Oligo(aryleneethynylene)s with Terminal Pyridyl Groups:Synthesis and Length Dependence of the Tunneling-to-Hopping Transitionof Single-Molecule Conductances.

The synthesis is reported of a newseries of oligo(aryleneethynylene)(OAE) derivatives of up to ca. 6 nm in molecular length (OAE9) using iterative Pd-mediated Sonogashira cross-coupling methodology.The oligo-p-phenyleneethynylene cores of the molecularwires are functionalized at both termini with pyridyl units for attachmentto gold leads. The molecular structures determined by single-crystalX-ray analysis are reported for OAE4, OAE5, OAE7, and OAE8a. The charge transportcharacteristics of derivatives OAE3–OAE9in single-molecular junctions have been studied using the mechanicallycontrolled break junction technique. The data demonstrate that thejunction conductance decreases with increasing molecular length. Atransition from coherent transport via tunneling to a hopping mechanismis found for OAE wires longer than ca. 3 nm. [ABSTRACT FROM AUTHOR]