Photoreduction efficiency

Wang, W. N. An, W. J. Ramalingam, B. Mukheijee, S. Niedzwiedzki, D. M. Gangopadhyay, S. Biswas, P. Size and Structure Matter Enhanced C02 Photoreduction Efficiency by Size-Resolved Ultrafme Pt Nanoparticles on Ti02 Single Crystals. J. Am. Chem. Soc., 2012,134, 11276-11281. 

The photoreduction efficiency of carbonyl compounds such as 9,10-anthraquinone, 1,4-anthraquinone, 6,13-pentacenequinone, and 2-phenyl-9.10-anthraquinone, in the presence of anthracene, pyrene, naphthalene, biphenyl, 1,4-benzoquinone, and 1,4-naphthoqulnone has been shown to depend on the relative positions and nature of the electronic levels of the substrate and acceptor. Photoreduction of aromatic ketones to alcohols or pinacols can be catalysed by CdS powders using triethylamine as sacrificial donor in MeCN, and morphology is reported by the same authors to affect the two-electron transfer photoreductions of aromatic ketones on CdS induced by visible llghti under analogous conditions olefins behave similarly. Aldehydes of the type RCHO (R -jB -tolyl, g-anlsyl, hexyl) have been photoreduced to the corresp>onding... 

The photoreduction efficiency of ortho-zSkyl benzophenone derivatives is greatly reduced by intramolecular enolization process, known as photoenolization reaction. For example, orf/io-ethyl benzophenone 2 on photoirradiation in deuterated hydroxylic solvents gives deuterated ethyl benzophenone 3 by photoenolization without reduction [3]. 

Liu L, Gao F, Zhao H, Li Y (2013) Tailoring Cu valtaice and oxygen vacancy in Cu/Ti02 catalysts for enhanced CO2 photoreduction efficiency. Appl Catal Environ 134 349-358... 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

Photoinduced ET at liquid-liquid interfaces has been widely recognized as a model system for natural photosynthesis and heterogeneous photocatalysis [114-119]. One of the key aspects of photochemical reactions in these systems is that the efficiency of product separation can be enhanced by differences in solvation energy, diminishing the probability of a back electron-transfer process (see Fig. 11). For instance, Brugger and Gratzel reported that the efficiency of the photoreduction of the amphiphilic methyl viologen by Ru(bpy)3+ is effectively enhanced in the presence of cationic micelles formed by cetyltrimethylammonium chloride [120]. Flash photolysis studies indicated that while the kinetics of the photoinduced reaction,... 

It is important to point out at this point that the rate constant k and the quantum yield for a photochemical reaction are not fundamentally related. Since the quantum yield depends upon relative rates, the reactivity may be very high (large kr), but if other processes are competing with larger rates, the quantum yield efficiency of the reaction will be very small. That there is no direct correlation between the quantum yield and the rate is clearly seen from the data in Table 1.2 for the photoreduction of some substituted aromatic ketones in isopropanol ... 

To obtain more information on this point, let us examine the data given in Table 3.6<42-47> for some substituted benzophenones. The data in Table 3.6 indicate that benzophenone derivatives having lowest triplet states of n->TT character undergo very efficient photoreduction in isopropyl alcohol. Those derivatives having a lowest it- -it triplet, on the other hand, are only poorly photoreduced, while those having lowest triplets of the charge-transfer type are the least reactive toward photoreduction. In additon, in some cases photoreduction is more efficient in the nonpolar solvent cyclohexane than in isopropanol. This arises from the solvent effect on the transition energies for -> , ir- , and CT transitions discussed in Chapter 1 (see also Table 3.7). 

