Quantifying Molecular Structure-Conductance Relationship in Nonlinear ?-Conjugated versus Linear ?-Conjugated Wire for Application in Molecular Electronics

We report a quantitative analysis of conductance in tunneling junctions based on r-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 r-conjugated TT32-OPI-n and nonlinear r- 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 (g) of 2.6 and 3.1 nm-1 for the linear r-conjugated TT32-OPI-n and nonlinear r-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 eh and the electronic coupling, Gamma. The I-V simulation of these junctions using eh 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 r-conjugated TT32-OPI-n junction is much greater (100-fold) than in the nonlinear r-conjugated TT23-OPI-n junction; the much larger conductance for the linear r-conjugated TT32-OPI-n junction is primarily due to a 10-fold increase in Gamma and not to any significant change in eh. Overall, the results provide insights into molecular structure-conductance relationships as well as a basis for rational design of molecular electronic devices.

Automated summary

We report a quantitative analysis of conductance in tunneling junctions based on r-conjugated oligophenyleneimine (OPI) wires incorporated with thieno[2,3-b]thiophene (TT23) or thieno[3,2-b]thiophene (TT32) units. The results show that the conductance in linear r-conjugated TT32-OPI-n junctions is much greater than in nonlinear r-conjugated TT23-OPI-n junctions, primarily due to an increase in electronic coupling Gamma.