Ultrafast Carotenoid to Retinal Energy Transfer in Xanthorhodopsin Revealed by the Combination of Transient Absorption and Two-Dimensional Electronic Spectroscopy

By comparing two‐dimensional electronic spectroscopy (2DES) and Pump‐Probe (PP) measurements on xanthorhodopsin (XR) and reduced‐xanthorhodopsin (RXR) complexes, the ultrafast carotenoid‐to‐retinal energy transfer pathway is revealed, at very early times, by an excess of signal amplitude at the associated cross‐peak and by the carotenoid bleaching reduction due to its ground state recovery. The combination of the measured 2DES and PP spectroscopic data with theoretical modelling allows a clear identification of the main experimental signals and a comprehensive interpretation of their origin and dynamics. The remarkable velocity of the energy transfer, despite the non‐negligible energy separation between the two chromophores, and the analysis of the underlying transport mechanism, highlight the role played by the ground state carotenoid vibrations in assisting the process.