Simultaneous Observation of Triplet and Singlet Cyclopentane-1,3-diyl Diradicals in the Intersystem Crossing Process

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Intersystem crossing is an important chemical process. In this study, the rate constant of intersystem crossing, kISC ~3 × 107 s–1, for cyclopentane-1,3-diyl diradicals was unequivocally determined by the simultaneous observation of the decay process of the triplet diradical (λobs = 320 nm) and the growth process of the corresponding singlet diradical (λobs = 560 nm). The two spin states were directly observed using a long-lived singlet 2,2-dimethoxy-1,3-diphenylcyclopentane-1,3-diyl diradical.