Quenching pathways

The photophysical properties of lanthanide ions are influenced by their local environment, the nature of the quenching pathways available to the excited states of sensitizing chromophores, and the presence of any available quenchers (as we have seen when discussing bioassay). All of these factors can be exploited for the sensing of external species. 

Since the excited state may act as an electron donor or as an electron acceptor, two electron-transfer quenching pathways, reductive and oxidative, are possible. 

The labeling can also be done by fluorescence lifetime differences, e.g., introduced by quenching pathways connected with different surroundings. This can be used for lifetime imaging methods (Chapter 1, this volume) or for distinguishing complexes of one and the same fluorescence label with, e.g., different DNA-bases. In this case, the fluorescence label is not only a label but incorporates a function which senses the environment and can therefore be regarded as a sensing fluorescence probe. 

In such reactions, both the reductive and oxidative quenching pathways are possible for the photocatalytic reactions between Dred and Aox. In general, one reaction pathway, either reductive or oxidative quenching, becomes dominant when the reactant pair Dox and Aox is fixed. However, acid catalysis can alter the operating quenching pathway by accelerating one pathway and/or retarding the other pathway (vide infra). 

From these expressions, we find the OER and HER are isoenergetic at pH 11 and 2.3 for the reductive and oxidative quenching pathways, respectively. More importantly, ... 

Fig. 8. (a) The irradiance dependency of qg (O) and the quantum efficiency of electron transport by open Photosystem II reaction centres (excl ) (b) the irradiance dependency of the quantum efficiencies for de-excitation by the basal, constitutive pathway and the inducible (NPQ or non-photochemical quenching) pathway at each irradiance the sum of these de-excitation pathways and the quantum yield for Photosystem II electron transport is one. 

Several interesting photoreductions of the excited uranyl ion have appeared. Irradiation of U02 in MeCN or PrCN solution using wavelengths above 400 nm brings about reduction by a first-order process, which is in competition with physical quenching pathways. No ground-state complex seems to be involved, and the reduction proceeds by an a-H-abstraction from the nitrile... 

The theoretical implications of the dependence of on A r,.Sj, have been discussed in Ref. 29. A theoretical treatment of triplet quenching by O2 had predicted that excitation transfer via the singlet encounter pair would be the dominant quenching pathway (33). In addition, it was pointed out that the large Franck-Condon factors for the (0-0) transitions for... 

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