Control of the Optical and Physical Characteristics of Conjugated Polyelectrolytes in the Solution and Solid Phase
Authors:   , Heeley Martin Edward Henry 
Published: November 2021

<p>Conjugated Polyelectrolytes (CPEs) are a branch of conducting polymers that combine the electronic and solution processability of conjugated polymers (CPs) with the ionic and self-assembling nature of polyelectrolytes. These systems have been shown to exhibit high sensitivity with changes in aggregation state and optical character dependant on the local environment. The ionic character of the CPEs can be used as scaffolds for post-synthetic alterations allowing for control of the optical and physical characteristics. In this thesis, the control of the optical and physical characteristics of the conjugated polyelectrolytes (CPEs) sodium poly[2-(3-thienyl)ethoxyl-4-butylsulfonate] (PTEBS) and poly(9,9-bis[6-(N,N,N-trimethylammonium)hexyl]fluorine-co-altphenylene] (FPQ-X, where X denotes the various counter-ions of the polymers) is investigated though the addition of various extrinsic ions to dilute solutions and concentrated solutions used for film casting, with the main focus being in the solution phase behavior. The CPE characteristics were studied primarily through UV/Vis absorption and fluorescence spectroscopy coupled with dynamic light scattering and surface tension techniques. Controlling the solution phase characteristics of the CPE was investigated through a study of through of solvent composition effects, monovalent and divalent ion addition, organic salt addition, and surfactant additions to dilute aqueous solutions of the CPEs. Solvent composition effects were shown to result in fluorescence enhancement with changes in the polarity of the solvent, while the addition of monovalent and divalent ions was shown to induce fluorescence quenching through ionic strength, ion condensation, and cross-linking of CPE molecules dependant on the concentration and valency of the metal ion. Organic salt additions of a range of concentrations were shown to result in both concentration and alkyl chain length dependant fluorescence intensity enhancements with little changes in the particle size of aggregates in solution. The lack of change in particle size suggested that the effects were localized to the aggregate surface with the size of the organic salt inducing a steric prying effect on the CPE aggregate. A proposed model of this was created to this effect. Large changes in the optical and physical characteristics of the CPEs were found with addition of surfactants to the CPE solutions. Fluorescence quenching and enhancements, particle size increases and decreases, and absorption hypsochromic shifts have been noted, with surfactant structure and concentration dependence. The resulting effects are shown to be hydrophobically, electrostatically, and self-assembly driven. Concentration control of the CPE aggregate size and optical characteristics is completed with surfactant micelles being noted at pre-CMC concentrations within the solutions. A model of interactions at the various concentration levels of surfactant has been developed explaining these results. Transferring this system to the solid state has been shown to exhibit both bathochromic and hypsochromic shifts in absorption and have two optically active phases. The dual phase absorption and emission was attributed to a CPE-surfactant complex where the CPE backbone and surfactant self assemblies result in lamellar type structures within the cast films. The optical overlap of the emission and absorption of the CPEs used was also shown to be favorable for FRET based transfer from FPQ-X to PTEBS. Films created by the layer-by-layer technique showed FRET based signal of PTEBS via excitation of FPQ-Br showing effective FRET based energy transfer between the two species. The absorption signatures of the films with multiple layer-by-layer processes showed that the films do not result in unique layers but rather interdigitated mixtures within the film. Proof of concept P3HT with DOD addition OFET devices were then created in the attempt to alter the electrode potentials using mobile ions. The devices were found to be less efficient than that of the controls due to the disruption of self assembled structures within the devices hampering electron movement.</p>