Bis-porphyrin copolymers covalently linked by pyridinium spacers obtained by electropolymerization from β-octaethylporphyrins and pyridyl-substituted porphyrins

Article abstract of DOI:10.1039/c1nj20790c

Five bis-porphyrin copolymers have been obtained by an electropolymerization process using zinc β-octaethylporphyrin (ZnOEP) type macrocycles and porphyrins functionalized by several pyridyl groups. This electropolymerization process is based on a nucleophilic attack of the pyridyl groups onto electrogenerated ZnOEP dications at their meso-positions. It is the first time that bis-porphyrin copolymers have been obtained. In this study, the copolymers have been characterized by electrochemistry, UV-visible absorption spectroscopy and atomic force microscopy. In particular, interactions between pyridinium spacers are thoroughly discussed. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c1nj20790c

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PAPER
Bis-porphyrin copolymers covalently linked by pyridinium spacers
obtained by electropolymerization from b-octaethylporphyrins and
pyridyl-substituted porphyrinsw
Yun Xia,a Delphine Schaming,a Rana Farha,bc Michel Goldmannbd and
Laurent Ruhlmann*ae
Received (in Montpellier, France) 13th September 2011, Accepted 3rd November 2011
DOI: 10.1039/c1nj20790c
Five bis-porphyrin copolymers have been obtained by an electropolymerization process using zinc
b-octaethylporphyrin (ZnOEP) type macrocycles and porphyrins functionalized by several pyridyl
groups. This electropolymerization process is based on a nucleophilic attack of the pyridyl groups
onto electrogenerated ZnOEP dications at their meso-positions. It is the first time that
bis-porphyrin copolymers have been obtained. In this study, the copolymers have been
characterized by electrochemistry, UV-visible absorption spectroscopy and atomic force
microscopy. In particular, interactions between pyridinium spacers are thoroughly discussed.
Concerning covalent polymers, they can be principally
obtained by a classical chemical way or by an electrochemical
Introduction
Porphyrins and porphyrin-containing materials are among the
most-studied chromophores, not only because of their roles as
biological photosensitizers, redox centers and oxygen carriers,
way. For example, several reports have exposed the possibility to
achieve direct polymerization of porphyrins via a meso–meso
coupling. Indeed, Osuka et al. have proposed a method based on
the chemical oxidation of zinc diarylporphyrins using AgIPF6 as
but also because of their attractive chemical properties and
oxidant.10,11 Moreover, a similar meso–meso coupled polymer
has also been recently obtained by electrooxidation of the
potential technological applications. In particular, conducting
porphyrin polymers are used in applications such as solar energy
transformation cells, potentiometric or amperometric sensors,
magnesium porphine at a potential corresponding to the first
catalysts, biomedicine, electronic devices and photonics.1–7
oxidation of the macrocycle.12 However, more currently, the
Different routes allowing the formation of porphyrin
formation of covalent polymers relies on the use of porphyrins
polymers have already been described in the literature. Among
with polymerizable substituents attached on the ring periphery.
them, the strategies most developed correspond to the formation
of coordination and covalent polymers.
chemical route4,13 or by an electrochemical process.2,13–19
A first general method to obtain metalloporphyrin polymers
is the use of bridging ligands which can coordinate the metal
For instance, polymerization of amino, pyrrole or thiophene-
substituted porphyrins has been allowed either by a classical
In this context, we have developed in our precedent works an
center. For instance, such coordination polymers have been
original methodology for electropolymerization of porphyrins,
obtained by coordination of nitrogenous ligands with ruthenium,
osmium or iron porphyrins.8,9
based on the nucleophilic attacks of di-pyridyl compounds
directly onto electrogenerated b-octaethylporphyrin (OEP)
dications.20,21 Indeed, compounds having two pendant pyridyl
groups present two accessible nucleophilic sites which can
a
both react with electrooxidized porphyrin rings in the meso-
position. A detailed study has been performed using 4,40-
bipyridine as a spacer. This methodology has further been
extended to the use of other pyridine-derived spacers, where
the organic links between the two pyridyl groups have been
changed according to their specific properties: rigid or not,
long or short, with conjugated p bonds or successive s bonds.
