OXIDATION OF AN ELECTRON-RICH OLEFIN INDUCED BY SINGLET OXYGEN - MECHANISM FOR TETRAPHENYLETHYLENE

The oxidation of an electron-rich olefin (tetraphenylethylene (TPE)) w as studied using methylene blue as sensitizer. The Stern-Volmer correl ation for methanolic solutions of TPE, using N-3(-) as O-1(2) quencher , and the measurement of the pseudo-first- order constants for TPE dis appearance (k(I)) in reactions with air and oxygen saturation, confirm that oxidation occurs via a singlet-oxygen-based mechanism. The forma tion of an exciplex state, as a metastable intermediate involving TPE and O-1(2), was observed indirectly by studying the effect of solvent polarity on the reaction. The rate constant for oxidation in methanoli c solutions (k(I) congruent to 10(5) M(-1) s(-1)) reveals that this me chanism involves a charge transfer intermediate. This intermediate can form a dioxetane oxidation product or be physically quenched. The obs erved second-order constant (k(a)= k(I)+ k(Q) = 6.76 X 10(7) M(-1) s(- 1)) shows that the physical quenching of O-1(2) due to TPE (k(Q) = 6.7 4 X 10(7) M(-1) s(-1)) is not diffusion limited, since it involves a c harge transfer intermediate. The O-1(2), quenching promoted by the sen sitizer is diffusion limited (k(s)=3.0 X 10(10) M(-1) s(-1)). Theoreti cal calculations provide additional evidence for the formation of a ch arge transfer intermediate, the reversibility of this state and the co mpetition of a similar process involving molecular oxygen in the groun d state.