DELAYED FLUORESCENCE OF ANTHRACENE AT LOW TEMPERATURES . FAILURE OF SQUARE DEPENDENCE ON EXCITING LIGHT

The intensity of blue delayed fluorescence of anthracene excited by red light in the triplet absorption region was investigated as a function of temperature over the range 2°-300°K. It was found that as the temperature was lowered the square relationship between fluorescence intensity and excitation intensity broke down at comparatively low excitation intensities. In order to explain the results the theory of Siebrand [J. Chem. Phys. 42, 3951 (1965) ] was extended to include the saturation of triplet traps and the decay of triplets by bimolecular annihilation. An expression was derived for the square law failure and this gave reasonable agreement with experimental results. It indicated that both saturation and bimolecular decay were present. Square law failure can provide a way to measure total" bimolecular annihilation rates and collision rates between free triplet excitons and trapped triplet excitons."