Alpha-methylstilbene and the duality of mechanism in the quenching of stilbene triplets by molecular oxygen.

Spin-exchange quenching of alpha-methylstilbene triplets by molecular oxygen and by the free radical di-tert-butyl nitroxide is shown to favor the cis isomer more than does natural decay. The effect of the two quenching events is an identical 7% decrease in the fraction of perpendicular triplets that decay to the trans isomer. The conclusion that relaxed stilbene triplets reside in a shallow minimum corresponding to a geometry in which the two benzyl moieties are orthogonal was based on the observation that their quenching by O2 does not alter the trans/cis photostationary ratio. Our results with alpha-methylstilbene confirm the hypothesis that in the case of stilbene spin exchange quenching by O2 at the twisted geometry favors the cis isomer but occurs in competition with excitation transfer from transoid triplets that leads to the trans isomer and to singlet oxygen. The opposite effects of the two oxygen quenching paths on stilbene isomer composition cancel accidentally leading to an overall insensitivity of benzophenone-sensitized photostationary states to the presence of oxygen. Quenching rate constants derived on the basis of this cancellation are close to diffusion-controlled and predict singlet O2 quantum yields of 0.08 and 0.13 in the presence of air and under an O2 atmosphere, in good agreement with experimental measurements.