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-β-carotenal occurring on the picosecond time scale, after excitation in the S state. In a (slightly) polar solvent like chloroform, one-dimensional (T1D-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 and the dark double-excited 2A states are involved. Specifically, the initially populated 1B relaxes toward 2A in 200 fs. In a nonpolar solvent 2A 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 state which then quickly relaxes to the GS (18 ps in chloroform). The 1B 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 → 1B 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 and 1B states are coupled. © 2014 American Chemical Society.