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SINGLET AND TRIPLET REACTIVITY IN THE PHOTOREDUCTION OF OXONINE(3,7‐DIAMINOPHENOXAZIN‐5‐IUM CHLORIDE) BY IRON (II)
Author(s) -
CREED DAVID,
FAWCETT NEWTON C.,
THOMPSON ROBERT L.
Publication year - 1991
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1991.tb08484.x
Subject(s) - chemistry , quantum yield , intersystem crossing , singlet state , photochemistry , electron transfer , quenching (fluorescence) , triplet state , protonation , superexchange , excited state , chloride , dication , reaction rate constant , ion , fluorescence , atomic physics , molecule , kinetics , physics , organic chemistry , quantum mechanics
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, S 1 , rather than from the triplet state, T 1 . The latter is formed with an intersystem crossing (isc) quantum yield of ca 1.7×10 ‐3 . The quenching of S 1 by Fe (II) has a rate constant k s Q = 2.2 ± 0.1 × 10 9 M ‐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 T 1 , generated by bromide ion quenching of S 1 or by diacetyl sensitization, occurs with k Q t 1.2×10 6 M ‐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 S 1 reduction are discussed. These are energy transfer from S 1 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 S 1 and T 1 , and spin allowed back electron transfer in an exciplex formed between S 1 and Fe (II). Evidence is also presented for a very low probability ( ca 1%) induced isc from the encounter of S 1 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 × 10 9 M ‐1 s ‐1 , oxidation of R by Fe (III), k R OX = 1.5 ± 0.1 × 10 5 M ‐1 s ‐1 , and oxidation of L by Fe (III), k L OX = 1.1 ± 0.1 × 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.