cycle II).a
time to full
conversion
entrya
photocatalyst
yieldc of 8
One-pot sequential C-O activation / C-C coupling pathway
of 6b
O
OMe
OH
1
2
3 h
1 h
61%
64%
69%
77%
0%
4CzIPN
3DPAFIPN
5CzBN
OMe
O
MeO
OH
MeO
8
6
3
1 h
4
2 h
3DPA2FBN
3CzClIPN
DIPEA
5
3 h
PC
PC
ArO
h
H
O
6
45 min
5 h
64%
0%
3DPAClIPN
4MeOCzIPN
5MeOCzBN
[Ir(ppy)2(dtbbpy)](PF6)
Eosin Y
DIPEA
I
II
7
PC
PC*
PC*
PC
MeO
7
8
5 h
0%
56%d
h
DIPEA
DIPEA
9
1 h
e
-
10
0%
proposed carbonyl activation
iPr
iPr
iPr
iPr
H
N
N
a conditions:
6 (0.5 mmol), DIPEA (1.5 mmol), HCO2H
H
O
O
[1,2]-H shift
N
iPr
N
2-center/3e
interaction
H-bond
iPr
iPr
(1.5 mmol), photocatalyst (1 mol %), MeCN (2.5 mL), irradiation
with blue LEDs. b conversion of 6 and yield of 7 monitored by
GC-FID with mesitylene as internal standard. c isolated yield after
18 h of irradiation. d 12 h of irradiation (cf. ref. 44). e after 24 h
only traces of acetophenone 7 could be detected.
iPr
Ar
R
Ar
R
activation
DIPEA
known for: R = H, CH2OR', CH3, Et
a Cycle I: C–O bond cleavage. Cycle II: ketyl radical-promoted
homocoupling (pinacol formation).
With respect to the required redox properties for these
transformations the initial C–O bond cleavage may only
be reached by high reductive potentials (cf. e.g.
E1/2 (red) = –1.74 V vs SCE for 2-oxo-2-phenylethyl acetate)
while the subsequent pinacol formation is even more
challenging (cf. e.g. acetophenone, E1/2 (red) = –2.64 V vs
SCE46). The large deviation compared to the reduction
potentials available for our catalyst family (table 1, Figure 4
and 5) suggests an additional LUMO-lowering activation of
the carbonyl group to be operative. DIPEA as tertiary amine
may not only act as a sacrificial electron donor in this reaction
sequence and HAT donor in cycle I (cf. scheme 3), but also
may considerably contribute to an additional activation of the
carbonyl group via H-bond interaction, respectively a related
LUMO-lowering 2-center/3-e– interaction (Scheme 3, lower
part) as suggested by Rueping.46 This requirement for
additional LUMO activation is in good agreement with our
previous work, where related reductive C–O-bond cleavage
reactions also required either DIPEA, ascorbic acid, or Lewis
Relying on the differing requirements for the two sequential
transformations, we hoped to use this system to single out the
specific differences in the catalytic performance of our
catalysts. To evaluate the reductive power of the catalyst series
we initially followed reaction conditions of Stephenson
(irradiation time: 12 h).44 However, we were not able to obtain
the expected acetophenone product 7 in high yields. In fact,
pinacol 8 was isolated as main product in most cases
(Table 4). In an additional control reaction we also
predominantely obtained the pinacol product 8 in 56% yield
with
Stephensons`
originally
applied
catalyst
Ir(ppy)2(dtbbpy)(PF6) (entry 9). This hence points to the
supposed, subsequent coupling reaction of the product of the
initial C-O bond cleavage after 12 h of irradiation,47 being in
in good agreement with the pinacol reaction published by
Rueping46 employing similar conditions. Notably, the
reductive C–O bond cleavage of lignin model derivative 6
(Table 4) was achieved in quantitative yields with all tested
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acids. respectively, as an additional activation tool.48 Such a
dual catalytic,3 synergistic activation would also well explain
Table 5: Detriflation of 2-naphthol triflate (9).
1
the discrepancy in the reduction potential of the weaker
catalysts and the required potential to mediate the C–O bond
breakage.
