Molecular Conductance. Synthesis, self-assembly, and electrical characterization of II-conjugated wires and switches

PhD ceremony: Ms. E.H. Valkenier-van Dijk, 16.15 uur, Doopsgezinde Kerk, Oude Boteringestraat 33, Groningen

Dissertation: Molecular Conductance. Synthesis, self-assembly, and electrical characterization of II-conjugated wires and switches

Promotor(s): prof. J.C. Hummelen

Faculty: Mathematics and Natural Sciences

The thesis of Hennie Valkenier-Van Dijk describes how the molecular structure determines the electrical conductance of rigid π -conjugated molecules: molecular wires. To connect the molecules to gold electrodes, Self-Assembled Monoalayers (SAMs) were formed. Valkenier-Van Dijk investigated the formation of SAMs from (bis) acetyl protected π -conjugated dithiols and optimized the procedures for the formation of high-quality densely-packed SAMs.

Valkenier-Van Dijk developed a library of molecular wires, synthesized by a general scheme, and studied molecular conductance by the ‘Matrix Approach’. In that, she studied trends in the molecular conductance of several series of molecular wires in five different junction geometries. All these experiments were performed in collaborations with different research groups.

Valkenier-Van Dijk found that the length of the molecules has a major influence on the conductance of oligo(phenylene ethynylene)-based molecular wires. Wires with a smaller HOMO-LUMO gap (or higher HOMO level) showed a higher conductance, although the length of the wire has a dominant role on the charge transport. Furthermore Valkenier-Van Dijk found that the π -conjugation pattern plays an important role in molecular conductance. Linear conjugated wires are about two orders of magnitude more conductive than cross-conjugated wires. The low conductance of cross-conjugated molecular wires (even in comparison with broken conjugated wires) can be explained by destructive quantum interference. Valkenier-Van Dijk showed that anthraquinone-based wires can be switched between a cross-conjugated and a linear conjugated state, which opens the way for redox-based conductance switching.