Ultrafast transient lens spectroscopy of various C40 carotenoids: lycopene, β-carotene, (3R,3′R)-zeaxanthin, (3R,3′R,6′R)-lutein, echinenone, canthaxanthin, and astaxanthin

Abstract

The ultrafast internal conversion (IC) dynamics of seven C₄₀ carotenoids have been investigated at room temperature in a variety of solvents using two-color transient lens (TL) pump–probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and—for the first time—echinenone, as well as new data for lycopene, β-carotene, (3R,3′R)-zeaxanthin and (3R,3′R,6′R)-lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S₂ band at 400 nm. For the S₂ lifetime solvent-independent upper limits on the order of 100–200 fs are estimated for all carotenoids studied. The S₁ lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthin/echinenone/β-carotene (13/12/11 double bonds) one finds τ₁ ≈ 5, 7.7 and 9 ps for the S₁ → S₀ IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on τ₁, as observed for canthaxanthin/astaxanthin (τ₁ ≈ 5 ps in both cases). For all carotenoids studied, τ₁ is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e.g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C₄₀ carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured τ₁.