Forster through-space

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... 

The energy transfer processes can occur by two mechanisms the Forster-type mechanism (through-space) [55], based on coulombic interactions, and the Dexter-type mechanism (through-bond) [56], based on exchange interactions. The energy transfer rate constants according to the Forster and Dexter treatments can be evaluated by Eqs. (4) [55] and (5) [56], respectively ... 

Frechet and coworkers [85,86] synthesized a series of dendrimers whose energy transfer mechanism is exclusively through-space. By designing dendrimers in which the donor periphery chromophores are effectively separated from the interior acceptors, the dendrimer architecture becomes simply a structural scaffold upon which chromophores can be placed. Chromophores were carefully chosen to satisfy the requirements of FOrster energy transfer (i.e. emission of donor overlaps absorbance of acceptor), so that any photons absorbed by a molecule on the periphery undergo intramolecular singlet energy transfer to the core moiety and emission from that core ensues. 

The energy is transferred through space via dipolar interactions to the acceptor and can occur over rather long distances of up to several nanometers. Consequently, in the Forster mechanism, for a D-A dyad in a given medium, the rate of energy transfer will be dependent on the distance r separating D and A, and is expressed as... 

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. 

As mentioned earlier, triplet energy transfer can occur by way of Forster-type through-space interactions [100] or via Dexter-type electron exchange [101], In the former case, the rate constant for Forster-type triplet energy transfer (kp) can be expressed as follows ... 

Luminescence resonance energy transfer (LRET) proceeds by radiationless dipole-dipole coupled energy transfer from an excited luminophore through space to another lumino-phore [30]. There are several criteria that must be met for LRET. First, the emission band of the excited luminophore donor must overlap the absorbance band of the acceptor luminophore. The distance between the donor and acceptor molecules can be measured by using Equation 8.2 [31]. In Equation 8.2, the efficiency of energy transfer is E, tda is the time-resolved luminescence lifetime of the donor-acceptor pair, tq is the lifetime of the donor, Rq is the distance for 50% energy transfer to occur (or Forster distance), and r is the calculated distance between the donor and acceptor. 

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