**2. Reactivity of porphyrin** π**-radical cations and dications with nucleophilic compounds**

It is well-known that the oxidation of the π-ring of a porphyrin proceeds *via* two oneelectron steps generating the π-radical cation and the dication (Fajer et al., 1970). The reactivity of these porphyrin π-radical cations and dications with nucleophilic compounds has been intensively studied. Dolphin and Felton have observed for the first time the reactivity of oxidized porphyrins with nucleophilic solvents or halides (Dolphin & Felton, 1974). In the case of β-octaethylporphyrin derivatives, stable *meso*-substituted porphyrins have been obtained. This is the first example of nucleophilic substitution at the periphery of porphyrin nucleus, allowing afterwards the development of a new and simple convenient synthetic route to obtain varied substituted porphyrins. Indeed, several further works report the nucleophilic attack of nitrogeneous, phosphorous or sulphurous nucleophiles (nitrite ion, pyridine, phosphine, thiocyanate…) onto π-radical cations in *meso*-position of β-

more stable than biomimetic enzymes in aqueous or organic media, particularly at extreme pH and temperature (Griveau & Bedioui, 2011). Finally, conductive porphyrin polymers

Different strategies for the formation of porphyrin polymers have already been described in the literature. A first common method to polymerize porphyrins consists in the use of bridging ligands which can coordinate the metal center of metalloporphyrins. For instance, such coordination polymers have been obtained by coordination of nitrogenous ligands with ruthenium, osmium or iron porphyrins (Collman et al., 1987; Marvaud & Launay, 1993). Nevertheless, more frequently, covalent polymers have been obtained. Generally, the formation of such polymers relies on the use of porphyrins with polymerizable substituents attached on the ring periphery. Even if classical chemical ways can be used to obtain such polymers (Li et al., 2004), electrochemical route appears as an elegant, attractive and easy

Indeed, one of the main advantages of electropolymerization lies in the non-manual addressing of polymers allowing formation of films with a good reproducibility and a controlled thickness. Electropolymerization is also an easy way to functionalize conductive surfaces with a good precision. Moreover, electropolymerization of porphyrins provides densely packed layers that facilitate the electron hopping process between macrocycles. Furthermore, the immobilization of such porphyrins-based materials on an electrode is a convenient way to design electrochemical sensing devices, catalytic electrodes, and to study the enzyme reactions by excluding the requirement of a chemical redox mediator to shuttle

Electropolymerization of porphyrins has been initially developed by Macor and Spiro, who have polymerized vinyl-substituted porphyrins, the coupling occurring after the formation of vinyl radicals by electrooxidation (Macor & Spiro, 1983). Afterwards, other methods of electropolymerization have been published, for example from hydroxy-, amino-, pyrrole- or thiophene-substituted porphyrins (Bedioui et al., 1995; Bettelheim et al., 1987; Li et al., 2005). In this context, we have developed an original methodology for the electropolymerization of porphyrins, based on nucleophilic attacks of di-pyridyl compounds directly onto electrooxidized porphyrins. This chapter presents an overview of this new method of

**2. Reactivity of porphyrin** π**-radical cations and dications with nucleophilic** 

It is well-known that the oxidation of the π-ring of a porphyrin proceeds *via* two oneelectron steps generating the π-radical cation and the dication (Fajer et al., 1970). The reactivity of these porphyrin π-radical cations and dications with nucleophilic compounds has been intensively studied. Dolphin and Felton have observed for the first time the reactivity of oxidized porphyrins with nucleophilic solvents or halides (Dolphin & Felton,

have been obtained. This is the first example of nucleophilic substitution at the periphery of porphyrin nucleus, allowing afterwards the development of a new and simple convenient synthetic route to obtain varied substituted porphyrins. Indeed, several further works report the nucleophilic attack of nitrogeneous, phosphorous or sulphurous nucleophiles (nitrite ion, pyridine, phosphine, thiocyanate…) onto π-radical cations in *meso*-position of


β-

electrons between the porphyrins and the electrode (Griveau & Bedioui, 2011).

allow interesting applications for molecular electronics (Wagner & Lindsey, 1994).

strategy to perform polymerization.

electropolymerization.

