Single-molecule junction conductance through diaminoacenes.

The study of electron transport through single molecules is essential to the development of molecular electronics. Indeed, trends in electronic conductance through organic nanowires have emerged with the increasing reliability of electron transport measurements at the single-molecule level. Experimental and theoretical work has shown that tunneling distance, HOMOLUMO gap and molecular conformation influence electron transport in both saturated and -conjugated nanowires. However, there is relatively little experimental data on electron transport through fused aromatic rings. Here we show using diaminoacenes that conductivity depends not only on the number of fused aromatic rings in the molecule, which defines the molecular HOMO-LUMO gap, but also on the position of the amino groups on the rings. Specifically, we find that conductance is highest with minimal disruption of aromaticity in fused aromatic nanowires. We recently reported on the improved reliability and reproducibility of conductance measurements using amines instead of thiols or isonitriles in metal-molecule-metal junctions. Junctions are formed by breaking Au point contacts in a solution of diamines. Conductance measurements for diaminoacenes were recorded in 1,2,4-trichlorobenzene solution by repeatedly forming and breaking Au point contacts with a modified STM tip (Figure 1b, inset). Typically, diamines were sublimed under vacuum prior to use. Conductance traces measured as a function of tipsample displacement reveal quantized conductance steps observed at integer multiples of G0 (2e/h), the fundamental quantum of conductance. Many of the traces reveal steps at moleculedependent conductance values below G0 (Figure 1a) due to conduction through a single molecule bridging the gap between the two Au point-contacts. Repeated measurements give a statistical assessment of the junction properties presented as conductance histograms (Figure 1b) with the peak representing the most probable measured conductance value for the molecular junction. Figure 1b shows representative conductance histograms resulting from several thousand conductance traces for three molecules, 1,4-diaminobenzene, 2,6-diaminonaphthalene and 2,6diaminoanthracene along with a control histogram measured in the solvent alone. Peaks in the conductance histograms determined by Lorentzian fits to the data correspond to the most prevalent single molecule junction conductance. Figure 2 shows the measured conductance values for 5 different acenes. For the upper series, 1,4-diaminobenzene, 1,4-diaminonaphthalene and 9,10-diaminoanthracene, the conductance increases with increasing number of benzo rings, whereas in the lower series, 1,4-diaminobenzene, 2,6-diaminonaphthalene and 2,6diaminoanthracene, the conductance decreases with increasing number of rings. Figure 1. Representative conductance traces (a) and histograms (b) for 1,4-diaminobenzene (1), 2,6-diaminonaphthalene (2) and 2,6diaminoanthracene (3). The lower inset illustrates 1,4diaminonaphthalene binding in the metal-molecule-metal junction.