**5.3 Scanning Electrochemical Microscopy (SECM)**

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 the reactivity is possible.

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).

427 nm *vs. n*.

al., 2012)

dichloro-

β

scans, no substitution in

experimental conditions.

**Porphyrins) process** 

of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 67

Fig. 13. UV-visible absorption spectra of ITO electrodes modified with the polymer obtained from ZnOEP and tzpy after different numbers *n* of iterative scans. Inset: absorbance at

**6. Formation of original copolymers from the EPOP (Easy Polymerization Of** 

As previously explained, all compounds having two pendant pyridyl groups can play the role of spacers between macrocycles. Consequently, more original organic or inorganic compounds, with specific interesting properties, can be used, if two pendant pyridyl groups are present. Thus, this novel easy way of electropolymerization of porphyrins appears as a

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

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-

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

One can underline that even if the 5,15-H2Py2Ph2P porphyrin is oxidized during the iterative

position is too slow compared to the kinetic of a substitution in *meso*-position (see part 2).

(Giraudeau et al., 1996), electropolymerization is very difficult and even impossible, because

H2Py2Ph2P macrocycle could occur. Indeed, the kinetic of a nucleophilic substitution in

in order to allow the formation of a linear copolymer as represented in Fig. 14.a.



β

β

β-



promising approach to elaborate new functional materials.

**6.1 Porphyrin–porphyrin copolymers** 

the reduction of the pyridinium spacers.

β

Consequently, while mono-substitutions are possible in

the rate of the iterative sweeps is too fast to let enough time for the

**6.1.1 Electropolymerization** 

Secondly, tetracyanoquinodimethane (TCNQ) and 4-nitrobenzonitrile (4NB), which work in reduction, have been chosen as mediator (Fig. 12.b). Their redox potentials are sufficiently negative to permit to the reduced forms (TCNQ•– and 4NB•–, respectively) to reduce the viologen spacers of the polymers (Fig. 12.a). Thus, they are good candidates to probe the reactivity of the polymeric films, because their electron exchanges with the non-coated ITO substrate are negligible (negative feedback). Nevertheless, a more important positive feedback is observed when the ITO electrodes are coated with a polymeric film, showing well the predicted reactivity of the polymers with these two mediators. But the positive feedback characters are lower in the case of the polymer obtained from the non-substituted ZnOEP. That can be explained by a lower conductivity of this polymer compared to the other one, because of an increase of the distance between the redox active centers also due to the formation of bipyridine-bridged zinc porphyrins.

Fig. 12. (a) Cyclic voltammogram of the polymer obtained from ZnOEP and bpy after 25 iterative scans onto ITO electrode in CH3CN and 0.1 M NBu4PF6 (*v* = 0.2 V s–1), with indications of the redox potential values of several mediators used. (b) SECM approach curves, in CH3CN and 0.1 M NBu4PF6 on non-coated ITO electrodes (full lines), ITO electrodes coated with the polymer obtained from ZnOEP(bpy)+ after 25 iterative scans (dashed lines) and ITO electrodes coated with the polymer obtained from ZnOEP and bpy after 25 iterative scans (dotted lines), with TTF (▬) and 4NB (▬) used as redox mediators. *L* is the normalized distance and *I*norm is the normalized current, where *i* corresponds to the current at a platinum microelectrode (with a radius *a* equal to 5 µm) localized at a distance *d* from the substrate and *i*inf corresponds to the steady-state current when the microelectrode is at an infinite distance from the polymeric substrate.

#### **5.4 Control of the polymeric film thickness**

It is also interesting to note that electropolymerization allows easily to control the thickness of the polymeric film deposited onto the electrode (Schaming et al., 2011b).

As represented Fig. 13, plot of the absorbance of the polymer at the maximum of the Soret band as a function of the number *n* of iterative scans shows a linear increase. The thicknesses of the films determined from AFM confirm this point since a linear increase of the thickness with the value of *n* is observed (Fig. 13, inset).

Oxidation of Porphyrins in the Presence of Nucleophiles: From the Synthesis of Multisubstituted Porphyrins to the Electropolymerization of the Macrocycles 67

66 Electropolymerization

Secondly, tetracyanoquinodimethane (TCNQ) and 4-nitrobenzonitrile (4NB), which work in reduction, have been chosen as mediator (Fig. 12.b). Their redox potentials are sufficiently negative to permit to the reduced forms (TCNQ•– and 4NB•–, respectively) to reduce the viologen spacers of the polymers (Fig. 12.a). Thus, they are good candidates to probe the reactivity of the polymeric films, because their electron exchanges with the non-coated ITO substrate are negligible (negative feedback). Nevertheless, a more important positive feedback is observed when the ITO electrodes are coated with a polymeric film, showing well the predicted reactivity of the polymers with these two mediators. But the positive feedback characters are lower in the case of the polymer obtained from the non-substituted ZnOEP. That can be explained by a lower conductivity of this polymer compared to the other one, because of an increase of the distance between the redox active centers also due to

Fig. 12. (a) Cyclic voltammogram of the polymer obtained from ZnOEP and bpy after 25 iterative scans onto ITO electrode in CH3CN and 0.1 M NBu4PF6 (*v* = 0.2 V s–1), with indications of the redox potential values of several mediators used. (b) SECM approach curves, in CH3CN and 0.1 M NBu4PF6 on non-coated ITO electrodes (full lines), ITO electrodes coated with the polymer obtained from ZnOEP(bpy)+ after 25 iterative scans (dashed lines) and ITO electrodes coated with the polymer obtained from ZnOEP and bpy after 25 iterative scans (dotted lines), with TTF (▬) and 4NB (▬) used as redox mediators. *L* is the normalized distance and *I*norm is the normalized current, where *i* corresponds to the current at a platinum microelectrode (with a radius *a* equal to 5 µm) localized at a distance *d* from the substrate and *i*inf corresponds to the steady-state current when the microelectrode

It is also interesting to note that electropolymerization allows easily to control the thickness

As represented Fig. 13, plot of the absorbance of the polymer at the maximum of the Soret band as a function of the number *n* of iterative scans shows a linear increase. The thicknesses of the films determined from AFM confirm this point since a linear increase of

of the polymeric film deposited onto the electrode (Schaming et al., 2011b).

the formation of bipyridine-bridged zinc porphyrins.

is at an infinite distance from the polymeric substrate.

the thickness with the value of *n* is observed (Fig. 13, inset).

**5.4 Control of the polymeric film thickness** 

Fig. 13. UV-visible absorption spectra of ITO electrodes modified with the polymer obtained from ZnOEP and tzpy after different numbers *n* of iterative scans. Inset: absorbance at 427 nm *vs. n*.