1974). In the case of

β

**compounds** 

octaethylporphyrins or in β-position of *meso*-tetraphenylporphyrins (Barnett & Smith, 1974; Barnett et al., 1976; Rachlewicz & Latos-Grażyński, 1995; Shine et al., 1979). However, it must be noted that the π-radical cations have been in most cases prepared by chemical ways, principally by oxidation of macrocycles with iodine, bromine or peroxide.

Giraudeau *et al.* have for the first time investigated the reactivity of the radical cation of the zinc *meso*-tetraphenylporphyrin (ZnTPP), obtained by a direct electrochemical oxidation (electrolysis), in the presence of pyridine as Lewis base, leading to a macrocycle substituted by a pyridinium in β-position (El Kahef et al., 1986). Afterwards, a similar work has permitted to obtain zinc β-octaethylporphyrin (ZnOEP) with pyridinium *meso*-substituted (Giraudeau et al., 1996, 2001) (Fig. 1.a).

When 4,4'-bipyridine is used instead of pyridine, dimers of porphyrins can also be electrosynthesized (El Baraka et al., 1998; Giraudeau et al., 1996). Indeed, 4,4'-bipyridine presents two accessible nucleophilic sites which can both react with porphyrin rings (Fig. 1.b and c).

More recently, dimers of porphyrins with pyridinium spacer have also been synthesized from macrocycles substituted by a pendant pyridyl group which can react with a ZnOEP radical cation (Schaming et al., 2011a) (Fig. 1.e).

Fig. 1. Examples of a multi-substituted porphyrin (a) and of several oligomers of porphyrins (b-g) obtained from the reactivity of the electrogenerated π-radical cation with Lewis bases such as pyridine-derived species (a-e) or phosphane-derived species (f-g).

β

Fig. 3. Electropolymerization scheme of the mono-substituted ZnOEP(bpy)+.

This polymer is obtained with iterative scans by cyclic voltammetry (Fig. 4). During this iterative process, the current increases progressively, showing the formation of a conducting polymer coating the electrode. Moreover, the oxidation waves of the macrocycle become irreversible, suggesting well the reactivity of the dication. Furthermore, these oxidation waves are progressively shifted to higher anodic potential values during electropolymerization. This shift is a consequence of the electron-withdrawing effect of the viologen spacers formed between porphyrins during the polymerization process. These viologen spacers are also responsible of the two reversible reduction waves which appear

**3. Towards an electropolymerization process of porphyrins** 

**3.1 Electrochemical investigation of the electropolymerization** 

group in *meso*-position (zinc *meso*-bipyridinium-

ZnOEP(bpy)+) (Fig. 3) (Ruhlmann et al., 1999b, 2008).

macrocycle (step CN

investigated.

β

position. Then, the spare proton in

order to re-aromatize the macrocycle.

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 57

During previous electrolyses, electrodes were systematically coated by a thin colored film, ascribed to polymers. Consequently, a more detailed study of these polymers has been

Thus, in a first report, we have published the formation of a polymer of porphyrins obtained from the electrogenerated dications of ZnOEP macrocycles substituted by a bipyridinium

) which leads to the loss of the pyridinium attached at the *meso*-

β



To perform all these electrosyntheses, electrolyses at a potential allowing the formation of the radical cation of the porphyrin have been carried out. Nevertheless, an increase of the electrolysis potential can allow multi-substitutions onto macrocycles. Indeed, a judicious control of the applied potential has permitted to obtain di-, tri- and tetra-*meso*-substitutions, leading to the formation of trimers, tetramers and pentamers of porphyrins, respectively (Ruhlmann, 1997; Ruhlmann et al., 1999a) (Fig. 1.d).

