A Study of Photochemical Reactions of Conjugated Trienes

Abstract

In the last decade there has been done considerable study of the photochemical isomerization and dimerization of conjugated dienes. Studies on both direct and indirect irradiation of conjugated dienes can be readily found in chemical literature and many of these are discussed in a review article by G. J. Fonken.1 On the other hand, there has been very little work done on the conjugated triene system, especially sensitized isomerization and dimerization. If simple examples of conjugated diene and triene are taken, such as pentadiene and heptatriene respectively, we see that the diene has only one possible way of isomerization but that of triene has two as shown below. The present work was initiated with the intention to determine whether in the photosensitized isomerization of a triene, reaction at the 1,2- or the 3,4 - position of the triene is favored. However, in our study of the isomerization of the conjugated trienes we observed efficient dimerization accompanying isomerization. Therefore, the photosensitized dimerization reactions of the trienes have also been studied. The two trienes we used in our studies were 2,6-dimethyl-1,3,5-heptatriene,I, and 1,3,5-hexatriene,II. We have attempted to determine the quantum yield and photostationary state compositions of photosensitized isomerization reaction and the mechanism of dimerization reaction and the structures of the dimers.