This novel easy way of electropolymerization opens up also
original synthesis routes for the elaboration of new functional
materials. Indeed, for instance, it has been possible to obtain
original hybrid organic–inorganic copolymers by using an
inorganic compound (such as polyoxometalate) functionalized
´
Laboratoire de Chimie Physique, UMR 8000 CNRS/Universite
´
ˆ
Paris-Sud 11, Faculte des Sciences d’Orsay, Batiment 349,
91405 Orsay, France. E-mail: laurent.ruhlmann@u-psud.fr;
Fax: +33 1 69 15 61 88; Tel: +33 1 69 15 44 38
b
c
´
Institut des NanoSciences de Paris, UMR 7588 CNRS/Universite
Paris 6, 4 place Jussieu, boıˆte courrier 840, 75252 Paris, France
`
Laboratoire d’Analyse et Controˆle des Systemes Complexes, ECE
Paris Ecole d’Ingenieurs, 37 quai de Grenelle, 75015 Paris, France
´
d Universite´ Paris Descartes, 45 rue des Saint Pe`res, 75006 Paris,
France
e Laboratoire d’Electrochimie et de Chimie-Physique du Corps Solide,
´
UMR 7177 CNRS/Universite de Strasbourg, 4 rue Blaise Pascal,
67081 Strasbourg, France. E-mail: lruhlmann@unistra.fr
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1nj20790c
c
588 New J. Chem., 2012, 36, 588–596
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by two pendant pyridyl groups.22 Such porphyrin–polyoxo-
metalate copolymers have presented interesting photocatalytic
properties.
In this paper, we show hereafter that this method of
electropolymerization also allows to easily obtain copolymers
of porphyrins containing two different types of macrocycles.
Indeed, porphyrins substituted by at least two pendant pyridyl
groups can be used as spacers to link a second type of
macrocycle (for instance, OEP-type). To our knowledge, it is
the first method of electropolymerization which allows for-
mation of such bis-porphyrin copolymers. Moreover, for these
copolymers, the bridging spacer between the macrocycles is
shorter compared to spacers previously used, because only
pyridinium groups bridge the porphyrins with another.
Results and discussion
Electropolymerization and electrochemical properties of the
copolymers
Scheme 2 Porphyrins used as monomers for the different copolymers
formed: (A) ZnOEP, (B) ZnOEP(Cl)2, (C) cis-H2Py2Ph2P, (D) trans-
H2Py2Ph2P, (E) H2Py3PhP and (F) H2Py4P.
We have recently shown that the use of a porphyrin substituted by
one pendant pyridyl group allows the formation of a dimer of
porphyrins in the presence of electrooxidized zinc b-octaethylpor-
phyrin (step E1) via an E1CNE2CB process (Scheme 1).23 Indeed, the
pyridyl group can attack in the meso-position of the electrooxidized
ZnOEP (step CN), leading first to the formation of an isoporphyrin.
Then, a second step of oxidation (step E2) allows the
re-aromatization of the ZnOEP macrocycle via an acid–base
reaction (step CB), leading to the substitution of the pyridyl group
onto the ZnOEP ring, i.e. to the formation of a dimer of porphyrins
with pyridinium as spacer between the two macrocycles.
The two possible isomers, i.e. the 5,10-dipyridyl-15,20-diphenyl-
porphyrin (cis-H2Py2Ph2P, Schemes 2C and 3A) and the 5,15-
dipyridyl-10,20-diphenylporphyrin (trans-H2Py2Ph2P, Schemes 2D
and 3B), have been compared. Fig. 1A and B show the voltammo-
grams recorded during iterative scans performed between ꢀ0.9 and
+1.6 V/SCE in the case of the use of cis-H2Py2Ph2P and trans-
H2Py2Ph2P, respectively, in the presence of ZnOEP (Scheme 2A).
