Significance of Excitation Wavelength in Pyrene Excimer Kinetics and Mechanism Studies
Open Access
- Author:
- Hanlon, Andrew Dennis
- Area of Honors:
- Chemistry
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Bratoljub Milosavljevic, Thesis Supervisor
Przemyslaw Maslak, Thesis Honors Advisor
John B Asbury, Faculty Reader - Keywords:
- Excimer
Excitation Wavelength
Photophysics
Photochemistry
Pyrene - Abstract:
- The effect of excitation wavelength on the observed kinetics of photochemical reactions was scrutinized. We examined the relation between the excitation wavelength and the spatial distribution of excited molecules (reactants) in the reaction cell and the relation between that spatial distribution of excited molecules and the observed kinetics. These effects are considered in the context of transient emission and transient absorption pulse laser photolysis. In particular, the experimental procedures of time-resolved laser photolysis studies of pyrene excimer formation in solution have been modified. Contrary to the experimental methods applied in all related previous works, 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 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 ºC are (2.38 ± 0.01) x 106 s-1, (2.78 ± 0.02) x 107 s-1, and (3.11 ± 0.06) x 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, 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. 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. This work was published in part in Photochem. Photobio. Sci. 2013, 12, 787.