Knowledge Bank

The decay of fluorescence of ""Intermediate case"" molecule can be described as the sum of two exponential components: $I(t)=Afaste-kfastt+Aslowe-kslowt$. We report here excitation frequency dependence of fluorescence decay $(Afast/Aslowandkslow)$ and quantum beats for three azabenzenes (pyrimidine, pyrazine and s-triazine). Comparison of the theory of the ""intermediate case"" radiationless decay with experiments indicates that rovibronic density based on the nuclear spin restriction (prohibiting coupling between singlet and triplet states with different nuclear spin symmetry) determines the number of effectively coupled triplet levels. The rotational state dependence of electronic relaxation is entirely consistent with the J dependence of the number of effectively coupled triplet levels.