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

Authors

David Creed, University of Southern MississippiFollow
Newton C. Fawcett, University of Southern MississippiFollow
Robert L. Thompson, University of Southern Mississippi

Document Type

Article

Publication Date

5-1-1991

Department

Chemistry and Biochemistry

School

Mathematics and Natural Sciences

Abstract

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 1.7×10^‐3. The quenching of S₁ by Fe (II) has a rate constant k^s_Q= 2.2 ± 0.1 × 10⁹M^‐1 s^‐1 and affords the one electron reduced product, semioxonine (R), with a limiting quantum yield, φ^S_R, 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_t1.2×10⁶M^‐1 s^‐1, extrapolated to zero ionic strength, and affords R with a limiting probability, φ^T_R= 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^R_DIS= 1.7 ±0.2 × 10⁹M^‐1 s^‐1, oxidation of R by Fe (III), k^R_OX= 1.5 ± 0.1 × 10⁵M^‐1 s^‐1, and oxidation of L by Fe (III), k^L_OX= 1.1 ± 0.1 × 10³M^‐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.

Publication Title

Photochemistry and Photobiology

Volume

53

Issue

5

First Page

573

Last Page

586

Recommended Citation

Creed, D., Fawcett, N. C., Thompson, R. L. (1991). Singlet and Triplet Reactivity in the Photoreduction of Oxonine(3,7-Diaminophenoxazin-5-ium Chloride) by Iron(II). Photochemistry and Photobiology, 53(5), 573-586.
Available at: https://aquila.usm.edu/fac_pubs/7020