Self-assembled oligomeric structures of an asymmetric molecular linker; 4-isocyanophenyl disulfide on Au(111)

Para-substituted benzenes, such as 1,4-benzene dithiol and 1,4-phenyl diisocyanide, have been observed to oligomerize on the Au(111) surface by incorporating gold adatoms extracted from the substrate. This work investigates if oligomerization occurs for an analogous but asymmetric linker, 4-isocyanophenyl disulfide (ICPD) on Au(111). This molecule is comprised of both disulfide and isocyanide terminal groups attached to the phenyl ring. The resulting surface structures formed on Au(111) following exposure to ICPD are studied using scanning tunneling microscopy (STM). 1,4-isocyanophenyl thiolate (ICPT), formed through scission of ICPD’s disulfide bond, was also found to oligomerize on the surface, and potential oligomer structures and binding geometries are proposed with the aid of density functional theory (DFT) calculations, along with simulated STM images of the resulting structures. It is observed in this work that ICPT forms oligomeric structures that cover large sections of the substrate and appear to create etch pits resulting from gold atom extraction. Numerous potential binding geometries are investigated based on the distances between substrate gold atom adsorption sites compared to the monomer length. Selected structural candidates were optimized using DFT and were used to generate simulated STM images using the Tersoff–Hamann method to compare with experiment. It has been shown previously that the isocyanide- and thiol-connected oligomers conduct electrons, suggesting the possibility that the asymmetric oligomers found here might form the basis for fabricating molecular diodes.