Combination of Transient 2D-IR Experiments and Ab Initio Computations Sheds Light on the Formation of the Charge-Transfer State in Photoexcited Carbonyl Carotenoids

The excited state dynamics of carbonyl carotenoids is very complex because of the coupling of single- and doubly excited states and the possible involvement of intramolecular charge-transfer (ICT) states. In this contribution we employ ultrafast infrared spectroscopy and theoretical computations to investigate the relaxation dynamics of trans-8'-apo-beta-carotenal occurring on the picosecond time scale, after excitation in the S-2 state. In a (slightly) polar solvent like chloroform, one-dimensional (TID-IR) and two-dimensional (T2D-IR) transient infrared spectroscopy reveal spectral components with characteristic frequencies and lifetimes that are not observed in nonpolar solvents (cyclohexane). Combining experimental evidence with an analysis of CASPT2//CASSCF ground and excited state minima and energy profiles, complemented with TDDFT calculations in gas phase and in solvent, we propose a photochemical decay mechanism for this system where only the bright single-excited 1B(u)(+) and the dark double-excited 2A(g)(-) states are involved. Specifically, the initially populated 1B(u)(+) relaxes toward 2A(g)(-) in 200 fs. In a nonpolar solvent 2A(g)(-) decays to the ground state (GS) in 25 ps. In polar solvents, distortions along twisting modes of the chain promote a repopulation of the 1B(u)(+) state which then quickly relaxes to the GS (18 Ps in chloroform). The 1B(u)(+) state has a high electric dipole and is the main contributor to the charge-transfer state involved in the dynamics in polar solvents. The 2A(g)(-) -> 1B(u)(+) population transfer is evidenced by a cross peak on the T2D-IR map revealing that the motions along the same stretching of the conjugated chain on the 2A(g)(-) and 1B(u)(+) states are coupled.