Mechanism of Photoreduction of Thiazine Dyes by Edta Studied by Flash‐photolysis‐i

The photoreduction of thiazine dyes by ethylene diamine tetraacetic acid (EDTA) was investigated by Rash photolysis. This reaction was found to occur according to a three‐step mechanism. the first being the formation of the dye triplet state followed, in weakly acid solutions, by protonation. During the second step, the triplet state of the dye disappears through two competing processes: spontaneous deactivation and reaction with EDTA, which leads to the semireduced dye. The third step leads to the leucodye. It is shown that the overall quantum yield of photoreduction is governed by the second step and can be calculated from the ratio of the rate constants of the two elementary processes involved in this step. This ratio was measured over a wide pH range.