New functional molecules in molecular junctions

The trend of decreasing the size of top-down fabricated silicon based electronic devices led to an increasing interest in alternative concepts which allow overcoming physical but also economical limits. The concept of integrating functional molecules into electronic devices, referred to as molecular electronics, might be a concept to fabricate smaller and cheaper devices in the future. The goal of this thesis was the design and synthesis of new functional molecules for molecular junctions. To achieve molecular electronic devices, molecules need to perform functions. Among them the most appealing ones are rectification and switching. However, to enable the design of electronic functions on a molecular level, it is equally important to develop a thorough comprehension of the relationship between the molecule’s structure and its transport properties. The contents of this thesis thus contribute to various aspects of molecular electronics.
A row of model compounds consisting of a non-varying oligo(phenylene ethynylene) (OPE) backbone was synthesized. As a reference molecule, an OPE with two terminal thiol anchor groups was inspected in a mechanical controllable break junction (MCBJ) on a single molecule level. Certain components were systematically varied to examine the role of contacts and intermolecular interactions in molecular junctions.
After the basic investigations the molecule’s function was further enriched and molecular switches and molecule based rectifiers moved into the focus of interest. Two redox switches based on ferrocene were proposed. In both switches, the ferrocene was functionalized in the 1,1’-position with two linkers comprising thiol anchor groups. Whereas in the first model compound the linkers consist of rather poor conducting alkyl groups, the linkers of the second switch are conjugated vinyl groups to provide an efficient electronic coupling to the electrodes. The latter switch was electrochemically characterized and a fully reversible redox reaction was observed. It was possible to immobilize single molecules of the switch in an MCBJ setup. Combining the MCBJ setup with an electrochemical cell, the conductance of the immobilized molecule was investigated in relation to an applied potential with respect to a reference electrode. Indeed, a change in the conductance correlating with the oxidation state was observed, thus the molecular junction can be considered as a reversible single molecule redox switch.
Furthermore a new switching concept based on the interplay of electrode and molecule was proposed. Therefore, different pathways for electrons in a cruciform type structure consisting of two crossed rods were considered. One of the rods bears terminal sulphur groups, whereas the second transversal rod is functionalized terminally with pyridine subunits. In an electrochemically controllable MCBJ the coordination of the pyridine nitrogen is expected to bind to the gold surface in dependence of the surface potential of the electrodes. The synthesis and characterization of several cruciform model compounds were synthesized to investigate the proposed coordination induced switch. The model compounds were investigated on a single molecule level in a MCBJ setup. No switching behaviour was obtained yet, nevertheless, the obtained results revealed that the molecules are functional and that electron transport is possible through both of the crossed rods.
Further a molecular rectifier based on two meta-substituted benzene units, one functionalized with an electron donating group, the other with an electron withdrawing group was proposed. The synthesis was performed, but no integration experiments have yet been performed.
An additional signal from a molecule other than the electrical response would be very appealing to obtain information from integrated molecules. We propose to profit from electroluminescence (EL) of a molecule connected to two electrodes. EL as signal from molecules is very appealing as it indicates that molecules are bridging the electrodes and the emitted light is characteristic for the fluorophore. Three target structures possessing fluorophores were designed to bridge a 5 nm gap of two single-walled carbon nanotube electrodes. The synthesis and characterization of three target structures with a length of around 7.5 nm was described. The photophysical investigations in solution confirmed the possibility of tuning the absorption and emission bands. The integration into a single-walled carbon nanotube device was successfully performed and the nanotube-molecule-nanotube junction was characterized by its I/V-characteristics. Furthermore, the characteristic emission signal of integrated molecules was detected upon electrical excitation.
The main work of this thesis was from synthetic nature, but in collaboration with physicists, it was contributed to several aspects of molecular electronics.