Tailored Coumarin Dyes for Photoredox Catalysis: Calculation, Synthesis, and Electronic Properties

Article abstract of DOI:10.1002/cctc.202001690

High level time-dependent density functional theory (TD-DFT) computational modeling of coumarin dyes has been exploited for guiding the design of effective photocatalysts (PCs). A library of coumarins were investigated from the theoretical point of view and photophysical/electrochemical properties (absorption and emission spectra, E₀₀, oxidation and reduction potentials) were evaluated. Comparison with literature values reported for a few candidates has been used for assessing the level of theory. On the basis of the results obtained, new strongly reducing PCs [E_ox(PC^.+/PC*)=?2.1 – ?2.0 V vs SCE] were discovered. Through the computational study of structure-properties relationships, a number of coumarins derivatives have been synthesized and evaluated in the pinacol coupling of aldehydes as the model reaction. The new organic photoredox catalysts show experimental photophysical and electrochemical data in accordance with the ones predicted by calculation, with excited state reduction potentials surpassing those of highly reducing transition metal-based PCs. A careful investigation of their behavior as PC has revealed crucial issues that need to be taken into consideration in the general photoredox catalysis, shedding light on the use of these PC in the pinacol, as well as, in other photoredox reactions.

Full text of DOI:10.1002/cctc.202001690

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Tailored Coumarin Dyes for Photoredox Catalysis: Calculation,
Synthesis, and Electronic Properties
Authors: Pier Giorgio Cozzi, Andrea Gualandi, Artur Nenov, Marianna
Marchini, Giacomo Rodeghiero, Irene Conti, Ettore Paltanin,
Matteo Balletti, Paola Ceroni, and Marco Garavelli
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ChemCatChem
10.1002/cctc.202001690
RESEARCH ARTICLE
Tailored Coumarin Dyes for Photoredox Catalysis: Calculation,
Synthesis, and Electronic Properties
Andrea Gualandi,*†[a] Artur Nenov,*†[b] Marianna Marchini, Giacomo Rodeghiero,
[a]
[a],[c]
Irene Conti,[a]
[
a]
[a]
[a]
[b]
[a]
Ettore Paltanin, Matteo Balletti, Paola Ceroni,* Marco Garavelli,* and Pier Giorgio Cozzi*
Dedication ((optional))
[
[
[
a]
b]
b]
Dr. A. Gualandi, Dr. M. Marchini, Dr. G. Rodeghiero, Mr. M. Balletti, Prof. P. Ceroni, Prof. P. G. Cozzi
Dipartimento di Chimica “G. Ciamician”,
Alma Mater Studiorum – Università di Bologna
Via Selmi 2, 40126, Bologna, Italy
andrea.gualandi10@unibo.it; piergiorgio.cozzi@unibo.it; paola.ceroni@unibo.it
Dr. A. Nenov, Dr. I. Conti, Mr. E. Paltanin, Prof. M. Garavelli
Dipartimento di Chimica Industriale “T. Montanari”,
Alma Mater Studiorum – Università di Bologna
Viale Risorgimento 4, 40136, Bologna, Italy
E-mail: marco.garavelli@unibo.it
Dr. G. Rodeghiero
Cyanagen Srl,
Via Stradelli Guelfi 40/C, 40138, Bologna, Italy
These authors contributed equally to this work
†
Supporting information for this article is given via a link at the end of the document.
Abstract: High level time-dependent density functional theory (TD-
DFT) computational modeling of coumarin dyes has been exploited
for guiding the design of effective photocatalysts (PCs). A library of
coumarins were investigated from the theoretical point of view and
photophysical/electrochemical properties (absorption and emission
spectra, E00, oxidation and reduction potentials) were evaluated.
