Excited state dynamics of β-carotene studied by means of transient absorption spectroscopy and multivariate curve resolution alternating least-squares analysis.

Determination of the excited-state dynamics of carotenoids has attracted considerable interest, engendering a number of controversial hypotheses because of the strongly overlapping spectral peaks and complicated dynamics of transient species. In the present work, aiming for better understanding the complexity of excited-state processes in carotenoids, excited-state dynamics of all-trans-β-carotene in ethanol was investigated by femtosecond pump-probe spectroscopy. Following the excitation of the strongly allowed S2 state of the β-carotene, transient absorption spectra were recorded in the visible spectral range. For comparison, the time-resolved transient absorption spectra are analyzed in a conventional way, fitting kinetic traces with a multi-exponential function at chosen wavelengths from obtained spectra, and then again by means of the soft-modeling multivariate curve resolution alternating least-squares analysis (MCR-ALS) method for modeling pure profiles and the generalized two-dimensional (2D) correlation spectroscopy data analysis for providing additional information on the dynamics of spectral features. MCR-ALS analysis shows that both the dynamics of the S* state, identified using the 2D correlation spectra, and the S1 state develop on a different timescale than the relaxation of the vibrationally hot S1v' state. Hot S1v' and S* states are shown to have different species associated difference spectra. Results of our analysis indicate that the S* state observed in this work is not the hot S1v' state but instead a separate singlet state.