transfer (DET) occurs through a simultaneous double electron exchange mechanism between the PC* and the substrate in its ground state. For this to be operational, the excited-state energies between the donor (the PC) and the acceptor (the substrate) must be close as there must be spectral overlap between the emission of the PC and the absorption of the substrate; the difference in triplet energies (ET) of the PC and substrate are commonly used as a surrogate for spectral overlap to predict whether a transformation is likely to occur via DET (Equation ). ∆𝐸 9 = 𝐸 9 (𝑃𝐶) -𝐸 9 (𝐴) (7) The matching of excited-state energies is encompassed in the spectral overlap requirement of DET, that is the overlap of the phosphorescence of the PC and the spin-forbidden absorption of the substrate. Practically, spectral overlap in these cases is challenging to quantify on account of the spin-forbidden nature of the transitions, especially the low absorptivity of the triplet absorption of organic substrates. Additionally, DET also requires orbital overlap between the two species involved in the encounter complex. The quantitative correlation of these prerequisites with the rate constant of DET, kDET, is described by Equation .