The final area which we are investigating involves the assembly of nanostructures by metal complexation of bis(terpyridyl) ligands. Recently, there has been much excitement about the development of such compounds, although this subject was introduced some thirty years ago. The field of nanoarchitecture is still in its infancy, but advances in characterization techniques, coupled with new synthetic methodology, have opened the area for significant development.
Structures which have been identified as being significant and potentially useful molecular ensembles include rigid rods, tubes, and monolayers. Rigid rods composed of conjugated compounds may be used as nanowires or may be inserted through a tube to form a rotating axle in a housing. Tubes themselves have been used to create microenvironments for heterogeneous catalysts, and as microvials for the slow release of medicinal agents and other materials contained within. Suitably-functionalized monolayers serve as templates for the alignment and organization of other molecules, while stacks of monolayers furnish the basis for organic zeolites.
Such nanoarchitecture requires readily-available building blocks with well-defined structures and properties. These should allow for the modular, stepwise construction of components and have the potential for one-step assembly; and they should be stable to heat and air. Many organometallic complexes are well-suited to this application, because of their ability to undergo ligand exchange under controlled conditions and because of their regular geometries (octahedral, etc.). Because metal-amine complexes, particularly those employing aromatic ligands, exhibit all of the aforementioned traits, we propose as a first approach that systems based on oligomers of substituted ethynylterpyridyl metal complexes represent ideal substrates having many desirable properties. The acetylene group will function as a linker with which to connect terpyridyl units, and the resulting structure will be used to synthesize dimers, trimers, and other small oligomers. The ultimate goal is the one step assembly of oligomers and polymers by treatment of poly(terpyridyl) ligands with the appropriate metal salt (shown below for rigid rods). An additional advantage of employing bis(terpyridyl)-metal complexes is their photo- and electrochemical behavior, which encourages their use in molecular wires and as components of models for light-harvesting systems in solar energy applications.
These rigid rods will also be synthesized in a stepwise manner analogous to that for the preparation of nanotubes, which is shown on the next page. Additionally, a stepwise synthesis of the rigid rods and "zig-zag" polymers based on the angular ligand 3, below, will be conducted on a solid support, which is anticipated to facilitate the rapid production and purification of these compounds.
The preparation of nanotubes will rely on hydrogen bonding groups to assemble the rings into tubes, as has been shown for peptide nanotubes. In addition to the carboxylic acid functionalities which cause the rings to associate into tubes, there are six of these groups which project into the interior of the tubes. This feature makes these structures excellent candidates for metal ion sequestration, in a manner similar to crown ethers and other ionophores. Additionally, these tubes may function as channels for the transport of ions across lipid bilayers, as has been reported for "tubes" of crown ethers, or for glucose, as has been published for peptide nanotubes. To facilitate the incorporation of the metal terpyridyl nanotubes into a lipid bilayer of a vesicle, the exterior carboxylic acid groups can be esterified with long alkyl chain alcohols. Since hydrogen bonding between rings will keep the metal terpyridyl units from rotating freely, these long chains are expected to remain on the exterior of the tubes. Once incorporated into the lipid bilayer of vesicles, proton transport can be monitored by the pH stat method, and glucose transport can be examined using an enzyme-coupled assay.
Our preliminary work in this area has resulted in the production of rigid rods containing five metal centers, which are the largest molecules of this type synthesized to date. We have also prepared the angular bis(terpyridyl) ligand 3, and are beginning to study its complexation behavior.