Uniting hundreds of thousands of atoms, this supramolecular structure is one of the most complex ever made via self-assembly, where molecules organize themselves into larger structures.

What's more, it's the first organic compound to assume an intricate structure previously seen only in metals such as uranium and various metal alloys.

The work is described in a paper published on the Web site of the journal Science.

"Understanding how self-assembly is controlled by molecular architecture will enable the design of increasingly complex nanostructures," said Virgil Percec, a professor of chemistry at Penn.

"The achievement of a lattice of this size is a significant step towards designing new synthetic molecules which would form even larger structures, with dimensions approaching the wavelength of light."

Among self-assembled structures, bigger is better. Percec says if this lattice can attain dimensions equaling the wavelength of light the material could represent a new class of photonic crystals and a new approach to telecommunications. Such work could also yield molecular-scale electronics.

To create these large nanostructures, Percec and his colleagues started with a supersized building block: a carefully designed, well-defined and highly branched molecule referred to as a dendron. When thousands of these tree-like molecules come together, they organize themselves, unaided, into discrete microscopic spheres.

In the liquid crystal phase, each sphere consists of 12 tapered dendrons linked at their narrow end. Percec and his colleagues observed 30 of these globular structures arrange themselves into a tetragonal lattice whose repeat unit is a rectangular prism containing 255,240 atoms and measuring 169 by 169 by 88 angstroms. This repeat unit size is comparable to the crystal form of some spherical virus particles isolated from plants.

"Some of the complex structures in metal alloys have 200 atoms per lattice, and uranium has 30 atoms per unit," Percec said. "This encourages researchers to aim for equivalent self-assembled structures, and our work gives some pointers to synthetic chemists on how to design new dendrons for specific 'crystal' structures."

Using increasingly sophisticated techniques, scientists engineer self-assembling molecules to arrange themselves into much larger, functioning objects. The field draws inspiration from nature, where proteins and cells are genetically encoded to arrange themselves into functional entities.

"We started our studies by trying to replicate the protein coat that surrounds a virus," Percec said. "We've designed these dendrons and other self-assembling molecules based on that model."

Self-assembly may prove useful in a wide range of fields, many involving encapsulation of materials: drug delivery, adhesives, pesticides, composites, coatings and paints, photographic and imaging media, catalysis, microfabrication and microelectronics. Percec's group is now tweaking the structure of their dendron molecules so they might assemble into hollow spheres.

Percec is joined in the Science paper by co-authors Wook-Dong Cho at Penn and Goran Unger, Yongsong Liu and Xiangbing Zeng at the University of Sheffield.

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