Naphthalene diimides with improved solubility for visible light photoredox catalysis

Article abstract of DOI:10.3762/bjoc.15.201

Five core-substituted naphthalene diimides bearing two dialkylamino groups were synthesized as potential visible light photoredox catalysts and characterized by methods of optical spectroscopy and electrochemistry in comparison with one unsubstituted naphthalene diimide as reference. The core-substituted naphthalene diimides differ by the alkyl groups at the imide nitrogens and at the nitrogens of the two substituents at the core in order to enhance their solubility in DMF and thereby enhance their photoredox catalytic potential. The 1-ethylpropyl group as rather short and branched alkyl substituent at the imide nitrogen and the n-propyl group as short and unbranched one at the core amines yielded the best solubilities. The electron-donating diaminoalkyl substituents together with the electron-deficient aromatic core of the naphthalene diimides increase the charge-transfer character of their photoexcited states and thus shift their absorption into the visible light (500-650 nm). The excited state reduction potential was estimated to be approximately +1.0 V (vs SCE) which is sufficient to photocatalyze typical organic reactions. The photoredox catalytic activity in the visible light range was tested by the α-alkylation of 1-octanal as benchmark reaction. Irradiations were performed with LEDs in the visible light range between 520 nm and 640 nm. The irradiation by visible light together with the use of an organic dye instead of a transition metal complex as photoredox catalyst improve the sustainability and make photoredox catalysis “greener”.

Full text of DOI:10.3762/bjoc.15.201

Naphthalene diimides with improved solubility for visible light
photoredox catalysis
Barbara Reiß and Hans-Achim Wagenknecht*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 2043–2051.
Institute of Organic Chemistry, Karlsruhe Institute of Technology
(KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
doi:10.3762/bjoc.15.201
Received: 20 May 2019
Accepted: 12 August 2019
Published: 27 August 2019
Email:
Hans-Achim Wagenknecht* - Wagenknecht@kit.edu
* Corresponding author
This article is part of the thematic issue "Dyes in modern organic
chemistry".
Keywords:
chromophore; dyes; electrochemistry; photochemistry; photoredox
catalysis
Guest Editor: H. Ihmels
© 2019 Reiß and Wagenknecht; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Five core-substituted naphthalene diimides bearing two dialkylamino groups were synthesized as potential visible light photoredox
catalysts and characterized by methods of optical spectroscopy and electrochemistry in comparison with one unsubstituted naphtha-
lene diimide as reference. The core-substituted naphthalene diimides differ by the alkyl groups at the imide nitrogens and at the
nitrogens of the two substituents at the core in order to enhance their solubility in DMF and thereby enhance their photoredox cata-
lytic potential. The 1-ethylpropyl group as rather short and branched alkyl substituent at the imide nitrogen and the n-propyl group
as short and unbranched one at the core amines yielded the best solubilities. The electron-donating diaminoalkyl substituents
together with the electron-deficient aromatic core of the naphthalene diimides increase the charge-transfer character of their photo-
excited states and thus shift their absorption into the visible light (500–650 nm). The excited state reduction potential was esti-
mated to be approximately +1.0 V (vs SCE) which is sufficient to photocatalyze typical organic reactions. The photoredox catalyt-
ic activity in the visible light range was tested by the α-alkylation of 1-octanal as benchmark reaction. Irradiations were performed
with LEDs in the visible light range between 520 nm and 640 nm. The irradiation by visible light together with the use of an
organic dye instead of a transition metal complex as photoredox catalyst improve the sustainability and make photoredox catalysis
“greener”.
Introduction
Photocatalysis couples the physical process of light absorption ated by a sensitizing species – a photocatalyst. If the interacting
to an organic-chemical reaction by means of time, space and mode between the sensitizer and the reactant is via charge
energetics. In order to apply visible light for photocatalysis transfer, it is named photoredox catalysis. This research field
despite its rather low energy this coupling requires to be medi- has been established over the past decade [1-20]. In principle, it
2043

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
is a sustainable method for catalysis because sunlight is an The cNDIs 2–6 differ by the alkyl groups at the imide nitro-
essentially unlimited and thereby “green” natural light source gens and at the nitrogens of the two core substituents at the core
and LEDs – conveniently used for irradiation experiments in the in order to enhance their solubility. In general, NDIs are elec-
laboratory – are cheap and energy-saving artificial sources for tron-poor chromophores. The diamino substituents of cNDIs
irradiations. The current “working horse” for photoredox catal- 2–6 are expected to increase the charge-transfer character of
ysis is mainly [Ru(bpy)3]2+ [21], due to its strong MLCT their photoexcited states in order to shift their absorption into
(metal-to-ligand charge transfer) absorption, the excellent yield the visible light and to improve their photoredox catalytic
of its triplet state and the long lifetime, the versatile redox be- power. The photoredox catalytic activity was representatively
havior (Ru3+ vs Ru+) in quenching processes and the chemical tested by the MacMillan benchmark reaction [53]. For this type
and photochemical robustness. Despite their positive of photoredox catalytic reaction, solubility of cNDIs in DMF is
photoredox catalytic behavior, transition metal complexes have a crucial prerequisite.
