Fig. 7 Two four-terminal molecular wires 19 and 20, with each terminal having a protected alligator clip and each molecular wire having two interior methylene group current barriers. Source: From Ref.[35].

Fig. 8 Molecular wires 21-26, each containing an interior methylene or ethylene group barrier to conduction. Source: From Ref.[35].

underscores that fact that varying test-beds can afford widely different results.

Oligo(phenylene vinylene)s

A third class of compounds that has been studied in our laboratories as well as in others' are the oligo-

Fig. 9 Unfunctionalized OPE 27 and functionalized OPEs 28-30. Source: From Refs.[24] and [38].

(phenylene vinylene)s (OPVs).[40-46] As mentioned earlier, the majority of others' works on OPVs have been geared toward applications in the optical and OLED field.[41-44,46] Our work produced molecular wires targeted for molecular electronics applications, and involved the synthesis of the three OPVs 31-33, shown in Fig. 10. Note that 31 is unfunctionalized, containing only a protected thiol alligator clip for later SAM formation. After formation of the SAM, Au or other metals would be deposited, under vacuum, for the formation of the top contact to complete the circuit through the p-framework. We functionalized 32 with a nitro group on the interior aromatic core to determine if 32 would then act as a switch. Compound 32 is undergoing testing in a collaborator's laboratory. Finally, 33 was synthesized with both a nitro functional group and protected thiol groups at both ends so that it could span two Au contacts and form a circuit via a self-assembly process. This type of self-assembly process is important in our nanocell research program,[1] and is quite different from the approaches of others, who have not necessarily designed OPVs that are meant to connect two proximal probes, an interface to the present solid state-based technology.

Detert and Sugiono[46] synthesized a series of readily soluble OPVs shown in Fig. 11 to study their electronic spectra. They found that appending various

Fig. 10 Three OPVs 31-33 synthesized by the authors. Source: From Ref.[40].

electron-withdrawing groups to the terminal aromatic rings allowed the tuning of the electron affinity of the chromophore without significant changes in the spectra. When those same substituents were placed on the vinyl-ene segments of the OPV molecules, strong bathochro-mic shifts were observed. Wong et al.[42] investigated similar substituent effects. Syamakumari, Schenning, and Meijer[41] and Gu et al.[43] synthesized large assemblies of OPVs attached to other molecules and measured their optical, electronic, and aggregation behaviors.

Sikes et al.[45] and Davis, Ratner, and Wasielewski[44] have used OPVs as core components of molecules (Fig. 12) synthesized to test electron tunneling and long distance electron transfer, respectively. Of the compounds Sikes et al. synthesized, 35 spanned the most distance, 3.3 nm. The OPVs were assembled on a Au electrode and the tethered ferrocene redox species at the other end of the OPV bridge was exposed to an aqueous electrolyte. They used laser-induced temperature jump techniques to measure the rate constants of thermal interfacial electron transfer through the system and observed transfer through the OPV. They found that OPVs up to 2.8 nm in length were good conductors, with the transfer limited by structural reorganization.

In their study of a series of compounds including 36 (Fig. 12), the lengthiest molecule tested, Davis, Ratner, and Wasielewski[44] found that electron transfer over long distances depends critically on the low-frequency torsional motions of the molecular wires (i.e., when the molecules twist and turn, their molecular orbitals do not line up in a fashion that favors fast electron transfer). But their tests were in solution rather than the more device-realistic molecule-surface attached patterns.

Aromatic ladder oligomers

Our group has synthesized a vast array of aromatic ladder polymers[47,48] for use in conducting polymer and optoelectronic applications. However, we realized that for molecular electronic applications, the molecules we synthesized needed to have defined length and composition to be commercially useful as molecular wires.

As shown in Fig. 13, Gourdon[49] have synthesized two classes of conjugated ladder oligomers that maintain similar guidelines for molecular wires, including a defined length, rigidity, extended p-conjugation for good electron transfer, and good electronic coupling with metallic contacts. The oligo(quinoxaline) derivative 37 and oligo(benzoanthracene) molecular wire 38 were synthesized using standard condensation and coupling reactions. The oligo(quinoxaline) 37 has

Fig. 11 A series of OPVs 34 synthesized by Detert and Sugiono[46] to study the effect that variation of substituents R1; R2, R3, and R4 had on the electronic spectra.
Fig. 12 Sikes et al.[45] synthesized a series of OPVs including 35 to test electron tunneling whereas Davis et al.[44] synthesized a series of OPVs including 36 to measure long distance electron transfer.

built-in alligator clips in the terminal 1,10-phenanthroline moieties. We have also synthesized molecular wires containing terminal pyridine and other nitrogen-containing functional groups.[36] Calculations have supported their use as alligator clips.[50] An iterative crossed divergent-convergent process that leads to rapid growth of the oligomers was developed to synthesize the oligo(benzoanthracene) 38.

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