In the case where DET occurs from the 3 PC* to sub, it is the spectral overlap of the phosphorescence of the PC* and the spin-forbidden absorption of the substrate that is relevant to generate sub*. Notably, as the spin-forbidden absorption spectrum is of negligible intensity in most organic substrates, the low energy onset, which is used as a surrogate estimation of the energy at which spectral overlap no longer occurs, is estimated from the onset of the low temperature phosphorescence spectrum, which is none other than the energy of the T1 state (ET) of the sub. It is generally accepted that DET will occur when ET(PC) > ET(sub), and the closer these two values are the more likely is the DET. Given that the sub* has biradical-like character, this enables a diversity of reactions such as E/Z isomerization, cycloadditions like [2+2], sensitization of metal complexes, and homolytic bond cleavages such as N2 release from benzoyl azides, scission of S -S bonds, and N -O dissociation of oxime esters to instigate the formation of carbon-centered and nitrogen centered radicals in concert with the loss of CO2. Figure . Simplified mechanism of the Förster Resonance Energy Transfer (FRET) and Dexter