disadvantages, including high costs due to limited availability,
toxicity [22,23] and polluting properties [24]. This thwarts the
Results and Discussion
principally “green” concept of photoredox catalysis. In order to Synthesis of cNDIs 2–6 and their solubility
avoid transition metals and enhance the sustainability further, The commercially available precursor for all NDIs is 1,4,5,8-
organic compounds, mainly eosin Y [25], rhodamine 6G [26], naphthalenetetracarboxylic acid anhydride (7) and the synthesis
9-mesityl-10-methylacridinium perchlorate [27], 1,2,3,5- of cNDIs follows a modular approach. Core-unsubstituted NDIs
tetrakis(carbazol-9-yl)-4,6-dicyanobenzenes [28] and are typically prepared by condensation of 7 with 2–3 equiva-
N-phenylphenothiazines [29] were applied as important alterna- lents of the respective amine. The corresponding reaction of 7
tive photoredox catalysts [30,31]. These studies conclusively with n-octylamine in DMF gave the reference NDI 1 in 90%
showed that there is no universal photoredox catalyst for differ- yield (Scheme 1) [54]. The synthetic module for cNDIs with
ent organic transformations. Instead, each photoredox catalyst two substituents at the core is the 2,6-dibromo anhydride deriv-
has its own reactivity profile and scope. In order to apply ative 8 that can be synthesized from 7 by elementary bromine,
organic dyes in advanced photoredox catalysts in a versatile dibromoisocyanuric acid, or 1,3-dibromo-5,5-dimethylhydan-
way, it is crucial that modifications can be easily incorporated toine (DBH) in concentrated sulfuric acid, or oleum in good
into the core structure in order to tune optical and redox proper- yields [55-57]. The regioselective introduction of just two sub-
ties. Naphthalene diimides (NDIs) as the smallest possible stituents can be controlled by stoichiometry. Accordingly, the
rylene dyes are such an important class of organic dyes. In 2,6-dibromo derivative 8 was prepared by 1.5 equivalents DBH
contrast to their bigger homologs perylene diimides which were and further used as crude product because it cannot be purified
rarely used for photoredox catalysis [32-34], NDIs have a lower due to its very poor solubility. The subsequent condensation
tendency to self-aggregate due to their smaller aromatic surface with n-octylamine, 1,4-dimethylpentylamine and 1-ethylpropy-
and thus are slightly better soluble in organic solvents [35-38]. lamine in refluxing CH2Cl2 gave the cNDIs 9–11 in 42%, 14%
NDIs are intensively applied as functional dyes [39,40], for arti- and 24% yields, respectively. Finally, the two bromo groups in
ficial photosynthesis [41,42], for molecular architectures by cNDIs 9–11 were substituted by n-propylamine as nucleophile
self-assembly [43,44], as molecular sensors [45-47] and for in refluxing CH2Cl2 to the cNDIs 2, 3 and 6 in 80%, 87% and
organic electronics [48,49], but yet nearly completely unex- 58% yields, respectively. The two cNDIs 4 and 5 carrying the
plored for photoredox catalysis. Core-unsubstituted NDIs are same alkyl groups at the imide nitrogens and at the nitrogens of
colorless compounds with high extinction coefficients at the the core substituents were directly synthesized (in one step) by
border between UV-A and visible light. Their fluorescence 1-propylbutylamine or 1-ethylpropylamine using pressure reac-
quantum yields are rather low and fluorescence lifetimes are tion vials. The yields were rather low, 14% for 4 and 25% for 5,
rather short due to ISC into the triplet state [50,51]. NDIs are but they should be regarded with respect to the fact that the ad-
reversibly reducible and their stable radical anions absorb in the ditional time-consuming isolations of the respective dibromo-
visible to NIR light range [37]. The aromatic core of NDIs can cNDI intermediates were omitted. After purification by column
be easily modified by substituents in order to tune their optical chromatography, all five cNDIs 2–6 were resolved in benzene
and redox properties as mentioned above [39,40,52]. The and lyophilized under reduced pressure.
common synthetic approach for core-substituted NDIs (cNDIs)
makes tailor-made dyes rather easily accessible. With respect to In general, NDIs are well soluble in CH2Cl2. As mentioned in
these unique properties, NDIs should also be explored for the introduction, solubility of cNDIs as potential photoredox
photoredox catalysis. We present herein the synthesis and char- catalysts in DMF or in mixtures of DMF with CH2Cl2 is a
acterization of NDI 1 as unsubstituted chromophore reference crucial prerequisite for their photoredox catalytic suitability.
and cNDIs 2–6 as potential visible light photoredox catalysts. The solubility was determined by preparing saturated solutions
2044

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
Scheme 1: Synthesis of reference NDI 1 and cNDIs 2–6; bottom: image of saturated solutions of cNDIs 2–6 in DMF.
of the respective NDI 1 or cNDIs 2–6 at 22 °C in pure DMF. concentration (cmax) of 2.5 mM. Saturated samples in DMF
The absorbance was measured after filtration of the solution and gave the first impression of cNDIs 2–6 (see bottom image in
after redilution if the optical density in the cuvette with 1 cm Scheme 1), and merely by visible inspection, the solubility of
path length exceeded 1. The reference NDI 1 with the two the cNDIs follows the order from 2 to 6. In fact, cmax of 2 is far
n-octyl substituents at the imide nitrogens showed a maximum below 1 mM and difficult to determine by this simple method.
