Rate measurements Stem-Volmer plots

Since both decay chatmels shown for the triplet encounter pair are multiplicity-allowed, a = 1 is expected for Eq. 20. Experimental rate constants for a large number of molecules have been obtained from Stem-Volmer plots of the effect of [O2] on the fluorescence intensity and by measuring the fluorescence lifetime in the absence and presence of O2 ... 

For charge transfer leading to a fluorescent exciplex, aU rate constants can be evaluated from the fluorescence decays, but particular attention should be paid to the possibility of occurrence of (1) transient effects, (2) the harpoon mechanism [70] (the electron goes first and then the exciplex is formed) and (3) ground-state charge-transfer complexes. All these phenomena lead to deviations from doubleexponential decays and/or differences between Stem-Volmer plots obtained from time-resolved (tq/t vs [Q]) and steady-state (Iq// vs [Q]) measurements. 

It is clear that, by changing the experimental conditions and/or detection wavelength, limiting values can be found for all of the quantities mentioned above from measurements of the fluorescence decay time. The effects of collisional and spontaneous processes can be separated by conventional Stem—Volmer analysis [36]. The concentration, [M], of quenching molecules is varied and the reciprocal of the observed lifetime is plotted against the concentration of M. The quenching rate coefficient is thus obtained from the slope and the intercept gives the rate coefficient for the spontaneous relaxation processes, which is usually the natural lifetime of the excited state. In cases where the experiment cannot be carried out under collision-free conditions, this is the only way to measure the natural lifetime from observation of the fluorescence decay. 

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