Appropriate excitation wavelength removes obfuscations from pyrene excimer kinetics and mechanism studies

Abstract

The experimental procedures of time-resolved laser photolysis studies of pyrene excimer formation in solution have been scrutinized and the appropriate modifications have been implemented. Contrary to the experimental methods applied in all related previous work, the selection of a suitable excitation wavelength (such that the corresponding pyrene absorbance is less than 0.5 absorbance units) utilized in our study results in simple homogeneous kinetics. Consequently, the rate parameters obtained and the mechanism proposed differ significantly from those published previously. The rate constant values of the unimolecular decay of the pyrene monomer, the unimolecular decay of the pyrene excimer, and the excimer formation in decane solution (η = 0.860 mPa s) at 25 °C are (2.38 ± 0.01) × 10⁶ s⁻¹, (2.78 ± 0.02) × 10⁷ s⁻¹, and (3.11 ± 0.06) × 10⁹ M⁻¹ s⁻¹, respectively. The dissociation of the excimer to form a singlet excited state pyrene and a ground state pyrene was shown to be negligible. The energies of activation corresponding to the monomer and excimer unimolecular decays were found to be 2.51 ± 0.07 and 25.7 ± 0.7 kJ mol⁻¹, respectively. Also, our temperature resolved laser photolysis data revealed that the excimer formation has a negative energy of activation equal to −11.2 ± 0.5 kJ mol⁻¹. This unique phenomenon may be attributed to steric effects in the collision of the reactants. The current findings are important for the correct data analysis and interpretation in many applications of the pyrene excimer.