Fluorescence resonance energy transfer decay constant

The maximum fluorescence quantum yield is 1.0 (100 %) every photon absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent. The fluorescence lifetime is an instance of exponential decay. Thus, it is similar to a first-order chemical reaction in which the first-order rate constant is the sum of all of the rates (a parallel kinetic model). Thus, the lifetime is related to the facility of the relaxation pathway. If the rate of spontaneous emission or any of the other rates are fast, the lifetime is short (for commonly used fluorescent compounds, typical excited state decay times for fluorescent compounds that emit photons with energies from the UV to near infrared are within the range of 0.5-20 ns). The fluorescence lifetime is an important parameter for practical applications of fluorescence such as fluorescence resonance energy transfer. There are several rules that deal with fluorescence. 

As long as r > 3R0, the fluorescence decay is close to exponential, the lifetime of the donor fluorescence decreases linearly with increasing concentration of A and fluorescence quenching obeys Stern Volmer kinetics (Section 3.9.8, Equation 3.36). However, the bimolecular rate constants ket of energy transfer derived from the observed quenching of donor fluorescence often exceed the rate constants of diffusion kd calculated by Equation 2.26, because resonance energy transfer does not require close contact between D and A. Finally, when r < 3R0, at high concentrations and low solvent viscosity, the kinetics of donor fluorescence become complicated, but an analysis is possible,109,110 if required. 

Time resolved fluorescence measurements have become an important tool in applied fluorescence spectroscopy. Recently, it has been pointed out that the controlled manipulation of fluorescence decay rates opens a new dimension in applied fluorescence spectroscopy. The fluorescence decay rate depends on two independent contributions, the pure rachative rate and the nonradiative rate. The latter one can be influenced by the well known Forster-type resonant energy transfer processes, while the radiative rate can be changed if the molecules are embedded or close to media comprising a dielectric constant markedly different from vacuum. Especially metal nanostructures have been used to alter both decay paths of fluorescent molecules. Apart from a change of those two rates, the absorption cross-section might also be altered. 

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