Kinetics excited state quenching

The spectroscopic properties of several rare-earth trihalide-aluminum chloride complexes and various rare-earth chelates has been studied (9) and optical gain observed for a Nd-Al-Cl vapor complex (10). Measurements of the fluorescence kinetics show evidence of strong excited-state excited-state quenching. This plus the low molecular densities achievable reduce the attractiveness of these systems for practical laser applications. 

Gain was measured for the F3/2" Il]/2 transition from one molecular vapor, a NdCl3-A1Cl3 complex (16.). Intense excited state-excited state quenching and low vapor pressures limit the attractiveness of this lasing medium. The excited-state kinetics for Nd(thd)3 chelate vapors have also been investigated and the prospects for laser action discussed (62). 

The relative rate constants for the fluorescence quenching of benzene and a series of its simple derivatives by some chloro- and fluoro-methanes and CDC13 are found to follow qualitatively but not quantitatively the hypothesis that the rate-determining step in the quenching process is formation of a donor-acceptor exciplex. The study examined the kinetics following excitation to S2 and S3 as well as Si. 115 The lack of quantitative correlation with the theoretical model used may be due to a deficiency in the model for excited-state quenching or due to the... 

For systems such as these, which consist of electron transfer quenching and back electron transfer, it is in general possible to determine the rates both of quenching and of the back reaction. In addition to these aspects of excited state chemistry, one can make another use of such systems. They can be used to synthesize other reactive molecules worthy of study in their own right. The quenching reaction produces new and likely reactive species. They are Ru(bpy)3+ and Ru(bpy)j in the respective cases just shown. One can have a prospective reagent for one of these ions in the solution and thereby develop a lengthy and informative series of kinetic data for the transient. 

FIG. 12 Simulation of fluorescent decays for dye species located in the aqueous phase following laser pulses in TIR from the water-DCE interface according to Eq. (38). A fast rate constant of excited state decay (10 s ) was assumed in (a). The results showed no difference between infinitely fast or slow kinetics of quenching. On the other hand, a much slower rate of decay can be observed for other sensitizers like Eu and porphyrin species. Under these conditions, heterogeneous quenching associated with the species Q can be readily observed as depicted in (b). (Reprinted with permission from Ref 127. Copyright 1997 American Chemical Society.)... 

Besides the mentioned thermodynamic losses, there always exist kinetic losses arising from the competitive non-radiative quenching of the excited state. For instance in photovoltaic devices, the undesired thermal recom-... 

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

A well-known example of an exciplex is the excited-state complex of anthracene and N,N-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene molecule. In nonpolar solvents such as hexane, the quenching is accompanied by the appearance of a broad structureless emission band of the exciplex at higher wavelengths than anthracene (Figure 4.9). The kinetic scheme is somewhat similar to that of excimer formation. 

In most of the cases studied bimolecular kinetics are followed, the rate constants of product formation depending on the rate of light absorption and on nucleophile concentration. Triplet lifetimes (as determined from quenching studies) also depend on nucleophile concentration. This means that the excited state is quenched by the nucleophile, accompanied by either product formation or reversal to starting material. In view of the inherently different triplet lifetimes of different substrates, it is highly desirable to rely on rate constants rather than... 

This represents a competitive, non-kinetic, method for determining relative rate constants for the photochemical system. The value of may be obtained after one direct determination of has been made. For an extensive compilation of quenching rate constants for excited states of metal complexes see Ref. 358. 

It is also possible to determine the nature of the excited molecule reaction leading to product formation by kinetic methods. For example, variation in the rate of formation of dimethyluracil hydrate with water concentration in acetonitrile-water mixtures is convex to the water concentration axis (Fig. 15).65 The rate of formation of uracil hydrate under similar conditions is linear with water concentration. The first of these is not the shape of curve to be expected if the function of the water molecules were simply to quench an excited state according to the common mechanism ... 

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