2045

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
At the far end, the best and measurable solubilities showed
cNDIs 5 and 6 with cmax of 0.6 mM and 2.7 mM, respectively.
The 1-ethylpropyl groups as rather short and branched alkyl
substituents at the imide nitrogens and the n-propyl groups as
short and unbranched ones at the core amines gave the best
combination to improve the solubility. This qualifies cNDI 6 as
the best soluble potential visible light photoredox catalyst.
Characterization of NDI 1, cNDI 2 and cNDI 6
The synthesized reference NDI 1 and the new cNDI 6 as poten-
tial photoredox catalyst were characterized in DMF and CH2Cl2
by means of optical spectroscopy and electrochemistry. The
UV–vis absorbance of the core-unsubstituted NDI 1 in CH2Cl2
show characteristic bands in the range between 300 nm and
400 nm with the maximum at 380 nm (Figure 1). The absor-
bance of NDI 1 in DMF is very similar to that in CH2Cl2, only
the extinction is slightly reduced. The charge transfer character
of the excited state of cNDI 6 should yield an absorbance in the
visible range for photoredox catalysis. In fact, the absorbance of
Figure 1: Optical properties of NDI 1 and cNDIs 2 and 6: UV–vis ab-
sorbance in CH2Cl2 and in DMF (normal lines), normalized fluores-
cence in CH2Cl2 (dashed lines); 1 in CH2Cl2 (black), 1 in DMF (gray),
2 in CH2Cl2 (green), 6 in CH2Cl2 (dark blue), 6 in DMF (light blue),
lexc = 350 nm for 1, λexc = 600 nm for 2 and 6.
the new cNDI 6 in CH2Cl2 shows considerably red-shifted File 1). The cyclic voltagram of NDI 1 shows two reversible
bands in the range between 500 nm and 650 nm with the reductions and one irreversible oxidation with E ≈ 0.85 V vs
maximum at 612 nm. The absorbance of 6 in DMF is very simi- SCE. The two potentials in the negative potential range can be
lar to that in CH2Cl2. The similarities of the UV–vis absor- assigned to the formation of the radical anion 1•−, E½(1/1•−) =
bance of the cNDIs 2 and 6 in CH2Cl2 exemplarily evidences −0.69 V, and the dianion 12−, E½(1•−/12−) = 1.10 V (Table 1).
that length and branching of the alkyl substituents both at the These values agree well with literature results [48,49,54]. In
imide nitrogens and at the core amino groups have only little in- comparison, the reductions of cNDI 2 to the radical anion 2•−
fluence on the optical properties but significantly modulate the were observed at E½(2/2•−) = −1.06 V and to the dianion 22− at
solubility in DMF. This result agrees with other cNDIs in litera- E½(2•−/22−) = −1.50 V and show the electronic effect of the two
ture [47,48,58]. NDI 1 shows weak fluorescence in CH2Cl2 n-propylamino substituents. Additionally, two reversible oxida-
with a maximum at 384 nm and a quantum yield of 7%. The tions were observed at E½(2•+/2) = +0.99 V and E½(22+/2•+) =
Stokes’ shift is small (413 cm−1). Based on these values, the ex- +1.40 V which can be assigned to the electron-donating effect
citation energy E00 for the singlet state which is an important of the n-propylamino groups. The key values for the photoredox
prerequisite for photoredox catalysis with 1 can be estimated to catalytic activity (vide infra) are the excited state potential for
be 3.25 eV. In DMF, the fluorescence of 1 is completely the reduction, E*red(2*/2•−) = +0.92 V, and for the oxidation
quenched. This is due to a photoinduced charge transfer be- E*ox(2•+/2*) = −0.99 V. According to the categories for
tween NDI 1 and DMF. DMF has an oxidation potential of strength of chemical redox agents [60], cNDI 2 in the excited
0.38 V vs SCE [59]. Together with the reduction potential of state is a strong oxidant and a mild reductant. We assume based
Ered = 0.69 V and E00 = 3.25 eV for NDI 1 (vide infra), this on literature-known cNDIs that the different alkyl groups of the
electron transfer is clearly exergonic (ΔG = Eox − Ered − E00 = other cNDIs 3–6 have no or only very little influence on the
−2.2 eV). NDI 6 shows a strong and broad fluorescence in electrochemical properties in comparison with those of cNDI 2
CH2Cl2 with a maximum at 640 nm and a quantum yield of [48,49]. Hence, the photoredox properties of the new cNDI 2–6
48%. The Stokes’ shift is rather large (715 cm−1). The excita- are comparable to those of eosin Y and rhodamine 6G as other
tion energy E00 of cNDI 6 in the singlet state is approximately organic photoredox catalysts.
1.98 eV and is significantly smaller than that of NDI 1 due to
the visible light excitation which delivers less energy to the Photoredox catalysis with NDI 1 and cNDI 6
excited state.
