Photoredox and photocatalytic characteristics of various iron meso-tetraarylporphyrins

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Abstract

The photoredox behavior of various iron meso-tetraarylporphyrins in ethanol-containing solvents is examined. The reduction of Fe(III) to Fe(II) is observed in every case under irradiation with light of 350-450 nm. The electron-drawing or -repelling power of the meso-aryl substituents are found to have a scarce influence on the energy and the reactivity of the excited states responsible for the photoreduction process. On the other hand, the photoreduction quantum yield is appreciably influenced by the composition of the solvent in proximity to the iron porphyrin molecules. The iron porphyrins examined are observed to catalyze the photoreduction of CCl4 by ethanol. The compounds containing bulky meso-aryl substituents are very resistant to degradation, so that they appear the best suited for catalytic purposes. A mechanism is proposed in which the primary process consists of an electron transfer from the axially ligated ethanolate to iron, giving an iron(II) porphyrin species and an ethoxy radical. These species are able to react with CCl4, initiating a catalytic cycle leading to the formation of Cl−, CH3CHO, and CHCl3. © 1991, American Chemical Society. All rights reserved.

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