Nobel prize winner Hoffmann to visit VUB for PhD defence

Wednesday, May 30, 2018 — Thijs Stuyver will defend his PhD dissertation on the electrical conductivity of molecules at VUB on Wednesday May 30th at 5:00pm. Among the jury members is American chemist and Nobel prize winner Roald Hoffmann. The famous chemist will travel to Belgium especially for this event, his first visit to the country in over 40 years.

Stuvyer’s doctorate deals with molecular electronics.

“My PhD studies how molecules can be used as electrical conductors. A field that is especially of importance for applications in computer science. Computers become ever smaller, while their computing power increases enormously. For years, the miniaturisation has followed the so-called Law of Moore, that posits that the number of transistor you can place on a certain surface will double every two years. Heretofore that law was more or less a good indicator for the diminution of components. These days lasers are used to print transistors onto silicon chips. However, miniaturisation in its current form is reaching its limits. That is why me, and my promoters and colleagues of the research group General Chemistry at VUB, are studying how in the future we can use molecules themselves as electronic components and how at the same time we can improve their conductivity.”

The fact that Roald Hoffmann is coming to Belgium shows the significance of Stuyver’s research. Hoffmann received the Nobel prize for Chemistry in 1981 for his innovative research in the field of theoretical chemistry and chemical bonding.

That chemical bond also exists between Hoffmann and Stuyver. For there are parallels in the work of the research group General Chemistry at VUB, where Stuyver and his promoters published a paper that drew the attention of Hoffmann. Last year Stuyver stayed at Cornell University in New York state for three months, during which he and Hoffmann - a professor there - got acquainted. “Hoffmann became one of my mentors, aside from my promoters”, according to Stuyver. “That is why he will be present at in the jury of my PhD defence.”

The research group heralded the arrival of Hoffmann with a symposium on chemical bonding at the Royal Academy for Sciences in Brussels on Monday 28th May. “It has been forty years since Hoffmann was in Belgium and that he spoke here publicly”, says Paul Geerlings, professor emeritus and former head of the VUB research group General Chemistry.

In the morning session Hoffmann gave a lecture on the current state of his field, after which the Dutch chemist Mathias Bickelhaupt (VU Amsterdam), who collaborated with Hoffmann as a postdoctoral researcher, provided a retrospective. After lunch, six young and promising Belgian researchers in theoretical chemistry presented their research. For VUB Mercedes Alonso spoke about her work “Aromaticity switches: From chemical Design to Simple-molecule electronic Devices”

Roald Hoffmann is more than a celebrated chemist. He is also a poet and playwright. “Hoffmann enjoys connection chemical theory with philosophy and the humanities”, Stuyver adds. He was born on 18 July 1937 in ZĹ‚oczów, Poland (now Ukraine) and was able to escape the Holocaust.

ir. Veronique van Speybroeck (Universiteit Gent)

Benoît Champagne (Université de Namur)

Abstract of the PhD research. Title of the PhD thesis: Qualitative Insights into the Transport Properties of Molecular Electronic Devices: A Quantum Chemical Approach

Single-molecule electronics, the study of the electrical transport through single molecules connected in a circuit, can be considered as the ultimate step in the down-sizing of electronic components and thus carries great technological potential. However, chemical understanding of the transport properties of molecules is still rather limited. This provides an impetus for theoretical and experimental chemists alike to investigate the phenomena associated with electric transport through molecules and the influence of the structure of these molecules on them.

Starting from the so-called Source-and-Sink Potential model at Hückel level of theory, an expression for the transmission probability of electrons through a molecule was obtained and cast in a graph-theoretical form. Clear structure-property relationships for the electric transport through molecules were obtained and connected to chemical concepts. With the help of the derived structure-property relationships, the occurrence of quantum interference - leading to an extremely low current when a small voltage is applied - could be predicted based on the structure of the molecule. Additionally, several guiding principles were formulated for tuning the conductivity of molecules, either through geometrical changes, through changes in the aromaticity patterns or through substitution.

With all of these tools at hand, a wide range of applications were considered throughout this thesis. We explored the switching properties of a number of light-, redoxand heat sensitive molecules, among which expanded porphyrins. Additionally, we investigated a wide range of molecular wires and all the different ways to make them more/less conducting. Furthermore, the paths the current takes through conjugated molecules has also been studied and a strategy was devised to "seal off" part of the molecule for this current. Finally, we contributed to the further development of the Source-and-Sink Potential formalism, by developing a computer code capable of visualizing the path the current takes through the molecule in real space at higher levels of theory (Hartree-Fock and Density Functional Theory).

Our results will hopefully guide experimentalists in their quest for the design of optimal functional electronic components at the single-molecule-level.

Curriculum Vitae

Thijs Stuyver (°1992) graduated as Master of Science in Chemistry at the VUB in 2014. Subsequently, he started his PhD on the topic of molecular electronics. His research was funded by the FWO through an aspirant fellowship. During his PhD, he stayed 3 months at Cornell University, USA (NY). His research has resulted in 12 publications in international peer-reviewed journals, of which 8 as first author. He supervised a bachelor thesis and presented his work at (inter)national conferences. Thijs was also involved in many of the governing bodies of the university as a representative­qualitative-insights-into-the-­transport-­properties-of-­molecular-­electronic-devices-a

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