Single molecule studies of charge transport in organic redox-active materials

Li, Jialing

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https://hdl.handle.net/2142/121434 Copy

Description

Title

Single molecule studies of charge transport in organic redox-active materials

Author(s)

Li, Jialing

Issue Date

2023-07-05

Director of Research (if dissertation) or Advisor (if thesis)

Schroeder, Charles M

Doctoral Committee Chair(s)

Schroeder, Charles M

Committee Member(s)

• Rodríguez-López, Joaquín
• Sing, Charles E
• Jackson, Nicholas E

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• molecular electronics
• scanning tunneling microscope-break junction (STM-BJ)
• single molecule conductance
• redox-active materials
• charge transport
• conjugated molecules
• radical molecules

Abstract

The development of next-generation batteries and smart electronic devices relies on the design of new organic redox-active materials. However, the performance of electronic devices critically relies on the functional properties of organic materials. Despite recent progress, key knowledge gaps remain in understanding charge transport in redox-active materials at the single molecule level. In this dissertation, I use single molecule techniques to understand charge transport in redox-active molecules, thereby bridging the gap between molecular-scale behavior and macroscopic properties. To achieve this goal, I designed and implemented a custom electrochemical scanning tunneling microscope-break junction (ECSTM-BJ) instrument that allows for the direct measurement of molecular conductance at different molecular states and electrochemical environments. Using this method, I characterize and investigate the intra- and intermolecular charge transport mechanisms of several types of materials containing redox moieties such as pyridinium, ladder-type conjugated molecules, and open-shell radical non-conjugated molecules. In Chapter 2, the role of the counterion and redox state on charge transport in viologen molecules is studied. We characterize the single molecule charge transport in viologen molecules paired with counterions of various sizes at two different redox states. Our results show that the molecular conductance of viologen varies more than 10-fold with different counterions and significantly increases upon reduction from dicationic state to the radical cationic state. Molecular modeling reveals that the extent of backbone rotation between two pyridinium rings contributes to the conductance differences observed in the experiments. These results highlight the importance of electrochemical environment on charge transport in charged redox-active materials. In Chapter 3, we investigate the charge transport properties of a cyclohexadiene-1,4-diimine derivative molecule modified with a ladder-type structure. The ladder-type structure effectively “locks-in” a planar molecular backbone conformation, thereby preventing backbone bond rotation and enhancing intramolecular charge transport. We characterize the charge transport properties of this ladder-type molecule at various protonation, lithiation, and oxidation states using single-molecule techniques. Our results show that a ladder-type oligoaniline derivative serves as a robust and reversible molecular switch with over two orders of magnitude changes in molecular conductance when controlled using chemical or electrochemical stimuli. Experimental results are complemented by molecular modeling using density functional theory (DFT) and non-equilibrium Green’s function-DFT (NEGF-DFT) to elucidate charge transport mechanisms. This work provides new strategies for advancing the stability, programmability, and efficiency of molecular charge transport using ladder-type single-molecule switches. In Chapter 4, we extend the single molecule techniques to understand the intermolecular interaction on charge transport in pyridinium-based molecule junctions. Large differences in junction displacement at different concentrations of pyridinium-based molecules are observed compared to the neutral counterpart as a result of the charge repulsion. We demonstrate that the relatively short pyridinium-based junctions can be stabilized and extended back to full-length by screening the charge repulsion via the addition of crown ethers to induce the formation of host-guest complex. These results highlight the essential role that intermolecular interactions play in charge transport of molecular junctions. In Chapter 5, we further utilize the host-guest chemistry to promote the intermolecular charge transport in pyridinium molecules. Discrete and well-defined supramolecular host-guest complexes are self-assembled in solution, enabling the study of intermolecular charge transport in π-stacked pyridinium dimers. The results show that the molecular conductance of pyridinium dimers is unexpectedly large via intermolecular charge transport compared to that of the free guest pyridinium molecules via intramolecular charge transport. Molecular modeling reveals that the efficient intermolecular charge transport arises from the reduced torsion angle in the pyridinium molecule and the strong π-π coupling in the pyridinium dimer upon encapsulation in host molecule. This work provides a new strategy to investigate and enhance the charge transport in stable π-stacked dimeric structures. In Chapter 6, a series of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical containing non-conjugated molecules is studied with single molecule conductance experiments and molecular modeling. The experimental results show that the TEMPO pendant groups enhance the molecular conductance compared to the quenched TEMPOH and neutral phenyl pendant groups. Temperature-dependent experiments are carried out to confirm that the charge transport is dominated by tunneling. Molecular simulation further indicates that the interaction between the radical pendant group and the gold electrode facilitates the high-conductance junction configuration. This work reveals the enhanced charge transport in non-conjugated molecules by incorporating open-shell radicals, which helps to inform the design of new materials for electronic devices. From a broad perspective, this dissertation demonstrates the power of using single molecule techniques to characterize charge transport in organic redox-active materials. This work holds the potential to provide a new set of molecular-scale material design rules for new device architectures for energy storage systems and organic electronics.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Jialing Li

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