Singlet Sigma: The "Other" Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O2(a1‐DLg), 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, O2(b1δg+), 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.