Finally, instead of the use of pyridyl-derived compounds as nucleophile, phosphanes have also been used to perform substitutions onto porphyrin rings, allowing the synthesis of dimers or trimers with more flexible di- or tri-phosphonium spacers (Ruhlmann & Giraudeau, 2001; Ruhlmann et al., 2003) (Fig. 1.f and g).

The mechanism of the *meso*-substitutions (onto zinc β-octaethylporphyrin (ZnOEP) for instance) has been described as an E1CN*meso*E2CB process (Giraudeau et al., 1996) (fig. 2). Indeed, after formation of the radical cation (step E1), nucleophilic attack can occur directly onto the *meso*-position (CN*meso*) where initially a proton is present. After a second oxidation (step E2), this proton can be removed (step CB), allowing the re-aromatization of the macrocycle. This mechanism will be more precisely described in the case of the electropolymerization process (see part 3.2).

Fig. 2. E1CNE2CB mechanism explaining the reactivity of a 4,4'-bipyridine (bpy) in *meso*position of a zinc β-octaethylporphyrin (ZnOEP).

In the case of a nucleophilic substitution in β-position (allowed when *meso*-positions are already substituted as in the case of the zinc *meso*-tetraphenylporphyrin (ZnTPP)), the mechanism is a little bit different. Indeed, an E1CN*meso*E2CNβCB process occurs with two different successive nucleophilic attacks (Rachlewicz et al., 1995). First an attack at a *meso*position (step CN*meso*), which is more favorable, occurs after the electrogeneration of the radical cation (step E1). After the second oxidation (step E2), as no proton can be removed from the substituted *meso*-position, a second nucleophilic attack occurs at a β-position of the

To perform all these electrosyntheses, electrolyses at a potential allowing the formation of the radical cation of the porphyrin have been carried out. Nevertheless, an increase of the electrolysis potential can allow multi-substitutions onto macrocycles. Indeed, a judicious control of the applied potential has permitted to obtain di-, tri- and tetra-*meso*-substitutions, leading to the formation of trimers, tetramers and pentamers of porphyrins, respectively

Finally, instead of the use of pyridyl-derived compounds as nucleophile, phosphanes have also been used to perform substitutions onto porphyrin rings, allowing the synthesis of dimers or trimers with more flexible di- or tri-phosphonium spacers (Ruhlmann &

instance) has been described as an E1CN*meso*E2CB process (Giraudeau et al., 1996) (fig. 2). Indeed, after formation of the radical cation (step E1), nucleophilic attack can occur directly onto the *meso*-position (CN*meso*) where initially a proton is present. After a second oxidation (step E2), this proton can be removed (step CB), allowing the re-aromatization of the macrocycle. This mechanism will be more precisely described in the case of the

Fig. 2. E1CNE2CB mechanism explaining the reactivity of a 4,4'-bipyridine (bpy) in *meso*-

β

already substituted as in the case of the zinc *meso*-tetraphenylporphyrin (ZnTPP)), the

different successive nucleophilic attacks (Rachlewicz et al., 1995). First an attack at a *meso*position (step CN*meso*), which is more favorable, occurs after the electrogeneration of the radical cation (step E1). After the second oxidation (step E2), as no proton can be removed


from the substituted *meso*-position, a second nucleophilic attack occurs at a

mechanism is a little bit different. Indeed, an E1CN*meso*E2CN

β



CB process occurs with two

β


β

(Ruhlmann, 1997; Ruhlmann et al., 1999a) (Fig. 1.d).

Giraudeau, 2001; Ruhlmann et al., 2003) (Fig. 1.f and g). The mechanism of the *meso*-substitutions (onto zinc

electropolymerization process (see part 3.2).

position of a zinc

β

In the case of a nucleophilic substitution in

macrocycle (step CNβ) which leads to the loss of the pyridinium attached at the *meso*position. Then, the spare proton in β-position can be removed (step CB) in a last step in order to re-aromatize the macrocycle.