The current increases progressively during these iterative
scans, showing the formation of a conducting polymer coating
the electrode. According to our previous studies, this poly-
merization can be described by a E(ECNECB)nE mechanism
(Scheme S1, ESIw).20,21 The process is different to the mecha-
nism described for the monosubstitution (Scheme 1). Indeed,
the formation of the dication of the ZnOEP (two steps E) is
needed to observe an electropolymerization. Thus, a pyridyl
group of the H2Py2Ph2P macrocycle can react with ZnOEP2+
in the meso-position to give an isoporphyrin (step CN). For
recall, an isoporphyrin corresponds to a porphyrin whose
aromaticity is broken because of the simultaneous presence
of two substituents (a pyridinium and a proton in our case) on
the same meso-carbon. Afterwards, this isoporphyrin can be
oxidized again (step E). Finally, the spare proton is lost,
leading to the re-aromatization of the macrocycle (step CB)
and allowing the formation of a dimer of porphyrins (ZnOEP-
Py+-H2Ph2P-Py in this case).
Scheme 1 Illustration of the E1CNE2CB mechanism for ZnOEP with
Then, the free pyridyl group can react in a similar way with
another oxidized ZnOEP macrocycle, and little by little, the
copolymer can grow.
H2PyPh3P.
Thus, the same type of reaction should lead to the formation of
oligomers and/or polymers of porphyrins if macrocycles substituted
by two (or more) pendant pyridyl groups are used. Indeed, both
pyridyl can attack different oxidized ZnOEP macrocycles. More-
over, several attacks can occur on one ZnOEP macrocycle, because
the four meso-positions 5, 10, 15 and 20 are free (Scheme 2A).
For this study, we have first employed dipyridyl diphenyl-
porphyrins (H2Py2Ph2P) as dipyridyl-substituted macrocycles.
One can also underline that even if the H2Py2Ph2P rings are
oxidized during the iterative scans, no substitution in b-positions
of these macrocycles by pyridyl groups of other H2Py2Ph2P
macrocycles could occur. Indeed, the kinetics of a nucleophilic
substitution in the b-position is probably too slow compared to
the kinetics of a substitution in the meso-position. A simple
explanation can be offered since the charge density at b-positions
c
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Fig. 1 Cyclic voltammograms recorded during the electropolymerization of (A) cis-H2Py2Ph2P, (B) trans-H2Py2Ph2P, (C) H2Py3PhP and (D)
H2Py4P in the presence of ZnOEP in CH3CN/1,2-C2H4Cl2 (1 : 4) with 0.1 M NEt4PF6. Working electrode: ITO; S = 1 cm2; scan rate: 0.1 V sꢀ1
.
is much smaller than that at meso-positions.24,25 Moreover,
Rachlewicz and Latos-Grazynski et al. have shown that oxidized
tetraphenylporphyrins react with pyridine to form meso-5-iso-
tetraphenylporphyrins and b-pyridinium substituted porphyrins
which suggested that the meso-isoporphyrin is an intermediate
product of the reaction of b-substitution.26 The zinc(II) iso-
porphyrin intermediate was also described by Dolphin et al.27
Thus, this reactivity shows that the pyridyl group is able to
attack at both meso- and b-positions of the ring, but due to the
charge density distribution, it prefers first the attack at
the meso-position. Then, a second nucleophilic attack at the
b-position (in concomitance with the release of the pyridinium
group at the meso-position) leads to b-pyridinium substituted
porphyrins.28,29 Thus, in the case of electropolymerization in
the presence of ZnOEP, a competitive attack at the meso-
positions onto the ZnOEP and the H2Py2Ph2P macrocycles
takes place. However, the nucleophilic attack at meso-positions
is preferred onto ZnOEP macrocycles because of a severely
restricted access to H2Py2Ph2P due to the presence of the phenyl
groups, even more in the case of the formation of oligomers and
polymers, rendering this nucleophilic attack less favorable.
Moreover, the phenyl groups of the H2Py2Ph2P macrocycles
being bad leaving groups, a second nucleophilic attack at the
b-position must be accomplished in order to have finally the
b-substitutions, which is probably impossible under our
experimental conditions. Indeed, the rate of the iterative
sweeps is too fast to let enough time for the b-attack and the
b-substitutions. Thus, nucleophilic attacks at the substitutable
meso-positions of ZnOEP would be preferred. Consequently,
electropolymerization of H2Py2Ph2P itself is very difficult and
maybe even impossible.
