Concentration quenching luminescence

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-... 

Bacon J.R., Demas J.N., Determination of oxygen concentrations by luminescence quenching of a polymer-immobilized transition-metal complex, Anal. Chem. 1987 59 2780. 

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. 

Finally, it is important to mention that besides the possibility of energy transfer, a high concentration of centers can lead to new kinds of centers, such as clusters formed by aggregation or coagulation of individual centers. Thus, these new centers can have a different level scheme to that of the isolated centers, giving rise to new absorption and emission bands. This is, of course, another indirect mechanism of concentration quenching for the luminescence of the isolated centers, as happens in the next example. 

Cf2 O j concentrated sample, the measured lifetime becomes L7 ms. Estimate the loss of efficiency due to this concentration quenching of the luminescence. 

If we consider now transfer between two identical ions the same considerations can be used. If transfer between S ions occurs at a high rate, in a lattice of S ions there is no reason why the transfer should be restricted to one step. This can bring the excitation energy far from the site where the absorption took place. If in this way, the excitation energy reaches a site where it is lost nonradiatively (quenching site), the luminescence will be quenched. This phenomenon is called concentration quenching. 

The mineral matrix is formed chiefly by La " or Ce ". The last one is widely regarded as the luminescence impurity center, but in rare-earth bearing minerals it is subjected to concentration quenching because of strong exchange... 

Figure 3.40 Example of the Perrin plot of static quenching. Luminescence of a metal complex [Ru (bpy)32+] in rigid glycerol in the presence of increasing concentrations of methylviologen (quenching by electron transfer)...

Pressure effects on the energy transfer between f elements of the same kind were studied by Merkle et al. (1981) for the case of Nd3+-Nd3+ pairs in Ndx Y xP50i4 (x = 1,0.1). This material was studied in detail because of its potential use as a stoichiometric laser material. An outstanding property is a very weak concentration quenching of the luminescence. The total luminescence decay rate of the 4F3/2 multiplet in Ndx Y xP50i4 (x = 1,0.1) underpressure is shown in fig. 17. Obviously the stoichiometric compound shows a much larger increase of the decay rate than the doped compound. 

It is well-known that concentration quenching of the 5D4 luminescence is not observed in usual glasses whose rare-earth concentration is limited to a few atomic percent. In some sodium fluorophosphate glasses, whose terbium concentration is as high as 35 atm%, 5D4 luminescence quenching is found to occur at concentration of 20 atm% (3.3 1021 ions cm-3) and greater [145]. 

Tables Minimum Nd-Nd distance, splitting of the ground state 19/2, and ratio of the luminescence lifetimes for x = 1 (r) and X = 0.01 (ro) for (a) Low or (b) High concentration quenching neodymium materials (1/r = l/xo + VFnr where VLir is the rate of nonradiative decay due to concentration quenching) ...

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