Spectroscopic properties of spheroidene analogs having different extents of π-electron conjugation

The spectroscopic properties of spheroidene and a series of spheroidene analogs with extents of π-electron conjugation ranging from 7 to 13 carbon-carbon double bonds were studied using steady-state absorption, fluorescence, fluorescence excitation, and time-resolved absorption spectroscopy. The spheroidene analogs studied here were 5′,6′-dihydro-7′,8′-didehydrospheroidene, 7′,8′-didehydrospheroidene, and 1′,2′-dihydro3′,4′,7′,8′- tetradehydrospheroidene and taken together with data from 3,4,7,8-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene already published (DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J.; Farhoosh, R.; Frank, H. A. Biochim. Biophys. Acta 1992, 1102, 107) provide a systematic series of molecules for understanding the molecular features that control energy transfer to bacteriochlorophyll in photosynthetic bacterial light-harvesting complexes. All of the molecules were purified by high-pressure liquid chromatographic techniques prior to the spectroscopic experiments. The absorption spectra of the molecules were observed to red-shift with increasing extent of π-electron conjugation. The room temperature fluorescence data show a systematic crossover from dominant S₁ → S₀ (2¹A_g → 1¹A_g) emission to dominant S₂ → S₀ (1¹Bu → 1¹A_g) with increasing extent of conjugation. The S₂ fluorescence quantum yields of all the carotenoids in the series were measured here and indicate that 3,4-dihydrospheroidene with nine carbon - carbon double bonds has an S₂ quantum yield of (2.7 ± 0.3) × 10^-4 which is the highest value in the series. The lifetimes of the S₁ states of the molecules were determined from time-resolved transient absorption spectroscopy and found to decrease as the conjugated chain length increases. The transient data are discussed in terms of the energy gap law for radiationless transitions which allows a prediction of the S₁ energies of the molecules. The implications of these results for the process of light harvesting by carotenoids in photosynthesis are discussed.