Comparison with literature values reported for a few candidates has
been used for assessing the level of theory. On the basis of the results
iridium complexes,[6] both capable of absorbing visible light and to
initiate electron or energy transfer reactions from their reactive
photoexcited states.[ These photoredox catalysts were found
quite effective due to their photophysical properties such as redox
stability, reversible oxidation and reduction processes, and ability
to adsorb visible light.[8] Furthermore, their excited states are long
lived and they can be engaged in both reductive and oxidative
electron transfers. By synthesis it was possible to vary the
7]
•
+
[9]
obtained, new strongly reducing PCs [Eox(PC /PC*) = - 2.2 — - 2.0 V
vs SCE] were discovered. Through the computational study of
structure–properties relationships, a number of coumarins derivatives
have been synthesized and evaluated in the pinacol coupling of
aldehydes as the model reaction. The new organic photoredox
catalysts show experimental photophysical and electrochemical data
in accordance with the ones predicted by calculation, with excited
state reduction potentials surpassing those of highly reducing
transition metal-based PCs. A careful investigation of their behavior
as PC has revealed crucial issues that need to be taken into
consideration in the general photoredox catalysis, shedding light on
the use of these PC in the pinacol, as well as, in other photoredox
reactions.
photophysical properties of ruthenium and iridium complexes,
covering a large spectrum of photophysical properties and
amplifying their use, particularly when strong reductants or strong
oxidants are needed. However, despite their successful use in
many reactions, these complexes are expensive and not all are
commercially available. In addition, the use of Ru and Ir based
photocatalysts presents a concern with regard to their availability
and sustainability, [10] as they are among the rarest metals on earth.
Furthermore, residual metal traces in the final product can limit
their employments, for example for bio-medical applications. To
overcome these drawbacks, several alternative PCs were
explored in recent years. Organic PC (OPC) families represent a
valuable alternative. Several families of organic molecules were
11]
benzophenones,[12]
studied—including
anthracenes,[
acridiniums,[ xanthene based dyes, perylene diimides,[15] and
many others.[16] Most of the mentioned organic dyes are operating
through a reductive quenching pathway generating a strong
organic reductant. Making an organic molecule a strong reductant
is a difficult task. Murphy described powerful organic reductants
able to promote radical coupling and other reactions of substrates
13]
[14]
Introduction
In recent years photoredox catalysis has led to the development
of interesting new catalytic transformations due to the mild
conditions in which operates.[1] Photoredox catalysis has been
applied to the synthesis of small molecules,[2] as well as to the
preparation of polymers,[3] both in academic and industrial fields.[4]
The Renaissance of photoredox catalysis and its rapid
advancements were related principally to the use of transition
metals complexes as photocatalyst (PC), such as ruthenium[5] or
with reduction potentials < -1.8 V (vs a saturated calomel
electrode, SCE).[17] Unfortunately, these organic reductants are
rather unstable and difficult to generate, although an interesting
catalytic variant was recently reported.[18] Miyake has developed
and studied UV light-absorbing N-aryl phenoxazines as strongly
reducing metal-free photoredox catalyst for Atom Transfer
1
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ChemCatChem
10.1002/cctc.202001690
RESEARCH ARTICLE
Radical Polymerization (ATRP),[19] that can access a highly Results and Discussion
reducing excited state and operate via an oxidative quenching
pathway, analogous to the one previously reported for iridium-
catalyzed reactions in ATRA reactions.[20] By the guidance of
In our previous publications we have reported the intriguing
photoredox properties of coumarins bearing a thiophen ring.[
However, many other coumarins that were tested did not exhibit
the desired catalytic activity. It was not clear how to design the
coumarin scaffold in order to produce an effective photocatalyst,
and more importantly, how the subtle modifications of the
electronic structure realized through functionalization affected the
photophysical properties. To guide the design and reduce the
synthetic efforts, we decided to use an ab-initio computational
strategy to model the photophysical properties of on silica
modified coumarins derivatives before their synthesis.
25]
computational methodologies Miyake designed new dyes with
interesting properties:[21] (i) strong visible-light absorption (high
molar absorptivity); (ii) long-lived excited state; (iii) sufficiently
negative excited state reduction potential for the reduction of alkyl
bromide,[22] employed as initiators in polymerization reactions; (iv)
sufficiently oxidizing, in their oxidized species, for oxidation of the
propagating radical; (v) redox reversibility, i.e., stable radical
cations or radical anions; (vi) low reorganization energy for the
transition from PC* to PC•+ and back to ground state PC; and (vii)
photoinduced charge-transfer excited states resulting from
spatially separated singly occupied molecular orbitals (SOMOs).
