Effect of Mechanical Strain on Electric Conductance of Molecular Junctions

Electromechanical properties of single molecular junctions are investigated using scanning tunneling microscopy based break junction method. Two types of molecular junctions consisting of pi-conjugated backbones with and without coordinative bonding (i.e., Co((4-aniline)-terpyridine)(2) complex and oligo(phenylene-ethynylene) derivative) are prepared between two Au electrodes. Electronic transport measurements revealed molecular conductance of ca. 10(-4) G(0) (G(0) = 2e(2)/h) for both of the molecular junctions. Then we assessed the electronic transport properties of the two types of molecular junctions under mechanical strain in their compression-elongation cycle. We found significant asymmetric electromechanical response for all covalent systems of the oligo(phenylene-ethynylene) derivative, while the Co complex with the coordinative bonding exhibits symmetric modulation of the electronic transport property in the compression-elongation cycle of the molecular junctions. The asymmetric and symmetric electromechanical behavior can be, respectively, ascribed to rigid covalent bonding in the pi-conjugated backbone and flexible coordinative bonding at the metal center. This study demonstrates potential tunability of the molecular conductance under mechanical stimulus.