Synthesis and spectroscopic and electrochemical properties of an axially symmetric fullerene-porphyrin dyad with a rigid pyrrolo[3,4-c]pyrrole spacer

Article abstract of DOI:10.1021/jo302763a

A new approach to porphyrinofullerene donor-acceptor dyads, based on consecutive 1,3-dipolar cycloaddition of azomethine ylides, generated from bis-aziridinedicarboxylate, to C₆₀ and to porphyrin with a maleimidophenyl substituent, was developed. A synthesis of the axially symmetric porphyrin-fullerene-C₆₀ ensemble 5 with a novel rigid pyrrolo[3,4-c]pyrrolic linker was realized. Theoretical, electrochemical, and spectroscopic studies of compound 5 showed that it is capable of forming a charge-separated state.

Full text of DOI:10.1021/jo302763a

Article
pubs.acs.org/joc
Synthesis and Spectroscopic and Electrochemical Properties of an
Axially Symmetric Fullerene−Porphyrin Dyad with a Rigid
Pyrrolo[3,4‑c]pyrrole Spacer
Alexander S. Konev,† Alexander F. Khlebnikov,*,† Tamara G. Nikiforova,† Alexander A. Virtsev,†
and Holm Frauendorf‡
†Department of Chemistry, Saint Petersburg State University, Universitetskii pr. 26, 198504 St. Petersburg, Russia
‡Institute of Organic and Biomolecular Chemistry, Georg-August-University, Tammannstrasse 2, D37077 Gottingen, Germany
̈
S
* Supporting Information
ABSTRACT: A new approach to porphyrinofullerene donor−
acceptor dyads, based on consecutive 1,3-dipolar cycloaddition
of azomethine ylides, generated from bis-aziridinedicarboxylate,
to C60 and to porphyrin with a maleimidophenyl substituent,
was developed. A synthesis of the axially symmetric porphyrin−
fullerene−C60 ensemble 5 with a novel rigid pyrrolo[3,4-
c]pyrrolic linker was realized. Theoretical, electrochemical, and
spectroscopic studies of compound 5 showed that it is capable
of forming a charge-separated state.
and acceptor moieties benefits the prolongation of the CS state
lifetime1 by decreasing the electronic coupling between donor
and acceptor. On the other hand, an increase in the distance
between the reacting species increases the reorganization
energy λ (eq 2), which has the opposite effect on the lifetime
of the CS state1,3
INTRODUCTION
■
Porphyrin−fullerene donor−acceptor dyads represent promis-
ing compounds for the creation of artificial photosynthetic
systems due to their ability to form long-lived charge-separated
(CS) states upon photoexitation.1 The long lifetime of the CS
state is crucial for the following intermolecular electron transfer
to occur.2 The energy deposited in such a CS state can be
further transformed either to electric power (solar cells) or to
the energy of chemical bonds (catalysis). According to Marcus
theory,3 the following factors affect the lifetime of CS states at a
given temperature: free Gibbs energy for charge recombination
ΔGBET (BET, back-electron transfer), reorganization energy λ,
and electronic coupling (V) between donor and acceptor
moieties (eq 1).
λ = λout + λin
(2)
−1
λout = e2(4πε0)−1(ddonor−1 + dacceptor−1 − Rcenter‐to‐center
(n−2 − ε−1)
)
(3)
where ddonor, d acceptor, Rcenter‑to‑center are diameters of donor and
acceptor counterparts and distance between their centers,
respectively.
This is one of the reasons why the search for optimal
geometry of molecular artificial photosynthetic systems is
mostly a matter of trial and error.
kBET = [2π3/2/(h(λkBT)1/2)]V2
exp(−(ΔGBET + λ)2/(4λkBT))
(1)
The synthesis and investigation of the photophysical
properties of various porphyrin−fullerene covalent1,2,4,5 and
supramolecular donor−acceptor ensembles,2,6−9 as well as
ensembles of fullerene C60 with other donors,10 have already
been performed. Different types of connection of the fullerene
core to the rest of the molecule have been tried. Thus, the
fullerene moiety was included in fullerene−porphyrin dyads as
methanofullerene11−14 aziridinofullerene,11 Diels−Alder cyclo-
adduct,15 and pyrrolofullerene.11 Various spacers have been
used to connect fullerene with porphyrin fragments, both
flexible and rigid. Among the first group are ethylene,12 alkynyl
Porphyrin−fullerene dyads were reported to often possess
small values of λ compared to ΔGBET (λ < −ΔGBET). This
moves the process of back-electron transfer into the Marcus
inverted region, where the larger the driving force (ΔGBET) the
slower the process. That means that increasing ΔGBET would
increase the efficiency of light energy conversion both by
increasing the relative amount of transformed energy and by
increasing the lifetime of the charge-separated state. Electronic
coupling between donor and acceptor moieties depends on the
length and nature of the bridge connecting the two fragments
and on the topology and symmetry of the molecule. The
dependence of the lifetime of the CS state on the distance
between donor and acceptor fragments is not straightforward.
On the one hand, a large separation distance between donor
Received: December 20, 2012
© XXXX American Chemical Society
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and naphthyl glycols,11and silanes.16 The second group
includes steroids,11 amides,17 anthracene,15 pyromellitimide,18
rigid silanes,16 and others.
Based on the bis-aziridine synthetic strategy recently
suggested for the preparation of dumbbell-like bis-C60-fullerene
triads,19 we envisioned a novel type of spacer for porphyrin−
fullerene donor−acceptor dyads, namely the pyrrolo[3,4-
c]pyrrole system (Scheme 1). This spacer would provide (a)
generating a charge-separated state. The simple approach for
the estimation of the ability of a molecule to form a long-lived
CS state is based on the frontier molecular orbital description
of photoinduced electron transfer.21 Application of this
approach to molecule 2 implies that the formation of the CS
state proceeds via the local acceptor photoexcitation (transfer
of an electron from 464MO to 467MO located on the fullerene
moiety), followed by electron transfer involving orbital
interactions between the HOMO localized on the donor
(466MO) and a partly filled orbital of the acceptor (464MO).
Charge recombination involves interactions between 467MO
(LUMO) and 466MO (HOMO) (Figure 1). The overlap of
the corresponding orbitals affects the value V in eq 1: the better
the overlap, the higher is the rate of the corresponding process.
