under ambient conditions an ideal strategy for the functional
modeling of difficult or energetically demanding chemical trans-
formations that are otherwise restricted to biocatalysis and
natural photosynthesis. In Table IV, some complementary
functions of biological systems and basic types of photochemical
reactions are compared.
The design principles for the rational construction of light-
driven counterparts of bioinorganic, bio-organometallic, and bio-
catalytic systems have recently been reviewed in more detail
( 5 ). Here, we will limit our discussion on the most important
aspects and advantages of photoreactive components in
bioinspired energy conversion and catalysis.
B.1. Electrons
Light absorption modifies the driving force for electron transfer
processes in all kinds of materials. As photoactivated species are
always better oxidantsandreductants than their ground state
equivalents, an enhanced redox reactivity is usually observed
in the excited state. Photoreactions are therefore ideally suited
to trigger, study, and mimic bioinorganic electron transfer.
The typical one-electron redox reactions resulting from
PET processes can be applied to control the generation of
reactive intermediates similar to the way radical enzymes are
performing ( 112 ). To achieve an accumulation of permanent reac-
tion products, as is the case with most oxidoreductase enzymes
and photosynthetic systems, it is very important to provide suit-
able (photo)-catalytic multielectron transfer (MET) pathways.
TABLE IV
EXCITEDSTATEPROCESSES ANDTHEIRFUNCTIONALCOUNTERPARTSLight-dependent reaction types Biosystems with related functions
Bond distortion, spin crossover Enzyme-substrate complexes
Sensitization, energy transfer Light-harvesting antenna systems
Photoinduced electron transfer Photosynthetic reaction centers
Hydrogen atom abstraction Radical enzymes
Photooxidation, photoreduction Oxidoreductase enzymes
Photoaddition, photosubstitution Transferases, kinases
Photodissociation, photo-cleavage Hydrolases, lyases, nucleases
Photoisomerizations Isomerases, mutases
Photodimerization, photo-
polymerization
Ligases, polymerasesPhototriggered release of compounds Hormones, neurotransmitters
Photodeposition of materials Biomineralization and storage
proteins
252 GÜNTHER KNÖR AND UWE MONKOWIUS