Oxygen, effect fluorescence lifetimes

While the covalent attachment of cyanine dyes such as Cy5 or Alexa 647 to proteins does not result in noticeable changes in their spectral properties, squaraine dyes (oxo-squarines and squaraines with substituted squaraine oxygens) behave quite the opposite the absorption and emission maxima of squaraines are in general red-shifted after binding to proteins and the quantum yields and fluorescence lifetimes are manifold increased [68-70]. In general hydrophobic squaraines exhibit more pronounced increases compared to hydrophilic dyes. These effects are even stronger in noncovalent dye-protein complexes. Importantly, the photostability of squaraines also increases after binding to proteins. 

What are the effects of a rigid polymer matrix and of oxygen on the fluorescence lifetime and quantum yield ... 

As mentioned above, phosphorescence is observed only under certain conditions because the triplet states are very efficiently deactivated by collisions with solvent molecules (or oxygen and impurities) because their lifetime is long. These effects can be reduced and may even disappear when the molecules are in a frozen solvent, or in a rigid matrix (e.g. polymer) at room temperature. The increase in phosphorescence quantum yield by cooling can reach a factor of 103, whereas this factor is generally no larger than 10 or so for fluorescence quantum yield. 

Carbon disulfide quenches the fluorescence of anthracene quite efficiently,145,149 but seems to have little effect on its triplet lifetime.147 Diphenylanthracene in benzene fluoresces with a quantum yield of 0.8 and shows a high sensitivity to the oxygen concentration in photooxygenation reactions. With about 1 vol% of CS2 present, AC>2 is practically independent of [02] (> 10"5 mole/liter). In jjoth cases, where carbon disulfide was either used as solvent or was added to an otherwise strongly fluorescent solution, the quantum yields of photooxygenation followed... 

Kropp and Windsor also examined the effect of oxygen on the fluorescence properties of rare-earth solutions. They found that the lifetimes remained constant to 0.5 per cent for (a) normal air-equilibrated solutions, (b) solutions that had been flushed with helium, and (c) pure-oxygen-flushed solutions. This last effect is in variance with the well-established fact that dissolved oxygen is an efficient quencher of fluorescent aromatic compounds. 

The effect of oxygen on cyclic 1,3-diradicals shows that conformation can affect the triplet state lifetime ST Time resolved resonance Raman spectroscopy has been used to examine triplet states produced from different isomers of p-carotene. A theoretical study has also been reported on the a-cleavage of the triplet states of symmetric and non-symmetric ketones S mechanism for triplet state relaxation of aromatic molecules has been used to explain experimental data for substituted benzenes. The decay kinetics of triplet-triplet fluorescence in the mesitylene biradical (two sub-levels) have been measured between 10 and 77K in Shpolski matrices triplet state of dimesityl... 

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