From this theoretical and experimental work, new organic dyes,
N,N-diaryldihydrophenazines (PhenN’s) and N-arylphenoxazines
In silico design strategy
Efficient reductive OPCs (Organic Photo Catalysts) have to
satisfy the following conditions
(
PhenO’s)[23] were reported to have a computationally predicted
2
•+ 3
highly reducing triplet excited states of [Eox( PC / PC*) < –2.0 V
vs SCE], together with the capability to adsorb visible light. Beside
these dyes, N-arylphenothiazine (PTH) derivatives are also
interesting and active photoreductants, with have reduction
a) absorption in the visible (> 400 nm) is preferred over
ultraviolet (UV) region of electromagnetic spectrum for
a selective excitation of the photocatalyst;
b) high molar absorption coefficients (ε) leads to a more
•+
[24]
efficient absorption of light;
potential in the order of Eox(PC /PC*) = -2.0 — -2.1 V vs. SCE.
c) lifetimes beyond the nanosecond range enable the
bimolecular collision events (at sufficiently high
concentration of the substrate) required for effective
electron transfer;
We have recently introduced in the field of photoredox catalysis
coumarins as new effective photocatalysts.[25] Remarkably,
although coumarins have been largely used in many
applications,[
26]
their systematic employment in photoredox
d) photoreactive excited state characterized by a strong
reductant ability (highly negative value of Eox*) is
required to catalyze reactions with organic substrates
featuring high reduction potentials;
reactions has not been explored yet. Coumarins can be accessed
by straightforward synthesis[27] that gives the possibility to vary
their photophysical and redox properties,[28] allowing to cover a
wide range of redox potentials in their ground and excited state.
e) a sufficiently positive potential value Eox is required to
regenerate the photocatalyst for the next catalytic cycle.
With all these potentialities, we have applied coumarin dyes as
powerful photoreductant in pinacol coupling,[25,29,30] replacing the
use of iridium complexes for this transformation.[31] In addition, we
A bathochromic shift of the absorption spectrum of UV-absorbing
compounds (such as most precursors utilized in the development
of OPCs) can be realized in a two-fold manner: (i) by introducing
on the chromophore electron-donating and/or -withdrawing
functional groups; (ii) by extending the conjugation.
have shown that a completely different mechanistic picture is
operating[25] and that the coumarin dyes, in their excited state, are
able to directly reduce carbonyls to the corresponding ketyl
radicals. We were interested in tailoring redox and photophysical
properties of coumarin dyes by introducing appropriate
substituents, for exploring all the possibilities offered by these
[20, 21, 32, 33, 34, 35]
Regarding coumarin, it has been demonstrated in the past that
the introduction of an electron-donating group (EDG) at position
7, in combination with aromatic group at position 3, induces a
bathochromic shift and increases the molar absorption coefficient
scaffolds. For this reason, as was recently explored by different
authors,[32] we decided to study the photophysical properties of a
virtual library of coumarin dyes by the state of the art theoretical
investigations. By the analysis of the results, a series of
coumarins were prepared and tested in pinacol coupling, chosen
as the model reaction. We were able to find suitable in silico
candidates, that were revealed to be powerful photoreductant. In
addition, by careful investigation of redox potential of the dyes
conducted by electrochemistry, we were able to shed light on the
ability of specific dyes to promote or not the reaction. The
theoretical investigation can be used not only to study and predict
photochemical properties of new designed dyes, but also to
(
see Figure 1, for the coumarin system and numerations).[36]
establish the properties necessary for setting
a catalytic
photoredox cycle. The investigations reported herein is a full
account of our studies.
Figure 1. General strategy adopted in the present work.
