Structure and chemical characterization of novel 2D homo- and hetero-organic architectures on a gold surface: a combined STM and photoemission spectroscopies study

This work focuses on the study of the electronic and structural properties of molecular architectures at metal surfaces formed by assembly of functionalized organics, in particular amino- and/or carboxylic-functionalized molecules. Several homo- and hetero-organic architectures are characterized on a gold (111) surface by means of combined low temperature scanning tunneling microscopy (LT-STM) and X-ray spectroscopy techniques, coupled with ab initio calculations performed in collaboration with theoretical groups. The aim is to identify the role of the molecule-molecule and molecule-substrate interactions in determining the electronic and structural configuration of the investigated systems. This thesis is subdivided in two parts. In the first part we report the homo-organic assembly of a simple solvent molecule, dimethyl sulfoxide (DMSO), and two different amino-functionalized molecules. In particular, DMSO is found to form molecular complexes unexpectedly trapping gold adatoms natively available on Au(111). Although such adatoms are not visible in the STM images, comparison between experiments and theoretical calculations shows that they are crucial for the stability of the observed DMSO complexes. Then, we have studied a small aminomethyl-functionalized molecule, 1-naphthylmethylamine (NMA), forming complexes of 3 or 4 molecules, and an amino-phenyl-porphyrin forming hydrogen bonded molecular chains. In the latter two works, we have pointed out the interplay between the amino-amino and amino-gold substrate interactions which drive the formation of such complexes. In the second part we exploit what we have learned about the amino-functionalized molecules on Au(111) to build novel hetero-organic supramolecular architectures based on the amino-carboxylic (A-C) interaction. Initially, we have studied the self-assembly of two small molecules, namely NMA and terephthalic acid (TPA, carboxylic-), whose 2D hetero-organic assembly is shown to be driven by the competition between the A-C and carboxylic-carboxylic recognition. In literature, organic templates are successfully synthesized on surfaces to build guest-host systems based on the shape matching between the host template and guest molecules. We instead exploit the competitive A-C bond to develop a novel guest-host method based on chemical affinity. By employing the monolayer of a carboxylic-functionalized porphyrin as the host template, we find that small guest aminomethyl-terminated molecules (like NMA) can be intercalated into this 2D template. These results show that the competitive A-C recognition is a powerful tool to build supramolecular assemblies.