The role of time-dependent measurements in elucidating static versus dynamic quenching processes

Fluorescence quenching provides a unique method for assessing the ability of quenching species to approach a fluorophore, The distance scale depends on the mechanism of quenching (e.g. compare Forster energy transfer with the external heavy atom effect and/or electron transfer). If one is dealing with amphiphilic systems or hydrophobic surface/water interfaces then ionic and organic quenchers can be compared. While the general concepts of ''static'' and ''dynamic'' quenching are well known and often used to characterize the mechanism of fluorescence quenching, it is this author's experience that for many systems the quenching mechanism is ''mixed.'' The signature of this situation is when the quenching of the steady-state intensity does not match the quenching of the average fluorescence lifetime. First a few of the standard quenching models are reviewed and then a simple approach to characterize the degree of static quenching is discussed, Finally, it is indicated that careful analysis of the fluorescence decay for the shortest timescale should be undertaken if one wishes to estimate accurately the rate of static quenching and that this may be expected to be particularly demanding in the case of fluorophores with longer lifetimes, Although fluorescence is stressed in the discussion that follows, the same concepts would apply to phosphorescence from triplet states, except for the generally longer timescale encountered.