Synthesis and study of the properties of polyaromatic organic compounds of interest in molecular electronics

Abstract

Molecular electronics is the field of science that studies the electron transport phenomena at the scale of one individual molecule. It allows us to evaluate the effect of different chemical functions; search for molecular structures able to emulate the functioning of components of electronic circuits, such as wires, transistors, switches, etc; and study small and more accessible molecular models of large area materials. In the last years, the research in this field has crossed beyond the limits of mere electronics, making possible studies on photovoltaics, thermoelectricity and spin filtering; broadening the spectra of potential functions and applications for nanoscale molecular systems. This thesis has been carried out between two complementary laboratories, namely the FQM-367 organic chemistry lab at the Universidad de Granada and the molecular electronics lab at the Fundación IMDEA Nanociencia in Madrid. It covers the design and synthesis of several novel organic compounds as well as the experimental characterization of their electron transport properties by means of Scanning Tunneling Microscopy Break Junction (STM-BJ) technique, including work in setup improvement implementation. As part of this thesis, a new home-built STM has been developed at IMDEA Nanociencia and it has been set up at the Centro de Instrumentación Científica from the Universidad de Granada. Supplementary collaborations with theoretician groups have expanded the studies, creating models able to explain the experimental observations. Consequently, this thesis comprehends an interdisciplinary research, in which molecular electronics has been tackled from both, chemical and physical points of views. The manuscript is divided into six chapters, with their corresponding references included at the end of each one. The first chapter consists on a general introduction to molecular electronics, where the principal techniques for single-molecule experiments are collected; the most relevant information they provide is explained; and the theoretical model for understanding the electron transport phenomena in nanoscale molecular systems is presented. Besides, a brief revision of the elements of a molecular junction is included, with special interest on the applications of different molecular backbones according to the properties they exhibit. Then, the research developed in this doctoral thesis is depicted in the next four chapters (Chapter 2-5). Each of these chapters contains the following sections: i) a background of each specific topic, ii) objectives of the research, iii) results and discussion, and iv) conclusions. In particular, Chapter 2 is focused on multistate molecular systems, designed for displaying more than two conductance values. The introduction of an in-backbone linker in an asymmetric position of the molecular bridge is proposed as strategy for achieving three different conductance paths in a controlled manner. In particular, an oligo-para-phenyletinylene (p-OPE) derivative, containing a pyrimidine ring in an asymmetric position of the backbone, is used as proof-of-concept for this hypothesis. The results show that this configuration opens two new conduction channels well distinguished from each other in addition to the end-to end pathway. Chapter 3 is focused on azaborines, a family of boron–nitrogen heteroarenes. This chemical function appears frequently in π-extended systems like graphene, and leads to isoelectronic and isostructural compounds of the all-carbon analogues, but originating a charge polarization in the system. For the first time, conductance measurements of azaborine-acene derivatives are performed, with the added incentive that heteroatoms are forced to be in the most probable electron pathway. Chapter 4 is dedicated to small polycyclic aromatic hydrocarbons (PAHs), which exhibit a negative curvature due to the presence of 7-membered rings into their structure. PAHs in general are raising an increasing interest due to their fascinating optoelectronic properties. However, the electrical properties of these curved PAH cores have not been studied yet, in spite of being attractive models for studying the effect of heptagons in graphene. The synthetic strategies for incorporating different anchoring groups into these structures are discussed, and the first conductance measurements of PAHs with negative curvature are presented. Finally, an exhaustive analysis of the behaviour of terminal acetylene groups as anchoring groups is developed, while comparing the conductance of one of the prepared curved PAHs with that of its corresponding planar analogue. In Chapter 5, the implementation of an electrochemical control system in our STM is detailed. Besides, the preliminary results obtained for simultaneous application of electrochemical control and magnetic electrodes, obtained during a short stay at Bristol University, are presented. In Chapter 6, a series of general conclusions from the results of the thesis are collected. Finally, an Experimental Section, containing a description of the STMs used for the single-molecule experiments, as well as the synthetic details for preparing the studied compounds, is included. An Acronyms and Abbreviations section and a List of Publications derived from the thesis results and from other collaborations are added at the end.