PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

Optical spectroscopic studies of polyenes and carotenoids having various -electron chain lengths. Harry Frank^*,1, Mary Grace Galinato¹, Emily Chu¹, Ronald Christensen², ¹ Department of Chemistry, Storrs, CT, USA² Department of Chemistry, Brunswick, ME, USA

ABSTRACT- Several dimethyl polyenes, including decatetraene, dodecapentaene, tetradecahexaene and hexadecaheptaene, having four to seven conjugated -electron carbon-carbon double bonds, have been synthesized from polyene aldehydes using Wittig reactions. Also, carotenoids of various -electron chain lengths have been obtained from photosynthetic bacteria grown under different conditions. Purification using reverse-phase high-performance liquid chromatography (HPLC) allows the separation and identification of the all-trans geometric isomers. Steady-state absorption, fluorescence and fluorescence excitation spectroscopy were performed at room temperature and 77 K in EPA (5:5:2 % v/v/v ether: isopentane: ethanol). The linear polyenes exhibit vibrationally well-resolved S₀ → S₂ (1¹A_g^- → 1¹B_u⁺) absorption and S₁ → S₀ (2¹A_g^- → 1¹A_g^-) fluorescence. Tetradecahexaene and hexadecaheptaene also display S₂ → S₀ (1¹B_u⁺ → 1¹A_g^-) fluorescence. The effect of conjugation chain length on the excited state energies of the molecules is revealed by the studies. The dependence of the spectra on solvent polarizability allows an extrapolation to the values of the transition energies in the absence of solvent. The polyenes provide the means of making a direct comparison between experimentally-determined transition energies and those obtained from quantum computations. They also provide a well-defined, systematic series of model compounds for understanding the molecular spectroscopic properties and photophysical behavior of the carotenoids.

KEY WORDS: light-harvesting, absorption, carotenoids, energy transfer