The α-alkylation of 1-octanal (12) by diethyl 2-bromomalonate
(13) yielding product 14 (Scheme 2) is one of the benchmark
The redox potentials of the reference NDI 1 and the cNDI 2 reactions for photoredox catalysis because it combines
(due to the better solubility) were determined in comparison by photoredox catalysis with organocatalysis [53]. Initially,
cyclic voltammetry in CH2Cl2 and in the presence of 0.1 M [Ru(bpy)3]Cl2 was applied by MacMillan et al. as photoredox
n-Bu4PF6 as conducting salt (see Supporting Information catalyst together with the chiral imidazolidinone 15 as organo-
2046

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
Table 1: Optical and electrochemical properties of NDI 1 and cNDI 2 (in CH2Cl2) in comparison to other organic photoredox catalyst X, in particular
eosin Y (EY), rhodamine 6G (Rh6G) and 9-mesityl-10-methylacridinium perchlorate (MesAcr) 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzenes
(4CzIPN) and N-phenylphenothiazine (Ptz).
X
λ
E00a
E½(X/X•−)
E*red(X*/X•−)a
E½(X•+/X)
E*ox(X•+/X*)a
[nm]
[eV]
[V]
[V]
[V]
[V]
1
2
380
612
539b
530d
425
3.25
1.98
2.31
2.32
2.67e
3.25
2.67
−0.69
−1.06
−1.08c
−1.14
−0.49
–
+2.56
+0.92
+1.23
+1.18
+2.08e
–
–
–
+0.99
+0.76c
+1.23
–
+0.75
+1.49b
−0.99
−1.58
−1.09
–
−2.50
−1.18
EY [61]
Rh6G [62]
MesAcr [63]
Ptz [29]
4CzIPN [28]
320
≈370b
−1.24b
+1.43
All potentials were converted from the ferrocene scale to the SCE scale [64]. aFor singlet state. bIn MeCN. cIn MeOH. dIn EtOH. eCT state.
catalyst to achieve enantioselectivity. Several proposal for the solvent mixture of DMF/CH2Cl2 = 1:1 was used. 20 mol % of
mechanism are found in literature ranging from a closed organocatalyst 15 were applied. 2,6-Lutidine was added as base
photoredox catalytic cycle [53,61] to a chain propagation mech- to trap protons that are potentially formed during the reaction
anism with photoredox initiation [65]. We evaluated the refer- and to ensure enamine formation with the organocatalyst 15.
ence NDI 1 and the cNDI 6 as new photoredox catalysts using Under these conditions, a moderate conversion of 13 (65%) and
this benchmark reaction. The samples were degassed by the a low yield of product 14 (25%) were obtained (Table 2). In
freeze-pump-thaw method, the catalysis was performed under pure CH2Cl2 as solvent there was only a small conversion, but
inert gas conditions (Argon), and the chemical conversions and no product 14 detectable. Only the debrominated diethyl
yields were determined by 1H NMR spectroscopy (see Support- malonate was identified as side product. This makes conclu-
ing Information File 1). The enantiomeric excess of the product sively clear that the solvent DMF is needed for this type of
14 was determined after conversion with (2S,4S)-(+)-pentane- photoredox reaction. The control reaction without light did not
diol into diastereomers and by integration of the corresponding show conversion at all. Control reactions with light, but with-
signals in the 1H NMR spectrum. In all experiments with NDI 1 out NDI 1 showed, however, a low conversion of 24% and a
the enantiomeric excess exceeded a value of 78%; in all experi- low yield of 18%. This is due to the UV-A absorption of the en-
ments with cNDI 6 the enantiomeric excess was higher than amine that is formed as intermediate between 1-octanal (12) and
81%. This parameter is omitted for clarity in the following para- the organocatalyst 15. Similar photochemical reactions by
graphs because enantioselectivity is not a matter of discussion direct excitation of the emanine intermediate were described by
in this work.
Melchiorre et al. [66]. In pure DMF this effect is even stronger
and increases the conversion to 80% and the yield to 58% even
in the absence of NDI 1. Obviously, photoredox catalytically
driven conversion by NDI 1 competes with the direct excitation
of the intermediate enamine. This is the reason why lowering
The photoredox catalysis with NDI 1 was performed by LEDs
with 387 nm maximum emission wavelength and an irradiation
time of 18 h. In order to ensure solubility of all components, a
Scheme 2: Photocatalytic α-alkylation of octanal (12): 500 mM 12, 250 mM 13, 50 mM (20 mol %) organocatalyst 15, 500 mM 2,6-lutidine, NDI 1 or
cNDI 2–6 as photoredox catalyst in 1.3 mL solvent, stirring, irradiation by LED, see Table 2.
2047

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
Table 2: Photoredox catalytic conversions of diethyl 2-bromomalonate (13) with 1-octanal (12) and yields of product 14 by different photoredox cata-
lysts (X).
