FOrster energy-transfer mechanism

The self-assembled diad Zn P-PH2P consisting of a zinc porphyrin donor and a free base porphyrin acceptor (Scheme 7.4) was studied by time-resolved fluorescence [21]. The driving force of the assembly is the site selective binding of an imidazole connected to a free base porphyrin. Evidence for Forster back transfer was obtained from the analysis of the fluorescence decay (Fig. 7.8) and the relevant rate was quantitatively evaluated for the first time. The transfer efficiency [13] is 0.98, and the rate constants for direct and back transfer were found to be 24.4 x 10 s and 0.6 X 10 s respectively. These values are consistent with the Forster energy transfer mechanism. 

The stoichiometry of ai-acid glycoprotein - hemin complex (1 1) indicates that the hemin binds to a specific site. Displacement of TNS by hemin gives the nature of this site a hydrophobic one. Therefore, hemin does not bind to the surface of ai-acid glycoprotein, a hydrophilic area. It binds to a hydrophobic domain of the pocket of the protein inducing a decrease of the fluorescence intensity of Trp residues of the protein via a Forster energy transfer mechanism. 

The rate constant of energy transfer kj between the donor molecule and the acceptor molecule in the Forster energy transfer mechanism can be described by the following equation ... 

The first polymer acts as donor and the second polymer acts as acceptor. The two polymers show a strong overlap between the donor emission spectrum and the acceptor absorption spectrum, both in solution and films. The emission decay of neat PVK is much slower than that of the acceptor, which indicates a non-radiative energy transfer process. The steady-state photoluminescence spectra of PVK exhibit an intensity decrease in the presence of the donor, however, the decrease in the PVK lifetime does not follow the same trend upon increasing the donor concentration. Therefore, it has been assumed that the intensity decrease is more strongly correlated with the trivial energy transfer than with a Forster energy transfer mechanism [92]. 

The fluorescence for adsorbed molecules was apparently quenched due to the reduced lifetime of the excited state in a molecule adsorbed at the metal surface as discussed earlier. Quenching due to the energy transfer to the metal can also be observed in molecules that are desorbed but reside in the electrolyte layer near the electrode surface. According to the so-called Forster energy transfer mechanism, observed in the membrane studies [35,40] and in Langmuir-Blodgett films [41], the change of the fluorescence intensity with separation of the fluorescent molecule from the quencher (metal) is described by the formula... 

In the particular case of conjugated polymers, the situation can be even more complex due to the presence of both Dexter and Forster energy transfer mechanisms (the former being dominant at short distances), and the possibility of interchain energy transfer steps these are favoured in polymer solid films due to a closer proximity between chains. 

Another important aspect that needs attention is the optical behavior of a chromophore near a metal surface. The loss in fluorescence intensity by Forster energy transfer mechanisms operating for chromophores in the immediate vicinity of the metal substrate has to be balanced against the decrease in the excitation probability for higher separation distances governed by the exponential decay of the evanescent surface mode normal to the metal substrate. ... 

The somewhat complex behavior is summarized in Figure 7 in a simplified scheme that contains, however, the essentials that are relevant for SPFS. Three different distance regimes are important. For dye molecules very close to the metal surface (Figure 7(a)) the classical FOrster energy transfer mechanism operating between an excited chromophore as the donor molecule and the metal substrate as the acceptor system leads to a dramatic loss in emission probability and, hence, decrease in fluorescence light intensity. In the... 

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