The cyclic voltammograms recorded during electropolymer-
ization of both H2Py2Ph2P in the presence of ZnOEP show
three waves in the cathodic part (well observed for instance in
Fig. 1A), at least during the first sweeps (indeed, sometimes,
the waves can merge each other when the number of scans
increases). These three waves are always absent during the first
scan, i.e. before the oxidation of the ZnOEP ring. The first
peak (peak*), around 0.0 and ꢀ0.2 V/SCE, corresponds to the
reduction of the isoporphyrin which is formed during the
electropolymerization process.21,30 The two other waves
between ꢀ0.2 and ꢀ0.8 V/SCE are assigned to the reduction
of the pyridinium spacers which are formed between each
macrocycle within the polymeric chain.20,21,30
In order to have an electrochemical characterization of these
copolymers, after 25 iterative scans, the coated electrodes have
been removed from the electrochemical cell, washed with
CH3CN to remove the weakly adsorbed macrocycles and used
as working electrodes in new electrolytic solutions containing
only the solvent and the supporting electrolyte (Fig. 2A and
B).
The shape of the waves is characteristic of a process where
the species involved do not diffuse from and toward the
electrode (non-diffusive species). The reduction potentials of
the copolymers are listed in Table 1. As previously observed,
three waves appear in the cathodic part. The first one, around
0.0 V/SCE, corresponds to the reduction of the isoporphyrin.
The two other well-distinct waves, observed between ꢀ0.2 and
c
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If we assume that in the case of our two copolymers the two
waves in the cathodic part are due to couplings between
pyridiniums onto the same ZnOEP rings, and noticing that
these two waves have nearly the same intensity, it will be
straightforward to conclude that only two substitutions occur
on each ZnOEP macrocycle within the polymeric chain,
and that these two substitutions should be performed in the
cis-position (5 and 10 meso-positions).
In order to validate this first assumption concerning the cis-
position substitutions onto the ZnOEP macrocycles occurring
when cis-and trans-isomers of H2Py2Ph2P are used, a similar
copolymer has also been prepared using the 5,15-dichloro-b-
octaethylporphyrin (ZnOEP(Cl)2, Scheme 2B; Fig. S3, ESIw)
instead of ZnOEP. In this case, two of the four meso-positions
of the macrocycle are ‘‘protected’’ by chloride groups (substituted
in trans-positions onto the ZnOEP ring). Consequently, only a
linear copolymer can be obtained,32 where the pyridinium spacers
within the polymeric chain are necessarily in trans-positions
(Scheme 3C). Fig. 2C exhibits the cyclic voltammogram
corresponding to this linear copolymer measured into a new
electrolytic solution containing only the solvent and the
supporting electrolyte. Two waves assigned to the reduction
of the pyridinium spacers are again observed in this case. Only
one should be expected in regard to our former work in the
case of substitutions of pyridiniums in trans-positions on a
ZnOEP ring. Indeed, a lack of any interactions between trans-
pyridiniums must be observed leading to only one wave of two
electrons for the reduction of the two pyridiniums. Consequently,
it seems that the couplings observed between pyridiniums within
these copolymers are not due to pyridiniums substituted in the
cis-position onto the same ZnOEP macrocycles. Indeed, even
if this type of interaction was widely observed in the case of
pyridiniums-substituted ZnOEP (5,10-ZnOEP(py)22+, 5,10-
ZnOEP(bipy)2
,
5,10,15,20-ZnOEP(py)44+),29,31,33 it is
2+
generally not observed in the case of oligomers34 or polymers
of porphyrins.20,30 That can be tentatively explained by a very
important change of the deformation of the ZnOEP rings
when substitutions onto macrocycles are too numerous. As a
consequence, the non-planarity of the ZnOEP macrocycle must
be very different and especially less flexible when entrapped in the
copolymers compared to the monomeric pyridinium-substituted
macrocycle where the dynamic macrocycle inversion process
between the non-planar conformations must be present and
function of the temperature.
Fig. 2 Cyclic voltammograms of ITO electrodes modified with
copolymers obtained after 25 iterative scans: (A) poly-cis-
H2Py2Ph2P-ZnOEP, (B) poly-trans-H2Py2Ph2P-ZnOEP, (C) poly-
trans-H2Py2Ph2P-ZnOEP(Cl)2, (D) poly-H2Py3PhP-ZnOEP and (E)
poly-H2Py4P-ZnOEP. Solvent: CH3CN/1,2-C2H4Cl2 (1 : 4) with
0.1 M NEt4PF6; scan rate: 0.1 V sꢀ1
.
