NON-A-PRIORI ANALYSIS OF FLUORESCENCE DECAY SURFACES OF EXCITED-STATE PROCESSES .3. INTERMOLECULAR EXCIMER FORMATION OF PYRENE QUENCHED BY IODOMETHANE

Abstract

The global compartmental approach to analyze the kinetics of excited-state processes is tested on simulated and real fluorescence data of the excimer formation of pyrene quenched by CH3I in cyclohexane. An identifiability study is presented to investigate the uniqueness of the set of estimated rate constants. It is shown that all the rate constants can be uniquely determined from the relaxation times in the fluorescence decays of four well-chosen sample preparations. The results of the identifiability study are evaluated by applying the global compartmental analysis on fluorescence decay data of the excimer formation of pyrene quenched by iodomethane in cyclohexane. The following rate constants are obtained at room temperature: k01 = 2.27 x 10(6) s-1, k21 = 6.7 x 10(9) s-1 M-1, k(Q1) = 110 x 10(6) s-1 M-1, k02 = 17.1 x 10(6) s-1, k12 = 3.42 x 10(6) s-1, and k(Q2) = 27.9 x 10(6) s-1 M-1. The calculation of the species associated emission spectra (SAEMS) of the monomer and the excimer demonstrates that the unsubstituted intermolecular pyrene excimer in cyclohexane is emitting in the monomer emission band at 375 nm. The absorbance of the monomer normalized to the total absorbance deviates from unity at pyrene concentrations higher than 2 x 10(-4) M, indicating the existence of ground-state aggregates between the pyrene molecules. The molar extinction coefficient for the dimer at 325 nm is about 2300 cm-1 M-1. The value for the equilibrium constant in the ground state between monomer and dimer is found to be 2450 M-1. The existence of ground-state aggregates is further demonstrated by the change of the ratio of fluorescence intensities at the monomer and excimer emission bands as a function of excitation wavelength.