An original electrochemical pathway for the synthesis of porphyrin oligomers

Article abstract of DOI:10.1002/chem.201203271

An electrosynthesis process has allowed the formation of four oligomers, containing three, four, or five macrocycles. This method is based on the nucleophilic attack of porphyrins substituted by several pendant pyridyl groups to the electrogenerated radical cation of zinc β-octaethylporphyrin (ZnOEP), according to an ECEC processes. Thus, a control of the number of macrocycles and of the geometry of the oligomers can be performed. These new compounds have been characterized by HRMS as well as ¹H NMR, UV/Vis, and fluorescence spectroscopy, and electrochemistry. The results show a strong influence of the pyridinium spacers on the macrocycles. Copyright

Full text of DOI:10.1002/chem.201203271

DOI: 10.1002/chem.201203271
An Original Electrochemical Pathway for the Synthesis of Porphyrin
Oligomers
Delphine Schaming,[a, c] Yun Xia,[a] Renꢀ Thouvenot,[b] and Laurent Ruhlmann*[a, d]
Abstract: An electrosynthesis process
has allowed the formation of four
oligomers, containing three, four, or
five macrocycles. This method is based
on the nucleophilic attack of porphyr-
ins substituted by several pendant pyr-
idyl groups to the electrogenerated
radical cation of zinc b-octaethylpor-
phyrin (ZnOEP), according to an
ECEC processes. Thus, a control of the
number of macrocycles and of the ge-
ometry of the oligomers can be per-
formed. These new compounds have
been characterized by HRMS as well
1
as H NMR, UV/Vis, and fluorescence
spectroscopy, and electrochemistry. The
results show a strong influence of the
pyridinium spacers on the macrocycles.
Keywords: electrochemistry
· nu-
cleophilic substitution · oligomers ·
porphyrinoids · pyridinium
Introduction
green plants contains up to 200 chlorophyll molecules, ab-
sorbing and transforming the energy coming from sunlight.
This is the primary energy source of a lot of living organisms
on earth, and applications of the photosynthetic system have
great potentials to solve energy and environmental problems
in the world.
A number of researchers have attempted to construct arti-
ficial light-harvesting systems, which provide important con-
tributions not only to understand the natural system and to
elucidate fundamental aspects of the reactivity, but also to
mimic Nature in creating new efficient light-harvesting sys-
tems for energy conversion.[3,4] Therefore, many artificial
systems consisting of multi-porphyrin systems or arrays in
which the chromophoric units are placed in a predefined
spatial arrangement have been developed to mimic the pho-
toinduced electron or energy transfer.[5]
One of the most used approaches is the construction of
multiporphyrin systems by using the methodologies of self-
assembling by electrostatic interactions,[6–9] metal coordina-
tion,[10–14] or hydrogen-bonding interactions.[15] Besides these
different types of interactions to build up porphyrin arrays,
considerable efforts have been devoted to the syntheses of
multiporphyrin arrays with covalent linkages.
Several elegant strategies have been already described for
the syntheses of such covalent multiporphyrin systems.[16]
Unfortunately, they always consist of multistep chemical
syntheses, which suffer from low overall yields of the desired
products. In this context, we have developed an easy
method to synthesize porphyrin oligomers based on an elec-
trochemical process. This method consists of the nucleophil-
ic substitution of compounds derived from pyridine in meso
position of the electrogenerated radical cation of zinc b-oc-
taethylporphyrin (ZnOEP).[17–19] Thus, when pyridyl-substi-
tuted porphyrins are used, they can play the role of a nucle-
ophile and their reaction with the radical cation of ZnOEP
can lead to the formation of porphyrin dimers and oligom-
ers.
