Nonradiative quenching

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 natural fluorescence lifetime, x0, is defined as the fluorescence lifetime that would be observed in the absence of any nonradiative quenching processes (%0=ke l). 

The crystallization behavior and nanophase transitions were further investigated by laser spectroscopy at 3.5 K. As shown in fig. 28a, no Eu3+ luminescence is recorded for the as-grown sample possibly due to nonradiative quenching by surface defects. In the sample annealed at 600 °C (fig. 28b), however, luminescence lines are observed, but the line width is much broader than for Eu3+ in a crystalline phase, thus suggesting that the Eu3+ ions have amorphous environments. In contrast, the narrow peaks in the emission spectrum of the sam-... 

The exdtons are created away from the electrodes, so they are protected against nonradiative quenching at the metal-polymer interfaces. 

The PL quantum yield r)pl. While r]pl of many dyes is close to 100% in solution, in almost all cases that yields drops precipitously as the concentration of the dye increases. This well-known concentration quenching effect is due to the creation of nonradiative decay paths in concentrated solutions and in solid-state. These include nonradiative torsional quenching of the SE,148 fission of SEs to TEs in the case of rubrene (see Sec. 1.2 above), or dissociation of SEs to charge transfer excitons (CTEs), i.e., intermolecular polaron pairs, in most of the luminescent polymers and many small molecular films,20 24 29 32 or other nonradiative quenching of SEs by polarons or trapped charges.25,29 31 32 In view of these numerous nonradiative decay paths, the synthesis of films in which r]PL exceeds 20%, such as in some PPVs,149 exceeds 30%, as in some films of m-LPPP,85 and may be as high as 60%, as in diphenyl substituted polyacetylenes,95 96 is impressive. 

In the nonradiative quenching, the efficiency (Ep) of depopulation by resonance energy transfer is... 

In conjugated polymers a similar nonradiative quenching process as observed for molecular crystals [2] happens if a SE encounters a trapped or free polaron—acting as a charged defect—during the migration process ... 

Nonradiative reiaxation and quenching processes wiii aiso affect the quantum yieid of fluorescence, ( )p = /cj /(/cj + Rsiative measurements of fluorescence quantum yieid at different quencher concentrations are easiiy made in steady state measurements absoiute measurements (to detemrine /cpjj ) are most easiiy obtained by comparisons of steady state fluorescence intensity with a fluorescence standard. The usefuiness of this situation for transient studies... 

M.L) it may be a MMCT state (see text). The arrow indicates the nonradiative transition from the charge-transfer state d to the ground state, so that c b and c - a emission is quenched... 

Temperature-dependent luminescence measurements in the range from 77 to 300 K show quenching of the peak luminescence by a factor of about 15. Similar behavior is observed in the lifetime quenching [665, 666], As the band gap of the PECVD a-Si H is about 1.6 eV, nonradiative deexcitation of Er may occur at elevated temperatures. The amount of quenching lies in between that of c-Si and LPCVD a-Si H, just like the bandgap. 

Bimolecular reactions with paramagnetic species, heavy atoms, some molecules, compounds, or quantum dots refer to the first group (1). The second group (2) includes electron transfer reactions, exciplex and excimer formations, and proton transfer. To the last group (3), we ascribe the reactions, in which quenching of fluorescence occurs due to radiative and nonradiative transfer of excitation energy from the fluorescent donor to another particle - energy acceptor. 

The next group of bimolecular interactions (3) shown in Table 1, includes noncontact interactions, in which fluorescence quenching occurs due to radiative and nonradiative excitation energy transfer [1, 2, 13, 25, 26]. Energy transfer from an excited molecule (donor) to another molecule (acceptor), which is chemically different and is not in contact with the donor, may be presented according to the scheme ... 

The lifetime, therefore, depends not only on the intrinsic properties of the fluorophore but also the characteristics of the environment. For example, any agent that removes energy from the excited state (i.e., dynamic quenching by oxygen) shortens the lifetime of the fluorophore. This general process of increasing the nonradiative decay rates is referred to as quenching. 

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