Self-organizing molecules can form structures with useful electronic properties. These supramolecular materials combine the benefits of polymers with those of organic crystalline systems.
To create new electronic devices, physicists and engineers have had to rely increasingly on the originality of chemists in designing, synthesizing and characterizing molecular systems possessing useful properties. Exciting results have been obtained with both polymer-based light-emitting diodes and organic transistors, in which molecular-scale layers are deposited from the vapour phase. But these two semiconducting classes of material have different virtues. The particular advantage of the polymer systems is that they are cheap and highly 'processable' — that is, easy to synthesize, manipulate and incorporate into devices. The advantage of the organic materials is the precise ordering of their thin crystalline layers, which supports the high charge-carrier (electron and hole) mobility essential to electronic performance.
The dream for many scientists is to bring these two features together in one class of materials, and to produce easy-to-process yet highly ordered molecular systems. On page 384 of this issue1, Percec et al. describe a novel and clever way of doing this. As with most such research, 'π-conjugated systems' are the key theme. Put simply, these are materials containing electrons that are not tied up in individual covalent bonds, so that they can travel freely over the molecular framework and hop from molecule to molecule. Percec et al. have used a remarkably simple strategy to 'program' various electronically active molecules into liquid-crystalline columns containing cores with 'π-stacks' that display high electron and hole mobility. By this they have taken an important step towards the creation of self-organizing systems with favourable carrier mobility.
It is 25 years since Chandrasekhar and co-workers published their seminal paper2 describing the properties of the first 'discotic' liquid crystals. These disc-like molecules self-organize into stacks or columns, which typically order themselves into hexagonal lattices. It was 1994 before the first example3 of fast light-induced conductivity in such systems was obtained. But since then the ability of synthetic chemists to bring molecules together selectively, in well-defined architectures, has increased tremendously.
The upshot has been that materials scientists have discovered the world of supramolecular chemistry, which has the potential to marry the best characteristics of high-molecular-weight polymers and low-molecular-weight organic molecules. As their name implies, supramolecular materials self-assemble and are therefore often easier to synthesize and process than crystalline materials. And provided the individual building-blocks are carefully 'programmed', the self-assembly should yield structures and molecular orientations that are suitable for the desired function. For example, the macroscopic orientation of the π-conjugated components in organic devices is of central importance for determining the direction of charge-carrier mobility, as has been demonstrated for semiconducting polythiophenes4.
Combining the strengths of classical polymers and single crystals will require new thinking. The way forward is to take functional, disordered molecules and allow them to self-assemble — through information programmed into the molecule — into well-defined arrays. To obtain a general scheme for making such self-processable organic materials, Percec et al.1 apply several different concepts from supramolecular chemistry. The details of the chemistry and the liquid-crystal assembly are shown in Figs 1 and 2, respectively, on page 385. In short, the authors have synthesized fluorine-carrying clusters of highly branched polymers called dendritic wedges. When electron donor and acceptor groups are attached, the wedges self-assemble into supramolecular columns. It is self-assembly of these wedges that drives the liquid-crystal formation of the organic component. Both low-molecular-weight organic materials and polymerized species can be used as donor and acceptor groups. By selecting the right ingredients, the molecules co-assemble into columns in which a donor–acceptor complex is formed in the core of the liquid-crystal column. The fluorinated periphery of the molecules shields the core of the column from external influences such as moisture. Remarkably, even disordered polymers self-assemble into well-defined columns.
Percec and colleagues' in-depth analysis of the liquid-crystalline phases, including the use of X-ray and NMR spectroscopy, provides a wealth of information about the organization of the molecules within the column, as well as showing that the columns are oriented perpendicular to the surface. These studies reveal that the regularly packed nanocolumns are of high density, with roughly 1012 columns per square centimetre. The charge-carrier mobilities, as determined by the time-of-flight method, fall in the range of 10−4–10−3 cm2 V−1 s−1, well within the values needed for molecule-based devices. The data showing at high resolution how the system is packed are particularly striking. Using an especially sophisticated NMR technique5, a group of the authors in Mainz resolved the distances between different building-blocks in the liquid-crystalline phase to the proton–proton level.
The true significance of the systems described by Percec et al.1 will only become clear when they are incorporated into electronic devices (for transistor applications, the column orientation will preferably be parallel, rather than perpendicular, to the surface). An even more exciting prospect would be the independent use of individual columns, with the goal of producing supramolecular electronics as an alternative to molecular electronics. Several groups, including our own, are currently engaged in such work. Columns with diameters as small as 3–5 nm and lengths of 50–100 nm should self-assemble between electrodes with a single-crystal-like packing, providing alternatives for single-walled carbon nanotubes or inorganic wires. All in all, the concepts outlined by Percec et al. constitute a good starting point in the search for supramolecular electronic materials.
Percec, V. et al. Nature 419, 384–387 (2002).
Chandrasekhar, S., Sadashiva, B. K. & Suresh, K. A. Pramana 9, 471–480 (1977).
Adam, D. et al. Nature 371, 141–143 (1994).
Sirringhaus, H. et al. Nature 401, 685–688 (1999).
Brown, S. P. & Spiess, H. W. Chem. Rev. 101, 4125–4155 (2001).
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Electronic circular dichroism imaging (CDi) maps local aggregation modes in thin films of chiral oligothiophenes
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