Abstract :
Radical cations and dications of two carotenoids astaxanthin and canthaxanthin were prepared by oxidation with FeCl3 in fluorinated alcohols at room temperature. Absorption and electroabsorption (Stark effect) spectra were recorded for astaxanthin cations in mixed frozen matrices at temperatures about 160 K. The D0→D2 transition in cation radical is at 835 nm. The electroabsorption spectrum for the D0→D2 transition exhibits a negative change of molecular polarizability, Δα=−1.2·10−38 C·m2/V (−105 A3), which seems to originate from the change in bond order alternation in the ground state rather than from the electric field-induced interaction of D1 and D2 excited states. Absorption spectrum of astaxanthin dication is located at 715–717 nm, between those of D0→D2 in cation radical and S0→S2 in neutral carotenoid. Its shape reflects a short vibronic progression and strong inhomogeneous broadening. The polarizability change on electronic excitation, Δα=2.89·10−38 C·m2/V (260 A3), is five times smaller than in neutral astaxanthin. This value reflects the larger energetic distance from the lowest excited state to the higher excited states than in the neutral molecule.
