Dissertation

|1.5 Photosynthesis|
To counter the low solar energy flux, materials for energy conversion without foregoing
concentration must be long-term stable and cheap to make the process economical. Today, the
cost of solar electricity is about ten times higher than conventional energy.
A direct conversion of solar energy into chemical energy (fuels) represents the third alternative for
its exploitation. Light-driven reactions can copy principles of nature in which chemical bonds are
broken and formed to keep the organism alive and to build up biomass. The basis for such artificial
photosynthesis process is the detailed understanding of important principles of photophysics,
photochemistry, electrochemistry and catalysis as well as of the natural photosynthesis.
1.5 Photosynthesis
Already about 3.5 billion years ago, nature learned how to utilize sunlight in a primitive way.
Starting from bacteria photosynthesis evolutionary processes led to the photosynthesis of higher
plants. Until today, bacteria, algae and green plants are capable of transforming vast amounts of
solar energy into biomass via different types of photosynthesis. The primary processes from light
to chemical energy conversion are very efficient and sustain the organism’s life. Besides, only a
small fraction (1% of the available light energy) is used by plants for biomass production. By that
means, solar energy conversion accounts for the formation of nowadays biomass as well as for the
formation of fossil fuels.
These primary steps of the so called light-dependent reactions involve light absorption, charge
separation and transfer as well as catalytic oxidation and reduction processes within two
photosystems at the thylakoid membrane of chloroplasts. [18] Figure 10 gives a résumé of these
extremely organized processes in the so called Z-scheme of photosynthesis. [19]
Photosynthesis starts with the simultaneous excitation of special pairs of chlorophyll(a) molecules
(P680) in photosystem II (PS II, see figure 9) and (P700) in photosystem I (PS I), the only
step where light energy is converted into chemical energy stored in excited electrons with high
reduction potential. This can be achieved by direct absorption of visible light by the special pairs
(λPS II
max = 680 nm and λ PS max I = 700 nm). However, more likely is the activation by excitation energy
transfer via Förster or Dexter mechanisms from nearby arranged chlorophylls and carotenoids.
A great number of these antenna pigments is precisely arranged in light harvesting complexes
(LH I and LH II) and increases the absorption area as well as the absorption band of P680 and
|10|