Luminescence Quenching Kinetics and Radiative Lifetimes

Bimolecular deactivation (pathway vii, Fig. 1) of electronically excited species can compete with the other pathways available for decay of the energy, including emission of luminescent radiation. Quenching of this kind thus reduces the intensity of fluorescence or phosphorescence. Considerable information about the efficiencies of radiative and radiationless processes can be obtained from a study of the kinetic dependence of emission intensity on concentrations of emitting and quenching species. The intensity of emission corresponds closely to the quantum yield, a concept explored in Sect. 7. In the present section we shall concentrate on the kinetic aspects, and first consider the application of stationary-state methods to fluorescence (or phosphorescence) quenching, and then discuss the lifetimes of luminescent emission under nonstationary conditions. 

Observable effects in the quenching of fluorescence are usually the result of competition between radiation and bimolecular collisional deactivation of electronic energy, since vibrational relaxation is normally so rapid, especially in condensed phases, that emission derives almost entirely from the ground vibrational level of the upper electronic state. The simplest excitation-deactivation scheme, which does not allow for intramolecular radiationless 

Solution of the steady-state equations for [X ] (i.e. with d[X ]/df = 0) provides an expression for the luminescence emission intensity, Iium, in terms of the intensity of absorbed radiation, iabs, where A is the Einstein coefficient for spontaneous emission  

Equation 24 can be inverted to give the Stern-Volmer relation 

therefore, l/iium is plotted as a function of [M], the ratio of slope to intercept provides a value of kq/A, even if Iium is measured in arbitrary units and Jabs is not determined. Thus, if the Einstein A factor is known, or can be measured, the value of the quenching rate constant can be calculated. The A factor can be calculated from the B factor by use of the v3 relationship presented as Eq. 9 (and B itself can be calculated from the measured integrated extinction coefficient for the absorption band, as implied by Eq. 15). It is also possible, under suitable conditions, to measure A directly by observation of the decay of emission after suddenly extinguishing the illuminating beam. As will be explained at the end of this section, the fluorescence or phosphorescence lifetime may be shorter than the natural radiative lifetime as a result of intermolecular and intramolecular nonradiative energy degradation, so that due care must be taken in the interpretation of emission decay measurements. 

---