As one can see from Figure 1, the overlap of 466MO and
464MO (corresponding to the formation of CS state) is
significantly larger than the overlap of 467MO and 466MO
(corresponding to charge-recombination), suggesting that VCS
Scheme 1
will be larger than VBET
The values ddonor = 18.1 Å, dacceptor = 7.1 Å, Rcenter‑to‑center
.
=
25.0 Å, taken from the optimized geometry of compound 2,
allows the estimation of the solvent reorganization energy λout
from eq 3. The λout value strongly depends on the solvent used,
e.g., for an unpolar solvent (benzene) it will be 0.007 eV, while
for a polar solvent (1,2-dichloroethane) it will be around 0.9
eV. The internal reorganization energy λin is reported to range
from 0.2 to 0.7 eV.21 Hence, the total λ for compound 2 should
range from 0.2 to 0.7 eV in a nonpolar solvent and from 1.1 to
1.6 eV in a polar solvent (Table 1).
previously undescribed axial symmetry to the molecule, which
can enhance electron-transfer, (b) rigidity of the molecule,
fixing donor and acceptor at a certain and large distance
preventing through space interaction and inhibiting back-
electron transfer, and (c) facility to tune the electronic
properties of the spacer and the solubility of the molecule by
varying the bridge X and substituent R (Scheme 1).
RESULTS AND DISCUSSION
■
In order to estimate the ability of the porphyrin−fullerene dyad
1 to generate a long-lived CS state, we performed quantum
chemical calculations on compound 2 (1, X = p-phenylene, R =
Me) using DFT, the B3LYP functional, and the 6-31G(d) basis
set. According to the calculations, the HOMO of molecule 2 is
located mostly on the porphyrin moiety, while the LUMO is
located on the fullerene cage (Figure 1). This is typical for
fullerene−porphyrin dyads20 and is the precondition for
Table 1. Calculated Values of λout in Various Solvents
solvent
benzene
n20
ε
λout, eV
D
1.5011
1.4969
1.4241
1.5514
1.4448
2.27
2.38
0.007
0.058
0.857
0.708
0.861
toluene
dichloromethane
o-dichlorobenzene
1,2-dichloroethane
8.93
9.93
10.36
Figure 1. FMO description of charge separation and charge recombination and visualization of MOs of molecule 2 (isovalue 0.001). MOs localized
mostly on C60 are shown in blue, while MOs localized mostly on porphyrin fragment are shown in red.
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Table 2. Experimental Data on Ered1/2 for Pyrrolofullerenes 3 and Ered1/2, Eox1/2 for Porphyrins 4, and FMO Energy Calculated in
the Gas Phase at the B3LYP/6-31G(d) Level
Another premise for the formation of a long-lived CS state is
that the back-electron transfer (BET) process takes place in the
inverted Marcus parabola region; i.e., λ should be less than
−ΔGBET (= −ΔGCR). The latter value can be determined from
eq 4.15
= 0.5) Marcus parabola region, in almost optimal conditions (λ
∼ ΔGET). It should also be noted that as ΔGCS has a negative
value, the charge separation process can occur spontaneously
from the thermodynamic point of view. Thus, the above
theoretical results indicate that the rate of generation of the CS
state in porphyrinofullerene 2 will be significantly larger than
the rate of its decomposition via charge recombination,
implying high probability of the formation of a long-lived CS
state. This motivated us to elaborate the synthesis of the first
compound of type 1 and investigate its spectral and
electrochemical properties.
Retrosynthetic analysis of the target molecule 5 (Scheme 2),
based on the bis-aziridine synthetic strategy,19 leads to fullerene
C60, bis-aziridine 6,19 and porphyrin 4a.
Unsymmetrical meso-tetraarylporphyrins of A3B type, which
porphyrin 4a belongs to, could be obtained by stochastic
−ΔGCR = Eox − Ered
(4)
The electrochemical oxidation and reduction potentials can
be estimated from the HOMO and LUMO energy levels of the
molecule, respectively, as it is known that a correlation exists
between these two pairs of values.22−27 However, as different
equations have been reported for such a correlation, we
synthesized, performed quantum-chemical calculations,29 and
measured electrochemical properties of a model series of
pyrrolofullerenes 328 and porphyrins 4 (Table 2) to find the
corresponding correlations. For the purpose of better
comparability of the results, the electrochemical potentials are
also reported vs Fc/Fc+ potential used as the external standard
Scheme 2
1/2
(Ered for Fc/Fc+ relative to Ag/AgCl was 0.475 mV, see the
Supporting Information). Least-squares approximation of the
resulting trend gives eq 5 for the correlation between calculated
MO energy level and experimental reduction or oxidation
potential with R2 having a value of 0.99.29
1/2
Ered = −0.85ELUMO(HOMO) − 3.19
(5)
Use of eqs 4 and 5 allows the estimation of the free energy of
charge recombination for compound 2 to be ca. −1.4 eV, which
means that in nonpolar solvents the BET process will proceed
in the inverted Marcus region (0.7 < 1.4). Estimation of ΔGCS
is possible from eq 6.15
−ΔGCS = E0−0 − (−ΔGCR
)
(6)
As only a minor interaction in the ground state between
donor and acceptor parts of the molecule is reported for
systems similar to 2,18 E0−0 can be taken from the absorption
spectrum of tetraphenylporphyrin 4b. This gives the value of
−0.5 eV for ΔGCS, suggesting that the forward electron transfer
process might proceed in the inverted (λ = 0.2−0.7 ∼ −ΔGET
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Table 3. Optimization of the Reaction Conditions for the Synthesis of Porphyrin 4a
yield, %
a
a
entry
1
catalyst
TfCl
oxidant
air
conc of pyrrole, mol/L
10−2
heating mode
(1) rt
time
4a
4b
(1) 1 h
3
6
5
13
(2) 40 °C
(1) rt
(2) 4 h
2
TfCl
TfCl
TfCl
I2
air
10−2
(1) 1 h
10
(2) 40 °C
(1) rt
(2) 16 h
3
MnO2
10−2
(1) 1 h
2
(2) rt
(2) 20 h
4
chloranil
DDQ
10−2
(1) rt
(1) 1 h
12
5
(2) rt
(2) 15 min
(1) 45 min
(2) 25 min
(1) 45 min
(2) 25 min
(1) 40 min
(2) 15 min
(1) 40 min
(2) 1 min
(1) 40 min
(2) 1 min
(1) 40 min
(2) 5 min
(1) 1 h
5
10−1
(1) μw 100 W/30 °C
(2) μw 100 W/30 °C
(1) μw 100 W/30 °C
(2) μw 100 W/30 °C
(1) μw 100 W/30 °C
(2) μw 100 W/30 °C
(1) μw 100 W/30 °C
(2) μw 100 W/30 °C
(1) μw 100 W/30 °C
(2) μw 100 W/40 °C
(1) μw 100 W/30 °C
(2) μw 100 W/40 °C
(1) 30−32 °C
(2) 30−32 °C
(1) 30−32 °C
6
17
6
I2
chloranil
chloranil
chloranil
chloranil
chloranil
chloranil
chloranil
chloranil
10−1
0.6
trace
12
7
I2
1.3 × 10−2
1.3 × 10−2
1.3 × 10−2
1.3 × 10−2
10‑2
10
8
I2
2−17
2−18
2
11
9
I2
1−15
trace
7−10
8
10
11
12
13
I2
I2
13−16
12
(2) 10 min
(1) 1.5 h
(2) 10 min
(1) 2 h
I2
10−2
(2) 30−32 °C
(1) 30−32 °C
I2
10−2
12
8
(2) 30−32 °C
(2) 10 min
a
(1) Condensation. (2) Oxidation.
