Conjugated Polymer Synthesis, Self-assembly Behavior And Its Optical And Electrical Properties

The research areas of this dissertation are focused on syntheses, photophysical characterization and self-assembly behaviors of light-emitting conjugated polymers.Since the 1990's,Ï€-conjugated polymers have attracted many interests owing to their wide-ranging application potentials in the development of organic light-emitting diodes, field effect transistors, solar cells, chem-/biosensors. A variety of conjugated polymers with different structures, different light-emitting wavelength are being developed. Due to the simple preparation process and some advantages in properties, hyperbranched structures have been introduced into conjugated polymers. Meanwhile, the copolymerization of lower band-gap monomers with higher band-gap chromophores is one of the most promising methods to tune the light wavelength and even realize white light-emitting from a single polymer chain. In this system, energy transfer process has remarkable influences on the light-emitting colors and fluorescence efficiency. However, nearly all present researches are focused on energy transfer in linear conjugated polymer backbones, therefore, we designed to make a systematic research on energy transfer in hyperbranched conjugated polymers.On the other hand, conjugated polymers and oligomers are the most promising candidates for the materials of opto-electronic molecular devices. And the structure, purity, morphology and supramolecular orientation of conjugated molecules show dominant effects on the properties of devices. The experience demonstrates that the properties of molecular devices will be further improved if the relation between macroscopic chemical structures and microscopic molecular ordering is established. And so the target of tuning material properties by molecular design will be realized. Therefore, we designed, synthesized a series of conjugated polymers and oligomers with functional groups and employed scanning probe microscopy to observe their self-assembly structures on the graphite surface. It provides information for the further design, synthesis and controlled nano-sized fabrication ofÏ€conjugated polymers and builds a solid base for the future development of organic opto-electronic molecular devices.In this dissertation, based on the previous reports and the research experience of our group, we did the systematic research on photophysical properties of conjugated polymers and their self-assembly behaviors at the interface, including three major aspects: In the first project, we introduced different contents of branching units (1,3,5-substituted benzene rings) into the poly(fluorene-co-benzothiazole) to construct the hyperbranched copolymers with branching degrees ranging from 5% to 40%. These highly branched polymers possessed good thermal stability and their decompostion temperatures increased with the increase of branching degrees. The spectra of these polymers did not show obvious changes after annealing them at 200℃in air, which also demonstrated highly branched structures led to better thermal stability. Furthermore, the energy transfer properties and their correlation with PL efficiencies of hyperbranched conjugated copolymers were studied. The narrow band gap comonomer 2,1,3-benzothiadiazole acts as a powerful exciton trap which allows efficient energy transfer of the exciton from fluorene segment to benzothiazole unit in the copolymers. As a result, the blue emission from the fluorene segment is completely quenched in concentrated solutions and in the solid state. But with the change of the solution concentration and the branching degrees; the efficiency of energy transfer of these hyperbranched polymers varied in solutions. Our experiment data demonstrated that the absorption and emission peaks of these polymers are blue-shifted gradually with the increase of branching degrees, which suggested that introduction of branching units interrupted the linearÏ€-system to some extent and reduced the effective conjugation length. Meanwhile, the critical concentration C* was used to scale the energy transfer properties in this system and its value increased with branching degrees. This indicates that intrachain interaction play a major role in the energy transfer in dilute solution and especially in these hyperbranched polymers with high branching degrees. However, as films, because of very close distance among all chromophores, only low band gap units (benzothiazole) emit luminescence around 520 nm. Cyclic voltammetry showed that hyperbranched structure in this system hindered the charge injection, which might result in low quantum efficiency for device application when. highly branching degrees. All these results demonstrated that highly branched structure would effectively impede the intra- and interchain energy migration, especially in solutions; and remarkably influence the energy transfer process in the solid state.The second project of this dissertation is the National Science Foundation supported research project cooperated with National Center for Nanoscience and Technology, China. A series of conjugated polymer and oligomers were synthesized and the highly ordered and stable supramolecular structures of these molecules at the liquid-solid interface were recorded by STM. We also summarized some effects which controlled their self-assembly behaviors.In chapter four, a series of PPEs with identical conjugated backbone but different side chains were synthesized and their two-dimensional assembly structure and chain foldings were studied by STM. The supramolecular fabrication influenced by the interactions between molecules, between molecules and substrate and the rigidity of polymer chains, were systematically discussed. The STM results demonstrated that the concentration and structure of side chains not only affected the supramolecular order on the interface, but also the chain folding in the adsorbed layer. The comparison of the chain folding of these rigid-rod polymers with that of the more flexible P3ATs illustrated that besides the interaction between polymer chains or the interaction between the side chains the rigidity of the conjugated backbone also plays an important role on the molecular assembling and chain folding at the interface. The degree of polymerization of these polymers are directly determined by STM and compared with that obtained from conventional gel permeation chromatography measurements. The statistics of the polymer contour length from the submolecularly resolved STM images shows satisfactory agreements with the theoretically expected Schulz-Zimm distribution, demonstrating the reliability of STM methods and the great importance of STM in the research of surface nano-fabrication.In chapter five, we synthesized star-shaped {C}₃-oligofluorenes end-capped with one, two, three carboxylic groups, respectively. Hydrogen bondings induced from carboxylic groups were intentionally designed at the end of the arm(s) of these star-shaped molecules. The hydrogen bonding positions of the so-perpared star-shaped oligofluorenes were similar to those of trimestic acids (TMA) which has been intensively studied, and this enlightened us to design these moleucles. Based on the simplest hydrogen bonding interactions from carboxylic acid groups, we initialize our research on hydrogen bonding induced well-ordered self-assembly of opto-electronic molecules at the interface. The assembly of these star-shaped oligomers at the interface between highly oriented pyrolytic graphite and octylbenzene or octanoic acid under ambient conditions was revealed by STM. The results demonstrated that the length of the side chains and the number of carboxylic acid groups considerably affected the final structure of self-assembly monolayers. Under the assistance of hydrogen bonding interactions,Ï€-Ï€interactions between the molecules and the substrate and the stablization effects from long side chains, every star-shaped oligomers with dedocane as side chains could form a stable assembly structure, on the contrary, those with butane as side chains could not. Moreover, the number of hydrogen bonds also led to completely different assembly patterns, especially as to star-shape oligofluorene with three -COOH end-groups which fabricated honeycomb structures like TMA only when they were dissolved in octanoic acid solution, and could not produce such an ordered network at the octylbenzene-HOPG interface unless the coadsorption with guest molecules. We also investigated these two conditions respectively.The research in the chapter 6 is based on my works on the operation of Matrix Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry as a research assistant in nearly two years. When employing this equipment to analyze some conjugated polymers, I found some novel and interesting results which was also related to the intrinsic properties of these opto-electronic materials. And I concluded these features and gave explanations in this chapter.