PHASE FLUOROMETRY AS A PROBE OF DIFFUSION-CONTROLLED MOLECULAR ENCOUNTERS IN DENSE FLUIDS

The Wilemski-Fixman model for diffusion-controlled molecular encounters in dense fluids is included as one step in a mechanism governing fluorescence quenching. The mechanism is analyzed by mathematics appropriate to a phase fluorometry experiment. As customary, time varying parts of the excitation and fluorescence are found to differ from one another by an amplitude and a phase. Expressions for this amplitude and phase are given in terms of the frequency of modulation of the excitation, the lifetime of the excited state, the diffusion coefficients and radii of the fluorophore and quencher, respectively, and the specific rate of collisions between fluorophore and quencher. There is obtained as a by-product of this analysis an expression for the Stern-Volmer coefficient in terms of the same set of variables. We analyze in some detail a combination of Stern-Volmer and phase fluorometry experiments which can be used to test the Wilemski-Fixman model. © 1979 American Institute of Physics.