Excited state characterization of carbonyl containing carotenoids: a comparison between single and multireference descriptions

TL;DR

This study compares single and multireference computational methods to characterize the excited states of six common carbonyl carotenoids, revealing the influence of the carbonyl group on their photophysical properties.

Contribution

It provides a benchmark of DFT and multireference methods for accurately describing the excited states of carbonyl carotenoids, highlighting the effectiveness of certain functionals.

Findings

The bright transition is mainly ππ* in character.

The C=O group lowers nπ* transition energies.

Meta-GGA functionals like M11L and MN12L perform best in benchmarking.

Abstract

Carotenoids can play multiple roles in biological photoreceptors thanks to their rich photophysics. In the present work, we have investigated six of the most common carbonyl containing carotenoids: Echinenone, Canthaxanthin, Astaxanthin, Fucoxanthin, Capsanthin and Capsorubin. Their excitation properties are investigated by means of a hybrid density functional theory (DFT) and multireference configuration interaction (MRCI) approach to elucidate the role of the carbonyl group: the bright transition is of {\pi}{\pi}* character, as expected, but the presence of a C=O moiety reduces the energy of n{\pi}* transitions which may become closer to the {\pi}{\pi}* transition, in particular as the conjugation chain decreases. This can be related to the presence of a low-lying charge transfer state typical of short carbonyl- containing carotenoids. The DFT/MRCI results are finally used to benchmark single- reference time-dependent DFT-based methods: among the investigated functionals, the meta- GGA (and in particular M11L and MN12L) functionals show to perform the best for all six investigated systems.