Triplet-Energy or Electron Transfer between Molecules

Triplet-Energy (or Electron) Transfer between Molecules... 

The exchange mechanism is available also for triplet-triplet energy transfer between donor and acceptor molecules in contact [37]. It has been studied in rigid solvents by phosphorescence emission, and in fluid solvents by flash absorption spectrophotometry (see Section 4.4.3.2). The results [37,a] show that for exothermic processes the rate constant in fluid solvents approximates to the encounter-controlled value. There are cases where it is smaller, possibly because of an activation requirement, which may be expected if orbital interactions are involved, or if activated vibronic interactions are required to make the energy transfer electronically feasible, or if steric factors prevent favourable electronic interactions. Another possibility is that in solvents of low viscosity the collision complex may dissociate before there has been enough time for the electron-exchange to occur this would account for the case of valerophenone -f- 2,5-dimethyl-2,4-hexadiene, where in a series of solvents k /ko falls well below unity as the viscosity is decreased [37,c]. 

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