THE ROLE OF TIME-DEPENDENT MEASUREMENTS IN ELUCIDATING STATIC VERSUS DYNAMIC QUENCHING PROCESSES

Fluorescence quenching provides a unique method for assessing the abil ity of quenching species to approach a fluorophore, The distance scale depends on the mechanism of quenching (e.g. compare Forster energy tr ansfer with the external heavy atom effect and/or electron transfer). If one is dealing with amphiphilic systems or hydrophobic surface/wate r interfaces then ionic and organic quenchers can be compared. While t he general concepts of ''static'' and ''dynamic'' quenching are well k nown and often used to characterize the mechanism of fluorescence quen ching, it is this author's experience that for many systems the quench ing mechanism is ''mixed.'' The signature of this situation is when th e quenching of the steady-state intensity does not match the quenching of the average fluorescence lifetime. First a few of the standard que nching 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 times cale should be undertaken if one wishes to estimate accurately the rat e 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 con cepts would apply to phosphorescence from triplet states, except for t he generally longer timescale encountered.