A Unified Picture of S∗ in Carotenoids

In π-conjugated chain molecules such as carotenoids, coupling between electronic and vibrational degrees of freedom is of central importance. It governs both dynamic and static properties, such as the time scales of excited state relaxation as well as absorption spectra. In this work, we treat vibronic dynamics in carotenoids on four electronic states (|S₀〉, |S₁〉, |S₂〉, and |S_n〉) in a physically rigorous framework. This model explains all features previously associated with the intensely debated S∗ state. Besides successfully fitting transient absorption data of a zeaxanthin homologue, this model also accounts for previous results from global target analysis and chain length-dependent studies. Additionally, we are able to incorporate findings from pump-deplete-probe experiments, which were incompatible to any pre-existing model. Thus, we present the first comprehensive and unified interpretation of S∗-related features, explaining them by vibronic transitions on either S₁, S₀, or both, depending on the chain length of the investigated carotenoid.