Blanchard-Desce and co-workers showed that the trend can be
amplified by introducing a heterocyclic fused benzenic ring in the
position 3.[33] An undesired side effect of the push-pull
functionalization, in particular of the use of EWGs, is the increase
of the ground state redox potential. To exemplify the dual role of
2
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10.1002/cctc.202001690
RESEARCH ARTICLE
EWGs, we refer to an early work by Bergmark et al. who
synthesized a coumarin having CF
3
(an EWG) at position 4 and
N,N-diethylamino (NEt
2
, an EDG) in position 7.[37] This push-pull
combination is capable of shifting the absorption spectrum to 396
nm, however at the expense of Eox of about 1.2 V (vs SCE),
leading to a moderate reported E*ox = -1.6 V (vs SCE). Therefore,
extension of the conjugation should be preferred with respect to
push-pull functionalization.
Long-lived (in the μs range) excited states are commonly realized
3
through population of triplet states ( CT) and a general computer-
aided-design strategy has been outlined by Kwon.[34] A drawback
of this protocol is the loss of reorganization energy associated
with the population of the triplet state from the locally excited (LE)
state, which inevitably reduces the reductive power of the catalyst.
Recently, Sakata and co-workers reported carbazole based
OPCs undergoing photocatalysis directly out of the bright LE state.
Their best candidate N-ethyl-3,6-bis(dimethylamino)carbazole
exhibits an impressive E*ox = -2.75 V vs. SCE.[35] In our previous
study on the photoredox activity of coumarins 1, 2, and 5 (Figure
2
) functionalized with benzene and thiophene rings,[25] we
Figure 2. Theoretically investigated coumarin derivatives.
demonstrated that the photoreactive state is the lowest singlet
state,[38] as demonstrated by the quenching of the coumarin
fluorescence by the organic substrates.
The requirement of absorption in the visible, constrains the
adiabatic excitation energy (E00) to values below 3.1 eV.[39] The
The excited state (S
1
) redox potential is related to the ground state
) redox potential through the formula:[16]
photoredox potential Eox is then tuned by the value of Eox,
*
[40]
(
S
0
which is associated with the ability of the catalyst to stabilize its
radical cation. Thus, for catalysts with similar adiabatic energies
E*ox = Eox – E00/nF
E00, those that are easier to oxidize in the ground state will be
with n is the number of electrons involved in the redox process
here equal to 1) and F is the Faraday constant (equal to the unit
charge e if the employed unit of energy is eV). E00 is the adiabatic
excitation energy given as the difference of the S and S states
in their respective minima. The excited state redox potential can
be also formulated as the energy inserted in the system upon
photoexcitation of the lowest absorption band Evert, diminished by
the reorganization energy Ere (i.e. E00 = Evert - Ere) and the ground
state redox potential Eox.[16]
more strongly reductant in the excited state. Naturally,
functionalization with EDGs decreases Eox (Figure 1).[32c]
(
Considering all the above points, we formulate a strategy for in
silico design of coumarin derivatives absorbing in the visible and
exhibiting high reducing potentials at excited states (Figure 2).
Bare coumarin absorbs in the near-UV at 314 nm (maximum of
the lowest peak independent of the polarity of the solvent) with a
0
1
4
-1
-1
molar absorption coefficient of 0.57 x 10 M cm . It exhibits a low
fluorescence QY (0.03%) and a few ps lifetime in non-polar
solvents.[41]
Table 1. Photophysical properties in DMF solution and electrochemical properties (E1/2 in V vs SCE) in CH
3
CN at 298 K of all synthesized coumarins.
Absorption
Emission
Electrochemistry
ε x 104
f
λ
Φ
τ
E
00/e
E
ox (PC /PC)
•+
E*ox (PC•+/PC*)
λ
abs
em
em
-
1
-1
Comp.
(nm)
(M cm )
exp.
(nm)
exp.
497[a]
482[a]
487
(ns)
exp.
3.3[a]
2.9[a]
3.2
(V)
(V)
(V)
exp.
427[a]
413[a]
426
442
400
400
402
406
415
398
408
theo.
409
398
416
432
377
383
377
394
411
386
389
theo.
0.98
1.04
1.01
1.26
0.94
0.96
0.97
0.92
0.90
0.43[e]
1.18
exp.
