Singlet-triplet transitions external heavy atom effect

Frosch(84,133) have explained the external heavy-atom effect in intersystem crossing by postulating that the singlet and triplet states of the solute, which cannot interact directly, couple with the solvent singlet and triplet states, which themselves are strongly coupled through spin-orbit interaction. Thus the transition integral becomes<134)... 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

The intensity of singlet-triplet transitions can be increased by the external and internal heavy-atom effect. It has been noticed by Kearns 8) that the Tn.n - So transitions were enhanced by a factor of about 2 on passing from an ordinary low-temperature glass, such as a 2 1 1 mixture of ether, ethanol, and toluene, to a heavy-atom glass, such as a 2 2 1 1 mixture of ethyl iodide, ether, ethanol, and toluene. 

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