Ultrafast dynamics of hydrophilic carbonyl carotenoids - Relation between structure and excited-state properties in polar solvents

We present a study of excited-state dynamics of water-soluble carbonyl carotenoids, crocin, norbixin, and astalysine in solvents with different polarity. While no polarity effects were observed in 2-propanol and methanol, polarity-induced lifetime shortening has been detected in water. For crocin and astalysine the S1 lifetime decreases from 135 ps to 61 ps (crocin), and from 4 ps to 2.2 ps (astalysine) when going from methanol to water. The S1 lifetime of norbixin is within the 15-18 ps range in all solvents, an effect attributed to its carboxylic group, which isolates the carbonyl group from the rest of conjugation. No spectral bands attributable to the intramolecular charge transfer (ICT) state have been found in any transient absorption spectra. The ICT-Sn transition is made forbidden, we suggest, by the symmetric location of the conjugated carbonyl groups. In astalysine, we have found a clear signature of the S∗ state with a lifetime of 7 ps (methanol) and 6.1 ps (water).