Modulation of Acridinium Organophotoredox Catalysts Guided by Photophysical Studies

Article abstract of DOI:10.1021/acscatal.9b03606

Control over redox states and spin multiplicity of photocatalysts throughout a catalytic cycle is crucial for selective and efficient photocatalytic processes. However, the rational design of photocatalysts is often hampered by the mechanistic complexity

Full text of DOI:10.1021/acscatal.9b03606

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Article
Modulation of Acridinium Organophotoredox
Catalysts Guided by Photophysical Studies
Christian Fischer, Christoph Kerzig, Bouthayna Zilate, Oliver S. Wenger, and Christof Sparr
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03606 • Publication Date (Web): 22 Nov 2019
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ACS Catalysis
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Modulation of Acridinium Organophotoredox Catalysts Guided by Pho-
tophysical Studies
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Christian Fischer,‡ Christoph Kerzig,‡ Bouthayna Zilate, Oliver S. Wenger* and Christof Sparr*
Department of Chemistry, University of Basel, Basel CH-4056, Switzerland
ABSTRACT: Control over redox states and spin multiplicity of photocatalysts throughout a catalytic cycle is crucial for selective
and efficient photocatalytic processes. However, the rational design of photocatalysts is often hampered by the mechanistic com-
plexity and low modularity of catalyst structure. Herein, we demonstrate a photophysical study of diverging photocatalytic path-
ways that guides the design of organic acridinium catalysts to complement polypyridyl transition metal systems. A combined halo-
gen-metal exchange / directed ortho-metalation provides reagents for a broad range of modular acridinium catalysts with fine-tuned
photophysical and photochemical properties such as excited-state lifetimes, redox potentials and photostabilities poised to refine
organocatalytic photoredox methodology.
KEYWORDS: acridinium salts, catalyst design, energy transfer, metalation, photoredox catalysis
INTRODUCTION
Me
N+
Synthetic methodology via radical intermediates has witnessed
a dramatic advance by the advent of preparative visible light
photoredox catalysis.1 A broad range of radical disconnections
and mild reaction conditions allowed progressive retrosynthet-
ic planning, expeditiously embraced by academic and industri-
al laboratories. The resulting versatility of visible light photo-
redox catalysis is reflected by numerous new reaction types
developed within the last ten years. A majority of these meth-
ods rely on polypyridyl transition metal complexes and in par-
ticular ruthenium and iridium catalysts.2 While polypyridyl
systems can be readily modulated by changes in the ligand
structure, organic photocatalysts,3 which render photoredox
methods sustainable and more amenable to scale up, remain
limited in their tunability. However, compared to polypyridyl
ligand modulations, molecular scaffold variation of organic
photoredox catalysts requires the development of more general
synthetic methods and therefore, most transition metal com-
plex catalyzed reactions are yet not complemented by organo-
catalytic methods.
For instance, the acridinium organophotoredox catalysts in-
troduced by Fukuzumi4 and further developed by Nicewicz5
are now established as particularly valuable catalytic motifs
for preparative organic synthesis, but their traditional assembly
severely limits synthetic modularity. Recently, we developed a
method to access acridinium salts directly from carboxylic
acid esters6 and observed the interesting photoredox activity of
diaminoacridinium salt 1 in a Ni-dual catalytic cross-coupling
reaction (Scheme 1, top pathway).7,6a This finding and the cap-
tivating prospect of a modular organophotocatalyst synthesis
prompted us to devise a detailed understanding of the photo-
physical characteristics of 1, guiding a versatile heterocycle
synthesis towards broadly applicable acridinium catalysts. A
comparative study of the novel acridinium catalysts together
with prototypical organic photocatalysts3,8 furthermore under-
lines the significance of complementarity in photoredox catal-
ysis.