Scheme 3
synthesis involving the Lewis acid catalyzed condensation of 4
mol of pyrrole, 3 mol of aldehyde A, and 1 mol of aldehyde B,
followed by oxidation of the resulting porphyrinogen.30 N-(4-
Formylphenyl)maleimide 7, necessary for the synthesis of 4a,
was prepared according to a published procedure.31
A literature survey shows that reactions catalyzed by
molecular iodine30 and triflyl chloride32 give better yields of
meso-tetraarylporphyrins than those catalyzed by other known
catalysts such as BF3−etherate,33 TFA,33 PCl5,34 propionic
acid,35 silica sulfuric acid/ZnCl2,36 benzoic acids,37 and
cellulose sulfuric acid.38 Therefore, the first two catalysts were
tried for the synthesis of porphyrin 4a. According to literature,
oxidation of the intermediate porphyrinogen can be performed
by DDQ,33 chloranil,39 molecular oxygen,35 and SeO2.40 We
have tried the first three oxidants. The results of the use of
various reaction conditions are summarized in Table 3. The
best result was obtained by I2-mediated condensation followed
by oxidation with chloranil (entry 11). In this case, porphyrin
4a was obtained in 13−16% yield along with 7−10% of meso-
tetraphenylporphyrin 4b as a byproduct. Conducting the
reaction under microwave irradiation gave an even higher
yield of the target porphyrin of up to 18%. However, these
experiments (entries 5−10) suffered from poor reproducibility,
giving under the same conditions yields of porphyrin 4a from 2
to 18% (entry 9). When the reaction was performed under
conventional heating, 2 h was required to consume all
benzaldehyde. However, the maximum yield of porphyrin 4a
was observed 1 h after the addition of the iodine, which is in
accordance with the report of Lindsey.33
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The use of the conditions originally reported for the TfCl-
catalyzed synthesis of tetraaryl-substituted porphyrins (1 h at
room temperature under an inert atmosphere followed by
oxidation by air in boiling dichloromethane for 4 h, entry 1)
gave a poor yield (3%) in our case. By increasing the time of
the oxidation to 16 h (entry 2) it was possible to double the
yield, indicating that oxygen is a very slow oxidant for this
system. A comparable result was obtained using manganese
dioxide as the oxidant (5% yield, entry 3), suggesting that
heterogeneous oxidation does not proceed well for this
reaction. Indeed, the use of a homogeneous oxidation with
chloranil improved this result and porphyrin 4a was obtained in
12% yield (entry 4). Thus, application of triflyl chloride gives
no advantage over iodine in our case while requiring use of a
more expensive reagent and a more complicated experimental
technique. All the newly obtained compounds were fully
characterized using standard spectral and analytical methods.
The key step of the synthesis of compound 5 involves a 1,3-
dipolar cycloaddition of the azomethine ylide generated from
the corresponding aziridine followed by aromatization of the
resulting cycloadduct. To find optimal conditions for cyclo-
addition of azomethine ylides from aziridine dicarboxylates to
maleimides the procedure was tested by reacting aziridine 10
and maleimide 11 (Scheme 3). Refluxing the mixture of these
compounds in toluene or chlorobenzene for 32 h gave
cycloadduct 12 in 65% yield. As microwave irradiation is
known to often improve both the reaction time and yields,41−44
we attempted to perform this reaction under microwave
irradiation instead of conventional heating. Indeed, the reaction
was complete after irradiation of a chlorobenzene solution of
compounds 10 and 11 for 45 min (160 W, maximum
temperature reached: 126 °C) to give cycloadduct 12 in 95%.
The stereochemistry of cycloadduct 12 was confirmed by the
observance of four signals (3.66, 4.01, 4.93, and 5.11 ppm) for
the pyrrolidine protons in the 1H NMR spectrum, meaning that
it has C1 symmetry and this is only possible if the relative
configurations of the stereocenters in the 2,4- and 3,5-positions
of the pyrrolidine ring are different. In addition, chemical shifts
and spin−spin coupling constants in compound 13 are very
close to those in similar compounds of known configuration.45
A number of reagents have been tested under various
conditions to find the optimal way for aromatization of
cycloadduct 12 to pyrrole 13. The reaction did not proceed
with sulfur as oxidant under conventional heating or microwave
irradiation. Heating with palladium on carbon or chloranil
resulted in formation of the desired product only in trace
amounts. The use of MnO2 gave compound 13 only in 35%
yield after heating in toluene under reflux for 32 h. Because of
the heterogeneity of the reaction, a large excess of MnO2 (25
equiv) was required to achieve a satisfactory yield. Application
of other solvents, such as benzene, DCM, or chlorobenzene
gave no improvement. Finally, DDQ proved to be an excellent
oxidant for the system under investigation. Thus, microwave
irradiation of a chlorobenzene solution of cycloadduct 12 for 45
min gave the desired pyrrole 13 in 97% yield.
