Excited-state propoerties of hydrophilic carotenoids

While a vast majority of carotenoids is hydrophobic, recent synthetic efforts led to a production of series of water-soluble carotenoids. Here we present a study of excited-state properties of two water-soluble carotenoids, crocin and astalysine. Crocin is a natural carotenoid found in saffron while astalysine is a synthetic carotenoid created by modification of astaxanthin by lysine. Since both carotenoids belongs to a family of carbonyl carotenoids having a conjugated carbonyl group, hydrophilic character of these two carotenoids offers possibility to study the polarity-induced effects attributed to the conjugated carbonyl group in a solvent with extreme polarity, water. Absorption spectrum of crocin, which has 7 conjugated C=C bonds and two conjugated C=O groups, exhibits well-resolved vibrational bands in methanol, but vibrational structure is significantly reduced in water. The polarity-dependent properties of the excited states are corroborated by measurements of S1 lifetime that is reduced from 135 ps in methanol to 61 ps in water. Interestingly, no bands attributable to a presence of an intramolecular charge transfer (ICT) state have been found in transient absorption spectra in both methanol and water. Astalysine has conjugation length of N=13 (11 C=C + 2 C=O) and its absorption spectrum shows no vibrational structure in any solvent. The S1 lifetime of astalysine in methanol, 4.5 ps, is very close to that of astaxanthin in methanol (4.9 ps), indicating that two lysine modifications do not alter excited-state properties. In water, however, the S1 lifetime is shortened to 2.2 ps. Thus, while no polarity-induced effects on the S1 lifetime were found for astaxanthin in a series of solvents with different polarities, it is obvious that extreme polarity of water can significantly shorten the S1 lifetime. As for crocin, however, no ICT bands were observed in the transient absorption spectra of astalysine in water. The presented results show that shortening of S1 lifetime, that was so far restricted only to carbonyl carotenoids with N