Phthalocyanines stability

Ballarin, B., M. Gazzano, J.L. Hidalgo-Hidalgo de Cisneros, D. Tonelli, and R. See-ber (2002). Electrocatalytic activity of cobalt phthalocyanine stabilized by different matrixes. Anal. Bioanal. Chem. 374(4), 891-897. 

Some recent results have revealed that the optimal activity for N4-chelate catalysts is normally obtained at a heat-treatment temperature range of 500-700 °C [30-32], However, it has also been discovered that a higher pyrolysis temperature (> 800 °C) is necessary in order to achieve stable performance in a PEM fuel cell enviromnent. A deleterious effect on electrode performance was observed at temperatures higher than 1100 °C [33], Even for some carbon-supported Fe- and Co-phthalocyanines, stability can also be considerably improved. For example, an almost 50 times greater enhancement in electrocatalytic activity was achieved at an electrode potential of 700 mV (vs. NHE) when carbon-supported Co-phthalocyanine was heat-treated in an environment of N2 or Ar at 700-800 °C [34]. Furthermore, in experiments with carbon-supported Ru-phthalocyanine, heat treatment at 650 °C could increase the catalytic activity by 20 times at 800 mV (vs. NHE). Unfortunately, there was no insignificant improvement in catalyst stability. Not all heat-treated carbon-supported metal phthalocyanines gave positive results. For example, the activities and stabilities of Zn- and Mn-phthalocyanines were not affected by heat treatment [34]. The duration of heat treatment for these complexes is usually around 0.3 5 hrs. 

Very unstable modifications, like the reddish, chlorine-free a-copper phthalocyanine, can be stabilized with amides or salts of copper phthalocyanine sulfonic acids (59—63). Mixture with other metal phthalocyanines, eg, tin, vanadium, aluminum, or magnesium, also inhibits crystallization change and poor performance in binders and prints (flocculation) due to the hydrophobic character of unsubstituted phthalocyanines. 

Electrochemical methods 78,79 and reactions under high pressure have also been investigated.80 Due to the high resonance stabilization of the macrocyclc, the formation of the phthalocyanine is strongly exothermic. Nevertheless, a high thermal activation and therefore usually a high temperature is necessary. 

Unsubstituted phthalocyanines can readily be purified by sublimation or by dissolution in concentrated sulfuric acid followed by precipitation in water. These classical methods of purification are applicable to phthalocyanines due to their high stability towards heat and acid. Simple washing or extraction procedures using water and organic solvents can also be used. 

The increased solubility of substituted phthalocyanines (vide infra) enables more common purifications as used for other organic compounds. Usually the purification is done by chromatography either on alumina or silica gel, but recrystallization and extraction procedures can also be used. In some cases, the methods used for unsubstituted phthalocyanines can also be practiced, although the increased molecular weight accompanied by a reduced thermal stability makes sublimation more difficult.97 98 However, for substituted phthalocyanines, the stability towards acid may be reduced97 and, therefore, purification by treatment with sulfuric acid cannot generally be recommended. 

In the following example, the low stability of lead phthalocyanines is used to prepare a metal-free 4,5-tetrabridged phthalocyanine 4 as the decomposition product. 

Calcium-binding proteins, 6, 564, 572, 596 intestinal, 6, 576 structure, 6, 573 Calcium carbonate calcium deposition as, 6, 597 Calcium complexes acetylacetone, 2, 372 amides, 2,164 amino acids, 3, 33 arsine oxides, 3, 9 biology, 6, 549 bipyridyl, 3, 13 crown ethers, 3, 39 dimethylphthalate, 3, 16 enzyme stabilization, 6, 549 hydrates, 3, 7 ionophores, 3, 66 malonic acid, 2, 444 peptides, 3, 33 phosphines, 3, 9 phthalocyanines, 2,863 porphyrins, 2, 820 proteins, 2, 770 pyridine oxide, 3,9 Schiff bases, 3, 29 urea, 3, 9... 

Organic dyes, aside from their role as sensitization agents for wide band gap semiconductors have been employed also for stabilization of narrow band gap semiconductors. The majority of such studies have considered metal or metal-free phthalocyanine films for both sensitization and electrode protection purposes [35]. 

Diaz AF, Logan JA(1980)Electroactive polyanihne films. JElectroanalChem 111 111-114 Noufi R, Nozik AJ, White J, Warren LF (1982) Enhanced stability of photoelectrodes with electrogenerated polyanUine films. J Electrochem Soc 129 2261-2265 Noufi R, Tench D, Warren LE (1981) Protection of semiconductor photoanodes with photoelectrochemicaUy generated polypyrrole films. J Electrochem Soc 128 2596-2599 Jaeger CD, Fan FRF, Bard AJ (1980) Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine. J Am Chem Soc 102 2592-2598 Gerischer H (1977) On the stability of semiconductor electrodes against photodecomposition. J Electroanal Chem 82 133-143... 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Baranton S, Coutanceau C, Roux C, Hahn F, Leger JM. 2005. Oxygen reduction reaction in acid medium at iron phthalocyanine dispersed on high surface area carbon substrate tolerance to methanol, stability and kinetics. J Electroanal Chem 577 223-234. 

Lalande G, Faubert G, Cote R, Guay D, Dodelet JP, Weng LT, Bertrand P. 1996. Catalytic activity and stability of heat-treated iron phthalocyanines for the electroreduction of oxygen in polymer electrolyte fuel cells. J Power Sources 61 227-237. 

Since iron phthalocyanine complexes can be activated and stabilized by a chlorine ring substitution (12), the activity of iron hexadecachlorophthalocyanine (7b) immobilized on silica was examined for the synthesis of 4, 5, and 6 with TBHP as oxidant. 

Phthalocyanines have attracted particular attention as potential surface modifiers due to their stability and tendency to form ordered structures directed by dispersion forces. They are inherently host-guest structures with a readily interchangeable coordinating metal ion, which in the solid state results in a tunable bandgap. At a surface, in addition to possibly interesting electronic... 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

Electroreduction of N2 to NH3 has also been examined using gas-diffusion electrodes modified by 14 different metal phthalocyanines.320 It was found that the Sn-Pc complex is the best catalyst in terms of current efficiency and stability of the electrode for the electrochemical dinitrogen activation. 

High stability metallo-organic compounds such as porphyrin, phthalocyanine, metallized... 

This reaction has been observed for porphyrins, and used to establish relative stabilities.51,53 It is of practical use in the preparation of metal complexes, e.g., metallophthalocyanines by metal exchange with dilithium(I) phthalocyanine.54... 

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