SPECTROSCOPIC PROPERTIES OF SPHEROIDENE ANALOGS HAVING DIFFERENT EXTENTS OF PI-ELECTRON CONJUGATION

The spectroscopic properties of spheroidene and a series of spheroiden e analogs with extents of pi-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'-dihyd ro-3',4',7',8'-tetradehydrospheroidene and taken together with data fr om 3,4,7,s-tetrahydrospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-d ihydrospheroidene already published (DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J.; Farhoosh, R.; Frank, H. A. Biochim. Biop hys. Acta 1992, 1102, 107) provide a systematic series of molecules fo r understanding the molecular features that control energy transfer to bacteriochlorophyll in photosynthetic bacterial light-harvesting comp lexes. All of the molecules were purified by high-pressure liquid chro matographic techniques prior to the spectroscopic experiments. The abs orption spectra of the molecules were observed to red-shift with incre asing extent of pi-electron conjugation. The room temperature fluoresc ence data show a systematic crossover from dominant S-1 --> S-0 (2(1)A (g) --> 1(1)A(g)) emission to dominant S-2 --> S-0 (1(1)B(u) --> 1(1)A (g)) With increasing extent of conjugation. The S-2 fluorescence quant um yields of all the carotenoids in the series were measured here and indicate that 3,4-dihydrospheroidene with nine carbon-carbon double bo nds has an S-2 quantum yield of (2.7 +/- 0.3) x 10(-4) which is the hi ghest value in the series. The lifetimes of the S-1 states of the mole cules were determined from time-resolved transient absorption spectros copy and found to decrease as the conjugated chain length increases. T he transient data are discussed in terms of the energy gap law for rad iationless transitions which allows a prediction of the S-1 energies o f the molecules. The implications of these results for the process of light harvesting by carotenoids in photosynthesis are discussed.