0.50
0.57
0.65
0.61
0.82
0.60
0.86
0.60
0.52
0.03
0.80
exp.
theo.
2.62
2.69
2.61
2.49
2.86
2.78
2.89
2.64
2.56
2.78
2.76
exp.
+0.79[a]
+0.83[a]
+0.72[d]
+0.81
+0.92
+0.90
+0.93
+0.70
+0.62
+0.71
+0.93
theo.
+0.74
+0.75
+0.61
+0.73
+0.88
+0.78
+0.88
+0.60
+0.53
+0.73
+0.87
exp.
-1.87[c]
-1.89[c]
-1.98
-1.78
-1.87
-1.87
-1.85
-1.97
-1.98
-2.07
-1.81
theo.
-1.88
-1.95
-1.99
-1.76
-1.99
-2.00
-2.00
-2.04
-2.04
-2.05
-1.89
1
2
3
4
5
6
7
8
9
3.30
3.03
2.90
4.25
2.97
2.94
3.35
3.23
2.88
3.07
3.63
2.66[b]
2.72[b]
2.70
2.59
2.79
2.77
2.78
2.67
2.60
2.78
2.74
515
3.1
476
3.0
477
3.0
475
2.9
513
3.5
524
3.7
1
0
1
480
2.6
1
483
2.5
[
a]
Data obtained from ref. [25]; [b] Computed from absorption and emission values as (λmax,abs+λmax,em)/2; [c] Computed according to E*ox = Eox – E00/e; [d] Chemically
irreversible one-electron transfer process; [e] The low value for the oscillator strength is due to two overlapping absorptions (ΔE = 0.15 eV) with oscillator strengths
0.43 and 0.5 which appear as a single band in the experiment. If both bands are considered the value for f would be in line with the overall trend.
3
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RESEARCH ARTICLE
The desired bathochromic shift towards 400 nm is achieved by
Design synthetic strategy for DFT calculated coumarins
Based on the results obtained by the in-silica screening, new
promising compounds were selected, synthesized, and used as
photocatalysts in a model reaction in order to test their efficiency,
and the affordability of the calculations. However, as stressed it is
possible to verify the comparison between the theoretical and
experimental data of coumarins, as in the case of 1, 2 and 5,
coumarins, that were already found as good catalysts for pinacol
coupling reaction,[25] and that exhibit the extreme accuracy of the
computational calculations.
introducing NEt
2
at position 7, in combination with
a
(
hetero)aromatic ring at position 3 such as phenyl (5) and
thiophene (1, 2),[ i.e. effectively extending the conjugated
system (Figure 2). Phenyl and thiophene achieve the desired red-
shift keeping Eox low with values of 0.92 V and 0.79 V vs SCE,
respectively.[ This results in a significant increase in the
photoredox potential with reported values of -1.87 V vs SCE for
both 1 and 5 (Table 1).[25] The aromatic rings offer various
42]
25]
possibilities for further functionalization. In particular, we studied
-
the effect of EDGs such as negatively charged groups (SO
groups with strong +M effect (OR: 3, 6, 7, NH : 8 or NMe
and fused heteroaromatic rings (thienothiophene, 4, carbazole,
1) regarding the fine-tuning of the spectroscopic properties. Our
3
, 2),
The synthesis of the selected candidates was performed through
different synthetical approaches.
2
2
, 9, 10)
In the case of coumarin derivatives 6, 7, 9, and 10, a Suzuki
coupling of 3-bromo-coumarin derivative 14 with corresponding
boronic acid was required (Scheme 1). Compound 14,[ was
1
44]
fully ab initio calculations (for details regarding the computational
protocol see supporting information) predict photoredox potentials
around -1.78 — -2.07 V vs SCE for all compounds (except 4)
which can be traced back mainly to the decrease of Eox to values
prepared in an excellent yield through
a Knoevenagel
condensation of 4-(diethylamino)salycilaldehyde (12) with diethyl
malonate, followed by acid hydrolysis/decarboxylation step to
as low as 0.5 V (Table 1). Notably, in the series MeO → NH
2
→
2
gives 13, and a subsequent treatment with Br . Boronic acids not
NMe , Eox decreases by ~0.3 V, facilitated by the ability of
stronger EDGs to stabilize the positive charge more effectively.
