Spectroscopy phosphorescence

Phosphorescence is more difficult to observe than fluorescence because of the possibility of quenching by impurities present in a sample. Phosphorescence is also less probable than fluorescence. On the other hand, many compounds that do not fluoresce do exhibit phosphorescence. Phosphorescence offers sometimes advantages with respect to UV spectrometry as to specificity however, in practice the sensitivity of phosphorescence usually does not exceed that of UV spectroscopy [389a]. Fluorescence is considerably more important than phosphorescence in analytical chemistry. Although application of phosphorescence to the analysis of additives in polymers is restricted, the direct determination is an asset. 

Various textbooks describe phosphorescence spectroscopy (c/r. Bibliography). 

Since the determination of inhibitors in polymers by Drushel et al. [13] little information has been added to the literature on the quantitative aspects of the direct examination of polymer films by phosphorescence spectroscopy. These authors examined phosphorescence (at liquid-nitrogen temperature) of thin EPR films containing Santonox R (2,2 -di-methyl-5,5 -di-r -butyl-4,4 -dihydroxydiphenyl sulfide) and V-phenyl-2-naphthyl-amine (PBN). The rather intense phosphorescence of PBN may be used to advantage when other additives interfere in the UV absorption method. As to quantitative phosphorescence analysis, several factors, e.g. film thickness, concentration quenching, and background absorption, etc., affect the linearity of the analytical working curves and precision of the measurements [13]. The reliability of a correlation between stabiliser concentration in the film and phosphorescence intensity at 77 K is also influenced by the degree of crystallinity [544]. 

Oxygen quenches phosphorescence of aromatic hydrocarbons in plastics at r.t. but not at liquid nitrogen temperature [547]. Luminescence spectroscopy (phosphorescence at 77 K and fluorescence at r.t.) may be used to evaluate oxidation processes in plastic materials, e.g. in LDPE films [548]. Recently, Allen et al. [549] have reported that prolonged melt oxidation of PET results in extensive discoloration and the formation of highly fluorescent hydroxy-lated terephthalate units which exist in equilibrium with highly phosphorescent stilbenequinone units. Phosphorescence characteristics of some common polymers are available [505]. 

Phosphorescence of dyes in PVAL can be observed at room temperature without prior evacuation to remove oxygen, which often quenches the emission in other systems [550]. Also phosphorescence of dyes in PVC has been observed. 

For compounds that are very weakly phosphorescent or that phosphoresce at wavelengths out of the normal range of sensitivity of the spectrometer this method of triplet energy determination cannot be applied. For these compounds triplet energies can sometimes be determined by measuring their E-type or P-type delayed fluorescence. 

One of the first explanations for the afterglow of organic compounds in rigid solutions after exposure to UV light was offered by Perrin. Perrin postulated that the excited molecules could undergo a transition to a metastable state of lower energy. Emission from this state was thouglht to be 

The flrst observations of P-type delayed fluorescence arose from the photoluminescence of organic vapors. It was reported that phenanthrene, anthracene, perylene, and pyrene vapors all exhibited two-component emission spectra. One of these was found to have a short lifetime characteristic of prompt fluorescence while the other was much longer lived. For phenanthrene it was observed that the ratio of the intensity of the longer lived emission to that of the total emission increased with increasing phenanthrene vapor 

Let us now return to the question of how E-type and P-type delayed fluorescence may be used to determine the triplet energy level. The efficiency of E-type delayed fluorescence is given by the following equation  

Since O should be independent of temperature in the temperature range 

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Figure 5.7 Layout of a typical fluorescence spectrometer. After Rendell [132]. Reprinted from D. Rendell, Fluorescence and Phosphorescence Spectroscopy. Copyright 1987 John Wiley Sons, Limited. Reproduced with permission...

D. Rendell, Fluorescence and Phosphorescence Spectroscopy, John Wiley Sons, Ltd, Chichester (1987). 

A number of experimental techniques are available for the determination of triplet energy levels. Those most commonly employed are phosphorescence spectroscopy, phosphorescence excitation spectroscopy, singlet to triplet... 

The fractions from elution chromatography were studied by a number of spectroscopic methods, n.m.r., i.r., u.v., fluorescence and phosphorescence spectroscopy. Equivalent fractions from chromatographic separation of the various oils showed no significant differences in their spectra and it appears that the composition of the fractions was independent of the catalyst concentration used to produce the oil. Though, as previously mentioned the amounts of the various fractions especially the polar fractions differ with the catalyst concentration. G.1.C. analysis of the saturate fractions also indicated no changes with different catalyst concentrations. 

SG Schulman. Fluorescence and Phosphorescence Spectroscopy, Physiochemical Principle and Practice. London Pergamon Press, 1977. 

Fluorescence spectroscopy forms the majority of luminescence analyses. However, the recent developments in instrumentation and room-temperature phosphorescence techniques have given rise to practical and fundamental advances which should increase the use of phosphorescence spectroscopy. The sensitivity of phosphorescence is comparable to that of fluorescence and complements the latter by offering a wider range of molecules for study. 

Schulman S. G. (1977) Fluorescence and Phosphorescence Spectroscopy Physicochemical Principles and Practice, Pergamon Press, Oxford. 

S. Subramanian, J. B. A. Ross, L. Brand, and P. D. Ross, Investigation of the nature of enzyme-coenzyme interactions in binary and ternary complexes of liver alcohol dehydrogenase with coenzymes, coenzyme analogs, and substrate analogs by ultraviolet absorption and phosphorescence spectroscopy, Biochemistry 20, 4086-4093 (1981). 

Tinti, D. S., El-Sayed, M. A. New techniques in triplet state phosphorescence spectroscopy Application to the emission of 2,3-dichloroquinoxaline. J. Chem. Phys. 54, 2429 (1971). 

Fluorescence and phosphorescence emission spectroscopy were employed to study the interaction of E. coli purine nucleoside phosphorylase (PNP) with its specific inhibitor, FA. The results show, for the first time, the application of phosphorescence spectroscopy to the identification of the tautomeric form of the inhibitor bound by the enzyme <2004MI377>. 

TABLE 6.10 Phosphorescence Spectroscopy of Some Organic Compounds... 

A wide range of possibilities exist for further improvements in the near future. Two of these are the use of a second-derivative attachment so that the techniques described in the previous section can be applied to synchronous spectroscopy, and the application of the synchronous method to phosphorescence spectroscopy which is described in the following section. 

When the sample is stimulated hy application of an external electromagnetic radiation source, several processes are possible. For example, the radiation can be scattered or reflected. What is important to us is that some of the incident radiation can be absorbed and thus promote some of the analyte species to an excited state, as shown in Figure 24-5. In absorption spectroscopy, we measure the amount of light absorbed as a function of wavelength. This can give both qualitative and quantitative information about the sample. In photoluminescence spectroscopy (Figure 24-6), the emission of photons is measured after absorption. The most important forms of photoluminescence for analytical purposes are fluorescence and phosphorescence spectroscopy. 

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