Relative quantum yield calculation

The energy transfer efficiency for this system can be calculated from relative quantum yield of the donor... 

Fig. 5.4. DMABN/propanol solution. Variation with T l of the viscosity (curve B) and of the relative quantum yield of the B state calculated with the best fit values of t" and t°...

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. 

Thus, in rigid solutions, the critical transfer distance R0 can be determined experimentally either from the observed relative quantum yields of D or A at various concentrations of A (Equation 2.44) or from time-resolved measurement of the fluorescence decay of D (Equation 2.42). The results are in good agreement with those calculated from the Forster Equation 2.37. 

The lifetime of the 450 nm fluorescence from methyl salicylate was measured over the range 40-353 K. The lifetime was nearly temperature independent between 40 and 160 K. Above 160 K the lifetime became shorter as the temperature was increased (see Table 1 and Fig. 6). Over a temperature range 253-333 K the relative quantum yield was found to decrease with increasing temperature. The change in the relative quantum yields of the 450 nm emission closely paralleled those observed in the lifetime data. Thus the radiative rate remains constant, while the nonradia-tive rate depends on the temperature. Using the fluorescence lifetime at 40 K as an estimate for a temperature-independent radiative rate it was possible to calculate the temperature-dependent nonradiative rate using the... 

Before developing it, we ought to mention that these two stages — MO s and electronic states — are "static levels of description. They should be followed by a third level which is the dynamical analysis of the behavior of the molecule(s) along the calculated potential energy surfaces. The static surfaces can be expected to predict the nature of many or most possible products. The relative quantum yields, the relative rates, and the possible wavelength dependence of the formation of these products can only be obtained from a dynamical study involving, in particular, a detailed analysis of the "radiationless" jumps between various surfaces. 

Since there is only one transfer rate, the donor decay remains a single exponential figure 14.2, top). It is this assumption of a single distance which allows calculation of the distance from the relative quantum yield of diedonor (Chapter 13). 

The transfer of energy from the donor to the acceptor results in the donor returning to the ground state and the acceptor entering an excited state. Relaxation from this state can then result in fluorescence from the acceptor. fret can therefore be calculated experimentally in a number of ways, using the relative quantum yields (4>), fluorescence intensities (f) or lifetimes (t) of the donor molecule in the presence (indicatedby superscript A) and absence of the acceptor ... 

Figure 5.69 summarizes the temperature behavior of decay time and quantum efficiency t of red benitoite luminescence at 660 nm in the forms of x and q as a functions of 1/T. In such case the luminescence may be explained using simple scheme of two levels, namely excited and ground ones. The relative quantum yield at a specific temperature (T) was calculated as ... 

Fig. 6 The magnitude of the initial flash-induced absorbance change at 815 nm in isolated wild type (solid triangles) and mutant (open circles) reaction centers as a function of light intensity. Excitation was at 600 nm with a 1 microsecond pulse. The wild type and mutant samples are those shown in the absorbance spectra of Fig. 1. A relative quantum yield was calculated from the shift in the intensity required to achieve half maximal saturation of the 815 nm absorbance change, essentially as described previously (Blankenship and Parson, 1979). The curves shown have been normalized at the highest light intensity used.

The lifetime is independent of temperature between 40 and 160 K however, above 160 K the hfetime becomes shorter, with the relative quantum yield decreasing as the temperature increases (see Table 68.5). These data make it possible to calculate the temperature-dependent nonradiative rates, which yield an activation energy of 3.7 kcal/mol, implying the existence of the excited zwitterions as well as the tautomer form. Nevertheless, interpretation of these experiments is comphcated by thermal and solvent effects, thus a proper description of the formation of the blue emission has not been unambiguously estabhshed. In order to eliminate these problems, supersonic jet excitation spectroscopy was found to provide new information regarding the spectra of the isolated molecule and dynamics at different excess vibrational... 

UV-radiation intensity was controlled with the help of ferrioxalate technique. Relative quantum yields of naphthalene eonsumption were calculated by the initial parts of kinetic curves. 

Quantum-chemical calculations which utilize the density functional theory (DFT) are now perhaps amongst the most frequently performed because of their relatively low cost and high accuracy. Structural results obtained from DFT based methods are often as good as those derived from MP2 calculations. It is well documented that DFT methods, especially those involving hybrid functionals such as B3LYP, B3P86 and B3PW91, yield reliable... 

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. 

The percent yield calculated in this way corresponds to the amount of product X relative to all products and is not the same as the percent of the starting material converted to X (this latter expression is given by x/a/Co, where C0 is the number of moles of reactant). It should be noted that similar to the relationship between rate and efficiency, a low quantum yield for a photochemical reaction does not necessarily mean that it will be formed in a low yield (the converse is true, however). If, for example, x = 0.005 and 2, t = 0.00005, then... 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

Using the molecular weight change and the number of photons required for the change, the absolute quantum yield at 2537 A of PMMA was obtained where the value of 0.9 was used as the density of PMMA film. Once the absolute quantum yield at 2537 A was obtained, the absolute quantum yields over all the spectral range can be calculated from the relative data of the spectral sensiti-... 

Many probes are now known that display changes in fluorescence lifetime on complexation of the analyte, photophysical properties some of them are summarized in Table 10.2. While we have listed the lifetimes of the free and the bound forms of the probes, there is no straightforward equation to calculate the analyte concentration using the mean lifetime as was in the case of the absorbance and intensity (Eqs. (10.14) and (10.15)). The mean lifetime depends not only on relative concentration of the probe species (free and complexed) but also on their decay times, quantum yields, and to some extent on the measurement (method or conditions). While the mean lifetime is independent of total probe concentration, this value generally depends not only on analyte concentration but also on excitation and observation wavelengths.03 ... 

Tryptophan at 77 K in rigid solution has a phosphorescence quantum yield of 0.17(20) and a lifetime of 6 s. These values at 77 K are relatively invariant from protein to protein and do not vary significantly between buried and exposed tryptophans.(21,22) If one assumes that the intersystem crossing yield is a constant, a calculation of the quantum yield of indole phosphorescence can be roughly estimated from the lifetimes. The phosphorescence yield is related to lifetime by... 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products < .,v(A)wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

While the relative importance of the various paths is not well established, it is expected that dissociation to the alkoxy radical, RO, and N02 will predominate. Luke et al. (1989) experimentally measured rates of photolysis of simple alkyl nitrates and compared them to rates calculated using the procedures outlined in Chapter 3.C.2. Figure 4.22 compares the experimentally determined values of the photolysis rate constants (kp) for ethyl and n-propyl nitrate with the values calculated assuming a quantum yield for photodissociation of unity. The good agreement suggests that the quantum yield for photodissociation of the alkyl nitrates indeed approaches 1.0. 

Although the above indirect method provided relatively accurate results, a new method of direct measurement of the IR phosphorescence of O2 ( A ) at 1270 nm was expected to improve the measurement [103]. The quantum yield of two-photon singlet oxygen generation, was calculated from... 

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