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July 10, 2006
Volume 84, Number 28
p. 5

Supramolecular Chemistry

Spontaneous Assembly Revealed

Study elucidates key steps in molecular nucleation and growth mechanism

Mitch Jacoby

Peruse a recent issue of your favorite chemistry journal and the phrase "self-assembled" is sure to show up repeatedly. The reason for the popularity is no secret: Molecules that arrange themselves spontaneously into neat films and other structures are convenient and useful in many research areas. Yet despite the ubiquity of self-assembling systems in science, little is known about the molecular mechanisms that govern assembly processes.

Image ?? Sevenheaven Design And Illustration

Manual Assembly Not Required OPV-x molecules form helical structures as a result of an assembly process that starts at the H-bonding side of the molecules (triazine, red), as depicted in this artistic rendering.

Now, scientists in the Netherlands have uncovered some of the elementary steps in a solution-phase system in which π-conjugated molecules organize into helical structures. The study probes the dependence of various states of molecular aggregation on temperature and reveals a crucial role played by solvent molecules in the assembly process. The investigation broadens understanding about the formation of supramolecular structures and may provide new routes to synthesizing complex nanostructured materials.

Like crafty kids with construction toys, researchers in chemistry and other fields have demonstrated a knack for using molecules to fashion a variety of structures such as rods, tubes, helical geometries, and other complex shapes. Unlike children, however, scientists rely on synthesis methods in which molecules form various structures spontaneously. But the molecular-scale events that underlie those methods have remained poorly understood. So chemists at Eindhoven University of Technology devised a model self-assembling system and used a number of analytical methods to probe the step-by-step manner in which the molecules nucleate into helical fiberlike structures.

The research team, which includes Pascal Jonkheijm, E. W. (Bert) Meijer, Albertus P. H. J. Schenning, and Paul van der Schoot, focused on paraffin solutions of oligo(p-phenylenevinylene) derivatives (OPV-x). The molecules are chiral rigid-rod chromophores that contain an ureidotriazine group tailored for hydrogen bonding at one end of the molecule and a bulky tridodecyloxybenzene group known to support columnar self-assembly at the other end.

On the basis of measurements made via circular dichroism spectroscopy, UV-visible spectroscopy, and other experimental and computational methods, the team observed that, as the solution cools slowly from 375 to 275 K, the molecules form hydrogen-bonded dimers that gather into small disordered stacks of 10 to 15 dimers. Upon further cooling, the molecules undergo a transition to form chiral nuclei—the "seeds" of helical structures—consisting of roughly 30 dimers. As the solution temperature continues to drop, the aggregates become elongated and ultimately assemble into clusters of helical nanometer-sized filaments (Science 2006, 313, 80).

A key finding of the study is that solvent molecules are not merely bystanders that remain passive during molecular assembly. In contrast, the team proposes that an organized shell of solvent molecules serves to lock in the aggregate structures and guide the molecules along the assembly process toward the final products.

In a commentary appearing in the same issue of Science, University of Pennsylvania chemists Virgil Percec and Mihai Peterca and Goran Ungar of the University of Sheffield, in England, note that the study illuminates important similarities between solution-phase self-assembly processes and phase transitions commonly associated with ordered solids and liquid crystals.

Additional studies along these lines will help self-assembly of complex functional nanosystems approach the level of structural and functional precision encountered in biological systems, they say.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2010 American Chemical Society