Br–
Me2N
NMe2
1[PC]*
O
N
N
CO2Me
Boc
1
Mes
OH
Boc
86%[Ref. 6a]
NiCl2 glyme,
2,2'-bipy or dtbbpy
A
+
Cs2CO3 or K2CO3, DMF,
LED light (λmax: 464 nm)
I
3[PC]*
O
Br
Br
CO2Me
O
NaO
Br
O
N
O
Boc
Br
CO2Me
CO2Na
B
96%
eosin Y
Scheme 1. Divergent photochemical pathways
RESULTS AND DISCUSSION
We initiated our studies with the dual catalytic decarboxylative
cross coupling and an array of most commonly employed or-
ganocatalysts was compared to the acridinium system 1. Nota-
bly, selectivity for the photoredox process to give A was ob-
served exclusively for diaminoacridinium catalyst 1. With a
large number of other routinely used organic photocatalysts
such as eosin Y, a diverging photochemical triplet-triplet ener-
gy transfer (TTET) pathway to sensitize nickel(II) species9
provided O-aryl ester B with up to 96% yield (Scheme 1, bot-
tom pathway).10 Furthermore, with established highly oxidiz-
ing acridinium catalysts such as mesityl-2,7-dimethyl-10-
phenylacridinium5 or the Fukuzumi catalysts, either a mixture
of photoredox- and TTET products (A 10% vs. B 16%) or no
product formation at all were observed. Given the unique reac-
tivity of the diaminoacridinium system catalyzing the selective
formation of decarboxylated A, comparative DFT computa-
tions of 1 and the Fukuzumi catalyst were performed, reveal-
ing the fundamentally different nature of lowest energetic elec-
tronic transitions in these two compounds (Figure 1). The in-
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troduction of the dimethyl amino groups evidently causes a
Page 2 of 7
photoisomerisation15a giving the cis isomer in 71% yield. This
conclusion is borne out by LFP studies on the TTET reaction
between 31 and pyrene,10 which has almost the same triplet
energy as trans-stilbene, and the fact that singlet excited 1
cannot oxidize stilbene for thermodynamic reasons.15b
change from charge transfer (CT)4b to π-π* character.10 The
CT states of the Fukuzumi and related catalysts are highly
reactive and even able to oxidize anthracenes and stilbenes to
their radical cations.4b,4c,5 In contrast to those highly oxidizing
properties often resulting in compromised yields,10 excited 1
with its less energetic π- π* states is well suited for dual catal-
ysis with nickel and compatible with the components of a
complex reaction mixture.
1
2
3
4
5
6
7
8
Me
9
Me2N
N+
NMe2
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1
Me
Me
–5.80 eV
–2.80 eV
Me
Me
N+
Fukuzumi
catalyst
Me
Me
Figure 2. Nanosecond laser flash photolysis (LFP) investigations
of the triplet state of 1 (20 µM in deoxygenated MeCN) using
532 nm laser pulses of ~10 ns duration (pulse energy, 16 mJ).
Main plot, transient absorption spectrum 500 ns after the laser
pulse and time-integrated over 200 ns. Lower inset, kinetic transi-
ent absorption trace monitored at 615 nm. Upper inset, transient
absorption spectrum in the presence of anthracene (0.4 mM) rec-
orded with a time delay of 3 µs. See SI for details.
–6.91 eV
–3.57 eV
Me
Figure 1. DFT calculations (B3LYP/6-31+G(d,p), IEFPCM solva-
tion with MeCN) of 1 and the Fukuzumi catalyst. See SI for de-
tails.
Fluorescence experiments to probe the excited singlet reactivi-
ties of catalyst 1 and eosin Y were carried out with Boc-
proline derived Cs- and K-carboxylates (Figure 3). Despite
identical thermodynamics for carboxylate oxidation and a
much longer natural lifetime of singlet-excited eosin Y, effi-
cient quenching occurs only with excited 1 (in accord with the
observed C-C coupling product A). The influence of Coulomb
interactions on the electron transfer kinetics may provide a
rational explanation for the notably different rate constants of
excited singlet state quenching by anionic substrates (see SI
page S6 for details).
Interestingly, a triplet energy (ET) for 1 of 1.89 eV was esti-
mated by computation, which is identical to the triplet energy
of eosin Y (1.89 eV).3e Since the triplet sensitization of aryl
nickel carboxylates requires an ET of >1.85 eV,9a both photo-
catalysts would be thermodynamically capable of promoting
ester formation via their triplet excited states. On the other
hand, only the singlet states of both catalysts (ES:
2.40 eV/2.30 eV),1f,6a which can both provide an excited-state
reduction potential of about +1.25 V vs. SCE,1f, 3e, 6a would
allow the oxidation of the carboxylate.11 We therefore envis-
aged that these comparable driving forces, for both triplet sen-
sitization of the intermediate nickel complex and excited sin-
glet state quenching by electron transfer from carboxylates,
provide a suitable platform to differentiate the relative contri-
butions and kinetics of singlet and triplet excited states.
