Triplet-to-singlet energy transfer

Furthermore, exchange transfer is also less sensitive to changes in spin multiplicity than resonance transfer. Therefore, it is the mechanism responsible for the triplet-to-singlet energy transfer in the spectral sensitization processes of organic photochemistry, as seen in organic resist materials. 

Although essentially any acceptor triplet reaction can potentially be used for determining 18C of the donor if its quantum yield is accurately known, some precautions must be observed in order to assure success. First, the lowest excited singlet level of the acceptor should be above that of the donor to eliminate the possibility of singlet energy transfer. Second, the acceptor should be chosen such that the donor molecules absorb all of the incident light. 

Since the photon emitted by D is absorbed by A, the same rules will apply to radiative energy transfer as to the intensity of absorption. Because singlet-triplet transitions are spin-forbidden and singlet-triplet absorption coefficients are usually extremely small, it is not possible to build up a triplet state population by radiative energy transfer. For this... 

D 1A —- 1D + 1 A (triplet-singlet energy transfer). This type of transfer leads to phosphorescence quenching of the donor (e.g. phenanthrene (T) + rhod-amine B (S)). 

The heterometallic system [(bpy)2Ru(234)] exhibits several intramolecular energy-transfer processes (i) ultrafast singlet-to-singlet transfer, (ii) fast triplet-to-singlet transfer and (iii) singlet-to-triplet transfer. Excitation into the Ru(bpy)3 " " domain is followed by rapid energy transfer to the triplet state of the Zn(porphyrin) fragment. There is no evidence for intramolecular electron transfer between the Ru(bpy)3 and Zn(porphyrin) units. 

In a careful study of cyclopentenone photocycloaddition reaction, DeMayo and coworkers215 have noted that ketone sensitizers of triplet energy less than 71.2 kcal did not sensitize cycloaddition to cyclohexene. Triphenylene (Et = 66.6 kcal) and acenaphthene (ET = 59.3 kcal) were exceptional since they resulted in quantum yields of 0.10 and 0.21, respectively. This behavior, as well as the fact that 0.1 M cyclopentenone quenched acenaphthene fluorescence by 90% in an EPA (ether-isopentane-alcohol) glass at 77°K strongly implicate singlet energy transfer. 

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