Ab Initio Study of Low-Lying Excited States of Carotenoid-Derived Polyenes.

The knowledge about excited states of carotenoids is essential for understanding photophysical processes underlying photosynthesis. However, due to the presence of a large number of optically dark states, experimental study of the excited state manifold is limited to a significant extent. In this paper, we apply high-level \textit{ab initio} quantum chemical methods to study low-lying excited states of polyenes containing from 8 to 13 conjugated double bonds which serve as a model for natural carotenoids. Vertical and adiabatic excitation energies from the ground 1A$_g^-$ state to the excited 2A$_g^-$, 1B$_u^+$, and 1B$_u^-$ states were evaluated by means of DMRG with NEVPT2 perturbative correction. The energies of all excited states are highly sensitive to nuclear geometry, especially the 2A$_g^-$ state. Thus, the 2A$_g^-$ and 1B$_u^+$ states interchange their relative positions upon geometry relaxation while the vertical excitation energy of (\hl{TO?}) the 2A$_g^-$ is rather high. At the same time, the 1B$_u^-$ state energy is shown to be higher than other studied excited states at any geometry. With relaxed geometries of the excited states, absorption and transient absorption spectra were calculated within Franck-Condon approximation bridging the gap between experimental spectroscopic data and computational results.