Porphyrins and porphyrin-containing materials have attract-
ed a considerable attention these last decades, porphyrin
macrocycles being intensely present in nature. For instance,
in the photosynthesis process, chlorophyll possesses attrac-
tive functions of sunlight energy conversion into chemical
energy to perform chemical reactions. Due to the structural
elucidation of photosystems I and II (PS I and II), the im-
portance of the spatial arrangement of the porphyrin units
in the light-harvesting system of the photosynthetic center
has been recognized.[1,2] For example, a PS II complex in
[a] Dr. D. Schaming, Dr. Y. Xia, Prof. L. Ruhlmann
Laboratoire de Chimie Physique
UMR 8000 CNRS/Universitꢀ Paris-Sud 11
Facultꢀ des Sciences d’Orsay, Bꢁtiment 349
91405 Orsay cedex (France)
[b] Dr. R. Thouvenot
Institut Parisien de Chimie Molꢀculaire
UMR 7201 CNRS/Universitꢀ Pierre et Marie Curie
(Universitꢀ Paris 06)
Case courrier 42, 4 Place Jussieu
75252 Paris cedex 05 (France)
[c] Dr. D. Schaming
Present address:
Laboratoire ITODYS
UMR 7086 CNRS/Universitꢀ Paris Diderot
-Sorbonne Paris Citꢀ
Bꢁt. Lavoisier, 15 rue Jean-Antoine de Baꢂf
Case 7107, 75205 Paris Cedex 13 (France)
[d] Prof. L. Ruhlmann
Present address:
Laboratoire d’Electrochimie et
de Chimie-Physique du Corps Solide
UMR 7177 CNRS/Universitꢀ de Strasbourg
4 rue Blaise Pascal, 67081 Strasbourg (France)
Fax :
ACHTUNGTRENNUNG(+33)3-68-85-14-31
E-mail: lruhlmann@unistra.fr
1712
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FULL PAPER
Indeed, in a previous report, we have presented the possi-
bility to synthesize such compounds by using porphyrins
bearing one pendant pyridyl group, thus, porphyrin dimers
with pyridinium spacers have been obtained; these pyridini-
um spacers have interesting electrochemical properties.[20]
However, such multi-porphyrin systems become even more
interesting when the number of macrocycles is increased.
Indeed, each macrocycle can act as an “antenna” to soak up
the light for the development of artificial photosynthetic sys-
tems. In this way, we have recently reported the use of
meso-tetraphenylporphyrin derivatives, in which two, three,
or four phenyl groups have been replaced by pyridyl groups.
Because such porphyrins have several pyridyl groups and
ZnOEP have several free meso positions, porphyrin copoly-
mers have been obtained through the electrochemical path-
way.[21] However, no control of the number of macrocycles
contained in the porphyrin polymeric chains can be per-
formed.
Results and Discussion
Electrosynthesis: In our previous works, we have shown that
in the presence of nucleophiles derived from pyridine, the
oxidation of ZnOEP (at its first oxidation wave) induces nu-
cleophilic attacks leading to the attachment of pyridyl
groups to the free carbon atoms of the porphyrins (i.e.,
meso carbon atoms).[17–20]
When pyridyl-substituted porphyrins are used, they can
play the role of a nucleophile and their reaction with the
radical cation of ZnOEP can lead to the formation of por-
phyrin oligomers. In order to obtain trimers, tetramers, and
pentamers, we have used substituted porphyrins bearing
two, three, and four pyridyl groups, respectively (Figure 1).
Thus, in this study, 5,10-dipyridyl-15,20-diphenylporphyrin
(cis-H2Py2Ph2P),
5,15-dipyridyl-10,20-diphenylporphyrin
5,10,15-tripyridyl-20-phenylporphyrin
(trans-H2Py2Ph2P),
(H2Py3PhP), and 5,10,15,20-tetrapyridylporphyrin (H2Py4P)
have been employed.
In this context, we report on the electrosynthesis of por-
phyrin oligomers with controlled sizes, namely two trimers,
one tetramer, and one pentamer.
These oligomers were obtained by carrying out an exhaus-
tive electrolysis of a mixed solution of ZnOEP and one of
the pyridyl-substituted porphyrins, in appropriate concentra-
tions. The electrolysis was performed at a potential equal to
0.75 V versus SCE (SCE=saturated calomel electrode), al-
Figure 1. Electrosynthesis scheme of the four obtained oligomers, that is, a cis- and a trans-trimer, a tetramer, and a pentamer.
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1713

L. Ruhlmann et al.
lowing the formation of the radical cation of ZnOEP
However, the protons released during the synthesis could
lead to a demetallation and a protonation of the macrocy-
cles. In order to trap them, a sterically hindered base such
as 2,6-lutidine (whose nucleophilicity is weakened due to
the presence of methyl substituents close to the nitrogen
atom) has been added to the solution before electrolysis.
(ZnOEP +, step E1, that is, a first electrochemical step).
C
Then the pyridyl groups of the second type of porphyrins
can achieve a nucleophilic attack onto the electrogenerated
+
C
radical cation ZnOEP in the meso positions (step CNmeso
i.e., a chemical reaction corresponding to the nucleophilic
attack in the meso position), which leads to the formation of
isoporphyrins (i.e., porphyrins for which the aromaticity is
broken, because of the simultaneous presence of a pyridini-
um group and a proton on the same meso carbon atom).
