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This paper describes the synthetic organic phase of a project directed toward the construction of molecular scale electronic devices. Outlined is a convergent synthetic route to orthogonally fused conjugated organic oligomers. The final systems are to have a potentially conducting chain fused perpendicularly to a second potentially conducting chain via a sigma bonded network. One of the core segments synthesized is based on a spirobithiophene moiety with a central silicon atom. It is formed by a zirconium-promoted bis(bicyclization) of a tetrapropargylsilane. The second core is a 9,9'-spirobifluorene system. Terminal halide groups provide the linkage points for further extension of the chains via Pd-catalyzed or Pd/Cu-catalyzed cross coupling methods. All four branching arms are affixed to the core in a single operation, thus making the syntheses highly convergent. In the cases of the larger functionalized systems, alkyl substituents on the thiophenes afford soluble materials. In order to prepare the molecules with >50 Å lengths, an iterative divergent/convergent approach had to be utilized for the construction of oligo(thiophene-ethynylene) branching arms. Organopalladium-catalyzed procedures are used extensively for the syntheses of the orthogonally fused compounds.

The syntheses of soluble oligo(3-ethyl-2,5-thiophene ethynylene)s via an iterative divergent/convergent approach starting from 3-ethyl-2-(trimethylsilylethynyl)thiophene are described. The monomer, dimer, tetramer, octamer, and 16-mer were synthesized. The 16-mer is 100 Å long in its minimized extended zigzag conformation. At each stage in the iteration, the length of the framework doubles. Only three sets of reaction conditions are needed for the entire iterative synthetic sequence:  an iodination, a protodesilylation, and a Pd/Cu-catalyzed cross coupling. The oligomers were characterized spectroscopically and by mass spectrometry. The optical properties are presented which show that at the octamer stage, the optical absorbance maximum is nearly saturated. The size exclusion chromatography values for the number average weights, relative to polystyrene, illustrate the tremendous differences in the hydrodynamic volume of these rigid rod oligomers verses the random coils of polystyrene. These differences become quite apparent at the octamer stage. Attachment of thiol end groups, protected as the thioacetyl moieties, was achieved. These serve as binding sites for adhesion to gold surfaces. In some cases, one end of the oligomeric chains were capped with a thiol group so that the surface attachments to gold could be studied. In other cases, thiol groups were affixed to both ends of the molecular chains so that future conduction studies could be done between proximal metallic probes. The rigid rod conjugated oligomers may act as molecular wires in molecular scale electronic devices, and they also serve as useful models for understanding analogous bulk polymers.

An iodination, a protodesilylation, and a Pd/Cu‐catalyzed cross coupling are the three reactions needed to successively double the length of molecular units to form 1, a 128 Å long linear 16‐mer, in a doubling sequence with only four iterations. Compound 1 could possibly serve as a molecular wire in molecular electronics studies.

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Eine Iodierung, eine Protodesilylierung und eine Pd/Cu‐katalysierte Cc‐Verknüpfung sind die einzigen drei für eine Verdopplung der Länge des Schlüsselmonomers von 1 erforderlichen Reaktionen. In nur vier Schritten ist so das 128 Ä lange, lineare 16mer 1 zugänglich, das ein aussichtsreicher Kandidat für einen potentiell leitenden molekularen Draht ist.magnified image

We report measurements of the scaling of the cubic polarizability, γ, with chain length in oligo(3-ethylthiophene ethynylene)s with up to 16 repeat units. We observe a saturation of the increase of γ in the longest molecules and compare our results with published work on related molecules.

The synthesis of oligo(2-ethylphenylene-ethynylene)s, oligo (2-(3 -ethylheptyl)phenylene-ethynylene)s, and oligo(3-ethylthiophene-ethynylene)s is described via an iterative divergent convergent approach. Synthesized were the monomer, dimer, tetramer, octamer and 16-mer of the oligo(3-ethylthiophene-ethynylene)s and oligo(2-3 -ethylheptyl)phenylene-ethynylene)s. The 16-mers are 100 Å and 128 Å long, respectively. At each stage in the iteration, the length of the framework doubles. Only three sets of reaction conditions are needed for the entire iterative synthetic sequence; an iodination, a protodesilylation, and a Pd/Cu-catalyzed cross coupling. The oligomers were characterized spectroscopically and by mass spectrometry. The optical properties are presented which show the stage of optical absorbance saturation. The size exclusion chromatography values for the number average weights, relative to polystyrene, illustrate the tremendous differences in the hydrodynamic volume of these rigid rod oligomers versus the random coils of polystyrene. These differences become quite apparent at the octamer stage. The preparation of thiol-protected end groups is described. These may serve as molecular alligator clips for adhesion to gold surfaces. These oligomers may act as molecular wires in molecular electronic devices and they also serve as useful models for understanding related bulk polymers.