Electron-Transfer and Heavy-Atom Quenching

According to the following scheme, electron transfer, intersystem crossing and energy transfer can all compete with fluorescence in deactivating an exciplex (DA) formed from the molecules A and D. 

In polar solvents, polar exciplexes (contact ion pairs) dissociate into non-fluorescent radical ions (loose ion pairs or free ions) due to the stabilization of the separated ions by solvation. It has been observed that exciplex emission decreases with increasing polarity of the solvent and that at the same 

Assuming the existence of a quasi-stationary state the rate constant of an exothermic electron-transfer reaction can be written as 

The temperature dependence of electron-transfer rate constants is interesting. In the normal region, it shows an activation energy as predicted from simple Marcus theory. In the inverted region, the activation energy is very small or zero. This agrees with the quantum mechanical version of the theory (Kestner et al., 1974 Fischer and Van Duyne, 1977), which makes it clear that the transition from the upper to the lower surface behaves just like ordinary internal conversion. 

Studies of the fluorescence quenching in acetonitrile have shown that the electron-transfer reaction 

---