Abstract
The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown. In order to contribute to this quest, here, we present a solvent-dependent 2D Electronic Spectroscopy (2DES) characterization of fucoxanthin, a carbonyl carotenoid involved in the light-harvesting process of brown algae. The 2DES technique allows probing its ultrafast relaxation dynamics in the first 1000 fs after photoexcitation with a 10 fs time resolution. The obtained results help shed light on the dynamics of the first electronic state manifold and, in particular, on an intramolecular charge-transfer state (ICT), whose photophysical properties are particularly elusive given its (almost) dark nature.
Highlights
The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown
The main challenge in characterizing the electronic states of carotenoids is connected primarily to the presence of dark states, one of their most intriguing features. It is well-known that the absorption properties of longer polyenes are dominated by the transition from the ground state (S0) to the second excited state (S2), since the transition to forbidden.[13−16] the first excited state (S1) Historically, the dark S1 state has is symmetry been studied, for example, through pump−probe spectroscopy where S1 can be populated indirectly from the S2 state, through ultrafast internal conversion,[17−19] and it can be probed thanks to an excited state absorption (ESA) promoted by the transition from S1 to higher excited states (Sn)
The attention is focused on fucoxanthin (Fx), a carotenoid typically found in diatoms antenna proteins called fucoxanthin-chlorophyll proteins (FCP), that belong to the family of intrinsic light-harvesting complexes
Summary
The electronic state manifolds of carotenoids and their relaxation dynamics are the object of intense investigation because most of the subtle details regulating their photophysics are still unknown. Signals in the transient absorption spectra of Fx dissolved in polar solvents has already been captured by pump−probe spectroscopy.[10,28] The high-energy ESA signal is common to all carotenoids, and it has been attributed to the S1 → Sn transition.[26] The cross in Figure 2a indicates the coordinates where the hot vibrational states of S1 are expected to contribute to the ESA signal through the S2 → hot S1 internal conversion and the hot S1 relaxation processes.
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