Über einige photochemische Reaktionen

Abstract

Two classes of light-induced reactions are discussed.

The first class concerns the photochemical reactions of nitrophenyl esters and -ethers. Some years ago it was discovered that upon irradiation of aqueous solutions of these compounds – that are stable in the dark – a remarkably smooth hydrolysis takes place. The meta-derivatives were found to react more rapidly than the ortho and para isomers. Evidently, the specificities of reaction in the photo-excited state are completely different from the ones observed in the ground state. Solutions of m-nitrophenylphosphate in methanol yield nitrophenol and methylphosphate on illumination. Addition of methylamine to the system lowers the rate of hydrolysis; N-methylnitroaniline is formed. The point of attack – that may be either phosphorus or carbon – in these, seemingly heterolytic, reactions has been studied by tracer technique (¹⁸O).

The second class of photochemical reactions occurs with hexatriene- and cyclohexadiene derivatives. A survey of the results recently obtained in the vitamin D field is given and a scheme of the essential photochemical isomerizations occurring is presented, together with their quantum yields. This scheme is corroborated by the fact that prolonged irradiation of the various isomers leads to the same quasi-photostationary state, the composition of which is in accordance with what is calculated on the basis of the data given in the scheme. No experimental evidence for the occurrence of a triplet state as an intermediate in these reactions was found.

The remarkable stereochemical specificities of the photochemical reactions are discussed and possible ways of understanding are suggested, based on the stereochemical features and electronic structures of the photo-excited states of the molecules. Special attention is paid to the phenomena observed with the photochemical cis/trans isomerization of the double bond.

The following tentative conclusions are presented:

1. Photochemical reactions are not necessarily of a homolytic nature but may also proceed by mechanism of heterolytic fission.
2. These reactions may also occur in the short-lived photo-excited singlet state and not exclusively in the triplet states as is frequently stated.
3. The reactions proceeding in the excited state show a remarkably high specificity which is different from that found in the ground state.
4. Combination of the results obtained from investigations of simple molecules and of complicated systems has proved its usefulness in elucidating problems in this field.