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

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 ab initio quantum chemical methods to study the 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 density matrix renormalization group (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 to the 2A_g^- state 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 the Franck-Condon approximation bridging the gap between experimental spectroscopic data and computational results.