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Covalent assembly of molecular building blocks - November 2007

Writing in the November 2007 issue of the journal Nature Nanotechnology, a team of German and British scientists (Leonhard Grill, Matthew Dyer, Leif Lafferentz, Mats Persson, Maike V. Peters, and Stefan Hecht) reported the rational assembly on a surface of building blocks linked by covalent bonds—probably carbon radicals. A "News & Views" perspective by Neil R. Champness in the same issue describes in more detail than can be provided here why this advance is important.

"Nano-architectures by covalent assembly of molecular building blocks" Nature Nanotechnology abstract
Nature Nanotechnology "News & Views" perspective "Surface chemistry: Building with molecules" abstract
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Almost all work so far on atomically precise assembly directed toward developing productive nanosystems has self-assembled nanostructures in solution through a designed or evolved network of noncovalent interactions. Such nanostructures are held together by many weak noncovalent bonds, such as hydrogen bonds, van der Waals, and hydrophobic interactions. As Eric Drexler pointed out more than 20 years ago, it would be advantageous to be able to engineer robust covalent bond formation in the necessary spatial configurations to build strong, rigid nanostructures. Those laboring to develop molecular electronics also desire to rationally link molecular building blocks via covalent bonds to form larger nanostructures, not only because covalent connections form more stable nanostructures, but also because covalent bonds allow the efficient electron transport required by electronic devices. Now researchers have succeeded in rationally assembling building blocks on a surface using covalent bonds probably formed by carbon radicals—very reactive species envisioned by Drexler for use in advanced molecular manufacturing systems. Prior to this achievement, many workers had self-assembled supramolecular structures on surfaces by engineering noncovalent interactions, but no one had succeeded in assembling covalent molecular networks.

Porphyrin molecules—approximately square planar molecules made from linked rings of carbon and nitrogen atoms—were chosen as building blocks. A "leg" made from a phenyl group was attached to the center of each side of the central building block. One, two, or four of the legs were terminated with a bromine atom because the carbon-bromine bond is weak enough to be broken at a temperature that will not break the other bonds. In different variations of the procedure, the bromine atoms were dissociated either before or after the building blocks were deposited on gold surfaces. Dissociation of the carbon-bromine bonds produces carbon radicals that quickly form carbon-carbon bonds to link building blocks together.

The resulting nanostructures were clearly revealed by scanning tunneling microscopy. As expected, the nanostructures produced depended on the number of bromine atoms in the building blocks: with one bromine-activated bond, only dimers were found; with two bromine atoms on opposite sides of the porphyrin, straight chains resulted; with four bromines on each building block, a grid was produced. The authors emphasize that the formation of these nanostructures is not a self-assembly process since the process is not reversible.

This paper reports a promising avenue toward building covalent atomically precise structures on surfaces. The authors recognize that building more useful nanostructures will require more complex molecular building blocks and more options for generating covalent connections. "In a next step, complex network architectures could be constructed by selective activation of different molecular sites at characteristic dissociation temperatures..."

—James B. Lewis