**4.1 A first example**

60 Electropolymerization

electropolymerization, the rate of the sweeps is too fast to perform nucleophilic attacks onto the radical cation, the reverse scan in the cathodic part (leading to its reduction) being too fast. Nevertheless, if an electrolysis is carried out at a potential corresponding to the formation of the radical cation, the electrode is coated, showing also the formation of oligomers and/or polymers. However, this polymeric film formation may also be due to the disproportionation of the radical cation (slow kinetic) leading to the non-oxidized ZnOEP and the porphyrin dication which can be attacked more rapidly by the nucleophile. Nevertheless, the advantage to perform electropolymerization by iterative scans instead of electrolysis at a fixed potential is double: the protons released during the polymerization can be reduced during the cathodic scans, and the iterative scans allow a better homogeneity of the polymeric film deposited onto the electrode, therefore enhancing its conductivity. Another point which can explain the need to apply a higher potential for electropolymerization concerns the degree of substitution of macrocycles. Indeed, into the polymers, porphyrins are at least substituted twice by positively charged groups. Moreover, when the chain of polymer grows, the quantity of positively charged groups increases. Consequently, porphyrins are more and more difficult to oxidize, needing an increase of the oxidative potential to perform the polymerization. This explanation is also supported by the fact that a higher potential has always been required to perform electrosyntheses of multisubstituted porphyrins by electrolysis. As a matter of fact, it can also be noted that the higher the limit potential during the anodic sweeps is, the longer the polymeric chains could be obtained. Otherwise, if the limit potential is too low, only small oligomers can be

The use of ZnOEP(bpy)+ as monomer allows multi-substitutions onto macrocycles because three *meso*-positions remain free (positions 10, 15 and 20, Fig. 3). Thus, nucleophilic attacks can occur on each of them, and consequently that can lead to the formation of zig-zag polymers and eventually of hyper-branched polymers. In order to permit a better control of the geometry of the obtained polymers, porphyrins substituted by two protecting groups as

ZnOEP(Cl)2(bpy)+) can be used (Ruhlmann et al., 1999b). In this case, linear wires of polymers are obtained because only one *meso*-position remains free. Transmission electron micrographs of this polymer show long wires having length of several micrometers and diameter of about

20 Å (Fig. 6). This is in agreement with the 19 Å molecular width of a ZnOEP molecule.

Fig. 6. (a) Transmission electron micrograph of a linear fiber obtained by

electropolymerization of ZnOEP(Cl)2(bpy)+. (b) Scheme of the linear chain obtained.

β


obtained.

**3.3 Control of the geometry of the polymer** 

chlorides (zinc 5-bipyridinium-10,20-dichloro-

The way of electropolymerization of porphyrins presented above requires a first step consisting in the synthesis of the starting monomeric subunit (ZnOEP(bpy)+). The synthesis of this monomer can be performed by the electrochemical process described before, (reaction of 4,4'-bipyridine with the electrogenerated radical cation of the ZnOEP, see part 2). Even if this synthesis appears easy, the further purification of the obtained compound is more difficult, due to the possibility of multi-substitutions onto the macrocycle. In order to avoid the synthesis of this monomeric subunit, we have recently proposed an alternative method consisting in the direct use of commercial and non-substituted ZnOEP, with the presence of free 4,4'-bipyridine (bpy) in the solution (Fig. 7) (Giraudeau et al., 2010).

Fig. 7. Electropolymerization scheme of the non-substituted ZnOEP with free bpy.

As previously, electropolymerization can be performed with iterative scans by cyclic voltammetry (Fig. 8). The current increases again during the iterative sweeps. Compared to the cyclic voltammograms obtained in the previous case (Fig. 4), an additional reversible wave is observed at –0.30 V/SCE (peak c in Fig. 8). This additional wave can be attributed to the reduction of the bipyridinium substituents (Schaming et al., 2011b). Indeed, in this case, many bipyridinium groups can be grafted on the macrocycles, without further polymeric chains growth, because of the too important hindrance between the macrocycles. Consequently, the polymer obtained from non-substituted ZnOEP and free bpy can have many bipyridinium groups substituted onto the porphyrins (represented in blue in Fig. 7), leading to the appearance of this new reversible wave corresponding to the reduction of these bipyridinium substituents. On the contrary, when ZnOEP(bpy)+ is used to carry out

water in order to remove the supporting electrolyte (NEt4PF6).

**5.1 UV-visible absorption and fluorescence spectroscopies** 

pyridinium groups on the macrocycles (Giraudeau et al., 1996, 2010).

(tzpy).

**5. Studies of the polymers** 

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 63

Fig. 9. Several Lewis bases (Py-R-Py) used as spacers: 1,2-bis(4-pyridyl)ethane (bpe), trans-1,2-bis(4-pyridyl)ethylene (tbpe), 4,4'-azopyridine (azpy) and 3,6-bis(4-pyridyl)-*s*-tetrazine

When the electropolymerization process is finished (stopped after a pre-defined number of scans: 25 scans for all studies described in this paragraph, unless otherwise indicated), the electrode is removed from the electrochemical cell. Then, the electrode is systematically coated with a brown thin film corresponding to the polymer. In order to study in more details these polymers, it is necessary to previously wash the electrode with CH3CN or

Firstly, the polymers can be characterized by UV-visible absorption spectroscopy. When an ITO electrode is used to perform the electropolymerization, the spectrum can be recorded directly onto this optically transparent electrode. Whatever the method of electropolymerization, and whatever the spacer presented before, the spectra are similar (blue spectrum, Fig. 10.a). They consist in a large Soret band whose maximum is red-shifted compared to the ZnOEP monomer. Similarly, Q bands are also red-shifted and larger. That can be attributed to the intra- and intermolecular interactions between the porphyrins subunits (Giraudeau et al., 2010; Ruhlmann et al., 2008). The red-shift of the Soret and Q bands can also result from the electron-withdrawing effect of the positively charged

Fig. 10. (a) Normalized UV-visible absorption spectra of ZnOEP in DMF (▬), of an ITO electrode modified with the polymer obtained from ZnOEP and bpy after 25 iterative scans (▬) and of this same polymer in solution in DMF (▬). (b) Luminescence spectra of ZnOEP

(▬) and of the polymer obtained from ZnOEP and bpy (▬) in DMF. λexc = 420 nm.

the electropolymerization, each chain of polymer can contain only one bipyridinium substituent (at the end of the chain, represented in blue in Fig. 3). In this case, the number of bipyridinium substituents is negligible compared to the number of viologen spacers (present between each macrocycle and represented in red in Fig. 3). As a matter of fact, the signal corresponding to the reduction of these bipyridinium substituents is not observed for the polymer obtained from the substituted monomer ZnOEP(bpy)+.

Fig. 8. Cyclic voltammograms recorded during the iterative scans between –0.90 and +1.60 V/SCE of ZnOEP (0.25 mM) in the presence of bpy (0.25 mM) in 1,2-C2H4Cl2 and 0.1 M NEt4PF6. Working electrode: ITO; *S* = 1 cm2; *v* = 0.2 V s–1.
