important step is to analyze the structural and mechanistic
details of the natural process to be copied as far as possible.
Then, instead of trying to create a synthetic blueprint of all the
molecular components involved, the next step should be to define
the most fundamental functional requirements for a certain pro-
cess and to find out, which photoreactive components could possi-
bly fulfill the same kind of function in a more easily accessible
way. It is important to note that such a bioinspired design
strategy is not at all limited to the bioavailability of certain
chemical elements or to the arsenal of biological ligands, which
had defined the evolution of the natural systems.
Once a working model for a functional analogue has been
identified and tested, available theoretical concepts can serve
as a helpful guideline for optimizing the performance of the
biomimetic compounds. This may include more detailed
insights in multielectron reactivity, proton-coupled steps, conical
intersections, stereoselectivity, and selection rule constraints
including spin catalysis effects. At this stage of the development,
the efficiency and selectivity of important mechanistic key steps
of biological systems can already be directly compared with the
photochemical reactions chosen to copy the same function.
Long-term stability criteria, undesired side reactions, and the
possibility of light-dependent regulation should be included in
the considerations to optimize the synthetic compounds.
Finally, the best building blocks identified can be coupled in a
modular way to complete photocatalytic reaction cycles, which
then should be able to mimic a certain biological process. If these
bioinspired photocatalytic systems are performing under identi-
cal conditions as their native counterparts, a direct comparison
of quantitative criteria such as turnover frequencies and the
total number of catalytic cycles is possible and should always
be the final goal to demonstrate the potential usefulness of the
biomimetic process.
A. IDENTIFYINGFUNCTIONALANALOGIES
Nature sometimes solves identical problems with apparently
quite different solutions. Important examples of such a conver-
gent evolution at the molecular level are the functional parallels
between iron and copper centers in bioinorganic chemistry.
The dioxygen-carrier proteins of different organisms may, for
instance, be based on mononuclear iron tetrapyrrole complexes
or in contrast may involve dinuclear copper sites( 10 ). Synthetic
chemistry can even go a step further and try to mimic basic
PHOTOSENSITIZATION AND PHOTOCATALYSIS 261