| Abstract
| - The radiationless decay mechanisms of cyclic alkenes CnH2n-2 (n = 4, 5, 6), norbornene, their phenylderivatives, and styrene in their lowest triplet state have been investigated by unrestricted density functional,ab initio CASSCF, and MRD-CI calculations. The potential energy surfaces of the ground (S0) and lowesttriplet state (T1) have been explored along double bond twisting and anti pyramidalization reaction pathwaysto explain the experimentally observed inverse proportionality between ring size and triplet-state lifetime.The calculations for the transition probabilities between T1 and S0 states are based on Fermi's golden ruleincluding spin−orbit coupling (SOC) constants. According to the older “free-rotor model”, the hindered twistaround the double bond in small ring alkenes has been assumed so far to be the main factor determining theT1-state lifetimes. All computational results show, however, that only a combined reaction coordinate of antipyramidalization and twisting at the double bond provides a low-energy pathway which reproduces theexperimentally observed transition probabilities. For the relative transition rates, the different Franck−Condon(FC) factors in the series of compounds are found to be much more important than the SOC constants (FC-controlled mechanism). On the basis of the theoretical model, the effect of substitution of vinylic hydrogenatoms by phenyl groups is discussed.
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