Photoreduction of Proflavine in the Aqueous Solution. I. A Flash Photolysis Study

The photoreduction of proflavine (D) in an aqueous solution at pH 4, was investigated by a flash technique. No intermediates were observed other than the triplet state (T) and the-semiquinone (DH·). This, with the finding that the triplet state decays according to the second order, rules out the occurrence of such reactions as T+D→D++D−, T+T→D++D−. In the presence of a large quantity of allylthiourea (RH2), the yield of leuco-proflavine (DH2) by one flash becomes constant independent of the ATU concentration, and its value is about one half of the yield of T. This indicates that almost all the T produced reacts with ATU. The rate constants obtained are: T+T→ k3=0.8–1.4×109 m−1 sec−1 T→D k46×103 sec−1 T+RH2→DH·+RH· k5=4.3×106 m−1 sec−1 DH·+DH·→DH2+D k6=7×108 m−1 sec−1 T+O2→D+O2 k7=1.5–4.7×109 m−1 sec−1 DH·+O2→D+HO2· k8=1–3×109 m−1sec−1 m-Phenylenediamine was found to produce an intermediate which decays according to the first order. Its decay constant is 1.5×102sec−1. The rate constant for the reaction of T with m-phenylenediamine is 3.5×106 m−1 sec−1, about 1000 times smaller than that of the encounter-controlled process. This suggests that some of the rate constants reported by Oster should be reinvestigated.