An intramolecular charge transfer state of carbonylcarotenoids: implications for excited state dynamics of apo-carotenals and retinal

Abstract

Excited state dynamics of two apo-carotenals, retinal and 12′-apo-β-carotenal, were studied by femtosecond transient absorption spectroscopy. We make use of previous knowledge gathered from studies of various carbonyl carotenoids and suggest that to consistently explain the excited-state dynamics of retinal in polar solvents, it is necessary to include an intermolecular charge transfer (ICT) state in the excited state manifold. Coupling of the ICT state to the A_g⁻ state, which occurs in polar solvents, shortens lifetime of the lowest excited state of 12′-apo-β-carotenal from 180 ps in n-hexane to 7.1 ps in methanol. Comparison with a reference molecule lacking the conjugated carbonyl group, 12′-apo-β-carotene, demonstrates the importance of the carbonyl group; no polarity-induced lifetime change is observed and 12′-apo-β-carotene decays to the ground state in 220 ps regardless of solvent polarity. For retinal, we have confirmed the well-known three-state relaxation scheme in n-hexane. Population of the B_u⁺ state decays in <100 fs to the A_g⁻ state, which is quenched in 440 fs by a low-lying nπ* state that decays with a 33 ps time constant to form the retinal triplet state. In methanol, however, the A_g⁻ state is coupled to the ICT state. This coupling prevents population of the nπ* state, which explains the absence of retinal triplet formation in polar solvents. Instead, the coupled A_g⁻/ICT state decays in 1.6 ps to the ground state. The A_g⁻/ICT coupling is also evidenced by stimulated emission, which is a characteristic marker of the ICT state in carbonyl carotenoids.