2
3
4
Taking hence the more demanding subsequent pinacol
coupling as a benchmark test for the reductive properties of
the donor-acceptor photocatalysts, we could well match the
observed performance with their redox potentials. For nearly
all catalysts the pinacol product 8 was obtained in good yields
(61-77%; isolated yield of the two-step process), except for
3CzClIPN (Table 4, entry 5), owing the lowest reduction
potential (E1/2(PC/PC●−) = –1.16 V vs SCE), and the methoxy-
carbazole substituted candidates 4MeOCzIPN and
5MeOCzBN. Despite being unserviceable to mediate the
pinacol coupling (and the oxidative decarboxylation as shown
before (Table 2)), these two MeOCz-substituted catalysts were
still able to catalyze the initial C–O bond cleavage, however
by slower rate. 3DPA2FBN, displaying the highest reduction
potential (E1/2(PC/PC●−) = –1.92 V vs SCE), in contrast
allowed for fast transformation providing the highest observed
yield (entry 4).
yieldb of
5
6
7
8
reaction
timeb
entry photocatalyst
conditions
10 │ 11
3DPA2FBN
3 equiv DIPEA
3 equiv HCO2H
MeCN
21% │ 60%
Σ 81%
1
1 mol %
8 h
9
c(PC) = 2 mM
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
fac-Ir(ppy)3
3 equiv DIPEA
3 equiv HCO2H
MeCN
22% │ 59%
Σ 81%
8 h
2
1 mol %
c(PC) = 2 mM
3 equiv DIPEA
3 equiv HCO2H
MeCN
3DPA2FBN
3
23% │ 63%
Σ 86%
12 h
8 h
1 mol %
c(PC) = 0.4 mM
3 equiv DIPEA
3 equiv HCO2H
MeCN
fac-Ir(ppy)3
4
26% │ 64%
Σ 90%
1 mol %
c(PC) = 0.4 mM
6 h
8 h
23% │ 65%
Σ 88%
3DPA2FBN
3 equiv DIPEA
3 equiv HCO2H
MeCN
(4) Photocatalytic, reductive detriflation of arene triflates.
With respect to the impressive reductive properties as
determined for catalyst 3DPA2FBN, we finally set out to
investigate its performance in a highly challenging detriflation
of unactivated aryl triflates (typical range of E1/2 (red): –2.65 V
to –1.95 V vs SCE).49
5
2 mol %
c(PC) = 4 mM
24% │67%
Σ 91%
2.2 equiv
DIPEA
Rh-6G
0% │ 0%
Σ 0%
24 h
6
DMSO
10 mol %
Importantly, the photocatalytic defunctionalization of such
non-activated substrates has previously not been described as
even stronger reducing potentials are required; related
photoredox catalytic transformations could by now only be
achieved employing activated aryl triflates (e.g. p-CN-benzene
derivatives).50 Current best-performing, visible light
photocatalytic reduction methods try to bypass the above
mentioned photophysical restrictions (in terms of providing
E > 3.0 eV). Among the recently reported, highly successful
concepts in this context are procedures relying on consecutive
electron-transfer approaches such as conPET either employing
perylene bisimides (PDIs)51 or rhodamine-6G (Rh-6G) as
photocatalyst.52 Other very powerful “super-reducing”
photocatalytic methods like SenI-ET (sensitization-initiated
electron transfer) using a combination of Ru(bpy)3Cl2 and
aromatic hydrocarbons such as pyrene,50a or TTA
(triplet-triplet annihilation)53 build on a favorable combination
of energy transfer processes together with successive electron
transfers. All these procedures provide unique, novel
opportunities to address transformations that require extremely
high reductive power.
degassed
[Ru(bpy)3]Cl2
1.4 equiv
DIPEA
1 mol %,
8% │ 34%
Σ 42%
7
12 h
DMSO
pyrene
degassed
5 mol %
a irradiation with blue LEDs. b determined by GC-FID with
mesitylene as internal standard.
While the selectivity of the bond-cleavage (C-O vs S-O) upon
SET to the triflate is the same for all investigated strategies
(according to the common SET-initiated mechanism), both the
overall yield of products 10 and 11 resulting from the initial
reduction step as well as the reaction time differ
considerably.54
Starting with typical conditions following
a reductive
quenching cycle 3DPA2FBN demonstrates its remarkable
reductive power with a total yield of 81% after 8 h of
irradiation (table 5, entry 1). Only fac-Ir(ppy)3 as commonly
used, strongly reductive precious metal-based catalyst could
afford the same overall yield of 10+11 (entry 2). SenI-ET
conditions only provided 42% yield after 12 h (entry 7), while
the in other respects highly successful alternative conPET
strategy completely failed to perform this reduction (entry 6);
even after 24 h irradiation no conversion of the starting
material could be observed.
