Polarization of the Lowest-Energy Allowed Transition of β-Ionylidene Crotonic Acid and the Electronic Structure of the Polyenes

Abstract

Polarized specular reflection spectra have been obtained for two faces of the all‐trans β‐ionylidene crotonic acid single crystal in the 700‐to‐220‐mμ region. Visual inspection of these results leads to the assignment of the absolute polarization of the lowest allowed transition, which is believed to be the first accurate absolute assignment for a polyene‐related compound. The polarization is confirmed by the integrated intensities of the absorption bands derived through a Kramers—Kronig analysis of the reflection data. The relationship between the experimental polarization and that predicted by various zero‐order theories that have been applied to the polyenes is examined and the effect of refinements in these theories on the direction and magnitude of the transition moment explored.