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Excited-state dynamics of 3,3′-dihydroxyisorenieratene and (3R,3′R)-zeaxanthin: Observation of vibrationally hot S0 species

Ehlers, Florian, Scholz, Mirko, Oum, Kawon, Lenzer, Thomas
Archives of biochemistry and biophysics 2018 v.646 pp. 137-144
absorption, beta-ionone, bleaching agents, cooling, solvents, wavelengths, zeaxanthin
We report on an ultrafast transient absorption study of all-trans-3,3′-dihydroxyisorenieratene (“DHIR”) and all-trans-(3R,3′R)-zeaxanthin in organic solvents covering the wavelength range 350–770 nm. The lifetime of the S2 state in both carotenoids is 160–170 fs. Upon internal conversion (IC) non-equilibrated S1 molecules are formed which internally relax on a 300–400 fs time scale. The time constant for IC from S1 depends on the type of terminal substituent: Replacement of the two terminal β-ionone rings of zeaxanthin by two aryl rings in DHIR results in an increase from 9.5 to 10.9 ps in THF. This suggests a mild decrease in the effective conjugation length of DHIR. IC to the ground electronic state prepares vibrationally hot S0* molecules which exhibit characteristic bleach and absorption bands. These are typically denoted as “S* features”. Collisional cooling of S0* happens with a time constant of 15 ps. Based on our results and the findings from previous studies for other carotenoids, such as macro-β-carotenes, β-carotenes and long-chain apocarotenals, we conclude that S0* spectral features are ubiquitous in carotenoid photophysics: They are particularly easy to observe in systems with a very short S1 lifetime and a high quantum yield for IC to the ground electronic state.