Quenching of emission

Many of the adsorbents used have rough surfaces they may consist of clusters of very small particles, for example. It appears that the concept of self-similarity or fractal geometry (see Section VII-4C) may be applicable [210,211]. In the case of quenching of emission by a coadsorbed species, Q, some fraction of Q may be hidden from the emitter if Q is a small molecule that can fit into surface regions not accessible to the emitter [211]. 

A third type of bimolecular reaction, summarized by equation (24), is that of excited state proton transfer where Q is now a proton acceptor. This type of process was proposed in the case of the OH" and CO32- quenching of emission from [RhCl(NH3)5]2+.48... 

Fig. 4.6.11. Reductive ET in pyrenyl-modified DNA duplexes. Excitation of the Py-dX group at 340 nm results in ET, yielding the Py 1 dX biradical. If ET to the adjacent DNA bases occurs as an alternative pathway, quenching of emission is observed.

Fig. 2. The isosbestic points at 446 and 556 nm in the absorption spectra are matched by an isoemissive point at 685 nm indicating only two species present in solution, both of which are emissive. The shift in emission maximum from 606 nm in neutral solutions to 728 nm upon addition of acid may have interesting sensor applications. The results for 4 stand in contrast with results from dppz-containing Ru(II) tris diimine complexes, where dppz = dipyrido-ipyridophenazine, in which reversible protonation of quinoxaline N atoms leads to quenching of emission. Luminescence in frozen solvent glasses for 4 at 77 K is much stronger ( = 0.044 for the qdt complex), but still broad and without resolved structure.

Heat treatment of as-grown crystals in Li2C03 at 400 to 700°C for 2 to 7 days causes a partial substitution of lithium for Zn sites causing considerable growth of resistivity to p 10 -10 ° Qcm because of the LTi = Li zn and noticeable lowering of the Fermi level. The luminescence spectra of such samples are characterized by enhanced yellow-orange emission noticeable quenching of emission due to excitons bound to neutral shallow donors. 

Figure 1. Fluorescence quenching of emission spectra due to Cu(II) ion titration of 15 ppm soil fulvic acid at 0.1 M ionic strength and pH 5. The FA sample was excited at 335 nm and emission wavelengths were scanned from 300 to 600 nm. Cu(n) concentrations of each emission spectra are (a) 0 uM, (b) 4.9 uM, (c) 9.8 uM, (d) 14.7 uM, and (e) 22 uM.

Figure 11. Stem-Volmer plots for the quenching of emission intensity ( ), emission lifetime ( ), and photoreaction quantum yield (O) of Pt(tpy)2 by anthracene. The inset shows a plot of r/° versus 7/7° or t/t°. From Ref. 117 with permission of American Chemical Society.

TABLE 2 The kq Values at Ambient Pressure and AFq Values for Quenching of Emission from 3[Cu(dpp)2t] in Dichloromethane"... 

Sometimes fluorescence emissions are not observed, even though the correct excitation is used. There are two causes of loss of fluorescence emission. One cause is quenching of emission photons. Quenching occurs when the energy of the excited... 

Alternatively, although Process II cannot be the rate determining process it may still be the emission process provided that the quenching of emission determines the rate of decay. If the bimolecular processes (Processes III and IV) are the important quenching processes, Equation 3 remains applicable. Also, as delayed fluorescence has been shown to be unimportant, then either k4 or the factor f is small and Equation 2 reduces to ... 

Two reports of a static contribution to the quenching of emission from Rum complexes have been published. Measurement of both phosphorescence lifetimes and intensities allowed Bolletta et al 7 to observe static quenching (through ion-pair formation) of 3[Ru(bipy)3]2+ by [Mo(CN)3]4- and [PtClJ2- in DMF. For [IrCle]3- only dynamic quenching was found. Demas and Addington78 77... 

Nd " ion has a disadvantage, which is called self-quenching of emission, due to its complex electronic stmcture. Por downconversion, an excited Nd " ion in the " p3/2 level could transfer part of the excitation to an unexcited Nd " ion. At room temperature, the process is dominated by the cross-relaxation process, i.e., (%3/2, " 19/2) C Ii5/2> " Ii5/2). Iti itiost Nd-doped laser materials, the final levels of the... 

The increased self-quenching of emission, as shown in Eig. 9.3, has been widely acknowledged as a disadvantage of Nd laser materials with increasing doping concentration [30]. Actually, the effect of Cn<3 should be evaluated by considering both the pump absorption efficiency rj and the emission quantum efficiency that have effects on the laser parameter, i.e., the incident pump power [29, 30, 32]. 

Excited States.—Several groups of workers have reported further studies of quenching of emission of excited states by the electron-transfer mechanism (83). In previous... 

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