Knowledge Bank

Some organic molecules in polymer matrices show enhanced fluorescence in the presence of molecular oxygen instead of the expected quenching. This phenomenon has been attributed to a singlet oxygen feedback mechanism in which there is cyclic repopulation of the singlet manifold of the organic molecule via energy transfer from singlet molecular $oxygen.1$ An alternate explanation is that the enhancement is the result of ground state $depopulation.2$ The relative merits of these two mechanisms will be considered. The results of equilibrium experiments which outline the general nature of the fluorescence enhancement and cross beam experiments which compare the amount of fluorescence enhancement with the amount of ground state depopulation will be discussed. The discovery of oxygen stimulated fluorescence under conditions in which the singlet manifold of the organic molecule can only be populated by energy transfer from singlet oxygen is reported. The results support the feedback mechanism.