Scheme 4
bis-aziridine 6 with fullerene C60.19 A more productive route to
the target compound 5 (Scheme 5) involves cycloaddition of
porphyrin 4a and azomethine ylide from aziridine 14, which
can be synthesized by the reaction of fullerene C60 and bis-
aziridine 6.19
We found that the use of a 10-fold excess of porphyrin 4a
and relatively mild reaction conditions (heating in chloroben-
zene at 90 °C) gives the maximum yield of the intermediate
diastereomeric cycloadducts 15 (Table 4). In this case, the yield
of the mixture of cycloadducts 15 comprised 80−85% as
opposed to the value of ∼50% typical for the experiments with
the lesser ratio of aziridine: porphyrin (1:4 or 1:2) and/or
higher temperatures (100 or 120 °C). The oxidation of the
intermediate cycloadducts did not proceed at 100 °C or lower
temperatures. The desired product is formed under oxidation
only at 120 °C or higher in 5−13% yield based on starting
aziridine. A significant improvement of this result was obtained
by the use of microwave irradiation. In this case, when the
reaction mixture was irradiated for 30 min (160 W, maximum
temperature attained during the reaction was 127 °C) the yield
of the target porphyrinofullerene 5 was 42%. Application of the
microwave irradiation also for the first step and performing the
reaction in a one-pot mode without isolation of the
intermediate cycloadducts gave a poorer yield of 8.5%. Thus,
the best conditions for the synthesis of the target porphyr-
inofullerene 5 are a result of performing the cycloaddition of
aziridine 14 to porphyrin 4a at 90 °C under conventional
heating, followed by isolation of the cycloadducts 15 by flash
chromatography and then oxidation of 15 with DDQ under
microwave heating (160 W, 127 °C).
Ionization of 5 was achieved by ESI to give low-intensity
molecular ion (calcd for C130H56N7O10+ [M + H]+, 1874.4083,
found 1874.4064). Field desorption (FD) ionization was,
therefore, applied for further mass spectrometric detection of
this compound. Under the soft ionization conditions used for
field desorption, the molecular ion was formed by removal of
an electron under exposure to a high voltage (10 kV) without
extensive fragmentation. The simulated isotopic pattern of the
molecular ion of 5 fits perfectly the experimental one (Figure
2).
The procedure was tested on porphyrin 4a using model
aziridine 1028 as a reaction partner. Microwave irradiation of a
solution of 4a and 10 in chlorobenzene followed by immediate
oxidation of the resulting adduct with DDQ gave porphyrin 4c
in 46% yield over two steps (Scheme 4).
The reaction of bis-aziridine 6 with fullerene C60 and
porphyrin 4a (Scheme 2) would lead to a complex mixture of
products, as one can conclude from a study of the interaction of
The 1H NMR spectrum of 5 contains signals for the
porphyrin NH protons at −2.82 ppm. Pyrrole rings A show two
doublets at 8.94 and 8.84 ppm (J = 4 Hz). The latter is poorly
resolved from the signals of pyrrole rings B, which shows a
singlet at 8.83 ppm. The ortho-protons of phenyl groups appear
E
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Scheme 5
Table 4. Optimization of Reaction Conditions for the Synthesis of Porphyrinofullerene 5
yield, %
a
a
entry
1
ratio 14:4a
heating mode
time,
h
15
5
1:4
(1) 100 °C
(1) 12
(2) 1
48
48
20
19
(2) μw 160 W/max temp 127 °C
(1) 120 °C
2
3
4
5
6
1:2
(1) 3
0
(2) 120 °C
(1) 100 °C
(2) 9
1:10
1:10
1:10
1:10
(1) 12
(2) 1
5
(2) μw 160 W/max temp 127 °C
(1) μw 160 W/max temp 127 °C
(2) μw 160 W/max temp 127 °C
(1) 90 °C
(1) 0.5
(2) 1.5
(1) 24
(2) 6
without isolation
8.5
84
79
13
42
(2) 120 °C
(1) 90 °C
(1) 22
(2) 0.5
(2) μw 160 W/max temp 127 °C
a
1) Condensation. (2) Oxidation.
Figure 2. FD-mass spectrum of porphyrinofullerene 5; experimental (left) and simulated (right) isotopic pattern of the molecular ion of 5.
as a multiplet at 8.15−8.25 ppm with an intensity of 6 protons,
meta- and para-protons appear as a multiplet at 7.70−7.80 ppm
with an intensity of 9 protons. The protons of the benzene ring
adjacent to the porphyrin core appear as a doublet of doublets
at 8.39 and 7.83 ppm (J = 8 Hz). The protons of the benzene
ring adjacent to the pyrrolidine unit appear as a doublet of
doublets at 7.50 and 7.39 ppm (J = 8.5 Hz). That these sets of
doublets belong to the same AB systems is confirmed by the
observance of the corresponding cross-peaks in COSY
spectrum (see the Supporting Information). Two pyrrolidine
protons of the pyrrolofullerene unit appear as a singlet at 6.49
ppm with an intensity of two protons. The value is
characteristic for trans-pyrrolofullerenes of this kind.19,28 Two
sets of signals are observed for the two kinds of ethyl groups
presented in the molecule. According to the COSY spectrum
(see the Supporting Information), ethyl groups of one kind
appear as a quartet at 4.36 ppm and a triplet at 1.40 ppm (J = 7
Hz), while the ethyl groups of another type appear as a
multiplet at 4.25−4.32 and a triplet at 1.20 ppm (J = 7 Hz).
The 13C NMR spectrum of 5 contains 57 signals (see the
Supporting Information). There are six signals in the aliphatic
region: two methyl groups at 14.4 and 14.5 ppm, two
methylene groups at 62.30 and 62.32 ppm, newly formed
sp3-carbons of the fullerene cage at 71.2 ppm, and remaining
pyrrolidine carbon at 74.9 ppm. The carbonyl groups give
signals at 169.9 and 161.1 ppm. Though there are three kinds of
carbonyl groups in the molecule, the observation of only two
signals is most probably caused by the overlapping of two
signals. Taking into account that for compound 4c (same as for
compound 4a) three phenyl rings of the triphenylporphyrin
fragment show four signals, and the porphyrin core shows up to
six signals, one of which is a broad singlet at ca. 130 ppm, one
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would expect for the trans-compound 5, which has C2
symmetry, 49 signals in the aromatic region of the spectrum.
That is in fact the case. If the pyrrolidine ring had cis-
configuration, the molecule would have Cs symmetry, and
therefore the number of signals in the aromatic region of the
spectrum would be 51.
