Through-space energy transfer

An alternative approach is that adopted by Horrocks and co-workers, where the aromatic residues in metal-binding proteins are used as sensitizers. Since the distance between the metal and the donor is effectively fixed, this provides a rigid scaffold for the experiment, and the absence of a directly conjugated pathway between the metals means that Forster (through space) energy transfer can be assumed. The r-6 distance-dependence of this means that the extent of sensitized emission from the lanthanide ion provides information on the spatial relationship between the metal-ion binding site (lanthanide ions often bind at Ca2+ sites) and nearby aromatic residues. 58-60... 

Figure 3.3 Self-complementary hydrogen bonding to yield a helical structure (a) and butterfly-type motion of a bis(porphyrin)terpyridine complex to give a species capable of through-space energy transfer (b).

For such a through-space energy transfer process, a Dexter-type mechanism[29] seems more likely than a Forster-type mechanism[30] because of the triplet character of the excited states involved and the close approach of the two partners. 

Figure 2.32. Structures of spiro-bridged binuclear complexes displaying efficient through-space energy transfer.

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