Single Tripyridyl-Triazine Molecular Junction with Multiple Binding Sites

We present an electronic characterization of a single molecular junction of 2,4,6-tris(2',2 '',2 '''-pyridyl)-1,3,5-triazine (TPTZ) with multiple metal-molecule binding sites using scanning tunneling microscopy-based break junction method under ambient conditions. The TPTZ molecule consists of a centered triazine moiety and surrounding three 2-pyridyl groups. The benzene rings containing a N atom in TPTZ act as molecular binding sites for bridging a gap between two Au electrodes to form a single molecular junction. Because the N atom at the ortho-position in the 2-pyridyl groups is spatially hidden from the electrode surfaces, the single molecular junction forms via direct metal-pi couplings. We demonstrated that the single TPTZ molecular junctions exhibit highly conductive character up to 10(-1) G(0) (G(0) = 2e(2)/h), which is due to the effect of the direct metal-pi coupling. We found three preferential conductance states of ca. 10(-1), 10(-2), and 10(-4) G(0), which suggests that the single TPTZ molecular junctions have three charge transport paths depending on the molecular anchoring sites on the Au electrodes. Analysis of electrode-gap distance in the molecular junction revealed that effective gap length is 0.5, 0.9, and 1.2 nm for the high, medium, and low conductance states, respectively. By combining the results of the measured conductance and the estimated electrode-gap distance, we proposed models of junction-structures for the observed three conductance states. This study demonstrates that a molecular junction consisting of multiple metal-pi binding sites provides high and tunable conductance behavior based on the multiple charge transport paths within a molecule.