Non-radiative quenching

Hoirocks and co-workers took advantage of this and derived an empirical equation, which allows the determination of the number of water molecules coordinated to Eu(Ill) and Tb(lII) by comparing emission lifetimes in water and deuterated water. This will be further discussed in Chapter 2 [76,77]. 

In addition to O—H other bonds, sueh as N—H (3300 cm ) and C H (2900 cm ), also contribute to vibrational quenehing of Ln(III)-eentred emission. 

Since the non-radiative quenehing proeesses are vibrationaUy mediated, they are temperature dependent and the temperature dependence of the rate constants is described by an Arrhenius-like Equation 1.84. 

Support through the National Science Foundation grant CHE-1058805 is gratefully 

AOM angular overlap model BT back energy transfer 

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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-... 

BCPDA is a tetradentate ligand, and forms 1 1 complexes in these conditions. The metal ion is thus coordinatively unsaturated, and also binds a number of water molecules, which can lead to non-radiative quenching of the metal-centered excited state (vide supra). These must be removed by drying to achieve optimal luminescence intensity. This form of assay is, in fact, slightly less sensitive than DELFIA. 

Non-radiative quenching therefore usually reduces both the intensity and the decay time. These relations apply only when the different recombination processes compete directly. The actual processes are usually more complicated, with some competing and non-competing paths and a distribution of rates. 

Recently, evidence has been presented that during steady state photosynthesis energy absorbed by PSII and not used for electron transport is dissipated via non-photochemical and non-radiative quenching of excitation energy [5] and that this high energy... 

Once the lanthanide excited state has formed, luminescence is not an inevitable outcome. Non-radiative quenching of the excited state can occur through vibrational modes of the array, and particularly through vibrational quenching by X-H oscillators in the ligand backbone or in aqueous media. To optimize emission from the lanthanide it is necessary to minimize the non-radiative quenching pathways... 

In a polynuclear or polymetallic lanthanide complex, additional processes become feasible. Figure 3 is a simplified scheme that shows the energy transfer pathways available in a bimetallic lanthanide complex singlet-mediated energy transfer and competitive non-radiative quenching pathways are not shown in the scheme to facilitate interpretation. Not only can both lanthanide ions be sensitized by the excited states of the chromophore, but energy transfer between lanthanide ions is also possible where appropriate spectral overlap exists between the two lanthanide ions. 

In particular, the art in excimer laser design is to minimize non-radiative quenching processes of the excited excimer, against which photoemission has to compete and which dimmish the available inversion in the upper laser level. 

Figure 7.3 Radiative (absorption, stimulated emission, fluorescence) and non-radiative (quenching, collisional energy transfer, elastic scattering) processes in a molecular system with electronic, vibrational and rotational energy levels...

In addition to the non-radiative quenching mentioned further above, an addition collisional energy transfer process can be observed, namely the transfer from the laser-excited level to neighbouring quanmm levels within the excited-state manifold. Hence, under the right conditions, one observes lines from levels that were not directly populated by the laser excitation. 

However, in contrast to surface enhanced Raman scattering (SERS), maximal enhancement of fluorescence is not observed from molecules that are directly adsorbed on the surface, but from those that are located at a short distance away from the metal surface. The reasons are two counterbalancing effects and can be deduced from Fig. 17a [117]. First, if positioned too close to the metal surface, Forster-type energy transfer can set in and Z((Weni) is rapidly reduced because of the onset of non-radiative quenching (Fig. 17a, dotted line for the influence on the radiative rate constant, see Sect. 2.1) [118]. Second, if the dye is placed too far from the metal surface, coupling is inefficient because the induced field is an evanescent field and its intensity decays exponentially with the distance from the surface (Fig. 17a, solid line). For instance, considering a gold surface and water as the dielectric, the field has decayed to 1/e at ca. 150 nm. Thus, a maximum enhancement Z(mem) exists only in a small zone at an optimum distance from the metal surface. For the Au/water system in... 

Potential upper laser levels are limited to those having a large energy gap to the next-lower level because of the large probability for non-radiative quenching in chelate laser solutions. 

Figure 1.12 Radiative transitions of Eu(lll) and non-radiative quenching through O—H and...

As mentioned in Section 2.2 of this chapter, the determination of excited state lifetimes relays important information regarding the coordination environment of the lanthanide ion. Since the lifetime of the excited state is strongly influenced by non-radiative quenching pathways, the number of oscillators leading to non-radiative quenching in the coordination sphere of the lanthanide ion can be determined with lifetime measurements, as detailed below. 

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