Excited State Dynamics of Selected All-trans C40 Xanthophyll Carotenoids

Abstract

Abstract We investigated the excited state dynamics of selected all-trans C40 xanthophyll carotenoids by ultrafast pump-supercontinuum probe (PSCP) spectroscopy in the spectral range 350–770 nm upon photoexcitation by a femtosecond laser (ca. 80 fs pulse length) near 500 nm in acetone. The following carotenoids were selected considering a systematic variation of the number and position of carbonyl (CO) and hydroxyl (OH) functional groups on the β-ionone rings and the change of the effective conjugation length of the polyene system: β-cryptoxanthin (1), echinenone (2), 3-hydroxyechinenone (3), 3′-hydroxyechinenone (4), canthaxanthin (5), adonirubin (6) and astaxanthin (7). The carotenoids featuring an increasing conjugation length, for example, (1), (3) and (6), showed a systematically shorter S1 lifetime of 8.7, 6.2 and 4.7 ps, respectively. Carotenoids with OH groups adjacent to a CO group showed slightly broader steady-state and transient absorption bands, but the influence on the S1 lifetime was minor compared to the case of CO substitution. Moreover, in all cases we observed clearly visible “S* signals”, namely a longer-lived characteristic S0 → S2 red-edge absorption and a negative signal in the S0 → S2 bleach region. This spectral signature is assigned to highly vibrationally excited molecules in the ground electronic state S0 * which are generated by internal conversion from S1. The S0 * species cool with a time constant of 9.2–9.6 ps in the case of the keto-substituted carotenoids (2)–(7), whereas we obtain a larger value of 13.2 ps for (1) which does not have a keto group. Our findings are supported by results from a global kinetic analysis procedure.