Chemical reactions can be studied spectroscopically at the single-molecule level with the use of fluorescent tracers or dyes. These fluorophores do not last forever; eventually they bleach and become invisible in the fluorescent microscope. Fluorophore destruction is known to be accelerated in the presence of oxygen, but our detailed understanding of the photobleaching process is limited.
Christ et al. have studied, one by one, the behavior of several hundred terrylene molecules in air, oxygen, and argon atmospheres. Most of the molecules succumbed to photobleaching, and an argon atmosphere slowed this process by several orders of magnitude. The remaining molecules first switched into a new fluorescent state, with an emission maximum shifted to a shorter wavelength, before subsequently becoming dark. Quantum chemical calculations of possible product molecules indicate that immediate photobleaching and switching into a second fluorescent state are both caused by the covalent addition of oxygen. In the latter case, a second oxidation event then leads to photobleaching, which is consistent with their observation that the lifetime of the secondary photoproduct increases when it is transferred to an argon atmosphere. -- JU
Angew. Chem. Int. Ed. 40, 4192 (2001).