To finally compare the reductive power of our best reducing
catalyst 3DPA2FBN (E1/2 (PC/PC●−) = –1.92 V vs SCE) to
such protocols, we chose the simple, but challenging
defunctionalization of naphthol triflate 9, whose detriflation
would require high reduction potentials (E1/2 (red) = –2.01 V
vs SCE for 9).49
With respect to the clearly limited solubility of fac-Ir(ppy)3 in
acetonitrile, we additionally evaluated the differences in
performance for the two best catalysts in this transformation
according to altering catalyst molarities. Changing from
c(photocat) = 2 mM (equivalent to c(substrate 9) = 0.2 M) to a
catalyst concentration of c(photocat) = 0.4 mM fac-Ir(ppy)3
shows improved conversion under these highly diluted
conditions (entry 4), while this lower concentration lengthens
the time for 3DPA2FBN to reach full conversion (entry 3).
Surprisingly, using 3DPA2FBN and also strongly reductive
metal-complex based photocatalyst fac-Ir(ppy)3, we were able
to achieve remarkably better results in the reduction of our
non-activated aromatic triflate than upon application of the
aforementioned SenI-ET and conPET strategies (Table 5,
entries 6 and 7).
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voltammetry, absorption and emission spectra, lifetime decay
curves.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Importantly, a high overall yield can still be obtained (86%
1
2
3
4
5
6
7
8
after 12 h), pointing to a sound catalyst stability. Doubling the
loading of photocatalyst 3DPA2FBN within the more
concentrated reaction set-up55 (entry 5) leads to a significant
acceleration providing high yields of around 90% only after
6 h reaction time. As demonstrated for this unprecedented aryl
detriflation the consideration of further aspects relevant to the
actual attractiveness of a process, such as costs, handling or
scale-up issues etc., clearly reveals obvious advantages for our
novel, strongly reducing organic photocatalyst 3DPA2FBN as
compared to other catalyts or strategies.
AUTHOR INFORMATION
▀
Corresponding Author
*E-mail: kzeitler@uni-leipzig.de
ORCID
Kirsten Zeitler: 0000-0003-1549-5002
9
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52
53
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55
56
57
58
59
60
Author Contributions
CONCLUSION AND OUTLOOK
▀
§ These authors contributed equally.
In summary, we have synthesized and studied eight donor-
acceptor molecules (including five completely new photocata-
lysts) to elucidate their structure-property relationships, which
we could beneficially use for targeted structural modification
to obtain remarkably high specific redox potentials based on
deliberate structural changes. Installation of halogen substitu-
tions at the arene core were crucial to reach the most highly
reducing and the most highly oxidizing catalysts (Figure 6).
Notes
The authors declare no competing financial interest.
Funding Sources
This work was generously supported by the Deutsche
Forschungsgemeinschaft (DFG, GRK 1626).
All catalysts stand out due to their balanced redox potentials
and their therefore broad applicability in concert with a sound
chemical stability. Moreover, we extensively evaluated all our
organic photocatalyst candidates in different challenging oxi-
dative and reductive photocatalytic transformations in com-
parison to current best-practice catalysts.
ACKNOWLEDGMENT
▀
We thank Regina Hoheisel (University of Regensburg) for her
assistance with cyclic voltammetry, M. Sc. Simon Pfeiffer (Uni-
versity of Leipzig) for his assistance with absorbance, emission
and lifetime measurements and Dr. Lothar Hennig (University
of Leipzig) for his support in terms of the structure determina-
tion via NMR.
ABBREVIATIONS
▀
General (selected). A: acceptor; conPET: consecutive
photoinduced electron transfer; CT: charge transfer; D: donor;
ET: energy transfer; HAT: hydrogen atom transfer; ISC:
intersystem crossing; MLCT: metal-to-ligand chargetransfer;
OLED: organic light emitting diode; PC: photocatalyst; ppy:
2-phenylpyridine; RSIC: reversed intersystem crosssing; SenI-ET:
sensitization-initiated electron transfer; SET: single electron
transfer; TADF: thermally activated delayed fluorescence; TTA:
triplet-triplet annihilation;
Figure 6. Structures and properties of our balanced, most
oxidizing (3CzClIPN) and reducing photocatalysts
(3DPA2FBN) in comparison to 4CzIPN.
Names and Nomenclature. BN: benzonitrile (A4); ClIPN:
5-chloro isophthalonitrile (A1Cl); Cz: carbazole (D3); DPA:
diphenylamine (D2); 2FBN: 3,5-difluoro benzonitrile
Owing to their extended redox windows we could identify
photocatalysts for all benchmark reactions, which clearly can
successfully compete with the hitherto known best catalysts
for the respective reactions, while offering the additional ad-
vantage of being easily accessible, inexpensive and metal-free.
