Fluorescence Forster mechanism

The Forster mechanism for non-tiivial transfer of excitation energy is well estabhshed 123), Transfer may occur over distances as large as 100 A or more, provided there is sufficient overlap between the emission band of the donor and absorption band of the acceptor. Weber showed by means of fluorescence polarization studies that transfer between phenol molecules and between indole molecules as well as phenol- indole transfer occurs over distances of A 124.125), ePR studies of frozen solutions indicated that energy transfer occurs between tyrosine and tT3q)tophan over distances of this order or larger .186),... 

UV-visible titrations confirmed that in the experimental conditions of the photophysical experiments, for which concentrations of free base are in the low micromolar range, 92% of the zinc dimers were chelated around pyrazines in the assembled structures of 86, 87, and 88. Upon excitation at 564 nm, fluorescence occurred primarily from the central free base energy acceptor, due to energy transfer according to the Forster mechanism. Array 86 shows a 77-fold increase of the free base emission compared to the individual free base component (where R = PY2) with the same number of energy collecting zinc dimers located at a longer distance, array 87 shows only a 20-fold increase of the free base emission. 

From Table 13.11, the variation of the lifetimes and fluorescence quantum yields in the series of compounds shows the clear increase of homo-chromophore interactions in the excited states when the distance between the chromophores diminishes. The rate and efficiency of the energy transfer in hetero-dimers does not seem to be metal dependent. The distance dependence of the energy transfer rate has been analyzed using Forster and Dexter theories. Harvey and Guilard have established that in 135-Zn-H2 and 136-Zn-H2, energy transfer is dominated by a Forster mechanism, while in the case of hetero-dimers 137, 138, and 139, it proceeds mainly via a Dexter mechanism. The critical distance at which the Dexter mechanism becomes inoperative is estimated between 5 and 6 By analogy with what has been discussed earlier in the case of linearly arranged covalent dimers, it should be noted that for compounds 135-139, no electron density should be present on the meso carbons involved in the covalent connection to the spacer. 

Meijer and coworkers have synthesized a family of poly(propylene imine) dendrimers of first, third, and fifth generations which contain 4, 16, and 64 Pfb moieties, at the periphery, respectively (Figure 16). Fluorescence anisotropy values led to mechanistic models and equations to describe the energy transfer processes. The models take into account the different morphologies of the dendrimers the disk-like nature of [Gl] and the spherical natures of [G3] and [G5]. In the [Gl] dendrimer, the Forster mechanism is sufficient to explain the energy transfer. Low... 

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