Molecular oxygen, O₂, plays a key role in the maintenance of life on earth, and in mechanisms by which life is extinguished and materials destroyed. Singlet oxygen - the lowest excited electronic state of molecular oxygen - has been known to the scientific community for more than 80 years. It has a characteristic chemistry that sets it apart from the triplet ground state of molecular oxygen and is important in fields that range from atmospheric chemistry and materials science to biology and medicine. For such a "mature citizen", singlet oxygen still remains at the cutting edge of modern science.

As an isolated molecule, oxygen provides a wonderful model to illustrate a number of fundamental chemical and physical phenomena. For example, differences in the triplet and singlet states reflect ways that two electrons can be placed in degenerate orbitals and, as such, provide an ideal system to examine phenomena that give rise to Hund's rules for orbital occupancy. Also, the near IR transition between the triplet and singlet states is not very probable and provides an excellent example of selection rules based on changes in spin and orbital angular momentum, symmetry, and parity.

But singlet oxygen is not just an important model for fundamental chemical and physical phenomena. Many will argue that singlet oxygen stays at the forefront of research because of its unique reactivity as a synthetic reagent, as an intermediate in oxygenation reactions of polymers and, perhaps most significantly, as a so-called "reactive oxygen species" in a range of biological systems. Consequently, research in the singlet oxygen field is very broad, particularly as the systems that can be addressed, such as mammalian cells or multicomponent solar energy collectors, become increasingly complicated.

Singlet oxygen can be produced using sunlight

Producing singlet oxygen can be done in many ways, a convenient method involves electronic energy transfer from an excited state of a given molecule, a so-called sensitiser, to triplet. Because excited electronic states are readily produced upon ultra-violet or visible light absorption, and because we live in a world of light, oxygen, and molecules that are efficient sensitisers, this method of singlet oxygen production is also extremely pertinent. Indeed, sensitised systems driven by sunlight alone provide sufficient justification for research in this field. However, the use of more controlled light sources (i.e lasers, lamps) not only facilitates mechanistic studies, but makes specific applications more tenable (e.g medical treatments such as photodynamic therapy, PDT, where singlet oxygen is used to kill cancer cells).

One can consider singlet oxygen from three perspectives: (1) It is a molecule that is directly responsible for unique phenomena. Examples include specific reactions that lead to the death of a biological cell. (2) Singlet oxygen serves as nice model and/or probe to investigate other general phenomena. Examples here include issues related to oxygen diffusion or phenomena that enhance a forbidden optical transition. (3) A study of singlet oxygen provides the basis for the development of new tools, techniques and materials. Examples include the construction of IR imaging systems and the implementation of appropriate high-level computational methods.

The study of singlet oxygen, particularly in photosensitised systems, provides a coalescence point for many topics currently at the cutting-edge of science. Indeed, with singlet oxygen in mind, seemingly disparate issues such as nanoparticle-dependent surface plasmon resonances and genetically engineered protein tagging experiments can be mentioned, with significance, in the same sentence.

Studies of singlet oxygen will remain at the forefront of science simply because singlet oxygen is an important species that plays a role in so many processes. As science evolves, so does the study of singlet oxygen. There will always be something new under the sun.