Product quantum yield

The study of quantum yields. The quantum yield is the fraction of absorbed light that goes to produce a particular result. There are several types. A primary quantum yield for a particular process is the fraction of molecules absorbing light that undergo that particular process. Thus, if 10% of all the molecules that are excited to the state cross over to the T state, the primary quantum yield for that process is 0.10. However, primary quantum yields are often difficult to measure. A product quantum yield (usually designated ) for a product P that is formed from a photoreaction of an initially excited molecule A can be expressed as... 

Product quantum yields are much easier to measure. The number of quanta absorbed can be determined by an instrument called an actinometer, which is actually a standard photochemical system whose quantum yield is known. An example of the information that can be learned from quantum yields is the following. If the quantum yield of a product is finite and invariant with changes in experimental conditions, it is likely that the product is formed in a primary rate-determining process. Another example In some reactions, the product quantum yields are found to be well over 1 (perhaps as high as 1000). Such a finding indicates a chain reaction (see p. 895 for a discussion of chain reactions). 

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

Table 4.6. The Effect of Solvent Viscosity on the Symmetrical and Unsymmetrical Product Quantum Yields for the Photolysis of p-Methoxydibenzyl Ketone...

The total product quantum yield is higher in acetonitrile than in hexane solution (0.34 vs 0.07) the adduct yield is lower in acetonitrile (9% vs 28% of total product). 

Nitropyrrole does not seem to undergo any photosubstitution with either methoxide, cyanate, cyanide or water 123> 2-Nitrothiophene 1 a) and 5-bromo-2-nitrothiophene (1b) undergo photocyanation smoothly and efficiently. The disappearance quantum yield for 7 a,b equals the product quantum yield. 

Table 3 Product Quantum Yield in the PFR of 20 in Solvents of Different Viscosities...

Likewise, the N derivatives of carbazoles undergo PFR [116-118], Upon irradiation in cyclohexane A-acetyl-carbazole (157) gives the following products (quantum yields in parentheses) [119] 1-acetylcarbazole (158) (0.10), 3-acetyl-carbozole (159) (0.09), and carbazole (160) (0.06) (Scheme 43). The analogous A-acetyldiphenylamine gives the ortho- and para-acetylated products along with diphenylaniline with quantum yields of 0.20,0.096, and 0.048, respectively [119], Table 10 shows that these values decrease from cyclohexane to ethanol, but less markedly than in the case of acetanilide (see Table 8). 

A primary quantum yield is the fraction of light-absorbing molecules that are converted in a particular process. Unfortunately, such values are often very difficult to measure. A product quantum yield is the ratio of the number of molecules of product formed per number of quanta absorbed by the reactant. That product formation is the rate-determining step is a strong possibility if the observed value does not vary with experimental conditions. 

TABLE 4.29 Some Product Quantum Yield Measurements in the Photolysis of C10N02 ... 

Goldfarb, L A.-M. Schmoltner, M. K. Gilles, J. B. Burkholder, and A. R. Ravishankara, Photodissociation of C10N02 1. Atomic Resonance Fluorescence Measurements of Product Quantum Yields, J. Phys. Chem. A, 101, 6658-6666 (1997). 

From the variation in product quantum yields, Heicklen and Knight82 were able to deduce the steps for the biradical mechanism ... 

The accurate determination of incident light intensity is of pivotal importance in any quantitative photochemical experiment. While various physical devices are available for making absolute intensity measurements,168 these devices can be difficult to calibrate and usually are rather expensive. A much simpler approach involves the use of a chemical actinometer. This type of system is based upon a photochemical reaction for which product quantum yields are reasonably insensitive to variations in reactant concentration, temperature, light intensity and excitation wavelength. Once the quantum yield is calibrated by an absolute method, a chemical actinometer becomes a rapid, inexpensive and highly accurate secondary standard for light intensity measurements. 

Stern-Volmer constant from product quantum yield data and limiting quantum yield for triplet sensitized reactions. 

Reactions using the anthracene carboxylate as the primary sensitizer are also known as for instance in the water reduction to hydrogen AC- triplet is quenched by MV2+. The obtained viologen radical cation initiates hydrogen formation mediated by platinum/polyvinyl alcohol. The AC- anion is regenerated from the corresponding radical by electron transfer from EDTA and the H2 production quantum yield is close to unity [165]. 

TABLE 4. Summary of the Previous CO Photolysis Studies in Which Product Quantum Yields Have Been Measured... 

Picosecond and nanosecond transient spectroscopic studies to elucidate the nature of various intermediates formed in the photoinduced electron-transfer reaction of amines have been most extensive for the ketone-amine systems. These studies have been described in detail in a recent review by Yoon et al. [10]. Some aspects of these studies are briefly described here. In the earlier studies of Cohen and coworkers, the photochemical reactions of benzophenone with aliphatic amines were probed by fluorescence quenching, determination of product quantum yields, and nanosecond laser-flash photolysis [143-147]. They proposed that the reactions of amines with... 

All these photocorrosion processes are, of course, undesirable and it is obvious that their relative importance depends strongly on the presence of surface states which may facilitate recombination or redox reactions with adsorbed substrates. It is well known from ESR [69, 70, 94] and emission spectra [94] that most of these metal sulfide powders contain surface states. They are introduced during preparation of the powder as a result of lattice defects [72, 96], trapped holes [94], surface impurities [97] and metallization [38], and during the actual catalytic reaction as a consequence of irradiation and substrate adsorption. The stabilizing effect of plati-nization is exemplified by Figure 6 for the ZnS-catalyzed reduction of water in the presence of sodium formate [98]. Note that platinum does not accelerate the reaction but doubles the time of constant catalytic activity from 1 to 2 days. Similarly, the apparent product quantum yield of the 2,5-DHF dehydrodimerization is not increased but slightly decreases when platinizes ZnS is the photocatalyst [97]. 

Figure 19. ZnS-photocatalyzed dehydrodimerization of 2,5-DHF dependence of apparent product quantum yield on substrate oxidation potential.

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