Spheroidene and a series of spheroidene analogues with extents of pi-electron conjugation ranging from 7 to 13 carbon-carbon double bonds were Incorporated into the B850 light-harvesting complex of Rhodobacter sphaeroides R-26.1. The structures and spectroscopic properties of the carotenoids and the dynamics of energy transfer from the carotenoid to bacteriochlorophyll (BChl) in the B850 complex were studied by using steady-state absorption, fluorescence, fluorescence excitation, resonance Raman, and time-resolved absorption spectroscopy. The spheroidene analogues used in this study were 5＇,6＇- dihydro-7＇,8＇-didehydrospheroidene, 7＇,8＇-didehydrospheroidene, and 1＇,2＇-dihydro-3＇,4＇,7＇,8＇-tetradehydrospheroidene. These data, taken together with results from 3,4,7,8-tetrahydsospheroidene, 3,4,5,6-tetrahydrospheroidene, 3,4-dihydrospheroidene, and spheroidene already published (Frank, H. A.; Farhoosh, R.; Aldema, M. L.; DeCoster, B.; Christensen, R. L.; Gebhard, R.; Lugtenburg, J. Photochem. Photobiol. 1993, 57, 49. Farhoosh, R.; Chynwat, V.; Gebhard, R.; Lugtenburg, J.; Frank, H. A. Photosynth. Res. 1994, 42, 157), provide a systematic series of molecules for understanding the molecular features that determine the mechanism of energy transfer from carotenoids to BChl in photosynthetic bacterial light-harvesting complexes. The data support the hypothesis that only carotenoids having 10 or less carbon-carbon double bonds transfer energy via their 2(1)A(g) (S-1) states to BChl to any significant degree. Energy transfer via the 1(1)B(u) (S-2) State of the carotenoid becomes more important than the S-1 route as the number of conjugated carbon-carbon double bonds increases. The results also suggest that the S-2 state associated with the Q(x) transition of the B850 BChl is the most likely acceptor state for energy transfer originating from both the 2(1)A(g) (S-1) and 1(1)B(u) (S-2) states of all carotenoids.