Last updated April 25, 2000
Department of Chemistry and Biochemistry, University of Bern
Freiestrasse 3, CH-3000 Bern 9, Switzerland
Plants are masters of transforming sunlight into energy. In the ingenious antenna system of the leaf, 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 cylinders adopt the antenna function. The light transport is made possible by specifically organized dye molecules that mimic the natural function of chlorophyll. Zeolites are materials with different cavity structures. Some of them occur in nature as a component of the soil. We use zeolite L crystals of cylindrical morphology which consist of a continuous 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 transported by the dye molecules inside the tubes to the cylinder ends. We expect that our system can also 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 about 3000 nm. A cylinder of 600 nm consists of, for example, about 100 000 parallel arranged tubes. Single red emitting dye molecules (oxonine) were put at each end of the tubes filled with green emitting dye (pyronine). This arrangement made an 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, visible to the naked eye. The artificial light harvesting system makes it possible to realize a device in which the dye molecules inside the tubes are arranged in such a way that the whole light 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 being 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.