Nonradiative processes chelates

Fig. 16. Energy transfer processes in Eu chelates (from Hayes and Drickamer (1982)). Wiggly arrows represent nonradiative processes. Solid and dashed arrows represent radiative processes, with a lesser probability for the dashed arrows a, b, c, d see text.

Windsor and his co-workers 10, 11) have shown, by direct excitation of the levels in a number of europium compounds, that the quantum yield of fluorescence decreases progressively as one successively excites Do, Di, and Do. While the yield on direct excitation of Do is quite dependent on the particular compound and medium (the value is 0.82 for the thenoyltrifluoroacetone chelate in acetone solution), the proportion of the energy lost between e.g. - Do and - Do within a given compound is rather insensitive to changes in the environment. These nonradiative processes from " D and Di, whose nature is not understood, would appear to be responsible for most of the ca. 40% energy loss in these materials. 

Once the energy arrives at Do, it is emitted to Fo with high efficiency. Nonradiative processes from Do are almost completely lacking in (BTFA)4Eu chelates in nitrile solvents as is indicated by the relative insensitivity of the lifetime of the Do Fo emission to changes in environment and temperature. 

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