**6.2.1 Electropolymerization**

68 Electropolymerization

Fig. 14. (a) Scheme of the linear bis-porphyrin copolymer obtained. (b) Cyclic

(4:1) and 0.1 M NBu4PF6. Working electrode: ITO; *S* = 1 cm2; *v* = 0.2 V s–1.

appears as tightly packed coils without alignment (Fig. 15.b).

micrograph of the same copolymer after washing with water.

**6.2 Porphyrin–polyoxometalate copolymers** 

**6.1.2 Characterization** 

voltammograms recorded during the iterative scans between –0.90 and +1.60 V/SCE of 5,15- ZnOEP(Cl)2 (0.25 mM) in the presence of 5,15-H2Py2Ph2P (0.75 mM) in 1,2-C2H4Cl2/CH3CN

The characterization of this copolymer has also been performed by UV-visible absorption spectroscopy and by atomic force microscopy. No important change in comparison with the previous polymers has been observed. Indeed, its spectrum is red-shifted and larger compared to the ones of the porphyrins alone (Fig. 15.a). Furthermore, the copolymer

Fig. 15. (a) Normalized UV-visible absorption spectra of ZnOEP in 1,2-C2H4Cl2 (▬), of 5,15-H2Py2Ph2P in 1,2-C2H4Cl2 (▬) and of an ITO electrode modified with the copolymer obtained from ZnOEP and 5,15-H2Py2Ph2P after 25 iterative scans (▬). (b) Atomic force

Polyoxometalates (POMs) are well-defined metal-oxygen cluster anions constituted of early metal elements in their highest oxidation state with a wide variety of structures and properties (Jeannin, 1998; Katsoulis, 1998). They have particularly attractive catalytic, electrocatalytic and photocatalytic applications. For instance, POMs are known to photocatalyze the reduction of noble or heavy metal cations (Costa-Coquelard et al., 2008; Troupis et al., 2001, 2006). Fig. 17.a shows the cyclic voltammograms obtained in the case of the use of the nonsubstituted ZnOEP macrocycle in the presence of the py–POM–py compound. As previously, the current increases progressively during the iterative scans. As already described, the new peak appearing around –1.00 V/SCE (peak \*) can be attributed to the reduction of the pyridinium spacers. Nevertheless, one can also notice the appearance of a new peak around +0.20 V/SCE (peak •) during the anodic scans. This one could be assigned to the oxidation of the adsorbed H2 formed upon the reduction of the protons during the cathodic scans, these protons being formed during the nucleophilic substitution onto the porphyrins. To explain the presence of this additional anodic peak (not observed previously), one can suggest that it is due to the presence of POMs which can be easily protonated and consequently the released protons are not dispersed in the solution but remain close to the electrode. As a matter of fact, they can be easier and in bigger quantity reduced during the cathodic scans, and the H2 formed can be further re-oxidized.

In order to confirm the assignment of this wave, the electropolymerization process has also been carried out with iterative scans performed only in the anodic part (scans stopped at 0 V/SCE), in order to avoid the reduction of the protons. As expected, the signal assigned to the oxidation of the adsorbed H2 disappears (Fig. 17.b). Surprisingly, in this case, the current decreases during the iterative scans. Thus, the copolymer obtained by this way seems less conductive. This decrease in the conductivity can be tentatively explained by the fact that

of 5,15-ZnOEP(Cl)2, non-agglomerated coils are observed (Fig. 19.a).

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 71

interesting to note that in this case the morphology of the obtained copolymer is quite different. Indeed, the coils appear more agglomerated (Fig. 19.b), while in the case of the use

Fig. 19. 2D and 3D atomic force micrographs of different porphyrin–POM copolymers obtained onto ITO electrodes after 25 iterative scans: (a) polymer obtained from 5,15- ZnOEP(Cl)2 by iterative scans between –1.30 and +1.80 V/SCE; (b) polymer obtained from 5,15-ZnOEP(py)22+ by iterative scans between –1.50 and +1.80 V/SCE and (c) polymer obtained from 5,15-ZnOEP(py)22+ by iterative scans between 0 and +1.80 V/SCE.

