Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

John. C. Price

Second Advisor

Charles T. Rogers

Third Advisor

Minhyea Lee

Fourth Advisor

Noel Clark

Fifth Advisor

Josef Michl

Abstract

Molecular rotors are considered one of the key components of molecular machinery. With permanent electric dipole moments on their rotating parts, dipolar molecular rotors also have other potential applications such as electro-optical materials and ferroelectric dielectrics. A major challenge in the development of molecular rotor systems is to install the rotors with the covalently attached rotating part into a solid and stationary frame, and in the meantime to reduce the steric interactions between the rotating parts and the environments which hinder the rotation. In this thesis, I will discuss arrays of dipolar molecular rotors formed using host-guest structures, in which the rotors are non-covalently bonded in a porous host - the hexagonal structure of tris(o-phenylenedioxy) cyclotriphosphazene (TPP) crystal. A variety of molecular rotors have been made in order to form 2-dimensional arrays on the surface of the TPP host or 3-dimensional arrays in the bulk of the TPP host. Variable temperature (liquid helium temperature to room temperature) dielectric spectroscopy is used to investigate the rotational dynamics of the rotor-host inclusion compounds. X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy are used to determine the structure of the inclusion compounds. 2-D surface rotor arrays are found in some inclusion compounds, and the barriers to rotation as low as 2.41 kcal/mol reveal the steric hindrances the rotors are experiencing. Other inclusion compounds intended for surface rotor arrays are less successful due to full insertion of the rotor molecules into the host channels, and conditions designed to prevent this have been analyzed. 3-D rotor arrays in the bulk of the host are found to have rotational barrier energies from 1.36 kcal/mol to 4.4 kcal/mol. The dependence of the barrier energies on the stationary moieties of the rotors are discussed.

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