Effect of carotenoid structure on excited-state dynamics of carbonylcarotenoids

Effects of introducing a carbonyl group and its position in the conjugated system of carotenoids were studied by means of femtosecond time-resolved spectroscopy. We have compared four naturally occurring carotenoids with comparable structures, β-carotene, echinenone, canthaxanthin and rhodoxanthin, which differ in the number and position of conjugated carbonyl group(s). The S1 lifetime is systematically shorter upon increasing the number of the conjugated CO groups, yielding 9.3 ps (for β-carotene, no CO group), 6.2 ps (echinenone, one CO group), 4.5 ps (canthaxanthin, two CO groups), and 1.1 ps (rhodoxanthin, two CO groups in s-trans configuration). Except for slight polarity-induced broadening of absorption and transient absorption spectra, no other polarity effects, such as shortening of the S1 lifetimes or transient features attributable to intramolecular charge transfer (ICT) state bands, were observed. The absence of these polarity-induced features is explained as due to the long conjugated chain (no lifetime shortening), and the symmetrical position of the carbonyl groups (no ICT bands). On the other hand, all carotenoids exhibit the characteristic spectral band attributed to the S* state, and for the two longest carotenoids, canthaxanthin and rhodoxanthin, decay of the S* state is markedly longer than that of the S1 state. Moreover, it is shown that the S* state is preferentially populated for a specific subset of ground state conformations, underlining the importance of carotenoid conformation in S* state formation.