Enhanced photoreactivity

As a general rule, electron-donating substiments attached to the phenolic ring enhance photoreactivity, whereas electron-withdrawing substituents produce the opposite effect [5,6,29], For instance, the relative photo-Fries quanmm yield is more than two times higher for para-hydroxyphenyl benzoate than for para-acetylphenyl benzoate [5,6], Esters with susbstituents such as Cl or CH3 at the para position, as well as the unsubstituted compounds, have intermediate... 

Epling GA, Qiu QW, Kumar A. 1989. Hydride enhanced photoreaction of chlorodibenzodioxins. Chemosphere 18 329-332. 

A particularly promising feature of the Ru(terpy)(phen)(L)2+ series, in relation to future molecular machine and motors, is related to the pronounced effect of steric factors on the photochemical reactivity of the complexes [84]. When the bulkiness of the spectator phenanthroline moiety was increased, the steric congestion of the coordination sphere of the ruthenium complex also increased. This increased congestion was qualitatively correlated to the enhanced photoreactivities of these complexes (Fig. 14). More specifically, changing phen for dmp increased by one to two orders of magnitude the quantum yield of the photosubstitution reaction of L by pyridine with L = dimethylsulfide or 2,6-dimethoxybenzonitrile. 

Photolysis of [Rh(tfacac)3] (tfacac is the unsymmetrically substituted 1,1,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths the relative efficiency of the two paths is dramatically solvent dependent.1140 In cyclohexane, mer- cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of wer-[Rh(tfacac)3] and suggests that the photo reactive states have considerable radical character .1140 Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved. 

Incorporation of titanium oxide species within the framework of mesoporous silicas has been shown to produce highly efficient photocatalytic materials. Extremely careful preparation conditions [84] leads to highly structured materials comprising anatase nanoparticles of dimension between 5 and 10 run. The channeled structure, together with the hydrophobic/hydrophilic character, are also key features controUing their enhanced photoreactivity. The photocatalytic activity of such mesoporous catalysts has been studied for the degradation of phenol in aqueous solutions [85]. It was observed that for structured mesoporous materials with low Ti content, the turnover frequency was four times greater than that for standard P25. 

The enhanced photoreactivity of meta- vs. para-substituted benzenoids has been rationalised in terms of selective transmission of electron withdrawal or donation. A new example of the meta effect has been reported for the excited... 

Ionic halocarbons, including halogenated carboxylic acids, may form pho-toreactive complexes with transition metals in the aquatic environment. Indeed, complexes of carboxylates with dissolved Fe(III) and iron oxides are very photoreactive under solar radiation (28, 29). Photoreactions of such complexes may help to explain the enhanced photoreactivity of chlorinated acetates in natural water samples. 

The enhanced photoreactivity of sorbed nonionic halocarbons may involve photoreactive complexes with amines and other electron-donating substances. The enhanced photoreactivity of ionic halocarbons (e.g., chloroacetates) may involve complexes with DOM and transition metals. Additional studies are needed to examine the role of complexation in the aquatic photochemistry of halocarbons. 

Mashraqui SH, Kellog RM (1985) 3-Methyl-2,3-dihydrobenzothiazoles as reducing agents. Dye enhanced photoreactions. Tetrahedron Lett 26 1453-1456... 

Turro N J and Cherry W R 1978 Photoreaction in detergent soiutions. Enhancement of regioseiectivity resuiting from the reduced dimensionaiity of substrates sequestered in a miceiie J. Am. Chem. Soc. 100 7431-2... 

Photooxidafions are also iudustriaHy significant. A widely used treatment for removal of thiols from petroleum distillates is air iu the presence of sulfonated phthalocyanines (cobalt or vanadium complexes). Studies of this photoreaction (53) with the analogous ziuc phthalocyanine show a facile photooxidation of thiols, and the rate is enhanced further by cationic surfactants. For the perfume iudustry, rose oxide is produced iu low toimage quantifies by singlet oxygen oxidation of citroneUol (54). Rose bengal is the photosensitizer. 

Assuming that the reaction probability of all the elementary processes is equal in the reaction of 1,4-DCB crystals, the calculated yields of unreacted 1,4-DCB, cyclophane, and oligomer by simulation, should be 1.8, 37.7, and 60.5% by weight, respectively. Furthermore, if all the photoexcited species of the monocyclic dimer are assumed to be converted into cyclophane, these yields should become 6.9, 65.6 and 27.5%. It is, therefore, rather surprising that in an extreme case of the experiment the yield of cyclophane is more than 90% while the amount of unreacted 1,4-DCB is less than 2%. One plausible mechanism to explain this result is that the first formation of cyclophane induces the successive formation of cyclophane so as to enhance its final yield. If such an induction mechanism plays an appreciable role, an optically active cyclophane zone may be formed, at least in a micro spot surrounding the first molecule of cyclophane, as illustrated in Scheme 13. The assumption of an induction mechanism was verified later in the photoreaction of 7 OMe crystals (see p. 151). 

For example, the same dimer complex which contains two molar equivalents of ethanol, underwent photoreaction to give a higher molecular weight polymer (M = 12 000). The formation of such inclusion complexes with the solvent is rather generally observed with similar types of dimers formed with alcohols and some other solvents and, consequently, this enhances photopolymerizability. Such complex formation with a solvent may be one of the promising techniques that can be used for diolehn compounds in order to obtain polymers with high molecular weights. 

These unexpected results suggest a modification of the conclusions reached in our earlier studies. Specifically, the observations indicate that in order to detect ODPM photoreactivity in p,7-unsaturated aldehydes, substiments should be present to stabilize intermediate biradicals in the rearrangement pathway, but they should not enhance alternative reactions, such as allylic homolytic cleavage. Further studies will be necessary to confirm this hypothesis and to determine the scope of these new reactions. 

If released to water, 1,3-DNB and 1,3,5-TNB may be subject to direct photolysis when exposed to sunlight because both compounds can absorb light at wavelengths greater than 290 nm (ERA 1976 Mill and Mabey 1985). However, no data were located regarding the photolysis of 1,3,5-TNB in water. The photolytic half-life of 1,3-DNB in pure water was calculated to be 23 days (Simmons and Zepp 1986). A three-to four-fold increase in the rate of photoreaction of 1,3-DNB was observed in ambient waters containing natural humic substances or in distilled water containing dissolved humic materials compared to reaction without humic substances (Simmons and Zepp 1986). This enhancement of the reaction rate has been attributed to catalysis of the photoreaction by photosensitization effects of humic substances. 

As described above, the presence of H20 not only retards the formation of the carbon deposits on the catalyst surface, but also enhances their oxidation to C02 and CO. H20 regenerates the Ti02 surface hydroxyl groups which are consumed in the photoreaction. Based on these results, the following reaction mechanism is proposed focusing on the role of the hydroxyl groups. 

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