Page 580 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Reactions on Polymers 543

16.9 PHOTOSYNTHESIS

The recent environmental issues related to the green house effect and atmospheric contamina-
tion heightens the importance of obtaining energy from clean sources such as photosynthesis.
Photosynthesis also acts as a model for the creation of synthetic light-harvesting systems that
might mimic chlorophyll in its ability to convert sunlight into usable energy. The basis of natural
14
photosynthesis was discovered by Melvin Calvin, one of my academic grandfathers. Using C as a
tracer, Calvin and his team found the pathway that carbon follows in a plant during photosynthesis.
They showed that sunlight supplies the energy through the chlorophyll site, allowing the synthesis
of carbon-containing units, saccharides or carbohydrates. Chlorophyll is a metal embedded in a
protein polymer matrix and illustrates the importance of metals in the field of photochemistry and

photophysics. A brief description of the activity of chlorophyll in creating energy from the sun
follows.
2
The maximum solar power density reaching Earth is approximately 1,350 W/m . When this
energy enters the Earth’s atmosphere, the magnitude reaching the surface drops approximately to
2
1,000 W/m due to atmospheric absorption. The amount that is used by plants in photosynthesis is
about seven times the total energy used by all humans at any given time, thus it is a huge energy
11
source. On Earth, organisms convert about 10 tons of carbon into biomass yearly.
Solar energy is clean and economical but it must be converted into useful forms of energy.
For example, solar energy can be used as a source of excitation to induce a variety of chemical
reactions.
Plants and algae are natural examples of the conversion of light energy and this energy is used to
synthesize organic sugar-type compounds through photosynthesis. This process has a great impor-
tance for survival of all life on our planet because light provides the energy and reduces the car-
bon dioxide as well as produces molecular oxygen necessary for oxygen consuming organisms.
In fact, this process can be considered as a vital link between material and energy cycling in the
biosphere.
Photoenergy transfer can be considered to occur through two primary mechanisms. The fi rst is
the Förster mechanism, which is also known as the columbic mechanism or dipole-induced dipole
interaction. Here, the emission band of one molecule (donor) overlaps with the absorption band of
another molecule (acceptor). In this case, a rapid energy transfer may occur without a photon emis-
sion. This mechanism involves the migration of energy by the resonant coupling of electrical dipoles
from an excited molecule (donor) to an acceptor molecule. On the basis of the nature of interactions
present between the donor and acceptor this process can occur over a long distances (30–100 Å).
The Dexter mechanism is a nonradiative energy transfer process, which involves a double electron
exchange between the donor and the acceptor. Although the double electron exchange is involved
in this mechanism, no charge-separated state is formed. For this double electron exchange process
to operate, there should be a molecular orbital overlap between the excited donor and the acceptor
molecular orbital. For a bimolecular process, intermolecular collisions are required as well. This
mechanism involves short-range interactions (~6–20 Å and shorter).
In photosynthesis, green plants and some bacteria harvest the light coming from the sun by means
of their photosynthetic antenna systems. The light harvesting starts with light gathering by antenna
systems, which consist of pigment molecules, including chlorophylls, carotenoids, and their deriva-
tives. The absorbed photons are used to generate excitons, which travel via Förster energy transfers
toward the reaction centers (RCs). This overall series of processes is represented in Figure 16.9. The
series can be remembered by the initials ARC, where A represents antenna pigments; R represents
the reaction centers; and C represents chlorophylls and carbohydrates.
In reaction centers, this energy drives an electron-transfer reaction, which in turn initiates a
series of slower chemical reactions and the energy is saved as redox energy inducing a charge sepa-
ration in a chlorophyll dimer called the special pair (chlorophyll) . Charge separation, which forms
2
the basis for photosynthetic energy transfer, is achieved inside these reaction centers (16.40).

9/14/2010 3:43:14 PM
K10478.indb 543 9/14/2010 3:43:14 PM
K10478.indb 543