Electronic energy triplet state

Energy transfer from a donor that is in an electronically excited triplet state to produce an electronically excited acceptor which is also in its triplet state. 

While the possibility of a very rapid electron reorganization from a low energy triplet state is ruled out by the observed insensitivity of the photo-Fries rearrangement to paramagnetic quenchers (vide supra) and the usual rate of intersystem crossing (kST = 10s—1011 sec-1 43a), the remaining alternative to singlet processes for both Path A and Path B, i.e., reactions from... 

Biradical (Synonymous with diradical) An even-electron molecular entity with two (possibly delocalized) radical centres which act nearly independently of each other. Species in which the two radical centres interact significantly are often referred to as biradicaloids. If the two radical centres are located on the same atom, they always interact strongly, and such species are called carbenes, nitrenes, etc. The low-est-energy triplet state of a biradical lies below or at most only a little above its lowest singlet state (usually judged relative to kT, the product of the Boltzmann constant k and the absolute temperature T). The states of those biradicals whose radical centres interact particularly weakly are most easily understood in terms of a pair of local doublets. Theoretical descriptions of low-energy states of a biradical display the presence of two unsaturated valences (biradicals contain one fewer... 

In some molecules, electrons in the singlet excited state may undergo a process known as intersystem crossing to form a lower energy triplet state in which the spin state of the electron becomes reversed (Figure 2.29). 

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