Lastly, we could demonstrate the unprecedented photocatalytic
reduction of a non-activated aryl triflate.
=
meta,meta´-difluoro benzonitrile (A3); FIPN: 5-fluoro isophthalo-
nitrile (A1F); IPN: isophthalonitrile (A2); MeOCz:
3,6-dimethoxy-9H-carbazole (D1); Mes-Acr-Me+: 9-Mesityl-10-
methylacridinium; PDI: perylene bisimide; Rh-6-G: rhoda-
mine-6G.
The sustained need for abundant and reasonably priced,
non-metal photocatalysts makes our new family of donor-
acceptor dyes a highly attractive alternative to commonly used
Ru- and Ir-based catalysts. Based on their wide redox win-
dows and their expedient stability as demonstrated in a broad
range of benchmark reactions, we expect these organic photo-
catalysts to be broadly applicable for developing new photore-
dox catalyzed, synthetic methodologies.
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ASSOCIATED CONTENT
▀
Supporting Information. Experimental procedures and
characterization data for all compounds, including copies of
1H NMR, 13C NMR, 19F NMR (PDF), HR MS spectra, cyclic
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competitive non-radiative relaxation processes to be operative as
maybe triggered by MeOCz as stronger donor substituent. A
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(54) For a detailed, additional graphical representation of the
course of the reaction for all tested reductive transformations,
please see supporting info.
(55) Under the tested diluted conditions (c(PC) = 0.4 mM) a
doubling of the catalyst loading did not prove to be beneficial; see
supporting information for details.
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Selected reviews: a) Zeitler, K. Angew. Chem. Int. Ed. 2009, 48, 9785-9789. b) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322-5363. c) Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016, 116, 10075-10166.
2 Photokatalyse books
3 Dual photocatalysis
4 Late stage functionalization
5 Photocontrolled polymerization and photocatalysis for advanced materials
6 a) Tucker, J. W.; Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617-1622. b) Devery III, J. J.; Douglas, J. J.; Nguyen, J. D.; Cole, K. P.; Flowers II, R. A.; Stephenson, C. R. J. Chem. Sci., 2015, 6, 537-541. (b) Devery III, J. J.; Douglas, J. J.; Nguyen, J. D.; Cole, K. P.; Flowers II, R. A.; Stephenson, C. R.
J. Chem. Sci., 2015, 6, 537−541.
7 Volz, D.; Wallesch, M.; Fléchon, C.; Danz, M.; Verma, A.; Navarro, J. M.; Zink, D. M.; Bräse, S.; Baumann, T. Green Chem., 2015, 17, 1988-2011.
8 Wenger review
9 Reviwe Oliver, collins und neues Angewandte
10 Philly evtl. PCET angewandte
11 Review Wärmark evtl Febpy3 Article
12 Nickel Doyle
13 Cer Komplexe
14 Joshi-Pangu, A.; Roth, H. G.; Oliver, S. F.; Campeau, L.-C.; Nicewicz, D.; DiRocco, D. A. J. Org. Chem. 2016, 81, 7244-7249.
15 Weitere organo photoredox reviews….
16 Weaver und coord chem rev
17 Eosin review
18 Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.; Lemmetyinen, H. J. Am. Chem. Soc. 2004, 126, 1600-1601.
19 Miyake, Read de Alaniiiiiz, Phenazine, Phenoxazine Phenthiazine
20 Porphyrine Gryko
21 Pyrylium beeler, nicewicz
22 For a seminal example of the employment of TADF materials in photocatalysis, see : ZHANG
23 a) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Nature, 2012, 492, 234-238. For review articles on organic TADF materials for OLEDs see also: b) Yang, Z.; Mao, Z.; Xie, Z.; Zhang, Y.; Liu, S.; Zhao, J.; Xu, J.; Chi, Z.; Aldred, M. P. Chem. Soc. Rev. 2017, 46, 915-1016. c) Wong, M. Y.; Zysman-
Colman, E. Adv. Mater. 2017, 1605444.
24 a) Nishimoto, T.; Yasuda, T.; Lee, S. Y.; Kondo, R.; Adachi, C. Mater. Horiz., 2014, 1, 264-269. b) Cho, Y. J.; Jeon, S. K.; Chin, B. D.; Yu, E.; Lee, J. Y. Angew. Chem. Int. Ed. 2015, 54, 5201-5204.