The electronic absorption spectra in the ultraviolet and
visible range of porphyrinofullerene 5 and model compounds 4
are presented in Figure 3. The Soret band lies at 417−418 nm
Figure 4. Fluorescence spectrum of compound 5 in CHCl3.
rate constant for charge transfer in 5 to be 6.2 × 107 s−1 (eq
7).21
kcs = (ΦF(4b)/ΦF − 1)/τF
(7)
(5)
(4b)
ELECTROCHEMISTRY
■
The electrochemical properties of porphyrinofullerene 5 were
studied by cyclic voltammetry in a 0.1 M solution of tetra-n-
butylammonium perchlorate [(TBA)ClO4] in o-dichloroben-
zene (o-DCB). The CVA of 5 is characterized by three peaks in
the region of negative potentials and one peak in the region of
positive potentials (Figure 5). The presence of the same
Figure 3. Electronic absorption spectra in CH2Cl2 of pyrrolofullerenes
3, c ≈ 10−5 mol·L−1, and porphyrins 4a−c, 5, c ≈ 10−3 mol·L−1.
for all four compounds, four Q-bands are observable at 514,
549, 589, and 645 nm. The absorption band of the fullerene
cage can be observed for compound 5 at 310 nm with a
shoulder at 325 nm. Analysis of the data obtained for a series of
model pyrrolofullerenes 3 shows that a different pattern can be
observed for these two bands. They can appear either as two
distinct bands with maxima at 312 and 329 nm, with the band
at longer wavelength being more intensive, or as a band at ca.
310 nm with a shoulder at ca. 325 nm (Figure 3), which
confirms the assignment made. No interaction can be observed
in the ground state between the donor (porphyrin) and
acceptor (fullerene) moieties of compound 5.
The fluorescence spectrum of compound 5 was recorded in
CHCl3 with excitation at the wavelength of maximum
absorption (419 nm) (Figure 4). It reveals an emission
spectrum typical for tetraphenylporphyrin 4b with two
emission maxima at 653 and 720 nm, similar to that reported
for 4b (650 and 714 nm).46 The quantum yield of fluorescence
of 5 is diminished compared that of 4b (0.07 vs 0.1147−51). The
fluorescence quenching might be caused by a charge transfer
process. The data obtained along with the literature reported
fluorescence lifetime for 4b52 (9.16 ns, CHCl3) (9.32 ns,
toluene)53 made it possible to evaluate the upper bound of the
Figure 5. CVA curves of compound 5 (various concentrations) in a
0.1 M solution of (TBA)ClO4 in o-DCB at scan rate 10 mV/s.
number of peaks both in cathodic and anodic waves and the
proximity of their values indicate that they correspond to
reversible redox processes. However, the observed dependence
of peak potentials (Ep) on the scan rate (Figure 6 and Table 5)
means that the electrochemical processes proceed in a quasi-
reversible way.
To assign the observed peaks, we have performed a CVA
study of model compounds: tetraphenylporphyrin 4b,
porphyrin 4a, pyrroloporphyrin 4c, and pyrrolofullerenes 3.
The following conclusions have been made from this study: (1)
The pyrrolo[3,4-c]pyrrole system is electrochemicaly inert in
G
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Figure 6. CVA curves of compound 5 (c = 3.2 × 10−4 M) in a 0.1 M
solution of (TBA)ClO4 in o-DCB at various scan rates.
Figure 8. CVA curves of pyrrolofullerenes 3a−b and 3e (R = Et, R′ =
I) in the same media, scan rate 10 mV/s.
1/2
Table 5. Peak Potentials Ep (V) and Selected Ered (V) for
Compound 5 at Various Scan Rates
peak in the positive potentials range corresponds to the first
oxidation of the porphyrin ring.
scan
rate,
mV/s
Positions of the fullerene peaks in compound 5 are almost
the same as those in fulleropyrrolidines 3, suggesting only a
minor influence of the porphyrin fragments to the fullerene
core in 5 (Table 6). Interestingly, the peak for oxidation of
porphyrin is significantly shifted to zero potential, implying that
the fullerene fragment facilitates oxidation of the porphyrin
moiety in compound 5.
1/2
1/2
1/2
Ep1
Ep2
Ep3
Ep4
Ered
Ered
ΔEred
1
2
10
20
30
50
70
0.59
0.56
0.58
0.55
0.56
−0.67
−0.70
−0.71
−0.74
−0.76
−1.07
−1.09
−1.10
−1.13
−1.15
−1.33
−1.35
−1.37
−1.39
−1.40
0.76
0.73
0.72
0.70
0.70
−0.59
−0.61
−0.62
−0.65
−0.66
1.34
1.34
1.34
1.35
1.36
The obtained experimental data (Table 6 and Figure 3) allow
the ΔGCR and ΔGCS of porphyrinofullerene 5 to be calculated
(eq 4 and 6, E0−0 = 1.92 eV from the electron absorption
spectrum of 5) to obtain the values of −1.26 and −0.66 eV,
respectively. These values of ΔGCR and ΔGCS are reasonably
close to our preliminary estimates (vide supra).
the range of −1.75 to +1.5 V, since only the peaks of the
porphyrin system are observed for pyrroloporphyrin 4c at 0.93
and −1.40 V (Figure 7). (2) The maleimide moiety in
CONCLUSIONS
■
A novel synthetic approach toward porphyrinofullerene
donor−acceptor dyads based on consecutive 1,3-dipolar
cycloadditions of azomethine ylides, generated from bis-
aziridine, to C60 and then to porphyrin with a maleimidophenyl
substituent was developed. The synthesis of a new axial
symmetrical porphyrin−fullerene−C60 dyad 5 with a novel rigid
pyrrolo[3,4-c]pyrrolic linker was performed. Preliminary
theoretical, electrochemical, and spectroscopic studies of
compound 5 show that it is capable of forming a charge-
separated state and therefore it might be interesting for solar
cell application. Synthesis of further derivatives of 1 and
determination of the lifetime of the charge-separated state for
compound 5 is currently underway.
Figure 7. CVA curves of porphyrins 4a (c = 2 × 10−3 M), 4b (c = 2 ×
10−3 M), and 4c (c = 1.4 × 10−3 M) in the same media, scan rate 10
mV/s.
EXPERIMENTAL SECTION
General Methods. Melting points were determined on a hot stage
■
microscope and are uncorrected. IR spectra were recorded on FTIR
1
Instrument. H (300 or 600 MHz) and 13C (75 or 125 MHz) NMR
spectra were determined in CDCl3, C6D6, or (CD2Cl)2. Chemical
shifts (δ) are reported in ppm downfield from tetramethylsilane.
HRMS was performed using a 7T-FTICR mass spectrometer
equipped with an electrospray ion source applying an electrospray
voltage of 4.2 kV, micrOTOF mass spectrometer, and MS Q-TOF.
