No abstract

The fluorescence characteristics of pyrene solutions in seven different solvents were studied at temperatures from – 100 to 90 °C. Observations were made of the relative quantum yields of excimer and monomer fluorescence and of the fluorescence spectra and time characteristics. The data, which were analyzed to determine the rate parameters and their associated activation energies, are consistent with an excimer formation process which is diffusion-controlled and reversible. The pyrene excimer binding energy B = 0·40 (± 0·01) eV and its radiative transition probability k f D = 1·2 x 10 7 s -1 are independent of the solvent and of the temperature. This indicates that the excimer has a stable steric configuration.

Observations were made of the fluorescence spectra of cyclohexane and n -hexane solutions of benzene, toluene, p -xylene and mesitylene and of the pure liquids as a function of concentration and temperature under standard excitation conditions. Excimer fluorescence is observed in benzene, toluene and mesitylene, and there is evidence for excimer formation in p -xylene. The data are analysed to determine the fluorescence quantum efficiencies, the rate parameters and other properties of the monomer-excimer systems. The following values are obtained for the excimer binding energy B ; benzene, 0.22 eV; toluene, 0.17 eV; p -xylene, > 0.11 eV; and mesitylene, 0.12 eV. The results are consistent with the excimer potential model, and the reduction in B in the methyl derivatives is attributed to steric hindrance.

No abstract

Observations of the fluorescence lifetimes and quantum yields of toluene solutions in n-hexane from -85 'c to 50 'C have been analysed to evaluate the molecular and excimer fluorescence and internal quenching rate parameters, their frequency factors and activation energies and other related parameters. An excimer binding energy of 0.29 ev is obtained. Excimer (D*) formation from singlet-excited molecules (M*) M * + M + D * in fluid solutions of molar concentration [MI is described by the following parameters (Birks et al. 1963, 1965):