ꢀ0.7 V/SCE and attributed to the reduction of the pyridinium
spacers, deserve more attention. Indeed, if all the pyridiniums
are not reduced at the same potential while they seem to be
chemically equivalent, that means that pyridiniums substituted
onto the same macrocycles are in mutual interaction. These
couplings can occur either between pyridiniums substituted onto
the same ZnOEP rings, or between pyridiniums substituted onto
the same H2Ph2P rings.
As a matter of fact, it seems that the presence of two
successive waves of one electron for the reduction of the
pyridiniums in the case of poly-trans-H2Py2Ph2P-ZnOEP(Cl)2
shows an unexpected coupling between pyridiniums in regard to
our previous work of monomers in solution. Consequently, the
second hypothesis based on the presence of couplings occurring
between pyridiniums substituted onto the same H2Ph2P rings
seems more appropriate by supposing a lack of interaction with
the ZnOEP rings. Thus, in this case, the two waves assigned to
the successive one-electron reductions of the pyridinium spacers
observed both for poly-trans-H2Py2Ph2P-ZnOEP and poly-cis-
H2Py2Ph2P-ZnOEP copolymers (obtained from the cis- and the
trans-H2Py2Ph2P macrocycles, respectively) indicate inter-
actions existing both for substitutions in cis- and trans-positions
onto the same H2Ph2P macrocycles.
According to our previous works concerning the substitution
of pyridiniums onto ZnOEP macrocycles, such a mutual
interaction between two pyridiniums is observed in the case
of cis-substitutions (substitutions in 5 and 10 meso-positions),
leading well to two successive reduction steps instead of only
one.21,29,31 In contrast, when the two pyridinium groups are
substituted in trans-positions (5 and 15 meso-positions), the
reductions of both pyridiniums are simultaneous.
c
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Table 1 Electrochemical data of the six porphyrins used as monomers and of the five copolymers obtained
Oxidation
Dication
Reduction
Compounds
Porphyrins
Radical cation
Radical anion
Dianion
ZnOEPa
0.94
0.95
1.46
1.45
1.56
1.68
0.68
0.73
1.27
1.26
1.34
1.38
ꢀ1.60
ꢀ1.41
ꢀ1.04
ꢀ1.04
ꢀ0.99
ꢀ0.98
a
ZnOEP(Cl)2
cis-H2Py2Ph2Pb
trans-H2Py2Ph2Pb
H2Py3PhPb
ꢀ1.37
ꢀ1.37
ꢀ1.33
ꢀ1.35
H2Py4Pc
Copolymers
Radical cation H2PyxPh(4ꢀx)
(x = 2, 3 or 4)
P
Radical cation ZnOEP Isoporphyrin ZnOEP (peak*) Py+ spacers
Poly-cis-H2Py2Ph2P-ZnOEPc
1.48
1.37
1.44
1.54
1.47
1.30
1.22
1.16
1.29
1.17
ꢀ0.11
0.00
ꢀ0.45 (1eꢀ)
ꢀ0.58 (1eꢀ)
Poly-trans-H2Py2Ph2P-ZnOEPc
Poly-trans-H2Py2Ph2P-ZnOEP(Cl)2
Poly-H2Py3PhP-ZnOEPc
ꢀ0.31 (1eꢀ)
ꢀ0.62 (1eꢀ)
ꢀ0.27 (1eꢀ)
ꢀ0.56 (1eꢀ)
c
0.05
ꢀ0.05
ꢀ0.13
ꢀ0.34 (2eꢀ)
ꢀ0.59 (1eꢀ)
Poly-H2Py4P-ZnOEPc
ꢀ0.39 (2eꢀ)
ꢀ0.55 (2eꢀ)
All potentials in V vs. SCE were obtained from cyclic voltammetry in CH3CN/1,2-C2H4Cl2 (1 : 4) containing 0.1 mol Lꢀ1 NEt4PF6 at 100 mV sꢀ1
a
b
c
(working electrode: Pt electrode, glassy carbon electrode and ITO electrode). In brackets: number of electrons exchanged per H2PyxPh4-x
unit (x = 2, 3 or 4).
