Tailor-made structures for molecular junctions: from linear wires to molecular loops

In 1974, Aviram and Ratner suggested implementing molecules as the smallest building block, still providing structural diversity and functionality, allowing them to act as functional devices into electronic circuits. This visionary concept still fascinates scientists worldwide, leading to the blossom of interdisciplinary research to understand charge transport through molecules in the electrode-molecule-electrode junctions. Over the past decades, several possibilities and techniques for probing and manipulating the molecules in the junctions were developed, for instance, scanning tunneling microscope break junctions (STM-BJ), mechanically controlled break junctions (MCBJ), electromigration breakdown junctions (EBJ), and graphene molecule-graphene junctions. Beyond the initial interest in understanding electronic transport, these techniques also allowed scientists to explore interference of electron waves, mechanics, optical effects, and thermoelectric phenomena of molecular junctions.
This thesis contains the preparation of several molecules for investigations in molecular junctions, which were done in the scope of a highly interdisciplinary project named Quantum Interference Enhanced Thermoelectricity (QuIET), involving scientists from different disciplines and countries (theoretical and experimental physics and chemistry).
I. The first chapter describes the design and synthesis of presumably suitable and stable molecular rods for the investigation of charge-transport properties of graphene-molecule-graphene junctions. It is a follow-up project to the one started during my master's thesis in the group of Prof. Dr. Marcel Mayor in collaboration with the experimental physicists from the group of Prof. Dr. Michel Calame at EMPA (Swiss Federal Laboratories for Materials Science and Technology) in Zurich, Switzerland. Due to the challenges that arose during the chip preparation and molecules immobilization, the synthesis was frozen in the next-to-the-last step.
II. The second part of this thesis deals with a deeper understanding of the relationship between conductivity, quantum interference, and mechanical response of molecules implemented in mechanically controllable break junctions in dependence on difference substitution pattern. For this purpose, six molecular wires bearing the [2.2]paracyclophane as a central moiety were synthesized as model compounds. The realization of this project was only possible due to the fruitful and inspiring collaboration with the experimental physicists from the group of Prof. Dr. S. J. Herre van der Zant from the University of Technology in Delft, Netherlands, and theoretics from the group of Prof. Dr. Fabian Pauly from the University of Physics in Augsburg, Germany. The results of the first four structures are presented in the form of a IV publication. The last two structures were successfully synthesized and are now under investigation; therefore, only synthesis is included in the thesis.
III. The third chapter deals with the design and synthesis of the envisioned structure implementing molecular wire and loop scaffold, which combines two conductivity pathways: through-space and through-bond. This project was synthetically most challenging and provided surprising results. Our efforts, progress, and all challenges are summarized and will be discussed in this chapter.
IV. The last chapter provides the elucidation of cyclic dimers. The initial structure was obtained as a by-product in the macrocyclization reaction in Chapter 3 and was then transformed into the thiophene analogue. This synthetic step, as well as topological evidence and preliminary optical investigations of both dimers, are summarized in this chapter.
All chapters are constructed similarly, providing the introduction on the first pages to ease the reader into the topic. Afterward comes the project description, molecular design, synthetic strategy, results, and discussion. Also, each chapter is supplied with a summary and outlook. All the experimental parts can be found in the supporting information of the corresponding chapter and the spectra in the appendix.