Using electrostatic interactions to control supramolecular self-assembly on metallic surfaces

Abstract

Supramolecular self-assembly on metallic surfaces is the ideal playground for studying a variety of physical and chemical phenomena. Adsorbed molecules will diffuse and self-organise to form assemblies dictated by their functionalities, while the more or less pronounced metal reactivity will
accordingly affect both the supramolecular patterns and the interfacial chemistry. Besides structural aspects, electronic properties are central in determining the energy level alignment at the heterojunction and, thus, the performance of organic-based devices. Notably, charge reorganisation at the metal-organic interface will produce surface dipoles, whose effect is to add electrostatic repulsion to the dispersion-driven supramolecular self-assembly and to change the work function of the surface.

Herein, the relation between charge migration (i.e., the creation of surface dipoles) and molecular self-assembly is addressed by studying the behaviour of on-purpose designed molecular units on selected metals. We will show that choosing the substrate on the basis of its work function can selectively allow or inhibit the transfer of charge from the organic material to the electrode. When
charge transfer occurs, the growing supramolecular structures exhibit a phase modulation driven by the presence of competing interactions. Moreover, the introduction of reactive moieties in formerly inert tectons will be identified as a suitable strategy for promoting the formation of interfacial dipoles upon surface-mediated chemical reactions. Our work paves the way for a more
rational approach to the design of metal-organic systems, as we speculate that charge transfer effects and surface chemistry can be predicted at the stage of molecular design, at variance with the current trial and error approach used in the field of organic electronics.

This thesis is based on multiscale theoretical modelling of selected metalmolecule couples and it is the result of a fruitful collaboration with the groups of Prof Davide Bonifazi (Université de Namur) and Prof Giovanni Costantini (University of Warwick).

Date of Award	2014
Original language	English
Awarding Institution	King's College London
Supervisor	Alessandro De Vita (Supervisor)