The Participation of Excited Oxygen in a Chemiluminescent Reaction†

Abstract

Some properties of silica gel adsorbates of the acridine dye, acriflavine, have been investigated in the presence of oxygen and inert gases. In addition to its well-known quenching of phosphorescence, oxygen stimulates additional luminescence, a chemiluminescence, at high concentrations of the dye triplet state molecules. The dependence of the chemiluminescence and of oxygen consumption on experimental variables have been explained in terms of a mechanism in which O,*, formed in the act of quenching of one photoexcited dye molecule by oxygen, diffuses to a second excited dye molecule where it may cause an irreversible oxidation. This oxidation may be accompanied by chemiluminescence. Both the intensity of the chemiluminescence and the rate of oxygen consumption were found to be reduced specifically by inert gases having vibrational frequencies close to the vibrational spacings of oxygen. It was concluded that 02* is probably vibrationally excited oxygen and that it is deactivated principally by resonance energy transfer processes. I N T R O D U C T I O NIN CONNECTION with studies on dye-sensitized photo-oxidation reactions, a system consisting of an adsorbate of purified acriflavine (3,6-diamino-10-methyl acridinium chloride) on silica gel has been under investigation in this laboratory. Several advantages of this system are the relatively long lifetime of the lowest triplet (photosensitizing) state of the dye, up to 0-2 sec at room temperature, and the relatively long lifetime of the oxygen intermediate formed at the dye and capable of diffusing toward an oxidizable substrate.'l) The present work was undertaken in an attempt to identify this oxygen intermediate, designated throughout this paper as 0,*.(2)From the fact that neither oxygen gas nor the sensitizing dye is consumed at a rate commensurate with the formation of 0," by the oxygen quenching of the excited dye at low light intensities, we concluded that 0," is an excited form of oxygen rather than a chemical reaction product. This conclusion was recently arrived at for other reasons by Foote and Wexlert3) and by Corey and Taylor.(4) Most of the O,* is eventually de-excited and becomes ordinary oxygen. There is some chance, however, that O,* may undergo some reaction before it is de-activated. In the presence of an oxidizable substrate 0," may participate in an oxidation reaction. In our acriflavine-silica gel system to which an oxidizable substrate ?

Oxygen quenching of acriflavine phosphorescence

“…Weiner and Seliger used 100-200 mesh silica gel whose pores they felt are impenetrable to acriflavine;18 accessibility of the internal pores to dye in our 14-28 mesh silica gel was definitely proved. 19 Restriction of adsorption sites to the outer surface might also be related to a restriction in the size of a polymeric unit that might be found. Perhaps a more significant experimental difference is the use by Weiner and Seliger of an excitation, 407 mg, far on the high-energy tail of the main absorption band.…”

“…23 A discussion of the nature of 02* has been given elsewhere. 19 There is a respectable body of evidence in the literature that 02 quenches triplet states of aromatic substances in fluid solution by a collisional mechanism. Most of the reported systems have been studied by the triplet-triplet absorption technique, with oxygen concentrations considerably above those used here.…”