Field desorption measurements were carried out on a TOF mass
spectrometer using a FD emitter with an emitter voltage of 10 kV. The
reactions under microwave irradiation were carried out in a sealed flask
in a Minotavr-2 microwave oven for laboratory experiments. Flash
porphyrin 4a reveals some electrochemical activity as minor
quasi-reversible peaks were observed at −0.64, −1.03, and
−1.13 V (Figure 7). (3) Three quasi-reversible peaks are
observed for pyrrolofullerenes 3 in the range −2 to 0 V (Figure
8). (4) The observed peaks for 5 in the negative potentials
range correspond to the first three reductions of the fullerene
cage and to the first reduction of the porphyrin system. The last
peak overlaps with the peak for the third reduction of C60. The
H
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1/2
Table 6. Peak Potentials Ep (V) and Ered (V) for Compounds 3−5 and C60 (Scan Rate: 10 mV/s)
1/2
1/2
compd
Ep1
Ep2
Ep3
Ep4
Ered
Ered
−0.59
−0.58
−1.29
−1.29
−0.58
1
2
5
0.59
0.95
0.94
0.93
−0.67
−0.64
−1.07
−1.13
−1.33
−1.38
−1.41
−1.40
−1.64
0.76
4a
1.04
1.03
1.00
4b
4c
C60
3a−e
−0.72
−1.16
−0.58 to −0.74
−1.03 to −1.18
−1.51 to −1.74
−0.52 to −0.63
chromatography was performed using silica (0.040−0.063 mm). TLC
analysis was performed on glass backed plates coated with a 0.2 mm
silica layer with UV-indicator 60F254. UV−visible absorption spectra
were recorded on UV−vis spectrophotometer, and fluorescence
spectra on a fluorescence spectrophotometer. Quantum chemical
calculations were performed using a Gaussian 09 package.
ethyl acetate 5:1) gave compound 12 (119 mg, 95% yield): yellowish
crystals; mp 124−126 °C; 1H NMR (CDCl3, 300 MHz) δ 1.14 t (3H,
J = 7.2 Hz,), 1.24 t (3H, J = 7.2 Hz,), 3.66 dd (1H, J1 = 8.5 Hz, J2 = 1.0
Hz), 3.75 s (3H), 4.01 dd (1H, J1 = 9.7 Hz, J2 = 8.7 Hz), 4.11 q (2H, J
= 7.0 Hz), 4.19 q (2H, J = 7.2 Hz), 4.93 d (1H, J = 9.7 Hz), 5.11 d
(1H, J = 1.0 Hz), 6.76 d (2H, J = 9.3 Hz), 6.82 d (2H, J = 9.3 Hz),
7.28 d (2H, J = 8.7 Hz), 7.45 d (2H, J = 8.7 Hz); 13C NMR (CDCl3,
75 MHz) δ 13.9, 14.1, 46.9, 48.6, 55.4, 61.5, 62.0, 64.3, 65.4, 114.6,
118.1, 127.8, 129.4, 129.9, 134.8, 137.9, 154.4, 169.9, 170.8, 174.3,
174.5; IR (KBr, cm−1) ν 1707 (CO), 1734 (CO), 1754 (CO);
Electrochemistry. The electrochemical measurements were
carried out using an AUTOLAB12 PGSTAT12, connected through
an interface with a PC. Cyclic voltammograms were recorded in a
hermetically sealed three-electrode glass electrochemical cell. A
platinum disk electrode (d = 6 mm) was used as the working
electrode. A platinum wire served as the counter electrode. An Ag/
AgCl electrode was used as the reference electrode, tetra-n-
butylammonium perchlorate (0.1 M) as supporting electrolyte and
dry o-DCB as a solvent. All solutions were purged prior to
electrochemical measurements using argon gas. CVA curves were
recorded under an argon atmosphere at various potential sweep rates
(10, 20, 30, 50, and 70 mV/s) for compounds 3a−e, 4a−c, and 5 (see
the Supporting Information). In most cases, potentials are referenced
to a Ag/AgCl electrode. Where ferrocene is specified as a reference, it
was used as an external standard with the CVA experiment being
conducted under the same conditions.
+
HRMS-ESI calcd for C25H26ClN2O7 [M + H]+ 501.1423, found
501.1416; calcd for C25H25ClN2O7K+ [M + K]+ 539.0982, found
539.0990.
Compound 13. Procedure A. A mixture of compound 12 (41 mg,
0.08 mmol) and MnO2 (178 mg, 2.05 mmol) in chlorobenzene (3
mL) was heated under reflux for 32 h. On completion of the reaction
the oxidant was filtered off, the solvent removed in vacuo, and the
product purified by column chromatography on silica gel (petroleum
ether−ethyl acetate 5:1) to give pyrrole 13 as colorless crystals (14 mg,
35%).
Procedure B. A mixture of compound 12 (50 mg, 0.1 mmol) and
DDQ (23 mg, 0.1 mmol) in chlorobenzene (3 mL) was subjected to
microwave irradiation (160 W, maximum temperature achieved: 130
°C) for 45 min. On completion of the reaction the solvent was
removed in vacuo and the product was purified by column
chromatography on silica gel (petroleum ether−ethyl acetate 5:1)
giving pyrrole 13 (48 mg, 97%): colorless crystals; mp 238−239 °C;
1H NMR (CDCl3, 300 MHz) δ 1.32 t (6H, J = 7.1 Hz), 3.89 s (3H),
4.28 q (4H, J = 7.1 Hz), 7.02 d (2H, J = 8.7 Hz), 7.24 d (2H, J = 8.7
Hz), 7.37 d (2H, J = 8.7 Hz), 7.48 d (2H, J = 8.3 Hz); 13C NMR
(CDCl3, 75 MHz) δ 13.9, 55.4, 62.1, 113.8, 123.8, 124.4, 128.2, 128.3,
129.2, 129.8, 130.7, 133.8, 158.2, 160.3, 160.5; IR (KBr, cm−1) ν 1778
(CO), 1737 (CO), 1719 (CO); HRMS-ESI calcd for
N-(4-Formylphenyl)maleimide (7). Synthetic procedure is
reported elsewhere.31 Light yellow crystals: mp 114−120 °C (lit.30
1
130−131 °C); H NMR (CDCl3, 300 MHz) δ 6.93 s (2H), 7.65
pseudo d (2H, J = 8.43 Hz), 8.01 pseudo d (2H, J = 8.43 Hz), 10.06 s
(1H); 13C NMR (CDCl3, 75 MHz) δ 125.6, 130.4, 134.4, 134.8, 136.5,
168.7, 191.1.
