M. Woeller et al., A theoretical study of the radiationless decay mechanism of cyclic alkenesin the lowest triplet state, J PHYS CH A, 104(22), 2000, pp. 5366-5373
The radiationless decay mechanisms of cyclic alkenes CnH2n-2 (n = 4, 5, 6),
norbornene, their phenyl derivatives, 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 groun
d (So) and lowest triplet state (T-1) have been explored along double bond
twisting and anti pyramidalization reaction pathways to explain the experim
entally observed inverse proportionality bt tween ring size and triplet-sta
te lifetime. The calculations for the transition probabilities between T-1
and S-0 states are based on Fermi's golden rule including spin-orbit coupli
ng (SOC) constants. According to the older "free-rotor model", the hindered
twist around the double bond in small ring alkenes has been assumed so far
to be the main factor determining the T-1-state lifetimes. All computation
al results show, however, that only a combined reaction coordinate of anti
pyramidalization and twisting at the double bond provides a low-energy path
way which reproduces the experimentally 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 SO
C constants (FC-controlled mechanism). On the basis of the theoretical mode
l, the effect of substitution of vinylic hydrogen atoms by phenyl groups is
discussed.