Photoinduced catalytic reactions

In addition, unlike the photoinduced catalytic reaction, which may continue to proceed after the flux of incident light quanta... 

Figure 6.13 Examples of photoinduced catalytic reactions (a) alkene hydrogenation by the [Fe(CO)3(alkene)f photocatalyst and (b) the double bond migration by the [Fe(CO)3(1-pentene)] photocatalyst both catalysts are generated in photosubstitution and photodissociation reactions of the [Fe(CO)5] precursor [28]...

The photoinduced catalytic reactions in which both light and catalyst are not consumed, are of importance in organic syntheses, but are of minor significance in environmental chemistry. Much more important are those in which the solar energy is consiuned... 

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 ... 

It should be emphasized that a criterion s > 1, applied earlier by us and other authors to characterize photoinduced catalytic reactions, does not adequately describe this category, because < )s may be well less than unity if (1)C 1 or the deactivation of the catalytically active species proceeds rapidly. 

Unfortunately, the terms photoassisted catalysis (here defined as photoinduced catalytic reaction) and photocatalysis (here described as photoassisted reaction, not including the photosensitized reactions) are recommended in the rather briefly commented "Glossary of terms used in photochemistry" [21]. 

An example of a light-induced chain reaction, which we assign also to the category photoinduced catalytic reactions, is shown in figure 4. Photo-... 

According to our definitions, the well studied valence isomerization of quadricyclane to norbomadiene in the presence of appropriate transition metal complexes is also regarded as a photoinduced catalytic reaction. This reaction was recently discussed by Kutal[35], [36] as photo-generated catalysis. The observed quantum yield exceeds unity (( )s = 1.6) because a chain reaction is involved. Photochemically formed [Ru(bpy)3] , e.g., initiates the chain reaction by oxidation of quadricyclane to the corresponding cation radical which acts as chain carrier. The quadricyclane cation radical undergoes an isomerization to the more stable norbomadiene cation radical. The oxidation of quadricyclane by the latter one leads to the formation of norbomadiene as the product upon regeneration of the chain carrier. 

The synthesis of the pheromon S(-)cw-verbenol in high enantiomeric excess based on a photoinduced catalytic reaction in the presence of a cyclodextrane-substituted iron(III) porphyrin complex (21) [43] may be... 

Nagle, J. K., Roundhill, D. M. Excited State Properties and Photoinduced Catalytic Reactions of Ru0>py)3 and Pt2(pop)4" A Comparison of Outer-Sphere vs. Inner-Sphere Photochemistiy/ Chemtracts, Inorganic Chemistry 1992,4,141. 

Hao H-G, Zheng X-D, Lu T-B (2010) Photoinduced catalytic reaction by a fluorescent active cryptand containing an anthracene fragment. Angew Chem Int Ed 49 8148-8151... 

Figure 1. Simplified Jablonski-type diagram of photoinduced catalytic (I), photoassisted (II), and sensitized photoreaction (III) as well as catalysed photolysis (IV). (MLn i + L) represents coordinatively unsaturated species, free ligands as well as ligands and/or coordination compounds with changed formal oxidation number generated by photo-redox and/or photodissociation reactions.

Considering the applications, it is advantageous to distinguish between two limiting cases of photocatalysis, photoinduced catalytic, and photoassisted reactions. 

UV irradiation of the MgO catalyst in the presence of trans-l-C Hs at 273 K led to a marked enhancement in the formation of ds-l-C Un as well as in the formation of I-C4H8. The activity of the MgO catalyst for such photoinduced reactions depends strongly on the evacuation temperature of the MgO. When MgO was degassed at higher temperatures, the yields of the photoinduced isomerization reactions increased linearly with the UV irradiation time, indicating that the reactions proceeded catalytically under UV irradiation at 273 K 96-98). 

This chapter is intended to focus on catalysis in both thermal and photoinduced electron transfer reactions between electron donors and acceptors by investigating the effects of an appropriate substance that can reduce the activation barrier of electron transfer reactions. It is commonly believed that a catalyst affects the rate of reaction but not the point of equilibrium of the reaction. Thus, a substance is said to act as a catalyst in a reaction when it appears in the rate equation but not in the stoichiometric equation. However, autocatalysis involves a product acting as a catalyst. In this chapter, a catalyst is simply defined as a substance which affects the rate of reaction. This is an unambiguous classification, albeit not universally accepted, including a variety of terms such as catalyzed, sensitized, promoted, accelerated, enhanced, stimulated, induced, and assisted. Both thermal and photochemical redox reactions which would otherwise be unlikely to occur are made possible to proceed efficiently by the catalysis in the electron transfer steps. First, factors that accelerate rates of electron transfer are summarized and then each mechanistic viability is described by showing a number of examples of both thermal and photochemical reactions that involve catalyzed electron transfer processes as the rate-determining steps. Catalytic reactions which involve uncatalyzed electron transfer steps are described in other chapters in this section [66-68]. 

Among various photoinduced catalytic redox reactions, tremendous efforts have been made in relation to artificial photosynthesis [373,736-738,741-744], The target reacdons in ardficial photosynthesis should not necessarily be restricted to those of natural photosynthesis. One of the most crucial points should be how the uphill reacdon can be induced with the use of water molecule as an ideal electron donor. Special attendon has been paid to a splitdng of water upon visible light iiradiadon [373,736-738,741-744] ... 

Fig. 2 Simplified representation of photocatalytic reaction pathways ) photo induced catalytic reaction, N denotes a nominal catalyst which yields photochemically the real catalyst C (2) photoinduced chain reaction, here I means any photoinitiator (3) photoinduced chain reaction initiated by a sacrificial agent I (4) photoassisted reaction with N as nominal catalyst (photo-assistor) (5) photoassisted reaction, the catalyst N is a short-lived intermediate in ground state (6) sensitized photoreaction, N represents a sensitizer (7) catalyzed photo-reaction, C is to be considered as a real catalyst.

The ratio is the criterion to decide whether a photocatalytic reaction can be assigned to the categories photoinduced catalytic and photoassisted, respectively. 

Fig. 3 Illustration of selected photoinduced catalytic (I) and photo-assisted reactions (II) ([MLpX] represents a transition metal complex and S are educts and P products of an appropriate photocatalytic cycle).

In the following part some examples are given to illustrate both photoinduced catalytic and photoassisted reactions. The experimental findings have been chosen somewhat arbitrarily and are partially focused to experimental results obtained in the authors laboratory. 

The aimed synthetic designing of transition metal complexes or organometallic compounds as well as the choice of the wavelength of irradiation (ligand field, charge transfer, and inner ligand excitation, respectively) allows for tuning photoinduced catalytic and photoassisted reactions. 

Naphthaleneamine. 1-Naphthylamine or a-naphth5iamine/7i5 -i2- can be made from 1-nitronaphthalene by reduction with iron—dilute HCl, or by catalytic hydrogenation it is purified by distillation and the content of 2-naphthylamine can be reduced as low as 8—10 ppm. Electroreduction of 1-nitronaphthalene to 1-naphthylamine using titania—titanium composite electrode has been described (43). Photoinduced reduction of 1-nitronaphthalene on semiconductor (eg, anatase) particles produces 1-naphthylamine in 77% yield (44). 1-Naphthylamine/7J4-J2-. can also be prepared by treating 1-naphthol with NH in the presence of a catalyst at elevated temperature. The sanitary working conditions are improved by gas-phase reaction at... 

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