SUMMARY: Plants are masters of transforming sunlight into chemical energy.
In the ingenious antenna system of the leaf, the energy of the sunlight is
transported by chlorophyll molecules for the purpose of energy transformation.
We have succeeded in reproducing a similar light transport in an artificial
system on a nano scale. In this artificial system, zeolite L cylinders adopt
the antenna function. The light transport is made possible by specifically
organized dye molecules which mimic the natural function of chlorophyll.
Zeolites are crystalline materials with different cavity structures.
Some of them occur in nature as a component of the soil. We are using zeolite L
crystals of cylindrical morphology which consist of a continuous one-dimensional
tube system and we have succeeded in filling each individual tube with chains of
joined but noninteracting dye molecules. Light shining on the cylinder is first
absorbed and the energy is then transported by the dye molecules inside the tubes
to the cylinder ends. We expect that our system can contribute to a better
understanding of the important light harvesting process which plants use for
the photochemical transformation and storage of solar energy.
We have synthesized nano crystalline zeolite L cylinders ranging in length
from 300 nm to 3000 nm. A cylinder of 800 nm diameter e.g. consists of about
150'000 parallel tubes. Single red emitting dye molecules (oxonine) were put
at each end of the tubes filled with a green emitting dye (pyronine).
This arrangement made the experimental proof of efficient light transport
possible. Light of appropriate wavelength shining on the cylinder is only
absorbed by the pyronine and moves along these molecules until it reaches
the oxonine. The oxonine absorbs the energy by a radiationless energy transfer
process, but it is not able to send it back to the pyronine. Instead it emits
the energy in the form of red light. The artificial light harvesting system makes
it possible to realize a device in which different dye molecules inside the
tubes are arranged in such a way that the whole visible spectrum can be used
by conducting light from blue to green to red without significant loss.
Such a material could conceivably be used in a dye laser of extremely small size.
The light harvesting nano crystals are also investigated as probes in near field
microscopy, as materials for new imaging techniques and as luminescent probes in
biological systems. The extremely fast energy migration, the pronounced anisotropy,
the geometrical constraints and the high concentration of monomers which can be
realized, have much potential in leading to new photophysical phenomena. Attempts
are being made to use the efficient zeolite based light harvesting system for the
development of a new type of thin layer solar cell in which the absorption of light
and the creation of an electron-hole pair are spatially separated as in the natural
antenna system of green plants.
Synthesis, characterization and applications of an artificial antenna for light
harvesting within a certain volume and transport of the electronic excitation
energy to a specific place of molecular dimension has been the target of research
in many laboratories in which different approaches have been followed.
To our knowledge, the system developed by us is the first artificial antenna which
works well enough to deserve this name. Many other highly organized dye-zeolite
materials of this type can be prepared with similar methods and are expected to
show a wide variety of remarkable properties. The largely improved chemical and
photochemical stability of dye molecules inserted in an appropriate zeolite
framework allows us to work with dyes which otherwise would be considered
uninteresting because of their lack of stability. We have developed two methods for
preparing well defined dye-zeolite materials, one of them working at the solid/liquid
and the other at the solid/gas interface. Different approaches for preparing similar
materials are in situ synthesis (ship in a bottle) or different types of
crystallization inclusion synthesis.
Solid State Sciences, 2000, 2, in press.
Solid State Sciences, 2000, 2, in press.
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