Electron Transfer in the Quenching of Triplet States of Zinc Phthalocyanine and Methylene Blue by the Use of Fe(III), Co(III), and Organic Oxidants

The mechanism of the quenching of the triplet state of zinc phthalocyanine (ZnPc) and Methylene Blue (MB+) by low-spin complexes of Fe(III) and Co(III) and aromatic oxidants was studied. The quenchers studied included FeIII(CN)63−, FeIII(CN)4(2,2-bipyridyl)−, FeIII(CN)2(2,2-bipyridyl)+, ferrocenium ion, CoIII(2,2′:6′-2″-terpyridyl)3+, CoIII(1,10-phenanthroline)3+, dimethylviologen, 1,4-benzoquinone, and 2,4,7-trinitro-9-fluorenone (TNF). The rate constants of the quenching of 3ZnPc varied from a diffusion-controlled one to 6×105 dm3 mol−1 s−1 in the mixed solvent of dimethylacetamide (DMA) and water (7:3 by volume). The metal complexes quenched 3MB+ and the triplet state of protonated MB+ with a rate constant of more than 2×106 dm3 mol−1 s−1 in an aqueous solution with 0.5 mol dm−3 of MgCl2. The fractions of electron transfer yielding ZnPc.+ in the quenching process (F1) were high except for TNF and the iron(III) compounds. The F1 fractions were also obtained in the quenching of 3MB+ and the triplet state of the protonated MB+. The absence of radical production in the quenching by the doublet iron(III) compounds can be explained in terms of spin-allowed reverse electron transfer in the life of the geminate radical pair. Fractions of the reverse electron transfer between the half-reduced quencher and ZnPc.+ or MB.2+ were also measured; they are very close to those of 1–F1. The data reported in this study are consistent with the following mechanistic features. (1) Every quenching encounter produces a geminated radical pair. (2) The geminated pair is common to the quenching of the triplet excited state by oxidants and to the reverse electron transfer between the free radicals formed in the quenching. The molar extinction coefficient of ZnPc.+ was determined to be 2.9×104 dm3 mol−1 cm−1 at 520 nm.