Singlet oxygen formation and photostability of meso-tetraarylporphyrin derivatives and their copper complexes

Time-resolved photochemical studies of five porphyrin derivatives and their copper complexes were carried out. Triplet lifetimes (room temperature) we re determined under argon, air, and oxygen. The presence of oxygen and copp er shortens the triplet lifetimes (argon: in the order of 10(2) mus) by up to three orders of magnitude. With the copper complexes, Phi [O-2 ((1)Delta g)] is lower than with the metal-free porphyrins. The photodegradation, ind icated by bleaching, of meso-tetraphenylporphyrin and meso-tetrakis(pentafl uorophenyl)porphyrin under air shows the latter to be more stable than the former, and their copper complexes to be more stable than the porphyrins th emselves. With meso-tetraphenylporphyrin, the effect depends on the solvent , especially in the absence of oxygen (bleaching is faster in toluene than in benzene). With meso-tetraphenylporphyrin in toluene (lambda (irr) = 405 nm), the quantum yields of bleaching, determined from the rate of fading of the maximum of the Soret band are versus similar to1.8 x 10(-4) (argon) an d similar to1.1 x 10(-5) (oxygen). Two rate constants have been roughly est imated: (i) for the H-atom abstraction from toluene by the triplet excited meso-tetraphenylporphyrin, in the order of 0.1 dm(3) mol(-1) s(-1); (ii) fo r the product-forming reaction of O-2((1)Deltag) with meso-tetraphenylporph yrin, 40 dm(3) mol(-1) s(-1). (C) 2001 Elsevier Science B.V. All rights res erved.