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The cyclization of bianthrone to helioanthrone in toluene occurs from the singlet state, since the quantum yield of intersystem crossing in this solvent is very low or zero. In bromoform, the triplet is formed and in the presence of oxygen generates singlet oxygen with a quantum yield of 0.42. A transient absorbing at 720 nm with a lifetime of 19 μs is also observed in this solvent and is attributed...