The T<inf>1</inf>³(π - π*)/S<inf>0</inf> intersections and triplet lifetimes of cyclic α,β-enones

The ground and triplet excited states of cycloheptenone, cyclohexenone, and cyclopentenone have been studied using CASSCF calculations. For these three molecules, the difference in energy (ΔE) between the twisted T13(π - π*) minimum and T13(π - π*)/S0 intersection increases as the flexibility of the ring decreases. A strong positive correlation between ΔE and the natural logarithm of the experimentally determined triplet lifetimes (In τ) is found, suggesting that ΔE predominantly determines the relative radiationless decay rates of T1.