Broadband 2DES Detection of Vibrational Coherence in the S State of Canthaxanthin

The nonadiabatic mechanism that mediates nonradiative decay of the bright S state to the dark S state of carotenoids involves population of a bridging intermediate state, S, in several examples. The nature of S remains to be determined definitively, but it has been recently suggested that S 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 potential surface. In this study, the electronic and vibrational dynamics accompanying the formation of S 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 in <20 fs following excitation of the main absorption band. S 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 undergoes displacements from the Franck-Condon S state along out-of-plane coordinates as it passes to the S state. The results are consistent with the conclusion that CAN's carbonyl-substituted β-ionone rings impart an intramolecular charge-transfer character that frictionally slows the passage from S to S compared to carotenoids lacking carbonyl substitution. Despite the longer lifetime, the S state of CAN is formed with retention of vibrational coherence after passing through a conical intersection seam with the S state.