Abstract
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 |Sn⟩) 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.
Highlights
Vibrational energy relaxation is a process of equilibration among excited vibrational states.[1]
Such processes are of fundamental importance to molecular dynamics: after photoexcitation, vibrational energy relaxation dissipates excess vibrational energy, a process indicated by wavy arrows in Chlorophylls and carotenoids are the two molecular building blocks of photosynthetic light harvesters
Their polyenic backbone holds a delocalized π-electron system, which explains the intense lowest lying absorption band, typically in the 400−500 nm range. This transition is between ground and second electronic excited singlet state, S0 → S2, while the S0 → S1 transition is optically forbidden due to symmetry reasons, at least in an idealized C2h symmetry.[4,5]
Summary
Letter conjugated π-electrons, electronic excited states in carotenoids deactivate back to ground state rapidly, with an S2 to S1 transfer time of sub-200 fs and S1 to S0 transfer ranging from hundreds of femtoseconds to tens of picoseconds, all depending on chain length. This means that by including NIR transitions to the vibrationally hot ground state, VERA fully explains PDP experimental results. Features previously associated with the intensely debated S* state are explained by vibronic transitions from either S1 (as shown for β-carotene31) or from vibrationally excited levels on S0 (see Zea[15] discussed above) This model readily incorporates results from pump-dump-probe experiments, which were irreconcilable with any previous model for carotenoid energy level schemes.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
