| This thesis represents five years of graduate study in physical chemistry. Three years of which were devoted to the design and construction of modern optical instrumentation, such as second-harmonic and Raman spectroscopy, near field scanning optical microscopy, and time resolved and temperature dependent photoluminescence. The final two years were devoted to using the before-mentioned techniques to study the structural and electronic nature and optical phenomena of a main-chain thermotropic liquid-crystalline random copolyester composed of 73% 4-hydroxybenzoic acid (HBA) and 27% 6-hydroxy-2-naphthoic acid (HNA), commonly known as copoly(HBA/HNA) or commercially as Vectra A. This study proved that copoly(HBA/HNA) possesses a strong nonlinear, or second harmonic, response, a broad band absorption spectrum, and possesses strong photoluminescence. The strong nonlinear response is based on the net additive effect of permanent molecular electric dipoles oriented collinear with the main chain nematic director of the copolyester. Extruded thin films of copoly(HBA/HNA) possess the strong second harmonic response because of resonance effects and the highly ordered orientation of the molecular dipoles. The broad absorption spectrum is based on a distribution of strong pi-orbital conjugation strengths and/or lengths of adjoining HNA monomeric subunits. This allows for photoluminescent excitation from the ultraviolet to the near infrared regions, which correspondingly leads to a strong fluorescence emission. The time-resolved relaxation of the fluorescence emission revealed a first-order stretched exponential decay profile. This is indicative of structural and/or energetic disorder inherent within the random copolyester and is based on the distribution of conjugation strengths and/or lengths between the adjoining HNA subunits. Temperature dependent studies of the time-resolved fluorescence revealed additional disorder at high temperatures which follow classical Arrhenius behavior to reveal a fluorescence emission relaxation barrier of 1.357 kcal/mole. |
