Phosphorescence excitation spectra

This fluorescence and phosphorescence (curve IVa and IVb) originate only from those excited molecules whose intramolecular hydrogen bond is broken. This is proven by the phosphorescence excitation spectrum where the long wavelength band (intramolecular hydrogen bond) is lacking, Fig. 5, curve II. 

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

Phosphorescence is also characterized by an excitation and an emission spectrum. Because the lowest triplet state invariably lies lower in energy than the lowest excited singlet state of the same molecule, phosphorescence will occur at wavelengths longer than those of fluorescence and, therefore, at wavelengths much longer than those of the excitation spectrum. As in the case of fluorescence, the phosphorescence spectrum and the phosphorescence excitation spectrum are distorted by the instrumental components and therefore do not represent true spectra. 

The phosphorescence excitation spectrum may be used to decide whether the lowest triplet state is a (n,.w ) or a state, if measurements are carried... 

Figure 5.17. Phosphorescence excitation spectrum of p-hydroxybenzophen-one in an ether-toluol-ethanol mixture with (—) and without (—) the addition of ethyl iodide. The curves are normalized in such a way that the S— T excitation of the (n, i ) state shows about the same intensity in both solvents (by permission from Kearns and Case, 1966).

Phosphorescence emission occurs in glassy ethanol (77 K) at 440 nm for ace-toveratrone and a-(2-methoxyphenoxy)-3,4-dimethoxyacetophenone (Figure 3.18) [145,192], and at 460 nm for vanillin [193], The excitation spectra observed for vanillin show an unusual feature. The fluorescence excitation spectrum is, as expected, identical to the absorption spectrum. The phosphorescence excitation spectrum, however, does not show the weak band at 350 nm and has enhanced intensity at 300 nm. This implies that intersystem crossing to T, occurs from S2 rather than Sj... 

Figure 6. Comparison of the phosphorescence excitation spectrum of polypropylene film (--) before and after irradiation for 250 hr in a Xenotest-150...

To determine the fluorescence excitation spectrum, one selects a wavelength on the emission monochromator (generally the fluorescence Ama ) and scans the exciting wavelength with the excitation monochromator. Normally the fluorescence excitation spectrum will have the same shape and wavelength distribution as the absorption spectrum. However, in cases where the absorption band for a fluorescent state is hidden beneath that of a nonfluorescent state or when insufficient sample is available to run a routine absorption spectrum, the fluorescence excitation spectrum can provide very valuable information. Similarly, for phosphorescence where direct absorption to the triplet level does not occur one can learn a great deal from the phosphorescence excitation spectrum. This will be discussed in more detail later. 

Some examples of luminescence characteristics In commercial polymers are given In Table 11. The effect of additives is Illustrated In Figure 35, from which it can be seen that addition of titanium dioxide to Nylon-66 Introduces a new excitation band in the phosphorescence excitation spectrum (136). 

The phosphorescence excitation spectrum may be used to decide whether the lowest triplet state is a Hn,jr ) or a jiy7t ) state, if measurements are carried out in two different solvents with or without the heavy-atom effect, respectively. The intensity of the phosphorescence excitation spectrum will in general increase due to the heavy-atom effect if T, is a state as in p-hydroxy-... 

Fig. 34. Laser phosphorescence excitation spectrum of thioacetone 2.4.4 Higher Excited States...

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