We report on a new, highly fluorescent aromatic compound for single molecule spectroscopy, dibenzanthanthrene, studied in an n-hexadecane matrix. Narrow homogeneous lines are compatible with the measured lifetime. From the two components in the fluorescence correlation function, we determined the intersystem crossing rates towards and from the triplet states. The very weak triplet yields indicate that the molecule behaves as a nearly perfect optical two-level system, with negligible buildup of triplet population.