Quenching mechanism fluorescence resonance energy transfer

Nonradiative relaxation from states can be effected via fluorescence resonance energy transfer. The rate of energy transfer in donor-acceptor pairs is directly related to their distance, which can of course be affected by analyte complexation. Excitation energy transfer leads to quenching of the donor, but also to enhancement of the acceptor fluorescence therefore, the ratio of fluorescence intensities at donor and acceptor emission wavelengths provides information on complexation via changes in donor-acceptor distance. The chapters by Valeur, Bouas-Laurent, and Krafft describe uses of the energy transfer mechanism. 

Quenching of the luminescence of the QDs by an electron transfer (ET) mechanism, or by a fluorescence resonance energy transfer (FRET) route, could reflect the formation of a recognition pair, or may be used to follow the dynamics of chemical processes that occur on the modified QDs. In these systems, the sensing events or the respective biocatalytic transformations involve the association/ dissociation of quencher/FRETacceptor units that electronically couple with the QDs. 

The fluorescence resonance energy transfer (FRET), also called noiuadiative excitation energy transfer (NRET) or direct energy transfer (DET), is one of the processes that quench the fluorescence of an excited fluorophore. In contrast to collision quenching, the excitation energy of the donor is transferred to another molecule (acceptor) over nanometer distances and the underlying mechanism does... 

Supporting evidence for the resonance mechanism comes from experiments in which the solvent has been varied. The predicted proportionality of the rate constant to the overlap Integral J has been directly shown in a study of a donor-acceptor pair (both steroids) in a series of solvents in which J varied 40-fold (Figure 6.19) [40]. The effect of solvent viscosity has also been studied. Since energy transfer by the resonance mechanism does not require collision, the rate would be expected to be independent of solvent viscosity. This has been verified for the quenching by perylene of fluorescence emission from excited 1-chloroanthracene [41] where the rate constant, which is about 2 x lO" M s (about ten times kp) in benzene, is no lower in liquid paraffin whose viscosity is around 100 times larger, nor even in a glass at 90 K. 

When the first formed excited state is quenched by a fluorescent molecule present in solution, with radiation from the acceptor, sensitised chemiluminescence results. This form of emission is most easily seen when the transfer of energy takes place between singlet states (i. e. by the resonance or Forster mechanism... 

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