1965 Volume 14 Issue 3 Pages 193-198
The variation of fluorescence efficiency with concentration for the 2, 5-diphenyloxazole (DPO) and for the carbon tetrachloride in the toluene was studied under the aerated condition. The ratio of the light yield in absence of the quencher, I0, to that in its presence, IQ, at fixed solute concentration and at fixed irradiation intensity for direct excitation of the solute by 302 mμ UV was found to follow the relationship I0/IQ= (1+K2 [Q] ), where (Q) is the quencher concentration and K2 the quenching constant by CCl4 relating to the DPO (Fig. 2) ; the K2 value obtained was 6 liters/mole. Under indirect excitation of DPO via primary excitaion of the toluene by 260 mμ UV, the I0/IQ vs. [Q] curve was found to be described by the Stern-Volmer relationship I0/IQ= (1+K1 [Q] ) (1+K2 [Q] ), where K1 is the que nching constant relating to the solvent as shown in Fig. 2. Author obtained thefollowing values for zero concentration of DPO; K1=640 liters/mole and kq=2.5×1010 liters/mole. sec, where kq is the specific rate of quenching calculated from the relation kq=K1/τ', τ' being the decay time of the fluorescence in the quencher free toluene solution. The specific rate, kt of excitation transfer from toluene molecules to DPO molecules was determined from the observed relationship between fluorescence efficiency and DPO concentration and combined with other relevant data reported by other authors; kt=8.56×1010 liters/mole⋅sec.
The theoretical curves of the fluorescence efficiency vs. DPO concentration based on the energy-exchange processes given in Table 1 and the experimental values of rate constants mentioned above were found to fit satisfactorily the experimental values of fluoresence except the low concentration range (Fig. 6) . The disagreement seems to be explained by the assumption that the solvent emission is absorbed by the solute molecules, giving solute fluorescence with quantum efficiency of about unity. The validity of the assumption should, however, be checked experimentally. The quenching constant K1 under 60Co-γ ray excitation was larger by the factor of 2.2 than that under 260 mμ UV excitation. Similar experiments, although not so extensive as the case of DPO mentioned above, were carried out for p-terphenyl and their results have been also presented in the text.