SYNTHESIS, CHARACTERIZATION AND NANOSTRUCTURATION OF D-A SYSTEMS BASED ON Fc-PTM DYADS.

Abstract

This Thesis is centered in the field of multifunctional molecular materials and more specifically in the area of molecular electronics. The work is focused on the design, synthesis and nanostructuration of molecular materials based on donor-acceptor (D-A) open-shell systems. More specifically, it has been studied how the electronic properties of D-A molecules are modified when moving from the tridimensionality (3D) solution and crystals to the bidimentionality (2D) obtained when the dipolar molecules are oriented and nanostructured on a metallic surface as self-assembled monolayer (SAM). The influence of the electronic open-shell nature of such D-A molecules on the overall electronic properties of the oriented nanostructured system has been also evaluated._x000D_ The first part of the work has been devoted to the study of the temperature and solvent induced electronic bistability phenomena in 3D (solution and crystals) of D-A systems based on ferrocene electron donor units bonded covalently to a polychlorotriphenylmethyl radical acceptor unit through an ethylenic spacer (Fc-PTM) in solution and also in solid state. Thanks to the methylation of the Fc units that enhances their donor-ability a higher percentage of the zwitterionic state in comparison with the non-methylated Fc-PTM dyad was found in solid state. In solution playing with the polarity of the solvent it has been also possible to stabilize the neutral) or the zwitterionic electronic state of the dyad and more interestingly, using solvents with intermediate polarity like acetone or THF it has been observed the coexistence of both electronic states._x000D_ The second part of the Thesis is focused on the use of different approaches for the bidimiensional nanostructuration of Fc-PTM D-A molecules on gold surfaces to study its electronic properties. Initially, a two-steps approach has been used to prepare SAMs which after a lot of effort were considered not robust enough for further reliable electronic characterization. Then, the work moves to the use of a one-step approach with successful results. All the SAMs were characterized using different surface analysis techniques like PM-IRRAS, XPS, ToF-SIMS, CA and AFM. The stability of these SAMs was evaluated by CV measurements._x000D_ To get further insight into the electronic structure of such SAMs of molecular oriented dipoles on surfaces, synchrotron radiation techniques (PES, RPES and NEXAFS) have been used. The influence of the open-shell electronic structure of dyad 1 over the electronic properties of 2D nanostructures has been analyzed in comparison to the closed-shell electronic structure of 1H. We conclude that the dimensionality of the Fc-PTM D-A system determines its electronic properties and behavior because no bistability was observed in the 2D nanostructures formed._x000D_ The surface potential modification of surfaces modified with SAMs S1 and S1-H have been studied by KPFM showing in both cases an increase of the WF of the gold substrates used. The lower modification of the WF caused by the open-shell SAM S1 in comparison with S1-H has been attributed to a higher effect of depolarization due to its larger polarizability giving place to a modification of its energy levels in order to preserve the equilibrium and stabilize the energy of the system formed by 2D oriented dipoles._x000D_ The last part of the Thesis has been devoted to the transport measurements through these D-A SAMs junctions of the type Au-SAM//Ga2O3/EGaIn. The current measurements have shown that junctions based on SAMs of open-shell S1 have a high current density but do not rectify. However, junctions based on the closed-shell S1-H SAM show a lower current density at ± 1.0 V but when the energy barrier imposed by the LUMO of the system is overcome at -2.0 V it rectifies, showing an interesting rectification ratio of 102.

Date of Award	6 Mar 2015
Original language	English
Supervisor	Jaume Veciana Miró (Director) & Imma Ratera Bastardas (Director)