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Introduction The study focuses on the light-harvesting function of carotenoids. How carotenoids capture the photon energy and efficiently transfer its singlet energy to the neighboring bacteriochlorophyll in the pigment-protein antenna complexes (LH2) has been studied. The 11Bu- state in the light-harvesting function are critically investigated, tracing the excited-state dynamics of carotenoids free in solution and bound to the LH2. Aim of the study The goal of this study is to reveal the detailed mechanisms of light-harvesting function. The research focuses on the roles of the 11Bu- state in the light-harvesting function of Cars. A series of investigations were carried out to critically examine the roles of the 11Bu- state in the light-harvesting function. Method The internal conversion processes in Cars were investigated by the use of subpicosecond time-resolved absorption spectroscopy for a series of Cars in solution and bound to the pigment-protein antenna complexes. including neurosporene (n = 9), spheroidene (n = 10), lycopene (n = 11), anhydrorhodovibrin (n = 12) and spirilloxanthin (n = 13). The SVD and global fitting were applied to of spectral-data matrices. Results The results lead to a branched relaxation scheme of energy deactivation including (1) the singlet internal conversion in the sequence of 11Bu- 11Bu- > 21Ag- > 11Ag- (ground), and (2) the singlet-to-triplet conversion of 11Bu- > 13Ag followed by triplet internal conversion of 13Ag > 13Bu. The relaxation scheme took place in subpicosecond to decapicosecond time scales. Conclusion The 11Bu- state plays essential roles in light-harvesting mechanisms as a mediator state in the process of internal conversion, an acceptor state in the Car-to-BChl singlet-energy transfer and a precursor state for the rapid formation of the Car triplet state.