Our previous studies have allowed to propose an
ECNmesoECB mechanism to finally explain the formation of
such substituted porphyrins.[17,20] Indeed, during the continu-
ous electrolysis, these isoporphyrins are spontaneously oxi-
dized (step E2, i.e., a second electrochemical step), and the
spare protons present onto the meso carbon atoms of the
isoporphyrins are lost (step CB,
,
HRMS spectra: The oligomers have also been characterized
by high-resolution mass spectrometry (HRMS). The mass
spectra of each cationic oligomer match very well with the
theoretical isotopic pattern, as shown in Figure 2 in the case
of the cis trimer.
Nevertheless, it can be noticed that the transfer of the
pentamer in the gas phase cannot be achieved, because its
molecular mass is too high.
i.e., a chemical reaction corre-
sponding to the loss of the
proton at the meso position).
It can be noticed that the ap-
plied electrolysis potential is too
low to allow oxidation of the
meso-tetraphenylporphyrin de-
rivatives, the first oxidation step
occurring between 1.2 and 1.4 V
versus SCE. Thus, substitutions
on these macrocycles were
avoided. Moreover, the poten-
tial is also too low to permit
multi-substitutions onto the
ZnOEP macrocycles. Indeed,
the first oxidation potential of
the mono-substituted ZnOEP is
higher than that of the non-sub-
stituted ZnOEP, because of the
electron-withdrawing effect of
the pyridinium spacer.
Thus, a total control of the
Figure 2. Obtained (top) and predicted (bottom) HRMS data for the cis trimer.
size of the synthesized oligom-
ers can be achieved according to the number of pendant
pyridyl groups onto the central porphyrin ring.
1H NMR spectra: 1H NMR spectra of the four oligomers
have been obtained in CD3CN (Figure 3). The cis and the
trans isomers of the trimer display similar NMR spectra,
except for the b-H protons. Firstly, for both of them, two
singlets (with integrations of four and two protons) are ob-
served in the d=10.5–10.3 ppm range, which are attributed
to the meso-H protons of ZnOEP. These signals are consis-
tent with the substitution of the two pyridyl groups of
H2Py2Ph2P onto one meso position of two ZnOEP. Two dou-
blets (4H) are also observed around d=10.9 and 9.8 ppm,
and are attributed to the protons of the two pyridinium
groups.
During each electrolysis, the current decreases exponen-
tially with time and the experiments were stopped when the
current reaches the residual current (measured in the ab-
sence of electroactive compounds). At the end of the elec-
trolysis, the number of transferred electrons was 4.1, 4.2, 6.2,
and 8.2 per molecule for the cis trimer, the trans trimer, the
tetramer, and the pentamer, respectively. Thus, the number
of electrons transferred is consistent with the proposed
ECEC mechanism, and the global reaction can be summar-
ized as given in Equation (1):
Concerning the b-H protons of the H2Py2Ph2P subunit,
they appear as large and ill-defined multiplets between d=
9.8 and 9.0 ppm, with a total integration equal to eight pro-
tons. Finally, the two signals observed around d=8.5 and
7.9 ppm (4H and 6H; respectively) are assigned to the pro-
n ZnOEP þ H2PynPhð4ꢀnÞP ! H2PyþnPhð4ꢀnÞPðZnOEPÞnþ
2n eꢀ þ n Hþ ðn ¼ 2, 3, or 4Þ
ð1Þ
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Synthesis of Porphyrin Oligomers
FULL PAPER
subunit, integrate for two and three protons, respectively,
which is consistent with the fact that this oligomer has only
ꢀ
one phenyl group. Finally, internal N H protons appear at
d=ꢀ2.26 ppm.
For the pentamer, no signal is observed in the d=9.0–
8.0 ppm range, as expected in the absence of phenyl groups.
Moreover, because the four ZnOEP subunits are equivalent,
the meso-H protons appear as two singlets around d=10.6
and 10.4 ppm, with integrations of eight and four protons,
respectively; and the protons of the four pyridinium groups
appear as two doublets around d=10.8 and 9.7 ppm (8H
each of them). Concerning the b-H protons of the H2Py4P
subunit, the signal appears as a singlet around d=9.9 ppm,
ꢀ
which confirms that the compound is symmetric. The N H
protons are also observable at d=ꢀ2.16 ppm.
Concerning the ethyl protons of the ZnOEP units for
each oligomer, they are observed between d=4.5 and
1.0 ppm.
UV/Vis absorption and fluorescence spectra: The UV/Vis
absorption spectra of the oligomers (Figure 4A) do not cor-
respond exactly to the addition of the spectra of the corre-
sponding individual porphyrins. Indeed, the absorption
bands show significant bathochromic shifts, which can be ex-
Figure 3. 1H NMR spectra of A) the cis trimer, B) the trans trimer, C) the
tetramer, and D) the pentamer in the d=11.5–7.5 ppm range (300 MHz,
CD3CN) (i=impurity).
tons of the two phenyl groups of the H2Py2Ph2P subunit. In-
ꢀ
ternal N H protons appear at d=ꢀ2.57 and ꢀ2.68 ppm for
the cis and the trans isomer, respectively.
