Quantum yields overall

Although the electrostatic field on the polyelectrolyte surface effectively impedes back ET, it is unable to retard very fast back ET or charge recombination of the primary ion pair within the photochemical cage. The overall quantum yield of photoinduced ET is actually controlled in most cases by the charge recombination. Hence, its retardation is the key problem for attaining high quantum yields in the photoinduced ET. 

Importantly, the overall quantum yield ( = 0.2) in equation (63) indicates that the fragmentation of bicumene cation radical occurs at a slightly slower rate (kc c 108 s 1) than the back electron transfer (/tbet 4 x 108 s—1) within the ion-radical pair in equation (64). 

If the reactive species in the chemical activation step initiates a radical chain with a chain length CL, then the overall quantum yield based on the ultimate product is X CL, and can be greater than 1. Photons are rather expensive reagents, and are only used when the product is of substantial value or when the overall quantum yield is large. Examples are the use of photoinitiators for the curing of coatings (a radical-polymerization process (Section 7.3.1)), and the transformation of complex molecules as medications. 

Note that the overall quantum yield is

[Pg.452]

The overall quantum yield is the number of molecules of reactant, R, consumed per photon of light absorbed. 

Irradiation of mixtures of hydrocarbons and chlorine at suitable wavelengths leads to chlorination of the organic molecule (Scheme 1.3). Reactions have overall quantum yields in excess of 106 (>106 propagation cycles for each termination step). 

The PFR also takes place with aryl alkyl carbonates [125]. Methoxy-substi-tuted dervatives can undergo substitution of MeO by the acyl moiety, as has been reported for esters and amides. This is shown in Scheme 47 for 2-methoxyphenyl ethyl carbonate (168) [126]. On the other hand, the overall quantum yield of photoproducts is 10-fold lower in the para-than in the ortho- or (meta- me-thoxyphenyl. Chlorophenyl ethyl carbonates do not rearrange, but undergo C—Cl homolysis. The efficency of photodechlorination follows the order para < meta < ortho [127],... 

Sensitized PET reactions are often very slow and have low quantum yields due to dominating back-electron transfer. In these cases, the addition of cosubtrates (e.g., biphenyl or phenanthrene to DCA- or DCN-sensitized reactions) is useful. The use of such an additive is called cosensitization. In these reactions, the substrate is not oxidized (or reduced) by the excited sensitizer but by the radical ion of the cosensitizer (ET, ). This is a thermal electron-transfer step without the problems of back-electron transfer. The key step is the primary PET process (ETJ in which the cosensitizer radical ion is formed. The main characteristic of cosensitization systems is the high quantum yield of the free-radical ion (e.g., [Pg.189]

In some cases, then, the overall quantum yield, rather than the primary quantum yield, is reported. The overall quantum yield for a particular product A, usually denoted by A, is defined as the number of molecules of the product A formed per photon ab-... 

An important measure of the luminescence is the quantum yield. In effect, this is the probability that a photon will be emitted by the lanthanide given that one photon has been absorbed by the antenna ligand. Since measurement of absolute quantum yields is particularly difficult, the overall quantum yield ( ) is normally measured with reference to certain standards (26) these are routinely [Ru(bpy)3]2+ in water or SulfoRhodamine 101 in methanol for Eu3 +, and quinoline sulfate in 0.1 M HC1 or fluorescein in 1 N NaOH for Tb3+ (27,28). A method has been developed that measures energy transfer from the lanthanide complex to an acceptor of known quantum yield (28). 

Due to the competing non-radiative decay routes for the lanthanide excited state, there is an intrinsic limit to the overall quantum yield in luminescent lanthanide complexes. It has been estimated that these values are 0.50 and 0.75 for europium and terbium, respectively (27). Although quantum yields exceeding these have been reported (31,32), care should be taken in analyzing quantum yield results in the literature, as these are often given for the energy transfer process alone, and not the overall quantum yield, and in other cases it is unclear as to which process(es) the quoted quantum yield refers to. 

The influence of these various effects may be manifested in measurable parameters of the reaction like the overall quantum yields (On) and the photoproduct ratios for fragmentation to cyclization (E/C) and for trans to cis cyclobutanol formation (t/c) as shown in Scheme 41. The values of these quantities and their variations as the media are changed can provide comparative information concerning the relative importance of solvent anisotropy on Norrish II reactions, also. Specifically, they reveal characteristics of the activity of the walls and the size, shape, and rigidity of the reaction cavities occupied by electronically excited ketones and their BR intermediates. 

To avoid purification of the reaction product from the (colored) sensitizer or its oxidation and photolysis products, the use of insoluble sensitizers has been proposed, in particular for sensitized oxidations [14]. Whereas sensitizers adsorbed on solid supports, such as ion exchange resins, silica, or alumina [15], show considerable leading rates [16] and must be discarded, potential application might be found for sensitizers that are chemically bound to inert surfaces [14-17]. However, a loss of efficiency of at least 50% has to be taken into account when comparing overall quantum yields with those determined in homogeneous reaction systems [17]. 

From the mechanism the overall quantum yield of 03 decomposition is 4. 

Some of these processes lead to chain reactions in which a new Cl atom is formed in a (dark) free radical reaction. A single Cl atom formed in the photochemical reaction can therefore lead to overall quantum yields of several thousands. 

On the other hand, in a photoassisted reaction a catalytical ly Inactive precursor ML is transformed photochemically into a photoassistor ( here MLn. or L ) which may bring about the transformation of the substrate A into the product E3, either thei— mally or by absorption of a second photon while the photoassistor itself is converted back to the precursor ML-n ln this process ML is not consumed, however, the overall quantum yield is 1. 

In the case of ligand sensitization, the overall quantum yield of a lanthanide-containing molecular edifice is given by... 

Thus if one wants to improve the overall quantum yield of /3-diketonate complexes, removal of coordinated water molecules is absolutely necessary. By means of the estimated nonradiative deactivation rate constants, calculations showed that removal of these water molecules allows one to reach a maximum quantum yield of 2.6% in toluene for the Ybm-tta complex. However, water molecules are usually replaced with a coordinating secondary ligand, such as phenanthroline, which also contributes to the nonradiative deactivation ( Phen 3.6 x 104 s-1), but to a much lesser extent than water molecules. Further improvement can be reached by deuteration of the central C-H group in the /i-dikctonatc in Yb(ttax/i )3(phcn) for instance, deactivation due to C-H oscillators occurs eight times faster when compared to C-D oscillators. 

The complexes of general formula [Ln(61)] (Ln = Nd, Er and Yb) (see fig. 60) exhibit the typical NIR luminescence of each Lnm ion upon excitation in the antenna chromophore. The overall quantum yields have been determined in deuterated methanol and by using the Ndm... 

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