1. Linear Polyenes: Models for the Spectroscopy and Photochemistry of Carotenoids
Ronald L. Christensen, Department of Chemistry, Bowdoin College Brunswick, Maine 04011
We have synthesized and purified the all-trans and 4-cis isomers of 2,4,6,8,10,12,14-hexadecaheptaene and obtained their room and low temperature absorption and emission spectra. The S1S0 fluorescence yields of the 4-cis isomer show the remarkable temperature dependence indicated below. Fits of the temperature data to a simple double well model suggest a 2-4 kcal barrier for the conversion of the cis isomer to the symmetric (C2h), non-fluorescent trans isomer on the S1 potential surface. This model accounts for the quenching of the 4-cis fluorescence and the similarity between the room temperature emission spectra of these species, which are assigned to distorted trans and/or cis isomers.
Distributions of conformers and geometric isomers, separated by low barriers on complex S1 energy surfaces, help to explain the complicated S2S1 relaxation kinetics of polyenes and carotenoids. The finding that fluorescence from linear polyenes is so strongly dependent on molecular symmetry also requires a reevaluation of the literature on the radiative properties of all-trans polyenes and carotenoids.
R.L. Christensen, M.G Galinato, E.F. Chu. R. Fujii, H. Hashimoto, and H.F. Frank, J. Am. Chem. Soc 129, (2007).