Employing laser flash photolysis (LFP) studies, we first ob-
served a long-lived (t: 330 µs) species after green laser excita-
tion of 1 (Figure 2); its TTET reactivity towards aromatic hy-
drocarbons such as anthracene (as opposed to the Fukuzumi
catalyst, which undergoes electron transfer) allowed us to
identify that species as the triplet state of 1 (see SI page S5 for
details). Quantitative LFP experiments12 on the sensitized
3anthracene formation with 1 as energy donor gave an ISC
3
Figure 3. Fluorescence quenching studies of 1 and eosin Y in
deoxygenated DMF upon 473 nm excitation. Main plot, response
function of the TCSPC instrument (IRF) and unquenched life-
times of both emitters. Inset, Stern–Volmer plots using either K+-
or Cs+-salt of Boc-L-proline as quencher.
(triplet) quantum yield for 1 of about 7%. A kinetic analysis of
our TTET experiments finally yielded the experimental triplet
energy of the acridinium dye 1 (1.91 eV), which is very close
to the calculated value (see above). Compared to eosin Y (tri-
plet quantum yield of 80%)13 whose triplet is usually consid-
ered as photoactive species,3e the importance of the excited
singlet state of acridinium photoredox catalysts can be further
substantiated in agreement with the studies by Nicewicz as
discussed below.14 Nevertheless, in a photochemical transfor-
mation devoid of competitive reaction pathways, the triplet
photochemistry of 1 remains operative for a classical stilbene
Based on these photochemical considerations, we conclude
that in comparison to eosin Y, both the lower triplet quantum
yield of 1 and the higher kinetic reactivity of its excited singlet
state for carboxylate oxidation give rise to the divergent pho-
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ACS Catalysis
tocatalytic reaction pathways illustrated in Scheme 1. Guided
Di- or triaryl amines 2a-e bearing a directing group and bro-
mide with or without amino substituents at distinct positions
are readily accessible, rendering this method particularly ver-
satile.10 Gratifyingly, the combined X-M exchange/DoM dou-
ble metalation with nBuLi at 65 °C afforded 1,5-dilithium
organyls 3a-e, that convert various esters into a broad range of
unsymmetric acridinium salts 4a-l.
1
2
3
4
5
6
7
8
by the notable photophysical differences between the aminoac-
ridinium system 1 and the established Fukuzumi and Nicewicz
catalysts,5 we set out to explore the generality of these charac-
teristics with an entirely modular acridinium core structure
assembly, providing a wide range of redox properties akin to
Ir- and Ru-catalysts. In contrast to our double X-M exchange6a
reactions to 3,6-diamino- and the double directed ortho-
metalation methods (dDoM) giving 1,8-dimethoxy-acridinium
salts,6b,c we investigated an integrated synthesis of amino- and
methoxy-acridinium salts by combining X-M exchange and
DoM processes (Table 1).
To our delight, the reagent from the combined X-M ex-
change/DoM double metalation gave the phenyl acridinium
product 4a in an excellent yield of 98% and also the sterically
demanding mesityl and vic.-m-xylyl substituted products were
reliably obtained (4e, 4i). Interestingly, both the dimethyla-
mino- and the triarylamine substitution further improved the
reaction outcome (4a vs. 4b and 4c vs. 4b) and even provided
atropisomeric10,6c 1-naphthyl substituted acridinium products
in high yields (4f, 4j, 4l). We next investigated the photophys-
ical and electrochemical properties of the products. The sin-
glet-excited states of all catalysts possess lifetimes of a few
nanoseconds, which is long enough for bimolecular substrate
activations. Notably, we observed catalyst tunability in terms
of oxidative character in their excited state (Table 2). In
agreement with an adjustable π-π* transition character be-
tween the frontier orbitals of the catalysts (as in Figure 1), the
novel acridinium dyes 4a-l span a wide range of triplet ener-
gies (ET) and excited state reduction potentials from E1/2(P*/
P–)= +1.81 V (4b) over E1/2 (P*/P–)= +1.40 V (4h and 4j) simi-
lar to Ru(bpz)32+, to E1/2(P*/P–)= +1.19 V vs. SCE (4l) with an
even lower E1/2(P*/P–) than Ir[dF(CF3)ppy]2(dtbbpy)+
(E1/2(P*/P–)= +1.21 V).
