Structural versus Electrical Functionalization of Oligo(phenylene ethynylene) Diamine Molecular Junctions

Abstract

We explore both experimentally and theoretically the conductance and packing of molecular junctions based on oligo(phenyleneethynylene) (OPE) diamine wires, when a series of functional groups are incorporated into the wires. Using the scanning tunnelling microscopy breakjunction (STM BJ) technique, we study these compounds in two environments (air and 1,2,4-trichlorobenzene) and explore different starting molecular concentrations. We show that the electrical conductance of the molecular junctions exhibits variations among different compounds, which are significant at standard concentrations but become unimportant when working at a low enough concentration. This shows that the main effect of the functional groups is to affect the packing of the molecular wires, rather than to modify their electrical properties. Our theoretical calculations consistently predict no significant changes in the conductance of the wires due to the electronic structure of the functional groups, although their ability to hinder ring rotations within the OPE backbone can lead to higher conductances at higher packing densities.

Unusual Length Dependence of the Conductance in Cumulene Molecular Wires

“…Them easured conductances for the alkene and allene vary slightly with distance across the plateau (z). [18] However, if we only consider the data points at longer z values,t he resulting histograms are more reproducible,i ndicating that the probability of multiple-molecule junctions diminishes as the junctions are stretched (as discussed in Sections S3.8 and S3.9). We had previously validated this method for afamily of porphyrins.…”

“…We often observed an apparent increase in the molecular conductance when the percentage of molecular junctions exceeded about 40 %, which was attributed to the formation of junctions containing several wired molecules. [18] However, if we only consider the data points at longer z values,t he resulting histograms are more reproducible,i ndicating that the probability of multiple-molecule junctions diminishes as the junctions are stretched (as discussed in Sections S3.8 and S3.9). The [ 3] and [5]cumulenes have essentially the same conductance,w hich is slightly larger than that of the alkene (see Figure S19 for ac omparison of multiple experimental runs showing high reproducibility).…”

Tuning the thermal conductance of molecular junctions with interference effects

“…7, have been synthesized. But closely related molecules exist, in which the central ring of an OPE3-diamine has been modified, among others, by the inclusion of several fluorine atoms and which have been studied in the context of singlemolecule junction experiments 67 . So, we think that there should not be any fundamental problem to synthesize the compounds discussed in this work.…”

Complex formation dynamics in a single-molecule electronic device

“…To rule out potential artifacts, we established another two kinds of charge transport pathways using (i) a control molecule [oligo(1,4-phenylene ethynylene)diamine (OPEDAM) ( 40 )] corresponding to the conjugated backbone of BPP34C10DAM (fig. S6), and (ii) a partially cleaved graphene ribbon device (fig.…”