Dark excited states of carotenoids: Consensus and controversy

Excited-state dynamics of carotenoids has attracted considerable recent interest, but it has also engendered a number of controversial hypotheses. This review summarizes the knowledge gathered from various ultrafast time-resolved techniques addressing the two most discussed dark excited states ( 1 B u - and S∗). These two states have been subjects of numerous earlier studies, because while the 1 B u - state was predicted by theory a long time ago but never clearly observed in experiment, the S∗ state is readily observed in transient absorption data, but no theoretical explanation of its origin has been available until recently. The major focus of this review is (1) the possible role of the 1 B u - state in excited-state processes and (2) the origin and mechanism of formation of the S∗ state in carotenoids.