Entry
X
mol %
LED
solvent
Conversion (%) of 13
Yield (%) of 14
1
2a
3
4
5a
6
7
8
9
1
1
1
1
1
–
1
1
–
6
6
6
5
5
2.5
1.25
1.25
–
5
5
–
387
387
387
387
387
387
387
387
387
520
597
637
DMF/CH2Cl2 1:1
DMF/CH2Cl2 1:1
DMF/CH2Cl2 1:1
DMF/CH2Cl2 1:1
DMF/CH2Cl2 1:1
DMF/CH2Cl2 1:1
CH2Cl2
DMF
DMF
DMF/CH2Cl2 1:1
DMF/CH2Cl2 1:1
DMF/CH2Cl2 1:1
65
87
67
67
99
24
30
69
80
77
46
n.d.
25
45
32
30
60
18
n.d.
44
58
69
43
n.d.
10
11
12
0.1
0.1
0.1
a40 mol % 15.
the concentration of the photoredox catalyst 1 from 5 mol %
over 2.5 mol % to 1.25 mol % did not reduce conversions and
yields, but increasing the amount of organocatalyst 15 from
20 mol % to 40 mol % finally improved the conversion to 99%
and the yield to 60% Table 2, entry 5).
These photoredox catalytic experiments with NDI 1 made
obvious that the excitation must be shifted from the UV-A
range into the visible range in order to achieve selective excita-
tion of the photoredox catalyst and not a mixture of different
photomechanisms. cNDI 6 with its absorbance between 500 nm
and 650 nm and a maximum at 612 nm fullfills this require-
ment. The corresponding photoredox catalytic experiment with
cNDI 6 and the 597 nm LED as irradiation source gives a
conversion of 46% and a product yield of 43%. The emission of
this LED overlays well with the absorbance of cNDI 6
(Figure 2). As expected, the photoredox catalytic reaction with
cNDI 6 is much “cleaner” than with NDI 1 and the substrate
Figure 2: Normalized absorbance of cNDI 6 in comparison to normal-
ized emission of the 468 nm, 520 nm, 597 nm, 638 nm and 683 nm
LEDs.
conversion differs only slightly from the product yield. Taken to the irradiation wavelength which was further studied by more
together, cNDI 6 is a suitable visible light photoredox catalyst detailed kinetic measurements.
for this reaction although its excited state potential (comparable
to E*red(2*/2•−) = +1.92 V) is much lower than that of NDI 1 Time-dependent product analysis was performed during the
(E*red(1*/1•−) = +2.56 V) but obviously still sufficiently high. photoredox catalytic conversions using different LEDs for irra-
However, irradiations at different wavelengths gave surprising diations at 468 nm, 520 nm, 597 nm, 638 nm and 683 nm (see
results: (i) If the 637 nm LED is applied for irradiation, there is LED emissions in Figure 2, results in Figure 3). These experi-
no conversion of substrate 13 detectable although the emission ments were performed in pure DMF, which further accelerates
of this LED also overlays well with the absorbance of cNDI 6. the photoredox catalytic conversion due to the better solubility
(ii) If the 520 nm LED was applied the conversion increased to of the cNDI 6. The product formation is almost finished after
77% and the yield to 69%, although excitation of only the side 4 h of irradiation. The most productive excitation provides the
absorption band of cNDI 6 is realized in this experiment. Obvi- 520 nm LED with a product yield of 82% although the emis-
ously, there is a strong difference between the absorbance of sion of this LED overlays only partially with the absorbance of
cNDI 6 and its photoredox catalytic activity profile with respect cNDI 6. The emissions of the 597 nm and 638 nm LEDs do
2048

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
Conclusion
The cNDIs 2–6 were synthesized as potential photoredox cata-
lysts that differ by the alkyl groups at the imide nitrogen and at
the two amino substituents at the core in order to improve their
solubility. Especially the cNDI 6 showed a good solubility in
DMF that is comparable to the unsubstituted NDI 1 and suit-
able for photoredox catalysis in solvent mixtures with DMF.
Due to the charge-transfer character in the excited state the ab-
sorbances of cNDIs 2 and 6 are shifted into the visible range
with a broad band between 500 nm and 650 nm. The reduction
potential to form the radical anion of such a cNDI is significant-
ly shifted to a more negative potential of E½(2/2•−) = 1.06 V.
Together with E00 = 1.98 eV an excited state potential of
E*red(2*/2•−) = +0.92 V was estimated for the singlet state
which renders such cNDIs to be suitable to photocatalyze
organic reactions. The photoredox catalytic activities of NDI 1
and cNDI 6 in comparison were successfully evaluated for the
MacMillan benchmark reaction. This photoredox catalytic reac-
Figure 3: Kinetic analysis of yields of product 14 in the presence (solid
lines) and in the absence (dashed lines) of cNDI 6: 500 mM 12,
250 mM 13, 50 mM organocatalyst 15, 500 mM 2,6-lutidine in 1.3 mL
DMF, stirring, irradiation by LEDs.
