title: Combination of transient 2D-IR experiments and ab initio computations sheds light on the formation of the charge-transfer state in photoexcited carbonyl carotenoids.
abstract: 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 S2 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 1Bu(+) and the dark double-excited 2Ag(-) states are involved. Specifically, the initially populated 1Bu(+) relaxes toward 2Ag(-) in 200 fs. In a nonpolar solvent 2Ag(-) decays to the ground state (GS) in 25 ps. In polar solvents, distortions along twisting modes of the chain promote a repopulation of the 1Bu(+) state which then quickly relaxes to the GS (18 ps in chloroform). The 1Bu(+) state has a high electric dipole and is the main contributor to the charge-transfer state involved in the dynamics in polar solvents. The 2Ag(-) → 1Bu(+) 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 2Ag(-) and 1Bu(+) states are coupled.
authors: LENS (European Laboratory for Nonlinear Spectroscopy) via N. Carrara 1, 50019 Sesto Fiorentino (FI), Italy.
authors: Mariangela
authors: M
authors: Di Donato
authors: Mireia
authors: M
authors: Segado Centellas
authors: Andrea
authors: A
authors: Lapini
authors: Manuela
authors: M
authors: Lima
authors: Francisco
authors: F
authors: Avila
authors: Fabrizio
authors: F
authors: Santoro
authors: Chiara
authors: C
authors: Cappelli
authors: Roberto
authors: R
authors: Righini
mesh_terms: D002338:Carotenoids / Q000737:chemistry*; D008027:Light; D015394:Molecular Structure; D055668:Photochemical Processes; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared
chemical_list: D002338:Carotenoids