**5. Studies of the polymers**

62 Electropolymerization

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

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

As previously said, this second method of electropolymerization allows to avoid the synthesis of the monomeric substituted porphyrin (ZnOEP(bpy)+ for example). Thus, this new way of electropolymerization more conveniently allows the use of the commercial free base porphyrin (H2OEP) or metalloporphyrins (MOEP) with different central metals (M = CoII, MgII, NiII, RuII(CO)…) (Giraudeau et al., 2010). For all of them, an electropolymerization process has been observed, even when porphyrins with oxidable central metal (CoOEP and NiOEP) have been used. Consequently, further applications could be envisaged. For example, polymer with CoOEP could be promising for dioxygen reduction, cobalt porphyrins being known to catalyze this reaction (Chen et al., 2010;

Moreover, it is also possible to modulate easily the nature of the bridging spacers between the porphyrin macrocycles. Indeed, instead of using free bpy, this process of electropolymerization can be extended to other spacers by varying the nature of the nucleophile, since each compound having two pendant pyridyl groups can be used as spacer. The spacers can be selected for their specific chemical and structural properties: rigid or not; long or short; electron conducting or not; electroreducible or not; with conjugated π bonds (aromatic/alkene/alkyne chains) or successive σ bonds (alkyl chains)… For instance, electropolymerization has been successfully carried out with different nucleophilic compounds (named Py-R-Py) as 1,2-bis(4-pyridyl)ethane (bpe), trans-1,2-bis(4 pyridyl)ethylene (tbpe), but also with reducible spacers as 4,4'-azopyridine (azpy) and 3,6 bis(4-pyridyl)-*s*-tetrazine (tzpy) (Fig. 9) (Giraudeau et al., 2010; Schaming et al., 2011b).

0.1 M NEt4PF6. Working electrode: ITO; *S* = 1 cm2; *v* = 0.2 V s–1.

**4.2 Extension to the use of other spacers** 

Collman et al., 1980).

the polymer obtained from the substituted monomer ZnOEP(bpy)+.

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

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

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

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.

5,10-dibipyridinium-

the reactivity is possible.

β

obtained from the mono-substituted ZnOEP(bpy)+.

process, the orientation effect disappearing in this case.

**5.3 Scanning Electrochemical Microscopy (SECM)** 

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 65

It can be noticed that similar electropolymerization has also been performed from the ZnOEP macrocycles substituted by two bipyridinium groups in *cis*-position (zinc

al., 2008). In that case, a regular arrangement of the coils aggregated in the form of "peanuts" (width of *ca.* 100 nm and length of *ca.* 900 nm), all oriented in the same direction, is observed when the washing is performed with water (Fig. 11.c). This enhancement of the self-alignment of the coils induced by the applied electric field could be explained by the increase of the number of positive charges in this case (because of the presence of two bipyridinium substituents onto each macrocycle) which would induce stronger coulombic repulsions between the different subunits. However a treatment with CH3CN induces also a homogeneous dispersion of the coils (Fig. 11.d) as previously observed for the polymer

In the case of the polymer obtained from the non-substituted ZnOEP with free bpy, tightly packed coils are also observed, but without specific alignment (Giraudeau et al., 2010), whatever the solvent used to wash the electrode (Fig. 11.e and f). That can be tentatively explained by the presence of free bpy which could lead to a disorganization of the film. Moreover, the free bpy can also axially coordinate the Zn central metal of the macrocycles (Giraudeau et al., 2010) which increases the distance between the macrocycles and consequently could lead to a decrease of the electrostatic effect between the macrocycles. As a result, effect of the applied electric field should be lesser onto the electropolymerization

Scanning electrochemical microscopy (SECM) in feedback mode has also been used to investigate electronic and permeation properties of the polymeric films obtained from the mono-substituted ZnOEP(bpy)+ and the non-substituted ZnOEP in presence of free bpy, in order to compare these two different electropolymerization ways allowing the formation of similar polymers (Leroux et al., 2010). Briefly, the SECM principle is based on the interaction of the polymeric film (deposited onto ITO electrode) under investigation with a redox probe (the mediator) that is electrogenerated at a microelectrode. This interaction is followed through the analysis of the current flowing at the microelectrode while it approaches the substrate. Depending on the nature of the mediator, the study of either the permeability or

Firstly, ferrocene (Fc) and tetrathiafulvalene (TTF), which work in oxidation, have been chosen as mediator (Fig. 12.b). According to their redox potentials, their oxidized forms (Fc+ and TTF•+, respectively) cannot react with the polymeric films (their redox potentials do not permit the oxidation of the porphyrin macrocycles) (Fig. 12.a). Thus, they are good candidates to probe the permeability of the polymeric films, because they can rapidly exchange electrons with the non-coated ITO substrate (positive feedback). Whatever the polymer studied as substrate onto the ITO electrode, a negative feedback is observed. These negative feedback characters are less important in the case of the polymer obtained from the non-substituted ZnOEP, showing that this polymer is more permeable than the one obtained from the mono-substituted ZnOEP(bpy)+. That can be explained by an increase of the distance between the macrocycles due to the formation of bipyridine-bridged zinc

porphyrins (bpy axially ligated to Zn) mentioned before (see part 5.2).


Moreover, the polymers can be removed from the electrode by dissolution in dimethylformamide (DMF). That allows to record the spectra of the polymers in solution (red spectrum, Fig. 10.a). Comparing the spectra obtained onto ITO electrodes, the red-shift of the Soret band for spectra recorded in solution appears smaller. Such an evolution can be explained by a decrease of the interactions between the macrocycles when the polymers are in solution. Indeed, in solution, polymeric chains are partially unfolded.

When polymers are in solution, it is also possible to study their luminescence properties: all the investigated polymers show a total quenching (Fig. 10.b).
