Browsing by Author "Harriman, Anthony"

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    Photoredox Reactions of Thionine
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (07), 1977) Ferreira, M. Isabel C.; Harriman, Anthony
    The photoredox reaction between thionine and iron (II) has been studied by flash photolysis techniques. The principal reaction intermediate is the semithionine radical anion which decays via a dismutation process forming ground state thionine and the fully reduced leucothionine. Subsequently, leucothionine is reoxidised slowly by iron (III) which is formed in the primary electron transfer reaction. Rate constants for all the reaction steps have been determined. The reaction has been extended to include reduction with cobalt (II) and manganese (II). Here, the ground state quenchers do not possess the necessary reducing power to form the semithionine radical anion, and reaction leads to enhanced deactivation of triplet thionine. However, when the transition metal ion is excited directly, full electron transfer takes place with concomitant formation of the radical ions.
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    Photoreduction of 1,10-Phenanthroline
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1977, 73 (04), 1977) Bandyopadhyay, Baida N.; Harriman, Anthony
    The effects of solvent polarity on the photophysical properties of 1,10-phenanthroline indicate the close proximity of (n, π*) and (π, π*) excited singlet states, whilst the lowest triplet state is (π, π*) in all solvents. In hydrocarbon solvent, the first excited singlet state is of (n, π*) character but in water a (π, π*) is situated ∼ 700 cm–1 below this state. Even in water, the (n, π*) state is capable of abstracting a hydrogen atom from the solvent forming an intermediate semidiaza radical. The radical can be detected by flash photolysis and e.s.r. techniques and, in organic solvents, it decays with second order kinetics due to a disproportionation reaction. The major product of reaction is dihydrophenanthroline which, in hydrocarbon solvents, is formed with a quantum efficiency of ∼ 25 %. In aqueous solvent, the yield and rate of decay of the semidiaza radical are pH dependent, and the pK for protonation of the radical is 7.3. Thus, irradiation in water at pH 5 results in formation of the stable diprotonated radical cation.