Fig. 20. Scheme of the coupling in 4-position of the electrogenerated pyridyl radicals leading

This agglomeration of the coils when 5,15-ZnOEP(py)22+ is used can be explained by the chemical reactivity of the pyridyl radicals obtained during the cathodic scans by reduction of the pyridiniums used as protecting groups. Indeed, these electrogenerated pyridyl radicals can react each other to give oligomeric species resulting from the coupling at the 4 position of the radicals (Fig. 20). This coupling has already been described in several works (Brisach-Wittmeyer et al., 2005; Carelli et al., 1998, 2002; Karakostas et al. 2010; Schaming et

to their oligomerization.

when the electropolymerization is performed by oxidative and reductive scans, the film should be more organized and consequently more conductive than when performed only by oxidative scans.

Fig. 17. Cyclic voltammograms recorded during the iterative scans (a) between –1.50 and +1.80 V/SCE and (b) between 0 and +1.80 V/SCE of ZnOEP (0.25 mM) in the presence of py–POM–py (0.25 mM) in 1,2-C2H4Cl2/CH3CN (7:3) and 0.1 M NBu4PF6. Working electrode: ITO; *S* = 1 cm2; *v* = 0.2 V s–1.

#### **6.2.2 Characterization**

Compared to the previous polymers, the UV-visible absorption spectrum of the porphyrin-POM copolymer appears less large and more structured (Fig. 18.a). That can be attributed to the presence of POM between each macrocycle, which avoids interactions between adjacent porphyrins. Concerning its morphology, this copolymer appears again as little coils (Fig. 18.b).

Fig. 18. (a) Normalized UV-visible absorption spectra of ZnOEP in DMF (▬) and of an ITO electrode modified with the copolymer obtained from ZnOEP and py–POM–py after 25 iterative scans performed between –1.50 and +1.80 V/SCE (▬). (b) 2D and (c) 3D atomic force micrographs of the same copolymer.

#### **6.2.3 Use of di-substituted ZnOEP porphyrins**

Different di-substituted ZnOEP macrocycles have also been used in order to control the geometry of the copolymers. As previously explained, 5,15-ZnOEP(Cl)2 allows the formation of linear polymers. But it can be noticed that the zinc 5,15-dipyridiniumβoctaethylporphyrin (5,15-ZnOEP(py)22+) can also be used in the same purpose. It is

when the electropolymerization is performed by oxidative and reductive scans, the film should be more organized and consequently more conductive than when performed only by

Fig. 17. Cyclic voltammograms recorded during the iterative scans (a) between –1.50 and +1.80 V/SCE and (b) between 0 and +1.80 V/SCE of ZnOEP (0.25 mM) in the presence of py–POM–py (0.25 mM) in 1,2-C2H4Cl2/CH3CN (7:3) and 0.1 M NBu4PF6. Working

Compared to the previous polymers, the UV-visible absorption spectrum of the porphyrin-POM copolymer appears less large and more structured (Fig. 18.a). That can be attributed to the presence of POM between each macrocycle, which avoids interactions between adjacent porphyrins. Concerning its morphology, this copolymer appears again as

Fig. 18. (a) Normalized UV-visible absorption spectra of ZnOEP in DMF (▬) and of an ITO electrode modified with the copolymer obtained from ZnOEP and py–POM–py after 25 iterative scans performed between –1.50 and +1.80 V/SCE (▬). (b) 2D and (c) 3D atomic

Different di-substituted ZnOEP macrocycles have also been used in order to control the geometry of the copolymers. As previously explained, 5,15-ZnOEP(Cl)2 allows the formation of linear polymers. But it can be noticed that the zinc 5,15-dipyridinium-

octaethylporphyrin (5,15-ZnOEP(py)22+) can also be used in the same purpose. It is

β-

oxidative scans.

electrode: ITO; *S* = 1 cm2; *v* = 0.2 V s–1.

force micrographs of the same copolymer.

