Phosphorescence in Aromatic Molecules with Nonbonding Electrons

Phosphorescence in Aromatic Molecules with Nonbonding Electrons 

In this section, we will apply the knowledge gained in Chapter 3 to the analysis of triplet state relaxation in aromatic molecules with nonbonding electrons. We have chosen diazaphenantrenes (DAPs) as examples for our consideration because they are representative of an important class of organic compounds for which the decay 

The electronic Hamiltonian of the system corresponding to the Ti So radiative transition involving interaction may be written as 

Triplet-singlet transitions are accompanied by emission of radiation. The probability is given by 

For notational convenience, we have set s(qo) = and so on. The indices a = X, y, z or -1,0,1 correspond to triplet sublevels by spin projection. At temperatures r 77 K, it can be treated as a threefold degenerate state since transitions between the three levels causing spin relaxation are very rapid and their spacing is small compared to ksT. At very low temperatures (T 1 K), spin relaxation is slow and the states behave independently. The summation in Equation 7.28 is taken over all singlet virtual states. This term represents the transition induced by direct spin-orbit coupling between T and So. The second term in (7.27) is given by 

---