On the use of dynamic fluorescence measurements to determine equilibrium and kinetic constants. The inclusion of pyrene in β-cyclodextrin cavities

An analysis of the kinetic identifiability of two-state excited-state processes goves the conditions which have to be fulfilled to make it possible to estimate the ground-state equilibrium constant from dynamic fluorescence data. For the aqueous system β-cyclodextrin:pyrene it turns out that the only kinetic parameters which can be estimated are (i) the deactivation rate constant of pyrene dissolved in the aqeuous mbulk, (ii) the rate of formation of a β-cyclodextrin:pyrene inclusion complex in the excited-state, which is negligibly slow, and (iii) the sum of the rate constats for deactivation to the ground-state and for exclusion into the aqueous bulk of the excited pyrene participating in inclusion complex formation. This sum cannot be separated into its individual rate constant contributions, and it is impossible to determine the ground-state equilibrium constant for the formation of β-cyclodextrin:pyrene inclusion complexes solely from fluorescence decay data, a fact not taken into account in the literature.