Sensitised photoreactions

In the following, four examples of different types of sensitised photoreactions are discussed first taking into account energy transfer. In all these cases lifetimes and rate constants of the intermediate steps, respectively, determine the concentrations necessary to obtain a noticeable turnover. The different parameters for these necessary concentrations are listed for these mechanisms in Table 3.3. Details are discussed in Appendix 6.6.1.2 for these four examples. 

A further example of a sensitised photoreaction is the case of an aromatic ketone (A), which acts as a chemical sensitiser in the photooxidation by molecular oxygen (C) of a hydrocarbon (B) to form a peroxide R-OOH (D) 

Survey on different mechanisms of physically sensitised photoreactions 

Applying the Bodenstein hypothesis to A and to the radical intermediates E, F, and G, one obtains 

In this case the quantum yield depends on the concentration of B and is independent of the concentration of C. 

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Some other simple uniform mechanisms are the basis of photochemical reactions. These are photoadditions, the photoreduction and some sensitised photoreactions which are discussed in the following. 

The integrals (3.43) and (3.44) can be simplified, if in a sensitised photoreaction just the sensitiser A absorbs the incident light. In this case the photokinetic factor (for its derivation and its meaning see Section 1.4.4), given by eq. (1.41) is constant... 

The chemically sensitised photoreaction according to Example 3.12, using eq. (3.9) for the reciprocal of the photochemical quantum yield,... 

Another example of such a mechanism is the photophysically sensitised photoreaction, which is derived in Section 3.1.1.4. In the following example, the mechanism via the triplet is chosen (see Table 3.3). However, according to Example 3.9 one can set up an equation in general coordinates which differs from eq. (3.74). One finds... 

Example 3.10 Physically sensitised photoreaction with another partner Another possible scheme for the physically sensitised reaction... 

When the light is adsorbed by the catalyst C, the system represents a sensitised photoreaction which may occur through two different ways ... 

Farid s group describes the results of an investigation of the quantitative aspects of electron transfer sensitised arene photoreactions in this article the fate of the radical ion pair generated by irradiation of an electron acceptor in the presence of an arene donor was probed by measurement of the quantum yield of separation of the ion pair. Combination of this with the rate constant for separation of the ion pair allowed the determination of the rate constant for back electron transfer... 

Further studies of the deoxygenation of alcohols by triplet sensitisation and SET to their S-methyl dithiocarbonates under a variety of conditions have been reported. The properties of 2,4,6-triphenylthiapyrilium tetrafluoroborate as an SET sensitiser have been discussed. The triplet excited state of 6H-purine-6-thione acts as an electron donor for p-dinitrobenzene and as an electron acceptor for tetramethylbenzidine. Photolysis of sulfuryl chloride in the presence of tetramethylsilane yields exclusively Me3SiCH2S02Cl. In the presence of added yttrium(III) chloride or sulfur, the reaction is less selective and Me3SiCH2Cl is also obtained. In contrast the presence of these photocatalysts in the photoreaction of sulfuryl chloride with hexamethyldisiloxane yields solely Me3SiOSi-Me2CH2S02Cl, whereas in their absence Me3SiOSiMe2CH2Cl is also obtained. ... 

Start from the molecules B or B", formed according to eqs. (1.19) and (1.20). If these reactions are followed by another chemical reaction, the photoreaction of B is called a (physically) sensitised reaction. On the other hand, if A shows a photoreaction, then the processes (1.19) and (1.20) are competitive reactions. Therefore the molecule B desensitises A and quenches the photoreaction starting at A. The process (1.20) is photochemi-cally very important. It allows the transfer of a precise amount of energy to the molecule B. 

Both the photo-parallel reaction and the consecutive photoreaction, being physically sensitised, form two linear independent steps of the reaction. Their mechanisms and the reduced reaction scheme are listed in Table 3.5. The overall reaction schemes are derived in Appendix 6.6.2. 

Mechanisms and reaction schemes of parallel and consecutive photoreactions, physically sensitised... 

As derived in Example 3.13 the mechanism of a photoreaction sensitised by radicals results in a quantum yield dependent on concentration and on intensity of irradiation. One finds according to eqs. (3.10) and (3.36) for the intensity independent factor Z ... 

Details for simple non-uniform photoreactions 6.6.2.1 Physically sensitised parallel photoreaction... 

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