**6.1 Porphyrin–porphyrin copolymers 6.1.1 Electropolymerization**

In a first example, porphyrins having two pendant pyridyl groups have been used in order to obtain copolymers of porphyrins containing two different types of macrocycles. (Xia et al., 2012)

Fig. 14.b illustrates the cyclic voltammograms obtained in the case of the use of the 5,15 dipyridyl-10,20-diphenyl free base porphyrin (5,15-H2Py2Ph2P) in the presence of zinc 5,15 dichloroβ-octaethylporphyrin (5,15-ZnOEP(Cl)2). These two macrocycles have been chosen in order to allow the formation of a linear copolymer as represented in Fig. 14.a.

As expected, the current increases progressively during the iterative scans. The new reduction processes appearing around –0.40 and –0.60 V/SCE (peaks \*) can be attributed to the reduction of the pyridinium spacers.

One can underline that even if the 5,15-H2Py2Ph2P porphyrin is oxidized during the iterative scans, no substitution in β-positions of this macrocycle by a pyridyl group of another 5,15- H2Py2Ph2P macrocycle could occur. Indeed, the kinetic of a nucleophilic substitution in βposition is too slow compared to the kinetic of a substitution in *meso*-position (see part 2). Consequently, while mono-substitutions are possible in β-position of porphyrins (Giraudeau et al., 1996), electropolymerization is very difficult and even impossible, because the rate of the iterative sweeps is too fast to let enough time for the β-substitutions in such experimental conditions.

able to transfer electrons to M*n*+ cations to give M0.

Fig. 16. Scheme of the porphyrin–POM copolymer.

**6.2.1 Electropolymerization** 

before (Fig. 16) (Schaming et al., 2010b).

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 69

Nevertheless, POMs are efficient only under UV light, which is a drawback for environmental applications. Indeed, it seems preferable to use solar light, principally in the visible domain. The development of photosensitized systems could overcome this issue. For instance, {POM– porphyrin} systems appear interesting for photocatalytic applications in the visible domain (Schaming et al., 2010a, 2011c; Schaming, 2010c). In these hybrid organic-inorganic systems, porphyrins act as photosensitizers able under visible illumination to transfer electrons to POMs, which are known to be good electron acceptors. Thus, the excitation of the porphyrins leads to their oxidation and to the simultaneous reduction of the POMs. Then, the reduced POMs are

Using an Anderson-type POM substituted by two pendant pyridyl groups ([MnMo6O18{(OCH2)3CNHCO(4-C5H4N)}2]3-, abbreviated py–POM–py), we have recently obtained a porphyrin–POM copolymer by the method of electropolymerization presented

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

Fig. 14. (a) Scheme of the linear bis-porphyrin copolymer obtained. (b) Cyclic 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 (4:1) and 0.1 M NBu4PF6. Working electrode: ITO; *S* = 1 cm2; *v* = 0.2 V s–1.

#### **6.1.2 Characterization**

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

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 micrograph of the same copolymer after washing with water.

#### **6.2 Porphyrin–polyoxometalate copolymers**

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). Nevertheless, POMs are efficient only under UV light, which is a drawback for environmental applications. Indeed, it seems preferable to use solar light, principally in the visible domain. The development of photosensitized systems could overcome this issue. For instance, {POM– porphyrin} systems appear interesting for photocatalytic applications in the visible domain (Schaming et al., 2010a, 2011c; Schaming, 2010c). In these hybrid organic-inorganic systems, porphyrins act as photosensitizers able under visible illumination to transfer electrons to POMs, which are known to be good electron acceptors. Thus, the excitation of the porphyrins leads to their oxidation and to the simultaneous reduction of the POMs. Then, the reduced POMs are able to transfer electrons to M*n*+ cations to give M0.

Using an Anderson-type POM substituted by two pendant pyridyl groups ([MnMo6O18{(OCH2)3CNHCO(4-C5H4N)}2]3-, abbreviated py–POM–py), we have recently obtained a porphyrin–POM copolymer by the method of electropolymerization presented before (Fig. 16) (Schaming et al., 2010b).

Fig. 16. Scheme of the porphyrin–POM copolymer.
