Concentration-quenching

It has been observed in many cases that the luminescence quantum yield in solution decreases as the solute concentration increases, and this applies also to the quantum yields of photochemical reactions. There must therefore be some process which can be written as 

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A high concentration of the fluorescent dye itself in a solvent or matrix causes concentration quenching. Rhodamine dyes exhibit appreciable concentration quenching above 1.0%. Yellow dyes, on the other hand, can be carried to 5 or even 10% in a suitable matrix before an excessive dulling effect, characteristic of this type of quenching, occurs. Dimerization of some dyes, particularly those with ionic charges on the molecules, can produce nonfluorescent species. 

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an cxciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Titus, for example, an electron trap in the ETL will capture and hold an election until a hole is injected nearby from the HTL. In this case, the dopant is the recombination mmo.-... 

These values equal 2.0, 1.05, and 0.5, respectively, for PP, DPAcN, and PPA. It is possible that the contribution of excited states caused by n - it transitions accounts basically for a bathochromic luminescence of some PCSs and for a shift of the maxima in the luminescence spectra of polymers of this kind when proceeding from the solution to the solid phase. PCS solutions reveal concentration-quenching accom-... 

For anthracene the concentration quenching process could be the decomposition of the excimer to two ground state anthracenes, as shown in... 

However, in most cases the observed intercept is greater than unity. This indicates that if our mechanism is correct thus far, we must add another process by which singlet energy is lost. If, without going into any conjectures as to the nature of the process, we include a concentration quenching step,... 

We should now examine the nature of the concentration quenching process that we proposed in our singlet dimerization mechanism. There are a number of possibilities, as follows. 

The rate constants for dimerization, concentration quenching, and heavy-atom quenching undergo an approximately two-fold increase as a result of heavy-atom substitution, as compared to the seven-fold increase observed for kd. 

Chen, R.F., and Knutson, J.R. (1988) Mechanism of fluorescent concentration quenching of carboxyfluo-rescein in liposomes Energy transfer to nonfluorescent dimers. Anal. Biochem. 172, 61. 

Y Kawamura, J Brooks, JJ Brown, H Sasabe, and C Adachi, Intermolecular interaction and a concentration-quenching mechanism of phosphorescent Ir(III) complexes in a solid film, Phys. Rev. Lett., 96 017404-1-017404-4, 2006. 

Concentration quenching. In this case, quenching occurs because of the formation of an association between excited state and ground state, which if homo-molecular is called the excimer formation. The subsequent processes can be radiationless or the complex can emit at much longer wavelength and effectively not be detected. 

Superquenching, Concentrational Quenching, and Directed Homo-FRET. 117... 

DASPE-TFPB), respectively. The obtained solid precipitates were brightly emissive whereas that of the native DASPE-I were almost nonemissive (Fig. 7a the photo is taken under normal illumination and UV-light irradiation). This indicates that, in the solid of the ion-pair species between DASPE+ and TPB (or TFPB ), concentration quenching is effectively suppressed, and more importantly, these ion-pair complexes can generate fluorescent... 

Figure 11.1. An example of concentration quenching. In ruby, the observed decay time of the fluorescence at 77K is independent of chromium concentration up to a concentration of around 03 weight% of CrjOj in the material/251...

Finally it is worth mentioning the relation between fluorescence properties and the concentration of activator content. The fluorescence intensity increases initially with increases in the concentration of activator content. However, the increase in the concentration above a certain critical value can lead to a reduction in fluorescence intensity. This is called concentration quenching. It can also be observed from the reduction in the lifetime with the increase in the concentration, as shown in Figure 11.1 in the case of ruby, where the activator is the Cr3+ ion. Therefore, it is advantageous to select an activator concentration short of the critical value to achieve a good level of fluorescence intensity in a practical sensor system. 

Figure 14.17. Liposome fluoroimmunoassay depiction, in which antigen-liposome conjugates containing fluorescent dye (concentration quenched) compete with analyte antigen for antibody binding sites, followed by wash and detergent lysis, to release the fluorophore for fluorescence measurement.

In principle, an increase in the concentration of a luminescent center in a given material should be accompanied by an increase in the emitted light intensity, this being due to the corresponding increase in the absorption efficiency (see expression (1.15)). However, such behavior only occurs up to a certain critical concentration of the luminescent centers. Above this concentration, the luminescence intensity starts to decrease. This process is known as concentration quenching of luminescence. 

Figure 5.22 Schemes of possible mechanisms for luminescence concentration quenching (a) energy migration of the excitation along a chain of donors (circles) and a killer (black circle), acting as nonradiative sink (b) cross relaxation (including an illustrative energy-level diagram) between pairs of centers. (Sinusoidal arrows indicate nonradiative decay or radiative decay from another excited level.)...

As the concentration quenching results from energy transfer processes, the decay time of the emitting ions is reduced when one concentration quenching mechanism occurs. In general, this decay-time reduction is much easier to measure than the reduction in the quantum efficiency. In fact, the easiest way to detect luminescence concentration quenching is to analyze the lifetime of the excited centers as a function of the concentration. The critical concentration is that for which the lifetime starts to be reduced. 

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