Porphyrins electrochemical activity

The thin films are electrochemically active in both the oxidation and reduction regions, due to their having the electrochemically active zinc porphyrin units. The electrochemical processes are accompanied by color changes of the film, between black and yellowish-green in the oxidation region, and between red and yellowish-green in the reduction region. 

The CV curves obtained for carbons with preadsorbed copper shown in Figs. 45 (curves b, b, c, c ) and 46 (a-a")) exhibit only slight peaks of the Cu(II)/Cu(I) couple and broad waves due to the redox reaction of surface carbon functionalities (.see Section IV). However, preadsorbed copper enhances the peaks of the redox process in bulk solution (especially the anodic peaks for D—H and D—Ox samples), as can be seen in Fig. 46 (curves c-c"). The low electrochemical activity of samples with preadsorbed copper species observed in neutral solution is the result of partial desorption (ion exchange with Na ) of copper as well as the formation of an imperfect metalic layer (microcrystallites). Deactivation of the carbon electrode as a result of spontaneous reduction of metal ions (silver) was observed earlier [279,280]. The increase in anodic peaks for D—H and D—Ox modified samples with preadsorbed copper suggests that in spite of electrochemical inactivity, the surface copper species facilitate electron transfer reactions between the carbon electrode and the ionic form at the electrode-solution interface. The fact that the electrochemical activity of the D—N sample is lowest indicates the formation of strong complexes between ad.sorbed cations and surface nitrogen-containing functionalities (similar to porphyrin) [281]. Between —0.35 V and -1-0.80 V, copper (II) in the porphyrin complex (carbon electrode modifier) is not reduced, so there can be no reoxidation peak of copper (0) [281]. 

The electrochemical activity of (por)Sn for a variety of porphyrins has been investigated and it has been shown that the first one electron reductions are all centered upon the ring system. Two-electron oxidation in the presence of supporting electrolyte comprising perchlorate leads to (por)Sn(sol)C104 (sol = FHF, CH3CN). ... 

With Co-facial cobalt porphyrins (Figure 2.5) adsorbed on pyrolytic graphite both cobalt centers are electrochemically active . However if one of the cobalt centers is replaced by iron, this metal shows no redox activity. This cofacial porphyrin with two different metal centers shows catalytic activity for the reduction of O2 and the foot of the reduction is observed at a potential where presumably the Fe(III)/(II) should appear. The fact that the Fe center is electrochemically silent is not clear . 

Griveau, S., V. Albin, T. Pauporte, J.H. Zagal, and F. Bedioui (2002). Comparative study of electropolymerized cobalt porphyrin and phthalocyanine based films for the electrochemical activation of thiols. J. Mater. Chem. 12, 225-232. 

Here we review our work aimed at correlating the reactivity of a series of M(II) N4-ligands, see Figure 12.1, (M = Co, Fe, Mn and N4 = porphyrin (P), phthalocyanine (Pc), teraphenylporphyrin (TPP), tetrabenzoporphyrin (TBP) and tetraazaporphyrin (TAP)) towards the electrocatalytic oxidation of 2-mercaptoethanol. Different effects will be analysed, namely the role of the metal atoms, the role of the N4 functionalisation, solvent and the impact of the adsorption on the electrode on the electrochemical activity. The whole machinery of DFT and the notions of hardness, chemical potential, intramolecular hardness and elec-trophilicity are used to better quantify these effects and discriminate between the examined molecular complexes. 

Yamamoto and co-workers have studied the synthesis of zinc porphyrin polymers (57) by polycondensation of 56 using Ni and Pd catalysts (128) (eq. 16). Functionalizing the zinc porphyrins and aromatic spacers with alkyl groups enhanced the solubility of these polymers. In addition, various aromatic spacers were studied in order to decrease the steric crowding surroimding the porphyrins. The molecular weights of these polymers were between 4600 and 37,900 (PDI = 1.3-1.8), and thin films of these polymers were electrochemically active and exhibited electrochromism. 

Cheng M-J, Head-Gordon M, Bell AT. How to chemically tailor metal-porphyrin-like active sites on carbon nanotubes and graphene for minimal overpotential in the electrochemical oxygen evolution and oxygen reduction reactions. J Phys Chem C 2014 118 29482-91. 

A particular electrocatalyst of Co-TPPA GCF (TPP 5, 10, 15, 20-tetrakis(phenyl)-21//, 23//-porphyrin, VGCT vapor growing carbon fiber) heat-treated in He at 1,073 K enhanced the formation of neutral H2O2 remarkably. The maximum concentration of 4.0 M was obtained with 90 mA cm and 42 % current efficiency at 278 K. Electrocatalysis of the Co-TPP/VGCF was excellent turnover frequency of Co as 14 s and total turnover number as 4 x 10 in 8 h. Various characterization data propose that electrochemical active site for reduction of O2 to H2O2 was C0N2CX structure on carbon surface [21]. Direct production of the neutral and halide-... 

Porphyrins represent an important class of dyes with promising apphcation in many areas, such as optoelectronics, chemosensors and catalysis [6,7]. These dyes exhibit strong absorption and emission in the visible region and show electrochemical activity. The self-assembly of porphyrins is mainly inspired by the photosynthetic systems in nature and is not only important for a better understanding of natural processes, but is also valuable for appUcations in optoelectronic devices. 

In the previous examples, porphyrin polymers were obtained by exploring the reactivity of oxidized porphyrin me o-positions with relatively strong nucleophiles such as pyridine derivatives, that attack the electrochemically activated C-atoms at... 

Gervaldo M, Liddell PA, Kodis G, Brennan BJ, Johnson CJ, Bridgewater LW, Moore AL, Moore TA, Gust D (2010) A photo- and electrochemically-active porphyrin-fuUeiene dyad electropolymer. Photochem Photobiol Sci 9 890-900... 

Ye, T. He, Y. F. Borguet, E. 2006. Adsorption and electrochemical activity An in situ electrochemical scanning tunneling microscopy study of electrode reactions and potential-induced adsorption of porphyrins. J. Phys. Chem. B 110 6141-6147. 

Dloxygen reduction electrocatalysis by metal macrocycles adsorbed on or bound to electrodes has been an Important area of Investigation (23 ) and has achieved a substantial molecular sophistication in terms of structured design of the macrocyclic catalysts (2A). Since there have been few other electrochemical studies of polymeric porphyrin films, we elected to inspect the dloxygen electrocatalytic efficacy of films of electropolymerized cobalt tetraphenylporphyrins. All the films exhibited some activity, to differing extents, with films of the cobalt tetra(o-aminophenylporphyrin) being the most active (2-4). Curiously, this compound, both as a monomer In solution and as an electropolymerized film, also exhibited two electrochemical waves... 

Cobalt porphyrin derivatives were also reported129 to be active for electrochemical reduction of C02 to formic acid at an amalgamated Pt electrode. More recently, Becker et al have reported130 that Ag2+ and Pd2+ metalloporphyrins acted as homogeneous catalysts for C02 reduction in dry CH2C12 oxalic acid and H2 (its source was not clear) were produced, but no CO was detected. 

A similar catalytic activity with a monomeric porphyrin of iridium has been observed when adsorbed on a graphite electrode.381-383 It is believed that the active catalyst on the surface is a dimeric species formed by electrochemical oxidation at the beginning of the cathodic scan, since cofacial bisporphyrins of iridium are known to be efficient electrocatalysts for the tetraelectronic reduction of 02. In addition, some polymeric porphyrin coatings on electrode surfaces have been also reported to be active electroactive catalysts for H20 production, especially with adequately thick films or with a polypyrrole matrix.384-387... 

---