Triplet-Mediated Energy Transfer

There are other cases in which triplet-mediated energy transfer might compete with LMCT-mediated energy transfer, particularly where the metal ion is either europium or samarium. In these cases, the redox potentials of the metal ions are such that LMCT is feasible, although the LMCT state will be lower in energy than the emissive state of the metal, precluding luminescence.65... 

Beeby, A., Faulkner, S., Parker, D., and Williams, J.A.G. (2000) Sensitised luminescence from phenanthridine appended lanthanide complexes analysis of triplet mediated energy transfer processes in terbium, europium and neodymium complexes. Journal of the Chemical Society, Perkin Transactions, 2, 1268. 

Figure 5 Triplet-mediated ligand-to-metal energy transfer in lanthanide complexes.

The transformation of monuron photosensitized by fulvic acids (X = 365 nm) was shown to yield the same products as direct photolysis or sensitization by hydroquinone [82]. It was concluded that in this case, sensitization was mediated by triplet energy transfer in contrast, the presence of a humic acid induced an H atom transfer and the products were different [82]. The fulvic acid example shows that long-wavelength sensitization may in specific... 

Figure 2. Schematic representation oF electron, hole, and triplet energy transfer between the localized HOMO and LUMO orbitals of the donor and the acceptor. Note that the above diagram also applies to the exchange-mediated singlet excitation transfer if the spin of the electron in the LUMO orbital is reversed.

The time scale for triplet energy transfer in 31 and 30 mediated in this way would be expected to be too slow to provide complete protection from the sensitization of singlet oxygen (see Eq. 5). However, studies of DPBF bleaching sensitized by 30 in aerated toluene demonstrated that a remarkable level of photoprotection does in fact exist [28]. The rate of DPBF bleaching was only around 5% that observed in a solution of methyl pyro-pheophorbide-a alone. 

The static contribution to the electron exchange mediated triplet energy transfer was determined by measuring the triplet carotenoid rise time for each compound in a polystyrene glass at ambient temperatures, where large-scale intramolecular motions are precluded. As expected for the similar covalent... 

If the operational triplet energy level for excited states of humic substances Is Indeed around 57 kcal/mole, then pollutants which have triplet energy levels below this value are likely to photoreact by an energy transfer mechanism mediated by humic substances or DOM. If a redox potential of an environmentally reactive transient produced by humic substances Is higher than that of a xenoblotlc, then the xenoblotlc may be oxidized, etc. If the reaction Is also klnetlcally favorable. 

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