Saltiel Jack, Klima Rodney F
Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, FL 32306-4390, USA.
Photochem Photobiol. 2006 Jan-Feb;82(1):38-42. doi: 10.1562/2005-05-09-RA-521.
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.
分子氧和自由基二叔丁基硝基氧化物对α-甲基芪三线态的自旋交换猝灭作用显示，相较于自然衰变，其更有利于顺式异构体的形成。这两种猝灭事件的效果是，垂直三线态衰变为反式异构体的比例相同地降低了7%。芪三线态弛豫后处于一个浅势阱中，对应的几何结构是两个苄基部分相互正交，这一结论是基于以下观察结果得出的：它们被O2猝灭不会改变反式/顺式光稳态比率。我们对α-甲基芪的研究结果证实了以下假设：在扭曲几何结构下，芪被O2进行自旋交换猝灭时有利于顺式异构体的形成，但它与反式三线态的激发转移相互竞争，后者会导致反式异构体和单线态氧的产生。两条氧猝灭路径对芪异构体组成的相反作用意外地相互抵消，导致二苯甲酮敏化的光稳态对氧气的存在总体上不敏感。基于这种抵消作用得出的猝灭速率常数接近扩散控制，并预测在空气存在和O2气氛下单线态O2的量子产率分别为0.08和0.13，与实验测量结果吻合良好。