P
This aspect has been initially studied by Evans,35 then by
Save
ant36,37 and by Amatore38,39 and more recently by Santi
isoporphyrin, and the other two, between ꢀ0.2 and ꢀ0.7 V/
´
SCE, are assigned again to the reduction of the pyridinium
spacers. Regarding the intensity of the two waves attributed to
the pyridiniums, the first one (around ꢀ0.4 V/SCE) corre-
sponds to the transfer of two electrons per H2Py3PhP while the
second one (around ꢀ0.6 V/SCE) corresponds to the transfer
of one electron per H2Py3PhP. Consequently, the voltammo-
gram is consistent with substitutions of the three pendant
pyridyl groups of the H2Py3PhP macrocycles. The apparent
two-electron wave should be in fact two successive one-electron
waves with a difference in potentials lower than 60 mV. Finally,
in the case of the polymer obtained from the H2Py4P
ring (Fig. 2E and Table 1), two successive two-electron waves
(per H2Py4P) assigned to the reduction of the pyridinium
spacers are observed.
et al.40 for monomeric systems in solution where conforma-
tional changes play a major role in these phenomena, as
well the solvent reorganisation, the nature of the supporting
electrolyte, the conjugation and of course electrostatic interactions
(more especially in the case of polymers). It is therefore not
surprising to observe different behaviours in the copolymers
obtained compared to monomer species dissolved in solution.
Thus, in the case of poly-trans-H2Py2Ph2P-ZnOEP(Cl)2, of poly-
trans-H2Py2Ph2P-ZnOEP and of poly-cis-H2Py2Ph2P-ZnOEP
(symbolized (Py+-H2Ph2P-Py+-ZnOEP)n), the two successive
one-electron reversible waves observed can be easily understood
as an electrostatic effect reflecting the fact that it is more difficult
to insert an electron into the pyridinium–ion radical system
((Py+-H2Py2Ph2P-Pyꢁ-ZnOEP)n) than it is to reduce the starting
di-cationic (Py+-H2Ph2P-Py+-ZnOEP)n species.
The waves corresponding to the reduction of the isopor-
phyrins deserve also attention (peaks*). Indeed, regarding the
five cyclic voltammograms recorded during electropolymeriza-
tion of the five copolymers (Fig. 1; Fig. S3, ESIw), an increase
in the intensity of this peak is observed from the use of trans-
H2Py2Ph2P to the use of cis-H2Py2Ph2P, and then for
H2Py3PhP and H2Py4P. One can also note that in this last
case, the isoporphyrin peak is so important that it completely
masks the pyridinium reduction waves after several scans. One
can tentatively connect this increase of the intensity of this
wave to the increase of the steric hindrance of the copolymers
obtained with each porphyrin. Indeed, the steric hindrance
of the copolymers naturally increases when the number of
pendant pyridyl groups onto the macrocycles increases. And
more the number of pendant pyridyl groups is high, more
isoporphyrins can be formed. Nevertheless, this increase of the
number of substitutions with the number of pyridyl groups
In order to modulate the steric hindrance of the copolymers,
porphyrins bearing three and four pendant pyridyl groups
have also been used, namely the 5,10,15-tripyridyl-20-phenyl-
porphyrin (H2Py3PhP, Schemes 1E and S2A, ESIw) and the
5,10,15,20-tetrapyridylporphyrin (H2Py4P, Schemes 1F and
S2B, ESIw). As previously observed, the evolution of the
cyclic voltammograms characterizes the formation of poly-
mers coating the electrodes (Fig. 1C and D). Fig. 2D and E
exhibit the cyclic voltammograms obtained at the end of the
electropolymerization of these two copolymers, after replacing
the ITO electrodes into new electrolytic solutions containing
only the solvent and the supporting electrolyte. In the case of
the polymer obtained from the H2Py3PhP ring (Fig. 2D and
Table 1), three waves are observed in the cathodic part, the
first one around 0.0 V/SCE corresponds to the reduction of the
c
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Scheme 3 Tentative representation of copolymers: (A) poly-cis-H2Py2Ph2P-ZnOEP, (B) poly-trans-H2Py2Ph2P-ZnOEP and (C) poly-trans-
H2Py2Ph2P-ZnOEP(Cl)2.
