Fluorescence, from excited species

Self-absorption Absorption of part of the fluorescence from excited molecular entities by molecular entities of the same species in the ground state. The mechanism operating is a radiative energy transfer... 

Fluorescence from absorbing species in solution may also contribute to interference. However, this kind of interference is rare and minimal because, first, the lower source intensities employed in UV-visible spectrophotometry imply that fewer fluorescent species become excited, and second, the fluorescence emitted, if any, will be too weak to significantly influence the accuracy of measurement. 

Selectivity The selectivity of molecular fluorescence and phosphorescence is superior to that of absorption spectrophotometry for two reasons first, not every compound that absorbs radiation is fluorescent or phosphorescent, and, second, selectivity between an analyte and an interferant is possible if there is a difference in either their excitation or emission spectra. In molecular luminescence the total emission intensity is a linear sum of that from each fluorescent or phosphorescent species. The analysis of a sample containing n components, therefore, can be accomplished by measuring the total emission intensity at n wavelengths. 

This emission occurs at longer wavelengths than the normal fluorescence. The exciplex emission, on the other hand, occurs from an excited associated complex formed between an excited species and a different ground-state species. 

Figure 5.6 Fluorescence. Absorption of incident radiation from an external source causes excitation of the analyte to state 1 or 2. Excited species can dissipate the excess energy by emission of a photon or by radiationless processes (dashed lines). The frequencies emitted correspond to the energy differences between levels...

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

Photon emission must be a favorable deactivation process of the excited product in relation to other competitive nonradiative processes that may appear in low proportion (Fig. 3). In the case of sensitized CL, both the efficiency of energy transfer from the excited species to the fluorophore and the fluorescence efficiency of the latter must be important. 

In multicomponent systems A"0 can be written as a sum of the individual absorption coefficients A ot = 2TA , where each AT,(A ) depends in a different way on the wavelength. If one or more of the components are fluorescent, their excitation spectra are mutually attenuated by absorption filters of the other compounds. This effect is included in Eqs. (8.27) and (8.28) so that examples like that of Figure 8.4 can be quantified. The two fluorescent components are monomeric an aggregated pyrene, Mi and Mn. The fluorescence spectra of these species are clearly different from each other but the absorption spectra overlap strongly. Thus the excitation spectrum of the minority component M is totally distorted by the Mi filter (absorption maxima of Mi appear as a minima in the excitation spectrum ofM see Figure 8.4, top). In transparent samples this effect can be reduced by dilution. However, this method is not very efficient in scattering media as can be seen by solving Eqs. (8.27 and 8.28) for bSd — 0. Only the limit d 0 will produce the desired relation where fluorescence intensity and absorption coefficient of the fluorophore are linearly proportional to each other in a multicomponent system. 

Another consideration in flames is radiatioiL The light that one sees in a flame is mostly fluorescence from the radiation of particular radical species formed in electronically excited states. (The blue color from CH4 flames is CH emission.) Gases also radiate blackbody radiation, primarily in the infiared. The glow from burning wood or coal is blackbody emission radiated from the surface. 

Conventional EPR techniques have been successfully used to measure the D and E values of matrix-isolated carbenes in the ground triplet state because the steady-state concentration of triplet species is sufficiently high in the system. The technique cannot be used, however, for excited species having triplet hfetimes of the order of 10-100 ns, since their steady-state concentration is too low. The D parameters are estimated from the external magnetic field effect on the T—T fluorescence decay in a hydrocarbon matrix at low temperamre. The method is based on the effect of the Zeeman mixing on the radiative and nonradiative decay rates of the T -Tq transition in the presence of a weak field. The D values are estimated by fitting the decay curve with that calculated for different D values. The D T ) values estimated for nonplanar DPC (ci symmetry) is 0.20... 

These reactions are, respectively, photostimulation [at a rate F(f) species per second], fluorescence from the excited fluorophor, non-radiative de-excitation, and fluorescence quenching. Consider, for instance, that the photostimulation only occurs at time t — f0 that is, F(t) = F05(f — f0). Then, the concentration of the excited fluorophor [A ] varies according to... 

Fig. 8 Serum PC molecular species distribution after 6 weeks of treatment for both the placebo control group (A) and the drug-treated group (B). The molecular species were derivatized with 7-methoxycoumarin-3-carbonyl azide prior to analysis and detected by fluorescence with excitation at 340 nm and emission at 370 nm. (Reprinted from Ref. 91 with the permission of the Journal of Lipid Research.)...

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