overlay better but show lower yields of 55% and 62%, respec- tion in the presence of cNDI 6 was much “cleaner” than with
tively, in photoredox catalysis. The lag phases of more than NDI 1 since the conversions differed only slightly from the
30 min in the latter experiments indicate that indeed the en- product yields. Irradiations were performed with LEDs in the
amine formation plays a crucial role. According to the mecha- visible light range between 520 nm and 640 nm. The substrate
nism suggested by Yoon et al. the photoredox catalytic cycle is conversion and product yields were significantly higher by LED
initiated by the oxidation of the enamine [65]. Control experi- irradiation into the absorbance shoulder at 520 nm which
ments were performed without cNDI 6 as photoredox catalyst to implies a non-Kasha-type photodynamic behavior. This makes
elucidate product formation by direct excitation of the interme- clear that future photoredox catalysts must not only by tuned by
diate enamine, as discussed above. In fact, by irradiation at their optical and electrochemical properties but also by their
468 nm there is a significant amount of product formed without photophysical dynamics. To the best of our knowledge this is
cNDI 6 (52% yield) that even exceeds the product formation in the first report on the usage of cNDIs as photoredox catalysts.
the presence of cNDI 6 (21% yield). By irradiation at 520 nm The irradiation by visible light from LEDs as energy-saving
there is only a small amount (10% yield) of product 14 formed light sources together with the use of an organic dye instead of
without cNDI 6, but only after longer irradiation times (approx- a transition metal complex as photoredox catalyst improve the
imately after 2 h). Irradiations at 597 nm and 638 nm are com- sustainability and make photoredox catalysis “greener”.
pletely unproductive without the cNDI 6 as photoredox catalyst.
It is obvious that the absorbance of cNDI 6 differs from the Experimental
photoredox catalytic activity profile. This activity profile shows All experimental details are described in the Supporting Infor-
highest values at 520 nm whereas the absorbance has the mation File 1.
maximum at 612 nm. The observed spectral sensitivity of the
photoredox catalytic product formation cannot be easily ex-
plained. One possible reason could be the different luminous
Supporting Information
flux of the applied high-power LEDs. Given that Kasha’s rule is
Supporting Information File 1
also applicable for photochemical reactions and not only fluo-
rescence this result is probably a “non-Kasha” photophysical
dynamic behavior which can be found also for other photo-
chemical reactions in the literature [67], but needs further inves-
tigations by time-resolved spectroscopy. The irradiations at
638 nm and 597 nm overlap with the emission range of cNDI 6
so that an inner filter effect cannot be excluded. Taken together,
it makes clear that tuning of optical and electrochemical proper-
ties of potential photoredox catalysts have to be combined with
their elucidation of their photophysical dynamic behavior.
Synthetic protocols, copies of 1H and 13C NMR spectra,
mass spectra, and cyclic voltammetry data.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-201-S1.pdf]
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft
(Wa 1386/16-1) and KIT is gratefully acknowledged. BR
thanks the GRK 1626 “Chemical photocatalysis” (funded by the
2049

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
26.Ghosh, I.; Marzo, L.; Das, A.; Shaikh, R.; König, B. Acc. Chem. Res.
2016, 49, 1566–1577. doi:10.1021/acs.accounts.6b00229
27.McManus, J. B.; Nicewicz, D. A. J. Am. Chem. Soc. 2017, 139,
2880–2883. doi:10.1021/jacs.6b12708
28.Speckmeier, E.; Fischer, T. G.; Zeitler, K. J. Am. Chem. Soc. 2018,
140, 15353–15365. doi:10.1021/jacs.8b08933
Deutsche Forschungsgemeinschaft) for participation in their
qualification program.
ORCID® iDs
Hans-Achim Wagenknecht - https://orcid.org/0000-0003-4849-2887
29.Speck, F.; Rombach, D.; Wagenknecht, H.-A. Beilstein J. Org. Chem.
2019, 15, 52–59. doi:10.3762/bjoc.15.5
30.Nicewicz, D. A.; Nguyen, T. M. ACS Catal. 2014, 4, 355–360.
doi:10.1021/cs400956a
31.Sideri, I. K.; Voutyritsa, E.; Kokotos, C. G. Org. Biomol. Chem. 2018,
16, 4596–4614. doi:10.1039/c8ob00725j
32.Ghosh, I.; Ghosh, T.; Bardagi, J. I.; König, B. Science 2014, 346,
725–728. doi:10.1126/science.1258232
References
1. Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. J. Org. Chem. 2016, 81,
6898–6926. doi:10.1021/acs.joc.6b01449
2. Marzo, L.; Pagire, S. K.; Reiser, O.; König, B. Angew. Chem., Int. Ed.
2018, 57, 10034–10072. doi:10.1002/anie.201709766
3. Romero, N. A.; Nicewicz, D. A. Chem. Rev. 2016, 116, 10075–10166.
doi:10.1021/acs.chemrev.6b00057
33.Weiser, M.; Hermann, S.; Penner, A.; Wagenknecht, H.-A.
Beilstein J. Org. Chem. 2015, 11, 568–575. doi:10.3762/bjoc.11.62
34.Gong, H.-X.; Cao, Z.; Li, M.-H.; Liao, S.-H.; Lin, M.-J.
Org. Chem. Front. 2018, 5, 2296–2302. doi:10.1039/c8qo00445e
35.Weil, T.; Vosch, T.; Hofkens, J.; Peneva, K.; Müllen, K.
Angew. Chem., Int. Ed. 2010, 49, 9068–9093.
doi:10.1002/anie.200902532
36.Würthner, F.; Ahmed, S.; Thalacker, C.; Debaerdemaeker, T.
