Singlet oxygen reactivity in water-rich solvent mixtures

The 3-methylindole (3MI) oxygenation sensitized by psoralen (PSO) has been investigated in 100%, 20% and 5% O2-saturated water/dioxane (H2O/Dx) mixtures. The lowering of the ¹O2* chemical rate when water (k chem∆3MI = 1.4 × 109 M-1 s-1) is replaced by deuterated water (k chem∆3MI = 1.9 × 108 M-1 s-1) suggests that hydrogen abstraction is involved in the rate determining step. A high dependence of the chemical rate constant on water concentration in H2O/Dx mixtures was found showing that water molecules are absolutely essential for the success of the 3MI substrate oxidation by ¹O2* in water-rich solvent mixtures.