A set of carotenoids, i.e., neurosporene, spheroidene, lycopene, anhydrorbodovibrin 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 1B(u)(+), IBu+ and 2A(g)(-) channels were open, whereas in carotenoids with n = 11-13 the 1B(u)(-) and 2A(g)(-) channels were closed, causing a sudden drop in the efficiency on going from n = 10 to 11. (C) 2004 Elsevier B.V. All rights reserved.