A team of German and American scientists have succeeded in combining conventional organic molecules and conductive polymers to form highly symmetric, structured materials with new electronic properties. After the attachment of specific functional groups, the disc-like or ring-shaped organic molecules organize into highly symmetric cylinders, three nanometers in thickness and 50-100 nanometers in length, just like a roll of coins. With powerful spectroscopic techniques, scientists from the Max Planck Institute for Polymer Research contributed significantly to the structural clarification of the nano-cylinders, whose core consists of conductive molecules or polymers and is covered with a molecular insulating "coating". Such new materials are important for optoelectronics and open up new possibilities for supramolecular electronics (Nature, Sep. 26th, 2002).
The discovery of conducting organic crystals and polymers, which resulted in the Nobel prize for Chemistry in 2000, significantly broadened the spectrum of materials that are useful for optoelectronics. The materials, however, must display a high charge carrier mobility and be easy to prepare and handle. Crystals have a precise structure and a high electron conductivity, but they are difficult to handle. Polymers, in contrast, are cheap to produce and easy to handle; their charge carriers, however, are comparatively immobile. Liquid crystals have a charge carrier mobility similar to crystals but their preparation and processing is very expensive. For these reasons, it has been a long-standing aim of many research groups to combine the advantages of both types of materials in order to produce highly ordered but easy to handle molecular systems.
Scientist at the Max Planck Institute for Polymer Research and their American collaborators have managed to combine the advantageous properties of classical polymers with those of crystals by synthesizing clusters of fluorine-containing dendritic polymers. If single electron donor or electron acceptor groups are attached to the end of the dendrons, wedge shaped building blocks arise, which organize themselves into tiny supramolecular cylinders. Both components, organic materials as well as polymers, can be used as donor- or acceptor groups.
In this way, supramolecular liquid crystals can be synthesized from different organic materials through self-organization. The donor-acceptor complexes in the center of these molecules display promising optoelectronic properties. Under these conditions, even disordered polymers assemble into well defined cylinders. The fluorinated periphery of the molecules protects the inner core from external influences, i.e. humidity, similar to a Teflon-coating.
Using their expertise in solid-state nuclear magnetic resonance (NMR) spectroscopy, the group of Prof. Hans Wolfgang Spiess at the Max Planck Institute in Mainz probed the stacking of the aromatic ring systems, which determines the optoelectronic properties of the nanocylinders. The researchers in Mainz developed NMR techniques that resolve the exact arrangement of the nanometer-scale cylindrical structures by measuring the distance between single hydrogen atoms (3.5Å). This information is essential to determine the degree to which electron conductivity can be achieved and, hence, for the functionality of the material. The analysis revealed a very regular packing in which the nanocylinders are always perpendicular to the surface. The materials also display a high density with 1012 nanocylinders per square centimeter.
NMR spectroscopy is vital to the success of the project. The desired results can be achieved with very little sample volume in a very short time - virtually overnight. Together with the information about the synthesis and functionality of these new materials, important details about the structure could be discovered. This will facilitate the use of these new materials in electronic components in the very near future. Particularly fascinating is the possibility of using each single cylinder of the molecular conglomerate separately, allowing supramolecular electronic devices to become a viable alternative to the molecular electronics used today.
Mainz has a rich tradition in the development of optoelectronic materials over the past few decades. The so-called discotic liquid crystals were a breakthrough in the 1980s. The self-organization of these disc-like molecules into stacks was the basis for their use in optoelectronic conductors on a molecular and nanoscopic level. Development of the first electronic devices (light diodes, transistors, solar cells, etc.) started in the late 1990s and continues today. The Max Planck Institute for Polymer Research in Mainz contributed significantly to this work in close collaboration with the University Mainz. In April 2001 the collaboration with the University Mainz resulted in the establishment of a "Center for multifunctional materials and miniaturized functional units" (Nanozentrum) funded with more than 9 million Euro by the Bundesministerium für Bildung und Forschung.
V. Percec, M. Glodde, T. K. Bera, Y. Miura, I. Shiyanovskaya, K. D. Singer, V. S. K. Balagurusamy, P. A. Heiney, I. Schnell, A. Rapp, H.-W. Spiess, S. D. Hudson & H. Duan: Self-organization of supramolecular helical dendrimers into complex electronic materials. Nature 419, 384-387 (2002)