Chem. – Eur. J. 2002, 8, 4742–4750.
doi:10.1002/1521-3765(20021018)8:20<4742::aid-chem4742>3.0.co;2-
l
37.Röger, C.; Ahmed, S.; Würthner, F. Synthesis 2007, 1872–1876.
doi:10.1055/s-2007-983718
38.Andric, G.; Boas, J. F.; Bond, A. M.; Fallon, G. D.; Ghiggino, K. P.;
Hogan, C. F.; Hutchison, J. A.; Lee, M. A.-P.; Langford, S. J.;
Pilbrow, J. R.; Troup, G. J.; Woodward, C. P. Aust. J. Chem. 2004, 57,
1011–1019. doi:10.1071/ch04130
39.Al Kobaisi, M.; Bhosale, S. V.; Latham, K.; Raynor, A. M.;
Bhosale, S. V. Chem. Rev. 2016, 116, 11685–11796.
doi:10.1021/acs.chemrev.6b00160
4. Skubi, K. L.; Blum, T. R.; Yoon, T. P. Chem. Rev. 2016, 116,
10035–10074. doi:10.1021/acs.chemrev.6b00018
5. Staveness, D.; Bosque, I.; Stephenson, C. R. J. Acc. Chem. Res.
2016, 49, 2295–2306. doi:10.1021/acs.accounts.6b00270
6. Buzzetti, L.; Crisenza, G. E. M.; Melchiorre, P. Angew. Chem., Int. Ed.
2019, 58, 3730–3747. doi:10.1002/anie.201809984
7. Meggers, E. Chem. Commun. 2015, 51, 3290–3301.
doi:10.1039/c4cc09268f
8. Zou, Y.-Q.; Hörmann, F. M.; Bach, T. Chem. Soc. Rev. 2018, 47,
278–290. doi:10.1039/c7cs00509a
9. Arias-Rotondo, D. M.; McCusker, J. K. Chem. Soc. Rev. 2016, 45,
5803–5820. doi:10.1039/c6cs00526h
10.Oelgemöller, M.; Hoffmann, N. Org. Biomol. Chem. 2016, 14,
7392–7442. doi:10.1039/c6ob00842a
11.Fukuzumi, S.; Ohkubo, K. Org. Biomol. Chem. 2014, 12, 6059–6071.
doi:10.1039/c4ob00843j
12.Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2013, 42, 97–113.
doi:10.1039/c2cs35250h
13.Zeitler, K.; Neumann, M. Synergistic Visible Light Photoredox
Catalysis. In Chemical Photocatalysis; König, B., Ed.; De Gruyter:
Berlin, 2013; pp 151–168. doi:10.1515/9783110269246
14.Pannwitz, A.; Wenger, O. S. Chem. Commun. 2019, 55, 4004–4014.
doi:10.1039/c9cc00821g
40.Suraru, S.-L.; Würthner, F. Angew. Chem., Int. Ed. 2014, 53,
7428–7448. doi:10.1002/anie.201309746
41.Wiederrecht, G. P.; Niemczyk, M. P.; Svec, W. A.; Wasielewski, M. R.
J. Am. Chem. Soc. 1996, 118, 81–88. doi:10.1021/ja953159y
42.Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.;
Svec, W. A.; Wasielewski, M. R. J. Am. Chem. Soc. 1996, 118,
10228–10235. doi:10.1021/ja961919e
43.Molla, M. R.; Ghosh, S. Chem. – Eur. J. 2012, 18, 9860–9869.
doi:10.1002/chem.201201299
44.Pandeeswar, M.; Avinash, M. B.; Govindaraju, T. Chem. – Eur. J.
2012, 18, 4818–4822. doi:10.1002/chem.201200197
45.Guha, S.; Saha, S. J. Am. Chem. Soc. 2010, 132, 17674–17677.
doi:10.1021/ja107382x
15.Majek, M.; Jacobi von Wangelin, A. Acc. Chem. Res. 2016, 49,
2316–2327. doi:10.1021/acs.accounts.6b00293
16.Hopkinson, M. N.; Tlahuext-Aca, A.; Glorius, F. Acc. Chem. Res. 2016,
49, 2261–2272. doi:10.1021/acs.accounts.6b00351
17.Zhou, Q.-Q.; Zou, Y.-Q.; Lu, L.-Q.; Xiao, W.-J. Angew. Chem., Int. Ed.
2019, 58, 1586–1604. doi:10.1002/anie.201803102
18.Wang, C.-S.; Dixneuf, P. H.; Soulé, J.-F. Chem. Rev. 2018, 118,
7532–7585. doi:10.1021/acs.chemrev.8b00077
19.Parasram, M.; Gevorgyan, V. Chem. Soc. Rev. 2017, 46, 6227–6240.
doi:10.1039/c7cs00226b
46.Doria, F.; Nadai, M.; Sattin, G.; Pasotti, L.; Richter, S. N.; Freccero, M.
