Excitation-Dependence of Excited-State Dynamics and Vibrational Relaxation of Lutein Explored by Multiplex Transient Grating

Multiplex transient grating (MTG) spectroscopy was applied to lutein in ethanol to investigate the excitation-energy dependence of the excited-state dynamics and vibrational relaxation. The transient spectra obtained upon low (480 nm) and high-energy (380 nm) excitation both recorded a strong excited-state absorption (ESA) of S-1 -> S-n as well as a broad band in the blue wavelength that was previously proposed as the S* state. By means of Gaussian decomposition and global fitting of the ESA band, a long-time component assigned to the triplet state was derived from the kinetic trace of 480 nm excitation. Moreover, the MTG signal with a resolution of 110 fs displayed the short-time quantum beat signal. In order to unveil the vibrational coherence in the excited-state decay, the linear and non-linear simulations of the steady spectrum and dynamic signals were presented in which at least three fundamental modes standing for C-C stretching (nu(1)), C=C stretching (nu(2)), and O-H valence vibrations (nu(3)) were considered to analyze the experimental signals. It was identified that the vibrational coherence between nu(1) and nu(3) or nu(2) and nu(3) was responsible for quantum beat that may be associated with the triplet state. We concluded that upon low-or high-energy excitation into the S-2 state, the photo-isomerization of the molecule and structural recovery on the time-scale of vibrational cooling are the key factors to form a mixed conformation in the hot-S-1 state that is the precursor of a long life-time triplet.