Ligand-controlled electronic properties in inorganic materials

Open Access
Kim, Juyeong
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
June 18, 2015
Committee Members:
  • Benjamin James Lear, Dissertation Advisor
  • Alexander Thomas Radosevich, Committee Member
  • Thomas E Mallouk, Committee Member
  • Thomas Nelson Jackson, Committee Member
  • Ruthenium complex
  • hydroxypyridine
  • TCNQ
  • pi-stacking
  • zinc oxide
  • coumarin 153
  • metal-organic framework
  • electronic property
Electron transfer is a widespread and important phenomenon studied across multiple fields of science and engineering such as biology, chemistry, and physiology. Specifically, understanding electron transfer in inorganic systems has become of significant interest since energy and health emerged as the most important focus to humankind in the 21st century. Inorganic materials and complexes function as an essential part in alternative energy conversion systems, molecular electronics, cell bodies, and medicinal treatments, and in many cases they are used as redox centers — places where electron potential energy can be stored and then extracted. In order to best-control their performance for specific applications, it is crucial to know fundamentals of how to modulate the electronic properties of inorganic materials and further have control over the electron transfer in different conditions. In this dissertation, a ligand-assisted approach is introduced to tune the electronic properties of different inorganic materials, and various types of electron transfer are investigated in such materials spectroscopically and electrochemically. I begin by demonstrating that the electronic properties of a series of ruthenium polypyridyl complexes with a hydroxypyridine ligand can be modulated by change in the electron density of the hydroxypyridine ligand. Protonation/deprotonation and ethylation of this ligand were implemented, and I found that electronic perturbation of the ligand influenced the electronic structure of the metal center as well as the bipyridine ligands via UV-visible spectroscopy, nuclear magnetic resonance spectroscopy and electrochemical measurements. The deprotonation enriched the electron density of the metal center and induced de-shielding of the protons in the bipyridine ligands, and the ethylation only increased the reduction potential for the hydroxypyridine ligand. Furthermore, the intramolecular electron transfer between the bipyridine ligands could be studied with the ethylated complex. There was no absorption band detected for the intramolecular electron transfer, indicating that the complex possesses poor electronic coupling between the ligands through the metal center. This study provided an easy strategy for controlling the electronic properties of a transition metal complex by coordinating a hydroxypyridine ligand and applying a different substituent to the ligand. Following this, I studied examining systems with redox active ligands. Coordination of organic radicals to metal centers and study of a related intermolecular electron transfer were performed by synthesizing a dinuclear zinc complex with tetracyanoquinodimethane (TCNQ) radicals, bridged with a TCNQ sigma-dimer. The individual molecules were easily stacked in crystals, forming pi-stacking of the TCNQ ligands. Unlike existing research, incorporation of the TCNQ sigma-dimer introduced a novel stacking pattern with different spacing between the TCNQ ligands. The intermolecular electron transfer was measured via diffuse reflectance spectroscopy, and the magnetic coupling between the adjacent TCNQ ligands was studied by electron paramagnetic resonance spectroscopy. The crystals showed a high-energy intermolecular charge transfer band, resulting in poor electronic conductivity. Also, I found antiferromagnetic coupling through the pi-stacks. Use of the TCNQ sigma-dimer demonstrated an example of how one could differentiate the stacking pattern in such pi-stacked systems and modify a degree of the intermolecular interaction electronically and magnetically. The ligand-assisted approach for modification of the electronic structure could be expanded from the molecular systems to nano-sized inorganic materials. I studied the effect of ligands on the surface electronic structure of semiconductor nanocrystals by using colloidal zinc oxide nanocrystals. In this case, the electronically excited species was the zinc oxide nanocrystal, and two different types of carboxylic acid ligands (aliphatic and aromatic) were adsorbed on the surface of the nanocrystals. Changes to the electronic properties were probed by examining change to the bandgap energy using UV-visible spectroscopy. The absorption spectra showed that the bandgap energy of the nanocrystal with the aromatic ligand became lower than that with the aliphatic ligand. It appears that pi orbitals of the aromatic ligand could have better coupling with the valence band of the nanocrystals, leading to decrease in the bandgap. Further studies such as precise size measurements will follow to confirm this tentative conclusion. Continuing the theme of using the effect of ligands to control electrons, ligand effect to the electronic properties of a solvatochromic molecule was demonstrated by investigating confined environment in which coumarin 153 was encapsulated in a metal-organic framework. Different solvent molecules were intercalated into the pore with coumarin 153, and the absorption and fluorescence emission spectra were acquired. After confined in the pore, coumarin 153 lost the original solvent-dependent photophysical properties in the absorption and emission process. However, the Stokes shift of coumarin 153 was observed to be solvent-dependent and increased in energy upon confinement in the pore. Based on these divergent results, I hypothesize that the framework dominantly affected the energetics of coumarin 153. Specifically, lattice flexibility of the framework, mainly from ligand rotation, might dissipate the excited state energy of the molecule. This confined system with a solvatochromic molecule can be a good platform to understand complex molecular interactions in restricted space and how to perturb the electronic properties of confined molecules through indirect contact. These studies suggest that the electronic properties of inorganic materials can be controlled by modifying the electronic structure of coordinated ligands and inducing electronic interactions between the inorganic substrate and the ligand. I believe that the ligand-assisted approach is useful for tuning the electronic properties of inorganic materials in that ligands can be easily combined with inorganic materials in the solution phase and simultaneously influence the energetics to a large degree. In addition, the fact that there are a large number of ligands to be explored provides a potential toolbox for adjusting the properties of inorganic materials.