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, kp, 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 kp determined for separated isomers and for isomeric mixtures are identical within experimental error. Deuteriation of solvents does not affect kp. 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 kp values for some nonvinyl porphyrins are also reported and compared to literature data. The quenching mechanism is discussed.