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Effects of introducing a carbonyl group and its position in the conjugated system of carotenoids were studied by means of femtosecond time-resolved spectroscopy. We have compared four naturally occurring carotenoids with comparable structures, β-carotene, echinenone, canthaxanthin and rhodoxanthin, which differ in the number and position of conjugated carbonyl group(s). The S1 lifetime is systematically shorter upon increasing the number of the conjugated CO groups, yielding 9.3 ps (for β-carotene, no CO group), 6.2 ps (echinenone, one CO group), 4.5 ps (canthaxanthin, two CO groups), and 1.1 ps (rhodoxanthin, two CO groups in s-trans configuration). Except for slight polarity-induced broadening of absorption and transient absorption spectra, no other polarity effects, such as shortening of the S1 lifetimes or transient features attributable to intramolecular charge transfer (ICT) state bands, were observed …