The objective of this project is to synthesize complexes in which wire-like sp-hybridized carbon chains span two transition metal fragments, and to conduct detailed studies of their chemical and physical properties. At very long chain lengths, such materials bear a strong conceptual relationship to diamond and graphite, the classical three- and two-dimensional sp3 and sp2-hybridized polymeric carbon allotropes. They are also much more stable than analogs with organic endgroups. Progressively more complex architectures based upon such building blocks will be sought. In one effort, complexes with C32-C40 chains, in which the metal-metal distances would surpass 5 nm, will be targeted; those with C24-C28 chains are easily isolable and stable to >140 C. In another effort, the Cx chain will be sterically protected via two flexible sp3 carbon chains that span donor atoms on each terminus. When sufficiently long, these adopt chiral double helical conformations, much like the insulation about household electrical wire. The redox properties of such species will be studied; it is anticipated that the stabilities of mixed valence radical ions will be enhanced. Other approaches to sterically shielded sp chains involving rotaxanes will be investigated. In another effort, lateral arrays or "bundles" of such complexes with parallel Cx chains will be synthesized. These can be viewed as multi-stranded molecular wires. In another effort, well-defined longitudinally extended arrays, such as PtCxPtCxPtCxPt species, will be prepared. "Alligator clips" will be introduced on the terminal platinum atoms such that single-molecule conductivities can be measured. Reactions with chelating diphosphines may introduce "kinks" that might allow (under appropriate conditions) reductive eliminations to PtC3xPt species or oxidative couplings to polygons that contain platinum atoms at each corner. Polygons will also be prepared by oxidative couplings of monoplatinum species. Attempts will be made to extrude the platinum atoms, generating a cyclocarbon Cx (a monomeric carbon allotrope). Reactions of the C=C moieties will be investigated, such as trimerizations to hexasubstituted benzenes and metatheses to give odd-carbon species M(C=C)n'C=M'. In collaborative efforts, the NLO and photophysical properties of each family of complexes will be characterized. Particular attention will be given to defining chain length effects and chain-chain interactions. A detailed knowledge of how to engineer these wire-like unsaturated molecules into a variety of sophisticated architectures will result.
With this award, the Organic and Macromolecular Chemistry Program is supporting the research of Professor John A. Gladysz, of the Department of Chemistry at Texas A&M University. Professor Gladysz and his students are exploring the synthesis and properties of molecules containing extended carbon chains capped by metal atoms. These systems mimic all-carbon systems of considerable current interest. They are expected to display a variety of unusual properties of potential relevance in the design and construction of molecular wires, nonlinear optical materials, and other more complex molecular aggregates. Students involved in this project will have the opportunity to collaborate with students in Germany and with physical chemists specializing in the characterization of molecular materials.
