Reaction of singlet oxygen with vinyl-substituted porphyrins. A kinetic study by laser flash photolysis

Abstract

The reactivity of several vinyl porphyrins towards singlet oxygen (¹O₂) in various solvents has been investigated using time-resolved detection of infrared luminescence at 1270 nm. The porphyrins studied are derived from deuterioporphyrin through 2- or 4-substitution. The singlet-oxygen quenching constant, k_p, is found to be of the order of 10⁷ dm³ mol^–1 s^–1, a value much higher than that derived from steady-state photolysis measurements reported previously. The quenching rate constant decreases with increasing electron withdrawing effect of substituents on the porphyrin ring. Values of k_p determined for separated isomers and for isomeric mixtures are identical within experimental error. Deuteriation of solvents does not affect k_p. Little effect of solvent electronic properties or viscosity can be discerned for aprotic solvents. On the other hand, an accelerating effect of protic solvents such as water, methanol, ethanol and N-methylformamide is evidenced. The k_p values for some nonvinyl porphyrins are also reported and compared to literature data. The quenching mechanism is discussed.