Synthesis and characterization of molecular wires with small bond-length alternation

Thesis

This thesis presents the synthesis, characterization, and single-molecule charge transport studies of two families of molecular wires, cumulenes and cyanines, both of which have small bond-length alternation (BLA) in their linear conjugated systems. Normally, the single molecule conductance of a family of molecular wires will decrease as the molecules become longer, which would hinder the molecular wires from mediating long distance charge transport. Our experimental and computational studies showed that a small BLA contributes to more efficient charge transport through molecular wires.

Chapter 1 introduces the research field of molecular electronics, including the main techniques for studying single-molecule charge transport, scanning tunnelling microscope break-junction (STM-BJ) and mechanical controlled break-junction (MCBJ), and the mechanism of charge transport through single molecules. We also discuss how BLA influences the conductance of the molecules.

Chapter 2 introduces our research into cumulene molecular wires. Cumulenes are molecules with consecutive double bonds. We synthesized a family of cumulene molecular wires, with 1, 2, 3 and 5 consecutive double bonds, then studied their charge transport properties by the STM-BJ technique and DFT calculations. Our results showed that the [3]cumulene and [5]cumulene have essentially the same conductance, suggesting that the structure of consecutive double bonds contributes to higher conductance of the molecular wires. We attribute this to the small BLA, which leads to a smaller HOMO-LUMO gap in cumulenes.

Chapter 3 introduces our research into cyanine dye molecular wires. Cyanines are molecules bearing a positive charge, which have ending groups connected by an odd-numbered polymethine bridge. For the short cyanine dyes, their positive charge fully delocalises through the molecule, leading to a ground state structure without BLA. When the molecules become sufficiently long, their positive charge starts localizing at one of the end groups, thus the ground state structure shows BLA in their polymethine chains. We synthesized a family of cyanine dye molecular wires, with 3, 5, 7, 9 and 11 carbon atoms in their polymethine chains, with PF6- and B(C₆F₅)₄- as counter ions, then studied their charge transport properties by the STM-BJ technique and DFT calculations. All cyanine dyes showed similar conductance, except the Cy11·PF₆, which has a lower conductance. Combined with the UV-vis-NIR spectroscopy and crystallographic studies, we conclude that a Peierls transition happens in the Cy11·PF₆ molecular wire, leading to charge localization and a lower conductance of the molecule.

Authors: Xu, W

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: organic synthesis; cumulene; molecular electronics; cyanine dye
• Subjects: Chemistry