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

Abstract

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 nanocrystalline zeolite L cylinders ranging in length from 300 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 the energy 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 nanocrystals 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 great 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 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 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.