Quantum yield values

Table 7.16 Fluorescence Spectroscopy of Some Organic Compounds Table 7.17 Fluorescence Quantum Yield Values...

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Solid-surface fluorescence and phosphorescence quantum yield values were obtained from +23° to -180°C for the anion of p-aminobenzoic acid adsorbed on sodium acetate (11). Fhosphorescence lifetime values were also obtained for the adsorbed anion from +23° to -196°C. Table 1 gives the fluorescence and phosphorescence quantum yield values acquired. The fluorescence quantum yield values remained practically constant as a function of temperature. However, the phosphorescence quantum yield values changed substantially with temperature. The phosphorescence lifetime experiments indicated two decaying components. Each component showed a gradual increase in phosphorescence lifetime with cooler temperatures, but then the increase appeared to level off at the coldest temperatures. 

Table I. Fluorescence and Phosphorescence Quantum Yield Values for the Anion of p-Aminobenzoic Acid Adsorbed on Sodium...

These quantum yield values appear to be much higher than unity and therefore demonstrate that carbonization occurs by a chain reaction process. The mechanism of the laser-induced dehydrochlorination of photodegraded C-PVC can be schematically represented by the follo-... 

The quantum yield (Q) represents the ratio between the number of photons absorbed and photons emitted as fluorescence. It is a measure of brightness of the fluorophore and represents the efficiency of the emission process. The determination of absolute quantum yield for a fluorophore is experimentally difficult. Therefore, usually relative quantum yield values are determined. To measure the relative quantum yield of a fluorophore, the sample is compared to a standard fluorophore with an established quantum yield that does not show variations in the excitation wavelength [5, 6]. 

The quantum yield values (<)>fnp) are corrected for the absorption of the excitation light by the non-fluorescent, planar component using Equation 2. 

Some of these problems can be overcome with a different calorimetric design (see later discussion). Other problems, which are more dependent on the chemistry and physics of the process under study than on the instrumentation, require careful attention. Unnoticed side reactions or secondary photolysis are examples, but one of the most serious error sources in photocalorimetry is caused by the quantum yield values, particularly, as explained, when they are small. Unfortunately, many literature quantum yields are unreliable, and it is a good practice to determine n for each photocalorimetric run. Errors in

inner filter effects, that is, photon absorption by reaction products. 

A few examples, emphasizing the anomalies, will serve to illustrate these points. The bulk of the evidence in support of these generalizations will be found in the section on scope and limitations and in the tabular survey of this review. Unfortunately, since there is a dearth of quantum yield values for the photocycloaddition reaction, only a qualitative comparison of the efficiencies of these reactions is usually possible. Table III summarizes typical results with some characteristic phenyl... 

In the mixed complexes, [Ru(i-bq)n(bpy)3 n]2+, i-bq is not involved in the emission252. The absorption spectra show shoulders where [Ru(bpy)3]2+ has its maximum and e at this point is slightly less than that of this compound. These i-bq mixed complexes do show room temperature-fluid emission, but unfortunately quantum yield values are not available. 

Due to the modifications of the electronic cloud induced by complexation, the quantum yield and the excitation spectrum are also modified. As the direct determination of the absolute quantum yield is very difficult to achieve, one usually finds in the literature quantum yield values determined by comparison to well-known standards, such as quinine sulfate. For example, some values can be found in Georges (1993) or in Klink et al. (2000) for some europium complexes but may be found also in many other papers on lanthanide luminescence. Studies on the correlations between the photophysical properties of a given type of europium complexes and the energy levels can be found in Latva et al. (1997), Klink et al. (2000). A correlation has been found between the excitation properties and the stoichiometry of various Eu(III) complexes (Choppin and Wang, 1997). Note that the changes in the excitation maximum induced by complexation usually amount to a few tenths of nanometers, which requires high resolution for detection. In the case of Eu(III), a correlation has been found between the frequency... 

Assuming rrad 1 ms (see table 5) the authors estimate ijsens > 0.6. As a consequence, the low quantum yield values are explained mainly by deactivation of the Ybm excited state occurring through high-frequency vibrational modes of both ligand and solvent. In order to minimize these vibrational deactivations, allyl derivatives 50a-c (fig. 47) of ligands 47b, 47f, and 48a (fig. 41) have been synthesized and luminescence intensities of Ndm and Ybm complexes have been studied either in their monomeric form or in their co-polymeric form with styrene or methylmethacrylate (MMA) (Meshkova, 2000). 

Fig. 14 Photosubstitution quantum yield with the hindered dmp spectator chelate compared to its unhindered analog phen. Quantum yield values are given at room temperature in neat pyridine...

Photophysical and photochemical processes are characterized quantitatively by the quantum yield value (< ), which determines the number of defined events occurring per photon absorbed by the system (X is the wavelength of absorbed radiation) [lj. Integral quantum yield is defined by ... 

Energy-dependent (but not state-dependent) lifetime and quantum yield data are available for some of the more complex carbonyls. With reference to those molecules for which the appropriate data do exist, it can be seen in Table 11 that the radiative lifetime is virtually unaffected over a wide range of energies. This is in sharp contrast to the observed behavior of formaldehyde. The tr values for acetone might be less reliable than the others listed they are calculated using the fluorescence lifetime data of Breuer and Lee (42) and the fluorescence quantum yield data of Heicklen (105). It can be seen that there is no substantial change in tr the apparent (perhaps the real) trend is in the direction opposite that observed for formaldehyde. The values for perfluorocyclobutanone have been calculated using the rp and relative quantum yield values of Lewis and Lee (141) and the absolute fluorescence yield of 0.021 as measured by Phillips (187) as a standard. 

Values reported are fractions of total yield and not absolute quantum yields. Values in parentheses are wavelengths used. 

The value of ((i(H2)=0.4 appears to be the most reliable. The higher values including 0.5 which are still currently used would give primary processes) > 1 for many systems such as N2O (166), methanol (127), ethanol (128), isopropanol (129), di-t-butyl ether (87) and methyl-n-propyl ether (88), an unreasonable result. However, using <(>(H2)=0.4, (Ksum of primary processes) approaches or equals but never exceeds unity. If normalized to the same actlnometer quantum yield value, results from different laboratories, for example, the H2 yield of 1M methanol in water ( cf. 127,166-168), agree within 10 percent. 

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