Ultrafast solvation dynamics of 12′-apo-β-carotenoic-12′-acid in [C6mim]+[Tf2N]−

Intra- and intermolecular relaxation pathways of 12′-apo-β-carotenoic-12′-acid, an apocarotenoid with terminal carbonyl substitution, were identified after photoexcitation using ultrafast pump-supercontinuum probe spectroscopy in the 350-750 nm range. The apocarotenoid was dissolved in 1-n-hexyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)amide and excited to the S2 state at 482 nm. Internal conversion occurred within 170 fs to a single S1/ICT state with intramolecular charge transfer character. This state exhibits characteristic excited state absorption band-shifts due to time-dependent solvation, which appears to be a general feature of carbonyl apocarotenoids with polarity-dependent S1/ICT lifetimes. We also demonstrate that higher pump beam energies open a route for the clean preparation of long-lived radical cations by resonant one-color two-photon excitation using S2 as the intermediate state. The radical cations are easily identified by a characteristic absorption peak in the region 700-750 nm and an absorption tail towards shorter wavelengths.