Dependence of singlet-energy transfer on the conjugation length of carotenoids reconstituted into the LH1 complex from Rhodospirillum rubrum G9

A set of carotenoids, i.e., neurosporene, spheroidene, lycopene, anhydrorhodovibrin and spirilloxanthin, having the number of conjugated double bonds n=9, 10, 11, 12 and 13, were incorporated into the LH1 antenna complex from Rhodospirillum rubrum G9, and the carotenoid–bacteriochlorophyll (Cars–BChl) singlet energy-transfer efficiencies were determined by subpicosecond time-resolved absorption spectroscopy to be 78%, 75%, 46%, 40% and 36%, respectively. In carotenoids with n=9 and 10, all the 1Bu+, 1Bu− and 2Ag− channels were open, whereas in carotenoids with n=11–13 the 1Bu− and 2Ag− channels were closed, causing a sudden drop in the efficiency on going from n=10 to 11.