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, 21Ag– and S2, 11Bu+) 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.