Solvent dependence of pyrene monomer and excimer fluorescence quenching by heavy atoms

Two independent methods have been used for the determination of pyrene monomer (M*) and excimer (D*) quenching rate parameters in eight common solvents ranging in viscosities between 0.3 and 5.9 cP. Heavy atoms (CCl₄) were used as quenchers. The discrepancies between the values of the quenching rate parameters reported in the two methods have been explained as due to formation of complexes between the pyrene molecule and CCl₄ molecules as a result of using a more intense source, which might lead to larger values of the quenching rate parameters. Assuming that the quenching processes are diffusion-controlled ones, the probabilities of the effective collisions between M* or D* and CCl₄ (P_P or P_P, respectively) were then calculated. These values were found to depend on the nature of the solvents used and in particular on their viscosities (η). At low values of η, the average values of P_P and P_P are approximately the same for each solvent used. For intermediate values of η, P_P becomes much larger than P_P. Finally for relatively large values of η, P_P becomes much smaller than P_P.