The nonadiabatic mechanism that mediates nonradiative decay of the bright S-2 state to the dark S-1 state of carotenoids involves population of a bridging intermediate state, S-x, in several examples. The nature of S-x remains to be determined definitively, but it has been recently suggested that S-x corresponds to conformationally distorted molecules evolving along out-of-plane coordinates of the isoprenoid backbone near a low barrier between planar and distorted conformations on the S-2 potential surface. In this study, the electronic and vibrational dynamics accompanying the formation of S-x in toluene solutions of the ketocarotenoid canthaxanthin (CAN) are characterized with broadband two-dimensional electronic spectroscopy (2DES) with 7.8 fs excitation pulses and detection of the linear polarization components of the third-order nonlinear optical signal. A stimulated-emission cross peak in the 2DES spectrum accompanies the formation of S-x in <20 fs following excitation of the main absorption band. S-x is prepared instantaneously, however, with excitation of hot-band transitions associated with distorted conformations of CAN's isoprenoid backbone in the low frequency onset of the main absorption band. Vibrational coherence oscillation maps and modulated anisotropy transients show that S-x undergoes displacements from the Franck-Condon S-2 state along out-of-plane coordinates as it passes to the S-1 state. The results are consistent with the conclusion that CAN's carbonyl-substituted beta-ionone rings impart an intramolecular charge-transfer character that frictionally slows the passage from S-x to S-1 compared to carotenoids lacking carbonyl substitution. Despite the longer lifetime, the S-1 state of CAN is formed with retention of vibrational coherence after passing through a conical intersection seam with the S-x state. Published under an exclusive license by AIP Publishing.
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