Molecular orientation and desorption kinetics by second harmonic generation and angle-resolved absorbance.

摘要

Organic thin films have many important applications including biopolymers, optoelectronics, and surface modifications that improve such properties as wettability and lubrication. Further development of thin film technology relies on an improved understanding of the relationship between thin film structure and function. In this thesis, new methodology for thin film characterization is tested using a variety of chemical systems, including both covalent and non-covalently bound molecular layers.;The orientation of an azobenzene chromophore was investigated for two covalent film types, monofunctional silane and trifunctional silane. The orientation was measured using angle-resolved absorbance with photoacoustic detection (ARAPD), a technique based on linear dichroism, and second harmonic generation (SHG), a non-linear technique. In both the monofunctional and trifunctional silane cases, the ARAPD result was much different than the SHG result if a narrow distribution was assumed. Relaxation of the narrow distribution assumption, to a Gaussian distribution, and the combination of the results yielded the mean orientation angle and the distribution width. Combining the two techniques resulted in an orientation mean of 61° with a width of 32° for the monofunctional silane. The trifunctional silane yielded an orientation mean of 52° with a width of 25°. In both cases, the assumption of a narrow distribution was not valid and the use of only a single technique would have lead to erroneous conclusions about the film structure.;The utility of an orientation-insensitive SHG method to measure desorption kinetics was demonstrated with a merocyanine dye. The measured rate constant for 1-docosyl-4-(4-hydroxystyryl)pyridinium bromide desorbing from fused silica was 3.9 x 10-4 sec-1 +/- 0.8 x 10-4 sec-1.;Since the optical properties of merocyanine dyes depend on interactions with a solvent, surface, or aggregates of dye molecules, the optical properties of the merocyanine dye were investigated as a function of solvent environment. In 25:75 methanol:water solution the merocyanine dye exhibited a blue shift in the absorbance spectrum as a function of concentration. The blue shift suggests the formation of aggregates of the dye at high concentrations. Additionally, evidence suggests that the absorbance spectrum of a thin film of the dye is sensitive to the pH of the substrate.