Low-Lying Excited States of Natural Carotenoids Viewed by Ab Initio Methods

Abstract

Low-lying excited states of carotenoids (the optically dark 2A g − and bright 1B u + ) are deeply involved in energy transfer processes in photosynthetic antennas, such as light harvesting and non-photochemical quenching. Though any ab initio modeling of these phenomena has to rely on precise energies of the carotenoid electronic states, the accurate evaluation of these states remains a challenging problem due to their different natures. The paper aims to study the accuracy of the excitation energies of the low-lying excited states of certain open-and closed-chain carotenoids obtained by a state-of-the-art multireference approach for electronic structure calculation. Here, density matrix renormalization group SCF (DMRGSCF) and a perturbative approach based on driven similarity renormalization group second-order multireference perturbation theory (DSRG-MRPT2) were used to treat the static and dynamic correlation, respectively. Nuclear geometries of the electronic states were optimized with DFT-based approaches. It is demonstrated that spin-flip TD-DFT can replace multiconfigurational methods for the geometry optimization of the 2A g − state but not for the calculation of the excitation energy. Adiabatic excitation energies to the 1B u + state were shown to be within a margin of 1000 cm −1 with an appropriate flow parameter value. Adiabatic excitation energies to the 2A g − state for the open-chain carotenoids lie within a range of experimental values (taking into account the broad range of experimental estimates); for the closed-chain ones, the error does not exceed 2000 cm −1 . Ab initio stationary (1A g − → 1B u + ) and transient (2A g − → 1B u +) absorption spectra were modeled for violaxanthin and lycopene, and these spectra showed good agreement with the experimental ones both in terms of the vibronic structure and the transition energies.

Understanding the Excited-State Relaxation Mechanisms of Xanthophyll Lutein by Multi-configurational Electronic Structure Calculations

“…As shown in Figure , our OM2/MRCI-optimized 1Ag – and 1Bu + match very well with those of 1Ag – by CAM-B3LYP/cc-pVDZ and 1Bu + by TD-CAM-B3LYP/cc-pVDZ, respectively. In addition, the bond lengths of 2Ag – by spin-flip TD-PBE0/cc-pVDZ show the same trends as those of 2Ag – computed at the OM2/MRCI level . This results further confirm the reliability of OM2/MRCI in structural optimizations.…”

“…BLA can be used to understand the chemical structure and how charge distribution and dipole moment shift after the effects of radiation or electrical field. In this respect, BLA is widely used in the literature to characterize the geometrical properties of long chain carotenoids, , which is defined as follows: BLA = ∑ b s i n g l e s N s i n g l e s − ∑ b d o u b l e s N d o u b l e s where b singles and b doubles [ N singles and N doubles ] mean the bond lengths [the numbers] of single and double bonds, respectively. BLA can also represent the wave function of the electronic state in terms of a linear combination of the two limiting resonance structures: a neutral form characterized by a positive BLA and a charge-separated (zwitterionic) form characterized by a negative BLA (since the single and double bond pattern is reversed relative to the neutral form), while a value of zero associated with a completely delocalized form.…”

“…The most straightforward approach would be the multi-configurational self-consistent field (MCSCF) method with further energy refinements by means of the perturbation theory or any other approach, such as the complete active space second-order perturbation theory (CASPT2), the N-electron valence state second-order perturbation theory, etc. − The large scale of the carotenoid molecules and the strongly correlated electron characters make the applications of these post-HF methods extremely costly. Alternatively, some affordable computational approaches, including DFT-based DFT/MRCI and multi-state DFT and TD-DFT-based TDA/TD-DFT and spin-flip TD-DFT, are applied to study the photophysical properties of the carotenoid molecules. ,− In addition, several other cheaper semiempirical models, such as MNDO and INDO/S, are carried out to explore the photoinduced dynamics of the carotenoid molecules as well. − …”

“…Xanthophyll lutein (Scheme ) is one of the most abundant carotenoids in LHC II of photosystem II in plants. Since it can internally convert to the ground state in a time scale of picosecond, lutein is always inferred as the most probable site for non-photochemical quenching in various experimental and theoretical studies. , Static electronic structure calculations and non-adiabatic dynamics simulations are conducted to reveal the photophysical mechanism of lutein from an isolated molecule to a solvent environment. ,,− Here, we only introduce several representative works for lutein. For example, Ostroumov and co-workers provided direct proof for strong coupling between the bright 1Bu + state and an electronic dark 1Bu – state by using femtosecond transient absorption dynamics and DFT/MRCI calculations .…”

R2022: A DFT/MRCI Ansatz with Improved Performance for Double Excitations

“…By setting up a relationship between diagonal and off-diagonal corrections only one additional parameter needs to be introduced. After a careful assessment of the new effective Hamiltonian, dubbed R2022, we focus on its performance for two types of extended π-systems, namely, polyacenes and carotenoids, which are well investigated in the literature by other quantum chemical methods. ,− …”