| Summary: | 博士 === 國立臺灣大學 === 化學工程學研究所 === 94 === Polyfluorenes (PFs) have been widely studied recently due to their potential applications in light-emitting diodes, thin-film transistors, and photovoltaic cells. Manipulation of their electronic structures and morphology of polyfluorenes play key roles for the above applications. In this thesis, both theoretical and experimental investigations were conducted on various fluorene-acceptor copolymers to investigate the effects of acceptor strength on their electronic and optoelectronic properties. Variation on the surface structures and photophysical prperties of fluorene-based rod-coil block copolymer brushes with selective solvents were investigated theoretically and experimentally.
In the first part of this thesis (Chapter 2), the theoretical geometries and electronic properties of fluorene-based alternating donor-acceptor conjugated copolymers and their model compounds were studied by the density function theory (DFT) at the B3LYP level with 6-31G or 6-31G** basis set. The torsional angle, bridge bond length, bond length alternation, and intramolecular charge transfer were simulated and correlated with the electronic properties, i.e. HOMO, LUMO level, and band gap. It was found that the geometries of fluorene-based donor-acceptor alternating copolymers and their model compounds are significantly affected by the structure of acceptors, particularly the ring size on the backbone. The electronic properties of the polymers and their model compounds are well correlated with the acceptor strength, coplanarity of the backbone, and intramolecular charge transfer. The theoretical study suggests that the electronic properties of alternating fluorene-acceptor conjugated copolymers could be tuned by the geometries or acceptor strength.
In the second part of this thesis, fluorene-based donor-acceptor copolymers was synthesized and utilized as the emissive layer of light-emitting diodes. In Chapter 3, a series of novel light-emitting copolymers consisted of 9,9-dihexylfluorene (F) and different acceptor segments, including quinoxaline (Q), 2,1,3-benzothiadiazole (BT) and thieno[3,4-b]-pyrazine (TP), were synthesized by the palladium-catalyzed Suzuki coupling reaction. Three fluorene-acceptor alternating copolymers (PFQ, PFBT, PFTP), six F-TP (PFTP0.5~PFTP35) random copolymers, four random copolymers with three emitting units (PFQTPs and PFBTTPs) were investigated and compared with the parent polyfluorene (PF). The experimental results suggest that the acceptor strength or content significantly affect the electronic and optoelectronic properties. The luminescence characteristics based on the prepared polymers depend on the Förster energy transfer or the intramolecular charge transfer, or heavy-atom fluorescence quenching. The tuning of the electronic and optoelectronic properties could be achieved by incorporating various acceptors or content into the polyfluorenes. The emissive color of these LEDs covers the entire visible region, including red, green, blue, and white. In Chapter 4, the photoluminescence and electroluminescence characteristics of a series of polymer blends based on fluorene-acceptor alternating copolymers were investigated. The dependences of energy transfer and quantum efficiency on the chemical structure of component and on the composition of polymer blend were rationalized. Precise control of the composition of polymer blends resulted in incomplete Förster energy transfer from donor to acceptors. Emission from all the components simultaneously resulted in efficient white emission.
In the third part of this thesis, a theoretical model based on dissipative particle dynamics was established to simulate the surface structure of rod-coil block copolymer brushes. The effects of different solvent polarities, block ratios, and grafting densities of brushes on the surface structures were investigated. For the justification of our theoretical model, a novel multifunctional amphiphilic rod-coil block copolymer of the type of PF-b-PPEGMA-b-PPOPS was synthesized by ATRP and the functional rod-coil polymer brush was successfully prepared using reactivity of the silanol-carrying PPOPS block. The comparison between simulated surface structures and AFM images shows highly similar surface structures indicating the feasibility of this theoretical model.
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