Theoretical Study of the Photoisomerization Mechanism of All--Retinyl Acetate.

The compound 9--retinyl acetate (9--RAc) is a precursor to 9--retinal, which has potential application in the treatment of some hereditary diseases of the retina. An attractive synthetic route to 9--RAc is based on the photoisomerization reaction of the readily available all--RAc. In the present study, we examine the mechanism of the photoisomerization reaction with the use of state-of-the-art electronic structure calculations for two polyenic model compounds: -octatetraene and -2,6-dimethyl-1,3,5,7,9-decapentaene. The occurrence of photoisomerization is attributed to a chain-kinking mechanism, whereby a series of S/S conical intersections associated with kinking deformations at different positions along the polyenic chain mediate internal conversion to the S state, and subsequent isomerization around one of the double bonds. Two other possible photoisomerization mechanisms are taken into account, but they are rejected as incompatible with simulation results and/or the available spectroscopic data.