2013 Theses Doctoral

Linear Conjugated Molecular Wires: Organic Materials and Single-Molecule Electronics

Meisner, Jeffrey

In this work, the synthesis and properties of different families of molecule wires are described. These families are made up of collections of linear conjugated oligomers, such as oligoenes and phenylenevinylenes and their derivatives. The bulk properties of each system were examined in order to establish structure-performance relationship between the intrinsic molecular properties of the bridging organic wire and the performance of their single-molecule junctions. The electrical as well as mechanical characteristics of single-molecular junctions were measured using the scanning tunneling-based break junction (STM-BJ) and atomic force microscope-based break junction (AFM-BJ) techniques.

In addition, stilbene molecular wires and their derivatives are ideal model compounds for both of these oligomeric families and have helped to isolate and quantify some of the factors that govern charge transport through linear conjugated molecules. After an introduction of molecular electronics, a highly tunable class of oligoenes, the α,ω-diphenyl−μ,ν-dicyano-oligoenes (DPDC) is described in the second chapter. They range from three to eleven linear C=C double bonds in length. Their synthesis is reported while their bulk solution properties show novel electronic structures, as well as broad optical absorptions and high extinction coefficients. Theoretical investigation using DFT calculations as well as strategies for functionalizing DPDCs are described.

We have found that functionalization of these intractable materials has opened new doors for their material applications. We envisioned functionalized oligoenes as molecular building blocks (i.e. conducting wires or rigid connectors) in the bottom up construction of new materials and devices. Their prototypical structure and variable length would make DPDCs ideal candidates for molecular wires especially in the field of single-molecule electronics. Molecular junctions of the form metal-oligoene-metal were formed using the STM-BJ method and their charge transport characteristics were quantified in Chapter 2. In addition, we utilize long DPDC oligomers (n > 5) as variable resistance single-molecule potentiometers.

In chapter 3, we synthesize and employ our oligoene model compounds, the stilbenes, to differentiate the mechanical from electrical properties in molecular junctions. This enabled the development of new tools for uncovering the transport mechanisms in other molecules. One example is demonstrated in chapter 4, where stilbenes proved useful as mono-functionalized molecular wires. Together with extended oligoenes, stilbene molecular wires helped us to understand how current flows through a conjugated scaffold having only one electrode binding functional group (chapter 5). We observed a π-Au interaction that is weak, however strong enough to couple electronically to the electrode and complete the molecular circuit. In the last chapter, we showcase a variety of new chemical structures that were prepared to probe the IV characteristics of organic single-molecule wires.

A series of end-functionalized (p-phenylenevinylene) (PPV) oligomers and DPDC molecular wires were prepared. Exotic end-groups were important modifications for PPV's, since they increase oligomer solubility; the singe-molecule STM-BJ measurements would not be possible on these otherwise insoluble compounds. PPV materials are very stable and can be further functionalized along their main-chains, however due to shorter effective conjugations lengths (smaller than that of the oligoenes), the range of electronic tunability is smaller in these materials. In addition to this family of symmetric molecules other asymmetric oligoene molecules were synthesized as candidates for single-molecule rectification. These molecules allow different electronic coupling to the right and left electrodes, which may modulate their IV characteristics.