Porphyrin 4a. A mixture of aldehyde 7 (50 mg, 0.25 mmol),
benzaldehyde (80 mg, 0.75 mmol), and pyrrole (67 mg, 1 mmol) in
dichloromethane (100 mL) was vigorously stirred while being heated
to 28 °C during 3 min. Iodine (25 mg, 0.1 mmol) was added, and the
reaction mixture was stirred at 30−32 °C for 1 h. After that, chloranil
(185 mg, 0.75 mmol) was immediately added and the stirring
continued for another 5 min at the same temperature. The reaction
mixture was cooled to room temperature, hydrolyzed with 10% aq
NaOH (18 mL), washed with water and brine, and dried over Na2SO4.
The desiccant was filtered off, the solvent removed in vacuo and the
residue separated by column chromatography (silica gel, 15 g, DCM)
to give porphyrin 4a (27.4 mg, 16% yield) along with tetraphenylpor-
+
C25H22ClN2O7 [M + H]+ 497.1110, found 497.1112; calcd for
C25H21ClN2O7Na+ [M + Na]+ 519.0929, found 519.0952; calcd for
C25H21ClN2O7K+ [M + K]+ 535.0669, found 535.0687.
Porphyrin 4c. A mixture of aziridine 10 (21 mg, 0.07 mmol, 1
equiv) and porphyrin 4a (49 mg, 0.07 mmol, 1 equiv) in dry
chlorobenzene (5 mL) was subjected to microwave irradiation (160
W, maximum temperature achieved during the reaction: 126 °C) for
60 min. An additional portion of aziridine 10 (9 mg, 0.03 mmol, 0.3
equiv) was added, and irradiation was repeated for another 30 min.
DDQ (35 + 10 + 5 mg, 0.22 mmol, 3 equiv) was added in three
portions as denoted in parentheses followed by microwave irradiation
under the same conditions for 1 h after each portion. Column
chromatography (silica gel, petroleum ether/EtOAc) gave porphyrin
4c (32 mg, 46% yield): violet crystals; 1H NMR (CDCl3, 300 MHz) δ
−2.74 broad s (2H), 1.43 t (6H, J = 7.1 Hz), 3.91 s (3H), 4.38 q (4H,
J = 7.1 Hz), 7.06 pseudo d (2H, J = 8.7 Hz), 7.32 pseudo d (2H, J =
8.7 Hz), 7.7−7.8 m (9H), 7.86 pseudo d (2H, J = 8.2 Hz), 8.20−8.35
m (6H), 8.39 pseudo d (2H, J = 8.2 Hz), 8.75−8.95 m (4H), 8.91
pseudo d (2H, J = 5 Hz), 8.98 pseudo d (2H, J = 5 Hz); 13C NMR
(CDCl3, 75 MHz) δ 14.0, 55.5, 62.2, 113.9, 118.9, 120.2, 120.3, 124.1,
124.5, 125.2, 126.7, 127.7, 128.3, 129.9, 131.2, 132.0, 134.6, 135.1,
141.9, 142.1, 158.0, 160.4, 161.1; IR (KBr, cm−1) ν 3313 (NH), 1731
1
phyrin 4b (14.6 mg, 10%). Porphyrin 4a: violet crystals; H NMR
(CDCl3, 300 MHz) δ −2.72 broad s (2H), 6.98 s (2H), 7.6−7.9 m
(11H), 8.2−8.35 m (6H), 8.37 pseudo d (2H, J = 8.28 Hz), 8.8−9.1 m
(8H); 13C NMR (CDCl3, 75 MHz) δ 118.7, 120.3, 120.4, 124.0, 126.7,
127.8, 131.3 (broad m,), 134.4, 134.6, 135.0, 141.8, 142.3, 169.6; IR
(KBr, cm−1) ν 3318 (NH), 1716 (CO); HRMS-ESI calcd for
+
C48H32N5O2 [M + H]+, 710.2551, found 710.2538.
Compound 12. Procedure A. A mixture of aziridine 10 (59 mg,
0.2 mmol) and imide 11 (48 mg, 0.23 mmol) in dry chlorobenzene (5
mL) was heated under reflux for 32 h until all of the starting aziridine
was consumed. After removal of the solvent under reduced pressure,
the crude product was purified by column chromatography (petroleum
ether/ethyl acetate 5:1) to give compound 13 as yellowish crystals (65
mg, 65% yield).
Procedure B. A mixture of aziridine 10 (74 mg, 0.25 mmol) and
imide 11 (57 mg, 0.27 mmol) in dry chlorobenzene (3 mL) was
subjected to microwave irradiation (160 W, maximum temperature
achieved: 125 °C) for 45 min until all of the starting aziridine was
consumed. Purification by column chromatography (petroleum ether/
+
(CO); HRMS-ESI calcd for C63H45N6O7 [M + H]+ 999.3501,
found 999.3488.
Porphyrinofullerene 5. A mixture of aziridine 14 (15 mg, 0.013
mmol, 1 equiv) and porphyrin 4a (91 mg, 0.128 mmol, 10 equiv) in
I
dx.doi.org/10.1021/jo302763a | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
dry chlorobenzene (10 mL) was heated at 90 °C for 21 h. The solvent
was evaporated in vacuo, and unreacted porphyrin 4a was removed by
column chromatography (silica gel, 10 g, DCM/EtOAc, gradient from
pure DCM to ratio 10:1). Using this procedure, a recovery of 89.5 mg
of porphyrin 4a was made and 19 mg of a crude mixture of
cycloadducts 15 was achieved. Cycloadducts 15 thus obtained were
dissolved in chlorobenzene (5 mL), DDQ (5 mg, 0.022 mmol, 2.2
equiv) was added and the reaction mixture was subjected to
microwave irradiation (160 W, maximum temperature achieved during
the reaction: 126 °C) for 30 min. Column chromatography (silica gel,
10 g, DCM/EtOAc, gradient from pure DCM to 10:1 ratio) gave
(10) Araki, Y.; Ito, O. Photochem. Photobiol. C: Photochem. Rev. 2008,
9, 93.
(11) MacMahon, S.; Fong, R., II; Baran, P. S.; Safonov, I.; Wilson, S.
R.; Schuster, D. I. J. Org. Chem. 2001, 6 (6), 5449.
(12) Schuster, D. I.; MacMahon, S.; Guldi, D. M.; Echegoeyn, L.;
Braslavsky, S. E. Tetrahedron 2006, 62, 1928.