A quasi similar behavior is observed for the tetramer,
with nevertheless as expected, a different integration of the
signals. Indeed, the two signals assigned to the meso-H pro-
tons of the ZnOEP (in the d=10.8–10.3 ppm range) can
now be integrated with six and three protons, respectively.
Moreover, contrary to the two trimers, the three ZnOEP
subunits are nonequivalent, and consequently, these two sig-
nals each appear split in two singlets. Similar observations
can be done for the protons of the three pyridinium groups.
Indeed, two pairs of doublets are observed, the first one
centered around d=10.5 ppm and the other one around d=
9.5 ppm, and each pair can be integrated by a total of six
protons. Concerning the b-H protons of the H2Py3PhP subu-
nit, they appear also as four doublets between d=9.8 and
9.4 ppm (with a total integration of eight protons). In addi-
tion, the signals observed around d=8.6 and 8.1 ppm, attrib-
uted to the protons of the phenyl group of the H2Py3PhP
Figure 4. A) UV/Vis absorption spectra and B) fluorescence spectra (ex-
citation wavelength l=540 nm) of trans-H2Py2Ph2P (c=4ꢄ10ꢀ6 molLꢀ1
,
dashed line), ZnOEP (c=4ꢄ10ꢀ6 molLꢀ1, dash-dotted line), and the
trans trimer (c=4ꢄ10ꢀ6 molLꢀ1, solid line) in 1,2-C2H4Cl2 (1 cm path
length cell).
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1715

L. Ruhlmann et al.
plained by the electron-withdrawing effect of the positively
charged pyridinium spacers.[21–24] The important shift ob-
served for the Soret band of the central meso-tetraphenyl-
porphyrin derivative can also be a consequence of the dis-
tortion of this macrocycle.[21–23] The absorption bands also
show significant broadening. This broadening can be ex-
plained by the exciton coupling theory developed by Kasha
et al.[25] Indeed, such broadening of the Soret band is com-
monly observed for porphyrin dimers or oligo-
ACHTUNGTRENNUNG
quenching of the luminescence is observed for the porphyr-
ins within the oligomers. A rapid electron transfer from the
excited porphyrins to the pyridinium acceptor could explain
this observation.[23,29]
Figure 5. CV curves of the cis trimer and the trans trimer at c=5ꢄ
10ꢀ4 molLꢀ1 in 1,2-C2H4Cl2, containing 0.1 molLꢀ1 NBu4PF6 at 100 mVsꢀ1
&
(working electrode: glassy carbon; =peaks attributed to the redox
*
processes of the ZnOEP macrocycles, =peaks attributed to the redox
Electrochemistry: The electrochemical behavior of free base
porphyrins and metalloporphyrins involving non-electroac-
tive metals is well documented.[30] In general, in the anodic
domain, the oxidation of the p ring proceeds through two
one-electron steps generating successively the radical cation
and the dication. Similarly, in the cathodic domain, two one-
electron steps yield successively the radical anion and the
dianion.
~
processes of the H2Py2Ph2P macrocycle, =peaks related to the reduc-
tion of the pyridinium spacers).
Concerning the cathodic part, the electrochemical behav-
ior of the two trimers is quite different. Indeed, five succes-
sive waves are clearly observed for the cis trimer, whereas
only two waves are observed for the trans trimer (Table 1
and Figure 5).
In the case of the cis trimer, the two first processes, at
ꢀ0.41 and ꢀ0.58 V versus SCE, can be attributed to the re-
duction of the two pyridinium spacers (Figure 5). The three
following processes can be assigned to the reduction of the
macrocycles. By comparison with the free porphyrins, the
processes observed at ꢀ1.04 and ꢀ1.24 V versus SCE can be
attributed to the two successive reductions of the cis-
H2Py2Ph2P macrocycle and the process observed at ꢀ1.48 V
versus SCE can be attributed to the first reduction of the
ZnOEP macrocycles.
The potential values of these redox processes attributed
to the five porphyrins and the four oligomers are gathered
in Table 1.
