Playing with dye molecules at the inner and outer surface of zeolite L

Plants are masters of transforming sunlight into chemical energy. In the in genious antenna system of the leaf, the energy of the sunlight is transport ed by chlorophyll molecules for the purpose of energy transformation. We ha ve succeeded in reproducing a similar light transport in an artificial syst em on a nano scale. In this artificial system, zeolite L cylinders adopt th e antenna function. The light transport is made possible by specifically or ganized dye molecules, which mimic the natural function of chlorophyll. Zeo lites are crystalline materials with different cavity structures. Some of t hem occur in nature as a component of the soil. We are using zeolite L crys tals of cylindrical morphology which consist of a continuous one-dimensiona l tube system and we have succeeded in filling each individual tube with ch ains of joined but noninteracting dye molecules. Light shining on the cylin der is first absorbed and the energy is then transported by the dye molecul es inside the tubes to the cylinder ends. We expect that our system can con tribute 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 nanocrystalline zeolite L cylinders ranging in length from 300 to 3000 nm. A cylinder of 800 nm diameter, e.g. consists o f 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 (pyroni ne). This arrangement made the experimental proof of efficient light transp ort possible. Light of appropriate wavelength shining on the cylinder is on ly absorbed by the pyronine and the energy moves along these molecules unti l 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 pyronin e. Instead it emits the energy in the form of red light. The artificial lig ht harvesting system makes it possible to realize a device in which differe nt 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 d ye laser of extremely small size. The light harvesting nanocrystals are als o investigated as probes in near-field microscopy, as materials for new ima ging techniques and as luminescent probes in biological systems. The extrem ely fast energy migration, the pronounced anisotropy, the geometrical const raints and the high concentration of monomers which can be realized, have g reat potential in leading to new photophysical phenomena. Attempts are bein g made to use the efficient zeolite-based light harvesting system for the d evelopment of a new type of thin-layer solar cell in which the absorption o f light and the creation of an electron-hole pair are spatially separated a s in the natural antenna system of green plants. Synthesis, characterizatio n and applications of an artificial antenna for light harvesting within a c ertain volume and transport of the electronic excitation energy to a specif ic place of molecular dimension has been the target of research in many lab oratories in which different approaches have been followed. To our knowledg e, the system developed by us is the first artificial antenna which works w ell enough to deserve this name. Many other highly organized dye-zeolite ma terials of this type can be prepared by 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 wh ich otherwise would be considered uninteresting because of their lack of st ability. We have developed two methods for preparing well-defined dye-zeoli te materials, one of them working at the solid-liquid and the other at the solid-gas interface. Different approaches for preparing similar materials a n in situ synthesis (ship in a bottle) or different types of crystallizatio n inclusion synthesis. (C) 2000 Editions scientifiques et medicales Elsevie r SAS. All rights reserved.