**1. Introduction**

52 Electropolymerization

Wang, J., Martinez, T. Daphna, Y. R. & McCormick, L.D. (1991). Scanning tunneling

313 ( 1-2): 129-140

microscopic investigation of surface fouling of glassy carbon surfaces due to phenol oxidation, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry,

> Porphyrins and porphyrins-containing materials have attracted a considerable attention these last decades, not only because of their roles as biological photosensitizers, redox centers and oxygen carriers, but also because of their attractive chemical properties and potential technological applications (Kadish et al., 2000). In particular, molecular engineering and design of controlled spatial assemblies and architectures of porphyrins are fields undergoing wide growth (Griveau & Bedioui, 2011). For example, macromolecular porphyrins-based systems can potentially mimic natural metalloenzyme structures. Indeed, in such systems, proteins are replaced by metalloporphyrins which can mimic the structure and/or the activity of the prosthetic groups of enzymes (Traylor, 1991). Another area of applications of such assembled porphyrins systems consists in the field of nanomaterials, the electronic communication between the macrocycles allowing developments of molecular photonic, electronic or optoelectronic devices (Jurow et al., 2010). Finally, nanocomposite porphyrins-based materials have also been investigated for applications involving energy storage systems, fuel cells and sensors (Di Natale et al., 2010; Ma et al., 2006).

> For this purpose, porphyrins-based polymers have received peculiar attention these last decades. For instance, it has been established that a polymeric matrix may provide the best arrangement for a catalytically active center. The immobilized porphyrins appeared also

<sup>\*</sup> Clémence Allain1, Jian Hao1, Yun Xia1, Rana Farha3, Michel Goldmann3, Yann Leroux4 and Philippe Hapiot4

*<sup>1</sup>Laboratoire de Chimie Physique, UMR 8000 CNRS / Université Paris-Sud 11, Faculté des Sciences d'Orsay, bât. 349, 91405 Orsay cedex, France* 

*<sup>3</sup>Institut des NanoSciences de Paris, UMR 7588 CNRS / Université Paris 6, 4 place Jussieu, boîte courrier 840, 75252 Paris cedex 05, France* 

*<sup>4</sup>Sciences Chimiques de Rennes, équipe MaCSE, UMR 6226 CNRS / Université de Rennes 1, campus de Beaulieu, bât. 10C, 35042 Rennes cedex, France*

principally by oxidation of macrocycles with iodine, bromine or peroxide.

β

β

radical cation (Schaming et al., 2011a) (Fig. 1.e).

(Giraudeau et al., 1996, 2001) (Fig. 1.a).

β

octaethylporphyrins or in

by a pyridinium in

and c).

permitted to obtain zinc

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 55

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,

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

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

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

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).




more stable than biomimetic enzymes in aqueous or organic media, particularly at extreme pH and temperature (Griveau & Bedioui, 2011). Finally, conductive porphyrin polymers allow interesting applications for molecular electronics (Wagner & Lindsey, 1994).

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 strategy to perform polymerization.

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

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 electropolymerization.
