Pseudo quantum yield

We insert Equations 3.19 and 3.21 into

differential equation used for the spectrophotometric determination of quantum yields the reason for introducing the so-called pseudo quantum yield Q = [Pg.116]

The rate of photolytic transformations in aquatic systems also depends on the intensity and spectral distribution of light in the medium (24). Light intensity decreases exponentially with depth. This fact, known as the Beer-Lambert law, can be stated mathematically as d(Eo)/dZ = -K(Eo), where Eo = photon scalar irradiance (photons/cm2/sec), Z = depth (m), and K = diffuse attenuation coefficient for irradiance (/m). The product of light intensity, chemical absorptivity, and reaction quantum yield, when integrated across the solar spectrum, yields a pseudo-first-order photochemical transformation rate constant. 

Most commonly absorption or fluorescence spectroscopy is used for detection of the changes in the concentration of G or HG. The monitoring wavelength is chosen so that the difference between the molar absorptivities, in case of absorption, or emission quantum yields, in the case of fluorescence detection, between G and HG is maximized. The amplitude of the relaxation process depends on the difference in the molar absorptivities or fluorescence quantum yields, but the observed rate constants are the same at all observation wavelengths when the kinetics are first- or pseudo-first order (Fig. 3). 

Many of the reactions discussed in the preceding pages are in fact bimolecular processes, which would normally follow second-order kinetics. However, as aheady discussed, under the regime of LFP they behave as pseudo-first-order reactions. The corresponding rate constants and lifetimes are independent of the initial concentration of transient, and therefore knowledge of extinction coefficients and quantum yields is not needed. Further, it is not important to have a homogenous transient concentration. 

Here and in what follows the subscripts in and out indicate the Chi molecules localized near the inner and outer surfaces of the vesicle membranes. If no more than one pair of Chl+ and A- particles is generated on the inner surface of each vesicle, the recombination by reaction (7) may be described in terms of first-order kinetics. Having in mind that the number of D, A and Chlout molecules considerably exceeds the number of 3Chl and Chl+ particles, one can treat all the remaining rate constants of reaction sequence (5)-(9) also as pseudo first-order ones. In accord with this reaction scheme, the quantum yield of the transfer of the first electron through the vesicle membrane can be expressed as ... 

Very little is known about the mechanism of the chemliminescent reaction. Even the identity of the excited species is unknown at pre-ent. It is known that the deuterium isotope effect (i.e., 4a-FlEt—O—O —CH(OH)R/4a-FlEt—O—O—CD(OH)R) on the quantum yield is approximately 2 the breaking of the C—H(D) bond is at least partially rate controlling. The proposal (50) that the pseudo base (i.e., 4a-FlEtOH) is formed as an excited species seems unlikely. 

In ketones with quantum yields lower than unity, some other decay mode is competing with hydrogen abstraction. Cyclobutyl ketones provide nice examples of low quantum yields being due to very low xf values. The rate of decay of triplet benzoylcyclobutane is 5 X 105 sec-1 in benzene 83> the quantum yield for type II elimination and cyclization is only 0.03 and varies sharply with substitution on the benzene ring 84>. Hydrogen abstraction can occur only when the benzoyl group is in the pseudo-axial conformation a. 

Pseudo-stilbenes may emit fluorescence that is, contrary to true stilbenes, generally weak at room temperature and often weak even at low temperatures. Protonated azobenzene-type molecules and many protonated azo dye molecules emit strong fluorescence in sulfuric acid at 77 K with quantum yields of about 0.1. Inclusion of azobenzene in the channels of AIPO4-5 crystals provides complexation of the n-electrons and space confinement. This leads to emission by protonated azobenzene at room temperature. For their cyclopalladated azobenzenes, Ghedini et al. " report quantum yields of ca. 1T0 and lifetimes of ca. 1 ns. In contrast, donor/acceptor pseudo-stilbenes, if emitting at low temperatures or when adsorbed to surfaces, are weak emitters. In textile chemistry, it has long been known that azo dyes adsorbed to fibers may show fluorescence. ... 

The photoisomerization mechanism in aminoazobenzene- and pseudo-stilbene-type compounds has attracted far less attention than the mechanism for azobenzenes. In pseudo-stilbenes, the (n,7t ) state is buried under the intense it —> Jt band and cannot be populated selectively. No state-specific quantum yields are available because the yields are independent of the exciting wavelength.There is only a very narrow experimental basis for a discussion of these two mechanisms. However, this may change when pseudo-stilbenes are subjected to ultrashort-time experiments. 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

In this mechanism, is a pseudo-first order rate constant that includes the effective solvent concentration. is the rate constant for geminate recombination, assumed to be the same for both the ground and excited state intermediates. From a steady state treatment the quantum yield for ML5S formation when all of the reaction originates in a is given by... 

Consider, now, how the energy is dissipated. In the absence of B, A may lose its energy either as fluorescence emission or in some non-radiative process such as interaction with the solvent. On the convention used by Forster and Weller the rate coefficients or probabilities for these two processes are denoted by and sec S respectively. The lifetime Tq of the exdted spedes is then ( f+nquantum yield < >o of the fluorescence process is f/( f+nthird method of energy dissipation from A is by reaction with B for this a pseudo-first order rate coeffident k2C sec may be assigned. The lifetime of A (x) is now given by... 

Because ,Z-isomerization is reversible and proceeds with quantum yields which are generally much higher than those for other productive decay processes, direct irradiation of polyenes in solution leads to the formation of a pseudo-equilibrium mixture of geometric isomers, whose composition is dependent on the quantum yields for isomer-interconversion, the extinction coefficients at the excitation wavelength (Xex) of the interconverting isomers and the quantum yields for fonnation of other products which do not revert to any of the geometric isomers of the original polyene ". 

A mathematical model for the measurement of pseudo first order rate constants in laser flash photolysis has been put forward. Another data treatment provides a method for determining quantum yields of reactions of the type A(+hv) = B(+hv,A)... 

The effect of humic materials on the photolytic micellar system was evaluated in DR s photodegradation. DR solubilized within Tween 80 micellar solution with or without humic materials was determined. In order to calculate the quantum yield, the molar absorptivity of DR was determined by spectrophotometry. The determination of the quantum yield and reaction rates was examined through a pseudo first-order decay rate expression. Quenching and catalytic effects resulting from the humic substances were examined through Stem-Volmer analysis. A reaction mechanism of photolytic decay of DR solubilized within surfactant micelles in the presence of various amount of humic materials was proposed for this purpose. The effect of high and low concentration of humic materials has been accounted for by a designed model. 

In order to quantitatively characterize the photochemical reaction, several mathematical procedures were performed. These included I) Determination of pseudo first-order razte constants and quantum yields and 2) Stem-Volmer analysis of photochemical kinetiacs (quenching and rate enhancement study by humic materials). 

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