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Press Release

Angew. Chem. Int. Ed. 2002, 41 (13), 2284 - 2288

No. 13/2002

New Building Blocks for Optronics/Photonics

Dye-filled zeolith minicrystals
for the capture and storage of light energy

Plant photosynthesis, the conversion of light into (bio)chemical energy, is endlessly fascinating. Just as a radio uses an antenna to absorb electromagnetic waves from the atmosphere, plants use their chlorophyll to capture certain wavelengths of visible light and pass the light's energy on to their photosynthetic apparatus. Such "photonic antennas" would also be great for technical applications, particularly for a new, more highly efficient generation of solar cells.

A research group at the University of Bern has now attained extremely promising results with their artificial photonic antenna; in order to capture light, Gion Calzaferri and Huub Maas are using molecules of a green fluorescent dye, which they have introduced into the linear channels in tiny, porous, zeolite crystals. When the fluorescent dye is irradiated, the molecules absorb quanta of light. With the help of these "packets of energy", the electrons of the dye can jump into an excited state - much like a different "orbit". After a short time, the electrons fall back to their original orbit. A small portion of the energy released in this process is dispersed as vibrations throughout the molecules. The rest of the energy is released as another quantum of light. Because this quantum has less energy than was originally absorbed, its color is different. This phenomenon is known as fluorescence. When the dye molecules are stuck into the channels that cross through the crystals parallel to their long axes, they are neatly packed right up against each other. This allows them to pass the packets of energy directly - without conversion into light - from molecule to molecule. The special twist is this: the Swiss researchers plugged up the openings of the channels with a second type of fluorescent dye. The two types of molecules are precisely tuned to each other; the "plugs" are thus also able to accept energy packets from the electronic excitation, but they are not able to pass them back to the "emitters" within the crystal. The "plugs" re-emit the energy as red fluorescence on the surface of the little crystals - where it can be recaptured.

This "receiving antenna" can alternatively be made into a "transmitter" if the two fluorescent dyes are more or less switched. The "plugs" capture energy from outside, which they then pass on to the molecules inside the crystals. These then fluoresce, which is useful for making such things as new light emitting diodes.




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