Zur photosensibilisierten sauerstoffübertragung: Untersuchung der terminationsschritte durch belichtungen bei tiefen temperaturen

Abstract

The oxygen containing intermediate X in photosensitized oxygen transfer-reactions as well as the significance of the preceeding diffusion processes have been investigated. In the range +50/−150° the quantum yield of the O 2 consumption by the acceptor ( A) follows a simple rate law. Rate constants, entropies and activation energies have been determined for the deactivation of X as well as for its reaction with A. The decay constant of X is practically independent of sensitizer ( k N ∼ 5 × 10 6 sec −1 in methanol) and temperature. Electronically excited O 2 is suggested as the reactive species ( X), whose reactivity is only slightly modified by different sensitizers and solvents. The activation energies ( E H ) for the formation of AO 2 decrease in the order: 2-butene, 2-methylpropene, 2-methyl-2-butene, 2,3-dimethyl-2-butene, 1,3-cyclohexadiene, cyclopentadiene (≅0). In general, EP H is lowered by methyl substitution of the double bonds; the influence of more voluminous groups is discussed. Δ S is −12/−18 cal/deg mole for 42 out of 46 different acceptors.