2
commercially available were synthesized in good yields using a
classic protocol reported in literature (see SI, for details).
Exemplarily, Figure 3, top, compares the electrostatic potential
(
ESP)-mapped electronic density of the radical cation for
coumarin functionalized with a phenyl (5) and with a 4-
dimethylamino phenyl (9) ring.[43] As can be seen, the additional
EDG leads to a more equilibrated distribution of the charge in the
molecule. At the same time, the introduction of an EDG produced
a red shift, as a consequence, E00 decreases by ~0.2 V (Table 1).
Specifically, the ESP-mapped electronic densities for the ground
and excited state of the neutral forms of 5 and 9 (Figure 3, center
and bottom) demonstrate that the intramolecular charge transfer
is enhanced in the first excited state. Notably, its direction is
inverted in 9. Thus, we note a subtle interplay between Eox and
E
00 which imposes constraints on the values that the excited state
*
redox potential E ox can adopt.
Scheme 1. Synthesis of coumarins 6, 7, 9, 10 by Suzuki Miyura cross coupling
reaction.
In the case of compound 8 the synthesis was carried out in three
steps including
a
nitration of benzeneacetonitrile,[45] the
Knoevenagel condensation of compound 15 with 12 and a
subsequent reduction of the nitro group (Scheme 2).[46]
Scheme 2. Synthesis of coumarin 8.
3, 4 and 11, were obtained using cross coupling reactions with
appropriate conditions and reagents, as depicted in Scheme 3.
For the compounds 3 and 11, general conditions of Suzuki
coupling were employed starting from commercially available or
Figure 3. Electrostatic potential (ESP)-mapped electronic density for the radical
cation (top) ground state (center) and first excited state (bottom) of compounds
5
(left) and 9 (right). Red and blue color indicate positive and negative values of
easily prepared aryl boronic acids (See SI for details). The
derivative 4 was obtained by CH activation strategy[47] as reported
the ESP, respectively. For the neutral species the ESP maps are plotted with
limits [-0.05 : +0.05] a.u., whereas for the cations the ESP maps are plotted with
limits [+0.05 : +0.15] a.u.
4
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ChemCatChem
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RESEARCH ARTICLE
in literature. In all cases, coumarins were isolated in satisfactory
yields.
To our surprise, although the reduction potentials were quite high
for almost all derivatives prepared, the results were quite
disappointing. Coumarin 7 provided the product 18 in yield of 88%,
a value comparable to the best photocatalysts 2.[25] All the amino
derivatives 8-10 were inactive, as well the electron rich substrate
3. Differently, the electron rich compounds 4 and 11 showed a
reduced reactivity. Clearly, despite the redox properties, the
catalytic cycle of the photoredox catalyst was in same way
hampered. A detailed analysis of the catalytic cycle, and of all
data were crucial in order to understand the failure, and to the
reprojection of experiments. The proposed catalytic cycle, that is
following the proposals discussed in publication by Rueping[
and us[25] is reported in Figure S8 in SI.
31]
The catalytic cycle of photoredox pinacol coupling occurs by
single electron transfer from the photoexcited coumarin PC* to the
chlorobenzaldehyde. The related studies and Stern-Volmer
analysis were fully described in our previous article.[ The
electron transfer generates the radical cation of the coumarin dye
PC• and the ketyl radical which undergoes dimerization providing
the diol 18. As stressed by Rueping and by us, it is important to
highlight the role played by the oxidized form of the sacrificial
reductant, in order to favor the endergonic pinacol coupling
activation of aldehyde. The sacrificial reductant, triethylamine, is
in charge of reducing the coumarin radical cation, and restore the
coumarin in its ground state. By our calculations, and by the
experimental value of the registered ground state oxidation
potential of coumarins (see further discussions), the oxidation
potential of the compounds 3, 8, 9, and 10 were found inferior to
25]
+
Scheme 3. Synthesis of coumarins 3,11, and 4.
