Singlet oxygen probes: Diversity in signal generation mechanisms yields a larger color palette

The detection and imaging of singlet oxygen produced by natural processes, or by photochemical and chemical reactions has become an important goal in probe design. The short half-life and solvent sensitive nature of the singlet oxygen makes this detection highly challenging. At the same time, since singlet oxygen can be the precursor of other reactive oxygen species (ROS), it is crucial to have a selective probe for singlet oxygen in the presence of other oxidizing species. The challenge is immense, yet there are already brilliant examples of smart probe design. This review mechanistically classifies and discusses what is being done in order to obtain a selective response and at the same time, push the signalling events towards longer wavelengths of the visible spectrum.