A. A. M. Roof

Mechanisms in sensitized photochemistry of environmental chemicals

Abstract This review consists of three main parts. After a short discussion of the theory of photochemistry, the three major mechanisms (radiative, short range and long range) for the transfer of electronic excitation energy from sensitizers (donors) to substrates (acceptors) are described. Criteria for the selection of. sensitizers are a high efficiency of intersystem crossing (in the case of triplet sensitization) and energy transfer, and strong absorption at wavelengths longer than those at which the acceptor absorbs. Two mechanisms of sensitized photo‐oxygenation, viz. the radical and singlet oxygen type, are discussed. The second part of this review deals with photosensitizers, with a strong emphasis on compounds of importance in environmental photochemistry on the one hand and those interesting from a mechanistic point of view on the other. The third part describes photo‐inducers and their role in photoreactions. Additional literature references (to January 1978) are presented at the end of this review.

Photochemistry of halogenated benzene derivatives. I. Trichlorobenzenes : reductive dechlorination, isomerization and formation of polychlorobiphenyls

Abstract Irradiation of the three trichlorobenzenes with λ > 285 nm in a few solvents results in reductive dechlorination as the major reaction, and isomerization and formation of polychlorobiphenyls in some cases.

Photochemistry of halogenated benzene derivatives. Part 2. Photoreactions of α-substituted p-chlorotoluenes

Solution-phase photoreactions of α-substituted p-chlorotoluenes of the general structure p-ClC6H4CH2X (X = H, Cl, CN, CO2H and OH) have been studied at wavelengths around 300 nm. The acetone-sensitized photolyses of the substrates with X = H, CN, CO2H and OH in deaerated CH3OH provided reductively dechlorinated photoproducts C6H5CH2X with yields of 10–52% based on photodecomposed starting material. The photoreactions of p-chlorobenzyl chloride (2) proceeded through homolytical and heterolytical cleavage of its side-chain C–Cl bond possibly via a triplet excited state. The main product of the photochemical reactions of (2) in deaerated CH3OH both with and without acetone present was p-chlorobenzyl methyl ether.In order to achieve a better understanding of some photochemical aspects of the photolyses of substrate (2), the photoreactions of three halogenobenzaldehydes have been studied. The irradiation of p- and m-chloro- and m-bromo-benzaldehyde in deaerated CH3OH gave rise to the formation of the corresponding acetals as the sole principal products. The photoreactivity of m-BrC6H4CHO was 93 times greater than that of m-ClC6H4CHO.

The photochemistry of α-aryl carboxylic anhydrides—II : Photolysis of some substituted phenylacetic anhydrides☆

Abstract The photolysis of phenylacetic acids and anhydrides with λ 254 nm was studied. The main product is bibenzyl, but with o- and p-methoxyphenylacetic anhydride, in addition substantial amounts of methoxybenzyl methoxyphenylacetates are formed. The photoreactivity of phenylacetic acid strongly reduces upon substitution of the phenyl ring, this in contrast to the behaviour of the corresponding anhydrides. The quantum yields for substrate conversion and product formation are reported. The mechanisms of the photo-formation of the products are discussed. Experimental evidence is presented to support the proposition that the ester formation occurs via a dipolar intermediate.

Photochemistry of α-aryl carboxylic anhydrides. Part 4. Photoreactions of some asymmetrical anhydrides derived from o- and p-methoxyphenylacetic acid, and of some of the phenylacetate ester photoproducts

