Spectroscopic properties of polyenes. I. The lowest energy allowed singlet-singlet transition for cis- and trans- 1,3,5-hexatriene

Abstract

The absorption spectra of cis‐ and trans‐ 1,3,5‐hexatriene are reported for the region of the first allowed singlet‐singlet electronic transition, 2600 to 2100 A. A least‐squares procedure in which a Lorentzian line shape is assumed for each band was used to fit the spectra. Progressions are found of 408, 1262, and 1626 cm⁻¹ for the cis isomer and of 350, 1232, and 1628 cm⁻¹ for the trans isomer. The 0–0 transition for the cis compound, 39668 cm⁻¹, lies about 130 cm⁻¹ lower than that for the trans, 39800 cm⁻¹. The fact that the 0–0 transition energy for cis is less than that for trans is interpreted as evidence that the cis isomer has a non‐planar geometry in the electronic ground state. The extinction coefficient for the 0–0 band of the trans isomer is about 1.5 times that for the cis.