Summary of Photophysical Processes

We have discussed a number of factors that influence the transitions between various states in the Jablonski diagram, and it may be useful to summarize them here. We introduced the notion of allowed vs. forbidden transitions, but quickly added that these are not hard and fast rules, but rather guidelines for the relative efficiencies of various processes. 

The factor that most strongly influences allowedness of a transition is spin. Transitions between states with different spins, such as singlet and triplet, are very inefficient. Direct So to T1 absorption is rarely important, and the reverse emissive process, phosphorescence, occurs quite slowly. The heavy atom effect can relax this selection rule through a spin-orbit effect. As a result, S-T interconversions are much more facile in molecules that contain atoms such as Br or 1. The second major factor that influences the efficiency of transitions is the general spatial overlap of the wavefunctions for the two states. This term favors it,it transitions over ,ir transitions, for reasons discussed above. Another important factor is a general energy gap law. For processes such as intersystem crossing, the smaller the gap between the 

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