Low-lying excited stated of natural carotenoids viewed by ab initio methods

Low-lying excited states of carotenoids (the optically dark 2Ag- and bright 1Bu+) 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, their accurate evaluation remains a challenging problem due to a different nature of the states of interest. The paper aims to study how accurate are 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, DMRGSCF and a perturbative approach based on DSRG-MRPT2 were used for treatment of static and dynamic correlation, respectively. Nuclear geometries of the electronic states were optimized with DFT-based approaches. It was demonstrated that spin-flip TD-DFT can replace multiconfigurational methods for the geometry optimization of the 2Ag- state, but not for the calculation of the excitation energy. Adiabatic excitation energies to the 1Bu+ state were shown to be within a margin of 1000 cm-1 with an appropriate flow-parameter value. Adiabatic excitation energies to the 2Ag- 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 (1Ag- → 1Bu+) and transient (2Ag- → 1Bu+) absorption spectra were modeled for violaxanthin and lycopene, and they show a good agreement with the experimental ones both in terms of vibronic structure and transition energies.