Excited state dynamics of carbonyl carotenoids investigated by ultrafast vibrational and electronic spectroscopies.

Carbonyl carotenoids, mostly present in marinae algae and dinoflagellates of deep waters, are well-known light-harvesting pigments, which absorb the green-blue wavelengths of the scattered sunlight and efficiently transfer energy to nearby chlorophylls in antenna complexes. In recent years, carbonyl carotenoids have been largely investigated as interesting dyes for organic photovoltaic devises. Due to the presence of a conjugated electron-withdrawing group to the polyene chain, absorption wavelengths and lifetime of photo-excited states show to be sensitive to the polarity of the surrounding. An intra-molecular charge transfer (ICT) state is generally invoked, whose nature and mechanism of formation have not been clarified yet. In this PhD thesis, I focus the attention on the photo-physics and photo-dynamics properties of two carbonyl carotenoids, Peridinin and the all trans-beta-apo-8’-carotenal, dissolved in several solvents, differing for both polarity and polarizability, and exciting the sample at different excitation wavelengths. By means of 3rd and 5th order non-linear spectroscopies, both in the infrared and in the visible region, I identify excited state vibrational marker bands of the ICT state, whose evolution suggests a strong mixing in the ionic and covalent character of the low-lying excited state. With the support of calculations, no evidences of a further electronic excited state in between the covalent S1(2Ag-) state and the ionic S2(1Bu+) state are found. Two relaxation pathways are finally presented for carbonyl carotenoids in non-polar and polar solvents: while in non-polar solvents the S2-S1 energy gap results in no differences with respect to non-carbonyl carotenoids, a strong mixing of the two low-lying states occurs in both strongly and moderately polar environments. In the latter case, also the solvent polarizability effect is evident and can be rationalized in terms of a dynamical stabilization of slightly distorted molecules, favorited by a solvent rearrangement. The presence in polar solution of slightly distorted molecules, and thus the removal from the Franck-Condon geometry along with relaxation, is further confirmed by the wavelength excitation dependence observed: exciting on the very red-tail of the absorption spectrum, in fact, a subpopulation of slightly distorted molecules is photo-selected and directly excited to the low-lying charge transfer state.