(13) Schlundt, S.; Kuzmanich, G.; Spanig, F.; deMiguel Rojas, G.;
̈
Kovacs, C.; Garcia-Garibay, M. A.; Guldi, D. M.; Hirsch, A. Chem.
Eur. J. 2009, 15, 12223.
(14) Kovacs, C.; Hirsch, A. Eur. J. Org. Chem. 2006, 3348.
(15) Yamada, H.; Ohkubo, K.; Kuzuhara, D.; Takahashi, T.;
Sandanayaka, A. S. D.; Okujima, T.; Ohara, K.; Ito, O.; Uno, H.;
Ono, N.; Fukuzumi, S. J. Phys. Chem. B 2010, 114, 14717.
(16) Shibano, Y.; Sasaki, M.; Tsuji, H.; Araki, Y.; Ito, O.; Tamao, K. J.
Organomet. Chem. 2007, 692, 356.
(17) Fukuzumi, S.; Imahori, H.; Yamada, H.; El-Khouly, M. E.;
Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Am. Chem. Soc. 2001, 123, 2571.
(18) Imahori, H.; Yamada, K.; Hasegawa, M.; Taniguchi, S.; Okada,
T.; Sakata, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2626.
(19) Konev, A. S.; Khlebnikov, A. F.; Frauendorf, H. J. Org. Chem.
2011, 76, 6218.
(20) Manna, T.; Bhattacharya, S. J. Theor. Comp. Chem. 2008, 7,
1055.
(21) Williams, R. M. Introduction to Electron Transfer, e-book adapted
from Fullerenes as Electron Accepting Components in Supramolecular
and Covalently Linked Electron Transfer Systems, Ph.D Thesis,
Amsterdam, 1996.
1
porphyrinofullerene 5 (10 mg, 42% yield): purple-brown solid; H
NMR (CDCl3, 300 MHz) δ −2.82 broad s (2H, NH), 1.20 t (6H, J =
7.1 Hz, CH3), 1.40 t (6H, J = 7.2 Hz, CH3), 4.34 q (4H, J = 7.4 Hz,
CH2), 4.40 q (4H, J = 6.8 Hz, CH2), 6.59 s (2H, CHCO2Et), 7.43
pseudo d (2H, J = 8.5 Hz, Harom), 7.52 pseudo d (2H, J = 8.4 Hz,
H
H
H
arom), 7.70−7.80 m (9H, Hm+p(Ph)), 7.88 pseudo d (2H, J = 7.5 Hz,
arom), 8.20−8.30 m, (6H, Ho(Ph)), 8.41 pseudo d (2H, J = 8.1 Hz,
arom), 8.85−9.05 m (8H, Hpyrr); 13C NMR (CDCl3, 125 MHz) δ
169.9 (CO), 161.1 (CO), 157.9, 153.1, 150.2, 147.5, 147.0, 146.4,
146.26, 146.25, 146.13, 146.12, 145.63, 145.5, 145.42, 145.35, 145.3,
144.43, 144.41, 143.1, 143.0, 142.8, 142.7, 142.38, 142.35, 142.17,
142.16, 142.1, 141.9, 141.8, 141.73, 141.68, 140.2, 139.6, 136.8, 136.0,
135.1, 134.7, 132.4, 131.5, 131.3 (broad s), 128.7, 127.8, 126.8, 125.4,
124.9, 124.3, 120.5, 120.4, 119.2, 118.7 (Carom), 74.9 (CHpyrr),
71.2(Cqpyrr), 62.32 (CH2O), 62.30(CH2O), 14.5 (CH3), 14.4 (CH3);
IR (KBr, cm−1) ν 3314 (NH), 1732 (CO); HRMS calcd for
C130H56N7O10+ [M + H]+, 1874.4083, found 1874.4064; MS-FD calcd
́
(22) Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am.
+
for C130H55N7O10 [M]+ 1874.4, found 1874.5.
Chem. Soc. 1983, 105, 6555 Though correlations between IP and EA
are described in the refs 22−26, these values are related to HOMO/
LUMO energies via Koopmans’ theorem.
ASSOCIATED CONTENT
* Supporting Information
■
S
(23) Miller, L. L.; Nordblom, G. D.; Mayeda, E. A. J. Org. Chem.
1972, 37, 916.
(24) Parker, V. D. J. Am. Chem. Soc. 1974, 96, 5656.
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Chem. 1979, 57, 638.
(26) Lee, Y.-Z.; Chen, X.; Chen, S.-A.; Wei, P.-K.; Fann, W.-S. J. Am.
Chem. Soc. 2001, 123, 2296.
(27) Mi, D.; Kim, H.-U.; Kim, J.-H.; Xu, F.; Jin, S.-H.; Hwang, D.-H.
Synth. Met. 2012, 162, 483.
(28) Konev, A. S.; Mitichkina, A. A.; Khlebnikov, A. F.; Frauendorf,
1H and 13C NMR spectra for all new compounds, the 2D-1H-
COSY spectrum for compound 5, CVA’s at various rates for
C60, ferrocene and compounds 3a−e, 4a−c, and 5. Computa-
tional details: energies of molecules 2, 3a−d, and 4a−c and
their Cartesian coordinates of atoms. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: alexander.khlebnikov@pobox.spbu.ru.
■
H. Russ. Chem. Bull., Int. Ed. 2012, 61, 860.
(29) Geometry optimizations of the molecules in the gas phase and
using the PCM solvent model for 1,2-dichlorobenzene were performed
at the B3LYP/6-31G(d) level. The optimized geometries were used
for single-point RHF calculations of FMO energies in the gas phase
and in 1,2-dichlorobenzene (see the Supporting Information). The
experimental reduction and oxidation potentials were correlated with
calculated FMO energies. The best correlation was found for the
B3LYP/6-31G(d) FMO energies calculated in the gas phase, with R2
having a value of 0.99. This correlation was used for further
evaluations.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support of the Russian
Foundation for Basic Research (Grant No. 11-03-00186) and
Saint Petersburg State University (Grant No. 12.38.78.2012).
This research used resources of the resource center “Computer
Center” and “Center for Chemical Analysis and Material
Research” of Saint Petersburg State University.
(30) Boens, B.; Faugeras, P. A.; Vergnaud, J.; Lucas, R.; Teste, K.;
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Zerrouki, R. Tetrahedron 2010, 66, 1994.
(31) Manecke, G.; Klawitter, J. Makromol. Chem. 1971, 142, 253.
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