Spectroscopic and photochemical properties of carotenoids, xanthophylls and model polyenes

The spectroscopic properties of nine all-trans-carotenoids, xanthophylls and model polyenes were studied using steady-state absorption, fluorescence, and fluorescence excitation spectroscopy. The molecules include all-trans isomers of the intermediate π-electron chain length polyenes, decatetraene, dodecapentaene, tetradecahexaene; the C30 carotenoids, 2,6,10,15,19,23-hexamethyl-2,6,8,10,12,14,16,18,22-tetracosanonaene, 2,6,10,15,19,23-hexamethyl-2,6,8,10,12,14,16,18,20-tetracosanonaene, 2,6,10,15,19,23-hexamethyl-4,6,8,10,12,14,16,18,20-tetracosanonaene referred to as heptaene, octaene, and nonaene; and the xanthophylls, violaxanthin, lutein, and zeaxanthin. High performance liquid chromatography (HPLC) was carried out immediately prior to the spectroscopic experiments to obtain isomerically pure samples devoid of fluorescent contaminants. The polyenes showed vibrationally resolved S0 → S2 (11Ag− → 11Bu+) absorption and S 1 → S0 (21Ag− → 11Ag−) fluorescence. The fluorescence from the C30 carotenoids exhibits a systematic crossover from dominant S1 → S0 (21Ag− → 11Ag−) emission to dominant S2 → S0 (11Bu+ → 11Ag −) with increasing extent of conjugation. The xanthophylls all emit primarily from their S2 states. The experiments at cryogenic temperatures provide enhanced resolution compared to room-temperature studies, reveal clearly the vibronic features of the fluorescence line shapes, and allow precise, direct assignments of the spectral origins and electronic-state energies of the molecules. The S2 − S1 energy gaps of all of the molecules increase with increasing length of π-electron conjugation. The lifetimes of the S1 states of the C30 carotenoids and xanthophylls were measured by transient absorption spectroscopy and were found to decrease as the conjugated chain length increases. The energy gap law for radiationless transitions is used to correlate the S1 energies with the dynamics. The data provide a systematic series of molecules for understanding the structural features controlling the photochemical properties of π-electron conjugated systems.