Singlet oxygen: there is still something new under the sun, and it is better than ever.

Singlet oxygen, O(2)(a(1)Δ(g)), the lowest excited electronic state of molecular oxygen, has a characteristic chemistry that sets it apart from the triplet ground state of oxygen, O(2)(X(3)Σ). Although singlet oxygen can be produced in a variety of ways, it is arguably best known as a product of the interaction between a light absorbing chromophore (i.e., a photosensitizer) and O(2)(X(3)Σ). In this way, it plays a key role in the many disciplines that comprise the "photosciences". As outlined in this Perspective, which is part of the program "Photosciences: a look to the future", the study of singlet oxygen provides a coalescence point for many topics currently at the cutting edge of modern science ranging from materials science to biology and medicine. As such, although singlet oxygen is a "mature citizen" and has been examined for many years in a variety of contexts, there is still something new under the sun, and the future looks exceedingly bright for this photo-related discipline.