Figure 5 shows the cyclo voltammetry (CV) curves record-
ed for the two trimers. In the anodic part, three successive
processes can be observed for each trimer (Table 1 and
Figure 5). The two first two-electron waves can be assigned
to the first and second oxidations of ZnOEP, whereas the
last one can be assigned to the first one-electron oxidation
of the H2Py2Ph2P subunit. Compared to the potential values
obtained for free ZnOEP and H2Py2Ph2P, the oxidation
waves are anodically shifted. The more difficult oxidations
of the ZnOEP and the H2Py2Ph2P macrocycles can be ascri-
bed to the electron-withdrawing effect of the substituted
positively charged pyridinium groups.[17,19,22]
In the case of the trans trimer, only two electrochemical
processes are observed. The first one at ꢀ0.33 V versus
SCE, corresponds to the simultaneous reduction of the two
pyridinium spacers (Figure 5) and the second one, at
ꢀ1.41 V versus SCE, can be assigned to the first reduction
Table 1. Electrochemical data for the free porphyrins ZnOEP, cis-H2Py2Ph2P, trans-H2Py2Ph2P, H2Py3PhP, and H2Py4P, and for the four oligomers.[a]
Compounds
Oxidation
ring
Reduction
Py+
ring
ZnOEP
1.04
0.65
ꢀ1.59
cis-H2Py2Ph2P
trans-H2Py2Ph2P
H2Py3PhP
1.46irr
1.45irr
1.56irr
1.68irr
1.27irr
1.26irr
1.34irr
1.38irr
ꢀ1.04rev
ꢀ1.04rev
ꢀ0.99rev
ꢀ0.98rev
ꢀ1.37rev
ꢀ1.37rev
ꢀ1.33rev
ꢀ1.35rev
H2Py4P
cis trimer
trans trimer
tetramer
1.50
1.49
1.30
1.30
1.37
1.37
0.95
0.94
1.03
1.00
ꢀ0.41rev (1eꢀ), ꢀ0.58rev (1eꢀ)
ꢀ0.33rev (2eꢀ)
ꢀ1.04
ꢀ1.24
ꢀ1.48
ꢀ1.41
ꢀ1.46
ꢀ1.56
ꢀ0.23rev (2eꢀ), ꢀ0.58rev (1eꢀ)
pentamer
ꢀ0.20rev (2eꢀ), ꢀ0.44rev (2eꢀ)
[a] All potentials in V versus SCE were obtained from cyclic voltammetry in 1,2-C2H4Cl2, containing 0.1 molLꢀ1 NBu4PF6 at 100 mVsꢀ1 (working elec-
trode: glassy carbon). Bold characters are used for potential values observed for ZnOEP, italic characters are used for potential values observed for the
pyridine-substituted porphyrins and underlined characters are used for pyridinium signals; rev=reversible, irr=irreversible; in brackets: number of ex-
changed electrons.
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Synthesis of Porphyrin Oligomers
FULL PAPER
of the ZnOEP macrocycles. Nevertheless, one cannot ob-
serve the electrochemical processes corresponding to the re-
duction of the trans-H2Py2Ph2P macrocycle.
signed to the reduction of the ZnOEP macrocycles. As pre-
viously discussed, the reduction processes corresponding to
the H2Py3PhP subunit are not observed, because of a hin-
dered approach of this macrocycle to the electrode. Similar-
ly, in the anodic part, only the oxidation processes attributed
to the ZnOEP macrocycles are observed, at 1.03 and 1.37 V
versus SCE.
In the case of the pentamer, the cyclic voltammogram is
of poor quality. Indeed, this compound is even more bulky,
and consequently, only the signals attributed to the electro-
chemical processes of the ZnOEP subunits are well defined.
The electrochemical processes corresponding to the cen-
tral H2Py4P p ring are totally missing. Concerning the pyri-
dinium groups, their reduction occur in two successive two-
electron waves (at ꢀ0.20 and ꢀ0.44 V versus SCE,
Figure 7).
Thus, two points can be discussed: 1) the different electro-
chemical behaviors of the reduction of the pyridinium
spacers between the two isomers and 2) the disappearance
of the electrochemical processes attributed to the reduction
of the trans-H2Py2Ph2P subunit in the case of the trans
trimer. Firstly, the fact that the pyridinium spacers are re-
duced in two steps in the case of the cis trimer could be the
consequence of a mutual interaction between the two pyridi-
nium groups, which allows a delocalization of the electronic
density onto the two pyridinium spacers.[18,22,23,31] Contrary
to the cis trimer, the pyridinium spacers of the trans trimer,
which are simultaneously reduced, seem independent of
each other. Similar behaviors have been previously observed
for ZnOEP substituted by two pyridinium or two bipyridini-
um groups in cis and trans positions: when the pyridinium
(or bipyridinium) are grafted in cis position, their reductions
proceeded in two successive steps, whereas when they are
grafted in trans position, their reductions proceeded in a
unique step.[32] Concerning the observation of the signals
corresponding to the reduction of the H2Py2Ph2P subunit,
one can explain this phenomenon by the different geometry
between the two isomers. Indeed, in the case of the cis
trimer, the two ZnOEP macrocycles are on the same side of
the H2Py2Ph2P macrocycle (on two adjacent meso positions),
allowing undoubtedly a better approach of the H2Py2Ph2P
subunit to the electrode than for the trans trimer, for which
the two ZnOEP macrocycles are on both sides of the
H2Py2Ph2P subunit. As a matter of fact, the electrochemical
processes concerning the H2Py2Ph2P subunit are more diffi-
cult in the case of the cis trimer than in the case of the trans
trimer.
