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The electronic excited states corresponding to singlet oxygen generation versus O–O splitting in o-fluorine-phenyl-9-anthracene-9,10-endoperoxide 1 and its 9,10-bisarylanthracene analog 2 have been investigated using theoretical methods. In the case of the smaller endoperoxide 1, the recently developed second-order perturbation theory restricted active space (RASPT2) method has been employed and the results are compared to those from the complete active space (CASPT2), second-order approximated coupled cluster (CC2), and time-dependent density functional theory (TD-DFT) approaches. In addition to the vertical excited states, the photochemical path leading to homolytic O–O dissociation has been computed. This process is governed by a point, where four singlet and four triplet states are almost degenerate and show substantial spin-orbit coupling. The results obtained …