employing quinone cofactors as the primary electron acceptors in
photosynthetic reaction centers, which helps to avoid unproduc-
tive charge-recombination processes ( 180 ).
From a functional point of view, this important property can be
readily built into low molecular weight chromophore assemblies
acting as artificial reaction centers( 8 ). In simple coordination
compounds, the population of CT states is directly related to
the concept of light-induced charge separation in photosynthesis.
Whenever such CT states are photoreactive and lead to the for-
mation of the same kind of permanent redox products as
observed in photosynthesis, the most essential features of the
primary light reactions have been successfully duplicated. In a
more strict sense, this is of course only true, if actinic red or
NIR-light of comparable wavelength is absorbed by both the nat-
ural and artificial photosynthetic systems.
Biologically relevant electron acceptors such as quinones are
able to act as redox-active chelate ligands(181,182). Frequently,
a coordination of these cofactors leads to intensely colored metal
complexes with low-lying CT excited states. With such types of
inorganic chromophores, very simple functional model compounds
for mimicking the charge separation cascade in photosynthetic
reaction centers can be constructed. As an illustrative example
( 181 ), the organometallic rhenium complex 23 carrying a loosely
coordinated 9,10-phenanthrene-quinone (PQ) moiety is presented
here (Fig. 20). The deeply colored compound [ReI(PQ)(CO) 3 Cl]
contains a low-valent diamagnetic metal site which can replace
the PET functionality of the primary donor of photosynthetic
reaction centers, which usually consists of a special pair of chloro-
phyll pigments ( 6 ). At the same time, this compound carries a
preorganized quinone cofactor, which can mimic the functional role
of the rigidly orientated primary quinone acceptor QApresent in
the PS II-type reaction centers( 180 ). The very simple biomimetic
system 23 displays a broad and intense CT band in the visible and
NIR-spectral region, which has been assigned as a MLCT transition
( 181 ). Interestingly, the maximum of this band at around 711 nm is
almost coinciding with the NIR absorption features of the recently
discovered cyanobacterial light-harvesting pigment chlorophyllf,
which is now believed to represent nature's best choice to match
the intrinsic threshold wavelength limits required for oxygenic
photosynthesis( 95 ).
C.2. Oxygen activation
Modeling the active site features and the mechanistic key steps
occurring in biological dioxygen activation is one of the most
270 GÜNTHER KNÖR AND UWE MONKOWIUS