Use of coumarins synthesized in the pinacol coupling
reaction
With all the desired derivatives in our hands, pinacol coupling of
[25]
4-chlorobenzaldehyde 17 was selected as model reaction, in
order to compare the reduction ability of all the synthesized
derivatives.
3
those of Et N (0.91 V vs SCE, see Figure S5 in SI for more
details). Therefore, the inefficient reaction observed with some of
the coumarins prepared could be ascribed to their low oxidation
potential which results in the inability of Et N to close the catalytic
3
cycle. In order to test our hypothesis and found another suitable
sacrificial reductant for the reaction, an inspection of table of
sacrificial agents used in photoredox catalytic reactions[
22,48]
was
conducted. After having discarded suitable but insoluble sodium
ascorbate, and various acid derivatives,[49] our attention was
drawn by commercially available 1,3-dimethyl-2-phenyl-2,3-
dihydro-1H-benzo[d]imidazole (BIH), a derivative widely used as
sacrificial agent in photoredox reduction of α-haloketones,[50] in
the ring opening of cyclopropyl ketones,[51] and in the
photocatalytic reduction of CO
2
.[52] According to the literature, BIH
+
shows an oxidation potential E(BIH /BIH) of +0.33V vs SCE in
CH
3
CN.[53] The oxidation potential is in all cases inferior to the
•+
oxidation potential E(PC /PC) and successful reaction with this
sacrificial agent was expected. Therefore, the model pinacol
coupling was tested in the presence of BIH as reducing agent.
Contrary to our expectation, the conversion was absent in all the
cases. The addition of oxalic acid, as Brønsted acid, to improve
the efficiency of the photoredox coupling reaction, was reported
by Rueping et al.[ The addition of a Brønsted acid was found
crucial in order to improve the pinacol coupling of ketones, and
oxalic acid, among all the derivatives employed, gave the better
conversion. The reaction involved the concerted shift of a single
electron and a single proton (proton-coupled electron transfer
31]
(
PCET) reactions.[54] Different photoredox reactions in which ketyl
radical intermediates were formed through Brønsted acids
activations were reported. In the reaction performed with BIH,
the absence of a Brønsted acid was probably responsible for the
observed failures, as the redox potential of chlorobenzaldehyde
was too negative for all the coumarins employed. On the other
Scheme 4. Comparison between first described coumarin dyes and new
[55]
synthetized coumarin dyes (yields evaluated by 1H-NMR). Isolated yields after
chromatographic purification given in parenthesis.
5
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ChemCatChem
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RESEARCH ARTICLE
hand, aldehydes are very weakly basic requiring strong acids to
generate the corresponding proto-oxocarbenium ions.[56] However,
the joint action of Brønsted acids and photo reductants could
facilitate efficient ketyl formation through a concerted PCET
reaction scheme. On the basis of this hypotheses the addition of
oxalic acid in the reaction mixture was evaluated and Scheme 5
reports the collected results.
Figure 4. Absorption (left, solid lines) and emission spectra (right, dashed lines)
of 1 (red line), 5 (green line), 9 (light-blue line) and 11 (black line) in DMF
solution at 298 K. λex = 400 nm.
The cyclic voltammetry of the synthesized coumarins (a selection
is reported in Figure 5) shows chemically reversible one-electron
transfer process with Eox value from +0.62 V (vs SCE) to +0.93 V
(
vs SCE). In the case of coumarins 8, 9 and 10 two reversible
oxidation processes are observed, and this can be attributed to
the presence of two amine functions in the coumarin skeleton.
