Singlet Sigma: The “Other” Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O 2 (a 1‐ DL g ), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well‐studied intermediate in many solution‐phase photosystems. However, the second excited electronic state of oxygen, O 2 (b 1 δ g +), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.