Figure 6 illustrates the CV curve obtained for the tetram-
er. The two first waves observed in the cathodic part at
ꢀ0.23 and ꢀ0.58 V versus SCE can be attributed to the re-
duction of two and one pyridinium, respectively. And finally,
the third wave observed at ꢀ1.46 V versus SCE can be as-
Figure 7. Differential pulse voltammograms of the reduction of the pyri-
dinium spacers of the four oligomers (c=2.5ꢄ10ꢀ4 molLꢀ1) at 25 mVsꢀ1
in 1,2-C2H4Cl2, containing 0.1 molLꢀ1 NBu4PF6 (working electrode:
glassy carbon).
Comparison of the reduction potential values of the first
pyridinium spacer for each oligomer can also be discussed.
Indeed, an anodic shift of the potential is observed when
the number of pyridinium spacers within the oligomer in-
creases (Table 1 and Figure 7). This observation can be ten-
tatively explained by the global charge of the oligomer,
which increases with the number of pyridinium spacers.
Consequently, these positive charges lead to an easier reduc-
tion of the pyridinium spacers.
Figure 6. CV curves of the tetramer at c=5ꢄ10ꢀ4 molLꢀ1 in 1,2-C2H4Cl2,
containing 0.1 molLꢀ1 NBu4PF6 at 100 mVsꢀ1 (working electrode: glassy
&
carbon; =peaks attributed to the redox processes of the ZnOEP mac-
~
rocycles, =peaks related to the reduction of the pyridinium spacers).
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L. Ruhlmann et al.
cis-H2Py2Ph2P: 1H NMR (300 MHz, CDCl3, 258C): d=9.06 (d, J=
5.7 Hz, 4H; 3,5-Py), 8.93–8.81 (m, 8H; b-H), 8.23 (d, J=9.3 Hz, 4H; 2,6-
Ph), 8.18 (d, J=5.7 Hz, 4H; 2,6-Py), 7.82–7.78 (m, 6H; 3,4,5-Ph),
ꢀ2.80 ppm (s, 2H; NH); UV/Vis (1,2-C2H4Cl2): lmax (e)=417 (354000),
514 (22000), 548 (11000), 593 (11000), 646 nm (8000 molꢀ1 dm3 cmꢀ1).
trans-H2Py2Ph2P: 1H NMR (300 MHz, CDCl3, 258C): d=9.05 (d, J=
6.0 Hz, 4H; 3,5-Py), 8.92 (d, J=4.8 Hz, 4H; b-H), 8.82 (d, J=4.8 Hz, 4H;
b-H), 8.21 (d, J=7.5 Hz, 4H; 2,6-Ph), 8.18 (d, J=6.0 Hz, 4H; 2,6-Py),
7.82–7.73 (m, 6H; 3,4,5-Ph), ꢀ2.82 ppm (s, 2H; NH); UV/Vis (1,2-
C2H4Cl2): lmax (e)=417 (262000), 514 (13000), 548 (6000), 585 (6000),
643 nm (3000 molꢀ1 dm3 cmꢀ1).
H2Py3PhP: 1H NMR (300 MHz, CDCl3, 258C): d=9.07 (d, J=6.0 Hz,
6H; 3,5-Py), 8.93 (d, J=4.8 Hz, 2H; b-H), 8.87 (s, 4H; b-H), 8.83 (d, J=
4.8 Hz, 2H; b-H), 8.22 (d, J=7.5 Hz, 2H; 2,6-Ph), 8.17 (d, J=6.0 Hz, 6H;
2,6-Py), 7.85–7.75 (m, 3H; 3,4,5-Ph), ꢀ2.86 ppm (s, 2H; NH); UV/Vis
(1,2-C2H4Cl2): lmax (e)=417 (143000), 511 (10000), 548 (5000), 582
(7000), 647 nm (2000 molꢀ1 dm3 cmꢀ1).