**6.2.3 Use of di-substituted ZnOEP porphyrins** 

**6.2.2 Characterization** 

little coils (Fig. 18.b).

interesting to note that in this case the morphology of the obtained copolymer is quite different. Indeed, the coils appear more agglomerated (Fig. 19.b), while in the case of the use of 5,15-ZnOEP(Cl)2, non-agglomerated coils are observed (Fig. 19.a).

Fig. 19. 2D and 3D atomic force micrographs of different porphyrin–POM copolymers obtained onto ITO electrodes after 25 iterative scans: (a) polymer obtained from 5,15- ZnOEP(Cl)2 by iterative scans between –1.30 and +1.80 V/SCE; (b) polymer obtained from 5,15-ZnOEP(py)22+ by iterative scans between –1.50 and +1.80 V/SCE and (c) polymer obtained from 5,15-ZnOEP(py)22+ by iterative scans between 0 and +1.80 V/SCE.

Fig. 20. Scheme of the coupling in 4-position of the electrogenerated pyridyl radicals leading to their oligomerization.

This agglomeration of the coils when 5,15-ZnOEP(py)2 2+ is used can be explained by the chemical reactivity of the pyridyl radicals obtained during the cathodic scans by reduction of the pyridiniums used as protecting groups. Indeed, these electrogenerated pyridyl radicals can react each other to give oligomeric species resulting from the coupling at the 4 position of the radicals (Fig. 20). This coupling has already been described in several works (Brisach-Wittmeyer et al., 2005; Carelli et al., 1998, 2002; Karakostas et al. 2010; Schaming et

obtained from 5,10-ZnOEP(bpy)2

SiW12O404– (1 mM) (right).

**7. Conclusion** 

**8. References** 

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 73

electropolymerization) by POMs which are large polyanions. Thus, electrostatic interactions between the cationic porphyrin polymers and the anionic POMs can occur, leading to a hybrid supramolecular assembly. For instance, the dipping of an ITO electrode coated with a polymer

studied (Hao et al., 2008). Atomic force microscopy studies of the deposit show strong modifications to its morphology (Fig. 22). Indeed, the initial regular arrangement of the coils aggregated in the form of "peanuts" (Fig. 11.c) has disappeared and regular cloudy assemblies which are still oriented in the same direction are now observed. Such supramolecular

Fig. 22. Atomic force micrographs of an ITO electrode coated with the polymer obtained from 5,10-ZnOEP(bpy)22+ after plunging for 10 hours in water (left) and in a solution of

In this chapter, we have shown that electropolymerization of porphyrins can be easily performed by nucleophilic attacks of di-pyridyl compounds directly onto electrogenerated

The first tests have been performed with porphyrins substituted with bipyridinium(s). According to the degree of substitution of the monomer (which is linked to its charge), different morphologies of the polymers can be observed, which seem to be induced by the

in the presence of free Lewis bases having two nucleophilic sites. This second way of electropolymerization allows to avoid the somewhat complicated synthesis of the monomeric substituted porphyrin, and consequently allows to modulate easily the nature of the bridging spacers between the macrocycles. Thus, original organic or inorganic compounds, with specific interesting properties, can be used, on condition that two pyridyl groups have beforehand been grafted. Consequently, this new easy way of electropolymerization of porphyrins appears as a promising approach to elaborate new functional materials, for example with catalytic properties if polyoxometalates are used as

Finally, it is interesting to note that as all the electropolymerization methods, it is easy to control the thickness of the polymers, according to the number of scans performed, which is also a great advantage compared to the classical chemical methods of polymerization.

Barnett, G.H. & Smith K.M. (1974). Reactions of some metalloporphyrin and metallochlorin

dications of octaethylporphyrins, by an ECEC-type mechanism.

electric field imposed during the electropolymerization process. We have further shown that it is also possible to use non-substituted

spacers. Other applications can be envisaged according to the spacer chosen.