25 Iupac gold book, Nicht Rehm weller!
26 electronegativity
27 Hinweis auf abbreviations
28
For details and experimental proof for our revised catalyst structure, please see supporting information. (vorher 24b)
29 For a selection see: a) Huang, H.; Yu, C.; Zhang, Y.; Zhang, Y.; Mariano, P. S.; Wang, W. J. Am. Chem. Soc. 2017, 139, 9799-9802. b) Matsui, J. K.; Primer, D. N.; Molander, G. A. Chem. Sci. 2017, 8, 3512-3522. c) Stache, E. E.; Rovis, T.; Doyle, A. G. Angew. Chem. Int. Ed. 2017, 56, 3679-3683. d)
Lévêque, C.; Chenneberg, L.; Corcé, V.; Ollivier, C.; Fensterbank, L. Chem. Commun. 2016, 52, 9877-9880.
30 Design Review TADF – ox potentials HOMO LUMO
31 Synthesis see SI
32 Hammett fuer Halogene…
33 Fac Ir varianten Oxpotentials
34
Correlation hammett und redox potentiale…..
35 Fluor in mEdChem und co
36 TADF Beispiele mit Fluor evtl. link zu review ergänzen. EVTL
37 Extra Fussnote für Cl, br, I BUNZ – Halogene im Donor! – shortend triplet lifetime avoiding material destruction
38 Adachi, C.; Uoyama, H.; Nomura, H.; Goushi, K.; Yasuda, T.; Kondo, R.; Shizu, K.; Nakanotani, H.; Nishide, J. WO2013154064 A1, 2013-10-17.
39 Zhang, D.; Cai, M.; Zhang, Y.; Zhang, D.; Duan, L. Mater. Horiz. 2016, 3, 145-151.
40 For the redox potentials of all metal based catalysts see ref. 1b; for Eosin Y see: Hari, D. P.; König, B. Chem. Commun., 2014, 50, 6688-6699.; for Mes-Acr+ and Mes-MeOAcr+ see ref 4 (Mes-MeOAcr+ titled as acridinium 7)
41 Chu, L.; Ohta, C.; Zuo, Z.; MacMillan, D. W. C. J. Am. Chem. Soc. 2014, 136, 10886.
42 INTRA CT Story mit Text und Referenzen
a) Kosower, E. M.; Dodiuk, H.; Tanizawa, K.; Ottolenghi, M.; Orbach, N. J. Am. Chem. Soc., 1975, 97, 2167-2178. b) Kosower, E. M.; Dodiuk, H.; Kanety, H. J. Am. Chem. Soc., 1978, 100, 4179-4188. c) Bizók, L.; Bérces, T.; Márta, F. J. Phys. Chem. 1993, 97, 8895-8899.
43 Ohkubo, K.; Mizushima, K.; Iwata, R.; Fukuzumi, S. Chem. Sci. 2011, 2, 715-722.
44 Nguyen, J. D.; Matsuura, B. S.; Stephenson, C. R. J. J. Am. Chem. Soc. 2014, 136, 1218-1221.
45 Luo, J.; Zhang, X.; Lu, J.; Zhang, J. ACS Catal. 2017, 7, 5062–5070.
46 Nakajima, M.; Fava, E.; Loeschner, S.; Jiang, Z.; Rueping, M. Angew. Chem. Int. Ed. 2015, 54, 8828-8832.
47 Determined only by isolation. Not quantifiable with GC-FID caused by a thermally initiated back reaction to acetophenone during the GC-oven heating.
48 a) Speckmeier, E.; Zeitler, K. ACS Catal. 2017, 7, 6821-6826. b) Speckmeier, E.; Padié, C.; Zeitler, K. Org. Lett. 2015, 17, 4818-4821.
49 Quelle ?
50 Kirsten du kennst die Quellen doch bestimmt auswendig ☺ (a) Ghosh, I.; Shaikh, R. S.; König, B. Angew. Chem. Int. Ed. 2017, 56, 8544–8549. (b) Shaikh, R. S.; Düsel, S. J. S.; König, B. ACS Catal. 2016, 6, 8410–8414.
51 Ghosh, I.; Ghosh, T.; Bargadi, J. I.; König, B. Science, 2014, 346, 725-728.
52 Shaikh, R. S.; Düsel, S. J. S.; König, B. ACS Catal. 2016, 6, 8410-8414.
53 Majek, M.; Faltermeier, U.; Dick, B.; Perez-Ruiz, R.; Jacobi von Wangelin, A. Chem. Eur. J. 2015, 21, 15496-15501.
54
55 See SI no effect
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