leads to an increase of the deformation of the macrocycles
within the copolymers, which should lead to a more difficult
rearomatization of the rings, i.e. an accumulation of isopor-
phyrins near the electrode. For the cyclic voltammograms
recorded at the end of the electropolymerization after replacing
the ITO electrodes into new electrolytic solutions (Fig. 2), the
isoporphyrin reduction peaks remain but decrease in intensity,
due to a poor stability of the isoporphyrins, for instance to
further reaction leading finally to the nucleophilic substitution.
After electropolymerisation, even after several weeks, the
wave assigned to the isoporphyrin reduction does not completely
disappear but a decrease of the intensity of the peak current has
been observed showing a good stability of such isoporphyrins
inside the copolymer compared to the isoporphyrin monomer.
Finally, after this description of the cathodic behavior, one
needs to discuss the anodic part, corresponding to the oxidation
waves of the macrocycles. During iterative scans, these oxidation
signals progressively shift anodically as the coating of the electrode
progresses (Fig. 1). This is due to the electron-withdrawing
character of the pyridinium spacers onto the macrocycles.
Indeed, the number of positively charged pyridiniums within
the copolymers increases with the number of cycles, leading
progressively to an increase in the difficulty to oxidize the
macrocycles. As a matter of fact, at the end of the electro-
polymerization, when the ITO electrodes are used as working
electrodes in new electrolytic solutions, the potentials of the
first oxidation waves of each macrocycle within the macrocycle
are significantly shifted compared to the oxidation waves of
these same free porphyrins (Table 1).
UV-visible spectroscopic characterization and AFM
morphology
The copolymers have also been characterized by UV-visible
absorption spectroscopy (Table 2).
A typical spectrum
recorded on a coated ITO electrode is depicted in Fig. 3 (see
also Fig. S2, ESIw), and is compared to the spectra of both
monomers. The Soret band of the copolymers is split, because
of the presence of two different kinds of macrocycles within
the polymeric chain. Moreover, each Soret band is larger and
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Table 2 Spectroscopic data of the six porphyrins used as monomers
and of the five copolymers obtained
Conclusions
In summary, we have successfully obtained five copolymers by
electropolymerisation of ZnOEP-type macrocycles and porphyrins
functionalized by two, three or four pendant pyridyl groups. The
electropolymerization process is based on the now well-known
nucleophilic attack of the pyridyl groups onto electrogenerated
ZnOEP dications at their meso-positions. To our knowledge, we
have developed a simple method which allows the formation of
copolymers having two different kinds of macrocycles perfectly
alternated. Moreover, these copolymers possess short spacers
between macrocycles, namely pyridinium spacers. Electrochemical
studies of these copolymers have allowed us to show that
couplings between pyridiniums occur only when they are
substituted onto same H2Ph(4ꢀx)P rings (x = 2, 3 or 4), but
not when they are substituted onto same ZnOEP macrocycles.
Spectroscopic measurements and AFM morphologies have
also been performed to obtain a better characterization of
these copolymers.
Soret bands/
nm
Compounds
Q bands/nm
Porphyrins
ZnOEP
ZnOEP(Cl)2
cis-H2Py2Ph2P
trans-H2Py2Ph2P
H2Py3PhP
402
402
417
417
417
417
531; 568
531; 568
514; 548; 593; 646
514; 548; 585; 643
511; 548; 582; 647
513; 546; 588; 644
H2Py4P
Copolymers
Poly-cis-H2Py2Ph2P-ZnOEP
Poly-trans-H2Py2Ph2P-ZnOEP
Poly-trans-H2Py2Ph2P-ZnOEP(Cl)2 436; 449
429; 464
435; 456
603; 675
597; 686
597; 687
601; 673
591; 665
Poly-H2Py3PhP-ZnOEP
Poly-H2Py4P-ZnOEP
431; 468
431; 458
Spectra of porphyrin monomers have been measured in solution in
1,2-C2H4Cl2 while spectra of copolymers have been measured directly
onto optically transparent ITO electrodes.