Conclusion
Four oligomers, containing three, four, or five macrocycles,
have been obtained by an electrochemical synthesis path-
way. This method is based on the nucleophilic attack of
meso-tetraphenylporphyrin derivatives, having several pend-
ant pyridyl groups to electrogenerated radical cations of
zinc b-octaethylporphyrin (ZnOEP), according to an ECEC
processes. Thus, a control of the number of macrocycles
within the oligomers can be performed, according to the
number of pendant pyridyl groups at the central macrocycle.
Moreover, these experimental conditions avoid multi-substi-
tutions to the ZnOEP macrocycle, and consequently, they
avoid polymerization.
The electrochemical study of these four oligomers has
been performed by cyclic voltammetry and differential pulse
voltammetry. A strong influence of the positively charged
pyridinium spacers onto the redox processes of the macrocy-
cle has been observed. Moreover, electronic couplings be-
tween the pyridinium groups have been observed in some
cases.
Electrosynthesis of the oligomers
General procedure: ZnOEP and the appropriate porphyrin with pendant
pyridine groups were dissolved in 1,2-C2H4Cl2/CH3CN (3:1, 100 mL),
containing 0.1 molLꢀ1 tetraethylammonium hexafluorophosphate. 2,6-lu-
tidine (10 mL) was added to the solution. Then an electrolysis was carried
out at 0.75 V versus SCE. Indeed, the electrolysis was stopped when the
current value had reached the residual current value. During this anodic
oxidation, the electrolyzed solution was continuously stirred and main-
tained under argon. Then the solvents were removed and the residue was
dissolved in 1,2-C2H4Cl2 (100 mL). This organic solution was washed ten
times with water in order to remove the supporting electrolyte. The or-
ganic layer was concentrated (ca. 5 mL) and purified by column chroma-
tography on silica gel. The first and the second fractions (eluted with
CH2Cl2) contained unreacted ZnOEP and the porphyrin having the
pendant pyridine groups, respectively. The third fraction (eluted with a
mixture of CH2Cl2/CH3OH;99.5:0.5) contained the desired oligomer.
Experimental Section
Chemicals: All solvents were of reagent grade quality and used without
further purification.
Zinc-b-octaethylporphyrin (ZnOEP) and 5,10,15,20-tetraphenylporphyrin
(H2Py4P) were purchased from Sigma–Aldrich.
cis Trimer: ZnOEP (21.17 mg, 0.035 mmol) and cis-H2Py2Ph2P (8.63 mg,
Electrochemistry: All electrochemical measurements were carried out
under argon with a standard three-electrode system by using a PARSTAT
2273 potentiostat. The working electrode was a glassy carbon disk elec-
trode (d=3 mm). A platinum wire was used as an auxiliary electrode.
The reference electrode was a saturated calomel electrode that was elec-
trically connected to the studied solution by a junction bridge filled with
the corresponding solvent/supporting electrolyte solution.
0.014 mmol) were used and the electrolysis was carried out for 24 h. The
cis trimer was obtained with a yield of 74%. H NMR (300 MHz, CDCl3,
1
258C): d=10.95 (d, J=6.0 Hz, 4H; py+), 10.45 (s, 4H; meso-H), 10.33 (s,
2H; meso-H), 9.84 (d, J=6.0 Hz, 4H; py+), 9.75–9.00 (m, 8H; b-H), 8.42
(m, 4H; Ph), 7.99 (m, 6H; Ph), ꢀ2.57 ppm (s, 2H; NH); UV/Vis (1,2-
C2H4Cl2): lmax (e)=410 (262000), 468 (66000), 540 (32000), 577 (36000),
659 nm (9000 molꢀ1 dm3 cmꢀ1); FAB-MS (NBA): m/z calcd for
[Zn2C114H114N14]2+: 905.51; found: 905.40 [M]2+
.
Electrosynthesis were performed in by using a large platinum wire as the
working electrode. The anodic and cathodic compartments were separat-
ed by a fritted glass disk to prevent diffusion of the electrogenerated spe-
cies.
trans Trimer: ZnOEP (25.02 mg, 0.042 mmol) and trans-H2Py2Ph2P
(8.63 mg, 0.014 mmol) were used and the electrolysis was carried out for
24 h. The trans trimer was obtained with a yield of 69%. 1H NMR
(300 MHz, CDCl3, 258C): d=10.87 (d, J=6.0 Hz, 4H; py+), 10.44 (s, 4H;
meso-H), 10.31 (s, 2H; meso-H), 9.74 (d, J=6.0 Hz, 4H; py+), 9.75–9.30
(m, 8H; b-H), 8.36 (m, 4H; Ph), 7.92 (m, 6H; Ph), ꢀ2.68 ppm (s, 2H;
NH); UV/Vis (1,2-C2H4Cl2): lmax (e)=408 (189000), 459 (74000), 538
(24000), 578 (34000), 670 nm (12000 molꢀ1 dm3 cmꢀ1); FAB-MS (NBA):
Spectroscopic measurements: UV/Vis spectra were recorded with a Hew-
lett–Packard HP8453 spectrophotometer. Steady-state luminescence
emission spectra were obtained by using a Spex fluorolog 1681 spectro-
fluorimeter.