Benzophenone has also been found to be photoreduced in the presence of amines as hydrogen donors, although less efficiently than in the presence of benzhydrol or isopropyl alcohol. The photoreduction of ketones in aromatic amines is thought not to go by the same mechanism as the photoreduction in alcohols, for the following reasons ... 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

Product distributions and reaction conversions of several different photochemical systems, irradiated by conventional UV source and by EDL in a MW-UV reactor (Fig. 14.5), were compared to elucidate the advantages and disadvantages of a micro-wave photochemical reactor [90], Some reactions, e.g. photolysis of phenacyl benzoate in the presence of triethylamine or photoreduction of acetophenone by 2-propa-nol, were moderately enhanced by MW heating. The efficiency of chlorobenzene photosubstitution in methanol, on the other hand, increased dramatically with increasing reaction temperature. 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

With good hydrogen donors photoreduction of nitrobenzene becomes more efficient the rate constant for hydrogen abstraction from tributylstannane by (n, 7i )-nitrobenzene has been determined as 4 x 10 1 mole s-i... 

The efficiency of nitrobenzene photoreduction may be increased remarkably in 2-propanol/hydrochloric acid mixtures. In 50% 2-propanol/water containing 6 moles l i HCl, acetone and a complex mixture of chlorinated reduction products are formed i ). Both HCl and 2-propanol (as hydrogen source) are needed. When sulfuric acid is substituted for HCl, enhanced photoreduction does not occtu . When using mixtures of HCl and LiCl to maintain a constant chloride concentration (6 M) and vary [H+], a constant disappearance quantum yield 366 =0.15 is found within the [H+]-range 0.05—6 moles l i. This strongly suggests that chloride ions play an essential role, probably via electron transfer to 3(n, tt )-nitrobenzene i > [Eq. (1)], but it is also evident from the data presented that the presence of add is probably important in subsequent steps, [Eq. (3)]. 

It should be noted at tins point that the mechanism of photoreduction in amine solvents is highly likely to be quite different from that in hydrogen donor solvents. In the former class of solvents, electron transfer seems to prevail. StericaJly hindered nitrobenzenes are not capable of hydrogen abstraction from hydrogen donors as ethers or 2-propanol (see Section A. 1.4) but are efficiently photoreduced in di- or triethylamine ). 

The following compound is expected or not to undergo efficient photoreduction ... 

A similar mechanism was proposed earlier by Adamson for photoreduction of Co(NH3)5Br2 +. 48 An observed 4>red of 1.97 for Co(NH3)5I2+ predicts a quantum efficiency of 0.97 for the primary process [reaction (20)]. The mechanism also predicts that 4>red will depend upon the Co(NH3)5I2+ concentration and inversely upon the intensity of the irradiating light in the case where recombination of I atoms is important. Support for the mechanism of Haim and Taube came from the observation that upon flash photolysis of Co(NH3)5I2 + solutions with 370-mp. light, unusually short-lived transient I atoms were observed.62 This was taken to indicate that paths [reaction (21), for example] other than I atom recombination accounted for loss of I atoms in this system. 

Since chloride and trifluoroacetate ions affect neither the efficiency nor the Co(II)/Co(III) stoichiometry of photoreduction of 1-p or 2-p, kinetic analysis is possible of the reactions of the cobalt(III) intermediate produced in the photoreduction with water or added anions as shown by reaction (34). Calculation of the ratio, R = kx-jkH20, has... 

One of the limitations of the photocycloaddition reaction is that the unsaturated system may itself act as a quencher. Conjugated dienes fall within this category since they quench the n,ir triplet of some carbonyl compounds. For the photoreduction of benzophenone in benzhydrol, the ratio, kqlka, for m-piperylene is 750 (Table II), which indicates that this diene is indeed an efficient quencher for the reaction. 

Table IV summarizes the pertinent characteristics of some of the naphthyl carbonyl compounds. All of these compounds emit from a it,7T triplet very similar to that of naphthalene. Those that have been studied are resistant to photoreduction in isopropyl alcohol and photocycloaddition with 2-methyl-2-butene25 and isobutylene.17 Significant oxetane formation was, however, observed with the aldehydes, albeit with only moderate efficiency (quantum yield approximately one-tenth that of benzaldehyde).25...

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