9
10
11
12
13
14
Table 1: Modular synthesis of amino- and methoxy-
acridinium saltsa
O
OMe
H
R' Br
R'
Br–
Li
R'
Li
MeO
N
N
nBuLi
R''
MeO
N
15
16
17
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R
THF
RT
hexane
Et2O
65 °C
R
R
MeO
R''
2a-e
3a-e
then
R = H, NMe2
R' = Me, Ph
HBr
4a-l
Product b
Product b
Product b
Ph
Ph
Me
N
N
NMe2
N
NMe2
Br–
vic.-m-xylyl
Br–
Br–
vic.-m-xylyl
MeO
MeO
MeO
vic.-m-xylyl: Me
Me
Table 2: Photophysical and electrochemical properties of
the catalysts
4a, 98%
4e, 71%
4i, 78%
Ph
Me
Ph
a
b
Dye E0,0
ET
E1/2(P/P–)
[V] c
E1/2(P*/P–)
t a
[ns]
HOMO-LUMO
transition b
N
NMe2
N
N
[eV]
[eV]
[V] d
Br–
Br–
Br–
16a
4a
4b
4c
4d
4e
4f
2.40
2.30
2.29
2.23
2.25
2.25
2.26
2.29
2.29
2.29
2.27
1.94
1.87
1.89
1.91
1.76
1.75
1.76
1.75
1.74
1.90
1.91
1.91
1.88
1.17
1.14
–1.15
–0.83
–0.48
–0.54
–0.57
–0.56
–0.53
–0.89
–0.89
–0.90
–0.87
–0.71
–0.68
+1.25
+1.47
+1.81
+1.69
+1.68
+1.69
+1.73
+1.40
+1.40
+1.39
+1.40
+1.23
+1.19
2.2
0.9, 4.4
p-p*
p-p*
*
*
MeO
MeO
MeO
4b, 53%
4f, 74%
4j, 95%
1.0, 3.0, 17.3
1.0, 9.9
p-p*
p-p*
mixed
mixed
CT
Ph
Ph
Ph
N
Br–
N
N
NMe2
1.4, 12.1
1.2, 3.3, 16.8
1.0, 4.5
Br–
Br–
NMe2
Mes
Me
MeO
MeO
MeO
Me
Mes:
Me
4g
4h
4i
1.0, 6.9
p-p*
p-p*
p-p*
mixed
p-p*
p-p*
4c, 78%
4g, 91%
4k, 78%
1.1, 6.8
Ph
Ph
Ph
N
Br–
NMe2
N
NMe2
N
1.1, 7.2
Br–
Br–
*
Mes
MeO
Mes
MeO
MeO
4j
1.0, 6.2
4k
4l
1.5, 5.3
4d, 71%
4h, 76%
4l, 94%
0.9, 5.0
a
The combined DoM/Br-Li exchange was carried out with 2a-e
a
b
Measured in MeCN (15 µmolL–1). DFT calculation, experi-
(160 µmol) in a mixture of Et2O (200 µL) and n-hexane (2.0 mL)
with n-BuLi at 65°C for 6 h. Ester substrates (100 µmol) were
treated with resulting reagents 3a-e at a temperature of
–20°C and for 12 h at RT followed by aqueous work-up with HBr.
b Isolated yields.
mental ET for 1: 1.91 eV. Measured in 0.1 µmolL–1 n-Bu4NPF6
in degassed, dry MeCN against SCE. d Details are described in the
SI.10
c
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As a rule of thumb, the introduction of NMe2 substituents low-
MeO
Me
ers the E1/2(P*/P–) by 0.3-0.4 V and OMe groups by 0.1-0.2 V,
while differences in substitution patterns show an expected
strong influence on the redox behavior.
