Attack of H-atoms on aqueous thionine studied by pulse radiolysis

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Abstract
The reaction mechanism of the H-attack on thionine (TH+) in aqueous solutions (5 × 10−6 to 2 × 10−5 mol dm−3 TH+; 2 × 10−2 mol dm−3t-butanol, pH 1 and 4) was investigated by pulse radiolysis in combination with simulation by computation. Applying a least-squares fitting procedure it was possible to state that at pH 1, 89.5% of the H-atoms react with TH+, k = (8.5 ± 0.3) × 109 dm3 mol−1 s−1, to form semiquinone (•TH32+, λmax = 390 nm, ε390 = 685 ± 20 m2 mol−1; ε720 = 720 m2 mol−1 and λmax = 790 nm, ε790 = 775 ± 20 m2 mol−1), which dismutates to TH+ and leucodye with 2k = (2.4 ± 0.1) × 109 dm3 mol−1 s−1. In addition to this a second H-attack (10.5% H) occurs on the ring carbon of the dye with k = (1.0 ± 0.1) × 109 dm3 mol−1 s−1, resulting in the production of H-adducts. These results are of interest for photogalvanic cells using thionine as a sensitizer.

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