Electrical and Thermoelectric Characteristics of Organic Single Molecular Junctions: The Role of Anchoring Groups

In this work, we present a computational study of organic single molecular junctions by analyzing the electrical and thermoelectric properties of the Thienoisoindigo molecule, which are symmetrically and asymmetrically anchored to gold electrodes through various edge groups. To this end, we employ an ab initio method with density functional theory (DFT). The transmission coefficient’s results display that the type of anchoring groups affects the behavior of the molecular junctions. Molecules with symmetric anchoring groups show the highest occupied molecular orbital (HOMO) dominates in the molecular configuration with an S−S anchor. However, the lowest unoccupied molecular orbital (LUMO) dominates in molecular systems with N−N and NH\(_2\)−NH\(_2\) anchors. Another influence of the anchoring groups on the molecule’s properties is their effect on the bandgap, and it shifts gradually toward the blue region by changing the edges from S−S to NH\(_2\)−NH\(_2\) and N−N, respectively. In the case of molecules with asymmetric anchors, the HOMO dominates in molecules with the S−NH\(_2\) one, while the LUMO is dominant in molecules with S−N and N−NH\(_2\) anchors. In addition, the bandgap decreases through changing the edges from N−NH\(_2\) to S−NH\(_2\) and reduces more (red-shift) with S−N anchors. Seebeck coefficient calculations also exhibit anchor-dependency. For symmetric anchors, the behavior of molecules with S−S shows p-type conduction. It can shift to an n-type one by replacing the edges with N−N or NH\(_2\)−NH\(_2\). However, molecules with asymmetric anchors transition from p-type to n-type behavior by exchanging the S−NH\(_2\) with S−N or N−NH\(_2\) anchors. The obtained results demonstrate the significant role of anchoring groups in controlling the behavior of organic single molecular junctions, which could contribute to developing innovative applications in molecular electronic devices.