Rate constants for both radiative and non-radiative relaxation of the fluorescent state of indole and several methyl-indoles in solution have been calculated from fluorescence lifetime and quantum yield values.Radiative relaxation for indoles is solvent dependent while non-radiative relaxation is attributed to at least two processes, one of which is both solvent and temperature dependent. The latter process measured in aqueous solvents manifests a pattern of compensation of activation enthalpy by activation entropy with an isokinetic temperature for indole of 283°K.A quantitatively consistent description of the excited-state behavior of 1-Me-indole in both free and exciplex state is provided. Small deviations in fluorescence parameters for indole suggest a more complicated set of events.

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 ?

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