Luminescent species lifetime

In dynamic quenching (or diffusional quenching) the quenching species and the potentially fluorescent molecule react during the lifetime of the excited state of the latter. The efficiency of dynamic quenching depends upon the viscosity of the solution, the lifetime of the excited state (x ) of the luminescent species, and the concentration of the quencher [Q], This is summarized in the Stern-Volmer equation ... 

In summary the most likely explanation for dual emission with different lifetimes in d6 compounds is the presence of more than one luminescent species in the sample. 

It is well known that luminescent molecules near metal surfaces can suffer dramatically reduced luminescent quantum yields and lifetimes. This is attributed to non-radiative energy transfer to the metal surface [31]. This phenomenon is thought to commence when the luminescent species lies up to 200 A from the metal surface. Therefore, this form of quenching is expected to be particularly prevalent in... 

If the loss of luminescent species after the exciting radiation is shut off is uni-molecular or pseudo-first-order, then we may define mean lifetimes for the decay of emission as the inverse of the sum of all the effective first-order rate... 

Fluorescence lifetime measurements can give information about collisional deactivation processes, about energy transfer rales, and about excited-slate reactions. Lifetime measurements can also he used analytically to provide additional selectivity in the analysis of mixtures containing luminescent species. The measure-mciil of luminescence lifetimes was initially restricted to phosphorescent systems, where decay limes were long enough to permit the easy measurement of eiiiil-ted intensity as a function of lime. In recent years, however, it has become relatively routine to measure rates of luminescence decay on the fluorescence time scale (10 Mo -r 10 M). 

Under stoichiometric conditions, fluorexon and its derivatives form 1 1 complexes with Ln ions. However when the ratio Ln Fx is increased, complexes with other stoichiometries are observed, the exact nature of which has not been determined. On the other hand, luminescence data of solutions with a ratio Yb Fx < 1 clearly indicate the presence of only one luminescent species, the 1 1 complex. Monoexponential luminescence decays are observed corresponding to a lifetime of 1.9 ps, whereas multi-exponential decays are measured when the Yb Fx ratio is increased. Further proof of the existence of 1 1 complexes has been brought by mass spectrometry. Competitive titration with edta has been followed by monitoring the Yb luminescence, since the edta complex is non-luminescent, contrary to the chelate formed with Fx. After addition of 5 equivalents of edta to a solution of [Yb(fx)] in Tris-HCl buffer, the Yb luminescence intensity decreases to 12% of its initial value. The thermodynamic stability of the fluorexon chelate is, therefore, comparable to [Yb(edta)] . In addition, the luminescence decay after addition of edta aliquots is relatively slow, the estimated rate constant being 7.1 x 10" s indicating a reasonably high kinetic stability of the fluorexon chelate. 

Stern-Volmer quenching constant (iTsv) Constant for a given quenching process it is the product of the bimolecular rate constant for the reaction of quencher and the luminescent species and the luminescence lifetime in the absence of quencher... 

Time-resolved luminescence spectroelectrochemistry (TRLS) is less common than but complementary to luminescence spectroelectrochemistry. TRLS can be used to monitor the lifetime of a luminescent species, provided it is sufficiently long-lived. TRLS may be useful in studying the photophysics of an electroactive species to understand its temporal behavior under application of potential and can be used to provide a unique perspective on the electrochemical interface. 

Multi-exponential decays can also be observed from a single luminescent compound. For example, the compound under examination can be present in two (or even more) different chemical environments (e.g., due to different solvent shielding), producing species with different lifetimes. Normally the radiative constant of a luminescent species is independent of the chemical environment, and in such a case the pre-exponential factors are related to the fraction of species experiencing different environments. In biochemical research it is very common to consider that a multiexponential decay can result from different conformers of a single biological molecule. 

Luminescence has been used in conjunction with flow cells to detect electro-generated intennediates downstream of the electrode. The teclmique lends itself especially to the investigation of photoelectrochemical processes, since it can yield mfonnation about excited states of reactive species and their lifetimes. It has become an attractive detection method for various organic and inorganic compounds, and highly sensitive assays for several clinically important analytes such as oxalate, NADH, amino acids and various aliphatic and cyclic amines have been developed. It has also found use in microelectrode fundamental studies in low-dielectric-constant organic solvents. 

The luminescence of an excited state generally decays spontaneously along one or more separate pathways light emission (fluorescence or phosphorescence) and non-radiative decay. The collective rate constant is designated k° (lifetime r°). The excited state may also react with another entity in the solution. Such a species is called a quencher, Q. Each quencher has a characteristic bimolecular rate constant kq. The scheme and rate law are... 

The luminescence lifetimes of ARRR, ASSS, ARRS, and ASSR diastereomers (188), R = Ph, show that the symmetric forms are longer lived by a factor of two than the less symmetric species and are solvent dependent.348 The X-ray structure of R = Me confirms the fac arrangement of the Si/N ligands.349... 

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