MeO
1
2
3
4
5
6
7
8
OMe
7
OH
Photocatalyst (2.5 mol%)
O
+
(±)-9
Me
(NH4)2S2O8, MeCN, 40 h
40 W blue LED
To test the performance of the modular acridinium catalysts,
we comparatively investigated the decarboxylative fluorination
of substrates 5a and 5b (Scheme 2).16 Gratifyingly, the dia-
minoacridinium catalyst 1 allowed to reduce the reaction time
to convert 5a from 60 h to 6 h and to halve the catalyst loading
to 2.5 mol% with respect to previous organocatalytic meth-
ods,16c approaching the efficiency of optimized polypyridyl Ir-
catalysts (1 mol%).16b Interestingly, the complementarity to
known acridinium catalysts (47–52%)10 and donor-acceptor
cyanoarene photocatalysts was particularly evident (4CzIPN,
65%).17 Cognizant of the complex interplay of the numerous
catalyst properties impacting their performance, tentative cor-
relations indicate the merits of a library based on an identical
core structure. The yield was found to increase from 58% with
catalyst 4e (E1/2(P/P–)= –0.56 V; E1/2 (P*/P–)= +1.69 V), 75%
with 6-aminoacridinium 4h (E1/2(P/P–)= –0.89 V; E1/2 (P*/P–)=
+1.40 V) to 80% with catalyst 1 (E1/2(P/P–)= –1.15 V; E1/2
(P*/P–)= +1.25 V). 7-Aminoacridinium catalyst 4k with a dif-
ferent substitution pattern eludes this trend, possibly due to the
weaker absorption of the catalyst at the peak emission wave-
length of the light source. Strikingly, changing to substrate 5b
required acridinium salts 4e or 4f (E1/2(P/P–)= –0.56 V; –0.53
V; E1/2 (P*/P–)= +1.69 V; +1.73 V) for optimal reaction condi-
tions (93% and 94% yield) akin to the described performance
of distinct polypyridyl transition metal catalyst in this reac-
tion,16a,b further underscoring the importance of modularity in
the acridinium catalysts.
43% with 4d
37% with 4k
61% with 4j
53% with 1
(59% with 4CzlPN)
OMe
55% with 4e
77% with 4h
57% with 4i
8
Scheme 3. [3+2]-Cycloaddition catalyzed by an aminoacri-
dinium catalyst.10
9
10
11
12
13
14
15
16
17
18
19
20
21
22
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42
43
44
45
46
47
48
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50
51
52
53
54
55
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57
58
59
60
Consistent with these observations, the decarboxylative dual
catalytic cross coupling (Scheme 1) was also observed to be
strongly impacted by the substitution pattern of the modular
acridinium catalysts (Table 3). In this reaction, the examined
6-aminoacridinium catalysts 1, 4h and 4i suitably provided
C-C cross-coupled product A with an increased yield when
using the milder catalyst 1 (E1/2(P/P–)= –1.15 V; E1/2 (P*/P–)=
+1.25 V), whereas the Fukuzumi catalyst, 4d and 4e appear as
too strongly oxidizing for this multicomponent system.
Table 3: Impact of modularity for acridinium-catalyzed
cross couplings a
Dye
1
A
B
-
Dye
4h
4i
A
57%
75%
-
B
86%
5%
6%
8%
4d
4e
-
-
3%
9%
4k
a Dual catalytic cross coupling as shown in Scheme 1.
F
CO2H
selectfluor
Photocatalyst (2.5 mol%)
66% with 4d
58% with 4e
75% with 4h
78% with 4i
56% with 4k
80% with 1
Notably the differences of 1 and the Fukuzumi catalyst ob-
served in the photophysical study were also confirmed in stil-
bene isomerization reactions using 4a-4l and a dimethylacri-
dinium catalyst,14 resulting in significant variation of the isom-
erization efficiency (17–84% yield for cis-stilbene, Table 4).10
While the distinction between TTET (triplet mechanism)9f and
electron transfer driven (radical cation mechanism)4c stilbene
isomerization reactions remains tentative, the most highly oxi-
dizing acridinium catalysts with CT excited states were found
to result in low cis isomer yields (4e, (E1/2(P/P–)= –0.56 V;
E1/2 (P*/P–)= +1.69 V, 16%). In contrast, low excited-state
reduction potentials, π- π* states and high triplet energies ap-
pear as promising catalyst properties for accumulating the
thermodynamically less stable cis-stilbene isomer (4k,
(E1/2(P/P–)= –0.71 V; E1/2 (P*/P–)= +1.23 V, 84%).
