Carotenoids are natural pigments with multiple roles in photosynthesis. They act as accessory pigments by absorbing light where chlorophyll absorption is low, and they quench the excitation energy of neighboring chlorophylls under high-light conditions. The function of carotenoids depends on their polyene-like structure, which controls their excited-state properties. After light absorption to their bright S 2 state, carotenoids rapidly decay to the optically dark S 1 state. However, ultrafast spectroscopy experiments have shown the signatures of another dark state, termed S X . Here we shed light on the ultrafast photophysics of lutein, a xanthophyll carotenoid, by explicitly simulating its nonadiabatic excited-state dynamics in solution. Our simulations confirm the involvement of S X in the relaxation toward S 1 and reveal that it is formed through a change in the nature of the S 2 state driven by the decrease in the bond length alternation coordinate of the carotenoid conjugated chain.

This work is dedicated to Jean-Paul Malrieu on the occasion of his 80 th birthday.We present a diabatization method of general applicability, based on the localization of molecular orbitals on user specified groups of atoms. The method yields orthogonal molecular orbitals similar to the canonical ones for the isolated atom groups, that are the basis to build reference spin-adapted configurations representing localized or charge transfer excitations. An orthogonal transformation from the adiabatic to the quasi-diabatic basis is defined by requiring maximum overlap with the diabatic references. We present the diabatization algorithm as implemented in the framework of semiempirical configuration interaction based on floating occupation molecular orbitals (FOMOÀ CI), but the same transformation can also be applied to ab initio wavefunctions, obtained for instance with state-average CASSCF. The diabatic representation so obtained and the associated hamiltonian matrix are particularly suited to assess quantitatively the interactions that account for charge and energy transfer transitions, and to analyze the results of nonadiabatic dynamics simulations involving such phenomena.

Note: This paper is part of the JCP Special Topic on Up-and Down-Conversion in Molecules and Materials.

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We present surface hopping simulations of the nonadiabatic dynamics in three covalently bound dimers of the methylene-locked 1,3-diphenyl-isobenzofuran (ML-DPBF) chromophore, a modification of a molecule previoulsly studied experimentally and computationally...