Optical sensing and imaging of trace oxygen with record response.

Dioxygen is one of the key molecules on Earth. It plays an essential role in the atmosphere, the hydrosphere, the geosphere and not the least the biosphere. Many physiological transformations, chemical reactions, and (bio)technological processes produce or consume O2, while anoxic species, numerous chemical syntheses, and manufacturing protocols demand its complete absence. Its extraterrestrial presence hints at the potential presence of life in the form we know. Trace oxygen detection is also important in aerospace research and from a safety standpoint, as oxygen leaks can cause fires and explosions and can be harmful in storage chambers and packaged food. Common trace oxygen sensors are based on amperometry (Clark electrodes). These instruments are sensitive and applicable over a wide temperature range but are difficult to miniaturize, invasive, and limited to discrete points. Optical sensors overcome these limitations. Most are based on the quenching of the long-lived luminescence exhibited by polycyclic aromatic hydrocarbons, transition-metal complexes, and metalloporphyrins. These compounds are typically placed in inert polymer membranes. Highly permeable matrices are employed in order to sense traces of O2. [8] Herein we show that an as yet unmatched sensitivity combined with an unmatched brightness at high temperatures can be achieved by exploiting the extremely efficient quenching of the delayed fluorescence of the ellipsoidal fullerene C70 embedded in two highly permeable polymer membranes, an organosilica, and an ethyl cellulose. The electronic states and transitions of C70 and other fullerenes, owing to the large number of p electrons, lie on the interface between discrete molecular orbitals and band structures. The absorbance spectrum of C70 displays a peak at 470 nm (e 20000m 1 cm ; Figure 1). The luminescence of C70 is very atypical in several ways. The fluorescence occurs from two excited singlet states. Weak prompt fluorescence (FF: 0.05%, t 650 ps) occurs in the red region (mainly 650– 725 nm). Strong energy overlap and many low-lying excited states lead to a quantum yield of triplet formation close to one (reported: 0.994). Multiple weak phosphorescence bands are observed between 750 and 950 nm, displaying lifetimes of 20–25 ms at room temperature. Triplet-state lifetimes of this magnitude or even longer are observed for many molecules and are known to be efficiently