Na2HPO4, MeCN/H2O
40 W blue LED, 6 h
N
N
Bz
Bz
(65% with 4CzIPN)
5a
6a
tBu
CO2H
O
tBu
F
90% with 4d
93% with 4e
83% with 4h
86% with 4i
89% with 4k
94% with 4f
85% with 1
selectfluor
Photocatalyst (2.5 mol%)
N
O
N
O
O
Na2HPO4, MeCN/H2O
40 W blue LED, 3 h
(84% with 4CzIPN)
5b
6b
Scheme 2. Comparative acridinium catalyzed decarboxyla-
tive fluorination.10
These findings were substantiated by an unprecedented organ-
ophotocatalytic variant of Yoon's [3+2]-cycloaddition induced
by the oxidation of phenol 7 to form 2,3-dihydrobenzofuran
(±)-9 (Scheme 3).18 The aminoacridinium catalyst 4h provided
(±)-9 with a suitably low catalyst loading of 2.5 mol%, thus
representing an efficient organophotocatalytic alternative for
this oxidative cycloaddition.10 While subtle effects likely also
impact the performance of this reaction, the aminomethoxy-
acridinium catalysts, which are readily accessible by the de-
scribed synthetic approach (Table 1) and characterized by re-
duction potentials of around (E1/2(P/P–)= –0.89 V; E1/2 (P*/P–)=
+1.40 V), appear as optimal catalysts. Highly oxidizing photo-
catalysts (4d, 4e) might also oxidize the olefinic co-substrate,
reducing the overall yield of the transformation.
Table 4: Acridinium stilbene isomerization
Dye
cis-
Dye
cis-
stilbene
Dye
cis-
stilbene
stilbene
1
78%
47%
17%
4e
4f
16%
45%
41%
4j
4k
4l
54%
84%
77%
4a
4b
4g
a D4ucal catalyt3ic7%decarboxylative cross couplidnimgeithnySl- cheme 1.
4h
72%
28%
acridinium14
4d
36%
4i
70%
Fukuzumi
catalyst
18%
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ACS Catalysis
The combined effects of the numerous catalyst properties,
ACKNOWLEDGMENTS
1
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8
such as excited-state energies, redox potentials, excited-state
lifetimes, extinction coefficients, catalyst stabilities and the
photoactivities of degradation products, underline the value of
tunability within a class of photocatalysts for the design and
optimization of photocatalytic reactions. Moreover, photosta-
ble catalysts significantly reduce further uncertainties, which
prompted us to investigate the inherent photostabilities of the
prepared acridinium dyes. Good to excellent intrinsic catalyst
stabilities that compare favorably to Ru(bpy)32+ were observed
in the photostability assay (see SI page S38 for details).
We gratefully acknowledge the Swiss National Science Founda-
tion (BSSGI0-155902/1 and 175746), the University of Basel, the
German National Academy of Sciences Leopoldina and the
NCCR Molecular Systems Engineering for financial support. We
thank Prof. M. Mayor for equipment usage and Thomas Buchholz
and Dragan Miladinov for skillful experimental support.
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CONCLUSIONS
In conclusion, divergent organophotocatalytic reaction path-
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modularity of organophotocatalysts to complement tunable
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versatile acridinium synthesis using bifunctional organodilith-
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and a broad range of excited state reduction potentials were
prepared. The virtues of modular organophotocatalysts were
evident by optimal catalytic features to refine photoredox
methodology. With the growing scope of complementary
families of organophotocatalysts, significant contributions to
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AUTHOR INFORMATION
Corresponding Authors
* Email: oliver.wenger@unibas.ch, christof.sparr@unibas.ch
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the manu-
script. ‡These authors contributed equally.
Funding Sources
Swiss National Science Foundation (BSSGI0-155902/1 and
175746), German National Academy of Sciences Leopoldina,
NCCR Molecular Systems Engineering
Notes
The authors declare no conflict of interest. Compounds 4a-l are
part of a filed patent (C. Fischer, C. Sparr. EP 17/188,288) li-
censed to Solvias. The catalysts will be commercially available.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Experimental procedures, compound characterization, divergent
pathways, DFT calculations, photophysical properties, quenching
studies, emission lifetime determinations, benchmarking reactions
and inherent photostability studies (PDF)
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Page 6 of 7
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Insert Table of Contents artwork here
C-N cross
coupling
X-M
exchange
N+
DoM
5
lithiation
R
R''
6
7
8
1.25
1.5
1.75
2.0 E1/2(P*/P–)
vs. SCE
direct transformation
of esters
R
9
Photophysical Studies Catalyst Design
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• π-π* and CT transitions
• singlet/triplet reactivity
• ns emission lifetimes
• inherent photostability
• modular and complementary
• up to 98% yield
• broad range of E1/2(P*/P–)
• singlet photoredox catalysis
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