Org. Biomol. Chem. 2012, 10, 3830–3840. doi:10.1039/c2ob07006e
47.Doria, F.; Folini, M.; Grande, V.; Cimino-Reale, G.; Zaffaroni, N.;
Freccero, M. Org. Biomol. Chem. 2015, 13, 570–576.
doi:10.1039/c4ob02054e
48.Nowak-Król, A.; Shoyama, K.; Stolte, M.; Würthner, F.
Chem. Commun. 2018, 54, 13763–13772. doi:10.1039/c8cc07640e
49.Sakai, N.; Mareda, J.; Vauthey, E.; Matile, S. Chem. Commun. 2010,
46, 4225–4237. doi:10.1039/c0cc00078g
20.Reckenthäler, M.; Griesbeck, A. G. Adv. Synth. Catal. 2013, 355,
2727–2744. doi:10.1002/adsc.201300751
21.Tucker, J. W.; Stephenson, C. R. J. J. Org. Chem. 2012, 77,
1617–1622. doi:10.1021/jo202538x
22.Kruszyna, H.; Kruszyna, R.; Hurst, J.; Smith, R. P.
J. Toxicol. Environ. Health 1980, 6, 757–773.
doi:10.1080/15287398009529895
23.Brabec, V.; Nováková, O. Drug Resist. Updates 2006, 9, 111–122.
doi:10.1016/j.drup.2006.05.002
50.Alp, S.; Erten, Ş.; Karapire, C.; Köz, B.; Doroshenko, A. O.; İçli, S.
J. Photochem. Photobiol., A 2000, 135, 103–110.
24.Nriagu, J. O. Science 1996, 272, 223.
doi:10.1126/science.272.5259.223
25.Hari, D. P.; König, B. Chem. Commun. 2014, 50, 6688–6699.
doi:10.1039/c4cc00751d
doi:10.1016/s1010-6030(00)00306-3
2050

Beilstein J. Org. Chem. 2019, 15, 2043–2051.
51.Barros, T. C.; Brochsztain, S.; Toscano, V. G.; Filho, P. B.; Politi, M. J.
J. Photochem. Photobiol., A 1997, 111, 97–104.
doi:10.1016/s1010-6030(97)00205-0
52.Kishore, R. S. K.; Kel, O.; Banerji, N.; Emery, D.; Bollot, G.; Mareda, J.;
Gomez-Casado, A.; Jonkheijm, P.; Huskens, J.; Maroni, P.;
Borkovec, M.; Vauthey, E.; Sakai, N.; Matile, S. J. Am. Chem. Soc.
2009, 131, 11106–11116. doi:10.1021/ja9030648
53.Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77–80.
doi:10.1126/science.1161976
54.Hasharoni, K.; Levanon, H.; Greenfield, S. R.; Gosztola, D. J.;
Svec, W. A.; Wasielewski, M. R. J. Am. Chem. Soc. 1995, 117,
8055–8056. doi:10.1021/ja00135a040
55.Gao, X.; Qiu, W.; Yang, X.; Liu, Y.; Wang, Y.; Zhang, H.; Qi, T.; Liu, Y.;
Lu, K.; Du, C.; Shuai, Z.; Yu, G.; Zhu, D. Org. Lett. 2007, 9,
3917–3920. doi:10.1021/ol701539z
56.Kishore, R. S. K.; Ravikumar, V.; Bernardinelli, G.; Sakai, N.; Matile, S.
J. Org. Chem. 2008, 73, 738–740. doi:10.1021/jo702392q
57.Sasikumar, M.; Suseela, Y. V.; Govindaraju, T. Asian J. Org. Chem.
2013, 2, 779–785. doi:10.1002/ajoc.201300088
58.Demmig, S.; Langhals, H. Chem. Ber. 1988, 121, 225–230.
doi:10.1002/cber.19881210205
59.Wasmus, S.; Vielstich, W. Electrochim. Acta 1993, 38, 185–189.
doi:10.1016/0013-4686(93)85127-k
60.Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877–910.
doi:10.1021/cr940053x
61.Neumann, M.; Füldner, S.; König, B.; Zeitler, K. Angew. Chem., Int. Ed.
2011, 50, 951–954. doi:10.1002/anie.201002992
62.Ghosh, I.; König, B. Angew. Chem., Int. Ed. 2016, 55, 7676–7679.
doi:10.1002/anie.201602349
63.Romero, N. A.; Nicewicz, D. A. J. Am. Chem. Soc. 2014, 136,
17024–17035. doi:10.1021/ja506228u
64.Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298,
97–102. doi:10.1016/s0020-1693(99)00407-7
65.Cismesia, M. A.; Yoon, T. P. Chem. Sci. 2015, 6, 5426–5434.
doi:10.1039/c5sc02185e
66.Silvi, M.; Arceo, E.; Jurberg, I. D.; Cassani, C.; Melchiorre, P.
J. Am. Chem. Soc. 2015, 137, 6120–6123. doi:10.1021/jacs.5b01662
67.Demchenko, A. P.; Tomin, V. I.; Chou, P.-T. Chem. Rev. 2017, 117,
13353–13381. doi:10.1021/acs.chemrev.7b00110
License and Terms
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0). Please note
that the reuse, redistribution and reproduction in particular
requires that the authors and source are credited.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(https://www.beilstein-journals.org/bjoc)
The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.15.201
2051