Further development of this polymerization process is the
next step in the development of new polymers with design
properties.
Experimental section
Chemicals
All solvents were of reagent grade quality and used without
further purification.
The zinc-b-octaethylporphyrin (ZnOEP) and the 5,10,15,
20-tetrapyridylporphyrin (H2Py4P) were purchased from
Sigma-Aldrich.
UV-visible spectroscopic measurements
Fig. 3 Normalized UV-visible absorption spectra of ZnOEP in 1,2-
C2H4Cl2 (red dotted line), of cis-H2Py2Ph2P in 1,2-C2H4Cl2 (blue
UV-visible spectra were recorded with a Hewlett-Packard
HP8453 spectrophotometer.
dashed line) and of
a modified ITO electrode with poly-cis-
H2Py2Ph2P-ZnOEP (black full line). Inset: atomic force micrograph
Atomic force microscopy (AFM)
of poly-cis-H2Py2Ph2P-ZnOEP.
AFM was performed directly on the surface of an ITO electrode
using a Dimension 3100 (Veeco) in the tapping mode under
ambient conditions. Silicon cantilevers (Veeco probes) with a
spring constant of 300 N mꢀ1 and a resonance frequency in the
range of 120–139 kHz were used. The scanning rate was 1.0 Hz.
red-shifted compared to the one of the corresponding monomer.
This evolution can be interpreted by the exciton-coupling theory
involving intra- and inter-molecular interactions between the
porphyrin subunits within the polymer.20,21,30 These two
changes compared to the free porphyrins can also be explained
by a greater deformation of the macrocycles within the polymers
due to the many substitutions onto porphyrin rings.41 Moreover,
the red-shift of the Soret band compared to the corresponding
monomer results also from the electron-withdrawing effect of
the pyridinium groups on the macrocycles.20,21,28
Syntheses
Porphyrins with pendant pyridyl groups (cis-H2Py2Ph2P,
trans-H2Py2Ph2P and H2Py3PhP). These porphyrins were
prepared and purified according to a published procedure.42
A mixed solution of benzaldehyde (3.05 mL, 30 mmol),
4-pyridinecarbaldehyde (2.83 mL, 30 mmol) and pyrrole
(4.2 mL, 60 mmol) was refluxed (130 1C) in propionic acid
during 1 h. The reaction mixture was then cooled and allowed
to stand overnight. After that, the reaction solution was
concentrated to about 50 mL and then acetone (500 mL)
was added with stirring. The crystalline products of the
porphyrin mixture were obtained after filtration. This crude
product, a mixture of six porphyrins, was dissolved in CH2Cl2
and loaded onto a column of silica gel. The six porphyrins
were separated by a dichloromethane/methanol solvent system
The morphology of the copolymers coated onto ITO electrodes
has also been scrutinized by atomic force microscopy (AFM).
Whatever the porphyrins used, the deposited copolymers appear
in the form of tightly packed coils with an average diameter of ca.
60–100 nm (Fig. 3; Fig. S4, ESIw). The rms roughness does not
change a lot with the steric hindrance of the copolymers, since the
values (calculated from an area of 1.0 mm2) are between 3.4 and
3.7 nm according to the porphyrins used. Only for the linear
copolymer obtained from ZnOEP(Cl)2 with trans-H2Py2Ph2P the
rms value is lower (equal to 2.4 nm).
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consisting initially of 100% CH2Cl2, and gradually ending
with 90% CH2Cl2/10% CH3OH. After separation, the desired
products include cis-H2Py2Ph2P (yield of 9.7%), trans-
H2Py2Ph2P (yield of 1.1%) and H2Py3PhP (yield of 4.3%).
1H NMR (300 MHz, CDCl3, 20 1C):
the synthesis of the ZnOEP(Cl)2, both from the Institut
Parisien de Chimie Moleculaire, UMR 7201 CNRS-Universite
´ ´
Paris 6, France.
cis-H2Py2Ph2P: d = 9.06 (d, 4H, 3,5-Py), 8.87 (m, 8H, b-H),
8.23 (d, 4H, 2,6-Ph), 8.18 (d, 4H, 2,6-Py), 7.81 (m, 6H, 3,4,
5-Ph), ꢀ2.80 (s, 2H, NH).
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