The only exception is coumarin 3, which shows a chemically
irreversible oxidation process (see Figure S2), likely due to the
dioxythiophene substituent that can easily undergo electro-
oxidative polymerization.[58]
Scheme 5. Pinacol coupling of 4-chlorobenzaldehyde with different coumarins
dyes in the presence of BIH as sacrificial reductant. (conversion evaluated by
1
H-NMR). Isolated yields given after chromatographic purification given in
parenthesis.
through 11. We note the excellent agreement with respect to the
adiabatic energies E00 (black lines in Figure 6) which reinforces
Gratifyingly, with most of the new photocatalysts we observed a
quite good conversion. The lacking of reactivity observed with the
photocatalyst 9 and 10 could be attributed in part to a photo-
oxidative degradation, reported for organic photocatalyst under
photoredox conditions.[57] Indeed, upon 24 hours of irradiation of
the reaction mixture, 60% of the photocatalyst 9 can be recovered
but no conversion was detect (Figure S5). The new conditions
were also tested for the photocatalyst 2 described in our
preliminary investigation, and to our delight, the new conditions
afforded with the established photoredox catalyst quite high yields.
Preliminary investigations with different Brønsted acids shown
that phosphoric acids as well as thioureas could be useful
molecules in order to promote the pinacol coupling, hoping for a
stereoselective variant that will be investigated in near future.
Photophysical and electrochemical properties and
comparison with computational results
Among all the synthesized coumarins, four of them were chosen
to be representative for the category (see SI for detailed
information). Figure 4 reports the absorption and emission
spectra of the selected coumarins: the absorption maxima are
between 400 and 430 nm and the shape of the band is similar for
all the species. The emission maxima are spread over a wider
range of wavelengths, between 476 and 524 nm and the emission
spectra do not present any vibronic feature.
Figure 5. Cyclic Voltammetry of an argon-purged solution of 1 (1.4 mM, red
line), 5 (1 mM, green line), 9 (1 mM, light-blue line) and 11 (1 mM, black line) in
CH CN in the presence of 0.1 M tetraethylammonium hexafluorophosphate
3
-
1
6
(TEAPF ). Scan rate=0.2 Vs ; working electrode: glassy carbon. The arrow
shows the scanning direction of the cyclic voltammetry diagrams.
Figure 6 compares the theoretical and experimental values for
the photophysical and electrochemical properties of coumarins 1
the versatility of the selected DFT functional M06[59] and, in
particular, the suitability of the Minnesota functional to describe
6
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RESEARCH ARTICLE
excited states where long-range correlation effects are of crucial
importance. The ground state redox potentials Eox (red lines in
Figure 6) are systematically underestimated by ca. 0.05 V (except
as representative photoredox reaction for evaluated the
performance of the catalyst. By careful analysis of the reaction
outcome, redox potentials, and electrochemistry of the obtained
coumarins, we have fully defined the criteria for the reaction and
shed light on the catalytic cycle. In addition, we have employed
for the reaction other sacrificial agents, such as BIH, showing the
limitation of the process. All insights gained in the analysis of
experimental data can clarify the role of oxidation and reduction
potentials of the photocatalyst, both in ground and in excited state,
helping to interpret and guide the development of similar
photoredox catalytic investigations.
1
0) which rationalizes the slightly overestimated values for the
photoredox potential E*ox (blue lines in Figure 6). Overall, the
similar trend exhibited by the computed and measured data sets
is an attestation for the utilized fully ab-initio protocol (for details
see SI).
Acknowledgements
Prof G. Bergamini is acknowledged for discussions. Cyanagen srl
is acknowledged for financial support for this research (PhD
fellowship to G. R) and for the generous supplying of coumarins
dyes. National PRIN 2017 projects (ID: 20174SYJAF,
SURSUMCAT and ID: 20172M3K5N, CHIRALAB) are
acknowledged for financial support of this research. Partial
financial support from Bologna University is acknowledged. We
thank Mr. Francesco Calogero for perform some catalytic tests.
Figure 6. Measured (squares) and calculated (dots) E00, Eox and E*ox for all
Keywords: Photoredox • DFT Calculation • Organic Dyes •
synthesized coumarins in DMF solution at 298 K.
Coumarins • Pinacol coupling
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Entry for the Table of Contents
"
Calculemus"! A theoretical and experimental approach was used to design new effective and performant coumarin organic
dyes, by combining calculations, simple and applicable synthetic methodology for an effective design. Electrochemical and
photophysical investigations confirmed the theoretical predictions.
Institute and/or researcher Twitter usernames: @LabStereoMetPh1; @Ceroni_GROUP
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