Energy transfer Forster radius

The sixth power dependence explains why resonance energy transfer is most sensitive to the donor-acceptor distance when this distance is comparable to the Forster critical radius. 

Here the chromophores can be farther apart (10-100 A long-range interactions) since this mechanism does not require any overlapping of orbitals. The Forster radius designates the distance between donor and acceptor at which the efficiency of energy transfer amounts to exactly 50%. Half of the excited donor molecules are then deactivated by fluorescence resonance energy transfer, and the other 50% by fluorescence or phosphorescence. 

In these multichromophoric cyclodextrins the fluorophores are randomly oriented. Excitation of one of the naphthoate fluorophores is followed by efficient dipole-dipole excitation energy transfer between the seven fluorophores, with a Forster radius of 14 A. This process is not detectable by fluorescence intensity measurements, as neither the intensity nor the decay law are affected by energy transfer between identical fluorophores (also called homotransfer. The dynamics of energy hopping are on the other hand reflected in the fluorescence anisotropy. To avoid depolarization by rotational motion of the fluorophores, experiments were conducted in a low temperature and optically clear rigid glass (9 1 ethanol-methanol at 110 K). 

In subsection (B) we will discuss the results with the anthracene energy trap more extensively, from the point of view of intrachain energy migration. This system is more difficult to analyze with simple models because of the large Forster radius for naphthalene to anthracene single-step energy transfer (25A ) ... 

It is useful in discussion of weak coupling between nanostructures to remember the nonradiative mechanism of Forster resonant energy transfer from an excited molecule (a donor) to some other molecule (an acceptor) which can be in the ground or in an excited state. The probability of such a transfer is determined by the Coulomb nonretarded (instantaneous) dipole-dipole interaction between molecules and is proportional to Rp/R6 where Rp is the Forster radius and R is the distance between molecules. For organic materials the Forster radius is usually about several nanometers and strongly depends on the overlapping... 

The real disadvantage of the Forster energy transfer is that it is proportional to r. Therefore, the decrease of the energy transfer as a function of distance occurs so rapidly that only two cases can be discerned the distance is either smaller or larger than the Forster radius. However, the measurement of the real distance between the photodonor and acceptor is virtually impossible. A typical Forster radius is 5 + 1 nm. 

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