Phthalocyanines metalloporphyrin

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

In general, for a particular metal state, metallophthalocyanines are more difficult to demetallate than are metalloporphyrins (the central hole is marginally smaller in the phthalocyanine ligand). Thus, copper(II) phthalocyanine, dissolved in concentrated sulfuric acid, is precipitated unchanged when the solution is poured into ice/water. However, metallochlorins are demetallated more readily than the corresponding metalloporphyrins (basicity chlorin < porphyrin). 

Porphyrins, 21 14, 36, 135 -based manganese complexes, 46 400-402 as cobalt complex ligants, 44 284-290 compared to phthalocyanines, 7 75 complexes, 19 144, 145, 147 complex stability, 42 135-137 degeneracy lifting, 36 206 metalloporphyrins, DNA cleavage and, 45 271-283... 

Much of the work on the photoreduction of carbon dioxide centres on the use of transition metal catalysts to produce formic acid and carbon monoxide. A large number of these catalysts are metalloporphyrins and phthalocyanines. These include cobalt porphyrins and iron porphyrins, in which the metal in the porphyrin is first of all photochemically reduced from M(ii) to M(o), the latter reacting rapidly with CO to produce formic acid and CO. ° Because the M(o) is oxidised in the process to M(ii) the process is catalytic with high percentage conversion rates. However, there is a problem with light energy conversion and the major issue of porphyrin stability. 

Porphyrin and its derivatives are well-known model compounds for photosynthetic processes that involve charge separation [87], A zinc porphyrin, a copper porphyrin, and a zinc phthalocyanine (Pc) are chosen and incorporated into PPV backbones. Four polymers are synthesized via the Heck polycondensation [88], The incorporation of these metal complex moieties in polymers XI to XIII is manifested by the appearance of Q bands from metalloporphyrin and zinc Pc... 

The soluble metalloporphyrin-containing polymers are formed by the copolymerization of MM A or 4-VP with macrocyclic MCM — an interaction product of acrylic acid chloride with tetra-p-aminophenylporphyrinate acetate manganese [96]. Copolymers obtained by the radical copolymerization of acryloyl derivatives of cobalt phthalocyanine with 9-vinylcarbazole [97] should also be mentioned. 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

Groves, J. T. (2000) Reactivity and mechanisms of metalloporphyrin-catalyzed oxidations, Journal of Porphyrins and Phthalocyanines 4, 350-352. 

This chapter focuses on the progress and challenges in the field of photocatalysis as applied towards the water splitting reaction (Eq. 1.). More specifically, homogeneous molecule based systems that mimic the natural photosynthetic system are examined for their potential to drive reaction 1. A number of molecules, including porphyrins, metalloporphyrins and phthalocyanines,17 transition metal complexes of Ru, Os, Re, Rh, Pt, Cu,811 and acridine and flavin derivatives,1214 have been examined as the chormophores and sensitizers for light driven processes. 

Porphyrins and metalloporphyrins have been studied extensively due to their importance in biology. The related phthalocyanine macrocycles are often considered for comparison to porphyrins and are also of interest because of their increased use in molecular electronic devices. Much attention has been given to electronic structme calculations on porphyrins and metalloporphyrins, and as discussed earlier, the quality of a computational method for describing the electronic wavefunction of any system can be judged by the method s ability to calculate accmate ionizations energies. 

The observation that considerably less energy is required to remove electrons from filled orbitals than to remove an electron from a singly-occupied orbital indicates that the Aufbau Principle is being violated in these systems. Similar behavior has been observed for cobalt(II) and cop-per(II) phthalocyanines and other metalloporphyrins with unpaired metal electrons. Non-Aufbau electron configurations are occasionally invoked in theoretical calculations, but experimental observation of non-Aufbau behavior for transition metal molecules has been Umited to the cases of porphyrins and phthalocyanines. 

The use of environmental conditions to improve the yield of photoredox products continues to be a popular area of research. The influence of electrostatic factors has been studied for zinc phthalocyanine photosensitizers, the conclusions reached being in agreement with earlier work. Negatively charged metalloporphyrins form strong ground-state complexes with but... 

An early example of surface Raman without field enhancement is shown in Figure 13.7. This measurement predated the development of CCD detectors and represents a major achievement in terms of sensitivity (12). The adsorption of nitrobenzene on a well-defined nickel surface was carried out in UHV then Raman scattering was observed with a single spectrograph and an intensified Vidicon detector. Unenhanced spectra were obtained from 7.5 x lO molecules cm of nitrobenzene (1.1 x 10 " mol cm ), corresponding to a submonolayer. Examples of surface Raman spectroscopy with resonance enhancement include monolayers of phthalocyanines on gold (17) and ordered graphite (18), and multilayers of metalloporphyrins (19). 

Again, alternatives for Pt (-alloys) are not available (in acid), although there has been an active search for them. For a time, transition metal porphyrins and phthalocyanins supported on carbon were thought to be promising candidates, where their use was motivated by nature s use of metalloporphyrins for oxygen activation. The catalytic action of these chelates can be described in terms of the following mechanism ... 

Radiative and Nonradiative Decay Processes - Due to the potential application of these compounds as photosensitizers for photodynamic therapy" the photophysical properties of porphyrins and phthalocyanines, and their corresponding metal complexes, have been investigated extensively over the past decade. The photophysical properties of water-soluble metalloporphyrins, and especially the tetraphenylsulfonates," have been re-examined but nothing new has been found. The disulfonated metallophthalocyanines (MPcS2, where M = Al ", Ga" , or Zn") form complexes with fluoride ions for which the fluorescence yields and lifetimes are decreased with respect to the parent dyes while there are... 

R525 L. K. Sanders, W. D. Arnold and E. Oldfield, NMR, IR, Mossbauer and Quantum Chemical Investigations of Metalloporphyrins and Me-talloproteins , J. Porphyrins Phthalocyanines, 2001, 5, 323... 

Chemical ionization mass spectrometric detection has been explored for the detection of methyl hydroperoxides However, fluorometry has dominated the current detection schemes for the organic peroxides. Typically, a nonfluorescent substrate is oxidized by the peroxide to generate a fluorescent product. These methods are sufficiently sensitive for accurate measurement of the peroxides in the low ppt by volume. For example, the peroxidase-catalyzed dimerization of p-hydroxyphenylacetic acid (POPHA) occurs in the presence of a peroxy group at elevated pH. The formation of the fluorescent dimer, detected by excitation at 310 nm and emission at 405 nm, is proportional to the concentration of the peroxide. The most common peroxidase catalyst used for this reaction is horseradish peroxidase (HRP). Cost and stability issues with the use of HRP led to the use of other catalysts, such as metalloporphyrins or phthalocyanine complexes. Another fluorescent reaction scheme involves the oxidation of the nonfluorescent thiamine (vitamin Bi) to the fluorescent thiochrome by the peroxide group. This reaction is catalyzed by bovine hematin. This reaction is 25-fold more sensitive for H2O2 than for the organic peroxides. 

---