π-cation radicals with nitrite. *Chem. Commun.*, pp. 772-773

assemblies are the consequence of an agglomeration of "peanuts" by the POMs.

2+ in an aqueous solution of SiW12O404– for 10 hours has been

β


al., 2009). To check this explanation, the electropolymerization with 5,15-ZnOEP(py)22+ has also been performed by iterative scans limited to the anodic part (cyclic voltammograms performed between 0 and +1.80 V/SCE) in order to avoid the reduction of the pyridinium substituents. As expected, the coils appear non-agglomerated in this case (Fig. 19.c).

#### **6.2.4 Photocatalytic tests**

As previously said, this porphyrin–POM copolymer has been prepared in order to use it as photocatalyst for the reduction of metal cations. For this purpose, we have chosen silver cations as a model system to perform photocatalytic tests (Schaming et al., 2010b).

To carry out this experiment, the copolymer has previously been removed from the electrode by dissolution in DMF, and then, a drop of the copolymer solution has been deposited on a slide of quartz and the solvent evaporated in air. Then, the covered slide has been plunged in an aqueous solution of Ag2SO4 (80 µM) containing 0.13 M of propan-2-ol used as sacrificial electron donor. Finally, the sample has been exposed during 8 hours to visible light. The reaction has been followed by UV-visible absorption spectroscopy (Fig. 21.a): during light irradiation, a very large plasmon band appeared progressively, suggesting the formation of silver aggregates. Transmission electron micrographs and electron diffraction patterns have confirmed this result: silver triangular nanosheets were principally obtained (Fig. 21.b and c). The mechanism explaining the reaction has been discussed in detail in our previous works (Schaming et al., 2010a, 2011c).

Fig. 21. (a) Change in the UV-visible absorption spectrum of a porphyrin–POM copolymer deposited on quartz in aerated aqueous solution containing 80 µM of Ag2SO4 and 0.13 M of propan-2-ol under visible illumination. (b) Transmission electron micrograph of a silver triangular nanosheet obtained. (c) Selected-area electron diffraction pattern of the previous silver nanostructure. The first spots (circled) correspond to the formally forbidden ⅓{422} reflections and the second spots (squared) correspond to the {220} reflections.

For this study, silver has been chosen as a model system for metal reduction, but this copolymer could be used in water depollution for reduction or recovery of valuable or toxic metals that could also be combined with degradation of organic pollutants (used instead of sacrificial donor) as already proposed (Troupis et al., 2006).

#### **6.2.5 Electrostatic {porphyrin–POM} systems**

It can be noticed that hybrid {porphyrin–POM} systems can be obtained for similar application purposes by plunging an ITO electrode coated with a cationic polymer of porphyrins, as described in the first parts of this chapter, into an aqueous solution of POMs. The aim of this process is to replace the counteranions of the polycationic polymers (PF6 – further to the electropolymerization) by POMs which are large polyanions. Thus, electrostatic interactions between the cationic porphyrin polymers and the anionic POMs can occur, leading to a hybrid supramolecular assembly. For instance, the dipping of an ITO electrode coated with a polymer obtained from 5,10-ZnOEP(bpy)2 2+ in an aqueous solution of SiW12O404– for 10 hours has been studied (Hao et al., 2008). Atomic force microscopy studies of the deposit show strong modifications to its morphology (Fig. 22). Indeed, the initial regular arrangement of the coils aggregated in the form of "peanuts" (Fig. 11.c) has disappeared and regular cloudy assemblies which are still oriented in the same direction are now observed. Such supramolecular assemblies are the consequence of an agglomeration of "peanuts" by the POMs.

Fig. 22. Atomic force micrographs of an ITO electrode coated with the polymer obtained from 5,10-ZnOEP(bpy)22+ after plunging for 10 hours in water (left) and in a solution of SiW12O404– (1 mM) (right).
