excitation transfer occurring as a result of an electron exchange mechanism. It requires an overlap of the wavefunctions of the energy donor and the energy acceptor. It is the dominant mechanism in triplet-triplet energy transfer. The transfer rate constant, kET, is given by: \[k_{\text{ET}}\propto \frac{h}{2\ \pi }\ P^{2}\ J\ \text{e}^{\frac{-2\ r}{L}}\] where r is the distance between donor (D) and acceptor (A), L and P are constants not easily related to experimentally determinable quantities, and J is the spectral overlap integral. For this mechanism the spin conservation rules are obeyed.