Biphenyl-cyclophanes : the molecular control over the conductivity of single-molecule junctions

The ongoing miniaturization trend in the semiconductor industry will soon reach its technical and physical limit. Alternative concepts are required which allow a further reduction in size of the electronic active components. The idea to build cheap semiconductor devices with molecular-scale components is fueling the current interest in Molecular Electronics. However, our ultimate goal is to understand and master single-molecule devices. Organic molecules are very attractive because they can be engineered with high complexity, and their intrinsic physical properties can be tuned by chemists. Electronic devices such as switches or rectifiers using molecular systems have already been demonstrated. Scientists have yet to develop a profound understanding of charge transport through a single molecular wire. The ultimate goal of this thesis is to develop, design and synthesize model compounds in order to contribute to the comprehension of structure-transport relationships in single molecule junctions.
The idea was to design and synthesize new “test sets” of model compounds and to characterize their electrical, electronic, optical and electrochemical properties in a multidisciplinary cooperation with physicists. In contrast to ensemble experiments, the observation of a single molecule uncovers molecular characteristics which would otherwise be averaged out in the bulk. To corroborate the molecular nature of observed transport characteristics a series of molecules with a single well defined variation of their torsion angle was investigated.
In order to gain access to parallel series of model compounds as “test sets”,
a synthetic strategy was developed allowing post-modification of the principal model structures with various chemical labels, namely anchoring groups. Biphenyl-cyclophanes, functionalized with acetyl-protected thiol groups in their terminal positions, as a series of molecules of similar length and substitution pattern and the π-backbone conformation as the only structural variable, were first proposed and synthesized.
Charge transport investigations using a STM-based break junction technique revealed that electronic hole transport through the HOMO linearly depends on the cos2Φ of the torsion angle. In order to explore the extent to which these findings also apply to electrons delocalized in the structure’s LUMO a series of dicyano-biphenyl-cyclophanes were synthesized and studied.
Transport investigation showed that the electron transport through the LUMO also follows the cos2Φ relation. Furthermore, a two step electrochemical reduction process was observed. Thereby, the potential gap between both reduction processes was found to follow the cos2Φ relation. Optical absorption spectroscopy of both series showed that the vertical excitation energy of the conjugation band correlates with the cos2Φ of the torsion angle. These correlations demonstrate that the angle measured in the solid state structure is a good proxy for the molecular conformation in solution. Furthermore, spectroelectrochemical investigations and DFT calculations support these observations. X-ray structures of nearly all model compounds, provided the torsion angles used in these correlations.
The conductance values of the fully planar model structures did not follow the trend. To understand this unexpected behavior of the fluorene derivates within the respective series several fully planer biphenyl structures were synthesized. While the axial length within the series remains similar, the type of chemical bridge in 2,2’-position of the biphenyl scaffold varies. Interesting candidates for charge transport investigation were the dibenzo-norcaradiene derivates which represent an unbent biphenyl system.
Furthermore, thiol and cyano groups were installed in the terminal position to investigate hole and electron transport. Optical absorption measurements showed that the π-systems are highly sensitive to the chemical nature of the second bridge in these planar biphenyl structures.
To conclude, this thesis discusses the relationship between molecular structure and electronic and electrical properties of several series of biphenyl systems with an emphasis on the spatial molecular conformation and the single-molecule conductance in solution. Due to the large number of related structures within a series the measured transport signatures could be assigned to their molecular origin.