On the photophysics of carotenoids: a multireference DFT study of peridinin

We present a quantum-mechanical investigation of the photophysics of a specific carotenoid, peridinin, which is present in light-harvesting complexes. The fundamental role played by the geometry in determining the position and character of its low-lying singlet electronic states is investigated using a multireference DFT approach in combination with a continuum solvation model. The main photophysical properties of peridinin appear to be governed by the lowest two singlet excited states, as no evidence points to an intermediate S* state and the energies of the upper excited states are too high to allow their population with excitation in the visible range. These two excited states (S1, 2(1)A(g)(-) and S2, 1(1)B(u)(+)) are highly connected through the conjugation path here characterized by the value of the bond length alternation (BLA). The S1 and S2 states present distinct natures for small BLA values, whereas for larger ones they become more similar in terms of both brightness and dipolar character and their energies become closer. The geometrical issue is thus of fundamental importance for a correct interpretation of the spectroscopic signatures of peridinin.