Multilayer Metal-Organic Films Go 3-D

By taking advantage of metal-ligand binding in one direction and intermolecular interactions in a different direction, Milko E. van der Boom at the Weizmann Institute of Science, Rehovot, Israel, and coworkers have devised a fundamentally new approach to building highly ordered multilayer thin films on a substrate. As van der Boom explained during a Division of Inorganic Chemistry presentation at the American Chemical Society national meeting in New Orleans earlier this month, the three-dimensional molecular assemblies could eventually be used in a variety of chemical-sensing, chemical-communication, or molecular-switch applications.

The method is "a very creative hierarchical combination of molecular interactions to craft crystallike ordered assemblies," commented John A. Gladysz, a chemistry professor at Texas A&M University.

Gladysz, whose work includes using metal coordination to build "molecular gyroscopes" and related helical metal-organic systems, noted that all sorts of monofunctional monolayers exist, including some that are cross-linked in various ways. Multilayer films also can be made by stacking monolayers on top of each other, he added. But those systems are not modular in nature nor are they continuous molecular assemblies like those made by van der Boom's group.

"This is a new concept that really expands the toolbox of reactions that chemists can use to build multilayer assemblies," Gladysz observed.

The 3-D assemblies came about as an extension of van der Boom's earlier work on monolayers containing metal complexes. Those traditional monolayer systems consist of redox-active osmium and/or ruthenium tris(bipyridine) complexes tethered to glass or silicon substrates via siloxane linkers, van der Boom explained.

The monolayers are proving to have potential as part-per-million-level chemical sensors to optically detect water, nitrogen oxides, and various metal ions, van der Boom said. He believes they could be used as moisture detectors for organic solvents and fuels or to detect toxic aqueous metal ions such as Cr⁶⁺ (J. Am. Chem. Soc. 2008, 130, 2744).

In another case, van der Boom's group created a system in which monolayers on separate substrates use a metal ion to transfer electrons (Angew. Chem. Int. Ed. 2008, 47, 2260). "The communication between the nanostructures over large distances could serve as a versatile platform for integrating boolean logic gates," thereby opening up a route to molecular computing, he said.

His group more recently began to explore making multilayers by linking organic groups and metal atoms into oligomers, which led to the novel 3-D systems (J. Am. Chem. Soc. 2008, 130, 5040).

The researchers first create a template by treating a silicon substrate with a phenyl-based chromophore containing vinylpyridine end groups. One of the pyridines sports a siloxane substituent, which attaches the chromophore to the substrate.

As a second step, the team exposes the template monolayer to a PdCl₂ solution in order to coordinate palladium atoms to the free pyridine nitrogen atoms. The bound palladium atoms then serve as the foundation for the assembly's second layer.

In the final maneuver, the researchers introduce anthracene functionalized with two vinylpyridine units. One vinylpyridine coordinates to the exposed palladium atoms, leaving the second pyridine free to accommodate the next round of palladium. Further iterations of anthracene and palladium create a set of aligned, wirelike oligomers attached to the substrate.

But van der Boom's choice of the functionalized anthracene as a ligand provides a unique twist. The anthracene units on adjacent chains align their three fused rings face-to-face as a result of aromatic π–π interactions. These sterically demanding secondary forces lock in lateral stability to the metal-organic chains, he explained, resulting in a highly ordered molecular framework.

The new hybrid metal-organic materials, which mimic the hierarchy of forces observed in biological systems, "provide new insight into molecular interactions and orientation in thin films," van der Boom told C&EN. Besides using the multilayers to study electron transfer and selective chemical reactions at thin film-solution interfaces, his group is exploring how to expand the utility of the materials by developing nonlinear (dendrimer-like) multilayer assemblies.