Linear polyenes: models for the spectroscopy and photophysics of carotenoids.

We have developed procedures for synthesizing dimethyl polyenes using living polymerization techniques and have initiated investigations of the spectroscopic properties of these molecules. Purification using high-performance liquid chromatography (HPLC) of the polyene mixtures resulting from the syntheses promises to provide all-trans polyenes with a wide range in the number of conjugated double bonds. Low temperature optical measurements on these model systems, both in glasses and in n-alkane mixed crystals, yield absorption and fluorescence spectra with considerably higher vibronic resolution than the spectra currently available for carotenoids with comparable conjugation lengths. The dimethyl polyenes thus allow a more precise exploration of the electronic properties of long, linearly conjugated systems. These studies can be used to verify the existence of low-lying singlet states predicted by theory and recently invoked to explain low-resolution fluorescence, Raman excitation spectra, and the transient absorption spectroscopy of carotenoids. Steady state and time-resolved optical studies of the dimethyl series will be used to better understand the energies and dynamics of the low energy electronic states relevant to the photochemistry and photobiology of all linearly conjugated systems.