Singlet and Triplet Reactivity in the Photoreduction of Oxonine(3,7-Diaminophenoxazin-5-ium Chloride) by Iron(II)

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Chemistry and Biochemistry


The oxazine dye, oxonine (3,7-diaminophenoxazin-5-ium chloride), 1, is photoreduced by Fe (II) sulfate in dilute sulfuric acid. The reaction mechanism is analogous to that for the photoreduction of thiazine dyes by Fe (II), the most important difference being that reduction of oxonine occurs predominantly from its excited singlet state, S1, rather than from the triplet state, T1. The latter is formed with an intersystem crossing (isc) quantum yield of ca 1.7 x 10(-3). The quenching of S1 by Fe (II) has a rate constant k(Q)S = 2.2 +/- 0.1 x 10(9) M-1 s-1 and affords the one electron reduced product, semioxonine (R) with a limiting quantum yield, phi-R(S) of 0.26 +/- 0.02. In contrast, quenching of T1, generated by bromide ion quenching of S1 or by diacetyl sensitization, occurs with k(Q)T almost-equal-to 1.2 x 10(6) M-1 s-1, extrapolated to zero ionic strength, and affords R with a limiting probability, phi-R(T) = 1.1 +/- 0.2. Three possible reasons for the lower quantum yield of the more exothermic S1 reduction are discussed. These are energy transfer from S1 to Fe (II), different rates of escape of R from the encounter complex as a consequence of the different states of protonation of R as initially formed from S1 and T1, and spin allowed back electron transfer in an exciplex formed between S1 and Fe (II). Evidence is also presented for a very low probability (ca 1%) induced isc from the encounter of S1 with paramagnetic Fe (II). Rate parameters for other processes important to the overall reduction mechanism such as disproportionation of R to leucooxonine L and oxonine, k(DIS)R = 1.7 +/- 0.2 x 10(9) M-1 s-1, oxidation of R by Fe (III), k(OX)R = 1.5 +/- 0.1 x 10(5) M-1 s-1, and oxidation of L by Fe (III), k(OX)L = 1.1 +/- 0.1 x 10(3) M-1 s-1, have also been measured. These results are contrasted with those for the closely related thionine/Fe(II) photoredox reaction, the most well understood system for photogalvanic energy conversion.

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Photochemistry and Photobiology





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