Delayed fluorescence as a probe of radiationless transitions in anthracene and algae

The delayed fluorescence emitted by both crystalline anthracene and chlorophyll a in algae is studied. In the case of crystalline anthracene, the blue light-excited delayed fluorescence is used to study the temperature dependence of monomolecular intersystem crossing. The associated rate constant k is found to obey an equation of the form [equation] where E = 900 ± 100 cm and k2/k1 = 800 + 200. A calculation of the relevant Franck-Condon factors using the method developed by Siebrand yields good agreement with the experimental value of k2/k1 . At temperatures below 90 o K, the temperature dependence of the delayed fluorescence intensity exhibits a pronounced maximum which is qualitatively explained in terms of triplet exciton traps. The delayed fluorescence emitted by algae is explained in terms of a triplet exciton fusion model in which the production of triplets is mediated by a photochemical reaction center (photosystem II). The evidence for this model is based on an extensive study of the time dependence of the luminescence decay and the observation that, at low light levels, the delayed fluorescence' intensity is proportional to the square of the excitation intensity. Direct proof in the form of a magnetic field effect on the delayed fluorescence was not obtained.U of I OnlyThesi