Multireference Excitation Energies for Bacteriochlorophylls A within Light Harvesting System 2

Abstract

Light-harvesting system 2 (LH2) of purple bacteria is one of the most popular antenna complexes used to study Nature's way of collecting and channeling solar energy. The dynamics of the absorbed energy is probed by ultrafast spectroscopy. Simulation of these experiments relies on fitting a range of parameters to reproduce the spectra. Here, we present a method that can determine key parameters to chemical accuracy. These will eliminate free variables in the modeling, thus reducing the problem. Using MS-RASPT2/RASSCF calculations, we compute excitation energies and transition dipole moments of all bacteriochlorophylls in LH2. We find that the excitation energies vary among the bacteriochlorophyll monomers and that they are regulated by the curvature of the macrocycle ring and the dihedral angle of an acetyl moiety. Increasing the curvature lifts the ground state energy, which causes a red shift of the excitation energy. Increasing the torsion of the acetyl moiety raises the excited state energy, resulting in a blue shift of the excitation energy. The obtained results mark a giant leap for multiconfigurational multireference quantum chemical methods in the photochemistry of biological systems, which can prove instrumental in exposing the underlying physics of photosynthetic light-harvesting.

Macrocycle ring deformation as the secondary design principle for light-harvesting complexes

“…0.02 eV. This is in line with what was expected, considering the change in the dihedral angle previously seen when comparing different BChl units in LH2 (24).…”

“…The high-quality MS-RASPT2 computed spectroscopical properties for the alpha and beta units of LH3, as shown in Table 1, give a dual indication. On the one hand, comparing these data with those we previously obtained for LH2 (24), we can observe a clear blue-shift (i.e., higher excitation energies and longer wave lengths). Such blue-shift is very pronounced (ca.…”

“…This can be rationalized as follows: As shown in table 2 of ref. 24, the main contribution to the electronic excitation in BChls comes from molecular orbitals residing on the MCR. In other words, these are the molecular orbitals mostly depleted or enriched of electronic population upon photo excitation.…”

“…previously been proposed that the rotation of the acetyl moiety (torsion d in Fig. 2) is responsible for the energy modulation (22)(23)(24), but according to ref. 16, this effect is much too small to account for the 30-nm shift.…”

Spectroscopical UV–Vis implications of an intramolecular η2–Mg coordination in bacteriochlorophyll–a from the Fenna–Matthews–Olson complex

“…Despite the fact that the application of some coupled cluster (CC) or second order perturbation theory methods to light harvesting pigments are no longer prohibitive, they remain quite demanding for which the use of TD‐DFT has positioned itself as a gold standard for calculating site energies in the EET process, even superseding ab initio methods such as CIS, and thus the TD‐DFT method is used throughout the present work.…”