Ruthenium(II) as Conductive Promoter To Alleviate Conductance Attenuation in Oligoynyl Chains

To unveil the influence of metal coordination on charge transport and length-dependent conductance attenuation in pi-conjugated organic molecules, trans-Ru-(dppe)(2) (dppe = 1,2-bis(diphenylphosphino)ethane) was implanted into oligoynyl chains with various number of ethynyl groups to construct Au molecule Au junctions using methylthiol (-SMe) as anchoring groups. The single molecule conductance measured by I(s) method gave high and low values of conductance. A direct comparison of the conductance indicates that implanting trans-Ru(dppe)(2) moiety into Jr-conjugated oligoynyl chains results in 2.6- to 5.8-fold enhancement of the conductance although the molecular length is increased by ca. 0.25 nm as a result of Ru insertion. More importantly, implanting trans-Ru(dppe)(2) moiety into pi-conjugated oligoynyl chains alleviates dramatically length-dependent conductance attenuation in view of the much smaller conductance decay constants for ruthenium(II)-implanted molecules (2.6 nm(-1) for high value and 2.5 nm(-1) for low value) than those for pi-conjugated oligoynyl chains (4.4 nm(-1) for high value and 4.2 nm(-1) for low value). Undoubtedly, ruthenium(II) is a conductive-enhancing element instead of a suppressing element upon implantation into pi-conjugated oligoynyl chains. As revealed by the theoretical studies and the fitting of I V results according to the Simmon's model, the enhancement of conductance as well as the slowdown of conductance attenuation can be ascribed to the decrease of effective carrier mass and the rise of highest occupied molecular orbital level upon the participation of Ru in electronic delocalization.