NMR spectroscopy: 1H NMR spectra were recorded with
a Bruker
m/z calcd for [Zn2C114H114N14]2+: 905.51; found: 905.40 [M]2+
.
AC300 spectrometer at 300 MHz and at 258C unless otherwise stated.
Tetramer: ZnOEP (34.09 mg, 0.056 mmol) and H2Py3PhP (8.88 mg,
0.014 mmol) were used and the electrolysis was carried out for 48 h. The
tetramer was obtained with a yield of 70%. 1H NMR (300 MHz, CDCl3,
258C): d=10.75 (d, J=5.7 Hz, 2H; py+), 10.55 (d, J=6.0 Hz, 4H; py+),
10.55 (s, 2H; meso-H), 10.52 (s, 4H; meso-H), 10.38 (s, 1H; meso-H),
10.35 (s, 2H; meso-H), 9.78 (d, J=5.5 Hz, 2H; b-H), 9.76 (d, J=5.5 Hz,
2H; b-H), 9.63 (d, J=5.7 Hz, 2H; py+), 9.53 (d, J=6.0 Hz, 4H; py+),
9.52 (d, J=5.5 Hz, 2H; b-H), 9.47 (d, J=5.5 Hz, 2H; b-H), 8.55 (m, 2H;
Ph), 8.10 (m, 3H; Ph), ꢀ2.26 ppm (s, 2H; NH); UV/Vis (1,2-C2H4Cl2):
lmax (e)=409 (360000), 460 (131000), 539 (49000), 577 (52000), 660 nm
Mass spectrometry: MS experiments have been carried out on an electro-
spray-ion trap instrument (Bruker, Esquire 3000). Solutions of oligomers
with c=50 mmolLꢀ1 were infused by using a syringe pump (160 mLhꢀ1).
Synthesis of the porphyrins with pendant pyridine groups (cis-H2Py2Ph2P,
trans-H2Py2Ph2P, and H2Py3PhP): A solution of benzaldehyde (3.05 mL),
4-pyridinecarbaldehyde (2.83 mL), and pyrrole (4.2 mL) was heated to
reflux (1308C) in propionic acid for 1 h. The reaction mixture was then
cooled and allowed to stand overnight. After that, the reaction mixture
was concentrated to about 50 mL and acetone (500 mL) was added with
stirring. The crystalline products of the porphyrins were obtained after
filtration. The crude product, a mixture of six porphyrins, was dissolved
in CH2Cl2 and loaded on a silica gel column. The six porphyrins were
separated by using a dichloromethane/methanol solvent system consisting
initially of 100% CH2Cl2, and gradually ending with 90% CH2Cl2:10%
CH3OH. After separation, the desired products include cis-H2Py2Ph2P
(9.7%), trans-H2Py2Ph2P (1.1%), and H2Py3PhP (4.3%).
(9000 molꢀ1 dm3 cmꢀ1); FAB-MS (NBA): m/z calcd for [Zn3C149H156N19]3+
803.05; found: 802.69 [M]3+
:
.
Pentamer: ZnOEP (45.06 mg, 0.075 mmol) and H2Py4P (9.30 mg,
0.015 mmol) were used and the electrolysis was carried out for 72 h. The
pentamer was obtained with
CD3CN, 258C): d=10.76 (d, J=6.0 Hz, 8H; py+), 10.57 (s, 8H; meso-H),
a
yield of 59%. 1H NMR (300 MHz,
1718
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ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1712 – 1719

Synthesis of Porphyrin Oligomers
FULL PAPER
10.40 (s, 4H; meso-H), 9.89 (s, 8H; b-H), 9.71 (d, J=6.0 Hz, 8H; py+),
ꢀ2.16 ppm (s, 2H; NH); UV/Vis (1,2-C2H4Cl2): lmax (e)=410 (202000),
456 (77000), 542 (32000), 576 (31000), 659 (8000 molꢀ1 dm3 cmꢀ1).
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Received: September 13, 2012
Published online: December 4, 2012
Chem. Eur. J. 2013, 19, 1712 – 1719
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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