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Issue 22, 2011
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An intramolecular charge transfer state of carbonylcarotenoids: implications for excited state dynamics of apo-carotenals and retinal

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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 Ag 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 Bu+ state decays in <100 fs to the Ag 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 Ag 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 Ag/ICT state decays in 1.6 ps to the ground state. The Ag/ICT coupling is also evidenced by stimulated emission, which is a characteristic marker of the ICT state in carbonyl carotenoids.

Graphical abstract: An intramolecular charge transfer state of carbonyl carotenoids: implications for excited state dynamics of apo-carotenals and retinal

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Article information


Submitted
31 Jan 2011
Accepted
04 Apr 2011
First published
06 May 2011

Phys. Chem. Chem. Phys., 2011,13, 10787-10796
Article type
Paper

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

T. Polívka, S. Kaligotla, P. Chábera and H. A. Frank, Phys. Chem. Chem. Phys., 2011, 13, 10787
DOI: 10.1039/C1CP20269C

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