Steady state and laser flash photolysis of acenaphthenequinone in the presence of olefins

The rate constants for the quenching of acenaphthenequinone triplet by olef ins, in degassed benzene solution, have been measured by laser flash photol ysis. The alkenes studied included acyclic, cyclic, isolated and conjugated dienes, and vinyl ethers. The quenching rate constant ranges from 2.1 x 10 (6) M-1 s(-1) for hexa-1,5-diene to 6.0 x 10(8) M-1 s(-1) for 2,3-dimethylb ut-2-ene. A plot of log k(q) versus the ionization potential for some of th e olefins employed is linear (r = 0.89), with a slope of -1.5/eV. The magni tude of this slope, as well as the inverse solvent effect found in the quen ching process, i.e. k(q)(ACN)/k(q)(benzene) = 0.3-0.5, are consistent with a mechanism involving a partial charge transfer complex. Steady state photo lysis of acenaphthenequinone in the presence of cyclohexene, 2-methylbut-1- ene, 2-methylbut-2-ene, trans-penta-1,3-diene, cis-penta-1,3-diene, trans-s tilbene, cis-stilbene, ethyl vinyl ether, and 2,5-dimethylhexa-2,4-diene le d only to products resulting from a photocycloaddition process.