Quantifying Molecular Structure–Conductance Relationship in Nonlinear π‑Conjugated versus Linear π‑Conjugated Wire for Application in Molecular Electronics

We report a quantitative analysis of conductance in tunneling junctions based on π-conjugated oligophenyleneimine (OPI) wires incorporated with thieno­[2,3-b]­thiophene (TT23) or thieno­[3,2-b]­thiophene (TT32) units grown layer-by-layer on Au substrates by click chemistry. Reflection–adsorption infrared spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy were used to monitor the stepwise growth. The electronic structure of the conjugated wires was studied with ultraviolet photoemission spectroscopy (UPS). The current–voltage curves (I–V) of the molecular wires, linear π-conjugated TT32-OPI-n and nonlinear π-conjugated TT23-OPI-n where n = 3–5, were measured with conducting probe atomic force microscopy (CP-AFM), in which the molecular layer bound to an ultrasmooth gold substrate was in a contact with a gold-coated AFM tip. By systematically measuring the low-bias junction resistance as a function of aromatic rings (3–5), we obtained the structure-dependent tunneling attenuation factors (β) of 2.6 and 3.1 nm–1 for the linear π-conjugated TT32-OPI-n and nonlinear π-conjugated TT23-OPI-n junctions. The high-bias I–V characteristic for these junctions was analyzed quantitatively with a previously validated single-level model (SLM) to extract key junction metrics, the energy offset barrier εh and the electronic coupling, Γ. The I–V simulation of these junctions using εh estimated from UPS matches well with the experimental I–V characteristic, which provides strong evidence for the validity of the SLM tool. Importantly, we find that the conductance in the linear π-conjugated TT32-OPI-n junction is much greater (100-fold) than in the nonlinear π-conjugated TT23-OPI-n junction; the much larger conductance for the linear π-conjugated TT32-OPI-n junction is primarily due to a 10-fold increase in Γ and not to any significant change in εh. Overall, the results provide insights into molecular structure–conductance relationships as well as a basis for rational design of molecular electronic devices.