One major class of photochemical reactions are photoinduced energy transfer (PEnT) reactions, which typically involves an electronically excited photocatalyst (PC*) transferring its energy to a substrate (sub). The substrate in its electronic excited state (sub*) then undergoes a photochemical transformation. PEnT reactions can be classified into two main types: Förster Resonance Energy Transfer reactions (FRET) and Dexter Energy Transfer reactions (DET). In FRET, the energy transfer proceeds through a dipole-dipole interaction between PC* and sub and occurs over distances on the order of 1-10 nm. The majority of PEnT FRET reactions involves an energy transfer from the S1 state of the PC* and generates sub* in its S1 state. This requires a spectral overlap between the emission of the PC and the absorption of the sub. Most of the documented cases of FRET occur from the S1 state of the PC* to the sub, which is then excited to its S1 state; there are some reports of FRET occurring from the T1 of the PC to generate sub* in the singlet state. PEnT photochemical reactions proceeding by FRET, and more generally photochemical reactions involving substrates in their singlet excited state, are not nearly as common as those proceeding by DET to excite the substrate to its triplet excited state. DET proceeds via a simultaneous double electron exchange mechanism between the PC* and the sub to generate a PC in its S0 state and a sub*. This can take place from the singlet state from the 1 PC* to generate 1 sub* or more commonly from the triplet state of 3 PC* to generate sub* (Figure ) and does not involve a change of the overall multiplicity. DET processes occur over shorter distances, typically less than 10 Å, given the exponential dependence on the rate of DET with PC-sub distance. DET requires not only spectral overlap, like FRET, but also orbital overlap between the PC and the sub, which implies a collisional interaction is needed for DET to occur intermolecularly (Eq. 1).