Molecular oxygen enhanced fluorescence of organic molecules in polymer matrices: A singlet oxygen feedback mechanism

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Abstract
The surprising observation that molecular oxygen, the universal quencher of luminescence of organic molecules, can reverse its role and act as an enhancer of fluorescence in polymer matrices was originally recorded by Geacintov et al. Bolton, Kenner, and Khan investigated the phenomenon of oxygen enhanced fluoresence of organic molecules in polymer matrices and attributed the enhancement to a singlet oxygen feedback mechanism in which there is a repopulation of the singlet manifold of the organic molecule via energy transfer from singlet molecular oxygen. In this paper we examine the preliminary results and consider the relative merits of the singlet oxygen feedback mechanism and a trivial mechanism involving ground state depletion of the organic fluorescer in the evacuated samples. Optical cross beam experiments, designed to compare the extent of ground state depopulation of the organic molecule with the amount of enhancement observed, indicate that the depopulation mechanism can account for only a small part of the fluorescence enhancement. Furthermore, we report the discovery of oxygen stimulated fluorescence in similar systems under conditions in which the singlet manifold of the organic molecule can be populated only by energy transfer from singlet oxygen. Oxygen stimulated fluorescence is a direct and unequivocal confirmation of the basic idea of the singlet oxygen feedback mechanism and raises a fundamental question about the mechanism of oxygen quenching of the excited singlet states of organic molecules.