The photophysics of lumazines leading to triplet formation and the effect of thionation are explored in the presence of near-degenerate electronic states. Wave packet simulations are performed on model potential energy surfaces to understand the nonadiabatic population transfer among close-lying excited states. Ultrafast population transfer among singlets opens up new intersystem crossing channels from the higher states. An increased spin-orbit coupling strength originating from thionation enhances intersystem crossing and populates the higher triplets first. The rapid internal conversion in the triplet manifold eventually brings the molecules to their respective low-lying long-lived triplet state.