Low-Lying Excited States of Natural Carotenoids Viewed by Ab Initio Methods

Low-lying excited states of carotenoids (the optically dark 2A^- and bright 1B⁺) are deeply involved in energy transfer processes in photosynthetic antennas, such as light harvesting and non-photochemical quenching. Though any ab initio modeling of these phenomena has to rely on precise energies of the carotenoid electronic states, the accurate evaluation of these states remains a challenging problem due to their different natures. The paper aims to study the accuracy of the excitation energies of the low-lying excited states of certain open- and closed-chain carotenoids obtained by a state-of-the-art multireference approach for electronic structure calculation. Here, density matrix renormalization group SCF (DMRGSCF) and a perturbative approach based on driven similarity renormalization group second-order multireference perturbation theory (DSRG-MRPT2) were used to treat the static and dynamic correlation, respectively. Nuclear geometries of the electronic states were optimized with DFT-based approaches. It is demonstrated that spin-flip TD-DFT can replace multiconfigurational methods for the geometry optimization of the 2A^- state but not for the calculation of the excitation energy. Adiabatic excitation energies to the 1B⁺ state were shown to be within a margin of 1000 cm^-1 with an appropriate flow parameter value. Adiabatic excitation energies to the 2A^- state for the open-chain carotenoids lie within a range of experimental values (taking into account the broad range of experimental estimates); for the closed-chain ones, the error does not exceed 2000 cm^-1. Ab initio stationary (1A^- → 1B⁺) and transient (2A^- → 1B⁺) absorption spectra were modeled for violaxanthin and lycopene, and these spectra showed good agreement with the experimental ones both in terms of the vibronic structure and the transition energies.