The requirement of spin conservation also seems to be
controlling the rates of energy transfer processes, which is
directly relevant for the field of bioinorganic and biomimetic pho-
tosensitization. Recently, it could be shown, for example, that
spin effects can result in significant changes in the rate of Dexter
transfer by up to two orders of magnitude at room temperature
( 169 ). Orbital-specific energy transfer in coordination com-
pounds, occurring preferentially from excited states with a favor-
able dipole orientation, has also been documented ( 170 ). These
findings especially should be considered for the optimization of
all kinds of biomimetic and artificial photosynthetic systems
based on polynuclear metal complexes.
Many other important theoretical concepts of paramount
importance for biomimetic photochemistry are still in their
infancy. This includes the strategies to mediate and accelerate
proton-coupled single and multiple electron transfer catalysis
( 171 – 173 ), and the fundamental aspects of catalytic hydride
transfer and hydrogen tunneling processes ( 174 – 176 ) among
several others.
C. PHOTOCHEMICALMODELING OFKEYSTEPS
Various basic reaction sequences occurring in biological sys-
tems can actually be simulated and completely replaced by bio-
mimetic model systems containing light-responsive metal
complexes as their active components. Only a few illustrative
examples of this strategy will be given below, which have been
selected according to our own research interests.
C.1. Charge separation
The primary processes in natural photosynthesis involve the
conversion of solar into electrochemical energy (6,8). Light
absorption by antenna chromophores is followed by energy trans-
fer to a reaction center, where the initial charge separation takes
place. Enormous efforts have therefore been made by many
research groups to create artificial photosynthetic reaction cen-
ter models, which are able to integrate light-harvesting and
PET properties in large and complex molecular assemblies
( 177 – 179 ). A main goal in this field had always been the forma-
tion and spectroscopic characterization of long-lived charge-
separated states. Coupling of such systems to catalytic sites
where multielectron redox processes can occur, however, was
largely neglected. Nature has solved this crucial problem by
PHOTOSENSITIZATION AND PHOTOCATALYSIS 269