Irradiation of the two asymmetrical carboxylic anhydrides R1C6H4CH2·CO·O·CO·CH2C6H5(1a and b) at 254 nm in acetonitrile leads to the formation of esters and bibenzyls. Among the esters produced, the asymmetrical ones with structure R1C6H4CH2·O(CO)CH2C6H5 are the most abundant. These are for the greater part formed intra-molecularly by decarbonylation of one excited anhydride molecule. This result is in line with a mechanism involving electron transfer after excitation of the anhydride molecule. The bibenzyls are formed by the recombination of benzyl radicals. In the initial stage of the reaction of the anhydrides, the three bibenzyls are formed with the asymmetrical one predominating [(1a) gives the three bibenzyls (3a, c, and e) in yields of 20, 35, and 10%, respectively]. In the later stages, when substantial amounts of esters are present, the formation of the bibenzyls also originates in part from the photodecomposition of the esters. The photochemistry of the various (symmetrical and asymmetrical) esters which are photoproducts from the anhydrides (1a and b) has also been studied. The symmetrical esters (2a and b) afforded quantitatively the bibenzyls (3a and b), respectively. The asymmetrical esters gave variable yields of the three (possible) bibenzyls, depending on the type of ester considered. For the asymmetrical esters for which bibenzyl formation could be quenched by (Z)-piperylene, the three bibenzyls were formed in a close to statistical ratio, e.g.(2c) yielded the bibenzyls (3a, c, and e) in a ratio of 1 : 2.5 : 1. With the esters for which the photodecarboxylation was not quenchable by (Z)-piperylene, a relatively higher yield of the asymmetrical bibenzyl was found, e.g.(2d) yielded the bibenzyls (3b, d, and e) in a ratio of 1 : 6 : 1. These results are rationalized in terms of a recombination of free radicals resulting from the triplet excited esters after spin inversion in the former case, and a recombination of a radical pair in the solvent cage resulting from the singlet excited or short-lived triplet excited ester in the latter case.

The photochemistry of α-arylcarboxylic anhydrides. Part 3. Photoreactions and luminescence spectra of 1- and 2-naphthylacetic anhydrides and 1,3-di-(1-naphthyl)propan-2-one

The photochemistry of 1- and 2-naphthylacetic anhydride and the luminescence of these anhydrides and of two naphthyl-substituted propanones has been studied. 1-Naphthylacetic anhydride upon irradiation with λ 300 nm yields mainly 1,3-di-(1-naphthyl)propan-2-one. With λ 254 nm, the main product is 1,2-di-(1-naphthyl)ethane. 1,3-Di-(1-naphthyl)propan-2-one proved to be photostable for λ 300 nm; it is, however, photoreactive upon irradiation with λ 254 nm and then yields 1,2-di-(1-naphthyl)ethane. Essentially similar results have been obtained with 2-naphthylacetic anhydride. With both anhydrides a characteristic emission at λca. 400 nm has been observed which is ascribed to intramolecular excimer fluorescence. The excimer emission is more pronounced with the naphthylacetic anhydrides (with 6 σ bonds between the two naphthyl groups) than with 1,3-di-(1-naphthyl)propan-2-one (with 4 σ bonds between them). The assignment of the emission of the ketone at λ 400 nm to excimer fluorescence was made on the basis of a comparison with 1-(1-naphthyl)propan-2-one, which cannot form an intramolecular naphthyl excimer. Molecular model studies suggest that both the anhydrides and 1,3-di-(1-naphthyl)propan-2-one can adopt a sandwich type of conformation required for intramolecular excimer formation. The absence of fluorescence of the naphthalene monomer type for 1,3-di-(1-naphthyl)propan-2-one as well as its photostability (λ 300 nm) may be explained in terms of intramolecular transfer of singlet energy from the naphthyl to the carbonyl group, followed by intersystem crossing and subsequent intramolecular quenching of the carbonyl triplet by the naphthyl moiety.

Synthesis of Retro-γ-Ionols and the Corresponding-Ionones1

Abstract Both the direct2 and the sensitized3,4 photolyses of (E)-β-ionol (2) have been studied in some detail. In a preliminary publication5 we have indicated that direct photolyses of (E)-β-ionol (2) with λ = 254 nm yields (Z)-retro-γ-ionol (3) as the primary product; upon further irradiation 3 is converted into the corresponding (E)-isomer (4) which rapidly yields the bicyclic alcohol 5. A quantitative study revealed that the photoconversion of (E)-β-ionol with λ = 254 nm to 3 is about 10 times faster than the conversion of 3 into (E)-retro-γ-ionol.6 This rate difference thus allows the photosynthesis of 3.