Luminescence competitive experiments

The laser coulostatic experiment on semiconductor electrolyte interfaces, in fact, has an early history dating back to ca. 1980. These and other similar works have been reviewed [2, 3, 9]. Luminescence decay experiments shed light on the competition between charge transfer, nonradiative recombination and luminescence, pathways. Once again, the reader is referred to the aforementioned reviews for further details. 

The rate constant for total deactivation of singlet oxygen by Vaska s Complex + kq) was determined by IO2 luminescence quenching and is listed in Table 1. In order to obtain the rate constant of chemical quenching (k ), competition experiments were carried out with 9,10-dimethylanthracene (DMA, same conditions as above). DMA is known to quench IO2 only chemically. " Loss of 1 and DMA were monitored spectrophoto-metrically and the results fitted to the equation of Higgins et al A second independent determination of was obtained by measurements of direct disappearance of 1 compared with a tetramethylethylene (TME) reference, which quenches singlet oxygen only by a chemical mechanism. Values of / r, and by difference, of kq, are listed in Table 1,... 

In a nonattaching gas electron, thermalization occurs via vibrational, rotational, and elastic collisions. In attaching media, competitive scavenging occurs, sometimes accompanied by attachment-detachment equilibrium. In the gas phase, thermalization time is more significant than thermalization distance because of relatively large travel distances, thermalized electrons can be assumed to be homogeneously distributed. The experiments we review can be classified into four categories (1) microwave methods, (2) use of probes, (3) transient conductivity, and (4) recombination luminescence. Further microwave methods can be subdivided into four types (1) cross modulation, (2) resonance frequency shift, (3) absorption, and (4) cavity technique for collision frequency. 

Luminescence titrations further demonstrate that the ruthenated duplex behaves as a 15 mer bearing one intercalator (76). As free [Ru(phen)2(dppz)]2+ is added to a solution of unmetallated 15 mer duplex, the luminescence increases linearly until the emission reaches saturation at about three equivalents of ruthenium(II) per duplex, consistent with competitive binding of [Ru(phen)2(dppz)]2+ to the 15-mer duplex and an average binding site size of a little more than four base pairs. When the analogous experiment is conducted with the ruthenated duplex, saturation of luminescence occurs after almost two equivalents of [Ru(phen)2(dppz)]2+ are added. This comparison indicates that the covalently bound ruthenium(II) complex is not displaced by additional intercalators. 

The competition titrations presented in the previous paragraph are interesting to establish the stability of a given complex compared to another. They are also useful tools to ensure the kinetic inertness of the complex over time. Any decrease of intensity would iudicate a modificatiou of the coordination sphere. On release of any ligand, the lanthanide ion would be exposed to water molecules in its vicinity, thus causing a decay of its luminescence intensity. Nevertheless, in snch long experiments, one has to ensure that the lamp is stable over time, that no evaporation of the solution or precipitation takes place over time, etc. 

The same kind of experiments can be applied to test varions conditions in the medium (e.g., pH effect, competition with anions, cations, etc.). It is of great importance to guarantee the integrity of the lanthanide complex in the composite enviromnent of the biological medium. Any decom-position/exchange reaction of the Ln ions would result in a loss of the luminescence intensity and a possible toxic exposme of the cells to the free Ln ions. This means that the stability of the complex has to be checked under conditions as close as possible to those encountered within cells. 

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