magine a jigsaw of nine pieces forming a 3 x 3 square. There would be four corner pieces with two adjacent straight edges, four mid-edge pieces with one straight edge, and a center piece with no straight edges. Place them on a tray and shake them about and it is just possible that they will come together to complete the 3 x 3 picture. The odds are heavily stacked against this happening by chance, but in paper #10 on the current Hot Ten a group of chemists from the City University of New York (CUNY), led by Charles Drain, has achieved it with a set of porphyrin molecules. The secret is to make the jigsaw automatically self-assembling, and the secret of self-assembly in this case is to use metal atoms to direct the porphyrins to the right locations.

Interest in porphyrins stems from their ability to convert light energy into electron motion, and chemists have put a lot of effort into making such molecules. In the porphyrin of chlorophyll the metal is magnesium, and life on this planet depends on its ability to convert sunlight, water, and carbon dioxide into carbohydrate. Not surprisingly, chemists have tinkered with the porphyrin molecule, producing thousands of variants and joining them together. The largest array so far has 21 porphyrins and was made by Ken-ichi Sugiura at Osaka University, Japan (see K. Sugiura, et al., Chem. Lett., [11]:1193-4, November 1999), but it requires 17 chemical reactions to construct and is obtained in only 0.15% yield. However, there are some multiple porphyrins which construct themselves, as Drain has shown (e.g., see C.M. Drain, et al., J. Chem. Soc. Chem. Comm., [3]:337-8, 1996).

Now Drain and his team have pushed self-assembly one step further: Paper #10 announced the construction of a nine-porphyrin supramolecule. Not only that, but it assembled itself from a kit of the three kinds of jigsaw porphyrins (dissolved in the right ratio in a solvent such as chloroform) with the help of a palladium complex—bis(benzonitrile)palladium(II) dichloride—that guides them into the right configuration. The palladiums link the porphyrins together by binding to the outsides of the molecules. A remarkable 90% yield of the desired product was achieved from the kit within 30 minutes of adding the palladium connector. The CUNY chemists were also able to make a self-assembling "tape" of linked porphyrins using the same approach with a platinum complex as the linker.

The new supramolecules were subjected to light-scattering experiments which showed that they existed as clusters of radius 5 to 7 nanometers (nm) and that these could be deposited as arrays on polished glass substrates which were then characterized by James Battens of CUNY using atomic force microscopy. This indicated that they had stacked up in layers as nanocrystals as tall as 39 nm, although most fell in the range of 4.5 to 6.5 nm. Electrospray mass spectroscopy also confirmed these highly unusual structures.

"In the long term some scientists envisage nanoscale machines and self-correcting materials," says Drain, "but if these are ever to be used in a practical sense then they must be robust enough for processing. Our work demonstrates that it is possible to self-assemble nanoscale supramolecular species with exquisite control, and then place them on surfaces and manipulate them."

Why did he chose porphyrin? "These are structurally related to haem and chlorophyll, they have extraordinary photo and electro chemistry, and they can be stable on a geological time scale. Here we have developed building blocks that can form a staggering variety of structures depending on the direction or shape of the porphyrin molecule, the orientation of the building sites, and the geometry of a metal ion linker. It is remarkable that 21 particles of four different kinds of molecules (9 porphyrins and 12 palladium complexes) can self-assemble into a predictable structure, and even more remarkable that the supramolecular assembly they form can be deposited on surfaces without altering their three-dimensional structure."

And what plans does Drain have for future research in this area? "Our continuing work focuses on the controlled ordering of surface-bound self-assembled structures derived from such building blocks. To expand the types of molecular architectures that can be built, we are designing new building blocks with different properties and shapes."

Dr. John Emsley is Science Writer in Residence
at the Department of Chemistry, University of Cambridge, U.K.