Exploring the Light-Capturing Properties of Photosynthetic Chlorophyll Clusters Using Large-Scale Correlated Calculations

Chlorophylls are
light-capturing units found in photosynthetic
proteins. We study here the ground and excited state properties of
monomeric, dimeric, and tetrameric models of the special chlorophyll/bacteriochlorophyll
(Chl/BChl) pigment (P) centers P700 and P680/P870 of type I and type
II photosystems, respectively. In the excited state calculations,
we study the performance of the algebraic diagrammatic construction
through second-order (ADC(2)) method in combination with the reduced
virtual space (RVS) approach and the recently developed Laplace-transformed
scaled-opposite-spin (LT-SOS) algorithm, which allows us, for the
first time, to address multimeric effects at correlated ab
initio levels using large basis sets. At the LT-SOS-RVS-ADC(2)/def2-TZVP
level, we obtain vertical excitation energies (VEEs) of 2.00–2.07
and 1.52–1.62 eV for the P680/P700 and the P870 pigment models,
respectively, which agree well with the experimental absorption maxima
of 1.82, 1.77, and 1.43 eV for P680, P700, and P870, respectively.
In the P680/P870 models, we find that the photoexcitation leads to
a π → π* transition in which the exciton is delocalized
between the adjacent Chl/BChl molecules of the central pair, whereas
the exciton is localized to a single chlorophyll molecule in the P700
model. Consistent with experiments, the calculated excitonic splittings
between the central pairs of P680, P700, and P870 models are 80, 200,
and 400 cm–1, respectively. The calculations show
that the electron affinity of the radical cation of the P680 model
is 0.4 V larger than for the P870 model and 0.2 V larger than for
P700. The chromophore stacking interaction is found to strongly influence
the electron localization properties of the light-absorbing pigments,
which may help to elucidate mechanistic details of the charge separation
process in type I and type II photosystems.