Energy Transfer I. The Dexter Mechanism— Sensitization

We ve seen that bimolecular processes involving excited states can take many forms. Collision can facilitate relaxation to the ground state (quenching) or formation of an excited state complex (exciplex or excimer). Alternatively, bimolecular association can occur prior to excitation, leading to an absorption complex. In this and the next two sections we consider a new outcome for the interactions of an excited state, D, with another molecule, A (Eq. 16.8). Now the result is energy transfer from one molecule to another, producing electronically excited A (A ). Different mechanisms are possible, and these energy transfer processes are very important in photochemistry and other fields. 

The Dexter or electron exchange mechanism of energy transfer. 

As discussed in the text, this is a highly schematic diagram. 

As such, tlie efficiency of electron exchange energy transfer falls off steeply as the separation between D and A increases. 

When D is used to produce A through energy transfer, the process is known as sensitization. Compound D is a sensitizer used to produce the triplet state of A. This can be a very useful process. As discussed above, there is considerable variation in the efficiency of ISC for different molecules. For many compounds, excitation does not produce any significant quantity of Ti, because other processes such as fluorescence are too fast. But what if we want to investigate the photochemistry of the triplet state of such a system As we will see later in this chapter, singlet and triplet excited states often lead to quite different photoproducts, and perhaps the products we want are those expected from triplet photochemistry. This is where sensitization comes in handy. We begin with a molecule with a very efficient ISC, such as benzophenone. We excite the benzophenone (the sensitizer), and allow it to collide with the molecule whose triplet state we wish to produce. Energy transfer occurs, and the desired triplet state is produced. This is a common and very useful technique in photochemistry. 

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