Physico-chemical characterization of ultra thin films of functional supramolecular systems on surfaces

Abstract

The aim of this PhD thesis is the development of new metodologies for surface anchoring of molecular and supramolecular systems for applications in molecular electronics and photonics, and for preparation of devices for molecular recognition. The systems are made up, as usually, of hybrid systems, divided in two parts, a molecular one and a solid support. The molecular part performs the desired function, while the solid support (metal or semiconductor on which the molecules are bound) allows to conjunct its properties with molecular system properties and, moreover, to help the correct orientation of the molecules at the surface. Such systems were compositionally and structurally characterized by means of surface analysis techniques, as time of flight of secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). The nodal point of this research work, so, is surface functionalization. Many kind of surface were used, in particular oxides like silicon oxide and quartz (as model systems), transparent semiconductor oxides like indium tin oxide (ITO), and metal surfaces like gold. Such surfaces were functionalized by means of a bottom up approach, in particular the formation of a self-assembled monolayer (SAM), based on the chemisorpion of molecules bearing suitable functional groups. For oxide surfaces, molecules for SAM formation were characterized by a phosphonate as surface active group, able to form monolayer exploiting zirconium-phosphate-phosphonate (ZP) chemistry. For gold surface, instead, the surface active group was a thiol. Functional molecules adopted for this purpose, bear a terpyridine group as tail. Such ligand was chosen because the chelant moiety 2,2 :6 ,2 -terpyridine (Tpy) is one of the most exploited because of the very high affinity for bivalent metal ions, and the ability to form stereospecific, achiral, and octhaedrical complexes. Mono- and bis-terpyridine based metal ion complexes were prepared in order to obtain, by means of a step-wise approach, multilayer systems that can be seen as assembled molecular wires. In particular conjucated polyphenilenic ligands properties were exploited. Moreover, great efforts were made in order to obtain systems for the construction of devices for molecular recognition. With this aim, attention was focused on gold surface funtionalization to anchor on it, on one hand oligonucleotides chains, on the other hand, supramolecular systems called pseudo-rotaxanes . Oligonucleotide sequences anchoring was carried out in order to have information about the possibility that probe and targed oligonucleotides chains could react depending on them complementarity, studing this process by hybridization directly in situ. Pseudo-rotaxanes assembly, was very interesting because it was possible, for the first time, to carry out complexation of molecular building blocks (host and guest) directly on surface, instead of in solution.