Photogenerated catalysis

Thus, photocatalysis and photogenerated catalysis indeed open up rather reach opportunities in fine organic synthesis, including some new reactions and nontraditional pathways for some known reactions. More efforts should be made in engineering of appropriate photocatalytic reactors for such synthesis. 

The first more comprehensive classification of the photocatalytic processes was proposed by Salomon, who divided the processes into two main classes (1) photogenerated catalysis, which is catalytic in photons, and (2) catalyzed photolysis, which is non-catalytic in photons [130] (Figure 6.11). 

Photoinduced catalysis means the photogeneration of a catalyst that subsequently promotes a catalyzed reaction. Photons are required to generate the catalyst only. Thus, the efficiency of such processes depends only on the activity of the catalyst produced photochemically and, in homogeneous photocatalysis, the turnover number (TON) is the useful tool. The TON is usually expressed as the number of moles of product formed per mole of catalyst and, for photoassisted catalysis, TON <1, whereas for photogenerated catalysis TON >1 and even 1 [135], Therefore, high turnovers of photochemically produced catalysts are one of the main criteria concerning efficient photocatalytic processes. Quantum yields (ratio of moles of product formed to the number of photons absorbed) >1 may occur. The same is true for photoinduced chain reactions. 

Catalyzed photolysis refers to catalysis of a photochemical reaction, for which there is a physical pathway for decay of the system back to its ground state (Figure 6.17). When the photocatalytic process occurs through photoexcitation of the catalyst, the physical decay may occur through recombination and/or thermal photoionization of the excited states, which ultimately leads to regeneration of the original state of the catalyst. Note that catalyzed photolysis is not catalytic in photons, contrary to photogenerated catalysis. 

Photocatalysis includes various reactivity types which may be divided into two categories depending on the role of light on (i) photogenerated catalysis consisting of reactions catalytic in photons and (ii) catalyzed photolysis, in which processes are non-catalytic in photons (Fig. 1). 

Several processes that are catalytic (a photon is not a substance) in photons and involve a catalytic quantity of one compound have been reported. Different labels were associated with such an overall situation electron transfer induced chain reactions [7], photoinduced catalytic reactions [8], or photogenerated catalysis [9]. The main experimental observations which characterize such processes are ... 

In this Scheme, pC stands for pro-catalyst, C for catalyst, CS for a complex between catalyst and substrate, CP for a complex between catalyst and product, I for an initiator. S for a structural variation of the substrate, R for an added reagent. In cases 1.1 and 1.2 the catalysis is based on a coordinative interaction between catalyst and substrate in case 1.1 the product is released to regenerate C (for example by reductive elimination) whereas in case 1.2 the regeneration of CS results from a substitution of the complexed product by S. It should be clear that cases 1.1 and 1.2 do not exhaust the formal possibilities offered to photogenerated catalysis. One may actually imagine a photogeneration of catalyst from a selected pro-catalyst for any of the multiple catalytic cycles identified in homogeneous catalysis centered on transition metal complexes [12]. 

In cases 1.1 to 1.4 the role of the photon is played outside the catalytic cycle, which explains why the quantum yield of P is usually greater than 1. In stoichiometric photogenerated catalysis [9] (Scheme 1.5), also labeled photoassisted [8, 14], photoenhanced [15], or photoactivated [16], the role of the photon is inside the catalytic cycle therefore every S —> P transformation consumes a photon and the overall quantum yield for the production of P is smaller than 1. Scheme 1, case 1.5 should not be confused with sensitization because in the step S + C —> P + pC both the substrate and the catalyst are in their ground state, which is not the case for photosensitization. 

Figure 5.9 Schemes representing various pathways for the case of photogenerated catalysis.

In the case of photogenerated catalysis, two different but equivalent models are worth considering the Langmuir-Hinshelwood photocatalytic process and the Eley-Rideal photocatalytic process. The former is described by Mechanism II, in which the reaction occurs at a photochemically active surface when light is absorbed by the catalyst and leads to the generation of surface electrons (e ) and holes (h" ). 

To complete the consideration of the previous examples, it is relevant to note that photogenerated catalysis (stage 15a ) does not require continuous irradiation since the excited state is reproduced at the end of each cycle. In this case, the excited state might be created during a pre-irradiation stage and there is no physical decay during the course of the reaction. As a result, stages 12 and 13 can be excluded from this consideration in Mechanism III. 

Even though the active (excited) state of the catalyst is restored in both catalysed photolysis and photogenerated catalysis, the kinetic parameters are different (Serpone et al, 2000), since we deal with a different excited state of the catalyst, i.e. effectively we deal with different catalysts. Indeed, under irradiation the state of the catalyst e.g. a semiconductor) may be characterised by the splitting of the Fermi level into two quasi-Fermi levels (in catalysed photolysis), whereas in photogenerated catalysis the state of the catalyst is characterised by a unique Fermi level (in the dark). This level may however be shifted compared with the original state of the catalyst because of the possibility of having different excited states after pre-excitation of the catalyst. 

At this time, it is appropriate to reflect further on the principal difference(s) between photogenerated catalysis and catalysed photolysis. As implied earher, any photochemical reaction (5.42) starts from light absorption by a reagent M to form an excited state of the molecule, M. ... 

Unfortunately, to date no data are available for the quantitative estimation of the role of photogenerated catalysis and photocatalytic reactions over halides and oxides with wide band gaps in the global chemistry of the atmosphere. Therefore, in the estimates made above, only photocatalytic reactions over the oxide semiconductors Fc203, Ti02, and ZnO with relatively narrow band gaps were taken into account. However, new compounds may be added in the future to the list of photocatalysts that are important to the chemistry of the atmosphere. 

Carbonyl-containing compounds such as acetophenone, benzophenone, and chloranil can sensitize not only the 1—>2 rearrangement but also the back reaction, though quantum yields for the latter (O 0.05-0.10) are much less than those for the direct photoreaction. - Photogenerated catalysis of the 2—>1 reaction with some metal complexes is a more effective process, since a single photon may produce several molecules of N according to the sequence... 

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